02020602 - Cafet Innova

484 International Journal of Earth Science and Engineering 

ISSN 0974-5904, Vol. 02, No. 06, December 2009, pp. 484-498 

Water Balance of Haromaya Watershed, Oromiya 

Region, Eastern Ethiopia 

NATA TADESSE and ABDUL AZIZ MOHAMMED 

Department of Earth Science, College of Natural and Computational Sciences, Mekelle 

University, P.O. Box 1604, Mekelle, Ethiopia 

Email: tafesse24603@yahoo.com, amennata@gmail.com 

Abstract: The main purpose of this study was to conduct the water balance of Haromaya 

watershed, located in Oromiya Regional State, eastern part of Ethiopia, having an area of 

about 53.8 km 2 . The hydrology of the area was characterized based on land use, soil, 

rainfall, temperature, evapotranspiration, runoff and infiltration. In order to determine the 

basic hydrologic parameters, meteorological data was collected from a station inside the 

watershed. The rainfall coefficient method was used to determine the monthly distribution 

of rainfall and then to distinguish between rainy and dry months. The Thornthwaite and 

modified Penman methods were used to compute the potential evapotranspiration. Water 

balance model was used for determination of the actual evapotranspiration. The catchment 

is characterized by one rainy season and two dry seasons during the year. The rainy season 

has eight months whereas the dry seasons have four months. The mean annual rainfall of 

the catchment is 770.5 mm. Rains in the rainy season accounts for 93% of the mean annual 

rainfall whereas the amount of rainfall that occurs during the dry seasons accounts only 7 

%. The mean annual actual evapotranspiration of the watershed is 693.14 mm. Haromaya 

watershed is a closed basin and the actual evapotranspiration is the only outflow 

component. The net total amount of water which is actually available to recharge the 

groundwater circulation within the watershed is 2.01 million cubic meter. In general, the 

watershed has very good groundwater potential, and needs proper development and 

utilization under a controlled scientific manner. 

Keywords: Evapotranspiration, Ethiopia, groundwater, rainfall, surplus, water budget. 

1. Introduction: 

1.1. General: 

The demand of water in developing 

countries like Ethiopia is ever increasing day 

by day with rapid population growth. 

Supplying adequate water with a reasonable 

quality is a major constraint in both local 

and regional planning for urban and rural 

areas. Domestic consumptions, industrial 

and irrigation activities mainly relay on 

water. In most cases, in Ethiopia, the 

demand for water exceeds the supply from 

various resources such as surface waters 

and groundwater. This is not due to lack of 

water resources but due to inefficient water 

resources utilization practices. Meaningful 

planning to utilize and conserve water 

resources in a particular area, should be 

based on the assessment of its available 

water resources. Ethiopia has abundant 

water resources with estimated 111 billion 

cubic meters of mean annual runoff from its 

major river basins, and preliminary studies 

and professional estimates indicate that the 

country has a yearly refreshing groundwater 

of 2.56 billion cubic meters (Ministry of 

Water Resource, 2001). However, the 

country is known by famine and drought. 

Assessment of available water resources can 

be done through a means of water balance 

analyses. The water balance is an 

accounting of the inputs and outputs of 

water (Ritter, 2006). In other words, 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 within the volume. In general, 

therefore, use of a water-balance technique 

implies measurements of both storages and 

#02020602 Copyright © 2009 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.

. 

Water Balance of Haromaya Watershed, Oromiya Region, 

Eastern Ethiopia 

485 

fluxes (rates of flow) of water, though by 

appropriate selection of the volume and 

period of time for which the balance will be 

applied, some measurements may be 

eliminated (UNESCO, 1974). 

The water balance equation for any natural 

area (such as a river basin) or water body 

indicates the relative values of inflow, 

outflow and change in water storage for the 

area or water body. In general, the inflow 

part of the water balance equation 

comprises precipitation (P) as rainfall and 

snow actually received at the ground 

surface, and surface and subsurface water 

inflow into the basin or water body from 

outside (Q si and Q ux ). The outflow part of the 

equation includes evaporation from the 

surface of the water body (E) and surface 

and subsurface outflow from the basin or 

water body (Q so and Q up ). When the inflow 

exceeds the outflow, the total water storage 

in the body (Δs) increases; an inflow is less 

than the outflow resulting in decreased 

storage. Consequently the water balance for 

any water body and any time interval in its 

general form may be represented by the 

following equation (UNESCO, 1974): P + Q si 

+ Q ux - E - Q so - Q up - Δs = 0 (1) 

For application to a variety of water-balance 

computations Eq. 1 may be simplified or 

made more complex, depending on the 

available initial data, the purpose of the 

computation, the type of body (river basin 

or artificially separated administrative 

district, lake or reservoir, etc.), and the 

dimensions of the water body, its 

hydrographic and hydrologic features, the 

duration of the balance time interval, and 

the phase of the hydrological regime (flood, 

low flow) for which the water balance is 

computed. 

