EVALUATING SEPARATE IMPACTS OF LAND USE, LAND COVER AND CLIMATE CHANGE ON STREAMFLOW IN UPPER GIDABO WATERSHED, SOUTHERN ETHIOPIA

Abstract

LULC and climate change are two factors that produce major impacts on stream flow and separation of these impacts is important for water resources management as well as policy adaptation and planning for sustainable watershed development. This study was mainly focused on separating the combined and isolated impacts of LULC and climate change on stream flow reduction in upper Gidabo watershed, Southern Ethiopia. The hydrological modeling with the one factor at a time (OFAT) analysis was employed to separate hydrological impacts of LULCC from those of climate change. The SWAT model and MK statistics test were used for the study. SWAT was calibrated and validated in the watershed. The results confirmed that SWAT was a powerful and accurate model for the watershed. The model assessment metrics: NSE, R2 and PBIAS in the data were 0.90, 0.91 and 5% respectively for the calibration period and 0.73, 0.75 and 12.4% respectively for the validation period. After the calibration and validation of the SWAT model, four different scenarios were developed based on one factor at a time (OFAT) experiment. By comparing, the simulated mean annual stream flow components (Qsurf, Qlat and Qbase) results of S1 vs. S2, S3 and S4 for evaluating the effect of LULC and climate change and both on stream flow. The assessment of MK statistics result of climate change showed that precipitation and stream flow exhibited a downward trend and air temperature exhibited upward trends significantly from 1985 to 2016. Evaluation of LULCC from 1985 to 2016 showed that the changes in growth of agricultural land and built-up areas have increased by the annual rate of change 1.81% and 2.33% respectively whereas the mixed forest, evergreen forest and grassland has reduced by the rate of change 1.27%,1.84% and 0.30% respectively. The combined effects of LULC and climate change reduced mean annual lateral flow and base flow values by 7.17mm/yr. and 94.74mm/yr. respectively. The LULCC increased surface runoff by 38.29mm/yr. (56.5%) decreased lateral flow and base flow by 6.23mm/yr. (86.89%) and 26.48mm/yr. (27.95%) respectively while climate change decreased surface runoff, lateral flow and base flow by 29.47mm/yr. (43.5%),0.94mm/yr. (13.11%) and 68.26mm/yr. (72.05%) respectively. Generally, the climate change had the higher impact on base flow reduction while LULCC had higher impact on surface runoff increment and lateral flow reduction in the study watershed. Therefore, to mitigate the negative impacts of climate change combined with LULC change, local and national policymakers are encouraged to apply science-based watershed and land use planning and implement appropriate management approaches in the upper Gidabo watershed