SCHOOL OF GRADUATE STUDIES FACULTY OF TECHNOLOGY ...

ADDIS ABABA UNIVERSITY
SCHOOL OF GRADUATE STUDIES
FACULTY OF TECHNOLOGY
DEPARTMENT OF CIVIL ENGINEERING
ASSESSMENT OF LAKE ZIWAY WATER BALANCE
Thesis Submitted to the school of Graduate Studies in Partial Fulfillment
of The Requirement for The Degree of
Master of Science
In
Civil Engineering
(Major In Hydraulic Engineering)
By
Amare Mazengia Wondifraw
January, 2008

DECLARATION
This thesis is my original work and has not been presented for a degree in any
university and that all sources of material used in the thesis have been dually
acknowledged.
Candidate
Name _______________________
Signature _____________________

ACKNOWLEDGMENT
I am grateful to express my sincere thanks to my advisor Dr. Yilma Seleshi, Assistant
professor of Civil Engineering, Addis Ababa University who devoted his time to guide
and assist me in the process of the thesis work.
I would like to thank all staffs in the Ministry of Water Resource especially to those in
the Department of Hydrology and National Metrological Service Agency for providing
me with related data and materials.
I thank the staff members of Addis Ababa University Civil Engineering Department
Staff and my classmates for their material and moral support.
Last but not least, the support of my family for their material and moral support.
i

Table of Contents
CHAPTER ONE .............................................................................................................1
1. INTRODUCTION.......................................................................................................1
1.1 Background ...............................................................................................................1
1.2 Main Objective.............................................................................................2
1.2.1 Specific Objective 2
1.3 Research Problems .....................................................................................4
1.4 Research questions.....................................................................................4
1.5 Methodology ................................................................................................5
CHAPTER TWO ............................................................................................................5
2. LITERATURE REVIEW ............................................................................................5
2.1 The Hydrologic cycle ...................................................................................5
2.2 Climatic Variability .......................................................................................6
2.2.1 Topography and elevation 6
2.3 The Water Balance......................................................................................7
2.3.1 Water Balance Equation 8
2.4 The Lake......................................................................................................9
2.5 Uncertainty in the Water Balance ..............................................................10
2.6 Previous Studies on the Area ....................................................................11
CHAPTER THREE.......................................................................................................13
3. Descriptions of the Study Area. .................................................................................13
3.1 Location and Accessibility..........................................................................13
3.2 Population..................................................................................................14
3.2.1 Human 14
3.2.2 Livestock 14
3.3 Physiography and Drainage ......................................................................14
3.3.1.Lake Ziway 15
3.4. Climate and Rainfall....................................................................................16
3.5 Water Resources.......................................................................................18
3.6 Land Use and Vegetations ........................................................................18
CHAPTER FOUR.........................................................................................................20
4. HYDRO METROLOGICAL DATA ANALYSIS .....................................................20
4.1 Hydrological Data ......................................................................................20
4.1.1 Gauged River Runoff 20
4.1.1.1 Meki River 20
4.1.1.2 Katar River 22
4.1.1.3 Bulbula River. 24
4.1.2 Un-gauged Sub-Catchments Runoff. 26
4.1.3 Bathymetry Survey of the Lake 27
4.1.3.1 Lake Volume-Area-Elevation Curves27
4.1.3.2 Sediment Depositions 29
4.1.4 Lake Level 30
4.1.5 Runoff and Lake Level 32
4.2 Rainfall Data Analysis................................................................................38
ii

4.2.1 Analysis of Point Precipitation Data 38
4.2.1.1 Identification of Homogeneous Rainfall Stations Based on Monthly Rainfall
42
4.2.1.2 Estimating Missing Data 46
4.2.1.3 Checking Consistency of Data: Double Mass Analysis. 47
4.2.2 Estimation of Areal Rainfall 47
4.2.2.1 Arithmetic Average Method 48
4.2.2.2 Weighted Average Method 48
4.2.2.3 Lake Ziway Area Rainfall Distributions 49
4.2.3 Evaporation 50
4.2.3.1 Penman Combination 51
4.2.3.2 Pitch Readings 52
4.2.3.3 Factors Affecting Evaporation 52
4. 3 Rainfall and Runoff Relation in Sub-catchments ......................................56
CHAPTER FIVE...........................................................................................................60
5. LAKE WATER BALANCE MODEL .......................................................................60
5.1 Lake Water Balance Simulation.................................................................60
5.1.1 Water Balance Recursive Formula 61
5.1.1.1 Reservoir Storage 61
5.1.1.2 Gauged Surface Runoff 62
5.1.1.3 Area Rainfall over the Lake Ziway 62
5.1.1.4 Lake Evaporation 62
5.1.1.5 Abstractions 62
5.2 The Simulation Process and Analysis of the Result ..................................63
Figure 5.4 Lake Ziway water Balance ComponentsChapter six......................................67
Chapter six ....................................................................................................................68
6. ASSESSMENT OF THE CAUSE OF CHANGE IN LAKE ZIWAY LEVEL AND ITS
IMPACT ON THE TERMINAL LAKE ABIJATA. ......................................................68
6.1 Climatological Factors ...............................................................................68
6.1.1 Precipitation 68
6.1.2 Temperature 69
6.1.3 Humidity 70
6.1.4 Evaporation 70
6.1.5 Ground water 71
6.2 Anthropogenic Factors...............................................................................72
6.2.1 Deforestation & Over-grazing 72
6.2.2 Water Abstraction 74
6.2.2.1 Abstraction from Lake Ziway 74
6.2.2.1 Abstraction from Meki River 74
6.2.2.2 Abstraction from Katar River 75
6.2.3 Sediment Depositions 75
6. 2.3 Impact of Ziway Lake Level Decreasing on Lake Abijata 77
6.2.3.1 Bulbula Runoff and Abijata Lake Level Relations 77
CHAPTE SEVEN .........................................................................................................82
7. Conclusions and Recommendations...........................................................................82
7.1 Conclusions ...............................................................................................82
7.2. Recommendation .....................................................................................90
REFERENCE:...............................................................................................................92
iii

APPENDIXES ..............................................................................................................95
Appendix 4.1 Meki River Runoff Gauged at Meki Town(m3/sc) .....................95
Appendix 4.2 Katar River Runoff Gauged at Abura (m3/sc) ............................96
Appendix 4.3 Katar River Discharge at Fite Station(m3/sc).............................97
Appendix 4.4 Bulbula River Runoff Gauging at Karkarsitu (m3/sc) .................98
Appendix 4.5 Bulbula River Runoff Gauged at Bulbula Town (m3/sc).............99
Appendix 4.6 Runoff for the Ungauged Sub-Catchment (MCM)....................100
Appendix 4.7 Ziway Lake Maximum Gauge Height (m).................................100
Appendix 4.8 Lake Ziway Minimum Gauge Height (m)..................................101
Appendix 4.9 Mean Monthly Gauge Height of Lake Ziway (Reference point
1635.10masl).................................................................................................102
Appendix 4.10 Lake height with Reference point 1628masl ..........................103
Appendix 4.11 Area of the Lake with the Bathymetry Best Fit Equations (Km2)104
Appendix 4.12 Volume of The Lake using the Best Fit Curve of Bathymetry(MCM)
......................................................................................................................105
Appendix 4.13 Katar River Basin point Rainfall Distribution (mm).................106
Appendix 4.14 Meki River Basin Point Rainfall Distribution(mm)...................106
Appendix 4.15 Katar River Basin Areal Rainfall Distribution(mm) .................107
Appendix 4.16 Meki River Basin Areal Rainfall Distribution (mm) .................108
Appendix 4.17 Ziway Basin Areal Rainfall Distribution(mm)..........................109
Appendix 4.18 Lake Ziway Areal Rainfall (mm).............................................110
Appendix 4.19 Monthly Total Pitche Evaporation at Ziway town station (mm110
Appendix 4.20 Evaporation mm/month by Cropwat.......................................112
Appendix 4.21 Mean Monthly Ziway catchment Wind Speed (m/sec) ...........113
Appendix 4.22 Annual mean Wind speed......................................................113
Appendix 4.23 Mean Monthly Ziway Catchment RH (%)...............................114
Appendix 4.24 Mean Annual Ziway Catchment RH(%) .................................114
Appendix 4.25 Mean Maximum Temperatures Of All Station Ziway Catchment (0C)
......................................................................................................................115
Appendix 4.26 Mean Minimum Temperatures Of All Station in Ziway Catchment
(0C) ...............................................................................................................115
Appendix 4.27 Mean Temperatures Of 7 Stations (0C)................................115
Appendix 6.1 Abijata Lake Level (m) ................................................................1
iv

LIST OF TABLES
Table 4.1 Mean Monthly Discharges of main rivers to and from
Lake Ziway (MCM)………………………………………………………..29
Table 4.2 Mean Monthly Ziway Lake Level………………………………………..30
Table 4.3 Mean Annual Rainfall of Ziway Basin………………………………… .36
Table 4.4 Lake Ziway Mean Monthly Calculated Evaporation by Different
Methods (mm).....................................................................................52
Table 5.1: Annual Water balance of the Lake………………………………….......61
Table 6.1 Irrigation Development Scenario Result…………………………….......74
Table 7.1 Annual water balance of Lake Ziway ……… ………………………..82
v

LIST OF FIGURES
Figure 2.1 Ziway town Weather Station ..............................................................11
Figure 3.1 Location map of Lake Ziway Catchment ............................................12
Figure 3.2 Topography of the catchment derived from digital elevation model ...14
Figure 3.3 Mean Monthly Evaporation – Precipitation on the Lake Ziway……....15
Figure 3.4 Mean Annual Temperature vers Altitude…………………………… …16
Figure 3.5 Land use/land cover map………………………………………………...17
Figure 4.1 Mean Monthly Hydrograph of Meki River………………………………19
Figure 4.2 Five years moving average of Meki River discharge………………….19
Figure 4.3 Hydrograph of Katar at Abura and Fite…………………………………21
Figure 4.4 Fife years moving average Katar River Discharge……………………21
Figure 4.5 Mean Monthly Bulbula River Discharge at Kakarsitu…………………23
Figure 4.6 Hydrograph of Bulbula River at the Gauging Stations of
Kakarsitu and Bulbula Town............ …………………………………24
Figure 4.7 Elevations – Area Curve of Lake Ziway…………… . ………………....26
Figure 4.8 Elevation- Volume Curve of Lake Ziway…………………………… .....26
Figure 4.9 Elevation-Cumulative Volume & Cumm. Area Lost By the Lake…….29
Figure 4.10 Mean Annual Ziway Lake Level………………………………………..29
Figure 4.11 Mean Annual adjusted Lake level for sediment using 2005/06 Bathymetry
Servey………………………… ..........................................................29
Figure 4.12 Long-term Mean Monthly Discharges of Main Rivers
in the Catchment………………… .…………………………………….30
Figure 4.13 Scatter Plot between Ziway Lake Level and Meki River Runoff……31
Figure 4.14 Scatter Plot between Ziway Lake Level and Katar River Runoff…...31
Figure 4.15 Scatter plot between Ziway Lake level and sum of
Meki and Katar Rivers runoff…………………………………….........32
Figure 4.16 Mean Monthly Flactuations of Lake Level & bulbula outflow………33
Figure 4.17 Scatter plot Between Ziway Lake Level and Bulbula
River discharge…….…………… .............. ……………………………34
Figure 4.18 Scatter plot Between Ziway Lake Volume and Bulbula
River discharge….............................................................................36
vi

Figure 4.19 Scatter Plot of Annual Rainfall-Elevation of the Stations………… ...36
Figure 4.20 Katar River Basin Annual Rainfall Distribution – Elevation
of the stations…………………………………………………………. ...37
Figure 4.21 Katar River Basin Monthly Rainfall Distribution………………………38
Figure 4.22 Meki River Basin Annual Rainfall- Elevation of the Stations………..38
Figure 4.23 Meki River Basin Monthly Rainfall Distributions………………..…….39
Figure 4.24 Homogeneity Test of the Ziway Lake Basin………………………….40
Figure 4.25 Homogeneity Test of Meki and Ejersalale Rainfall
Stations in Ziway Lake Basin……………… ...................... …………41
Figure 4.26 Homogeneity Test of the Three Stations in the Ziway Lake Basin…41
Figure 4.27 Homogeneity Test of the Three Stations in the Lake Ziway Basin…42
Figure 4.28 Homogeneity Tests of the Five Stations in the Lake Ziway Basin… 42
Figure 4.29 River Basin Monthly Areal Rainfall Distributions………………… . …46
Figure 4.30 Comparisons of River Basin Rainfall Distribution………………… …47
Figure 4.31 Average Wind Speed- Elevations of the Stations in
Ziway Catchment……………………………………………………. .....49
Figure 4.32 Mean Annual RH- Elevations of the Stations…………………………50
Figure 4.33 Mean Annual RH of Lake Ziway Basin………………………………..50
Figure 4.34 Temperatures – Altitude Relations……………………………………. 51
Figure 4.35 Meki- Sub Catchment Wet Season Rainfall Runoff Relations……...53
Figure 4.36 Meki- Sub catchment Dry season Rainfall-Runoff Relation…… . ….54
Figure 4.37 Katar sub – catchment All Season Rainfall-Runoff Relation………..55
Figure 4.38 Katar-Sub Catchment Dry Season Rainfall-Runoff Relation ……….55
Figure 5.1 Lake Ziway Water Balance Simulation………………………………… 59
Figure 5.2 Scatter plot of Observed verse Simulated Lake Level……………… ..60
Figure 5.3 Temporal Distribution of Squared Difference between
Observed and Calculated Levels……………………………………….60
Figure 5.4 Lake Ziway Water Balance Components………………………...…….62
vii

Figure6.1Mean Annual Precipitation of Asella, Butajira and
Weighted Catchment… ................................................................... 64
Figure 6.2 Ziway town Station Temperature………………………….... ………….64
Figure 6.3 Humidity of the catchment………………………… …………………….65
Figure 6.4 Ziway Lake Surface Evaporation……………………………………… ..66
Figure 6.5 Meki and Katar River Discharge during dry period………………… …67
Figure 6.6 Rainfall and runoff of Meki Sub- Catchment during the wet season...68
Figure 6.7 Rainfall and Runoff Katar River sub-Catchment during
the wet season…..............................................................................68
Figure 6.8 Trends in catchment water use from lake and rivers……………….. 70
Figure 6.9 Human beings How Directly and Indirectly Affects the Lake,
Rivers to and out from the Lake………………………………… ........71
Figure 6.10 Practice of irrigation in the Ziway Lake Catchment enhancing
for more Abstraction……………..……………………… ................ ….71
Figure 6.11 Elevation- Cumulative Volume & Cummulative Area lost of Lake Ziway
due to Sedimentation ……………………………………….72
Figure 6.12 Mean monthly Abiata, Ziway Lake level and Bulbula Runoff……. ...75
Figure 6.13 Scatter plot of Abijata verses Ziway Lake Level………………… …..76
Figure 7.1 Comparisons of Rainfall to Evaporation ……………………………….77
Figure 7.2 Long-term Mean Monthly Discharges of Main Rivers in the
Catchment ……………………………………………………………. 79
Figure 7.3 River Basin Monthly Areal Rain fall distribution…..……………………80
Figure 7.4 Lake Ziway Water Balance Components ………………...……………83
viii

ABSTRACT
Lake Ziway, naturally existing exorheic reservoir located in the Ethiopian central Rift
valley with an average water surface area of 445km2 and estimated drainage area of
7444km 2 . It is drained with two main rivers flowing in to the lake are Meki and Katar.
And Bulbula River which spills form the lake. Global change of climate, future
uncontrolled abstraction of water and human interference of the catchments will
inevitably change the hydrologic balance of the lake. Currently the lake is used for
irrigation, water supply, fishing, transportation and recreation.
The objective of this study is to assess the water balance components of the Lake
Ziway. This activity is performed applying the continuity equation. The monthly water
budget of lake Ziway is determined from rainfall, evaporation, estimated inflow from
un gauged and gauged inflow and out flow from the lake. Average rainfall for the area
was estimated using Thiessen polygon. Evaporation was estimated using Cropwat,
Penman combination and Pitche reading. Inflow from the un gauged is developed
using area ratio method and for the gauged data was taken from ministry of Water
Resources, department of hydrology.
The model was developed using the values from each water balance component and
the main components of Lake Ziway water balance quantified are Katar, Meki &
Bulbula River runoff, runoff from un-gauged sub-catchment of the lake, precipitation
on the lake surfaces, Evaporation from the lake surface and abstractions from the
lake. The simulation of lake level variations (1980-2005) has been conducted through
modeling at monthly time stapes. The total annual inflow to the lake is equals
1106.91MCM and the total annual outflow from the lake equals 1,050.35MCM. Final
result of the water balance simulation for the lake has shown that 77.5% Of the inflow
is lost as evaporation.
It is recommended to look for an engineering solution that could be economically be
used to reduce evaporation, over utilizations and sedimentations.
ix

CHAPTER ONE
1. INTRODUCTION
1.1 Background
The Ethiopian rift valley is one of the great east African rift valleys and also called the
Afro-Arabian rift, which extends from Jordan in the Middle East, East Africa to
Mozambique in Southern Africa and from Kenya border up to the read sea and divides
the Ethiopian Highlands in to the Northern and Southern halve. The Ethiopian rift valley
is created by volcanic and faulting activities that formed volcano-tectonic depressions in
the floor of the rift, which later becomes lakes.
The floor of the rift valley encompasses three major water basins from north east to
south west (HALCROW, 1989)
• Awash basin with koka, Beseka, Gemari, and Abe as most important lake
• Central Ethiopia Rift (CER) valley with Ziway, Langano, Abijata and Shala lakes
as most important lakes
• Southern basins with Awasa, Abaya, Chamo and chew-Bahir as most important
lakes
Lake Ziway is located about 160kms south of the capital city, Addis Ababa, between
7 0 51'54’’N to 8 0 7'56’’N and 38 0 43'02’’E to 38 0 57'01’’E. It has catchment area of 7444
km 2 , an open water area of 445km 2 and average depth of 2.5m.
The lake catchment, which covers an area of about 7444km 2 , is drained with two main
rivers flowing in to the lake are Meki and Katar River . And Bulbula River which spills
form the lake.
The area is characterized by semi-arid at the rift floor to sub- humid climate at the high
land areas with mean annual precipitation, humidity, wind speed, evaporation, sunshine
1

and temperature of 758.76mm, 70%, 1.42m/sec, 1808.33mm/year, 8.52hrs/month and
20.41 0 C respectively.
Lake Ziway is one of the lakes in the rift valley used for multi purposes like irrigation,
fishing, domestic water supply, transportation, recreation and feeding fresh water to
Lake Abiyata through Bulbula River. Although its importance is in the wide range of
purposes, the water balance of the lake is poorly understood.
Proper assessments of the component of hydrological, meteorological and land use in
terms of water balance is extremely essential for any present and future water resource
development, for better managements of the lake water, understanding lake level
fluctuation and then the behavior of the lake at large. Any future sustainable utilization
of the water resources of the catchment and proper mitigation measures of the existing
problems (related to over utilization and misuse) demands the establishment of a proper
conceptual hydrologic assessment of the catchment. In this regard, a water balance is
one of the most important components of such a study to quantify the activities which
affects the lake more and at the same time shows the mitigation measures.
This study basically focuses on the components of Lake Ziway water balance.
1.2 Main Objective
The main objective of the study is to assess the water balance component of Lake
Ziway on monthly basis.
1.2.1 Specific Objective
• To estimate the various water balance components of the Lake Ziway which
includes, Surface runoff (Inflow and Out flow estimation for gauged and Un-gauged
catchment to the Lake Ziway), Evaporation and precipitation over the surface of
Lake Ziway.
2

• Assess the water abstraction and future scenario.
• Asses the impact of Lake Ziway on Lake Abiyata.
3

1.3 Research Problems
The lake water would potentially be used in the past and planned to be used in the
future extensively for agricultural, domestic water supply, floricultural purposes and the
like. For utilizing the lake water or any water from the catchments (springs, wells or
rivers) requires base line survey so that there will be no grave consequences on the
fragile rift valley environment and will lead:-
• To restrict the over utilization of the lake to the optimum level of the lake
capacity of the base line survey.
• To protect the environmental degradation by restricting the over utilization of
lake water for irrigation, domestic needs and decreasing sedimentation by
changing the land use practice in the catchment area etc
• To Improve sustainability of the water resources of the lake by utilizing proper
irrigation practice.
• To understand the combined effect of climatic, land and lake use change during
the past year and in the future.
These problems require proper hydro-meteorological studies, assessing water
resources potential of the lake and the extent to which it is to be utilized, so that there
will be no serious consequences on the rift valley environment (for future sustainable
utilization of the water resources of the lake)
1.4 Research questions
1. What are the dominant components of the hydrological cycle controlling the
hydrological behavior of the lake?
2. Does the Lake volume increase or decrease? If it is so can it be explained in terms
of climatic or anthropogenic factors?
3. What is the futurity of the Lake Ziway with the development of the human activities?
4. What is the hydrologic relation of Lake Ziway with the adjacent Lake Abijata?
4

1.5 Methodology
The methodologies used in this study to achieve the objective of the research are;
1. Literature review
2. Collecting relevant secondary data from all sources i.e. hydro-meteorological data
(rainfall, evaporation, temperature, wind speed, relative humidity, river discharge,
lake level and water abstractions from respective offices,
3. Data organization, data pre-processing and producing relevant maps; soil map and
land covers maps are from relevant hard copy maps,
4. Data quality checking and plotting, application of CROPWAT for estimation of lake
evaporation,
5. Estimating runoff for un-gauged catchments using area ratio methods,
6. Compare the lake water balance terms and understand the dominant components
on the lake level changes,
7. Application of spread sheet lake water balance model, and estimate the water
balance component of the lake.
5