On the other hand, depending on the 

specific problem, the terms of Eq. 1 may be 

subdivided also. For example, in the 

compilation of water balance for short time 

intervals, the change in total water storage 

(Δs) in a small river basin may be 

subdivided into changes of moisture storage 

in the soil (ΔM), in aquifers (ΔG), in lakes 

and reservoirs (Δs l ), in river channels (Δs ch ), 

in glaciers (Δs gl ) and in snow cover (Δs sn ). 

Thus in this case the water balance equation 

becomes; 

P + Q si + Q ux - E - Q so - Q up - ΔM - ΔG - Δs l - 

Δs ch - Δs gl - Δs sn = 0 (2) 

where Q si represents the net surface water 

diversion from other basins. 

Determining the components of the water 

balance has a great contribution in the 

water budgeting of an area. A water 

resource development which is carried out 

on the basis of water budget information will 

definitely minimize and/or avoid the 

wastage of water resources that are going to 

be exposed to risk and mismanagement, 

and hence will led to a proper utilization of 

this precious resource. 

In Ethiopia, water balance study was 

conducted only in few areas (Kebede et. al., 

2006; Kebede and Travi, 2004; Johnson and 

Curtis, 1994; Nata, 2006; Shahin, 1988; 

Valet-Coulomb et. al., 2001). Most of these 

studies were conducted for purpose of 

academic achievement rather than being 

utilized for the proper development of water 

resource of the country. 

The Haromaya watershed is a source of 

water supply for three major towns (Harar, 

Alemaya and Awaday) and also to one main 

University of the country called Haromaya 

University. Currently the groundwater is 

extracted through twenty boreholes and 

more than 180 hand dug wells that are 

drilled and dugged in different parts of the 

watershed. The boreholes were drilled to 

supply water to the three towns and the 

university, whereas the hand dug wells were 

constructed by the farmers for the domestic, 

irrigation and livestock purposes. 

Even though such a huge exploitation of 

groundwater is going on there, the water 

balance which accounts the whole Haromaya 

watershed has not yet been established so 

far. Consequently, the total amount of water 

which is actually available to recharge the 

groundwater circulation in the watershed is 

not yet been determined. This research work 

is designed to fill such a gap and by then 

give quantitative information about the 

water budget components and its analyses 

that can be used for further development 

and utilization plan for this precious 

resource in the area. 

International Journal of Earth Science and Engineering 

ISSN 0974-5904, Vol. 02, No. 06, December 2009, pp. 484-498

486 NATA TADESSE and ABDULAZIZ MOHAMMED 

1.2 Objective: 

The major objective of the research was to 

conduct the water balance of the Haromaya 

watershed. 

2. Methodology: 

2.1 Description of the Study Area: 

2.1.1 Location: 

The study area, Haromaya watershed, is 

located in Oromiya Regional State, eastern 

part of Ethiopia, about 505 km east of Addis 

Ababa, which is the capital city of country. 

Geographically, it is bounded between 9° 21' 

40'' to 9° 27' 13'' E and 42° 05' 16'' to 43° 

55' 12'' N. The Haromaya watershed is a 

closed basin, and has an aerial coverage of 

about 53.8 sq. km (Fig. 1). 

SUDAN ERITREA 

Red 

Sea 

N 

UGANDA 

Mekelle 

Tan a 

Gulf of 

H ayk DJIBOUTI 

Aden 

B lu e N ile 

SOMALIA 

Addis Ababa 

ETHIOPIA 

L ake R u d o lf 

KENYA 

0 200 KM 

SOMALIA 

Indian 

Ocean 

TIG RAY 

AMHARA 

AFAR 

BENSHANGUL 

DIRE DAWA 

ADDIS ABABA HARARI 

GAMBELA 

S.N.N.P.R.S. 

SOMALI 

O RO M I YA 

N 

1044000 

1041000 

1038000 

165000 168000 171000 174000 177000 180000 

Legend 

0 4 Kilometer 

Swampy area 

Watershed boundary 

Intermittent Streams 

Figure 1: Location map of the Haromaya watershed (After Nata et. al, 2006). 





