Olufayo O.A , F.A.O Otieno And G.M.M Ochieng - eWISA

ASSESSMENT OF SAND RESERVOIR WATER STORAGE
(SRWS) USING RAINFALL ANALYSIS AT DIFFERENT
EPISODIC DRY DAY THRESHOLDS
O.A Olufayo*, F.A.O Otieno ** and G.M.M Ochieng***
*Department of Civil Engineering, Tshwane University of Technology, Private Bag X680,
Pretoria 0001, South Africa, Tel.: 0123824434; e-mail: olufayooa@tut.ac.za
**Faculty of Engineering and the Built Environment, Tshwane University of Technology
*** Department of Civil Engineering, Tshwane University of Technology
ABSTRACT
Water from sand reservoir water storage for domestic and agricultural purpose needs good
understanding of complex event of rainfall and its inherent characteristics. Thus simple
direct method approach of calculating likelihood of event was used for assessment of
water availability in sand reservoir water storage (SRWS) for Limpopo province. This gives
an idea of available water in sand reservoir water storage. 30 years daily rainfall were
used for analysis at different dry day threshold values of 1-,2-,3-,4-,5-, and >5days. At
threshold value of >5day dry lengths the March through November match critical periods in
SRWS where groundwater level recedes because of increasing chance of >5day dry
lengths with continual water abstraction. It is peaked in July at over 90% chances of dry
days. All these have various implications in SRWS and can be used in advising stepping
construction of sand water storage barrier and buffer against climate variability and
associated droughts or floods
Keyword: sand water storage; rainfall; thresholds; SRWS
INTRODUCTION
Water from sand river beds are renewable natural water bodies. Abstraction of water from
sand river beds at the rates higher than its recharge has been a problem of receding
groundwater level in sand water reservoir areas. However, to ensure sustainability in sand
water reservoirs assessment of water availability is therefore essential. Several methods
have been adopted in assessing subsurface water storage (10, 11, 12, and 16). None
address issues of water availability towards episodic dry day lengths. This holds especially
for semi arid Limpopo Province of South Africa which is attributed to the vagaries of rainfall
and episodic dry length frequencies. Streamflow data may provide a clue of water
availability in subsurface water storage but not always available at location of interest for
analysis and often rainfall event-based methods used (17). For rainfall event-based,
understanding and predicting water availability in SRWS may need long-term historical
reconstructions and projections of rainfall records. Using historical statistical tool, the risk
or chance of water availability in SRWS in certain weather trends such as dry days of
certain durations during particular month can be assessed.
The erosion and deposition in rivers and streams are determined by the hysteretic
between discharge and yield of sediments depending on rainfall and its runoff (22), and
these influence SRWS. Therefore, when making SRWS development plans, it is important

to understand the characteristics of rainfall. Rainfall impact directly on sediment transport
capacities of the surface runoff as the grain size of the sediments is dependent on river
flow velocity. The quality of the sediments deposited determines the storage capacities of
SRWS as this form the aquifer for water storage. Therefore, there is a need for analysis of
inherent nonoccurrence of rainfall to assess likelihood of water availability in SWRSS.
The main objective of this study is thus to provide a tool for assessing SRWS at any
particular period of the year by using rainfall analysis.
WATER AVAILABILITY
Water from the sand reservoir water storage (SRWS) depends on recharge from rainfall
and percolation from runoff. Runoff can be magnified significantly by the effect of climate
variability on hydrological system and catchment characteristics. Ephemeral river flow is
sensitive to variations in rainfall as runoff is the residual after soil moisture replenishment
(13). However, for semi-arid regions where rainfall is limited and erratic the water
availability in SRWS needs to be appraised to assess the recharge capacities of the
hydrogeological formations. Water can be assessed with non-committed runoff flowing
unutilised. This can be achieved by analysing rainfall under different episodic dry day
thresholds. The variations in rainfall trend in space and time, and its relevance on the
scope of artificial recharge to subsurface reservoirs can be considered for assessing the
surplus surface water availability (18).
HYDROLOGY OF SAND WATER STORAGE
Erratic patterns of rainfall combined with different rain and dry periods, as found in arid
(semi) arid regions, has increased the incidence of ephemeral streams (20). This holds
especially in the northern region of South Africa. Ephemeral stream is a stream that flows
only for short periods in direct response to precipitation. Usually, ephemeral stream
experience heavy water runoff for short periods of time after rain which can be several
metres high. During such periods of high flow, large amount of sediments are washed
downstream and water stored in deposited sediments. However, when impermeable layer
sealed off leakages of water from deposited sands. Water is retained to form an aquifer
that can provide clean water lasting through dry periods. Thus such water from sand river
form an important groundwater source (7) for rural and semi arid areas of South Africa and
often the only dependable water source in some rural areas.
South Africa is not water rich and among the driest countries in the world (14). Surface and
groundwater have been reported to be relatively scarce because of sparsely events of
rainfall. However, groundwater is often considered dependable source of freshwater in arid
and semi arid zones because of its ability to regenerates (6) and is a source of supply to
many users in rural communities. South Africa hard rock geology further constrained
availability of groundwater (8) and recharge is often limited to rainfall causing the
intermittent flow in dry river sand beds.
(9) observed hydrological process in arid areas such as South Africa differs substantially
from humid environments. Surface runoff in arid environments occurs during short periods
of heavy precipitation (4). The ponding point for infiltration is reached within 10mins of first
rain and overland flows forms the major part of the basin runoff (3) carrying much
sediments load. Although, it is probable that reservoirs can be developed to capture water,
but they are vulnerable to loss of water through evaporation (4) and surfer severe siltation
problems. However, Sand water storage stores water runoff in sediments eliminating

