WEAP modeling of surface water resources allocation in Didessa Sub-Basin, West Ethiopia

The extensive water resources development plan that Ethiopia has recently embarked on encompasses all major river basins, including the Abbay Basin where Didessa Sub-basin is situated. The main objective of the study was to assess the impact of water resources development on instream and downstream water availability, and identify intra-sub-basin locations vulnerable to shortage of surface water, in Didessa Sub-basin. The Water Evaluation And Planning (WEAP) Tool was used to model the surface water resources allocation of the sub-basin, based on the water demand data extracted mainly from the Abbay Basin integrated development master plan. Existing demand sites were collected from irrigation and water supply sectors of the government. To estimate the crop water requirement and instream flows requirement (IFR), respectively CROPWAT 8.0 and Indicators of Hydrologic Alteration (IHA) programs were used. The shortage of recorded data of streamflow was supplemented by the outputs of SWAT hydrological model. Three development scenarios, viz., current development (2014), medium-term future development (2015–2030) and long-term future development (2031–2050), were built. It was demonstrated that the model sufficiently represented surface water flow of the sub-basin. The results indicated that at the last year of scenarios (2050), there will be a 1.101 billion cubic meters (BCM), i.e., 10.3 %, reduction in the total annual flow of Didessa River. Besides, it was found out that some of the schemes in Anger, Dabena and Upper Didessa watersheds will have unmet demands at the last year of scenarios. Nonetheless, it was shown that IFR of 5.46 BCM will be fully delivered at the outlet of Didessa River; hence, no sub-basin wide absolute scarcity of water would develop.