487 

2.1.2 Physiography and Drainage: 

The Haromaya watershed is located in the 

Harerghe plateau in the southeastern 

highlands and lowlands physiographic unit of 

the country. The watershed consists of vast 

depressed area bounded by adjacent 

highlands. The surrounding mountains are 

characterized by gentle to steep slopes 

covered with scattered bushes. The 

elevation of the area ranges from 2420 m 

above sea level at extreme northeastern 

parts to 2020 m above sea level near to the 

central depressed area. Steep to very steep 

slope, hilly and mountainous area, which 

covers 18% of the total area, characterizes 

the eastern and northeastern parts of the 

watershed. Flat to gentle slopes, which 

cover 82% of the total area and has a slope 

ranging 0-15%, characterizes the remaining 

parts of the watershed. The watershed 

includes a swampy area that lies in its 

southwestern parts. The slope of the 

watershed rises slowly in all directions away 

from the swampy area. The slopes, hills and 

mountains surrounding the study area have 

created a drainage network, which takes the 

surface flow towards the swampy area (Fig. 

1). All the streams that drained the 

watershed are intermittent, and the 

drainage pattern of the area is dendritic type 

(Fig. 1) 

2.1.3 Geology: 

The geology of the watershed is constituted 

by the rocks ranging in age from 

Precambrian to Recent. Stratigraphically, 

from the bottom to the top are the 

Precambrian basement rock (Granite), 

Mesozoic sedimentary rocks (Sandstone and 

Limestone) and Recent Quaternary 

sediments. The Precambrian basement rock 

of the watershed is granite, which is an 

Archean intrusive volcanic rock that 

penetrates the gneissic rocks. The fresh 

granite is light pinkish, massive, and coarse 

grained, having a batholithic size. It is 

characterized by well developed white 

colored quartz crystals and pinkish colored 

angular orthoclase crystal. It covers 46.3% 

of the total area of watershed, having a 

maximum thickness of more than 100m in 

the southeastern parts (Nata et. al, 2006). 

The Mesozoic sedimentary successions of 

the watershed consist of two formations, the 

sandstone and limestone.The sandstone, 

which occupies 14.3% of the total area, is a 

yellowish to pink, fine to medium grained 

quartz sandstone with well sorted, rounded 

grains weathering to red on the surface. It is 

non-calcareous except at the top near the 

contact with the overlying limestone, where 

thin beds of limestone have developed. The 

thickness of the sandstone ranges from 20 

to 200 meters, and shows deep vertical 

jointing (Nata et. al, 2006). The limestone, 

which occupies 5.1% of the mapped area 

having a maximum thickness of 180m, is 

fossiliferous, micritic limestone having very 

fine grain calcite crystals containing thin 

beds of light brownish marl bands having a 

thickness ranging from 3-5m (Nata et. al, 

2006). It has gray color when fresh and light 

yellowish to black color after weathering. 

Alluvium occurs in the central part of the 

watershed and as thin strip along the 

margins of the major rivers and their 

tributaries, and constitutes about 16.7% of 

the total area (Nata et. al, 2006).The lake 

deposits are found occupying the dried and 

disappeared Haromaya lake position in the 

current swampy area in the depressed part 

of the watershed, and constitute about 

17.7% of the total area (Nata et. al, 2006). 

Compositionally the deposits are comprised 

of clay, silty clay and silt in different 

proportion. 

2.1.4 Soil: 

On the basis of USDA soil textural 

classification scheme, the soil in the 

watershed was grouped in to four different 

classes: clay, clay-loam, sandy clay loam 

and sandy-loam. In addition to these soil 

covers swampy area is found at the central 

part of the area. The spatial distribution and 

areal coverage of these soil types is given in 

Table 1 and Fig. 2. 

Table 1: Types of soil and their 

corresponding area coverage. 




1044000 N 

1041000 

1038000 

165000 168000 171000 174000 177000 180000 

Legend 

Clay 

Clay loam 

Sandy clay loam 

Sandy loam 

0 4 Kilometer 

Swampy area 


2.1.5 Land Use and Vegetation: 

Figure 2: Soil map of the watershed (Nata et. al, 2006). 

Six major land use types were identified 

from the present land use during the field 

assessment made on the watershed. These 

are cultivated land, grazing land, forest, 

settlement (homestead), shrub and swampy 

area. The distribution and areal coverage of 

each land use is given in Table 2 and Fig. 3. 

Table 2: Land use type with their area 

proportion. 