CHAPTER TWO
2. LITERATURE REVIEW
2.1 The Hydrologic cycle
Considering the earth system as whole, water in the earth ocean is overwhelmingly the
largest component of the system. The amount of water in the atmosphere at any instant
of time is very small, but traffic of water through the atmosphere in the course of a
season or a year is large. Water on or near the surface of land areas, therefore, may be
regarded as an incidental effect of the interactions at the interface between the ocean of
water below and the ocean air above.
Both surface and groundwater flows originate from precipitation, which includes all
forms of falling on the ground from clouds, including rain, snow, dew, hail, and sleet.
Precipitation at any place is distributed as follows.
• A portion known as the interception is retained on buildings, trees, shrubs,
and plants. This is eventually evaporated.
• Some of the remaining precipitation is evaporated back into the atmosphere
directly.
• Another portion is infiltrated to the ground. A part of the infiltration in the root
zone is consumed by plants and trees and ultimately transpired into the
atmosphere.
• The water that percolates deeper into the ground constitutes the ground
water flow. It may ultimately appear as the base flow in streams.
• If the precipitation exceeds the combined evaporation and infiltration, puddles
known as depression storage formed. Evaporation takes place from these
puddles.
• After the puddles are filled, the water begins flowing over the surface to join
the stream channel. With reference to the precipitation this is termed the
5

excess. From the consideration of the surface water flow, this is known as
direct runoff. Some evaporation takes place from the stream surface.
• A layer of water is formed as runoff occurs. The water in this layer is known
as detention storage. Evaporation takes place from this storage as well.
When precipitations ceases, the water in detention storage eventually joins
the stream channel.
• The destination of all streams is open bodies of water, such as ocean seas,
and lakes which are subject to extensive evaporation.
The evaporation from all the sources above together with the transpiration, caries the
moisture into the atmosphere. This results in the formation of clouds that contribute to
the precipitation when steps above repeat. This chain process, driven principally by
energy from the sun, is known as the hydrologic cycle. The complete cycle is global in
nature. Sub cycles with smaller bounder limits also exist. (Ram S.Gupta Second
Editions ;)
2.2 Climatic Variability
Because of a variety of local controls the climate of a small area can differ significantly
from that of the larger surrounding region. Local difference in terrain (slope, elevation),
land surface characteristics (lakes, forests, cultivated land, Urban areas), and air
pollution affect the air flow, cloudiness, temperature, and even precipitation through
their effects on surface roughness, and the surface heat and water balance.
These small-scale variations in climate introduce significant uncertainties in the
characterization and mapping of climate from scattered point observations.
(R. Maidment, 1993)
2.2.1 Topography and elevation
Topography gives rise to a variety of meso-scale and micro scale climate variations.
Regionally, the blocking effect of mountain ranges results in forced ascent,
condensation, and heavy precipitation as the air moves up the windward slopes. To the
6

lee of the crest, the subsiding air is relatively dry owing to the dawnward movement
(subsidence) of the air and the upstream loss of the moisture. This rain shadow effect of
the mountains may extend for hundreds or even thousands of kilometers downstream.
Because of the atmospheric lapse rate, elevation has a profound effect on temperature
and precipitation type. (R. Maidment, 1993)
2.3 The Water Balance
The study of water balances is the application in the hydrology of the principle of
conservation of mass, often referred to as the continuity equation. This states that for
any arbitrary volume and during any period of time, the difference between total input
and output will be balanced by the change of water storage within the volume. In
general, therefore use of water balance technique implies measurements of both
storage and fluxes (rate of flow of water, through by appropriate selection of the volume
and the period of time for which the balance will be applied, some measurements may
be eliminated. For example, if the balance is computed for a year over a number of
years, the change in storage in any component of the hydrological cycle will usually be
smaller than errors in measurement input and outputs.
A common use of water balance equation is to infer one term from measurement of the
others, as when the evaporation from the earth’s surface is determined from
measurement of changes in soil moisture. But if all relevant terms are measured their
balance, or lack of it, can be used to check the accuracy of measurements and the
understanding of principles.
The world water balance project is mainly concerned with the measurement of all
relevant terms on the global scale. It is a central problem of the hydrology, involving the
entire hydrological cycle. Thus the world water balance constitutes the general
framework for the study of water balance in various areas (River basins, continents etc.)
and for different periods of time. The water balance equation can be usually employed
7

on its own in studies of the land components of the hydrological cycle. When
atmospheric water is to be considered, difficulties of measuring fluxes and phase
changes of water vapor usually lead to the use of techniques for indirect measurement
or estimation, based on the principle of conservation of energy. Consideration of water
balances involving phase changes, therefore, normally requires study of heat balances;
that is, the measurement of all, or all but one, of the terms in the energy-balance
equation. (R. Maidment, 1993)
The concept of global water balance is most easily envisaged for the atmosphere, the
oceans and the polar ice carps. For the land components of the hydrological cycle, not
all space and time scales are equally meaningful. There is little physical significance, for
example, in the volume of stored soil moisture for continent, or of ground water storage
in a small section of an aquifer, or of stream flow out of a region of uncoordinated
drainage. Meaningful space and time scales for any component of the hydrological
cycle may possibly derive from consideration of the hydrological regime expressed in
terms of the distribution residence time. (R. Maidment, 1993)
2.3.1 Water Balance Equation
In quantitative terms the hydrologic cycle can be represented by a closed equation
which represents the principle of the conservation of mass, often referred to in
hydraulics as a continuity equation. Many forms of this expression, called the water
balance equation, are possible by subdividing, consolidating, or eliminating some of the
terms, depending on the purpose of computation.
The Water balance can be expressed (1) for a short interval or for a long duration; (2)
for a natural drainage basin or an artificially separated boundary or the phase above the
ground surface, that below the surface, or the entire phase.
8

Three applications of the water balance equation are common: (1). a water balance
equations for large basin areas, (2). a water balance equations for water bodies, and
(3). a water balance equation for the direct runoff. In the first two cases the entire phase
above and below the ground surface is considered in the equation in terms of the
stream flows. In its general form, the equation may be represented by. (R. Maidment)
P + Q Si + Q Gi – E – Q So – Q Go – DS – n = 0 (2.1)
Where,
P = Precipitation
Q si , Q Gi = Surface and ground water inflow into the boundary from outside
E = Evaporation (including transpiration)
Q So , Q Go = Surface and ground water outflow from the boundary.
DS = Change of storage volume with in the boundary
n = Discrepancy term.
2.4 The Lake
Lakes are particularly vulnerable to change in climate parameters. Variation in air
temperature, precipitation, and other meteorological components directly cause change
in evaporation, water balance, lake level, ice events, hydro chemical and hydro
biological regimes, and the entire lake ecosystems. Under some climatic conditions,
lakes may disappear entirely. There are many types of lakes classified according to lake
formation and origin, the amount of water exchange, hydrochemistry, and so forth.
An important distinction is drawn between closed (endoreic) lakes with no out flow, and
exorheic lakes, which are drained by outflowing rivers. This also means that they are
very important indicators of climatic change and can provide records of past hydro
climatic variability over large area.
Small endoreic lakes are most vulnerable to a change in climate; there are indicators
even relatively small changes in inputs can produce large fluctuations in water level
9

(and salinity) in small closed lakes and exoreic lakes also may be sensitive to changes
in the amount of inflow and the volume of evaporation.
Climate changes also likely to have an effect on lake water quality, through change in
water temperature and the extent and duration of ice cover. (UNESCO, 1974)
2.5 Uncertainty in the Water Balance
A comprehensive analysis with regard to uncertainties in estimating the water balance of
the lake is presented by (Winter 1981), as follows; Estimates of precipitation can have a
wide range of errors, depending on gauge placement, gauge spacing, and aerial
averaging technique.
The amount of rainwater collected and measured by a raingauge may not always
represent the exact amount, which would have been caught. For example there may be
instrumental errors in the gauges, or in their recording or measuring arrangements;
some rainwater may get lost due to splash from the collector; some water from an initial
rain may got lost in moistening the gauge funnel and other inside surfaces; blowing
winds may tilt the rains from vertical, thus bringing lesser catch in the vertical gauge;
dents in the collector rim may change its receiving area; vertical upward air currents
may impart upward acceleration to precipitation this bringing lesser catch in the gauge;
etc. All such factors try to introduce errors in the measured catches. Some of them may
increases catch, and some of them may decrease the catch.
However, in general, it can be stated that almost all the errors that are introduced in the
rain catch measurements have a tendency to yield measurements which are too low. In
other words, the observed rain catch needs to be increased for the likely errors
introduced in its measurement. Of all the possible errors the most serious errors is
introduced by wind, which may result in a vertical acceleration of air, forced upward over
the gauge.
10

Higher the gauge, greater will be the wind errors, and hence more deficient will be the
rain catch.
Errors in estimate of evaporation can also vary widely depending on the instrumentation
and methodology. The energy budget is the most accurate method of calculating
evaporation with errors of the orders of 5% when applied to periods less than a week.
If pans are used that are located at a distance from the lake of interest, errors can be
considerable.
Annual pan-to-lake coefficient should be used for monthly estimates of evaporation
because they differ from the commonly used coefficient of 0.7 by more than 100%.
Errors in estimates of stream discharge are often considered to be within 5%. (R.
Maidment, 1993)
Figure 2.1 Ziway towns Meteorological Station
The pan is not covered by mesh wire, the compound is full of toll grass, to the west
direction the station is protected by houses greater than six meters, and near the road
and market area where more susceptible to dust.
2.6 Previous Studies on the Area
A number of studies have been carried out in Central Ethiopian Rift Valley that includes
the study area. These studies have been focusing on geology, volcano-tectonics,
hydrogeology, hydrology and water resource potential assessment which directly or
11

indirectly related to the current study. Some of the works are briefly described as
follows:
Makin (1976), in the work entitled ‘Prospects for irrigation Development around Lake
Ziway’, Evaluation of Water Resources, Evaluation of Agricultural potentials and Water
Balance of Lake Ziway and Abijata were analyzed.
JICA and OIDA (2001), in the project study of Meki irrigation and rural development,
the primary emphasis was given to the assessment of water resource potential in Meki–
Abijata basin. Accordingly, hydrological analysis and lake water balance were part of
the study.
HALCROW (1992), in the work entitled ‘Reconnaissance Master Plan for the
Development of the Natural Resources of the Rift valley lakes Basin’, Water Resource
Assessments, Surface Water Resources and Water balance Simulation Model were
analyzed.
12

3. Descriptions of the Study Area.
3.1 Location and Accessibility
CHAPTER THREE
The study area is located between latitude of 7 0 18’ to 8 0 25’ N and longitude of 38 0 15” to
38 0 22’E in the northern part of Central Ethiopian Rift Valley catchment; partly in Oromia
and partly in Southern Nations and Nationality states. It extends from Gurage Mountain
in the west via main Ethiopian rift valley to Mount Chilalo, Galema and Kakka of Arsi on
its eastern side. The total catchment area is about 7444km 2 .
Figure 3.1 Location map of Lake Ziway Catchment
The catchment is accessed by Addis Ababa-Mojo-Ziway, Addis Ababa-Alem Gena-
Butajira or Addis Ababa-Asela asphalt roads. Intra catchment areas are accessed by
numerous gravel and dry weather roads.
13

3.2 Population
3.2.1 Human
According to statistical abstract prepared by Central Statistical Agency, the total
population of the study area is estimated to be 1,479,451 as of July 30, 2005.
3.2.2 Livestock
Livestock population of the area was estimated using statistical report of Central
Agricultural Census Commission, 2003. Based on the data, total population of cattle,
sheep, goat, horse, asses, mule, poultry and beehives residing in the area are 452791,
164707, 93637, 30361, 44799, 3633, 481579 and 26107 respectively.
3.3 Physiography and Drainage
Generally, Lake Ziway basin is divided into three physiographic areas: the high plateau
on either side of the rift, the transitional escarpment and the rift floor. There is an
elevation difference of about 2550m between the rift floor and mountains.
The study area is bounded in the east by Chilalo (4056masl), Galama(4153masl) and
Kakka (4167masl) mountains and from the west by Guraghe mountains( 3609masl).
Lake Ziway is fed principally by Meki and Katar rivers; from its western and eastern
sides respectively (Figure 3.1). Most parts of plateau area are sources for perennial
rivers while the tributaries in the escarpments are intermittent sources. Rift floor sources
are disappearing before reaching the lake. In addition, the highland is characterized by
higher drainage density than the escarpment due differences in rock permeability,
climate and slope.
14

Figure 3.2 Topography of the catchment derived from digital elevation model
3.3.1 Lake Ziway
Lake Ziway is located at relatively higher elevation (1636masl) than the other three
Lakes Abijata, Langano and Shala. It is the largest and the shallowest lake in the central
Main Ethiopian Rift. It lies in a shallow down-faulted basin flanked in the east by a large
basalt field. Lake Ziway overflows into Lake Abiyata via the Bulbula River. The lake
level of Ziway is maintained by a lava threshold at 1636masl and the overflowing
Bulbula River descends some 50m over a distance of 30km between Lake Ziway and
Abijata. Ziway receives the bulk of its fresh water from the highlands through the Katar
and Meki River. (LRDA).
15

3.4 Climate and Rainfall
The rainfall pattern is largely influenced by the annual oscillation of the inter-tropical
convergence zone, which results in warm, wet summers (with most of the rainfall
occurring from June to September) and dry, cold and windy winters (Halcro and
Partners Ltd).
The main rainy season accounts for 60% of the total annual rainfall. Minor rain events,
originating from moist south-easterly winds, occur between March and May. Due to their
nature, these rainfall events are more pronounced in the highlands.
Rainfall in Ethiopia is erratic and subject to large special variability, which is largely
determined by altitude. Areas above 2500m may receive 1400-1800mm/year, mid
altitude regions (600-2500) may receive 1000-1400mm/year, and costal lowlands
generally receive less than 200mm/year. (Halcrow & Parteners Ltd)
Lake Ziway catchment is located in mid-altitude regions; mean annual rainfall varies
from 700mm-800mm in the valley (weather stations at Ziway town, Admi tulu, Ogelcho)
to 1150mm on the plateau (weather stations at Asella and Butajira)
Highlands flanking the Lake Ziway in both directions intercept most of the rainfall in the
basin. Open water evaporation (lake evaporation) is in the order of 1800-2000mm
per/year. This shows the total evaporation is greater than rainfall in the basin.
16

mean monthly evaporation
and Perciptation
200
180
160
140
120
100
80
60
40
20
0
Evaporation - Perciptation on the Lake Ziway
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Time
Evaporation
Perciptation
Figure 3.3 Mean Monthly Evaporation – Precipitation on the Lake Ziway
In Ethiopia, five agro-climate zones can be distinguished, ranging from alpine-type to
desert. Lake Ziway is located in sub-tropical (monsoon) agro-climatic zone. The
prevailing temperature largely depends on the altitude.
25
Mean Annual Tempreture(0C)
20
15
10
5
0
1640 2020 2100 2200 2350 2480 2940
Altitude
Tempreture vers Altitude
Linear (Tempreture vers Altitude)
Figure 3.4 Mean Annual Temperature vers Altitude
17

The number of daily sunshine hours varies from 5.7 hours in September to 9.5 hours in
November/December with average 8.3 hr. per day
3.5 Water Resources
Naturally Lake Ziway outflow level is at 1635.58masl which is the rock sill level at the
outflow of Bulbula River (LRD, 1976). The lake has a maximum depth of 7.58m and an
average depth of 2.5m.
The inflow into and the out flow from the lake has been estimated using the run-off
gauging at Meki town for Meki River, at Abura for Katar River and at Kakarsitu for the
Bulbula River.
3.6 Land Use and Vegetations
Most of the natural vegetation consists of woodland and savannas. In the highlands
(between 2000 and 3000masl) forests are found. Cultivated lands is mostly located in
the valley floor and major field crops are teff, barley, maize, lentils, horse beans,
chickpeas and field peas. Most important vegetables are grown under irrigation and
include haricot beans, tomatoes, onions, cabbage and broccoli (Woreda Agricultaural
Office).
18

Figure 3.5 Land use/land cover map
19

CHAPTER FOUR
4. HYDRO METROLOGICAL DATA ANALYSIS
4.1 Hydrological Data
4.1.1 Gauged River Runoff
4.1.1.1 Meki River
The Meki River drains an area of 2350km2 of the Gurage Mountains to the West and
North West of the Lake Ziway. Although the head water of Meki River is at an altitude of
about 3500m, the river rapidly descends the rift valley escarpment to below 2000m
before being joined by several major tributaries, including the Lebu, the Akomoja and
the Weja. The latter driving partially from saline swamps to the north of the Lake Tufa,
contributes some salinity to the main river (LRD, 1976). Downstream of its confluence
with the Weja, the Meki is incised in a steep-sided valley until it reaches Meki town at
the head of its delta. During the wet season, several shallow overflow channels carry
floods water from the Meki towards Lake Ziway in the variety of direction. So, impeding
access and causing serious local flooding which raises the water table. The Meki has
been gauged at Meki town since 1963.
On the average (using the data from 1975-2005 years) on monthly basis, maximum
flows occurs in August with a minor secondary peak in April and minimum flows
between December and Feb. As it can be observed from the hydrograph of the Meki
River, discharge at Meki town near the confluence to Lake Ziway, during Dec-Jan the
river bed may dry. It can be said that the base flow of the river dry out during the savior
dry years which is impossible to depend on runoff of the river throughout the year for
irrigation and domestic water supply.
The total annual contribution of the Meki River to the Lake Ziway is 277.81MCM.
20

Mean Monthly Meki River Discharge
35
Long term average
Discharge(m3/sec)
30
25
20
15
10
5
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Figure 4.1 Mean Monthly Hydrograph of Meki River
160
140
120
100
80
60
40
20
0
1975-1979 1980-1984 1985-1989 1990-1994 1995-1999 2000-2004
5 Years moving average Poly. (5 Years moving average)
Figure 4.2 Five years moving average of Meki River discharge
Moving 5 years average, the discharge of the Meki River decreasing continuously for
consecutive 15 years and come to the original position after 15 years. There are dry
years for consecutive 9 years and wet years for the consecutive 9 years using the mean
value as the reference point. Therefore, the wet and dry years of the Meki River
discharge varies within 7-9 years.
21

For the future developments and type of developments to be implemented in this area
the season should be taken into consideration as far as there is variation in rainfall from
year to years.
4.1.1.2 Katar River
The Katar with a large catchment of 3400.km2 rises in Arsi High lands to the East of the
lake. The catchments of the Katar River ascend to over 4100m on the summits of
mounts Badda and Kaka. Consequently, the gradient of the river is generally steep
throughout its course to Lake Ziway, and it is often deeply incised up to 50m below the
surrounding.
Gauges were installed at two stations; one at Fitte on the middle reaches of the Katar,
and the second near Ogelcho.
The hydrograph of the Katar at Ogelcho (Katar Abura) and Fitte is shown on the figure
4.3 below.
As it can be observed from the hydrograph of the river at two stations (figure 44.3), the
discharge measured at Katar Fitee is greater than the discharge at Katar Abura and the
difference is very pronounced during the high rainfall months of the country i.e. August.
This due to an abstraction either through the faults or evapo-transpiration between the
two observation stations and equal discharge measurements observed at the two
stations in the month of October & November.
The annual inflow of the Lake Ziway from the Katar River is 401.3MCM gauged at
Abura (confluence to Lake Ziway) and 562.20MCM gauged at Katar fite (sagure) at mid
of Katar River.
22

90
Comparision of Mean monthly Katar river discharge at Abura and Sagure
Mean Monthly discharge(m3/sec)
80
70
60
50
40
30
20
10
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Katar Abura
Monthes
Katar at Sagure
Figure 4.3 Hydrograph of Katar at Abura and Fite
Mean and Runoff Katar River (m3/sec)
180
160
140
120
100
80
60
40
20
0
1975-1979 1980-1984 1985-1989 1990-1994 1995-1999
Moving Average Mean Linear (Moving Average)
Figure 4.4 Fife years moving average Katar River Discharge
To see the recurrence interval of the discharge of the Katar River the 5 years moving
average was used, and the result obtained is that similar with the flow of the Meki River
which has a recurrence interval of 15 years.
23

From 1985-1989 and 1995-2004 the curve is below the average that is the dry years
and from 1975-1984 and 1990-2004 the curve is above the average that is the wet
years. From this it can be concluded that after nine wet years there is about 8 dry years.
The over all Katar River discharge is appears to be decreasing.
Although the overall pattern of flow of Katar is similar to that of the Meki, the peak flow are
more clearly defined, the base flows in the dry seasons are rather higher and it seems
most unlikely that the Katar would never dry up.
4.1.1.3 Bulbula River.
The entire outflow from Lake Ziway is carried by the Bulbula River, which flows south for
30km before discharging to Lake Abijata, a terminal lake.
The Bulbula descends some 58m over a distance of 30km between Lake Ziway and
Abijata. Except periodically during the wet season, the flow in the Bulbula usually
derives entirely from Lake Ziway. However, the Bulbula does have a significant
catchments of its own with ephemeral tributaries from the east occasionally contributing
to the flow.
The Bulbula has been gauged at a site near Bulbula village in 1968 but now it is
abandoned and the new one installed near Ziway Lake at Kakarsitu in 1980. The gauge
of Bulbula River near the Bulbula town is used for filling the missing data for the recently
gauged at Kakarsitu where there are no any intermittent tributaries to the rivers and
where there is no abstraction from the river is carried out in between.
The discharge of the Bulbula River almost dried out in 1980-1981, 1984-1990 and 2003-
2005 especially down of Bulbula town. Figure 4.5
24

50
45
40
35
30
25
20
15
10
5
0
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2002
2003
2004
2005
Bulbula river Discharge(m3/sec)
Time
Figure 4.5 Mean Monthly Bulbula River Discharge at Kakarsitu.
In actual sense the discharge at Bulbula town should greater than the discharge at
Kakarsitu during the rainy seasons since there are many ephemeral rivers between the
two stations which drains the runoff to the river from the catchments of Bulbula river.
However discharge observed at Kakaritu station is greater than measurements taken at
Bulbula town throughout the observed years. This is due to the river loss, abstraction,
along the stretch of the river and evaporation even during the rainy seasons.
The discharge outflow from the Ziway Lake is controlled by the height of the rock sill in
the Bulbula channel near Adamitulu. Outflow ceases when the lake falls to this level
which is 1635.56masl or 0.46m on the staff gauge at Ziwai town.
The total mean annual outflow from the Lake Ziway measured at Kakarsitu and Bulbula
Town is 161.33MCM and 127MCM respectively.
25

Mean Monthly Bulbula River Discharge at Kakarsitu and Bulbula town
14
12
Monthly Discharge in m3/s
10
8
6
4
2
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Bulbula at Kakarsitu
Bulbula at Bulbula Town
Figure 4.6 Hydrograph of Bulbula River at the Gauging Stations of Kakarsitu
and Bulbula Town
4.1.2 Un-gauged Sub-Catchments Runoff.
Apart from the Meki and Katar, Lake Ziway has its own catchment covering about
1354km2 (18.20% of the total catchment area) part of the lake catchments referred to
as un-gauged. The un-gauged area covers a sub-catchment adjoined to Lake Ziway
almost on all sides of the lake except a very small part to the north which is the subbasin
of the Meki River.
The gauged Katar and the Meki sub-river basin which their runoff is directly entered to
the Lake Ziway are used to estimate the runoff from the un gauged sub-catchments
using the Area Ratio Method.
The runoff from the ungauged catchments considered in this study is only in the rainy
seasons and the estimated amount is about 106.8MCM.
26

4.1.3 Bathymetry Survey of the Lake
4.1.3.1 Lake Volume-Area-Elevation Curves
On the basis of the bathymetric profile some bathymetric maps at a scale of 1:50,000
(LRD, 1976) and Ministry of Water Resource; 2005/2006) were developed. Using this
map the Volume-Area-Elevation relationships of the lake reservoir were estimated and
associated curves were established. The measuring staff gauge (zero water levels) of
the Lake Ziway is located at 1635.10masl and the outflow discharge at Bulbula River is
controlled by the height of the rock sill level of 1635.56masl or 0.46m above the staff
gauge at Ziway town is used in both Bathymetry study. The dead storage of the Lake
Ziway is the volume below the elevation 1635.56m. Then area – Volume- Elevation
above this point’s is used in this study for estimation of evaporation from the lake and at
the same time precipitations on the lake.
The time gaps between the two bathymetric surveys were 31 years. And the difference
of Volume-area-Elevations between the two bathymetry data set by LRD, 1976 and
ministry of Water resource, 2005/2006 is large. However, the graphs are shifted parallel
and makes the data reasonable. And the shifting of the graph is happened due to
sedimentation in the lake during 3 decades.
The Volume-Area-Elevation relationships are established for both bathymetry survey by
the curve fitting techniques. Accordingly the equation of the curve fitting used is
polynomial
Y = -0.074x^4 - 0.4169x^3 + 19.682x^2 – 36.599x + 8.042 with R 2 = 0.9915
Y= -0.6527x^4 + 9.6516x^3 – 4.4095x^2 + 6.0865x + 13.023 with R 2 = 0.9998
for Elevation-Area and Elevation-Volume (1976) respectively and
Y=0.0727x^6-1.7613X^5+14.953x^4-51.945x^3+72.74x^2-27.401x-0.6012withR 2 = 0.9973
Y=-0.1289x^5+2.202x^4-7.6764x^3+3.1549x^2+13.673x-2.3472 with R2= 0.9996
27

for Elevation-Area and Elevation-Volume (2005/2006) respectively
The water balance computations in this thesis work is done using the recent bathymetry
survey i.e. is by Ministry of Water Resources (2005/2006).
600
Elevation-Area
500
Area (10E6KM2))))
400
300
200
100
0
1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638
-100
Lake level above 1628masl
Elevation-Area(1976) Elevation-Area(2005/2006 Poly. (Elevation-Area(1976)) Poly. (Elevation-Area(2005/2006)
Figure 4.7 Elevation- Area Curve of Lake Ziway
2500
Leke Level-Volume
2000
Volume(mcm))m
1500
1000
500
0
1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638
-500
Elevation masl
Level-Volume(1976)
Poly. (Level-Volume(1976))
Level-Volume(2005/06)
Poly. (Level-Volume(2005/06))
28

Figure 4.8 Elevation- Volume Curve of Lake Ziway
4.1.3.2 Sediment Depositions
Using the bathymetry survey made for the different period it can possible to compute the
volume of sediment deposited during the interval of the period. Accordingly using the
bathymetry survey of the 1976 and that of 2005/206 as shown on the graph, the total
volume of sediment deposited for the last 30 years was 2076.24MCM which means the
depth of the lake reduced by 1.058m.
When there is no data about the rate of increase of erosion on a particular catchment
which is a usual case it is probably wise to assume at least in the developing countries
where there is annual population growth of 2-3% that the sediment production is
increasing at a rate in the vicinity of 50% per decade.(Soil erosion & sediment transport,
Mr.C.Aberenthy, University of Galway). Similarly in the case of lake Ziway since there is
no sediment transportation and deposition data available, the rate of increases of
sedimentation were estimated by dividing the total years into three decades and an
increment of sedimentations by 50% per decade.
Cummulative
Volume & area lost lost
Lake Level vers Cumm. Volume & Cummulative Area Lost by sediment deposits
2500.00
250.00
2000.00
200.00
1500.00
150.00
1000.00
100.00
500.00
50.00
0.00
1628 1629 1630 1631 1632 1633 1634 1635 1636 1637
Elevations asl
0.00
Lake level -Cumulative Sediment volume
Lake Level-Cummulative sediment area
29

