Assessing the impact of existing and future water demand on economic and environmental aspects (case study from Rift valley lake basin, Ethiopia
Swiss Institute of Technology
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In the development of water resource projects there is an increase and extensive use of water resources which causes exploitation of the existing systems and ecosystem of the natural environment. The Water Evaluation and Planning (WEAP) model is used to assess water demand by considering the existing development situation and future water resources development with scenarios analysis in the study area (Ziway_Meki Sub Basin, Ethiopia). Three different development scenarios were developed to simulate Water use at demand sites. In the simulations, the catchment was divided into 5 main sub-catchments where the supply and demand nodes were spatially located. The competing water sectors were irrigation development, domestic users, soda ash industry and environmental flow requirements. Hydro Meteorological data, net evaporation from Lake Reservoir, and monthly water demand from user sectors were the basic inputs to the model. The results of the reference scenario were validated using observed flows and the simulation result revealed that the total average annual inflow volume into the study area is decline significantly for reference scenarios and water availability is limited in the month of January (17Mm3) and December (171Mm3) while in the other months the availability is efficient and all users have 100% coverage; except langano irrigation site which have between 33.33% to 86.5% coverage in average during the month of Feb to May (2.57Mm3) and April in Bulbula 95.2% coverage. The minimum reliability observed mostly in the ongoing and likely future development scenarios at Bulbula irrigation demand sites which have 92.11% and 66.67% reliability in langano irrigation demand sites throughout over all development scenarios while in Sher_Ethiopia expansion 51.75% reliability is observed in ongoing and likely future development scenarios and in demand site of Katar irrigation .diversion and Meki irrigation from dam 51.75% is observed in likely future development scenarios. The simulation result revealed that the total average annual inflow volume into Lake Ziway might decline significantly for Reference scenarios. This inflow volume reduction is likely to drop the lake level of Ziway. This combined with the unbalanced supply-demand equation in the watershed is expected to have significant impact on the lake water balance. Hence, in Lake Ziway, runoff is likely to decrease in the future and be insufficient to meet future demands for water of the ever increasing population in the region.Key Words: Central Rift Valley, WEAP Model, Water Allocation, Demand Sites, demand coverage, reliability, Scenario Analysis.