Investigation Of The Existing Surface Water Potential And Allocation Of Future Water Demand: The Case Of Upper Gilgel Gibe Sub-Basin, Ethiopia.

Abstract

The upper Gilgel Gibe sub-basin is located in Oromiya regional state; the South-west of Ethiopia has large amount of water resource potential. However, rapid population growth and increased agricultural activities were led to Water demand and allocation stress in the sub-basin. Therefore, study was required to allocate water among water users sectors which avoid a water-based conflict and overutilization surface water resource in the sub-basin. To solve such problem this study was aimed to investigate the existing surface water potential and allocation of future water demand in the upper Gilgel Gibe sub-basin. The required data for this study was collected from different governmental institutions. The homogeneity and consistency of rain fall data for this study was done by Pi (non -dimensional) value and double mass curve methods respectively. The result of data checking shows, the rainfall data was homogeneous and consistent. Due to availability of data, the rainfall runoff method (simplified coefficient method) was used to simulate the catchment in the model. The findings of the study indicated that, surface water potential in Gilgel Gibe sub-basin was 1276 MMC. The model was simulated for the input rainfall data of year 1990-2018. The calibration of the model was assessed by simulated runoff from sub-basin and observed stream flow at Asendabo gauge station from 1990-2018. The statistical parameters values were R2 =0.9986, Nash-Sutcliffe Efficiency (NSE)=0.976, Percent of Bias (PBIAS)=-14%, Ratio of Standard Deviation of Simulated Versus Observed (SDR)=1 and Ratio of the Root Mean Squared Error to the Standard Deviation (RSR)=0.141 respectively. The result of the calibration indicates that there is good match pattern between simulated and observed stream flow. Hence, the WEAP model is applicable for water resource management. The current account for the Water Evaluation and Planning model was 2019 while last year of scenarios studies were extended up to year 2050. Under the current the water demand situation for rural accounts 70.88 MCM which was the largest water utilized sectors followed by agricultural which consumed 43.91 MCM. Livestock and urban water demand were also 28.64 MCM and 24 MCM respectively. These results indicated that under the current year all demand sites were fully satisfied. Also 20.8 % of the mean annual flow (265.83 MCM) was allocated for Environmental flow requirement to maintain the basic ecological functioning in the sub-basin which intern regulates the permanent flow of the downstream region. Four scenarios were created for future water demand estimation namely; reference, high population growth, current irrigation potential and irrigation projection scenarios. The annual water demand for reference scenario (RS), high population growth scenario (HPGS), current irrigation potential scenario (CIPS) and irrigation projection scenario (IPS) were estimated 167.72, 194.7, 176.89 and 188.32 MCM respectively. High shortage of water demand for agriculture was faced under CIPS and IPS which is 1.87 MCM and 3.47MCM respectively. Although rural and urban demand sites have got unmet demand of 8.31 and 7.52 MCM in HPGS respectively.