Figure 4.9 Elevation- Cumulative Volume & Area Lost of Lake Ziway
4.1.4 Lake Level
The main water source for the lake is the flows of the Katar and Meki rivers. The mean
annual flows of these rivers are 401.26 MCM and 277.81MCM respectively. The total
catchment area of Lake Ziway is 7444Km2.
The total annual average runoff inflow in the lake can be safely estimated by the sum of
the Katar, Meki river and runoff water from the un-gauged Sub-catchment (during the
rainy seasons).
Due to the sedimentation problem of the lake the mean annual water level time series of
Lake Ziway seems an increasing. However, after an adjustment made for the sediment
deposited in the lake using the bathymetric surveys of 1976 by Over Land Seas
Development and 2005/2006 by Ministry of Water Resources the annual water level of
time series of the Lake is decreasing.
The datum or the zero gauging height taken for measuring the lake level is at
1635.10m.The level of the river bed at confluence to Bulbula River from the lake is
0.46m greater of this reference point i.e. Bulbula river bed level is 1635.56masl.
30

1.4
Mean Annual Lake Level
1.2
1
Lake Height (m)
0.8
0.6
0.4
0.2
0
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
Years
Figure 4.10 Mean Annual Ziway Lake Level
adjested Lake Level using 2005/2006 bathymetry
9
8
Lakelevel (m)
7
6
5
4
3
2
1
0
1976
1978
1980
1982
1984
1986
1988
1990
1992
Time
1994
1996
1998
2000
2002
2004
Adjested for Sediment
Figure 4.11 Mean Annual Lake Level adjusted for sediment using 2005/06 bathymetry
31

4.1.5 Runoff and Lake Level
As described above Ziway Lake is fed by Meki and Katar Rivers from its western and
eastern sides respectively; and finally outflows to Bulbula River in southwest.
To analyze the relation of the runoff- Lake Level, 30 years data of Meki, Katar, Bulbula
and Ziway Lake level are used.
Table 4.1 Mean Monthly Discharges of main rivers to the Lake Ziway and from the
Lake Ziway (MCM)
Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Katar
Abura 5.8 6.4 8.9 14.3 15 16 48 145 85 39 12 6.33 401.3
Meki at
town 2.3 3.5 10 17.1 16 14 45 87 52 23 7.2 2.27 277.8
Bulbula at
Kakarsitu 13 7.7 5.2 3.73 3.1 2.9 4.9 16 32 34 25 15.7 161.3
32

Table 4.2 Mean Monthly Ziway Lake Level
Lake Level (m) above 1628masl after an Adjustments made for Sediment deposited with in 30
years
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
7.74 7.62 7.52 7.48 7.446 7.41 7.5 7.858 8.17 8.201 8.07 7.9061
River Runoff (mcm)
160
140
120
100
80
60
40
20
0
Catchment River Runoff
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
month
Katar Runoff (mcm)
Meki River Runoff(mcm)
Bulbula River Runoff(mcm)
Figure 4.12 Long-term Mean Monthly Discharges of Main Rivers in the Catchment.
As it can be observed from figure 4.12 peaks of lake level and Bulbula River flow do not
coincide with peak discharges of rivers inflow (Meki and Katar) to the lake. There is about
30days (one month) lag time between peak of inflow and that of lake level, this is due to
lake routing. Lake level and Bulbula outflow is more dependent on Katar than Meki river
runoff.
33

Meki Runoff Vers Lake Level
Meki Runoff Ru
100
y = 60.263x - 40.811
90
R 2 = 0.4716
80
70
60
50
40
30
20
10
0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Lake Level above 1635.1masl
Series1
Linear (Series1)
Figure 4.13 Scatter plot between Meki Runoff and Lake level (lagged by one month)
Katar runoff r
160
140
120
100
80
60
40
20
Katar-Runoff Vers Lake Level
y = 110.41x - 83.746
R 2 = 0.5759
0
-20 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Lake Level above 1635.10masl
Series1
Linear (Series1)
Figure 4.14 Scatter plot between Katar Runoff and Lake level (lagged by one month)
34

Katar+Meki runoff(mcm)
700
600
500
400
300
200
100
0
Katar+Meki vers Bulbula
y = 2.2566x + 26.15
R 2 = 0.2844
0 20 40 60 80 100 120
Bulbula out flow(mcm)
Series1
Linear (Series1)
Figure 4.15 Scatter plot between Bulbula out flow(lagged by one month) and sum of
Meki and Katar Rivers runoff
Like the other lake levels the outflow of Lake Ziway through Bulbula rivers shows a
marked variations or amplifying a change in hydro-meteorological conditions. The Lake
responded not only hydrological conations but also on the geometry, particularly that of
the relations of outflow and lake levels. The relation between the outflow and intern is the
function of geometry of the lakes near its outflow zone.
For natural unregulated lakes the relation is expressed as outflow (Rout) = aH b . The b
value varies between 0 to 3 in many natural lakes. The value b=1 indicates a lake whose
outflows and lake levels are related linearly. The b values close to 0 indicates the lakes
outflows constantly through out the year. (Tana Lake Water Balance, S.Kebede).
The regration analyses of lake out flow to lake levels is, Outflow (R out )=0.2H 1.6 . In the
case of lake Ziway b = 1.6 which shows the lake levels and outflow more or less linearly
related.
35

160
Bulbula Outflow & Lake Level
Values
140
120
100
80
60
40
20
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Time(month)
Bulbul outflow(mm)
Lake Level(cm)
Figure 4.16 Mean Monthly Fluctuation of Lake Level and Bulbula outflow
The correlation coefficient of Lake Level verses Bulbula Outflow,is 0.99 which shows a
great dependence of the Bulbula outflow on the lake level.
36

Mean Bulbula Outflow Discharge (MCM)
40
35 y = 27.606x 2 - 90303x + 7E+07
R 2 = 0.9966
30
25
20
15
10
5
0
1635.7 1635.8 1635.9 1636 1636.1 1636.2 1636.3 1636.4 1636.5 1636.6 1636.7
Monthly Mean Gauge Hieght at Bulbula Outflow amsl
Figure 4.17 Scatter plot Between Ziway Lake Level and Bulbula River discharge
40
Outflow from Bulbula (MCM)
35
30
25
20
15
10
5
0
y = 6E-05x 2 + 0.0429x - 4.6456
R 2 = 0.9871
0 100 200 300 400 500 600
Volume of the lake above Bulbula outflow datum (MCM)
Figure 4.18 Scatter plot Between Ziway Lake Volume and Bulbula River discharge
37

4.2 Rainfall Data Analysis
4.2.1 Analysis of Point Precipitation Data
The data from a precipitation gauge are subject to two regular problems. A gauge site
(station) may have a short break in the record of instrument failure or absence of the
observer. It is often necessary to estimate the missing record. Another problem is that
the recording conditions at a gauge site may have changed significantly some time
during the period of the record, due to relocation or up grading of the station in the same
vicinity, difference in observational procedure, or any other reason. The problem is
resolved in both cases by comparisons with the neighboring gauge site. Table 4.3
shows the distribution of the mean annual rainfall in the study area.
Rainfall is the lowest in the vicinity of the lakes, on the valley flanks and on hill masses,
rainfall rises steadily with elevation to a maximum of 1158 mm at around 2100 masl.
38

Table 4.3 Mean Annual Rainfall of Ziway Basin Gauging Stations
Years Station Elevation Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Sum
1989-
2006 Merero 2975 19.1 37.56 65.46 73.47 50.8 76.79 145.13 168.08 75.76 43.5 15.7 17.97 789.43
1976-
2006 Bokoji 2793 30.5 42.37 88.41 107 99.1208 112.9 178.84 193.85 89.18 57.4 17.4 13.89 1030.9
1976-
2006 Kersa 2760 25.7 40.7 73.6 103.9 90.2488 84.01 118.26 125.47 107.8 56.3 17.7 12.78 856.42
1976-
2006 Digalu 2689 29.2 39.11 75.23 97.58 84.1124 104 177.41 178.57 96.83 52.1 13.7 11.78 959.54
1976-
2006 Sagure 2516 13.5 28.31 61.23 78.92 83.9479 97.18 154.09 143.82 73.2 39.8 8.8 6.413 789.24
1976-
2006 Asella 2396 18.6 41.84 90.36 120.4 118.397 119.8 182.63 195.25 150.9 64.5 19.7 14.86 1137.3
1976-
2006 Kulumsa 2153 19.2 41.63 84.23 88.48 86.6896 90.6 122.83 131.89 97.19 44.7 11.5 10.64 829.57
1978-
2006 Katar.Ge 2149 11.2 30.32 58.57 71.94 92.8731 92.6 140.05 126.37 100.8 38.5 6.23 4.661 774.15
1975-
2005 Butajira 2088 44.6 65.57 124.3 136.6 111.314 119.8 186.18 168.54 120.4 53.9 13.6 13.96 1158.9
1976-
2006 Bui 2020 27.6 51.62 90.92 90.34 87.1474 123 202.89 178.81 92.58 40.3 11.5 10.47 1007.3
1976-
2006 Tora 2012 24.9 47.5 78.2 126.7 91.9389 92.48 129.35 126.57 111.7 51.1 8.27 6.012 894.71
1976-
2006 Koshe 1873 23 48.42 80.22 98.06 90.9742 94.21 163.85 165.04 104.5 47.4 5.46 7.721 928.84
1975-
2005 Ejersalale 1779 15.4 31.42 57.77 71.1 64.364 81.01 188.53 160.13 85.39 32.3 4.99 6.285 798.72
1978-
2006 Arata 1760 17.7 34.36 66.43 77.05 82.0108 90.28 136.08 117.22 100.4 41.1 12.1 6.033 780.72
1975-
2005 Ogelcho 1690 14 28.26 72.34 69.63 62.9468 77.08 149.78 109.11 92.4 30.7 7.69 3.789 717.72
1975-
2005 Meki 1663 16.1 38.2 57.05 69.36 63.208 74.4 172.4 145.93 79.9 36.2 5.24 5.081 763.05
1975-
2005
Adami
Tulu 18.5 35.29 47.85 76.2 62.1608 74.84 128.11 125.53 84.35 39.3 14 5.091 711.16
1975-
2005 Ziway 1646 19.7 35.43 57.63 77.16 75.2032 83.85 152.51 122.12 86.64 42 2.6 3.977 758.76
Annual Rainfall of the
Stations
1400
1200
1000
800
600
400
200
Rainfall-Elevation relation of the Stations
R 2 = 0.0955
0
0 500 1000 1500 2000 2500 3000 3500
Elevation of the Stations
Rainfall-Elevation of the Stations
Linear (Rainfall-Elevation of the Stations)
Figure 4.19 Scatter Plot of Annual Rainfall-Elevation of the Stations
39

Precipitation in the catchment varies with altitude. High altitude areas like Chilalo,
Galama, and Guraghe mountains receive mean annual rainfall of over 1200mm while
the lake area gets average depth of about 735 mm. However, the correlation coefficient
between precipitation and altitude is not very strong due orographic effect and is found
to be 0.096.
There is significant orographic effect on the spatial distribution of precipitation over the
area. Areas close to mountains of eastern highland get higher mean annual
precipitation than areas found far away from the mountainous region even if the later
ones are in higher altitudes. One good example of this effect is the difference between
Asela and Sagure; where the mean annual precipitation and altitude of the Asela is
1137.30mm and 2396masl respectively, while that of Sagure, is 786.10mm and
2517masl. In addition, western half of the area gets higher spring (March to May) rainfall
than the eastern half because the Guraghe Mountains act as windward direction to the
northward movement of moist air while the eastern mountains are rain shadow at that
time.
Annual
Rainfall(mm)
1200
1000
800
600
400
200
0
Katar river basin Annual Rainfall Distribution
2975 2793 2760 2689 2516 2396 2153 1760
Elevatiion Of the Station
Figure 4.20 Katar River Basin Annual Rainfall Distribution – Elevation of the stations
40

250
Katar River basin Monthly Rainfall
200
Monthly Rainfall
150
100
50
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Merero Bokoji Kersa Digalu Series5
Sagure Asella Kulumsa Arata Ogelcho
Figure 4.21 Katar River Basin Monthly Rainfall Distribution
1400
Meki river basin annual Rainfall
1200
Yearly Rainfall
1000
800
600
400
200
0
2149 2088 2020 2012 1873 1779 1663
Elevation of theStations
Figure 4.22 Meki River Basin Annual Rainfall- Elevation of the Stations
41

250
Meki river basin Monthly Rainfall
200
Rainfall (mm)
150
100
50
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Katargenet Butajira Bui Tora Koshe Ejersalale Meki
Figure 4.23 Meki River Basin Monthly Rainfall Distributions
4.2.1.1 Identification of Homogeneous Rainfall Stations Based on Monthly Rainfall
The objective of this treatment is to preliminary classify the basin in to sub-basin which
helps various studies such as filling missing values, rainfall elevation and runoff
correlation, as well as categorizing streams in to this regions. In order to find similar
regions monthly rainfall values where non-dimensional zed and plotted to compare the
stations with each others.
The non- dimensional zing of the monthly values where carried as
P i = 100 %(P vi /P) (4.1)
Where
P i = non-dimensional value of precipitation for month i
P vi = Over years averaged monthly precipitation of the station i
P= the over years average yearly precipitation of the station
The dimensionless computations of all stations where carried out for all 17 stations used
in analyses and the profile plotted.
42

3
Homoginity test
2.5
2
Pi Values
1.5
1
0.5
0
-0.5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Arata Asela Katargenet
Koshe Kulumsa Merero
Sagure Tora+'All stations'!$C$13:$N$13 Bui
Bokoji Kersa Digalu
Ziw ay Butajira Adamitulu
Ejersalale Ogelcho Meki
Figure 4.24 Homogeneity Test of the Ziway Lake Basin
Even though the basin has somewhat similar rainfall characteristics there is slight
difference according to the river basin and elevation of the station with respect to each
other. The rainfalls at Kersa, Tora and Butajira are Bimodal, peak in April and second
peak in between July to September. Kersa gauge stations in Katar River basin and Tora
& Butajira found in Meki River basin.
43

3
Homoginity Test
2.5
Pi Values
2
1.5
1
0.5
0
Months
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Meki
Ejersalale
Figure 4.25 Homogeneity Test of Meki and Ejersalale Rainfall Stations in
Ziway Lake Basin
Strongly peaked unimodial pattern with peaks in July and dry between Novembers to
January. For the case of Meki and Ejersalale both stations are found in the same river
basin Meki River basin and both have almost the same characteristics of rainfall and the
availability of the data of one station can be used directly for the missed data of the
other.
2
Homoginty Test
1.5
Pi Vales
1
0.5
0
Months
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Kersa Tora Butajira
Figure 4.26 Homogeneity Test of the Three Stations in the Ziway Lake Basin
44

2.5
Homoginity Test
2
Pi Values
1.5
1
0.5
0
Months
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Koshe Arata Asela
Figure 4.27 Homogeneity Test of the Three Stations in the Lake Ziway Basin
The same is true for Tora and Kersa which is found in different river basin Meki and
Katar River basin respectively. Koshe in the Meki River basin and Arata and Asella in
the Katar River basin are skewed bimodal with small peaks in April and high peaks
between July to August.
Homoginity Test
3
2.5
2
Pi Valu
1.5
1
0.5
0
-0.5
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Months
Meki Ejersalale Tora Kersa Ziw ay
Figure 4.28 Homogeneity Tests of the Five Stations in the Lake Ziway Basin
45

The rainfall distribution is divided in to three patterns in the Ziway catchment i.e. is
bimodal for the high land areas, strongly peaked unimodal for the rift floor and the
transition for the escarpment. This also proves that the rainfall far away from the water
body during the dry season is higher than the rainfall near the water body and the
reverse is true during the rainy seasons.
4.2.1.2 Estimating Missing Data
Some times, the rainfall amounts at a certain rain gauges for a certain months may be
missing due to the absence of some observer or instrumental failures. In such causes, it
is needed to estimate the missing rainfall amount by approximating the value from the
data of the nearby rain gauge and homogenous rain gauge stations. The precipitation
value missing at a site can be estimated from concurrent observations at three or more
neighboring stations and homogenous stations, known as index stations, located as
close to and evenly spaced from the missing data stations as possible.
The method adopted for computing the missing rainfall data is the Normal Ratio method
which is recommended to estimate missing data in regions where annual rainfall among
stations differ by more than 10%. This method is used to fill in missing data on rainfall
station in the study area since the difference in annual rainfall between most of the
station exceeds 10% due to large elevation difference between the stations that is from
the high mountains of about 4000masl to the floor of the rift valley 1640masl at Ziway
stations. This approach enables to estimate missing data by weighing the observation at
n gauges by their respective annual average rainfall values as expressed by the
equations;
Px/Nx = 1/n (P1/N1 + P2/N2 + P3/N3 + ----+Pn/Nn) [dimensionless] (4.2)
Where
Px= missing precipitation values for station X
P1, P2, P3, …..,Pn = precipitation values at the neighboring stations for the
concurrent period
46

Nx = Normal long term mean precipitation at station X
N1, N2, …,Nn = Normal long term precipitation for neighboring stations
N = Number of index (neighboring stations.
Selection of the station for the estimating the missing rainfall data is based on the
homogeneity and similarity of the stations.
4.2.1.3 Checking Consistency of Data: Double Mass Analysis.
The double –mass analysis is a consistency check used to detect whether the data at a
site have been subjected to a significant change in magnitude due to external factors
such as tempering with the instrument, change in the recording conditions or shift in the
observation practices.
If the data are consistent, the plot will be a straight line. On the other hand, inconsistent
data will exhibit a change in slope or break at the point where the inconsistency
occurred. Double mass curve analysis was carried out for 17 rainfall stations in the
catchment. The curves show that all stations are consistent according to the criteria set
above. The grouping of the stations for the consistency checking is made according to
the homogeneity test and topographic locations. Kersa station is representing the high
lands, Katar Gent station to represent the escarpments and Adami Tule representing
the rift floor.
4.2.2 Estimation of Areal Rainfall
The representative precipitation over a defined area is required in any water Resource
Development and Management applications, where as the gauged observation pertains
to the point precipitation. The areal precipitation is computed from the record of rain
gages with in the area by the following methods.
1. Arithmetic or station average method
47

2. Weighted average method.
a. Thiessen polygon method
b. Isohyetal method
4.2.2.1 Arithmetic Average Method
This simple method consists of computing the arithmetic average of the values of the
precipitations for all stations within and in proximity to the area. It assigns equal weight
to all stations irrespective of their relative spacing and other factors. If the stations are
uniform over the area, and the rainfall rate does not differ much at various stations, then
this method is quit satisfactory. However, the Ziway catchment has different rainfall
patterns, highlands with high mean annual point rainfall, lowland (rift floor) with low
mean yearly point rainfall and escarpments having in between of the two rain fall. Then
due to the rainfall intensity vary considerably in the catchment this method may lead to
errors.
4.2.2.2 Weighted Average Method
4.2.2.2.1 Thiessen Polygon Method
In this method, the weight is assigned to each station in proportion to its representative
area defined by a polygon. It is assumed that the entire area with in a polygon is nearer
to the rainfall station that is included in the polygon than to any other rainfall station. The
rainfall recorded at that station is, therefore, assigned to that polygon. If P is the mean
rainfall on the basin, and area of the basin is A, then
P= (A 1 P 1 + A 2 P 2 + A 3 P 3 + ……………..+ A n P n )/A (4.3)
Where
48

P 1 , P 2 , P 3 … P n represent rainfalls at the respective stations, and
the
surrounding polygons have areas A 1 , A 2 , A 3 … A n
Accordingly the Ziway watershed divided into 17 polygons, the monthly and annual
rainfall areal distributions are computed for the total catchment, Katar River basin, Meki
River basin and Lake Surface.
4.2.2.3 Lake Ziway Area Rainfall Distributions
In order to evaluate the rainfall component of the water balance which is falling on the
lake, gauging stations on the shore and inside the lakes are required. The number
depends on climatic condition, relative importance of gauging stations, accuracy of
measurements and other factors.
According to Ferguson and Znamensky(1981), (citing Nezikloviski (1976) for monthly
periods in flat areas, minimum of 7 gauges for water bodies of 1000km2 – 5000 km2
and at least 3 gauges for water bodies of 100 km2 to 500km2 and 1 or 2 gauges for
water bodies 10km2 to 50km2 are needed. Further more the precipitation on the lake
surface can be different from the surrounding areas and this difference can be up to 15-
20% less in the warm seasons, and more than the surrounding area during the cold
seasons. (Sileshi, Abaya-Chamo, 2001)
For Ziway Lake, there are five rainfalls gauging station in the surrounding shore of the
lake. With the Thiesson polygons method, the rainfall station located at Ziway Town,
Meki, Adamitulu, Arata and Ogelcho have been selected by virtue of their location with
the lake, class and long period of observation consistency of the data base.
49

Mean monthly River Basin Rainfall
200
180
160
140
120
100
80
60
40
20
0
River Basin Monthly Areal Rainfall Distribution
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Monthes
Katar River Basin
Lake Ziway
Meki River Basin
Ziway Water Shed
Figure 4.29 River Basin Monthly Areal Rainfall Distributions
Comparision of River basin Rainfall Distribution
Mean rainfall(mm)
1200
1000
800
600
400
200
737.40
836.67
965.08
864.46
0
Basin 1
Lake Katar Meki Average
Figure 4.30 Comparisons of River Basin Rainfall Distribution.
4.2.3 Evaporation
Evaporation is considered from two aspects: evaporation from an open water surface
and evapo-transpiration, which is the evaporation of intercepted water and transpiration
from vegetation. Knowledge of evaporation is a major importance in water resources
50

assessment among others to determine the amount of water lost through the process of
evaporation in the water balance computations of land, rivers, lakes and reservoirs.
There are many ways of measurements and estimation of evaporation from large water
bodies. Water balance approach, Mass transfer approach, Energy balance approach,
pan evaporation approach, pitche and lysimeter measurements are some to mention.
In order to compute potential evaporation or reference evapo-transpiration, a number of
methodologies are available which include Penman and its modification based type
equations like Penman-Monteith, Temperature type equations like; Blanely-Criddle
method and Thornthwaite method. (Maidment;1993).
As it was reported in FAO revised methodology, the Penman- Moteinth equation provides
the best method for the evapo-transpiration and evaporation computation.
4.2.3.1 Penman Combination
Penman (combination approaches) is an approach which does not require surface
water temperature and is recommended for estimating free water evaporation.
(Maidment, 1993). The Penman equation reads
E
O
[ H ] + γ /( ∆ + γ )[ 0.35( 0.5 + u / )( e − e )]
= ∆l
∆ + γ )
100
(
2
a
d
(4.4)
Where
Eo = Potential evaporation that occurs from free water evaporation
[mm/day]
∆ = Slope of saturation vapor pressure curve at air temperature [kpa/co]
γ = psychometric constant [kpa/c]
U 2 = Wind speed measured at 2m [m/s]
es – ed = vapor pressure deficit [mm of Hg]
H= R I (1-r)-Ro
51