78.25% of the total area of the watershed is 

constituted by cultivated land. The major 

crops grown in the area are sorghum and 

maize. Moreover, chat, a cash crop 

intercropped within sorghum and maize is 

also produced by farmers. There is also a 

good amount of vegetable production 

specifically around the swampy part by 

using irrigation water from hand dug wells. 





489 

1044000 N 

1041000 

1038000 

165000 168000 171000 174000 177000 180000 

Legend 

Cultivated land 

Grazing land 

Forest 

Shrub 

0 4 Kilometer 

Settlement (Homestead) 

Swampy area 


2.2 Data Collection: 

Figure 3: Land use map of the watershed (Nata et. al, 2006). 

Extensive work was carried out by collecting 

pertinent primary data of the area in the 

field and secondary data from different 

offices. 

The topographic map with a scale of 

1:50,000 was used as a base map. Different 

thematic maps of the area were prepared 

using this as a base map. The hydrology of 

the catchment was characterized by land 

use, soil, rainfall, temperature, 

evapotranspiration, runoff and infiltration. In 

order to determine the basic hydrologic 

parameters, meteorological data was 

collected from a meteorological station. Data 

for land use, soil, geology and root depth 

were collected in the field with the help of 

GPS. Using ArcView GIS 3.3 and CorelDRAW 

12 software’s, various thematic maps such 

as location, soil and land use were 

produced. 

Meteorological data were collected from 

Haromaya meteorological station, which is 

located in the study area. The Haromaya 

meteorological station, which is owned by 

the National Meteorological Service Agency, 

is the first class station. 

2.3 Data Processing: 

For textural analyses nineteen disturbed soil 

samples were collected and analyses was 

made by Pipette method. The analyses were 

carried out at the soil laboratory of the 

Haromaya University. The soil classification 

was done on the basis of USDA soil texture 

classification system. The average depth of 

the root zone was measured in the field. 

In this study the suggested values of 

available water capacities for combinations 

of soil texture and vegetation by 

Thornthwaite and Mather (1957) was used 

to determine the available water capacity of 

the different soils in the watershed. 

The rainfall coefficient method (Daniel, 

1974) was used to determine the monthly 

distribution of rainfall in the studied area 

and then to distinguish between rainy 

months and dry months. This method 

involved the calculation of "rainfall 




coefficient" for each month at the station, 

the coefficient being the ratio between the 

mean monthly rainfall and one-twelfth of the 

annual mean (the latter referred to as 

"rainfall module"). 

Potential evapotranspiration was estimated 

by modified Penman and Thornthwaite 

methods. 

The Penman equation, which was later 

modified by MAFF (Shaw, 1994) and called 

modified Penman equation, is given by: 

 

H T 

E at 

 

 

PET 

………………. (3) 

 

1 

 

Where 

PET = Potential evapotranspiration 

(mm/day) 

Δ = Slope of saturated vapor pressure Vs 

temperature curve (mmHg/ O C) 

γ = Hygrometric constant (mmHg/ O C) 

H 0 .75R 

(1 r) 

R (4) 

T 

I 

 

r 0. R f 

 

RI 75 

a 

a 

o 

n 

 

N 

1 (5) 

r = Albedo (for the study area it was taken 

to be 0.25) 

n 

 

N 

f a 

for the study area (within 

latitudes south of 54/2 0 N) equals 

 

 

 

n 

.16 0.62 

N 

0 (Shaw, 1994) 

(6) 

0.47 

0.075 e (0.17 0.83n 

/ ) 

R 

a 

 

4 

0 

T 

correction N factor. 

(7) 

= Stefan-Boltzman constant (5.67*10 -8 

Wm -2 K -4 ) 

T = Mean monthly temperature ( O C) 

E 

at 

u2 

 

0.351 

( es 

ea 

) (8) 

100 

U 2 = wind speed at 2 m height (mile/hr) 

e a = Actual vapor pressure (mm Hg) 

e s = Saturated vapor pressure (mm Hg) 

N = Maximum possible sunshine hours (hr) 

n = Sunshine hours (hr) 

The Thornthwaite method (Thornthwaite and 

Mather, 1957) uses air temperature as an 

index of the energy available for 

evapotranspiration, assuming that air 

temperature is correlated with the 

integrated effects of net radiation and other 

controls of evapotranspiration, and that the 

available energy is shared in fixed 

proportion between heating the atmosphere 

and evapotranspiration. There is no 

correction for different vegetation types. 

The Thornthwaite's empirical equation is: 


ISSN 0974-5904, Vol. 02, No. 06, December 2009, pp. 484-498 

E 

t 

10Tn 

 

1 .6 

J 

 

 

a 

(9) 

Where 

E t = Potential evapotranspiration in 

centimeter per month. 

