Evaluation Of Land Use Land Cover Change Impacts On Stream Flow; In Case Of Fetam Watershed, Ethiopia

Abstract

Land use land cover has been the major factors alters flow regime. The evaluation of the impacts of land use land cover change on flow regime, and understanding influence of it on river flow regimes was important for sustainable watershed management. This study is used to evaluate the land use change from 1993 to 2013, and the effect changes on the stream flow of Fetam watershed. Geographic Information system was integrated with the Soil and water assessment tool (SWAT) model to carry out the study. Arc GIS10.1 and ERDAS imagine2015 were used to process soil data set and prepare land use/cover map data (Landsat-7 ETM+ and Landsat-8 OLI_TIRS, for the year 1987and 2017 respectively) acquired from the website of USGS. The Land use classification was performed using a supervised classification system and accuracy assessment was done using a confusion matrix. Using the two land use/cover map SWAT model was set up and run and the default simulation was compared with the observed data. Then sensitivity analysis was made on a monthly basis using 20 input flow parameters, twelve flow parameters were used for model calibration. Runoff curve number (CN2),base flow alpha factor (ALPHA_BF)and ground water delay (GW_DELAY) are the most sensitive parameters ranking from one up to three, respectively. The model calibration was done from 1993 to 2005 years and the validation was carried out from 2005 to 2013 period. The model performance was checked using performance indicators, coefficients of determination (R 2 ), Nash-Sutcliff efficiency (NSE), and percent of bias (PBIAS). The performance indicators results in R 2 = 0.89, NSE = 0.87, and PBIAS = 12.7 for calibration, and R 2 = 0.84, NSE=0.72, and PBIAS=-7.5 for validation. The results indicated a well performance of the model. The annual simulated stream flow through the study period is increased for wet season from 34.58m 3 /s in 1993 to 40.37m 3 /s in 2013), the short rain season increased from 0.0113m 3 /sec in 1993 to 17.994m 3 /se in 2013) and dry season decreased from 14.75m 3 /s in 1993 to 7.405m 3 /s in 2013). Generally, the study result indicated flow during wet season and short rain season increased whereas during dry season decreased though the watershed. Therefore, curving the changes of LULC towards increasing vegetation cover is very necessary in order to reduce surface runoff that contribute to wet season flow and increase infiltration that supply groundwater from which dry season/base flow is contribute. l