Where r is the albedo and equals 0.05 for waters. RI is a function of Ra, the solar
radiation (fixed by latitude and season) modulated by the function of the ratio, n/N, of
measured to maximum possible sunshine duration. Using r=0.05 gives
Where
RI (1-r) = 0.95R a f a (n/N)
Penman used fa(n/N) = 0.18+0.55n/N
Ra and N are obtained from standard meteorological tables.
RI(1-r) = 0.95Ra(0.18+0.55n/N)
Ro = δ T
4
a
(0.56 − 0.09
⎛ 0.10 + 0.90n
/ N ⎞
ed
) ⎜
⎟
⎝
⎠
4
δ T a
= the theoretical black body radiation at Ta which is then modified
by function of humidity of the air(ed) and the cloudiness (n/N).
N = Mean monthly values of possible Sunshine hours [h/day]
n = bright sunshine over the same period [h/day]
In our case the Lake Ziway is found at latitude of about 8 o 00’
4.2.3.2 Pitch Readings
Unfortunately, the expertise in the National Meteorological Service Agency (NMSA) has
advised not to use Class A pan readings at Ziway town station instead Pitch readings
have been collected from the Agency. The Agency have no any coefficient that would
relate the pitch reading with open water surface evaporation rates.
4.2.3.3 Factors Affecting Evaporation
Wind speed
Wind speeds are measured in the study area at Ziway, Bui, Kulumsa and Meraro. The
windiest periods are June- July for the Ziway Lake area and from October – May for the
rest sub-catchment. Merero represents the high land area, Bui and Kulumsa represent
the escarpments and Ziway station represents the rift floor. Except the Ziway station
52

wind speed, the rest have almost equal wind speed in the month of January. The
magnitude of the annual mean average of the basin wind speed is increasing with
elevation (Figure 4.31). Ziway sation data is used for the study.
Mean average wind speed(m/s)
3
2.5
2
1.5
1
0.5
0
K
Merero
Bui
Ziw ay Lake
1640 2020 2200 2940
Elevations
Figure 4.31 Average Wind Speed- Elevations of the Stations in Ziway Catchment
Relative Humidity (RH)
Humidity is the highest in the wet season, and least in November – March. The Ziway
data seems to overestimate the natural relative humidity due to the fact that the
meteorology station is located near the lake and the RH of the Bui is higher than Merero
and Kulumsa from December – March due to the bi-modal rainfall distribution of the
Meki River basin. The mean annual RH of the basin is decreasing specially in the rift
floor. Ziway station data is used for the study.
53

Mean annual RH (%)
72
70
68
66
64
62
60
Ziw ay Lake
Merero
Bui
Kulumsa
1640 2020 2200 2940
Elevation of the stations
Figure 4.32 Mean Annual RH- Elevations of the Stations
RH Values (%)
100
90
80
70
60
50
40
30
20
10
0
Ziway Basin Mean annual RH
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Years
Ziway Bui Kulumsa Merero
Figure 4.33 Mean Annual RH of Lake Ziway Basin
54

Temperature
Temperature data has been investigated for seven stations in the basin to derive
important parameters as evaporation from the water bodies and relationship between
elevation and temperature. The temperature of the rift floor is very high and that of
Merero representing the high land is relatively very low. Comparing the values of each
station with the elevation, temperature is inversely proportional with the altitude.
The highest temperatures occur between March and June prior to the start of the rainy
season, though seasonal variation in monthly temperature is relatively insignificant.
Spatial variations in temperature are largely as a result of the difference in altitude. In
the catchment mean monthly temperatures fall with increasing altitude at a rate 0.58 0 C
per 100 m.
The minimum and maximum temperature recorded at Ziway town depict that monthly
variations in temperature are relatively low which characterize the dry-sub-humid nature
of the climate prevailing in the area. The annual mean temperature of Ziway station is
increasing. Ziway station data is used for the study.
25
Mean Annual Tempreture(0C)
20
15
10
5
0
1640 2020 2100 2200 2350 2480 2940
Altitude
Tempreture vers Altitude
Linear (Tempreture vers Altitude)
Figure 4.34 Temperatures – Altitude Relations
55

Sunshine Hours
The basic source of energy to run the hydrologic cycle and for its existence is sunshine.
And to evaluate the evapo-transpiration from the land surfaces and evaporation from
the water body the data of the sunshine hours is mandatory.
The lowest sunshine hours are recorded in the rainy season while the highest values
are generally observed in November through February.
The highest sunshine hours duration is observed in the month between November to
February and the peak for all stations is in the month of November and the list is in the
month of July when the precipitations in the basin attains maximum. Ziway town station
data is used for the study.
There for using the components of evaporations, the values obtained by different
methods are shown in the table 4.4.
For this study the result obtained by Cropwat method is used for further analysis.
Table 4.4 Lake Ziway Mean Monthly Calculated Evaporation using Different Methods
(mm)
Methods Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec sum
Penman 135 135 154 147 156 144 106 125 119 145 136 124 1626
Cropwat 153 152 172 164 174 133 122 142 135 163 155 143 1808
Pitche 170.20 209 185.10 181 180.10 164.3 123.2 113.4 118.3 184.4 198.2 192.2 2019.2
LRD 163 154 194 179 201 179 154 154 142 175 159 156 2010
4. 3 Rainfall and Runoff Relation in Sub-catchments
Rainfall runoff analysis technique is very important to evaluate the relation between
runoff and rainfall in the catchments. One of the common and simple approaches in the
56

assessment of rainfall-runoff relationships is the use of pattern analysis (USGS, 2006).
The simplicity of this method lies in its basic data requirements, specifically area rainfall
and runoff records at gauging stations.
Three periods are considered in the analysis:
• Wet season ( Jun, July, August and September )
• Dry season (before and after wet season
• All Season
The catchment has two main sub-catchments: Katar and Meki. Rainfall runoff analysis
conducted in both sub-catchments.
Meki Sub-Catchment: 28 years monthly mean rainfall at Meki sub-catchment and flow
data from Meki stations are considered in the analysis.
In wet season the analysis shows that Rainfall is in a decreasing pattern but flows are in
an increasing pattern, where as, for dry season rainfall and flows are decreased
progressively. Increasing of runoff during wet season innerves to rainfall is due to
deforestation and over grazing in the sub-catachment which clearly shows there is
human being interventions in the sub-catchment.
350
300
Meki-Sub Catchment Wet Season Rainfall - runoff Relation
y = 0.2701x + 32.219(Runoff)
y = -0.0194x + 137.43(ranfall)
Rainfall-Runoff values
250
200
150
100
50
0
1976
1977
1978
1979
1981
1982
1983
1984
1986
1987
1988
1989
1991
1992
1993
1994
1996
1997
1998
1999
2001
2002
2003
Time(Monthes)
Rainfall(mm) Runoff(mcm) Linear (Rainfall(mm)) Linear (Runoff(mcm))
2004
57

Figure 4.35 Meki- Sub Catchment Wet Season Rainfall Runoff Relations
Values
Meki-sub catchment Dry Season Rainfall-Runoff Relation
300
250
200
150
100
50
0
1976
1977
1978
1979
1981
1982
1983
1984
1986
1987
1988
1989
1991
1992
1993
Time(monthes)
1994
1996
1997
1998
1999
2001
2002
2003
2004
Rainfall(mm)
Linear (Rainfall(mm))
Run-off(mcm)
Linear (Run-off(mcm))
Figure 4.36 Meki- Sub catchment Dry season Rainfall-Runoff Relation
Katar Sub-Catchment: This sub-catchment is the largest with in the lake catchment
and it has large rainfall and flow contribution for the lake basin. Rainfall and runoff
analysis for 16 years monthly mean rainfall and flow data are used.
From pattern analysis it has been observed that in Katar sub-catchment rainfall as well
as runoff decreased in different amount. The dry season rainfall and Runoff is
decreasing, where as during the wet seasons the rainfall is decreasing considerably but
runoff is increasing very slightly.
Rainfall to runoff ratio is also less than that of Meki sub catchment. This shows that
Katar sub-basin is more favorable for the development than Meki sub catchment.
Factors that could have caused the decreasing patterns in runoff was changes in
watershed characteristics due to human activities. And the effect is more pronounced in
the Meki than Katar sub-catachment.
58

250
Wet Season Rainfall-Runoff
200
Values
150
100
50
0
-50
1989
1989
1990
1991
1992
1992
1993
1994
1995
1995
1996
Time
1997
1998
1998
1999
2000
2000
2001
2002
2003
2003
Rainfall-Runoff Series2 Linear (Rainfall-Runoff) Linear (Series2)
Figure 4.37 Katar sub – catchment Wet Season Rainfall-Runoff Relation
Values
180
160
140
120
100
80
60
40
20
0
Katar-Sub Catchment Dry Season Rainfall Runoff
1989
1989
1990
1991
1992
1992
1993
1994
1995
1995
1996
1997
1998
1998
1999
2000
2001
2001
2002
2003
2004
2004
2005
Time(month)
Rainfall(mm) Runoff(mcm) Linear (Rainfall(mm)) Linear (Runoff(mcm))
Figure 4.38 Katar-Sub Catchment Dry Season Rainfall-Runoff Relation
59

CHAPTER FIVE
5. LAKE WATER BALANCE MODEL
5.1 Lake Water Balance Simulation
In order to understand the basic hydrological process, water balance computation of the
lake is made using excel spread model. This model approach stems on solving the lake
water balance equations.
The study of the water balance is the application in hydrology of the principle of
conservation of mass, often referred to as the continuity equation. This states that, for
any arbitrary volume and during any period of time, the difference between total input
and output will be balanced by the change of water storage with in the volume.
∆ s = P − E + G − G − Abs + Q
(5.1)
v
in
uot
in
Where, P= direct precipitation on the lake (mm)
E= Evaporation from the lake surface (mm).
R= Surface runoff into the lake (mm)
G in = Ground water inflow to the lake (mm)
G o = Ground water outflow from the lake (mm)
Q in = Surface water inflow into the lake (mm)
Abs = Abstraction from the lake (mm)
∆G = Net ground water flow (mm)
∆S = Change in lake storage (mm)
As quantification of G in and G out is difficult with the available data, the net ground water
flux (G in - G out ) is estimated as residual to the model output.
60

5.1.1 Water Balance Recursive Formula
Equation 5.1 is the general water balance equation formulated for the lake. Using
equation 5.1 and knowing the initial storage (at some reference time) and assuming
ground water inflow is equal with out flow from the lake, the recursive continuity
equation is formulated as:
S t =S t-1 + Q cat + P lake – E lake - Q out – Q abs (5.2)
Where
S t = Storage at the current month (mm),
S t-1 = Storage at the end of the preceding month (mm),
Q cat = inflow to the lake at the current month (Katar&Meki River in mm),
P lake = is the mean areal precipitation on the lake at the current month,
Q out = outflow discharge at the current month (Bulbula River in mm),
Q abs = is the abstraction of water at the current month, and
E lake = evaporation loss from lake at the current month (mm).
As it can be seen from the recursive formula, the components of the water balance for
Lake Ziway are :
Reservoir storage;
Surface Runoff;
Lake Evaporation;
Areal Precipitation for Lake Region
Outflow
Abstraction from the lake
5.1.1.1 Reservoir Storage
Reservoir Storage, denoted as S t is obtained from the records of the lake level at Ziway
Lake level record at Ziway town is available for 31 years (1974 to 2005). Area Vs
Elevation and Reservoir Storage Vs Elevation curves as presented on Figure 4.7 and
4.8, section 4.1.3 are used for the water balance simulation of the lake. During the
61

simulation process, storage is read from these curves applying the data obtained from
lake level records.
5.1.1.2 Gauged Surface Runoff
After infilling the missing data surface runoff for two gagging stations Abura and Meki,
including the outflow record of Lake Ziway through Bulbula River at Kakarsitu, is
considered for Water balance simulation of the reservoir.
5.1.1.3 Area Rainfall over the Lake Ziway
Ziway town gauging station near the lake and sub-basin of the lake, Meki gauging station
at the confluence of Maki river basin to the Ziway Lake, Ogelcho (Abura) at the out let of
Katar river basin drainage and at the confluence to the Lake Ziway , Arata and Adami-
Tulu were selected to calculate the precipitation on the lake surface using Thiessen
polygon method. The available rainfall records for the period 1975-2005 have been
considered for the inflow magnitude to the lake. Since the water balance is conducted on
the basis of monthly time interval, monthly rainfall series have been adopted.
5.1.1.4 Lake Evaporation
Ziway meteorological station with 16 years (1990- 2005) monthly data are used to
estimate open water surface evaporation. Result of evaporations obtained using
Cropwat method is used in the simulation processes.
5.1.1.5 Abstractions
Irrigation activities in the lake catchments are common all year round due to scarcity of
rainfall. According to OIDA (2004) report 1071 ha are directly dependent on Lake Ziway,
855ha on Katar River, 300ha on Meki and about 800ha from Bulbula River. Then at
62

present a total of 4026ha of land directly or indirectly is irrigated by the water from Lake
Ziway with common crops of Tomato, Potato and Maize.
The abstraction for irrigation from the lake has been estimated using CROPWAT model.
5.2 The Simulation Process and Analysis of the Result
The water balance recursive formula as presented in section 5.1 equation 5.2 is used to
run the simulation. Simulation is made using EXCEL soft were. The recursive formula
has been adjusted to simulate reservoir storage on monthly basis and compare the
result with the recorded values. The simulation has been conducted on a monthly time
scale for different scenarios. The firs scenario is considering the data during the Durge
Regime only and has the correlation of 0.65, the second scenario is considering the dry
seasons only and has the correlation of 0.85, the third scenario excluding un-gauged
runoff and abstraction and has the correlation of 0.61 and the fourth scenario
considering all the components and has the correlation of 0.87 continuously starting
from Jan 1980 to Dec 2005.
The reasonable agreement between the measured and simulated lake level is achieved
for the simulation period is the fourth scenario. The output result as shown on figure 5.2,
there is no as such deviations between the simulated and observed lake levels. The
scatter plot between the observed and simulated lake level has the correlation of 87%.
The more misfit between the simulated and observed lake level from 1981-1983, 1991-
1996 and that of 2003-2005 may be due to uncertainty on the hydro-meteorological
records, estimating on un-gauged runoff, abstractions and due to an assumptions made
groundwater inflow and outflows taken equal in the simulation.
63

3500
Observed Vers Simulated Lake Level Relation
Reference Level 1635.10masl used as Zero Level
3000
2500
Lake Level (mm)
2000
1500
1000
500
0
-500
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
Simulated
Time(month)
Observed
Figure 5.1 Lake Ziway Water Balance Simulations
Observed Lake Level(mm)
2500
2000
1500
1000
500
R 2 = 0.8665
Obserbed Vers Simulated Lake Level
0
0 500 1000 1500 2000 2500
Simulated Lake Level(mm)
Observed Vers Simulated
Linear (Observed Vers Simulated)
Figure 5.2 Scatter plot of Observed verse Simulated Lake Level.
64

Temporal Distribution of squared difference between observed and
calculated levels
1200000
1000000
Lake level difference
square(mm)
800000
600000
400000
200000
0
1980
1981
1982
1983
1984
1985
1987
1988
1989
1990
1991
1992
1994
1995
Time(month)
1996
1997
1998
1999
2001
2002
2003
2004
2005
Figure 5.3 Temporal Distribution of Squared Difference between Observed and
Calculated Levels.
The annual average water balance is tabulated on table 5.1 and presented in figure 5.4
The balance has an error of + 127.72mm/year. The source of this error can be attributed
to the lake of direct measurements of accurate evaporation rate, abstractions and un
gauged runoff, or the fact that we assumed net groundwater flux and inaccuracy of data
for the gauged
65

Table 5.1: Annual Water balance of the Lake
Inflow (mm)
Outflow (mm)
Month Katar Meki
Lake
surface P
Un
guaged
Bulbula at
Karkarsitu
Abstractions
Lake
Surface
E
Balance(mm)
Jan 13.26 5.26 16.87 - 29.71 4.12 158.59 (157.04)
Feb 14.76 8.07 35.92 - 17.75 11.74 157.79 (128.54)
Mar 20.62 23.17 61.10 - 12.05 23.38 178.12 (108.67)
Apr 33.19 39.69 74.23 - 8.66 30.78 169.03 (61.36)
May 34.86 37.19 67.15 - 7.21 8.87 180.59 (57.46)
Jun 37.23 32.57 76.67 - 6.75 0.40 168.37 (29.05)
Jul 111.30 104.34 142.98 49.64 11.36 0.10 142.89 253.90
Aug 327.98 196.79 128.25 105.83 36.19 0.10 145.08 577.47
Sep 180.29 110.29 86.51 60.03 67.87 0.26 143.75 225.24
Oct 82.41 48.60 35.70 23.35 71.84 0.70 167.93 (50.41)
Nov 26.20 15.72 6.92 - 54.59 1.10 159.49 (166.33)
Dec 14.23 5.10 5.11 - 35.30 1.15 158.03 (170.04)
Total
(annual) 896.32 626.79 737.41 238.85 359.29 82.70 1,929.66 127.72
.
66

350.00
Water Balance Components (mm)
Balance Components(mm)
300.00
250.00
200.00
150.00
100.00
50.00
-
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Time
Katar Meki Lake Surface Preciptations Ungauged area
Bulbula Abstractions Lake surface evaporations
Figure 5.4 Lake Ziway water Balance Components
67

CHAPTER SIX
6. ASSESSMENT OF THE CAUSE OF CHANGE IN LAKE ZIWAY LEVEL AND ITS
IMPACT ON THE TERMINAL LAKE ABIJATA.
6.1 Climatological Factors
6.1.1 Precipitation
Precipitation is the main driver of variability in the water balance over space and time
and change in precipitation have very important implication for hydrology and water
resources. Hydrological variability over time in a catchment is influenced by variations in
precipitation over daily, seasonal, annual and decade time scale.
Many of the level of the rift lake fluctuate according to the precipitation trend in the
adjacent high lands (street, 1979). Accordingly, in this study monthly records of Butajira
(Middle altitude), Asela (high altitude) and weighted average of the Lake Ziway
catachment since the start of the declining water level of the lake were used for analysis
of the impact.
From the figure 6.1 the slope of the time serious rainfall of the two stations and the
weighted catchment is negative which shows that precipitation is apparently
decreasing.
68

Rainfall according to altitude
mean annual prcipitation (mm)
1600
1400
1200
1000
800
600
400
200
0
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Time(year)
Asella station rainfall(mm)
Ziway areal rainfall (mm)
Linear (Asella station rainfall(mm))
Butajira station rainfall(mm)
Linear (Butajira station rainfall(mm))
Linear (Ziway areal rainfall (mm))
Figure 6.1 Mean Annual Precipitation of Asella, Butajira and Weighted Catchment.
6.1.2 Temperature
The main component of the evaporation is temperature. Then in this study the
temperature of the Ziway station is used for analyzing the effects.
Ziway station mean annual temp.
Mean annual Temp.
22
21.5
21
20.5
20
19.5
19
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Time(year)
Figure 6.2 Ziway towns Station Temperature.
The trend of temperature is increasing continually which increase the rate of
evaporation and then affect the water balance of the catchment.
69

6.1.3 Humidity
Humidity of the four stations in the catchments are taken and shown in the figure 6.3. All
the graphs shows apparently decreasing which means the main components of the lake
water balance, evaporation is increasing ultimately the lake level is decreasing.
RH Values (%)
100
90
80
70
60
50
40
30
20
10
0
Ziway Basin Mean annual RH
1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Years
Ziway Bui Kulumsa Merero
Figure 6.3 Humidity of the catchment.
6.1.4 Evaporation
Increasing temperature generally results in an increasing in potential evaporation largely
because the water holding capacity of air is increased.
In this study change in evaporation in the study area was assessed using Cropwat
methods with the Ziway station meteorological data.
As it can be observed from the figure 6.4 the main component of climate and affects the
water balance component greatly in the arid area, i.e. the rate of the evaporation is
continuously increasing which means the lake level ultimately decreasing.
70

Mean annual Evaporation(mm)
2200
2150
2100
2050
2000
1950
1900
1850
Evaporation of Lake Ziway
1989
1990
1991
1992
1993
1994
1995
1996
Figure 6.4 Ziway Lake Surface Evaporation
6.1.5 Ground water
1997
1998
Time(year)
1999
2000
2001
2002
2003
2004
2005
The hydrograph of the river is the representation of runoff of a river. The runoff is the
sum of surface runoff and ground water flow. When surface runoff is zero, the runoff is
equal to the ground water flow. Therefore, during the periods of zero surface runoff, the
hydrograph of a river will represent the hydrograph of the ground water flow into the
river, i.e. ground water depletion curve.
As a recharge is a function of precipitation, evapo-transpiration, land use riverbed
morphology and its mode of flow, any change in these parameters could result in
change of ground water recharge, although the magnitude may vary over time and
space.
Change in ground water recharge can be resulted from change in land use. According
to Makin 1976, there is land degradation in the rift valley especially in the Lake Ziway
area due to its accessibility, freshness of the water and over population; impacting the
environment by deforestation and poor soil management practice. The effect of land
degradation on recharge was analyzed by the change in sustainability of base flow.
71

In this study the discharge of river Katar, and Meki during the dry season (after adding
the abstractions during the dry period) was analyzed in order to visualize the trend of
ground water flow.
Meki & Katar River Discharge during dry month
Monthly Discharge (m3/sec)
3.5
3
2.5
2
1.5
1
0.5
0
1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
Time (month)
Meki discharge (Dec.)
Linear (Katar discharge (Jan.))
Katar discharge (Jan.)
Linear (Meki discharge (Dec.))
Figure 6.5 Meki and Katar River Discharge during dry period
It is clear that from the figure 6.5 both rivers parallely decreasing significantly during dry
period which is an indication of ground water depletion in the catchment.
The decreasing of the ground water is either due to climatic change or human induced
activities.
6.2 Anthropogenic Factors
6.2.1 Deforestation & Over-grazing
Deforestation and overgrazing are related to many factors, and also it is the most
important. The pressure of a dense vegetative cover over a soil, increase the infiltration
capacity of a soil to a considerable extent. In the presence of a vegetative cover the rain
will not fall directly on the soil, and hence not able to compact a soil. The vegetation
cover also provides a layer of decaying organic mater which promotes the activity of
burrowing insects and animals, which in turn produces spongy permeable soil structure.
72

Both these factors help in increasing the infiltration capacity, and hence the presence of
a vegetative cover causes the infiltration capacity to increase predominantly. But in the
case of Ziway Catchment, the rate of runoff is increasing in the contrary to rainfall
during the rainy season(June, July, August & September), which shows the decreasing
of infiltration that is due to deforestation and overgrazing predominantly.
Rainfall-Runoff values
350
300
250
200
150
100
50
Meki-Sub Catchment Wet Season Rainfall - runoff Relation
y = 0.2701x + 32.219(Runoff)
y = -0.0194x + 137.43(ranfall)
0
1976
1977
1978
1979
1981
1982
1983
1984
1986
1987
1988
1989
1991
1992
1993
1994
1996
1997
1998
1999
2001
2002
2003
2004
Time(Monthes)
Rainfall(mm) Runoff(mcm) Linear (Rainfall(mm)) Linear (Runoff(mcm))
Figure 6.6 Rainfall and runoff of Meki Sub- Catchment during the wet season
250
200
Katar Sub-Catchment Wet Season Rainfall-Runoff Relation
y = 0.0982x + 63.528(runoff)
y = -0.1856x + 126.64(rainfall)
Values
150
100
50
0
-50
1989
1989
1990
1991
1992
1992
1993
1994
1995
1995
1996
1997
1998
1998
1999
2000
2000
2001
2002
2003
2003
2004
Time(month)
Rainfall(mm) Runoff(mcm) Linear (Rainfall(mm)) Linear (Runoff(mcm))
Figure 6.7 Rainfall and Runoff Katar River sub-Catchment during the wet season.
73

The effect of land degradation in hydrology is manifested by increasing the rate of
surface runoff and evaporation. R. Maidment, 1993). In both sub-catchments, the runoff
of the sub-catchment is increasing in the contrary to rainfall. The effect is more
predominant in Meki than Katar Sub-Catchments.
6.2.2 Water Abstraction
6.2.2.1 Abstraction from Lake Ziway
Since 1970, major irrigation activates were introduced around Lake Ziway and its
catchments (Halcro & partners).
A land total of about 4012ha is irrigated using the lake and rivers enclosed between
gauging stations around the lake as of August, 2007. (Source: East Showa Zone and
Ziway Woreda Agriculture and Rural Development Offices).
Onion, tomato, cabbage, pepper, sorghum, and maize are the major crops grown in the
area. Assuming 600mm average water requirement (FAO Irrigation, Drainage and Salinity
Paper, 1973) for single season; and if two phases of irrigation are considered annually,
the total annual abstraction of lake water estimated will be about 50MCM.
In addition, a total of 4.5mcm water is annually taken for Ziway town domestic water
supply from the lake since April 2003 (Source OWRB, 2004).
6.2.2.1 Abstraction from Meki River
Based on data collected during field work from Agriculture and Rural Development
offices, about 462ha.of land is currently under cultivation using Meki River and its
tributaries up stream of Meki town.
74