T n = Mean monthly air temperature (ºC). 

n = 1,2,3,......12 is the number of the 

considered months. 

J = Annual heat index and it is given by the 

equation: 

12 

 

J j 

(10) 

n1 

j = is the monthly heat index and it is 

expressed as: 

 

 

 

T n 

 

5 

1.514 

j (11) 

a = 0.49 + 0. 0179J - 0.0000771J 2 + 

0.000000675J 3 (12) 

The computed monthly potential 

evapotranspiration in Eq. 9 is for a standard 

month with 360 hours of daylight. It must 

be corrected for the varying length of day 

with latitude using the appropriate 

Actual evapotranspiration data are not 

available in the stations employed in this 

study. Due to the almost complete lack of 

field instruments such as lysimeters, the 

Thornthwaite water balance model (Leopold 

and Dunne, 1978) was used to estimate the 

actual evapotranspiration of the watershed. 

The required parameters to determine 

actual evapotranspiration using this model 

are mean monthly precipitation, mean 

monthly potential evapotranspiration, water 

holding capacity of the dominant soil type 

and monthly soil moisture storage. The 

actual evapotranspiration, AET, for the



491 

dominant soil types and the respective land 

use in the area was weighted according to 

the proportion of the area it represents, and 

the weighted AET that is the over all actual 

evapotranspiration of the watershed was 

calculated as 

AET 

T 

AETi 

* a 

 

A 

i 

……………… 

(13) 

Where AET T = Total Evapotranspiration; 

AET i = Annual evapotranspiration 

from each soil type 

a i = Area of each soil coverage: 

A = Total watershed area 

Since the watershed is a closed basin where 

the surplus entirely goes to infiltration, the 

water balance for the study area was 

determined by using the following equation. 

P - ET - I = 0 (14) 

Where ET = Evapotranspiration 

I = Infiltration 

P = Precipitation 

3. Results and Discussions: 

3.1 Hydrology: 

The hydrology of the catchment has been 

examined in terms of rainfall, temperature, 

potential evapotranspiration, actual 

evapotranspiration and runoff. Detailed 

sections on each of these topics follow. 

3.1.1 Rainfall: 

3.1.1.1 Mean Annual Rainfall: 

The rainfall data of the study area was taken 

from Haromaya University meteorological 

station. The data was collected from the last 

26 years records (1979-2005), and the 

mean is tabulated and presented in the 

Table 3. Accordingly, the mean annual 

rainfall of the study area is 770.5 mm. The 

mean monthly rainfall averaged over the 

twenty-six years period of record for the 

Haromaya University station is shown in Fig. 

4. The highest rainfall of the area is 

recorded in April, May, August and 

September which accounts above 62.5% of 

the total mean annual rainfall of the area 

whereas the minimum rainfall is exhibited in 

and January, February, November and 

December, which only accounts about 7 % 

of the total mean annual rainfall. 

3.1.1.2 Seasonality of Rainfall: 

The areal pattern of the seasonality of 

rainfall in the study area was determined by 

analyzing mean monthly rainfall data for one 

station in the study area. To compare the 

monthly distribution of rainfall at this 

station, the method employed here was 

adapted from a study of precipitation data 

for the Awash River Basin (Daniel, 1974). 

This involved the calculation of "rainfall 

coefficient" for each month at the station, 

the coefficient being the ratio between the 

mean monthly rainfall and one-twelfth of the 

annual mean (the latter referred to as 

"rainfall module"). To distinguish between a 

"rainy" month and a "dry" month in the 

Awash Basin study, a month is designated 

"rainy" when the monthly rainfall coefficient 

reaches 0.6 (60 % of the rainfall module), 

and distinctly rainy when it exceeds 0.8. 

Extremely rainy months have a coefficient of 

more than 1 (that is, the rainfall exceeds the 

module value) (Daniel, 1974). 

In this study, a month was designated 

"rainy" if the rainfall coefficient is 0.6 or 

over, as in the Awash Basin study. The term 

"small rains" is employed to refer to those 

months with a rainfall coefficient of 0.6 to 

0.9; and the term "big rains" to those 

months where the coefficient is 1.0 and 

above. The "big" rainy months are further 

classified into three groups: those with 

"moderate concentration" of rainfall 

(coefficient of 1.0 to 1.9); those with "high 

concentration" of rainfall (coefficient of 2.0 

to 2.9); and those with "very high 

concentration" of rainfall (coefficient of 3.0 

and above). This scheme of classification is 

presented in Table 4. 

