HYDROLOGICAL RESPONSE TO LAND USE LAND COVER CHANGE: THE CASE OF AWASH-AWASH SUB BASIN, AWASH RIVER BASIN, ETHIOPIA

Abstract

Land use land cover change is a global phenomenon that affects the watershed hydrological process and is subject to changes causing the area to form an impervious surface that affects the hydrological processes. Therefore, this study aimed to investigate the impact of land use/land cover change on the stream flow of the Awash-Awash Sub-Basin, from 1990 to 2019. Geographic Information System (Arc GIS10.4.1) and Earth Resources Data Analysis System (ERDAS) imagine (2015) were used to prepare land use/cover map from Land sat image acquired in 1990, 2000, and 2019 respectively through the Maximum Likelihood Algorithm of Supervised Classification. The results indicated that, cultivated land, built-up area and bare land expanded (by 7.28 %, 2.62%, and 3.46%), (by 2.59%, 0.57%, and 8%) and (by 9.87%, 3.19%, and 11.46%) for LULC of 1990-2000, 2000-2019 and 1990- 2019 respectively. Whereas, there was a decline in forest (by -7.48%, -1.74% and - 9.22%), rangeland (by -0.1%, -5.41% and -5.51%) and grassland (by -5.82%, -4% and -9.82%) for land use /cover change (LULC) of 1990-2000, 2000-2019 and 1990- 2019 respectively at which water body shows fluctuation. Using the three-land use/cover map Soil and Water Assessment Tools (SWAT) model was set up, run and the default simulation was compared with the observed data. Then sensitivity analysis was made on monthly basis using 20 flow parameters where only 10 parameters were identified as influencing the flow. The model calibration was done from 1990 to 2006 and the validation period from 2007 to 2013. The calibration and validation results showed good match of simulation with observation with Nash-Sutcliff efficiency (NSE) 0.68 , 0.65 , 0.74), coefficient of determination (R2 )( 0.81 , 0.78, 0.82)and Percent bias (PBIAS) values of (-20.1,-21.5,-14) for calibration and (NSE) (0.76 ,0.62,0.64) , (R2 ) (0.84 , 0.78 , 0.79) and (PBIAS) (-12.8,-16.7,-18.5) for validation 1990, 2000 and 2019 respectively . The result showed that the mean wet monthly flow increased by 12.54% (from 36.23m 3 /s in1990 to 39.45m 3 /s in 2019) and the mean dry monthly flow decreased by 59.50% (from 10.37m 3 /s in 1990 to 4.2m 3 /s in 2019). Overall, these findings provide water management authorities with useful information to face land use/cover change