According to the reports, potato, tomato, onion and pepper are the major vegetables
grown twice a year in the area. Assuming 60% of the irrigable areas to be developed
twice a year and 600mm average water requirement during single growing season the
total annual abstraction from the river and its tributaries is 6MCM.
6.2.2.2 Abstraction from Katar River
Large scale irrigation was started in Katar catchment in mid 1980’s by Katar irrigation
Project. Since then, irrigation demand has been increasing by using the river and its
tributaries.
In the highland area potato is the most dominant vegetable while tomato, cabbage, onion
and papaya are produced additionally in lowland areas. Assuming 60% of the irrigable
areas to be developed twice a year and 600mm average water requirement during single
growing season, the total water abstracted currently to satisfy the demand of 2115ha
vegetable will be 20.3MCM.(Data source: Oromia Irrigation Development Authority)
Annual abstraction(mcm)
60
50
40
30
20
10
0
1980 1990 2000 2006
Years
Lake Ziway Katar River Meki River
Figure 6.8 Trends in catchment water use from lake and rivers
6.2.3 Sediment Depositions
Using the bathymetry survey made for the different period it can possible to compute the
volume of sediment deposited during the interval of the period. Accordingly using the
bathymetry survey of the 1976 and that of 2005/206 as shown on the graph, the total
75

volume of sediment deposited for the last 30 years was 2076.24MCM which means the
depth of the lake reduced by 1.058m. This is manifested by day to day human beings
activity in the catchments by deforesting, overgrazing and disturbing the sediments in the
rivers outflow to the lake for the extractions of sands.(figure 6-9 & 6.10)
Figure 6.9 Human beings How Directly and Indirectly Affects the Lake, Rivers to the Lake
and out of the Lake
Figure 6.10 Practice of irrigation in the Ziway Lake Catchment enhancing for
more Abstraction
76

Cummulative
Volume & area lost lost
Lake Level vers Cumm. Volume & Cummulative Area Lost by sediment deposits
2500.00
250.00
2000.00
200.00
1500.00
150.00
1000.00
100.00
500.00
50.00
0.00
1628 1629 1630 1631 1632 1633 1634 1635 1636 1637
Elevations asl
0.00
Lake level -Cumulative Sediment volume
Lake Level-Cummulative sediment area
Figure 6.11 Elevation- Cumulative Volume & Cumulative Area Lost of Lake Ziway by
sedimentations.
Therefore Lake Ziway is vulnerable to changes in climatic parameters and anthropogenic
(human induced) factors which cause change in water balance and the lake level. There
fore both climatological and Anthropogenic are the factors for decreasing Ziway lake
level.
6. 2.3 Impact of Ziway Lake Level Decreasing on Lake Abijata
6.2.3.1 Bulbula Runoff and Abijata Lake Level Relations
The elevation with in the rift valley varies in a wide range from close to 2000masl at
Lake Abaya and around 120m below sea level in the Dalol depression (Makin.et.1976).
Many of the Lakes are located with in the closed basin fed by perennial rivers. The
major rivers in the region are Awash, Meki_Katar, Dijo and Bilate feeding lakes Abhe,
Ziway, Shala and Abaya respectively. Lakes Abaya and Chamo are seasonally
connected by overflow channels, Ziway and Abijata by the Bulbula River, Langano and
Abijata by the Horakalo River. Awasa, Abiyata, Shala, Beseka and Afrera are terminal
lakes.
77

Two major irrigation development scenarios are considered in this study to evaluate,
how irrigation activities affect the Ziway lake level and the outflow to downstream of the
lakes. They are:
• Irrigation expansion to 5,000 ha
• Irrigation expansion to 10,000 ha
Scenario one: Irrigation expansion to 5000 ha in the lake catchments needs additional
8.30 MCM water per year in addition to the present. Accordingly, this amount of water
abstraction contribute to the lake level to reduce by 18.66 mm and the out flow reduced
by 197.91MCM.
Scenario Two: If the irrigation development expands by 10,000 ha, it requires
additional 42.55 MCM water per year and this also reduced the lake level and the out
flow to downstream lakes by 95.64 mm and the outflow reduced by 201.71MCM.
78

Table 6.1 Irrigation Development Scenario Result.
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Bulbula flow at
present (MCM) 28.54 17.5 11.9 8.6 7.11 6.764 11.3 34.8 68.58 72 53.6 34.8
Lake Ziway Level
at present(m) 1.065 0.95 0.84 0.8 0.75 0.717 0.81 1.17 1.494 1.51 1.38 1.21
Lake Ziway
Area(km2) 444.1 440 436 434 433 431.4 435 448 464.3 465 458 450
Lake Abijata level
at present (m) 3.134 3.1 3.02 2.84 2.77 2.719 2.75 2.79 2.877 3.03 3.1 3.08
Scenario -1
abstraction(MCM) 2.2 6.15 12.1 15.7 4.45 0.2 0.05 0.05 0.15 0.4 0.6 0.6
Scenario-2
abstraction(MCM) 4.4 12.3 24.1 31.3 8.9 0.4 0.1 0.1 0.3 0.8 1.2 1.2
Scenario- 3
abstraction(MCM) 18.92 52.9 104 135 38.3 1.72 0.43 0.43 1.29 3.44 5.16 5.16
Bulbula flow
(MCM) after
Scenario-1 13.51 8.57 4.03 2.08 1.21 0.155 3.75 17.6 30.37 31 25.8 19.4
Scenario-2 13.32 8.02 2.96 0.68 0.81 0.137 3.74 17.6 30.36 31 25.7 19.3
-
Scenario-3 12.05 4.43 -4.1 -8.6 1.83 0.018 3.71 17.6 30.28 30.8 25.4 19
Lake Ziway Level
after(m)
Scenareio-1 1.06 0.93 0.82 0.77 0.74 0.717 0.81 1.17 1.494 1.51 1.38 1.21
Scenareio-2 1.055 0.92 0.79 0.73 0.73 0.717 0.81 1.17 1.493 1.51 1.37 1.21
Scenareio-3 1.023 0.83 0.61 0.49 0.67 0.713 0.81 1.17 1.491 1.5 1.37 1.2
Lake Abijata level
after (m)
Scenareio-1 3.051 3.05 2.97 2.8 2.74 2.683 2.71 2.69 2.665 2.8 2.94 2.99
Scenareio-2 3.05 3.04 2.97 2.8 2.74 2.683 2.71 2.69 2.665 2.8 2.94 2.99
Scenareio-3 3.043 3.02 2.93 2.74 2.72 2.682 2.71 2.69 2.664 2.8 2.94 2.99
The abstraction scenarios in the Ziway-Abijata Lake watershed affect not only the
Abijata Lake but also the domestic water users along the river since it is the only fresh
water available in between the two lakes.
79

If all the proposed irrigated areas are developed, the estimated annual water
requirement will be 365.9MCM. This would result in a 0.822m reduction in the level of
Lake Ziway which is -0.24m below the level to which the Bulbula River is flowing to
Abijata. Ultimately this will lead to a drastic reduction in the level of Lake Abijata and
drying up of the feeder Bulbula River which is the source of domestic water supply.
The maximum reductions in the level of Abijata Lake coincide with the time of largescale
reduction of Lake Ziway and Bulbula runoff. Figure 6.12
In wet years, 50% of time between November- June, Ziway shows a net loss of storage
due to outflow of water to the Lake Abijata. During August and September a net gain to
storage occurs because large inflows from the Katar and Meki rivers. The gain is
transferred to Abijata and at time reaches as much as 17% of the total volume of the
Lake (Halcrew, 1989)
120
Ziway, Abijata Lake Level & Bulbula Runoff Relations
100
80
Values
60
40
20
0
-20
1990
1990
1991
1992
1992
1993
1994
1994
1995
1996
1996
1997
1998
1998
1999
2000
2000
2001
2002
2002
2003
2004
Time(month)
Bulbula Runoff(mcm)
Abijata Lake Level(cm)
Ziway Lake Level(cm)
Linear (Bulbula Runoff(mcm))
Linear (Abijata Lake Level(cm))
Linear (Ziway Lake Level(cm))
Linear (Ziway Lake Level(cm))
Figure 6.12 Mean Monthly Abiata, Ziway Lake level and Bulbula Runoff
2004
2005
The apparent decreasing of Abijata Lake level and Bulbula runoff is parallel this shows
that the Abijata Lake level depends on the Bulbula runoff which spills out from Ziway
lake.
80

6
Abijata vers Ziway Lake Level
ABIJATA lAKE LEVEL(m)
5
4
3
2
1
0
0 0.5 1 1.5 2 2.5
ZIWAY LAKE LEVEL(m)
ABIJATA Vers Ziway Lake lEVEL
Linear (ABIJATA Vers Ziway Lake lEVEL)
Figure 6.13 Scatter plot of Abijata verses Ziway Lake Level
81

7. Conclusions and Recommendations
CHAPTE SEVEN
7.1 Conclusions
The study area is located between latitude of 7 0 18’ to 8 0 25’ N and longitude of 38 0 15” to
38 0 22’ E in the northern part of Central Ethiopian Rift Valley catchment and bounded in
the east by Chilalo (4056masl), Galama(4153masl) and Kakka (4167masl) mountains
and from the west by Guraghe mountains( 3609masl).The total catchment area is about
7444km 2 .
The basin is divided in to three physiographic areas: the high plateau on either side of
the rift, the transitional escarpment and the rift floor. There is an elevation difference of
about 2550m between the rift floor and mountains.
Lake Ziway catchment is located in mid-altitude regions; mean annual rainfall varies
from 700mm-800mm in the valley to 1150mm on the plateau.
Highlands flanking the Lake Ziway in both directions intercept most of the rainfall in the
basin. Open water evaporation (lake evaporation) is in the order of 1800-2000mm
per/year.
82

Evaporation & Perciptation
Value (mm)
200
150
100
50
0
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Time(months)
Evaporation
Percipitation
Figure 7.1 Comparisons of Rainfall to Evaporation.
Using the data from 1975-2005 years on monthly basis, Meki sub-Catchment maximum
flows occurs in August with a minor secondary peak in April and minimum flows
between December and Feb. The river bed discharge may dry out during Dec-Jan. It
can be said that the base flow of the river dry out during the savior dry years which is
impossible to depend on runoff of the river through out the year for irrigation and
domestic water supply. And the total annual contribution of the Meki River to the Lake
Ziway is 277.81MCM.
The discharge measured at Katar Fite is greater than the discharge at Katar Abura and
the difference is very pronounced during the high rainfall months of the country i.e.
August. This is due to an abstraction either through the faults or evaporation between
the two observation stations and equal discharge measurements observed at the two
stations in the month of October & November. And the annual inflow of the Lake Ziway
from the Katar River is 401.3 MCM gauged at Abura and 562.20 MCM gauged at Katar
fite mid of Katar River.
Although the overall pattern of flow of Katar is similar to that of the Meki, the base flow
are more clearly defined, the base flows in the dry seasons are rather higher and it
seems most unlikely that the Katar would never dry up.
83

The total mean annual outflow from the Lake Ziway measured at Kakarsitu and Bulbula
Town is 161.33MCM and 127MCM respectively.
The runoff from the un gauged catchment estimated using area ratio methods and
considered in this study is only during in the rainy season and the estimated amount is
106.8MCM.
The total annual average inflow to the lake can be safely estimated by the sum of the
Katar, Meki river and runoff water from the un-gauged Sub-catchment (during the flush
time only) and precipitations on the lake and the outflow from the lake can be estimated
from evaporation, abstraction and Bulbula river.
River Runoff (mcm)
160
140
120
100
80
60
40
20
0
Catchment River Runoff
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
month
Katar Runoff (mcm)
Meki River Runoff(mcm)
Bulbula River Runoff(mcm)
Figure 7.2 Long-term Mean Monthly Discharges of Main Rivers in the Catchment
Due to lake routing, peaks of lake level and Bulbula River flow do not coincide with
peak discharges of rivers inflow (Meki and Katar) to the lake. There is about 30days
(one month) lag time between peak of inflow and that of lake level. And the correlation
of Lake Level verses Bulbula Outflow is 0.99.
84

Precipitation in the catchment varies with altitude. High altitude areas like Chilalo,
Galama, and Guraghe mountains receive mean annual rainfall of over 1200mm while
the lake area gets average depth of about 735 mm. However, the correlation coefficient
between precipitation and altitude is not very strong.
To classify the basin in to sub-basin which helps various studies such as filling missing
values, rainfall elevation and runoff correlation, as well as categorizing streams in to
these regions, identifications of Homogeneous Rainfall Stations based on Monthly
Rainfall is used. In order to find similar regions monthly rainfall values where nondimensionalzed
and plotted to compare the stations with each others.
There fore rainfall distribution is divided in to three patterns in the Ziway catchment i.e. is
bimodal for the high land areas, strongly peaked unimodal for the rift floor and the
transition for the escarpment. This also proves that the rainfall far away from the water
body during the dry season is higher than the rainfall near the water body and the
reverse is true during the rainy seasons.
Double mass curve analysis was carried out for 17 rainfall stations in the catchment for
consistency checking. With the Thiesson polygons method, the rainfall stations have
been selected by virtue of their location with the lake, class and long period of
observation consistency of the data base to calculate the monthly and annual rainfall
areal distributions for the total catchment, Katar River basin, Meki River basin and Lake
Surface
85

Mean monthly River Basin Rainfall
200
180
160
140
120
100
80
60
40
20
0
River Basin Monthly Areal Rainfall Distribution
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Monthes
Katar River Basin
Lake Ziway
Meki River Basin
Ziway Water Shed
Figure 7.3 River Basin Monthly Areal Rain fall distribution
The main objective of the thesis is to quantify water balance components of the lake
ziway. In order to quantify and understand the basic hydrological process, water
balance computation of the lake is made using excel spread model. This model
approach stems on solving the lake water balance equations.
Accordingly the components of the water balance for Lake Ziway are:
The reservoir storage, Surface Runoff (Katar and Meki river) and Ungauged runoff,
Lake evaporation, areal precipitation for Lake region, outflow through Bulbula river and
abstraction from the lake
The simulation has been conducted on a monthly time scale for different scenarios. The
fourth scenario considering all the components and has the correlation of 0.87
continuously starting from Jan 1980 to Dec 2005 is in a reasonable result.
86

The annual balance has an error of + 127.72mm/year. The source of this error can be
attributed to the lack of direct measurements of accurate evaporation rate, abstractions
and un gauged runoff, or the fact that we assumed net groundwater flux and inaccuracy
of data for the gauged.
87

Table 7.1: Annual Water balance of Lake Ziway.
Inflow (mm)
Outflow (mm)
Month Katar Meki
Lake
surface P
Un
guaged
Bulbula at
Karkarsitu
Abstractions
Lake
Surface
E
Balance(mm)
Jan 13.26 5.26 16.87 - 29.71 4.12 158.59 (157.04)
Feb 14.76 8.07 35.92 - 17.75 11.74 157.79 (128.54)
Mar 20.62 23.17 61.10 - 12.05 23.38 178.12 (108.67)
Apr 33.19 39.69 74.23 - 8.66 30.78 169.03 (61.36)
May 34.86 37.19 67.15 - 7.21 8.87 180.59 (57.46)
Jun 37.23 32.57 76.67 - 6.75 0.40 168.37 (29.05)
Jul 111.30 104.34 142.98 49.64 11.36 0.10 142.89 253.90
Aug 327.98 196.79 128.25 105.83 36.19 0.10 145.08 577.47
Sep 180.29 110.29 86.51 60.03 67.87 0.26 143.75 225.24
Oct 82.41 48.60 35.70 23.35 71.84 0.70 167.93 (50.41)
Nov 26.20 15.72 6.92 - 54.59 1.10 159.49 (166.33)
Dec 14.23 5.10 5.11 - 35.30 1.15 158.03 (170.04)
Total
(annual) 896.32 626.79 737.41 238.85 359.29 82.70 1,929.66 127.72
.
88

350.00
Water Balance Components (mm)
Balance Components(mm)
300.00
250.00
200.00
150.00
100.00
50.00
-
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Time
Katar Meki Lake Surface Preciptations Ungauged area
Bulbula Abstractions Lake surface evaporations
Figure 7.4 Lake Ziway water Balance Componentss
Rainfall, Humidity, infiltration and ground water flow are decreasing where as
temperature, evaporation, runoff(wet seasons), abstractions and erosion are increasing
in the Lake Ziway catchment, there for Lake Ziway is vulnerable to changes in climatic
parameters and human induced effects.
Two major irrigation development scenarios are considered in this study to evaluate,
how irrigation activities affect the Ziway lake level and the outflow to downstream of the
lakes, i.e. Irrigation expansion to 5000 ha in the lake catchments needs additional 8.30
MCM water per year in addition to the present and reduce the lake level by 18.66 mm
and will reduce the out flow by 197.91MCM and If the irrigation development expands by
89

10,000 ha, it requires additional 42.55 MCM water per year and this will reduce the lake
level and the out flow to downstream lakes by 95.64 mm and 201.71MCM respectively.
The abstraction scenarios in the Ziway-Abijata Lake watershed affect not only the
Abijata Lake but also the domestic water users along the river since it is the only fresh
water available in between the two lakes.
7.2. Recommendation
• More detailed resource assessment of water should be done, including
sustainable abstractions (safe yields) and the special variability of water quality
and quantity. Particularly, the potential impact of large-scale water abstraction for
cut-flower production near the eastern and western shore of Lake Ziway on the
regional hydrology of the lakes should be assessed in detail, with special
attention to the impacts of rural drinking water, small scale irrigation and Lake
Ziway sustainability.
• Measures should be taken to reduce loss of water due to evaporation in lakes
• Since the large trees are almost cleared out for the fuel woods and charcoal
production, any wind breakers are almost absent. Growing tall trees on the
windward side of the reservoir is recommended which act as wind breakers and
hence reduce evaporation.
• As the lake periphery and inside the lake is full of the large grasses and some
trees increases the rate of evaporation, so the trees and tall grasses should be
removed
• Proper irrigation scheduling, focusing on high value crops, using limited area and
water, and detail crop-water requirements study has to be made in irrigation
fields to protect the over abstraction of the lake.
• Upstream use of water must only be undertaken in such away that it does not
affect water quality and quantity to down stream users.
90

• Provisions of control network of river monitoring stations in order to establish
short and long term fluctuations in relations to basin characteristics to dictate
water quality change and to determine seasonal short and long term trends in
relations to demographic changes, water use changes and management
interventions for the purposes of water quality and quantity evaluations.
• Provision of storage structure at higher altitude enables storage of water under
significantly lower evaporation. If provided with adequate storage size it can be
considered as sediment retentions basin, which reduce impact of deposited
sediment in the lakes.
• Program of erosion control measures in the basin such as forestation terracing
prevention of overgrazing avoidance of poor farming.
• Replacement of wood construction materials with other source such as clay
bricks, soil cement blocks and stones there by reduce deforestation and its
consequences.
• Environmental education of the community through various associations,
organization, schools and etc.
• Irrigation activities should combined in an integrated approach to sustain future
development activates based on the lake water resources.
• Investigations of groundwater flux and directions should be very important to
improve the results obtained and issues identified.
91

REFERENCE:
CSA,2005. StatisticalAbstract.Central Statistical Authority, Addis Ababa,Ethiopia.
Dingman,S.L., 1994. Physical hydrology. Prentice-Hall, NewJersy, 557pp.
EthiopianValley Development Studies Authority (EVDSA), 1992. Reconnaissance Master
Plan for the Development of the Natural Resources of the Rift Valley
Lakes Basin. Sir Williams Halcrow & Partners Ltd. in association with ULG
Consultants, Addis Ababa, Ethiopia.
FAO/UNESCO, 1973. Irrigation, Drainage and salinity. An international source book.
Hutchinson, London, 510p.
Ferguson, L.H, Znamensky, A.V. (ed), 1981. Methods of computition of the water balance
of large lakes and reservoirs. Vol. 1, UNESCO- Studies and reports in
hydrology, No 31, 119pp.
Halcrow, 1989. Master Plan for the Development of Surface Water Resources in the
Awash Basin, vol. 6. Ministry of Water Resources, Addis Ababa.
JICA & OIDA, 2001. The study for Meki Irrigation and Rural development Project.
technical report, vol. 2.
Maidment, D.R.1993. Hand book of hydrology. McGraw Hall, New York, 660pp.
Makin, M.J., Kingham, T.J., Waddams, A.E., Birchall, C.J., Eavis, B.W.,1976. Prospects
for irrigation development around Lake Ziway, Ethiopia. Land Resources
Study Division, Ministry of Overseas.
92

UNESCO. 1971. Scientific framework of world water balance. Paris,
UNESCO. 27p. (Technical papers in hydrology, No. 7).
UNESCO, 1981. Methods of computation of the water balance of
reservoirs. UNESCO, Paris.
large lakes and
UNESCO, 1974. Methods for Water Balance computations, an International
Guide for research and Practice. UNESCO, 2 nd Paris.
WMO, 1986: Manual for estimation of probable maximum precipitation, Operational
Hydrology Report 1, WMO No.332, ed. Secretariat of the World Meteorological
Organization, Geneva Switzerland.
WMO, 1971: Problems of Evaporation Assessment in the Water Balance (C. E. Hounam).
WMO/IHD Report No. 13, WMO–No. 285, Geneva.
WMO. 1962. Guide to climatic practice. Geneva, WMO. (WMO Publ. NO. 100, TP44).
WMO. 1970a. Guide to hydrometeorological practices. Geneva, WMO. (Publ. No. 168, TP
82).
Ram, S.Gupta Hydrology and Hydraulic Systems Second Edtion
Winter,T.C. 1981. Uncertainties in Estimating the water Balance of The Lake. Water
Resource Bulletin 17, No.1:82-115
93

Dr. Derek Clark Crop Wat for Windows: user Gide University of South ampten Crop Wat 4
Windows version 4.2 .0013 October 1998
Huib Hengsdijk & Herco Janson .2006. Agricultural Development in the Centeral Ethiopia
Rift valleys : A desk Study on Water Related Issues and Knowledge to Support A
policy Dialogue.
Seleshi Bekele. 2001. Investigation of Water Resources Aimed at Multi Objective
Development With Respect to Limited Data Situations The Case of Abaya-Chamo
Basin, Ethiopia. Ph.D Thesis Technische Universitat Dresden Institute Fur
Wasserbaw und Technische Hydromechanik D-01062 Dresden.
94