Table 3: Mean monthly rainfall at Haromaya University station (mm). 

Months Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec MAR 

MMR 12 20 54 109 108 46 95 151 114 41 15 7.3 771 

Where MMR is Mean monthly rainfall: and, MAR is Mean annual rainfall. 



Mean Monthly Rainfall (mm) 


160 

140 

120 

100 

80 

60 

40 

20 

0 

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

Monthes 

Figure 4: Mean monthly rainfall at Haromaya University station (1979-2005). 

Table 4: Classification scheme of monthly rainfall values. 

Source: Daniel, 1974. 

Table 5: Rainfall coefficient at the Haromaya University station. 

On the basis of this classification, as 

depicted in Table 5, the watershed is 

characterized by one rainy season during 

the year, i.e., at this station rainy months 

are not separated into more than one group 

of rainy months by dry months. There are 

two dry seasons during the year. The rainy 

season in total have eight months: March, 

April, May, June, July, August, September 

and October. 

The rains in March, June and October are 

small rain and accounts 18.14 % of the 



Temperature (ºC) 



493 

average annual rainfall of the watershed. 

Big rains with moderate concentration occur 

in April, May, July and September and these 

accounts 55.24 % of the average annual 

rainfall of the watershed. Big rains with a 

high concentration occur in August, and this 

accounts for 19.57 % of the average annual 

rainfall of the watershed. 

The watershed is characterized by two dry 

seasons. The first dry season starts in 

January and ends in February. The second 

one starts in November and ends in 

December. The amount of rainfall that 

occurs during the four months of dry 

seasons in total accounts for 7.05% of the 

average annual rainfall of the watershed. 

The study area has not experienced very 

high concentration of rainfall. 

3.1.2 Temperature: 

Like the rainfall data, temperature data 

were taken from Haromaya University 

meteorological station. A twenty-six years 

(1979-2005) maximum and minimum 

temperature data were taken and analyzed. 

The mean annual minimum temperature of 

the study area is 9.6 ºC and the mean 

annual maximum temperature is 23.8 ºC 

.The mean annual air temperature of the 

area is 16.7 ºC. The mean monthly 

maximum and minimum temperature are 

observed in March and December, which are 

25.2°C and 3.8°C, respectively. 

Table 6: Mean monthly temperatures at Haromaya University meteorological station. 

Where, MMMxT=Mean monthly maximum temperature (ºC); 

MMMnT=Mean monthly minimum temperature (ºC); 

MMT=Mean monthly air temperature (ºC); and, 

MA: Mean Annual (ºC). 

As it is shown in the above table, the minimum air temperature is 13.2ºC in December and 

the maximum air temperature is 19.1ºC in June. The annual range of temperature is 5.9ºC. 

MMMxT MMMnT MMT 

100 

10 

1 

J F M A M J J A S O N D 

Months 

Figure 5: Mean temperatures at Haromaya University meteorological station. 



MMR (mm) 

Temperature (ºC) 


Where, MMMxT=Mean monthly maximum temperature (ºC); MMMnT=Mean monthly 

minimum temperature (ºC); and, MMT=Mean monthly air temperature (ºC). 

MMR 

MMT 

160 

140 

120 

25 

20 

100 

80 

60 

40 

20 

0 

J F M A M J J A S O N D 

Months 

15 

10 

5 

0 

Figure 6: Temperature and rainfall relation at Haromaya University meteorological station. 

Where, MMT=Mean monthly air 

temperature; and, MMR is Mean Monthly 

Rainfall. Figure 6 shows the temperature 

and rainfall relations over the period (1979- 

2005) for the study area. Even though the 

maximum air temperature occurs only in 

May and June, high temperature values are 

observed during the rainy seasons. Months 

in the rainy seasons are warmer than 

months in dry seasons. The minimum 

temperatures as well as rainfall are also 

occurring almost in a similar range of 

months. 

Table 7: Other important climate data of Haromaya watershed. 

Months Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

WS 1.8 2 2 1.9 2 2.7 2.6 2.2 1.4 1.2 1.4 1.6 

RH 60 61 63 67 72 75 78 77 76 66 62 64 

SH 9.4 8.9 8.3 7.2 7.8 7.4 6.9 6.9 6.9 7.8 9.6 9.4 

SR 21 22 22 21 21 20 19 20 20 21 22 21 

Where 

WS is Mean monthly wind speed (m/s); 

RH is Mean monthly relative humidity (%); 

SH is Mean monthly sunshine hours (Hrs); 

and, SR is Mean monthly solar Radiation 

(MJ/m 2 /d). 