problems of siltation and evaporation. A subsurface reservoir created this way holds water
during wet season and used throughout the dry season (5). Because water is stored under
the ground; contaminations and submergence of land are avoided and land above can be
used.
MATERIALS AND METHODS
Polokwane formerly known as Pertersburg was selected for the study area. A typical
characteristic of the rainfall regime in the study area is a long dry period during the
summer. Polokwane is situated on latitude 23 0 56’ S and longitude 29 0 23’ with elevation of
1284m (16). Thus, it is found in the central part of the Limpopo province with mean annual
rainfall of 478 mm per annum. The intensity of rainfall varies between the rainy and nonrainy
seasons by 11mm and 7mm (1). The average annual air temperature range from
12,2 0 C in June/ July to 22,6 0 C in January (19).It is known as one of the driest areas in
South Africa (15) with several scattered rural settlements. Figure 1 shows the map of
Limpopo Province.
Figure 1. Map of Limpopo Province South Africa
30 years records (1978-2007) of daily rainfall occurrence totalling 10 957days were
collected from South African Weather Services. A long period of record is one of the needs
for frequency analysis. The data were generated from rainfall measurement using
standard rain gauge. The records of rainfall are taken and entered against the day
preceding that on which they were read. Computer codes were written in MATLAB for
analysis of the data. In analysing the data, direct approach using simple arithmetic and
elementary statistical methods were used. This was used so the results can easily appeal
to users and interpreted by non-professional and allowing flexibilities to adapt to any other
water resources problems in the area.
Total frequency of occurrence of specific threshold of Dd
Relative frequency (%) = X 100 [1]
Total number of days in the month

Table 1. Calculated probability of occurrence of various thresholds of dry days
Dry
day(Dd) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
1 0.21 0.22 0.17 0.19 0.05 0.04 0.02 0.02 0.11 0.14 0.28 0.33
2 0.17 0.14 0.10 0.06 0.04 0.03 0.03 0.01 0.07 0.11 0.19 0.13
3 0.10 0.09 0.18 0.07 0.05 0.04 0.01 0.03 0.05 0.14 0.07 0.10
4 0.09 0.10 0.09 0.08 0.04 0.01 0.03 0.04 0.04 0.08 0.13 0.10
5 0.05 0.12 0.06 0.05 0.04 0.02 0.01 0.04 0.02 0.11 0.08 0.11
>5 0.37 0.33 0.39 0.55 0.77 0.86 0.92 0.88 0.71 0.42 0.25 0.23
RESULTS AND DISCUSSION
Ten thousand nine hundred and fifty-seven (10, 957) run of daily rainfall records were
processed with MATLAB for use in the SRWS assessment. The probabilities are
calculated at six different threshold levels of dry days of 1-,2-,3-,4-,5-and >5 days. The
thresholds were chosen to enable user to decide on the suitable dry days sufficient for
assessment. Dry day lengths and water abstraction yield the product of receding
groundwater level at all-time without recharged from rainfall. This holds especially for
Limpopo Province in which ephemeral sand river channel is prevalent (1). Table 1 shows
the computed probabilities of dry days while figure 2 shows the graphical representations
of the probability distribution at various thresholds of dry day lengths.

1-day dry length represented intermittent periods of rainfall. It can be shown in figure 2 that
sediment transport and higher concentration of sediment will agree more to the periods
from September through December to around May because of the rising limb of 1 dry day.
This is peaked in December with a value of 33% and minimum in August with a value of
2%. On the other hand, same can be said for thresholds of 2-, 3-, 4-, and 5days which all
have their rising limbs in September and declined around May. The results agree with (1)
who found the period October to March receiving 86% of the rainfall, while 50% occurring
from November to January. The winter months June to August receive only 2% of rainfall.
Studies have pointed out that sediment-rating curve is positively correlated between the
sediment load and the water discharge (21). The suspended sediment load is generally
higher in the rising limb of any flood hydrograph compared with the recession limb (1).
Theoretically this infers that groundwater level will be raised as deposited sand stored
more water in SRWS. The highest expected periods are December and February with
values of 33% and 22% respectively. At threshold of >5 days the March through November
match to critical periods in SRWS (1) where it is expected that groundwater will be
reduced because of increasing prolonged dry days. Maintenance can be scheduled
around this period as well as phased increment of height of barrier if the reservoir has
been fully silted up.
CONLUSION
It is fundamental to be sure of availability of source of water for recharging purposes in
SRWS and planning must include hydrological analysis. Although SRWS are site specific
and thus similar areas are to be based on local environmental characteristics.
The component of rainfall, dry length frequency, has been analysed and the probability of
occurrence established in other to assessed SRWS. The February to November match the
periods of high probability of greater than 5 dry days. This represents the declining periods
of groundwater level because of limited opportunity to be recharged by rainfall/ runoff with
a continual abstraction of water from sand and localized leakages. SRWS periodic
maintenance can be scheduled to match this period. Increment of dam heights can be
done these periods in a silted up reservoir.
ACKNOWLEDGMENT
Authors thank Tshwane University of Technology for sponsorship and financial support for
the ongoing research.
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