APPENDIXES
Appendix 4.1 Meki River Runoff Gauged at Meki Town(m3/sc)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1975 0.11 0.24 0.01 0.601 0.44 4.47 27.2 28.11 44.9 12.07 1.96 0.49 310.57
1976 0.26 0.09 2.39 2.041 5.87 2.08 14.8 18.86 13.9 2.041 5.43 0.74 179.06
1977 2.5 3.94 0.66 3.521 9.95 6.45 31.5 28.75 18.8 17.92 24.5 2.84 395.88
1978 0.46 1.3 5.27 1.204 0.74 3.86 13.2 29.34 16 14.27 3.32 2.37 239.03
1979 2.97 5.86 7.91 20.89 13.2 2.98 23.4 30.57 14 12.85 3.53 1.16 365.22
1980 0.81 0.96 1.27 2.186 1.54 4.95 16.7 21.07 9.53 4.352 0.82 0.53 169.71
1981 0.4 0.49 13.7 20.03 6 2.1 9.43 30.14 22.2 5.11 0.62 1.14 290.23
1982 0.92 1.77 1.49 7.026 7.12 2.53 8.26 30.07 9.64 16.62 2.58 1.74 236.07
1983 0.53 2.76 4.62 11.87 18 14 9.92 33.42 21.4 7.531 1.61 0.89 329.92
1984 0.62 0.49 0.45 0.318 2.6 4.58 9.81 9.818 13 1.083 0.35 0.29 112.2
1985 0.22 0.18 0.13 1.534 6.65 1.25 8.96 27.42 15.3 2.77 0.34 0.17 169.67
1986 0.09 0.52 0.87 4.894 2.56 7.94 8.24 16.84 11.3 1.896 0.2 0.04 144.19
1987 0.01 0.28 7.7 18.25 17.3 14.6 7.52 6.256 7.31 7.259 0.41 0.04 227.94
1988 0.04 0.41 0.15 3.066 2.22 2.85 15.8 23.26 22.5 12.62 2.91 0.92 225.42
1989 0.14 2.43 3.3 9.65 2.93 3.9 15.2 15.77 17.6 9.722 1.65 0.89 216.09
1990 0.31 11.3 20.2 21.69 5.28 5.82 17.1 19.81 15.2 6.423 1.86 0.87 327.54
1991 0.5 2.6 7.17 2.84 0.89 3.76 24.5 34.9 20.1 3.483 0.87 0.66 267.37
1992 1.5 1.83 1.3 2.257 2.27 2.4 6.07 48.97 40.1 18.61 3.38 2.1 338.3
1993 4.92 0.98 0.98 13.44 17.5 12.3 25.2 57.52 19.9 11.61 4.45 0.89 446.33
1994 0.8 1.42 0.6 1.157 1.5 3.17 17 60.03 47.1 4.388 0.98 0.62 358.66
1995 1.16 1.18 5.89 5.362 4.04 1.88 9.35 35.93 44 3.408 1.74 1.57 296.67
1996 1.84 1.26 2.12 2.919 5 13.3 16 47.58 24.5 5.549 1.99 1.21 321.64
1997 0.32 0.27 0.99 9.947 2.89 4.36 16.6 16.6 5.4 4.139 6.18 0.92 180.37
1998 1.6 0.74 12.1 3.272 11.8 5.23 27.6 70.11 29.2 23.12 2.55 0.42 494.16
1999 0.09 0.07 2.84 0.115 0.51 2.84 20.6 22.42 10.2 23.55 4.73 0.28 232.99
2000 0.02 0 0 0.045 0.83 0.64 6.31 14.02 11 8.245 2.96 0.94 117.23
2001 0.01 0.18 3.06 1.908 4.61 10.9 19.3 21.76 10.9 0.323 0 0 191.38
2002 1.05 0.55 0.13 1.545 2.07 2.55 5.41 11.6 12 2.159 1.69 1.42 109.04
2003 0.86 0.51 7.85 6.234 1.96 4.61 42.1 46.95 20.9 3.666 0.33 0.73 359.48
2004 0.84 0.26 0.62 19.13 0.65 2 18.5 56.2 21.3 8.723 0.39 0.11 337.33
2005 0.33 0.08 1.15 5.976 23.6 12.9 24 87.29 50.5 8.624 1.76 1.16 567.02
Mean 0.845 1.451 3.77 6.61 5.883 5.394 16.63 32.3 20.64 8.5205 2.777 0.907 276.022
MCM 2.264 3.511 10.1 17.13 15.76 13.98 44.54 86.52 51.71 22.821 7.198 2.273
95

Appendix 4.2 Katar River Runoff Gauged at Abura (m3/sc)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1975 1.92 1.72 1.32 2.03 1.98 4.59 24.55 90.35 67.31 14.33 3.25 2.52 215.9
1976 2.16 1.95 1.92 2.54 4.02 2.72 13.46 46.42 26.51 4.654 4.54 3.31 114.2
1977 3.28 3.22 2.59 5.69 5.06 4.92 20.56 50.21 43.07 32.9 24.7 4.1 200.3
1978 2.73 2.98 5.71 2.78 3.59 3.64 34.69 57.95 21.49 17.18 4.48 3.19 160.4
1979 4.48 8.58 4.02 2.58 3.03 3.77 25.83 48.41 20.08 12.55 3.67 2.82 139.8
1980 2.57 2.59 2.33 2.37 2.47 3.89 16.96 38.87 18.67 7.914 2.86 2.46 103.9
1981 2.24 2.2 4.11 21.9 8.08 2.83 12.71 64.24 57.12 33.9 3.6 2.75 215.7
1982 2.72 2.6 2.46 5.41 5.47 3.97 12.48 50.53 20.19 15.39 4.55 4.43 130.2
1983 1.13 1.61 2.33 9.67 16.5 23.8 11.91 99.98 41.71 23.61 3.21 1.97 237.4
1984 1.75 1.49 1.3 1.3 3.15 6.28 17.37 25.52 22.66 3.56 2.37 2.28 89.03
1985 2 1.77 1.71 3.26 7.95 3.23 16.76 34.02 28.57 8.35 1.94 1.82 111.4
1986 1.85 2.79 3.41 5.1 5.74 9.9 28.18 51.54 34.52 13.13 3.39 2.51 162
1987 1.84 1.81 4.87 18.1 9.79 14.8 10.02 19.34 17.06 7.373 2.37 2.01 109.4
1988 1.85 2.12 2.02 2.3 2.52 2.73 22.99 99.43 34.52 20.32 5.17 2.69 198.6
1989 2.4 2.35 2.33 6.39 5.8 3.93 12.84 25.06 29.16 12.1 3.43 4.01 109.8
1990 1.39 10.9 20 21.3 5.3 4.31 11.95 27.67 18.71 3.761 1.06 2.39 128.8
1991 2.16 2.24 3.76 5.39 2.77 3.44 14.05 45.78 36.07 6.142 2.56 2.31 126.7
1992 2.02 2.46 1.76 3.04 3.07 3.23 8.183 65.98 53.99 25.08 4.56 2.83 176.2
1993 2.71 6.55 2.19 5.1 11.5 11.6 16.41 52.83 35.71 20.12 7.17 2.7 174.6
1994 2.14 1.92 1.64 1.56 2.02 4.27 22.93 68.87 49.09 7.022 2.81 2.04 166.3
1995 1.56 1.58 7.94 7.23 5.44 2.54 12.6 48.41 59.32 4.593 2.35 2.11 155.7
1996 2.48 1.69 2.86 3.93 6.74 17.9 21.5 64.12 24.38 8.54 2.44 2.16 158.8
1997 2.91 1.67 1.57 6 2.6 2.74 13.57 20.98 11.05 5.577 6.96 2.94 78.57
1998 2.19 3.45 5.01 2.38 5.99 3.66 13.4 69.19 49.6 28.12 6.76 2.36 192.1
1999 1.99 0.43 1.71 1.65 1.69 3.19 17 33.86 21.21 44.47 7.45 2.34 137
2000 1.58 1.58 1.56 1.49 4.67 2.75 9.732 49.31 28.69 25.27 8.71 2.51 137.8
2001 1.76 1.55 2.17 2.57 7.57 13.8 34.32 80.06 37.86 10.45 2.45 1.79 196.4
2002 1.84 1.9 2.77 2.08 2.8 3.43 7.29 26.57 13.45 3.334 1.48 1.92 68.86
2003 2.44 1.39 1.55 3.93 3.8 2.44 16.62 51.94 28.99 7.495 1.84 1.85 124.3
2004 1.47 1.34 1.35 8.71 3.52 3.32 20.04 38.03 26.83 13.57 2.22 1.41 121.8
2005 1.51 1.39 2.29 2.74 13.9 19.3 35.78 130.2 75.26 12.86 2.62 1.73 299.5
Mean 2.16 2.64 3.31 5.5 5.44 6.35 17.96 54.05 33.96 14.63 4.42 2.52 152.9
MCM 5.79 6.39 8.86 14.3 14.6 16.5 48.1 144.8 85.09 39.2 11.5 6.33 401.3
96

Appendix 4.3 Katar River Discharge at Fite
Station(m3/sc)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1975 2.55 2.27 1.747 2.69 2.623 6.09 32.56 119.8 89.26 19 4.31 3.34 286.27
1976 2.86 2.59 2.545 3.36 5.326 3.6 17.85 61.56 35.15 6.17 6.02 4.39 151.43
1977 4.35 4.27 3.44 7.54 6.708 6.53 27.26 66.59 57.12 43.6 32.8 5.43 265.68
1978 3.62 3.95 7.571 3.69 4.754 4.82 46.01 76.86 28.5 22.8 5.94 4.23 212.72
1979 5.94 11.4 5.329 3.42 4.012 4.99 34.25 64.2 26.63 16.6 4.87 3.74 185.4
1980 3.41 3.43 3.086 3.14 3.27 5.16 22.49 51.55 24.75 10.5 3.79 3.26 137.85
1981 2.97 2.92 5.451 29.1 10.72 3.75 16.86 85.19 75.75 45 4.77 3.65 286.06
1982 3.61 3.44 3.256 7.18 7.26 5.26 16.55 46.74 11.49 3.93 3.25 3.09 115.05
1983 1.49 2.12 3.075 6.54 27.36 41.9 31.61 171.6 36.97 33.4 4.23 2.61 362.91
1984 2.32 1.96 1.72 1.72 2.19 5.56 33.31 21.02 28.1 2.79 1.64 1.64 103.95
1985 1.57 1.41 1.495 2.41 7.4 2.45 50.98 65.23 27.73 5.66 1.72 1.41 169.45
1986 1.27 1.67 2.05 3.43 3.495 2.34 46.27 50.63 41.92 18.8 2.47 1.49 175.78
1987 1.41 1.5 7.765 24.9 17.66 10.9 8.955 34.5 17.62 13.5 1.72 1.64 142.06
1988 1.41 2.45 1.805 2.64 2.345 2.35 42.72 85.42 35.14 56.3 5.73 3.12 241.41
1989 2.5 2.73 2.73 8.61 6.125 4.54 18.88 32.48 43.05 7.86 3.52 4.12 137.12
1990 2.57 24.2 41.01 32.5 8.23 3.74 22.14 51.24 34.64 6.97 1.96 1.64 230.8
1991 1.41 2.81 3.495 4.62 2.265 1.92 15.6 81.28 42.72 3.42 1.41 1.27 162.21
1992 1.67 1.74 1.22 4.21 1.92 1.99 10.47 70.01 38.37 28.2 2.76 1.96 164.48
1993 2.41 5.84 1.695 6.02 14.22 23.4 21.02 88.04 35.04 52.5 5.75 2.13 258.07
1994 1.96 1.71 2.065 2.16 3.745 3.75 30.42 91.34 65.1 9.31 3.72 2.48 217.75
1995 2.32 2.23 2.875 10.6 10.6 4.13 39.5 77.56 80.3 7.08 2.25 2.45 241.87
1996 2.73 0.24 3.566 1.53 11.53 17 42.87 178.2 29.41 10.8 0.73 70.5 369.13
1997 1.92 1.09 2.362 5.91 2.964 2.44 16.36 23.69 7.711 4.39 4.71 1.7 75.25
1998 2.22 4.56 5.913 12.2 5.193 3.33 40.4 136.3 76.9 57.2 6.4 1.49 352.03
1999 1.68 3.2 4.216 7.95 3.904 2.38 26.62 100.8 53.11 40.4 4.52 1.39 250.12
2000 1.75 3.37 4.428 8.48 4.066 2.5 28.34 105.2 56.09 42.5 4.75 1.4 262.86
2001 2.33 2.05 2.883 3.41 10.04 18.3 45.52 106.2 50.21 13.9 3.24 2.37 260.4
2002 2.44 2.52 3.668 2.76 3.707 4.55 9.668 35.24 17.83 4.42 1.96 2.55 91.325
2003 3.23 1.84 2.058 5.21 5.034 3.23 22.04 68.88 38.45 9.94 2.44 2.46 164.82
2004 1.95 1.78 1.788 11.6 4.672 4.4 26.57 50.44 35.58 18 2.95 1.87 161.54
2005 2 1.84 3.038 3.63 18.39 25.6 47.45 172.6 99.81 17.1 3.47 2.3 397.18
Mean 2.45 3.52 4.495 7.52 7.152 7.51 28.76 79.69 43.24 20.4 4.51 4.74 213.97
97

Appendix 4.4 Bulbula River Runoff Gauging at Karkarsitu
(m3/sc)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1976 3.4 0.91 0.55 0.35 0.31 0.15 0.2 1.54 7.09 5.85 4.38 1.53 26.3
1977 8.66 5.78 6.21 3.4 1.35 0.66 2.29 8.88 14 14.7 11.2 8.13 85.3
1978 8.66 5.78 6.21 3.4 1.35 0.66 2.29 8.88 14 14.7 11.2 8.13 85.3
1979 5.14 4.3 3.69 3.04 1.74 1.56 3.97 12.4 15 15.2 12 8.25 86.4
1980 3.99 2.21 0.9 0.2 0 0.18 0.19 0.84 2.04 2.03 0.92 0.38 13.9
1981 0.21 0.19 0.21 0.2 1.38 0.2 0.22 2.39 12.7 18 13.1 7.81 56.6
1982 3.21 1.77 1 0.62 0.54 0.31 0.26 2.1 6.89 7.99 5.95 4.11 34.7
1983 1.97 1.31 0 1.3 3.14 6.49 7.11 21.2 43.9 6.24 4.06 2.52 99.2
1984 11 6.25 3.17 1.42 0.68 0.56 0.67 2.94 5.64 4.48 2.17 0.92 39.9
1985 0.39 0.32 0.25 0.19 0.18 0.18 0.18 0.32 2.02 2.58 0.99 0.48 8.08
1986 0.25 0.24 0.22 0.22 0.22 0.19 0.23 1.27 4.25 6.24 3.84 1.68 18.8
1987 0.66 0.27 0.23 0.59 0.81 2.44 1.64 2.43 4.2 5.66 3.33 1.95 24.2
1988 1.19 0.87 0.65 0.68 0.64 0.66 0.66 1.5 5.19 14.2 10.8 6.16 43.2
1989 3.36 1.98 1.13 1.46 0.77 0.82 1.03 2.8 6.55 7.01 3.3 1.46 31.7
1990 3.41 0.46 1.39 3.35 2.72 0.79 1.21 10.1 26 23.6 14.5 7.47 94.9
1991 3.46 1.36 0.68 0.27 0.31 0.13 0.41 4.59 19.4 17 11.4 6.78 65.8
1992 3.45 4.08 0.65 0.4 0.14 0.08 0.21 5.66 23.6 25.5 20.4 13.2 97.4
1993 8.48 6.8 3.35 1.99 4.85 7.42 10.3 21.2 27.9 24.7 18.6 12.8 149
1994 7.94 5.01 3.32 1.19 0.05 0.04 0.03 3.94 15 18.1 14 10.2 78.9
1995 5.21 2.31 1.5 0.47 1.79 1 1.07 3.8 9.34 11.5 7.97 3.12 49.1
1996 1.84 4.54 0.28 0.32 0.86 2.7 7.19 20.9 35.8 28.8 18.7 12.5 134
1997 8.59 6.78 3.76 7.26 5.75 3.16 5.39 9.01 10.1 8.49 7.39 4.71 80.3
1998 2.7 1.48 1.19 0.36 0.44 0.57 0.76 9.14 24.4 31.9 27.8 20.7 121
1999 14.7 10.1 7.75 4.39 1.2 0.35 0.74 3.48 7.33 17.2 22.5 15.7 106
2000 10.7 5.8 1.66 0.67 0.51 0.23 0.63 1.7 6.2 14 15.1 11.5 68.8
2001 8.43 7.1 3.63 2.87 1.18 0.53 3.99 7.47 25.1 23.8 14.5 12.2 111
2002 9.63 6.73 4.01 2.26 1.07 1.12 1.53 1.86 2.7 1.43 1.1 0.98 34.4
2003 0.63 0.28 0.18 0.19 0.27 0.34 0.27 0.73 1.93 1.76 1.13 0.59 8.3
2004 0.19 0.09 0.03 0.05 0.04 0.08 0.1 0.36 1.04 0.99 0.58 0.45 3.98
2005 0.43 0.27 0.2 0.11 0.2 0.21 0.26 1.21 1.89 1.39 0.94 1.19 8.3
Mean 4.73 3.18 1.93 1.44 1.15 1.13 1.84 5.82 12.7 12.5 9.46 6.26 62.2
Runoff
(MCM) 12.7 7.69 5.18 3.73 3.08 2.92 4.92 15.6 31.8 33.5 24.5 15.7 161
98

Appendix 4.5 Bulbula River Runoff Gauged at Bulbula Town (m3/sc)
Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1976 2.93 0.78 0.48 0.305 0.265 0.13 0.175 1.325 6.1 5.04 3.77 1.32 22.6
1977 7.45 4.98 5.35 2.925 1.16 0.57 1.975 7.64 12.09 12.7 9.61 7 73.43
1978 7.45 4.98 5.35 2.925 1.16 0.57 1.975 7.64 12.09 12.7 9.61 7 73.43
1979 4.43 3.7 3.18 2.615 1.495 1.35 3.415 10.72 12.92 13.1 10.35 7.1 74.33
1980 4.04 2.13 0.73 0.175 0 0 0 0 1.665 2.2 0.775 0.395 12.09
1981 0.17 0.15 0.17 0.163 1.19 0.16 1.305 2.84 8.52 10.8 7.53 4.66 37.67
1982 4.28 1.61 1.02 0.756 0.725 0.31 0.287 2.628 5.24 5.91 5.125 3.54 31.43
1983 2.34 1.14 0 1.12 2.705 5.59 6.12 18.24 37.79 5.12 3.334 2.066 85.56
1984 9.04 5.12 2.74 0.81 0.145 0.43 0.75 2.325 5.14 3.67 1.814 0.575 32.55
1985 0.26 0.26 0.2 0.159 0.146 0.15 0.145 0.262 2.023 2.19 0.815 0.391 6.998
1986 0.2 0.38 0.5 0.828 0.473 0.61 0.703 1.38 3.66 5.37 3.305 1.443 18.85
1987 0.72 0.48 0.47 1.377 1.6 3.07 1.8 2.355 3.78 3.49 1.701 0.702 21.54
1988 0.91 0.66 0.5 0.519 0.487 0.51 0.506 1.146 3.963 10.9 8.235 4.704 32.99
1989 2.57 1.51 0.86 1.118 0.587 0.62 0.783 2.136 5.002 5.35 2.523 1.111 24.18
1990 2.61 0.35 1.06 2.556 2.079 0.6 0.926 7.748 19.83 18 11.04 5.707 72.50
1991 2.65 1.04 0.52 0.205 0.235 0.1 0.312 3.509 14.81 13 8.679 5.18 50.23
1992 2.64 3.12 0.5 0.305 0.106 0.06 0.16 4.327 18 19.5 15.6 10.05 74.37
1993 6.48 5.19 2.56 1.518 3.702 5.67 7.901 16.23 21.34 18.9 14.24 9.793 113.5
1994 6.07 3.83 2.53 0.91 0.038 0.03 0.024 3.01 11.45 13.8 10.69 7.817 60.24
1995 3.98 1.77 1.14 0.358 1.369 0.77 0.817 2.901 7.137 8.78 6.087 2.386 37.49
1996 1.41 3.47 0.22 0.247 0.658 2.06 5.494 15.93 27.34 22 14.31 9.552 102.6
1997 6.56 5.18 2.87 5.549 4.389 2.41 4.118 6.88 7.683 6.49 5.648 3.599 61.37
1998 2.06 1.13 0.91 0.272 0.332 0.43 0.578 6.982 18.67 24.4 21.23 15.84 92.79
1999 11.3 7.74 5.92 3.352 0.915 0.26 0.565 2.661 5.599 13.1 17.22 11.98 80.60
2000 8.21 4.43 1.27 0.51 0.39 0.17 0.478 1.302 4.735 10.7 11.53 8.821 52.56
2001 6.44 5.43 2.77 2.193 0.904 0.4 3.05 5.708 19.21 18.2 11.08 9.295 84.65
2002 7.36 5.14 3.06 1.728 0.815 0.85 1.169 1.419 2.062 1.09 0.839 0.748 26.29
2003 0.48 0.21 0.14 0.144 0.203 0.26 0.204 0.558 1.474 1.35 0.862 0.453 6.338
2004 0.15 0.07 0.02 0.04 0.029 0.06 0.073 0.273 0.792 0.75 0.445 0.342 3.038
2005 0.33 0.2 0.15 0.087 0.154 0.16 0.197 0.927 1.442 1.06 0.715 0.907 6.339
Mean 3.85 2.54 1.57 1.192 0.949 0.95 1.533 4.7 10.05 9.65 7.29 4.816 49.09
Runoff
(MCM) 10.3 6.14 4.21 3.09 2.541 2.45 4.107 12.59 25.18 25.9 18.9 12.06 127.4
99

Appendix 4.6 Runoff for the Ungauged Sub-Catchment (MCM)
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
PPt 0 0 0.06 0.074 0.07 0.08 0.14 0.128 0.087 0.04 0.007 0.01
C 0.1 0 0.04 0.062 0.07 0.06 0.1 0.261 0.244 0.23 0.338 0.13
Q(MCM) 1.4 2.1 3.65 6.258 6.25 5.81 20.1 45.34 28.61 11.3 3.165 0.93
Appendix 4.7 Ziway Lake Maximum Gauge Height (m)
year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1974 0 0 0 0 0.44 0.38 0.55 0.87 1.22 1.24 1.14 0.94
1975 0.84 0.69 0.54 0.38 0.3 0.19 0.51 1.12 1.69 1.77 1.54 1.33
1976 1.15 0.97 0.88 0.76 0.7 0.65 0.73 1.1 1.29 1.24 1.1 0.97
1977 0.88 0.84 0.72 0.58 0.57 0.54 0.92 1.31 1.56 1.73 1.81 1.74
1978 1.6 1.45 1.43 1.29 1.08 0.96 1.15 1.61 1.74 1.81 1.63 1.49
1979 1.27 1.26 1.19 1.21 1.11 1.08 1.33 1.61 1.77 1.73 1.66 1.44
1980 1.26 1.11 0.98 0.82 0.7 0.55 0.65 0.91 1.07 1.14 0.91 0.75
1981 0.59 0.49 0.52 0.7 0.68 0.58 0.7 1.23 1.67 1.7 1.54 1.35
1982 1.15 1.03 0.94 0.81 0.83 0.75 0.76 1.29 1.32 1.47 1.38 1.26
1983 1.1 0.95 0.88 1.04 1.15 1.23 1.32 2.13 2.23 2.22 2.1 1.85
1984 1.54 1.42 1.2 1.01 0.84 0.8 0.96 1.18 1.38 1.35 1.14 0.93
1985 0.82 0.68 0.58 0.38 0.42 0.38 0.44 0.86 1.16 1.16 0.98 0.84
1986 0.65 0.56 0.53 0.43 0.52 0.46 0.75 1.02 0 0 1.17 1.02
1987 0.92 0.8 0.82 0.71 1 1.09 1.13 1.23 1.24 1.19 1.1 0.91
1988 0.73 0.58 0.5 0.31 0.28 0.27 0.36 0.98 1.3 1.46 1.43 1.3
1989 1.08 0.96 0.85 0.88 0.86 0.73 0.87 1.12 1.36 1.4 1.31 1.13
1990 0.98 0.96 1.14 1.28 1.26 1.14 1.26 1.68 1.76 1.78 1.66 1.34
1991 1.2 1.06 1.03 0.97 0.88 0.74 1 1.5 1.74 1.7 1.44 1.26
1992 1.13 1.05 0.94 0.79 0.7 0.7 1.1 1.51 1.79 1.8 1.64 1.49
1993 1.38 1.26 1.17 0.98 1.1 1.15 1.36 1.79 1.86 1.82 1.76 1.54
1994 1.36 1.24 1.04 0.9 0.77 0.74 0.98 1.66 1.78 1.82 1.58 1.41
1995 1.24 1.22 1 0.98 1.02 1.02 1 1.34 1.56 1.56 1.26 1.12
1996 1.02 0.92 0.75 0.82 0.98 1.2 1.54 2.14 2.28 2.24 2.05 1.8
1997 1.58 1.46 1.3 1.3 1.3 1.14 1.3 1.44 1.46 1.4 1.36 1.26
1998 1.12 1 0.96 0.9 0.86 0.86 1.12 1.76 2.06 2.19 2.16 1.9
1999 1.68 1.48 1.27 1.16 1.04 0.94 1.1 1.37 1.48 1.88 1.86 1.7
2000 1.5 1.35 1.13 0.94 0.86 0.8 0.9 1.31 1.48 1.64 1.6 1.5
2001 1.34 1.2 1.08 1.08 0.96 1 1.43 2.11 2.3 2.26 2.04 1.82
2002 1.64 1.48 1.38 1.24 1.14 1.06 0.96 1.18 1.26 1.23 1.08 0.86
2003 0.76 0.62 0.57 0 0.42 0.26 0.6 1.06 1.26 1.26 1.1 0.96
2004 0.92 0.8 0.66 0.62 0.6 0.44 0.54 0.9 1.1 1.12 1.06 0.96
2005 0.8 0.7 0.56 0.54 0.66 0.66 0.7 1.14 1.54 1.54 1.41 1.25
2006 1.09 0.96 0.82 0.98 1.1 1.02 1.39 2.1 2.28 0 0 0
100