3.1.3 Evapotranspiration: 

Evapotranspiration is that portion of the 

precipitation which returns back to the 

atmosphere through evaporation from a free 

water surface, a bare soil or interception on 

a vegetal cover and other objects and 

transpiration from plants. In this study an 

attempt was made to estimate both the 

potential evapotranspiration and actual 

evapotranspiration for the Haromaya 

watershed. 

Potential Evapotranspiration: 

Both Thornthwaite method (Eq.9) and 

modified Penman method (Eq.3) were used 

to estimate the potential evapotranspiration 

of the watershed. 





495 

Thornthwaite Method: 

Potential evapotranspiration rates calculated 

for the station employed in this study is 

given in Table 8. Based on this method the 

mean annual potential evapotranspiration of 

the study area is 756.3 mm. 

Modified Penman Method: 

Based on this method (Table 9) the mean 

annual potential evapotranspiration of the 

study area is 1081.17 mm. 

Table 8: Potential evapotranspiration in Haromaya watershed. 

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual 

T 14 16 18 18 19 19 18 18 18 16 13 13 16.7 

j 4.9 5.7 6.9 7.1 7.6 7.6 7 7 6.8 5.6 4.4 4.3 74.9 

LCF 

at 1 1 1 1 1.1 1.1 1.1 1 1 1 1 1 

10º N 

CPET 46 55 68 74 81 82 74 74 70 54 41 40 756.3 

Where T = Mean Monthly Air Temperature 

(ºC); j = Monthly Heat Index; LCF at 10º N 

= Latitude Correction Factor at 10º N; 

CPET = Corrected or Adjusted Potential 

Evapotranspiration (mm). 

Table 9: Potential evapotranspiration (PET) of Haromaya watershed. 

Actual Evapotranspiration: 

The actual evapotranspiration was computed 

based on water balance method. In this 

method, the input variables are precipitation 

and temperature; the only parameter is the 

available water capacity of the soil. The 

available water capacity (AWC) of the soil is 

the total amount of water held in the soil 

between the -15 bar (wilting point) and 0.33 

bar (field capacity) potentials (Stephen, 

1999). The available water capacity is 

determined per unit depth of soil. The 

available water capacity can be measured, 

or it can be estimated from figure or table. 

The amount of water available to a crop can 

be determined from the product of rooting 

depth and available water capacity. In this 

study the suggested values of available 

water capacities for combinations of soil 

texture and vegetation by Thornthwaite and 

Mather (1957) was used. The different land 

use together with their respective soil and 




aerial coverage, and available water capacity 

of the root zone is summarized and given in 

the table below. 

The available water capacity for each soil 

type and the respective land use in the area 

was weighted according to the proportion of 

the area they represent, and the weighted 

available water capacity was calculated by 

utilizing Eq. 15 to determine the available 

water capacity of the soil for the entire 

watershed. Accordingly, the AWC of the soil 

in the watershed is 100 mm. 

AWCw = ∑AWCi * ai/A …….(15) 

AWCw = AWC of the soil in the watershed 

AWCi = AWC of each soil type and its 

respective land use 

Ai = area of each soil type and its respective 

land use 

A = area of the watershed (irrespective of 

the swampy and settlement areas) 

Table 10: Available water capacity of root zone of each land use with the respective soil 

type 

Land use 

Soil type 

AWC Root 

Area 

(Km 2 Depth 

) (%) 

(mm) 

AWCRZ 

Cultivated 

land 

Sandy loom(fine sand) 4.5 10 350 35 

Cultivated 

land 

Clay loom 21.7 25 350 87.5 

Cultivated 

land 

Clay 12.8 30 350 105 

Cultivated Sandy clay loom 

land 

(fine sand loom) 

3.2 15 350 52.5 

Forest Clay loom 0.3 25 2000 500 

Grazing land Clay loom 0.5 25 250 62.5 

Grazing land Clay 1.3 30 250 75 

Grazing land 

Sandy clay loom 

(fine sand loom) 

2.3 15 250 37.5 

Settlement On different soil types 2.4 …. …. … 

Shrub Sandy loom(fine sand) 0.01 10 1500 150 

Shrub Clay loom 2.4 25 1500 375 

Swampy area Sandy clay loom 2.4 … … …. 

Total 53.8 

Through the calculation of an average water 

balance, actual evapotranspiration was 

estimated for the watershed irrespective of 

the swampy and settlement area. The result 

is summarized and is given in Table 11. 

Accordingly, the mean annual actual 

evapotranspiration of the watershed without 

considering the swampy and settlement 

area is 659.4 mm. 

