HYDROLOGICAL RESPONSES TO CURRENT AND PROJECTED LAND-USE/LAND COVER CHANGES OF THE WELMEL RIVER WATERSHED, GENALE DAWA BASIN, ETHIOPIA

Abstract

Land use/land cover change is one of the important concerns in many regions of the world. It is recognized that dramatic LULC change can significantly impact regional climate, ecosystem stability, water balance, stream silt up, socioeconomic practices, and biodiversity. The main objective of this study was to assess the hydrological responses to historical and future Land Use/Land Cover Change at basin and sub-basin levels of the Welmel River watershed, which is located in the Genale-Dawa Basin South Eastern Ethiopia using hydrological SWAT model. The study analyses the historical LULC change between the years 1990, 2005 and 2020 and the future year of 2035 and 2050. The hydrological responses to LULC changes in the Watershed were analyzed using the historical and future LULC maps. Images were processed using ERDAS Imagine 2014 and CA-Markov chain model was used for the prediction of the LULC map of 2035 and 2050. Discharge data from 1990 to 2006 and 2007 to 2014 were used for calibration and validation respectively with three years of warm-up period and climate data from 1990 to 2020 time period. The main finding of this study revealed that the coverage of agriculture/settlement increased by a rate of change of 6.85km2 /year, while forestland was declined by the rate of change of 9.16km2 /year over the last 31 years between 1990 and 2020. In the coming 31 years (by 2050), if the current trend of LULC change continues, agriculture/settlement land is expected to increase by the rate of change of 6.73km2 /year, while forestland is expected to diminish by a rate of change of 8.78km2 /year. As a result of LULC change, surface runoff has increased by 25.32% while lateral flow, groundwater flow, water yield, evapotranspiration and percolation declined by 19.91%, 17.17%, 2.38%, 0.36% and 17.17% respectively between 1990 and 2020. If the current rates of LULC change continue, surface runoff is expected to increase by a relative change of 18.47% while lateral flow, groundwater flow, water yield, evapotranspiration and percolation are expected to decline by 26.84%, 17.51%, 2.09%, 1.91% and 17.47% respectively by 2050. Average annually, surface runoff in all 29 sub-basins has increased by 39.90mm and groundwater flow decreased by 34.57mm. The average annual stream flow increased with a relative change of 6.18% from 1990 to 2020 and is expected to increase by 12.69% by 2050. The average wet annual flow from 1990 to 2020 increased by 10.21%, while the average dry annual flow decreased by 6.34%. The average wet and dry annual stream flow is expected to increase and decline by 19.67 % and 6.86% respectively in 2050. Therefore, the Woredas in and around the Welmel River Watershed and the Bale Mountains National Park should integrate to design and implement a proper strategy for protecting and managing the existing forest and woodlands in addition to rehabilitating the degraded areas to maintain the hydrological balance of the watershed.