Appendix 4.8 Lake Ziway Minimum Gauge Height (m)
year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1974 0 0 0 0 0.33 0.29 0.29 0.52 0.88 1.12 0.95 0.8
1975 0.56 0.54 0.38 0.24 0.15 0.1 0.12 0.51 1.14 1.53 1.3 1.12
1976 0.98 0.85 0.75 0.67 0.65 0.52 0.57 0.74 1.14 1.07 0.97 0.86
1977 0.79 0.72 0.57 0.51 0.52 0.47 0.52 0.94 1.34 1.5 1.74 1.61
1978 1.45 1.28 1.29 1.05 0.94 0.8 0.88 1.17 1.6 1.64 1.49 1.26
1979 1.22 1.14 1.05 1.09 1.06 1.02 1.05 1.33 1.62 1.61 1.43 1.27
1980 1.11 0.97 0.83 0.7 0.53 0.48 0.49 0.66 0.91 0.94 0.75 0.57
1981 0.49 0.38 0.32 0.52 0.56 0.4 0.36 0.72 1.24 1.54 1.34 1.16
1982 1.04 0.91 0.77 0.75 0.71 0.62 0.62 0.77 1.27 1.27 1.22 1.1
1983 0.94 0.87 0.76 0.77 0.87 1.16 1.19 1.35 2.04 2.1 1.86 1.59
1984 1.19 1.18 1 0.79 0.76 0.73 0.72 0.96 1.16 1.12 0.93 0.78
1985 0.64 0.52 0.34 0.32 0.29 0.2 0.18 0.44 0.92 0.96 0.84 0.72
1986 0.52 0.49 0.41 0.39 0.33 0.29 0.48 0.78 0 0 1.02 0.93
1987 0.79 0.65 0.6 0.64 0.71 1.02 1.03 1.11 1.17 1.1 0.92 0.77
1988 0.58 0.48 0.3 0.22 0.16 0.15 0.23 0.3 0.98 1.3 1.28 1.08
1989 0.96 0.84 0.78 0.74 0.7 0.68 0.69 0.87 1.14 1.3 1.13 0.92
1990 0.84 0.84 0.98 1.09 1.1 1.02 1.02 1.28 1.68 1.55 1.36 1.16
1991 1.06 0.97 0.92 0.89 0.75 0.63 0.63 0.98 1.52 1.46 1.26 1.12
1992 1.03 0.94 0.8 0.69 0.63 0.6 0.7 1.11 1.52 1.65 1.48 1.35
1993 1.2 1.16 0.98 0.92 0.97 1.1 1.12 1.38 1.76 1.76 1.54 1.38
1994 1.24 1.06 0.9 0.76 0.7 0.69 0.72 0.98 1.66 1.58 1.4 1.24
1995 1.19 1 0.9 0.86 0.98 0.84 0.84 1 1.34 1.24 1.13 1
1996 0.92 0.76 0.68 0.73 0.81 0.94 1.22 1.58 2.14 2.08 1.8 1.54
1997 1.46 1.3 1.2 1.2 1.12 1.08 1.15 1.32 1.38 1.3 1.24 1.12
1998 1.02 0.94 0.9 0.8 0.8 0.72 0.72 1.09 1.76 2.02 1.92 1.69
1999 1.46 1.27 1.16 1.03 0.89 0.78 0.82 1.12 1.36 1.46 1.72 1.5
2000 1.34 1.14 0.92 0.64 0.65 0.7 0.7 0.88 1.34 1.5 1.5 1.36
2001 1.2 1.08 0.98 0.9 0.9 0.94 0.98 1.47 2.15 2.06 1.83 1.66
2002 1.5 1.38 1.24 1.14 1 0.92 0.9 0.9 1.2 1.08 0.88 0.76
2003 0.62 0.56 0.4 0 0.28 0.22 0.24 0.6 1.06 1.11 0.94 0.9
2004 0.76 0.68 0.52 0.44 0.44 0.34 0.3 0.56 0.82 1.08 0.96 0.8
2005 0.7 0.54 0.52 0.34 0.4 0.6 0.56 0.73 1.19 1.42 1.27 1.08
2006 0.96 0.82 0.75 0.8 0.98 0.96 0.98 1.44 2.12 0 0 0
101

Appendix 4.9 Mean Monthly Gauge Height of Lake Ziway (Reference point 1635.10masl)
year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
1975 0.728 0.621 0.456 0.321 0.215 0.132 0.282 0.781 1.462 1.645 1.45 1.227 0.77667
1976 1.07 0.916 0.814 0.718 0.679 0.596 0.647 0.948 1.233 1.158 1.044 0.915 0.89483
1977 0.832 0.767 0.628 0.552 0.545 0.51 0.725 1.124 1.477 1.564 1.771 1.68 1.01458
1978 1.531 1.367 1.356 1.191 1.006 0.862 1.001 1.398 1.683 1.744 1.552 1.366 1.33808
1979 1.241 1.202 1.126 1.159 1.092 1.057 1.178 1.489 1.718 1.66 1.525 1.366 1.31775
1980 1.175 1.047 0.91 0.761 0.613 0.517 0.555 0.803 0.996 1.012 0.825 0.654 0.82233
1981 0.538 0.432 0.395 0.599 0.635 0.499 0.491 0.987 1.452 1.635 1.418 1.238 0.85992
1982 1.11 0.976 0.863 0.786 0.773 0.683 0.666 0.992 1.294 1.381 1.313 1.172 1.00075
1983 1.022 0.91 0.81 0.888 0.974 1.21 1.235 1.649 2.167 2.175 1.955 1.691 1.3905
1984 1.444 1.313 1.094 0.898 0.801 0.769 0.809 1.139 1.307 1.239 1.036 0.872 1.06008
1985 0.719 0.601 0.456 0.355 0.353 0.262 0.268 0.68 1.039 1.065 0.912 0.78 0.62417
1986 0.58 0.52 0.484 0.407 0.386 0.361 0.632 0.888 1.124 1.1065 1.099 0.985 0.71438
1987 0.846 0.724 0.688 0.687 0.782 1.055 1.083 1.162 1.209 1.148 0.999 0.845 0.93567
1988 0.655 0.532 0.387 0.245 0.201 0.187 0.28 0.672 1.133 1.396 1.332 1.189 0.68408
1989 1.019 0.922 0.812 0.817 0.776 0.7 0.766 0.987 1.258 1.361 1.218 1.047 0.97358
1990 0.922 0.876 1.095 1.216 1.193 1.078 1.115 1.46 1.741 1.675 1.482 1.266 1.25992
1991 1.123 1.002 0.979 0.934 0.816 0.693 0.79 1.246 1.653 1.574 1.35 1.175 1.11125
1992 1.073 0.983 0.875 0.737 0.673 0.662 0.894 1.281 1.706 1.744 1.566 1.413 1.13392
1993 1.297 1.214 1.071 0.953 1.048 1.128 1.248 1.6 1.805 1.796 1.649 1.469 1.3565
1994 1.295 1.148 0.959 0.835 0.72 0.707 0.817 1.329 1.714 1.764 1.463 1.341 1.17433
1995 1.211 1.089 0.954 0.897 1.006 0.926 0.914 1.201 1.466 1.407 1.202 1.063 1.11133
1996 0.969 0.829 0.718 0.756 0.89 1.069 1.354 1.843 2.242 2.164 1.885 1.68 1.36658
1997 1.526 1.396 1.243 1.237 1.223 1.107 1.204 1.36 1.422 1.339 1.308 1.181 1.2955
1998 1.063 0.965 0.929 0.868 0.828 0.794 0.896 1.444 1.968 2.132 2.045 1.778 1.30917
1999 1.565 1.369 1.225 1.093 0.958 0.824 0.936 1.234 1.43 1.646 1.805 1.601 1.30717
2000 1.405 1.223 1.032 0.818 0.79 0.738 0.802 1.025 1.398 1.585 1.55 1.41 1.148
2001 1.274 1.124 1.03 1 0.94 0.974 1.211 1.696 2.25 2.154 1.927 1.747 1.44392
2002 1.568 1.411 1.296 1.2 1.084 0.985 0.926 1.03 1.224 1.151 0.972 0.805 1.13767
2003 0.685 0.582 0.45 0.8725 0.359 0.235 0.374 0.858 1.17 1.194 1.032 0.925 0.72804
2004 0.842 0.756 0.568 0.545 0.524 0.398 0.373 0.743 0.951 1.106 1.011 0.897 0.72617
2005 0.742 0.604 0.53 0.413 0.56 0.622 0.634 0.994 1.382 1.472 1.327 1.157 0.86975
2006 1.019 0.889 0.785 0.92 1.063 0.998 1.171 1.803 2.191 x x x 1.20433
Mean 1.0653 0.9472 0.8443 0.802453 0.765813 0.72931 0.82116 1.182688 1.50828 1.522339 1.3878 1.2237097 1.06534
102

Appendix 4.10 Lake height with Reference point 1628masl
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1975 7.828 7.721 7.556 7.421 7.315 7.232 7.382 7.881 8.562 8.745 8.55 8.327
1976 8.17 8.016 7.914 7.818 7.779 7.696 7.747 8.048 8.333 8.258 8.144 8.015
1977 7.932 7.867 7.728 7.652 7.645 7.61 7.825 8.224 8.577 8.664 8.871 8.78
1978 8.631 8.467 8.456 8.291 8.106 7.962 8.101 8.498 8.783 8.844 8.652 8.466
1979 8.341 8.302 8.226 8.259 8.192 8.157 8.278 8.589 8.818 8.76 8.625 8.466
1980 8.275 8.147 8.01 7.861 7.713 7.617 7.655 7.903 8.096 8.112 7.925 7.754
1981 7.638 7.532 7.495 7.699 7.735 7.599 7.591 8.087 8.552 8.735 8.518 8.338
1982 8.21 8.076 7.963 7.886 7.873 7.783 7.766 8.092 8.394 8.481 8.413 8.272
1983 8.122 8.01 7.91 7.988 8.074 8.31 8.335 8.749 9.267 9.275 9.055 8.791
1984 8.544 8.413 8.194 7.998 7.901 7.869 7.909 8.239 8.407 8.339 8.136 7.972
1985 7.819 7.701 7.556 7.455 7.453 7.362 7.368 7.78 8.139 8.165 8.012 7.88
1986 7.68 7.62 7.584 7.507 7.486 7.461 7.732 7.988 8.224 8.2065 8.199 8.085
1987 7.946 7.824 7.788 7.787 7.882 8.155 8.183 8.262 8.309 8.248 8.099 7.945
1988 7.755 7.632 7.487 7.345 7.301 7.287 7.38 7.772 8.233 8.496 8.432 8.289
1989 8.119 8.022 7.912 7.917 7.876 7.8 7.866 8.087 8.358 8.461 8.318 8.147
1990 8.022 7.976 8.195 8.316 8.293 8.178 8.215 8.56 8.841 8.775 8.582 8.366
1991 8.223 8.102 8.079 8.034 7.916 7.793 7.89 8.346 8.753 8.674 8.45 8.275
1992 8.173 8.083 7.975 7.837 7.773 7.762 7.994 8.381 8.806 8.844 8.666 8.513
1993 8.397 8.314 8.171 8.053 8.148 8.228 8.348 8.7 8.905 8.896 8.749 8.569
1994 8.395 8.248 8.059 7.935 7.82 7.807 7.917 8.429 8.814 8.864 8.563 8.441
1995 8.311 8.189 8.054 7.997 8.106 8.026 8.014 8.301 8.566 8.507 8.302 8.163
1996 8.069 7.929 7.818 7.856 7.99 8.169 8.454 8.943 9.342 9.264 8.985 8.78
1997 8.626 8.496 8.343 8.337 8.323 8.207 8.304 8.46 8.522 8.439 8.408 8.281
1998 8.163 8.065 8.029 7.968 7.928 7.894 7.996 8.544 9.068 9.232 9.145 8.878
1999 8.665 8.469 8.325 8.193 8.058 7.924 8.036 8.334 8.53 8.746 8.905 8.701
2000 8.505 8.323 8.132 7.918 7.89 7.838 7.902 8.125 8.498 8.685 8.65 8.51
2001 8.374 8.224 8.13 8.1 8.04 8.074 8.311 8.796 9.35 9.254 9.027 8.847
2002 8.668 8.511 8.396 8.3 8.184 8.085 8.026 8.13 8.324 8.251 8.072 7.905
2003 7.785 7.682 7.55 7.9725 7.459 7.335 7.474 7.958 8.27 8.294 8.132 8.025
2004 7.942 7.856 7.668 7.645 7.624 7.498 7.473 7.843 8.051 8.206 8.111 7.997
2005 7.842 7.704 7.63 7.513 7.66 7.722 7.734 8.094 8.482 8.572 8.427 8.257
103

Appendix 4.11 Area of the Lake with the Bathymetry Best Fit Equations
(Km2)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1975 431.8 428 422 416.8 412.5 408.9 415.26 433.66 462.5 473.95 461.82 450.8
1976 444.3 438 434.8 431.4 430.1 427.1 428.93 439.65 451.1 447.83 443.26 438.44
1977 435.5 433 428.3 425.5 425.3 424 431.68 446.43 463.3 468.57 483.46 476.45
1978 466.5 457 456.9 449.2 441.8 436.5 441.62 459.02 476.7 481.29 467.82 457.37
1979 451.4 450 446.5 447.9 445.1 443.8 448.67 464.03 479.3 475.01 466.16 457.37
1980 448.5 443 438.3 433 427.7 424.3 425.65 434.43 441.4 442.04 435.21 429.18
1981 425 421 419.7 427.2 428.5 423.6 423.32 441.09 461.9 473.26 460.08 451.29
1982 445.9 441 436.6 433.8 433.4 430.2 429.6 441.28 453.9 458.14 454.76 448.42
1983 442.4 438 434.7 437.5 440.6 450.1 451.15 474.23 525.2 526.23 500.31 477.25
1984 461.5 455 445.2 437.8 434.4 433.2 434.64 447.04 454.5 451.33 442.95 436.89
1985 431.5 427 422 418.2 418.1 414.4 414.69 430.1 443.1 444.08 438.33 433.62
1986 426.5 424 423.1 420.2 419.4 418.4 428.4 437.46 446.4 445.72 445.42 441.02
1987 436 432 430.4 430.3 433.7 443.7 444.78 448 450 447.42 441.54 435.92
1988 429.2 425 419.4 413.7 411.9 411.3 415.18 429.82 446.8 458.91 455.68 449.14
1989 442.3 439 434.8 434.9 433.5 430.8 433.13 441.09 452.2 457.12 450.4 443.38
1990 438.7 437 445.3 450.3 449.3 444.6 446.07 462.38 481.1 476.08 463.63 452.55
1991 446.4 442 440.8 439.1 434.9 430.6 433.97 451.65 474.5 469.21 456.57 448.55
1992 444.4 441 437 432.1 429.9 429.5 437.68 453.25 478.4 481.29 468.7 459.81
1993 454 450 444.3 439.8 443.4 446.6 451.74 470.9 486.3 485.53 474.23 462.88
1994 453.9 447 440.1 435.6 431.5 431 434.93 455.53 479 482.89 462.55 456.12
1995 450.1 445 439.9 437.8 441.8 438.8 438.4 449.66 462.7 459.49 449.7 444
1996 440.4 435 431.4 432.8 437.5 444.2 456.77 489.6 535.6 524.76 493.44 476.45
1997 466.2 459 451.5 451.2 450.6 445.7 449.79 457.07 460.3 456.02 454.52 448.8
1998 444 440 438.9 436.7 435.3 434.1 437.75 461.49 501.7 520.6 510.07 484.03
1999 468.6 458 450.7 445.2 440 435.2 439.21 451.11 460.7 474.02 486.29 470.96
2000 459.4 451 442.8 435 434 432.1 434.4 442.53 459 469.92 467.69 459.65
2001 452.9 446 442.7 441.6 439.4 440.6 450.09 477.62 536.8 523.44 497.49 481.53
2002 468.8 460 453.9 449.6 444.8 441 438.84 442.72 450.7 447.54 440.53 434.5
2003 430.3 427 421.8 436.9 418.3 413.3 418.9 436.39 448.3 449.36 442.8 438.8
2004 435.8 433 426.1 425.3 424.5 419.8 418.87 432.32 439.8 445.7 442 437.79
2005 432.3 427 424.7 420.4 425.8 428 428.47 441.36 458.2 463.05 455.43 447.79
104

Appendix 4.12 Volume of The Lake using the Best Fit Curve of Bathymetry(MCM)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1975 1499 1450 1375 1314 1267 1230 1296.5 1523.3 1843 1929.3 1837.1 1731.8
1976 1658 1586 1539 1494 1476 1438 1461.6 1600.9 1735 1699.3 1645.8 1585.5
1977 1547 1517 1453 1418 1415 1399 1497.4 1683.3 1850 1891 1988.9 1945.9
1978 1875 1798 1793 1715 1628 1561 1625.6 1812.5 1947 1976.1 1885.3 1797.4
1979 1738 1720 1684 1700 1668 1652 1708.7 1855.5 1964 1936.4 1872.5 1797.4
1980 1707 1647 1583 1514 1446 1402 1419.5 1533.5 1623 1630.8 1543.7 1464.8
1981 1412 1364 1347 1440 1456 1394 1390.4 1619.1 1838 1924.6 1821.9 1737
1982 1677 1614 1561 1526 1520 1478 1470.3 1621.4 1763 1804.5 1772.3 1705.9
1983 1635 1583 1537 1573 1613 1724 1735.5 1931.2 2175 2178.6 2075.6 1951.1
1984 1834 1772 1669 1578 1533 1518 1536.2 1690.4 1770 1737.4 1642 1565.5
1985 1495 1440 1375 1329 1328 1288 1290.2 1476.7 1643 1655.6 1584.1 1522.8
1986 1431 1404 1387 1352 1343 1332 1454.7 1572.9 1683 1675.1 1671.6 1618.2
1987 1553 1497 1480 1480 1524 1651 1664.1 1701.2 1723 1694.6 1624.7 1552.9
1988 1465 1409 1343 1280 1261 1254 1295.6 1473 1688 1811.5 1781.3 1713.9
1989 1634 1589 1538 1540 1521 1486 1516.4 1619.1 1746 1795 1727.5 1647.2
1990 1589 1567 1670 1727 1716 1662 1679.1 1841.8 1975 1943.5 1852.2 1750.2
1991 1683 1626 1615 1594 1539 1483 1527.5 1740.7 1933 1895.7 1789.8 1707.3
1992 1659 1617 1567 1503 1473 1468 1575.7 1757.2 1958 1976.1 1891.9 1819.6
1993 1765 1726 1658 1603 1648 1685 1741.7 1908 2005 2000.7 1931.2 1846.1
1994 1764 1695 1606 1548 1495 1489 1540 1779.9 1962 1985.5 1843.2 1785.6
1995 1724 1667 1604 1577 1628 1591 1585 1719.5 1845 1816.7 1720 1654.7
1996 1611 1546 1494 1512 1574 1657 1791.7 2022.8 2210 2173.5 2042.6 1945.9
1997 1873 1812 1739 1736 1730 1675 1720.9 1794.5 1824 1784.6 1770 1710.1
1998 1655 1609 1592 1564 1545 1529 1576.7 1834.2 2082 2158.5 2117.8 1992.2
1999 1891 1799 1731 1669 1606 1543 1595.3 1735.1 1828 1929.8 2004.9 1908.5
2000 1816 1730 1640 1540 1527 1503 1533 1636.9 1812 1900.9 1884.4 1818.2
2001 1754 1683 1639 1625 1597 1613 1724.2 1953.4 2214 2168.8 2062.4 1977.5
2002 1893 1819 1764 1719 1665 1618 1590.6 1639.2 1730 1696 1612.1 1534.4
2003 1479 1432 1372 1566 1331 1276 1337.6 1559 1705 1716.2 1640.2 1590.2
2004 1552 1512 1425 1415 1405 1348 1337.2 1505.7 1602 1674.9 1630.3 1577.1
2005 1505 1442 1408 1355 1422 1450 1455.6 1622.4 1805 1847.5 1778.9 1698.8
105

Appendix 4.13 Katar River Basin point Rainfall Distribution (mm)
Years Station Elv. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1989-
2006 Merero 2975 19.13 37.6 65.5 73.5 50.8 76.79 145.1 168.08 75.764 43.53 16 18 789.43
1976-
2006 Bokoji 2793 30.49 42.4 88.4 107 99.1 112.9 178.8 193.85 89.175 57.38 17 13.9 1030.9
1976-
2006 Kersa 2760 25.67 40.7 73.6 104 90.2 84.01 118.3 125.47 107.81 56.27 18 12.8 856.42
1976-
2006 Digalu 2689 29.2 39.1 75.2 97.6 84.1 104 177.4 178.57 96.831 52.06 14 11.8 959.54
1976-
2006 Sagure 2516 13.5 28.3 61.2 78.9 83.9 97.18 154.1 143.82 73.199 39.82 8.8 6.41 789.24
1976-
2006 Asella 2396 18.59 41.8 90.4 120 118 119.8 182.6 195.25 150.91 64.52 20 14.9 1137.3
1976-
2006 Kulumsa 2153 19.15 41.6 84.2 88.5 86.7 90.6 122.8 131.89 97.192 44.68 12 10.6 829.57
1978-
2006 Arata 1760 17.66 34.4 66.4 77.1 82 90.28 136.1 117.22 100.45 41.1 12 6.03 780.72
1975-
2005 Ogelcho 1690 13.99 28.3 72.3 69.6 62.9 77.08 149.8 109.11 92.398 30.71 7.7 3.79 717.72
Appendix 4.14 Meki River Basin Point Rainfall Distribution(mm)
Years Station Elv. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1978-
2006 Katar.Ge 2149 11.2 30.3 58.6 71.9 92.9 92.6 140.1 126.37 100.78 38.54 6.2 4.66 774.15
1975-
2005 Butajira 2088 44.64 65.6 124 137 111 119.8 186.2 168.54 120.45 53.95 14 14 1158.9
1976-
2006 Bui 2020 27.61 51.6 90.9 90.3 87.1 123 202.9 178.81 92.577 40.34 12 10.5 1007.3
1976-
2006 Tora 2012 24.92 47.5 78.2 127 91.9 92.48 129.3 126.57 111.67 51.12 8.3 6.01 894.71
1976-
2006 Koshe 1873 23.03 48.4 80.2 98.1 91 94.21 163.9 165.04 104.46 47.38 5.5 7.72 928.84
1975-
2005 Ejersalale 1779 15.4 31.4 57.8 71.1 64.4 81.01 188.5 160.13 85.395 32.32 5 6.28 798.72
1975-
2005 Meki 1663 16.09 38.2 57.1 69.4 63.2 74.4 172.4 145.93 79.905 36.17 5.2 5.08 763.05
106

Appendix 4.15 Katar River Basin Areal Rainfall Distribution(mm)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1989 6.472 39.47 85.4 119.7 36 99.31 115 135.4 116 45.43 9.135 45.71 853.068
1990 2.124 147 80.7 95.15 44.9 50 157 159.9 99.4 17.88 9.667 1.2622 864.563
1991 6.397 45.46 121 43.08 48.3 90.46 189 153.8 89.8 12.95 0.719 14.54 815.857
1992 48.71 59.6 29.5 69.06 49.6 73.21 139 193.6 103 77.59 18.52 12.857 873.7
1993 27.38 52.78 15.7 137.7 146 72.15 141 156.8 100 79.12 1.216 2.2935 931.893
1994 0.146 3.874 42.8 78.25 59.3 154.9 175 138.2 103 4.885 23.3 4.2172 787.866
1995 0 37.09 94.9 155.5 56.6 65.71 163 153.2 97.9 18 0.382 29.94 871.908
1996 51.57 5.301 106 60.01 125 157.6 136 151.9 91.8 15.86 2.876 1.5472 905.8
1997 19.91 7.38 76.7 114 38.5 98.17 154 96.3 64.6 87.21 26.73 0.5464 784.403
1998 49.29 45.62 54 64.81 91.9 91.42 140 175.8 107 93.08 6.882 0.0512 920.075
1999 16.28 6.023 50.2 20.48 49 102.8 165 107.7 80.3 168.3 3.573 0.3708 769.884
2000 0.221 0.52 8.45 94.84 89.2 103.9 134 147.3 153 58.89 55.79 8.2341 854.275
2001 8.129 35.89 124 46.11 157 134.2 143 162.7 68.7 18.22 3.316 3.3251 904.677
2002 18.88 34.21 80.7 46.53 68.5 79.23 96.1 147.2 58.9 4.055 0.911 28.761 663.977
2003 11.84 20.81 70.8 119 40.3 90.71 170 114.9 109 4.714 4.114 28.558 784.897
2004 37.7 1.248 44 104.2 32 88.29 124 129.5 114 67.62 7.744 11.67 762.674
2005 21.74 26.04 92.8 108.6 104 92.04 163 162.2 68.8 21.78 10.45 2.2251 873.891
Mean 19.22 33.43 69.3 86.89 72.7 96.72 147 146.3 95.6 46.8 10.9 11.536 836.671
107