Table 11: Water balance for Haromaya watershed (without swampy area and settlement). 

All values are in millimeter. 





497 

Where P = Mean Monthly Precipitation; CPET 

= Corrected Potential Evapotranspiration; 

P - CPET = is Difference by Subtraction; 

ACPWL = Accumulated Potential Water Loss; 

SM = Soil Moisture; ΔSM = Change in Soil 

Moisture During the Month; AETactual = 

Actual Evapotranspiration. 

The land uses that are not taken into 

consideration through the water balance 

calculation are settlement and swampy 

areas. It is difficult to quantify the 

evapotranspiration from settlement areas. 

According to Reichert (2001), the results of 

different modeled land use set-ups reveal 

that the mean annual evapotranspiration 

difference between the extreme-agricultureand 

extreme-settlement-scenario is only 16 

mm, which is insignificant to affect the over 

all results. On the basis of Reichert (2001) 

idea, the actual evapotranspiration of the 

settlement area, which is 4.46 % of the 

total, is considered as negligible. The 

swampy area was assumed to have 

evapotranspiration at potential rate all over 

the year. The actual evapotranspiration for 

the entire watershed was computed using 

Eq. 13. Accordingly, the mean annual actual 

evapotranspiration of the watershed is 

693.14 mm 

3.1.4 Runoff: 

The study area is a closed basin. There is no 

runoff that leaves the watershed. 

Consequently, the surplus water, which is 

readily available for both runoff and 

infiltration, entirely goes to the infiltration 

component: 

3.2 Water Balance: 

The main purpose of this computation is to 

make a quantitative evaluation of the 

amount of water that percolate into the 

ground to recharge the groundwater 

circulation occurring in the investigated 

area. Various assumptions have been made 

to derive the water balance equation for the 

studied area and these are summarized 

below: 

1. Since the computations are made on 

annual basis, net change of soil moisture 

and groundwater storage is assumed to be 

zero. 

2. Subsurface water exchange with 

neighboring basins is assumed to be zero. 

3. Assuming no artificial diversion from 

other basins. 

Since the basin is a closed basin, the main 

outflow pathway of the water that comes in 

the form of rain is chiefly through 

evapotranspiration. Consequently, the water 

balance equation for the studied area is 

written as follows, which is Eq. 14; 

P - ET - I = 0 

Where ET = Evapotranspiration; 

I = Infiltration; and P = Precipitation. 

From the budget equation, Eq. 14, the 

amount of water that percolates into the 

ground in the Haromaya watershed as a 

groundwater accretion has been calculated 

as follows: 

P = 770.5 mm, ET = 693.14 mm 

From Eq. 14 , 

I = P - ET (16) 

Therefore, 

I = (770.5 – 693.14) mm = 77.36 mm 

In terms of water volume measured in cubic 

meter, I expressed as: 

A s = Area of the watershed = 53.8 x 10 6 m 2 

ET = 37.29 x 10 6 m 3 

P = 41.45 x 10 6 m 3 

Therefore, 

I = (41.45 – 37.29) x 10 6 m 3 

= 4.16 x 10 6 m 3 

Annual discharge of groundwater from 

boreholes and hand dug wells for domestic 

and non-domestic uses in the watershed is 

2149131 cubic meter. Therefore, the net 

amount of water that percolates into the 

ground, In, is given by: 

In = I - 2149131 m 3 (17) 

In = 2.01 x 10 6 m 3 

Based on the above calculation, therefore, it 

is possible to say that the net total amount 

of water which is actually available to 

recharge the groundwater circulation within 

the Haromaya hydrological basin is 2.01 

million cubic meter. 

4. Conclusion: 

The study area receives a mean annual 

rainfall of 770.5mm. The mean annual 

actual evapotranspiration is 89.96 % of the 

mean annual rainfall. Since the watershed is 

a closed basin, the main and the only 

outflow component is evapotranspiration. 




The mean annual surplus, which is available 

only for infiltration is 10.04% of the mean 

annul rainfall. The net total amount of water 

which is actually available to recharge the 

groundwater circulation within the 

Haromaya hydrological basin is 2.01 million 

cubic meter. In general, the Haromaya 

watershed has very good groundwater 

potential, if it is developed and utilized in 

controlled manner. 

5. Acknowledgment: 

The Hare Town Water Supply and Sewerage 

Authority is duly acknowledged for funding 

this research project and also providing the 

necessary data. Thanks are also due to all 

those friends who helped during field, and 

lab and for going through the manuscript 

and providing constructive suggestions. 

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