Assessment Of The Impact Of Land Use/ Land Cover Changes On Runoff And Soil Erosion Using Soil And Water Assessment Tool (Swat) Model. (Case Study Of Borkena River Watershed Awash Basin, Ethiopia)

Abstract

Land use/land cover change has been responsible for altering the hydrologic response of watersheds leading to impacting river flows. Various water resources projects planning and implementation will require knowledge of the extent of these changes on watershed hydrology. Soil erosion and land degradation is a serious problem in the Borkena watershed on upper Awash sub basin. This is located in the south Wollo zone, North East part of Ethiopia in Amhara Regional State. There was rapid growing population and also lack of awareness of the effective watershed management and agricultural practices. The main objective of the study is to assess the effect of land use/land cover change on runoff and soil erosion at Borkena River watershed using soil and water assessment tool (SWAT) model. In this paper the effect of land use changes on catchment’s hydrology is observed particularly on surface runoff. To carry out this SWAT model 2012 was used for simulation. The delineated watershed was divides into 5 sub basins and 51 HRUs by the model. Model calibration and validation was done at Kombolcha flow gauge station. The semi-automated Sequential Uncertainty Fitting (SUFI2) calibration process built in SWAT calibration and uncertainty program (SWAT-CUP) were used to calibrate the model parameters using time series of flow data of 1982 to 1991 and validated with the observed data from years 1992 to 2001. In addition to this the model efficiency was checked at this station. Based on this values for coefficient of determination (R2 ) and Nash–Sutcliffe efficiency (NSE)were found to be in the acceptable range i.e.(R2=0.91,NSE=0.89) during flow calibration and(R2=0.89,NSE=0.82) for validation period. The annual surface runoff 272.78mm, lateral flow 241.63mm, total water yield 841.8mm ground water flow (shallow) 310.54mm and sediment yield 165.71ton/ha at the study watershed was found from the existing land use map. Then two scenarios were developed to observe the impact of land use changes. Based on this at scenario1garss land changed to cultivation land has resulted in 6.41%, 15.96%, 9.62%, and 41.73% increase in surface runoff, lateral flow ,water yield and sediment yield respectively, but decrease ground water by 0.68%. At scenario 2 grass land changed to plantation forest have resulted in decrease by 1.3%, 8.4%, 5.5% for surface runoff, sediment yield respectively and increase lateral flow, total water yield by 17.5%, 9.8% and ground water by12.5%.