MODELING WATER RESOURCES USING WEAP MODEL FOR EFFECTIVE WATER MANAGEMENT: A CASE STUDY IN LEGEDARA RIVER CATCHMENT, SNNPR, ETHIOPIA

Abstract

Water is the life blood of all living things yet its availability and management is not well understood and quantified at the catchments scale. Legedara River catchment water resources availability is not well known since the catchment is ungauged and also the water demands within the catchment is not quantified and properly allocated. This study attempted to model the water resources of Legedara river catchment in SNNPR for effective water management through scenario analysis using WEAP model.Hydro-meteorological, spatial, and water demand data were the basic inputs to the model. FAO-Rainfall-Runoff (Simplified Coefficient) method was used to estimate runoff of Legedara River catchment by transferring calibrated model parameters from gaugedWaleme River catchment. However, before transferring calibrated model parameters different procedures were done, since selection of representative catchment, modeling of gauged catchment and calibration and validation. Catchments areal rainfall and reference evapotranspiration are estimated by Thiessen Polygon method and Penman Monteithmethod respectively. Domestic, industrial and environmental water requirements were estimated using WEAP model. Current situation of water demands for the selected water users were simulated. Five different scenarios for future water demands were developed and in addition to these climate scenario was developed and evaluated. During the model setup, all demand sites were assigned equal priority. As a result of parameter derivation hydro meteorological Waleme River catchment is almost similar to Legedara River catchment. The model calibration and validation results were found satisfactory for the gauged catchment (ENS = 0.81; R2 = 0.86 and RVE = -2.64% for calibration and (ENS = 0.77; R2 = 0.81 and RVE = - 7.73% for validation). The water balance components of Legedara River catchment were estimated and mean annual values of rainfall and reference evapotranspiration found to be 1316.42mm and 1142.07mm, while the actual evapotranspiration and runoff constitutes; 538.78mm and 778.65mm respectively. The modeling result revealed that, all the selected demand sites satisfied fully in the current situation and for reference scenario, scenario one, two and four, even though the remaining river flows for the months of December, January and February were almost zero after deduction. However, the result of catchment water resources and demands showed that at the last year of scenario three (2040), there will be a 26.71MCM, i.e., 36.17%, reduction in the total annual flow of Legedara river catchment.Moreover, it was found in the catchment that it is having unmet demands in scenario three. Nonetheless, it was shown that environmental flow requirement of 14.77MCM will be fully delivered at the outlet of Legedara River catchment. Hence, no absolute scarcity of water would develop. In scenario five, the simulating value showed that, increase and decrease of current mean monthly rainfall values by 10% and 5% have an impact on increase or decrease of catchment runoff.Further researches on groundwater availability were also suggested to meet the unmet demands