Assessment of Surface Water Potential and Evaluation of Demands: In Case of Bilate River Sub-Basin: Rift Valley Lakes Basin: Ethiopia
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Date
2020-06
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Addis Ababa University
Abstract
This study was carried out in Bilate River sub-basin to assess the surface water potential and
demands. A DEM, LULC and soil data, slope, and climatic data were used to describe the
hydrology of study area. The annual activity level, annual water use rate and the supply data used
to estimate the current and the future water demands. ArcSWAT and WEAP models were used to
evaluate the surface water potential and demands. The Bilate River Sub-Basin receives an
estimated mean annual precipitation of 1106.2mm. The estimated mean annual runoff in the whole
watershed is 851.5MCM which correspond to 162.3mm (14.7%) mean annual depth of surface
runoff is equivalent to 14.7% of mean annual precipitation. The mean annual water balance shows
that the significant number of the mean annual precipitation (69.4%) received by the watershed
were lost via evapotranspiration. The remaining parts of annual precipitation (172.2mm which
accounts about 15.9%) received by the watershed infiltrates to the soil to recharge GW aquifers
as well as to contribute to baseflow (24.2mm as lateral flow and about 140.35mm as return flow).
The model was calibrated and validated by using 14 years streamflow data. The performance was
found good during calibration [correlation coefficient (R
2
= 0.75), Nash-Sutcliffe efficiency (NSE)
= 0.62), PBIAS =18.8] and validation (R
=0.78, NSE = 0.61, and PBIAS = 23). The water demand
part of this study was considers environmental flow requirements, domestic, agricultural, and
livestock by generating different scenarios based on a different set of assumptions in the sub-basin
up to 2035. The study found that the watershed is highly sensitive to the Increasing water demand
scenario and Irrigation command area projection, suggesting that the slight changes will change
the current and future water demands. The estimated surface water potential was 851.5MCM in
the sub-basin and the current utilization of these potential was about 322MCM for consumptive
use and non-consumptive (EFR). The scenarios were developed to evaluate the future demands
based on different sets of “what if” situations up to the 2035. Therefore, the estimated that total
annual consumptive water demand may be expected to be 235.2MCM, 298.4MCM, 447.6MCM
and 541.5MCM for the reference scenario, high population growth, projected irrigation area and
increased water demand scenarios and 170.3MCM for environmental flow requirement. Hence,
this study consider the environmental flow requirement of 20% of the mean annual flow volume to
maintain the basic ecological functioning and water balance in Lake Abaya.
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Keywords
Bilate River Sub-Basin, SWAT Model, WEAP Model, Surface Water Runoff, Water Demands