Appendix 4.16 Meki River Basin Areal Rainfall Distribution (mm)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1976 2.13 33.05 141 46.3 106 89 163 150.9 55.49 18.02 59.6 3.15 867.97
1977 106 33.596 63.1 110 87.9 126 172 130.2 78.09 232.6 27.3 0.52 1166.5
1978 2.01 151.34 44.1 84.1 37.5 131 136 197.7 103.1 54.95 2.15 15.8 959.12
1979 84.2 38.159 157 55.6 157 117 191 234 138.2 57.48 0.4 4.79 1234.3
1980 86.9 11.067 37.4 81.5 71.7 106 180 155.3 103.1 19.99 2.26 0.07 855.77
1981 0 7.1661 251 124 33.1 28 206 200.2 113.7 12.35 2.84 1.1 979.74
1982 54.3 69.626 35.3 115 150 65 119 140.2 86.9 107.7 30.9 18.8 992.1
1983 39.7 54.613 121 150 201 75 199 291.6 125.5 31.37 3.96 1.83 1294.2
1984 0.22 3.289 18.1 9.14 191 136 204 210.6 111.1 1.184 6.13 2.48 893.85
1985 5.67 0.6343 38 142 102 48 206 198.6 116.9 9.198 1.15 0.59 869.09
1986 0.44 125.95 22.2 132 101 133 113 123.5 116.9 36.87 0.11 5.26 910.43
1987 0 79.029 213 82.4 214 50 89.1 129.8 91.26 13.09 0 0 961.67
1988 7.64 60.272 29.6 109 38.4 96 170 132.7 134 51.12 0 3.34 831.51
1989 3.03 94.226 156 107 21.4 152 180 176.3 115.2 24.77 0 18.5 1049.2
1990 0.37 263.1 65.8 130 65.8 67 197 163.2 87.06 12.24 0 4.66 1056.1
1991 13.3 88.05 169 10.2 13.2 55 195 200.4 85.47 7.021 0 12.2 848.83
1992 36.3 54.336 11.2 78.5 53.8 93 273 210.5 90.69 55.91 0.76 7.94 966.38
1993 28 64.579 5.69 220 120 69 212 176.4 101.1 102.2 0.07 0.53 1099.1
1994 0 1.376 57 40.7 51.3 179 223 126.1 116.4 0.795 6.54 3.47 805.73
1995 0 30.699 110 236 85.2 58 142 103.2 91.68 10.15 0 46.3 914.11
1996 105 1.5341 165 71.9 158 186 137 211.3 85.11 4.867 10.3 0 1135.8
1997 62.5 2.8152 89.7 141 19.4 146 142 133 58.64 110 22.4 0 927.35
1998 78.9 68.211 109 55.9 105 85 174 200.3 91.84 77.93 0.16 0 1045.9
1999 6.08 11.678 70 35 32.5 110 215 133.4 77.14 155.3 0 0 846.58
2000 3.35 10.828 7.39 92.7 63.6 70 132 139.3 134.7 55.78 67.8 56 833.12
2001 3.98 59.18 190 38.3 161 145 192 164.5 77.19 13.42 2.49 8.15 1054.7
2002 25.4 30.233 81.3 75.5 60.9 96 127 169.1 82.47 0 0 18.7 766.66
2003 28.1 25.1 81.6 135 24.4 118 226 122.8 121.5 0.134 2.9 47.3 932.28
2004 51.4 2.8179 45.9 148 2.51 77 135 115.9 103.3 51.68 1.49 0.05 736.15
2005 19.3 24.203 70 138 147 106 256 150.4 148.1 48.75 10.3 0.52 1118.2
Mean 28.5 50.025 88.5 99.9 89.1 100 177 166.4 101.4 45.9 8.74 9.4 965.08
108

Appendix 4.17 Ziway Basin Areal Rainfall Distribution(mm)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1989 5.49 61.19 125 113.9 26.5 121.9 140.16 151 117 33.5 4.56 33.08 933.33
1990 1.2 192.4 67.36 105.6 52.7 56.37 170.99 158 93.2 13.7 4.82 2.448 918.94
1991 8.77 68.34 141.7 26.44 33.3 75.65 181.09 171 83.5 9.9 0.36 14.21 813.88
1992 40.8 54.44 18.68 69.78 52.1 85.21 198.34 194 88.3 72.1 9.65 9.875 893.74
1993 31 66.22 8.86 164.2 132 70.84 173.24 161 93.4 87 0.64 1.362 990
1994 0.07 2.461 44.61 56.03 53.9 163.9 186.3 131 107 2.74 14.8 3.42 766.08
1995 0 32.71 97.12 183.4 63.9 61.25 144.01 130 88 13.4 0.19 33.54 847.71
1996 70 3.817 123 68.33 136 162.4 134.25 174 89.4 9.76 7.76 0.775 979.31
1997 35.3 4.726 80.41 132.7 27 118.6 149.97 106 58.5 95.9 22 0.276 831.14
1998 57.2 52.04 72.86 59.99 90.4 87.44 156.34 189 99.4 86 3.58 0.026 954.38
1999 12 8.662 55.09 25.01 43.2 104 174.56 113 78.8 159 1.78 0.184 775.75
2000 2.11 5.37 5.841 93.81 78.9 82.57 136.09 138 141 54.9 55.2 29.1 822.75
2001 5.5 41.57 152.3 41.67 155 130.4 159.65 161 72.3 14.3 2.62 4.96 941.02
2002 20.1 31.04 76.6 58.32 63.8 83.01 107.22 147 68.1 2.03 0.46 22.27 679.77
2003 18.6 20.26 74.96 123 33 100.4 195.03 123 112 2.4 3.36 35.44 841.18
2004 46.3 1.752 41.35 125.5 16.9 83.63 133.26 122 109 57.6 4.6 5.845 747.9
2005 23.3 23.62 81.62 118.1 127 92.76 193.39 149 108 31.4 9.58 1.309 958.96
Mean 22.2 39.45 74.55 92.1 69.8 98.84 160.82 148 94.5 43.9 8.59 11.65 864.46
109

Appendix 4.18 Lake Ziway Areal Rainfall (mm)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1978 5.44 120 55.32 40.63 17.21 149.7 94.024 183.72 102.6 50.97 12.98 2.699 835.47
1979 58 41.7 106.8 42.58 104.3 102 156.7 98.702 89.69 56.12 0.065 2.632 859.18
1980 36.7 11.6 13.65 45.2 29.4 69.49 127.04 110.73 64.29 33.22 0.594 0 541.89
1981 0 9.17 174.2 99.05 24.79 8.631 190.52 160.92 165.2 25.32 3.858 0.145 861.78
1982 38.6 25.7 53.77 80.05 71.81 33.76 122.66 168.79 76.81 97.02 13.93 3.871 786.82
1983 20.4 56.1 70.8 114.3 142.8 52.51 153.38 151.69 82.12 25.35 1.403 0.608 871.43
1984 0 2.51 11.51 31.88 136.2 76.56 166.89 155.12 60.64 0.553 3.085 1.97 646.93
1985 2.73 0.97 36.28 90 80.35 27.96 166.34 140.63 85.29 4.746 53.82 0.03 689.13
1986 0 90.5 76.26 63.14 99.92 112.6 134.59 66.447 104.1 29.48 1.642 0.843 779.49
1987 0 21.3 82.02 53.84 192.2 22.75 65.566 79.72 43.26 7.75 0.045 0.76 569.3
1988 2.76 35.4 11.05 95.52 24.13 126.2 131.97 134.46 148.8 70.99 0 0.144 781.47
1989 4.89 40 139.4 98.87 8.427 126.9 112.51 146.22 120.8 17.76 0 13.14 828.96
1990 0 182 21.77 73.04 33.42 37.59 194.69 142.29 92.58 5.433 0.933 0.541 784.22
1991 2.52 71.3 148.4 22.67 34.87 57.16 126.23 137.93 53.43 9.269 0.562 17.98 682.34
1992 30 41.6 14.43 55.7 53.21 73.19 215.66 176.61 78.04 73.73 2.801 7.988 822.97
1993 40.9 70.6 9.539 116.3 110.7 57.32 197.46 133.16 58.91 68.9 0.521 0.627 864.85
1994 0 0.17 26.33 22.29 48.66 155.3 157.17 104.26 76.75 0.126 6.215 0.067 597.36
1995 0 19.9 69.46 139.9 37.72 49.55 86.878 106.57 47.1 12.48 0 17.94 587.53
1996 43.1 2.69 69.42 69.07 128.8 146.7 121.38 157.41 99.18 3.884 13.92 0 855.55
1997 17.7 0.87 80.53 162.3 19.18 100.5 161.75 68.172 58.31 81.63 7.516 0 758.42
1998 29.6 37.1 40.27 54.96 49.98 73.86 151.21 179.86 104.7 87.18 4.366 0 813.19
1999 35.4 23.1 39.62 24.21 35.42 87.1 125.61 88.157 71.76 160 2.706 0 693.06
2000 2.09 8.48 1.136 80.87 83.54 52.87 148.07 121.29 128.3 26.44 53.65 17.14 723.88
2001 0 19 134 23.15 119.4 78.09 136.87 130.29 52.37 2.614 0 1.223 697.02
2002 4.57 30.1 48.06 52.14 46.44 61.84 99.779 122.64 48.49 0.582 0 14.25 528.89
2003 15.2 26.2 74.27 116 30.08 83.77 207.14 119.2 78.35 1.732 3.914 36.03 791.93
2004 42.6 0.3 27.55 131.5 8.991 71.8 126.49 105.64 120.2 35.91 1.825 2.25 675.03
2005 39.2 17 74.9 79.37 108.3 50.96 125.03 100.28 110.3 10.43 3.541 0.08 719.35
Mean 16.9 35.9 61.1 74.23 67.15 76.67 142.98 128.25 86.51 35.7 6.924 5.106 737.41
Appendix 4.19 Monthly Total Pitche Evaporation at Ziway town station
(mm)
Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
1995 148.5 172 167.9 125 172 148.3 104.1 96.9 102.9 234.4 219.7 181.6 1872.5
1996 147.5 229 174.9 154 122.5 84.8 92.9 77 81.7 195.2 179.1 197 1735.2
1997 144.9 229 203.8 139 174.5 126.7 100.4 103.1 136.7 159.8 141.5 181.1 1841
1998 143.5 152 180.8 208 167.9 167.6 102.5 73.4 83.5 116.4 215.2 212.1 1822.8
1999 211.1 256 68.4 199 210.6 187.1 116.5 115.9 126.3 93.1 182.4 184 1949.9
2000 200.1 243 269.2 190 162 190.6 120.5 107.6 107.8 131.5 137.1 143.2 2002.5
2001 163 136 128.5 208 165.7 164.9 142 129.9 117 190.4 239.6 213.2 1997.7
2002 169.1 194 164 221 190 204.8 179.4 146.6 161.3 264.3 218.8 183.7 2297.3
2003 203 238 245.7 197 241.2 182.9 120.1 119.9 125.6 234.3 215.4 195.6 2318.5
2004 160.3 217 242.9 143 249.9 187.5 136.9 131.8 120 195.5 209 193.4 2187.1
2005 181 228 189.8 213 124.8 162.2 139.7 145.1 138.3 213.3 222.6 229.5 2187.2
Mean 170.2 209 185.1 181 180.1 164.3 123.2 113.4 118.3 184.4 198.2 192.2 2019.2
110

111

Appendix 4.20 Evaporation mm/month by Cropwat
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec annual
1980 138 139.4 132.8 154 161.1 138 123.13 133.9 126 146.9 128.5 121 1642
1981 125.7 140.8 127.6 128 156.2 138 114.58 128.3 107 149.2 133.6 125 1575
1982 133.5 112.6 152.9 136 143.2 140 121.64 107.9 108 128.3 115.9 122 1522
1983 135 129.4 142.8 136 134.7 140 126.11 112 120 127.6 128.9 132 1566
1984 136.2 141.8 167.8 169 134.7 129 132.43 129.5 127 157 138.2 132 1694
1985 138.4 136.8 155.1 122 138.8 136 111.97 122.4 120 144.3 135.4 129 1590
1986 130.6 128.7 160 139 157 123 128.71 137.6 131 147.3 182.2 183 1747
1987 174.5 166 170.4 181 161.4 172 183.77 168.9 168 184.9 178.2 183 2092
1988 178.2 163.3 225.4 180 219.5 187 102.3 150.7 143 167.4 127.8 169 2014
1989 166.3 162.3 177.8 150 196 177 146.94 156.2 145 174.5 178.6 154 1985
1990 183.8 135.7 178.2 177 210.2 204 155.87 152.1 149 187.1 181.1 181 2096
1991 186.4 153.9 174.8 192 194.9 188 143.96 138.8 154 183.4 175.7 168 2054
1992 152.5 149.9 207.2 193 189.7 179 148.43 135 145 165.9 164.9 164 1995
1993 160.7 145.5 205 167 182.7 172 142.1 154.4 145 168.9 167 166 1977
1994 170.4 167.7 189 176 194.9 164 136.52 149.9 147 195.3 172.1 173 2035
1995 178.6 168 184.1 163 198.3 196 151.78 152.1 154 202.4 180 172 2100
1996 160.3 183.1 186 170 168.1 149 144.34 142.8 144 183.8 164.5 167 1964
1997 154 191.5 197.5 162 193.4 172 153.26 159.6 168 167.4 156.6 169 2044
1998 163.3 157.9 184.9 195 194.6 190 150.66 145.5 150 151.8 168.8 170 2023
1999 173.7 191.2 187.9 204 210.9 190 143.59 148.8 159 146.6 165.2 161 2082
2000 174.1 188.2 210.6 185 184.9 183 156.61 154.4 141 157 157 157 2049
2001 158.8 163.6 162.6 195 182.7 167 162.56 152.9 159 178.2 164.5 164 2010
2002 161.1 168.3 174.1 201 201.6 193 187.86 161.8 162 195.3 175.3 159 2140
2003 170.4 177.7 197.2 170 209.8 185 138.01 143.2 151 188.2 169.9 154 2055
2004 153.6 169.3 190.5 165 215 188 156.98 162.2 152 174.8 173.9 164 2066
2005 165.2 170 189 184 161.1 176 151.03 171.1 161 192.7 163.1 169 2054
mean 158.6 157.8 178.1 169 180.6 168 142.89 145.1 144 167.9 159.5 158 1930
Lake area 444.1 439.6 435.9 434 432.7 431 434.66 448.2 464 465.4 457.9 450
Volume of
evapo.(MCM) 70.43 69.37 77.64 73.4 78.14 72.6 62.109 65.02 66.7 78.15 73.03 71.1 857.8
112

Appendix 4.21 Mean Monthly Ziway catchment Wind Speed (m/sec)
YEARS Elev. Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1979-2005 1640 Ziway 1.35 1.37 1.29 1.24 1.43 1.9 1.79 1.53 1.09 1.22 1.42 1.46
1991-2005 2020 Bui 2.71 2.2 2.01 1.91 1.94 1.55 1.49 1.36 1.28 1.86 2.137 2.17
1980-2004 2200 Kulumsa 2.57 2.46 2.19 2.13 2.15 2.4 2.14 1.72 1.32 2.71 2.98 2.83
1991-2005 2940 Merero 2.64 2.89 2.8 2.7 2.68 2.02 1.79 1.66 1.81 2.68 2.971 2.9
Appendix 4.22 Annual mean Wind
speed
Years Ziway Bui Kulumsa Merero Average
1991 1.95 2.125 2.683333 2.908 2.416667
1992 1.91 1.9833333 2.441667 2.8 2.283333
1993 1.89 1.8833333 2.275 2.708 2.189583
1994 2.06 1.7833333 2.75 2.506 2.274313
1995 1.93 1.875 2.458333 2.634 2.223177
1996 1.8 1.95 2.258333 2.74 2.187021
1997 1.84 1.9362292 2.291667 2.72 2.197491
1998 1.84 1.77429 2.1 2.493 2.052209
1999 1.91 1.8094942 2.141667 2.542 2.100458
2000 1.88 1.8916667 2.141667 2.342 2.064583
2001 1.73 1.7166667 1.875 2.143 1.866875
2002 1.95 1.8166667 1.883333 2.167 1.954167
2003 1.78 2.5583333 2.891667 2.142 2.34375
2004 1.78 1.6333333 1.829167 1.926 1.793
2005 1.74 1.5166667 2.131 1.347313
Mean 1.87 1.8835564 2.287202 2.46
Elevation 1640 2020 2200 2940
113

Appendix 4.23 Mean Monthly Ziway Catchment RH (%)
years Elev. Station Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1991-2004 2940 merero 58.984 52.7508 60.65 65.72 68.1 72.82 82.28 83.37 75.06 72.57 61.58 58.95
1989-2004 2200 Kulumsa 56.885 53.7292 58.13 61.29 61.45 68.7 78.39 81.35 78.08 62.31 54.06 58.4
1990-2005 2020 Bui 61.63 57.8953 60.97 61.25 62.58 70.43 78.07 78.94 73.76 63.98 59.21 59.76
1975-2005 1640 Ziway 66.247 65.0323 65.9 67.62 67.25 69.15 96.08 75.7 74.26 65.26 63.48 64.03
Appendix 4.24 Mean Annual Ziway Catchment RH(%)
Years Ziway Bui Kulumsa Merero
1991 62.42 83.56 61.89 68.944
1992 87.31 83.11 65.97 67.75
1993 77.24 79.75 67.5 71.056
1994 64.67 58.21 62.15 65.074
1995 61.19 57.43 64.86 67.91
1996 65.64 62.19 67.36 70.527
1997 63.03 61.96 67.78 70.963
1998 63.28 61.17 68 71.196
1999 58.86 56.9 61.44 64.332
2000 54.31 58.94 61.36 64.245
2001 61.83 61.9 65.61 69.859
2002 60.53 59.09 61.22 68.528
2003 59.36 59.15 63.28 68.361
2004 59.08 60.53 62.76 59.556
2005 57.86 59.31
Mean 63.77 64.21 64.37 67.736
Elevation 1640 2020 2200 2940
114

Appendix 4.25 Mean Maximum Temperatures Of All Station Ziway
Catchment (0C)
Staion Years Of Data Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean
Ziway 1975-2005 26.2 27.5 28.4 28.3 28.5 27.3 24.8 24.9 25.8 26.7 26.4 25.9 26.7
Butajira 1972-2006 25.7 26.1 26.4 26.1 26 25.2 23.7 23.9 25.1 25.6 25.8 25.6 25.4
Kulumsa 1972-2006 23.3 24.4 25 24.8 24.9 23.5 21.5 21.1 21.6 22.9 23.1 22.8 23.2
Sagure 1981-2006 23.4 23.9 24.3 22.8 22.8 21.3 19.5 19.4 20.5 21.9 22.8 22.9 22.1
Asella 1973-2006 22 22.7 23.2 23 23 21.6 19.8 19.5 19.7 20.9 21.2 21.2 21.5
Merero 1983-2006 18.7 19.2 19.1 18 18.6 18.4 16.3 16.3 16.7 16.4 17.4 18 17.8
Bui 1990-2005 25.5 26.4 27 26.6 27.3 25.9 23.3 23.3 24.6 25 25.3 25.2 25.4
Appendix 4.26 Mean Minimum Temperatures Of All Station in
Ziway Catchment (0C)
Staion Years Of Data Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean
Ziway 1975-2005 12.8 13.9 15.2 15.4 15.7 15.3 14.9 14.8 14.3 12.7 12.1 11.7 14.1
Butajira 1972-2006 10.8 11.2 12.1 12.5 12.2 11.8 11.8 11.8 11.8 11.4 10.9 10.3 11.5
Kulumsa 1972-2006 8.33 9.38 10.4 11.8 11.6 11 10.9 10.8 10.3 10.4 8.96 8.08 10.2
Sagure 1981-2006 5.04 6.73 7.76 9.18 8.64 8.49 9.09 8.63 8.15 6.59 4.41 4.39 7.26
Asella 1973-2006 6.96 7.97 9.43 10.4 10.4 10.1 10.2 10.2 9.76 9.04 7.22 6.59 9.02
Merero 1983-2006 4.54 5.28 6.24 7.53 6.88 6.27 6.82 6.81 6.24 5.91 4.92 4.93 6.03
Bui 1990-205 7.92 8.63 10 10.8 10.5 9.53 9.49 9.73 9.37 8.08 6.56 6.83 8.96
Appendix 4.27 Mean Temperatures Of 7 Stations (0C)
Staion Years Of Data Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec mean
Ziway 1975-2005 19.5 20.7 21.8 21.8 22.1 21.3 19.9 19.9 20.1 19.7 19.3 18.8 20.4
Bui 1990-2005 16.7 17.5 18.5 18.7 18.9 17.7 16.4 16.5 17 16.6 15.9 16 17.2
Butajira 1972-2006 18.1 18.5 19.2 19.3 18.3 17.9 17.6 17.7 18.1 18.5 18.3 17.9 18.3
Kulumsa 1972-2006 15.8 16.9 17.6 18.3 18.2 17.3 16.2 16 16 16.6 16 15.5 16.7
Asella 1973-2006 14.5 15.1 15.6 15.5 16.5 15.9 14.4 14.6 14.6 14.8 14.2 13.9 15
Sagure 1981-2006 13.6 14.8 15.5 14.2 15.1 14.3 14.1 13.8 14.1 14.1 13.5 13.5 14.2
Merero 1983-2006 11.3 11.8 12.7 12.7 12.7 10.9 11.6 11.6 10.6 10.7 10.8 11.1 11.5
115

Appendix 6.1 Abijata Lake Level (m)
Years Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Mean
1989 2.12 2.06 2.075 2.079 2.022 2.21 2.331 2.269 2.28 2.418 2.44 2.451 2.23
1990 2.4 2.373 2.388 2.359 2.312 2.29 2.244 2.27 2.4 2.877 3.13 3.119 2.513
1991 2.97 2.95 2.996 2.98 2.76 2.61 2.596 2.649 2.66 2.898 2.96 2.871 2.825
1992 2.84 2.917 2.677 2.443 2.43 2.38 2.476 2.556 2.66 3.065 3.3 3.58 2.777
1993 3.6 3.595 3.543 3.437 3.435 3.5 3.519 3.621 3.82 4.197 4.39 4.364 3.752
1994 4.3 4.303 4.114 3.217 3.128 2.94 2.901 2.964 3.22 3.444 3.47 3.484 3.457
1995 3.45 3.454 3.4 3.438 3.425 3.3 3.219 3.225 3.25 3.509 3.69 3.532 3.407
1996 3.54 3.418 3.236 3.173 3.11 3.14 3.291 3.57 4.08 4.525 4.72 4.792 3.717
1997 4.72 4.652 4.521 3.596 3.505 3.48 4.216 4.272 4.42 4.42 4.49 4.393 4.224
1998 4.3 4.225 4.158 4.018 3.899 3.82 3.736 3.806 3.88 3.473 3.56 3.666 3.878
1999 3.7 3.648 3.65 3.62 3.481 3.39 3.364 3.338 3.33 3.546 3.76 3.83 3.555
2000 3.85 3.764 3.588 3.427 3.366 3.25 3.203 3.173 3.17 3.306 3.43 3.458 3.415
2001 3.46 3.362 3.276 3.174 3.101 3.06 3.03 3.062 3.32 3.6 3.72 3.773 3.328
2002 3.79 3.724 3.678 3.591 3.459 3.32 3.245 3.215 3.18 3.053 2.85 2.688 3.316
2003 2.61 2.497 2.37 2.166 2.104 1.98 1.97 1.968 1.96 1.879 1.69 1.535 2.061
2004 1.41 1.297 1.086 1.052 0.897 0.88 0.829 0.796 0.75 0.728 0.57 0.404 0.891
2005 0.22 0.412 0.53 0.507 0.675 0.67 0.607 0.599 0.54 0.531 0.5 0.426 0.518
mean 3.13 3.097 3.017 2.84 2.771 2.72 2.752 2.785 2.88 3.028 3.1 3.0804 2.933
1