Modeling Surface Water Supply And Demand Balances Using Water Evaluation And Planning Model: The Case Of Katar Watershed, Oromiya, Ethiopia

Abstract

Katar watershed which is located in the Central Rift Valley basin of Ethiopia is gifted with plentiful amounts of water resources. However, there is no clear current figure of water resources potential between water utilized sectors in the watershed. This leads to conflict and allocation struggle near the future. Therefore, modeling the surface water resources of the watershed which satisfy the projected water demands and, give clue for future plan which is used to generate wise decisions for surface water resource based questions was very important. To achieve this, the Water Evaluation and Planning (WEAP21) model was applied as a Decision Support System (DSS). Checking consistency and homogeneity of rain fall station were done by double mass curve and non-dimensional pi value respectively. The analysis shows that the stations were consistent and homogenous. About 26.90% of the katar river flow was allocated for the environment needs to maintain the ecological services as well as the natural channel habitat associated to the historic flow regimes of the river. For simulation process, the population data were projected and hydro metrological data were cycled for scenario analysis. The model was structured according to four scenarios of the periods from (2019-2050) with a current account year 2018. The four scenarios were: reference scenario, high population growth scenario, increasing irrigation activity scenario and improved irrigation efficiency scenario. In year 2018 most amount of water was consumed by irrigation (37.18%), livestock (28.16%) and rural (34.50%) relative to urban water demand which consumed about, 0.165% of total water demand in the watershed. Between 2019 and 2050 in all scenarios the maximum water demand is consumed for irrigation activity scenario. Under high population growth scenario, the water demand will be increased by double on 2050 when compare to the year 2018 water demand situation. On the other hand, if compare high population growth scenario with respect to the reference scenario there is an increment of water demand by 20% on the year 2050. The average annual unmet demands of four scenarios were 5.87 Mm3 (1.70% of the total annual demand), 6.403 Mm3 (1.86% of the total annual demand), 10.84 Mm3 (3.14% of the total annual demand) and 0.24 Mm3 (0.069% of the total annual demand) for reference scenarios, high population growth scenario, increasing irrigation activity scenario and improved irrigation efficiency scenario respectively. The average annual demand coverage over the year (2019-2050) of the study periods would reached 92% which shows good demand coverage condition. Generally, the model results over all scenarios except for improving irrigation efficiency scenario shows that the water demand coverage problem was experienced during dry season of the year. Calibration of the WEAP model was based on the stream flow data observed at Abura gauging station with relative to the simulated runoff from the entire watershed for the period 1987-2017. The statistical parameters seen between the simulated and observed stream flow values indicates that the WEAP model can be efficiently applicable for water resource management system and best strategic plan for enhancing economic improvement, if the situation examined in this study would have been perfectly put into practice for the future development in Katar watershed. Generally, this study output can be used for the different stakeholders in the watershed specially to encourage well fitted irrigation activities and water supply division on equal basis, so that food security can be achieved in the watershed as well as in the region.