Assessing impacts of land use/land cover changes on the hydrology of Upper Gilgel Abbay watershed using the SWAT model

Population and economic growth, and the consequent increase in food demands have put great pressures on environmental resources and they are driving rapid LULC changes. This study is aimed to examine the impacts of land use/land cover LULC changes on the hydrology of the Upper Gilgel Abay watershed. Landsat images (Landsat 5 TM 1986, Landsat ETM+ 2003, and Landsat 8 OLI-TIRS 2021) were categorized into LULC classes following the supervised image classification technique, and the Soil and Water Assessment Tool (SWAT) model was used to evaluate the hydrological impacts of LULC changes. The result of image analysis revealed an increase in cultivated land (11.7%) and a decrease in forest (39.6%), shrubland (17.5%), grassland (5.6%), and water bodies (25%) during 1986-2003 periods. On the other hand, between 2003 and 2021 periods, cultivated land, grassland and water bodies were decreased while woodland and shrubland were grew up. Consequent to the LULC changes taking place between 1986 and 2003 periods, surface runoff (6%) and water yield (1.8%) were increased, but groundwater flow (37.9), lateral flow (4.1), soil water (1.85), and evapotranspiration (2.2%) were decreased at a watershed-scale. In contrast, the average watershed surface runoff and water yield were decreased between 2003 and 2021 time scales, while lateral flow, groundwater flow, soil water, and evapotranspiration increased, owing to the growth of forest and shrubland and the decline of cultivated land. The study also indicated the hydrological impacts of the LULC changes at the sub-watershed scales. The impacts of LULC change on status of hydrological components during 1986-2003 and 2003-2021 periods were different compared with the findings at the watershed scale. The findings suggests improvements of vegetation covers to reduce surface runoff and increase groundwater flow and soil water content in the study area in particular and in the Ethiopian Highlands in general.