IMPACTS OF CLIMATE AND LAND USE CHANGE ON HYDROLOGICAL PROCESSES OF ABELTI WATERSHED, OMO-GIBE BASIN, ETHIOPIA

Abstract

Climate and land use change are two factors that produce major impacts on hydrological processes.This study was conducted to examine the likely impacts of climate and a land-use change on hydrological processes in the Abelti watershed, Omo Gibe Basin, Ethiopia, using the Soil and Water Assessment Tool (SWAT) model. To reach this goal, long-term hydrological and meteorological parameters were used as an input for the analysis. Future climate scenario projection were made using Statistical Downscaling Model (SDSM) which was employed to convert the canESM2 (Canadian Earth system Model second Generation) general circulation climate model output into watershed level under two emission scenarios, the medium (RCP4.5) and the higher (RCP8.5). Evaluation of land use/cover change for the baseline period between (2001 and 2014) showed that the changes of cultivated land, including settlement, water body have increased gradually while the forest land and range-grass land area has decreased. In this study, based on base period data, average monthly minimum and maximum temperature were likely to change in the range of +1.5oC to +2.7oC and +1.23oC to +3.2oC. Regarding to the monthly temperature changes, there is an increment of Tmin from +1.5oC to +2.4oC in (2020s) and from +1.7oC to +2.7oC in (2050s) under RCP4.5 emission scenarios and likewise under RCP8.5 emission scenario from +1.5oC to +2.6oC in (2020s) and from +1.54oC to 2.7oC in (2050s). Likely, there is a rise of Tmax from +1.1oC to 2.6oC in (2020s) and from +1.23oC to 2.72oC in (2050s) under RCP4.5 and under RCP8.5 from +1.17oC to 3.15oC in (2020s) and from +1.32oC to 3.22oC in (2050s) from the baseline period. Concerning the rainfall change, under RCP4.5 scenario the study shows an annual average decreasing trend for both scenarios (RCP4.5 and RCP8.5). The decrement of annual percentage change of Areal rainfall ranges from 2.9% to 5.6% (2017-2076) and 5.6% to 8.3% (2017-2076) for RCP4.5 and for RCP8.5 scenarios respectively. Calibration and validation of SWAT were performed using monthly stream flow with two years of warm up periods. The results confirmed that showed the agreement between observed and simulated values in monthly flow of the Abelti watershed. The model assessment statistics, Nash Sutcliffe Efficiency (NSE) and coefficient of determination (R2) values were 0.63 and 0.65, for calibration period and 0.62 and 0.67, for the validation period. An assessment of climate change showed that decrement of runoff by 0.9%, and increment of temperature exhibited upward trends with a rate of 0.5°C per decade for the 1987 to 2076 period, respectively. Future climate changes were investigated under two emission scenarios (RCP4.5 and RCP 8.5) for the near (2017-2046) and midcentury (2047-2076). In conclusion, the outcomes of this study show that climate factors accounted for the notable effects more significantly than LULC on hydrological processes in Abelti watershed