MODELING THE IMPACTS OF LAND USE CHANGES ON WATER QUANTITY AND QUALITY IN KIMWARER RIVER CATCHMENT
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University of Eldoret
Kimwarer River basin has an area of 138.2 km2. The basin has experienced ecosystemdegradation due to extensive farming therefore impacting on water yield and nutrientloading in the basin. The study was undertaken to assess the impacts of land use changeson the quantity and quality of river flow by application of SWAT model. SWAT is amathematical model that has potential of predicting the impact of land managementpractices on water and chemical yields at catchment scale. The specific objectives of thestudy were; i) To determine land use changes for the past 30 years; 1985 to 2015,ii) Tocalibrate and validate SWAT model for prediction of stream flow and nutrient loading invarious land use scenarios and iii) To simulate stream flow and nutrient losses (nitratesand phosphates) in various land uses. Input data to SWAT model were assembled fromrelevant sources. Current and historical flow data were collected for the purpose of modelcalibration and validation. The model was then used to simulate quantity and quality ofstream flow for different land use and land cover scenarios by interchangeably varyingforest cover and agriculture. Results showed that from the year 1986 to 2010 for whichimages were obtained, there were; increase in agricultural land from 62.68% in 1986 to84.52% in 2010, decrease in forest cover from 17.33% in 1986 to 9.66 % in 2010 and thedecrease of wetland from 19.99% in 1986 to 5.84% in 2010. The model was successfullycalibrated and validated for stream flow. The model was capable of predicting flowduring calibration with R2and NSE values of 0.79 and 0.31 respectively. Duringvalidation, the model predicted flow with R2and NSE values of 0.70, 0.50 respectively.For scenario analysis to determine the effect of land use change on stream flow, it wasobserved that surface runoff decreased with increase in forest cover while base flowincreased with increase in forest cover. Introduction of terraces as management operationoption in agricultural land showed a reduction in runoff by 46%. The model was capableof predicting nitrate and phosphate losses with R2of 0.72 and 0.79 respectively. Theobserved mean monthly basin loading levels were 0.10 kg/ha and 0.092 kg/ha for nitratesand phosphates respectively. Nutrient losses decreased with increase in forest cover.From the research it is evident that the current trend of land use change affects the qualityand quantity of water from the catchment. The trend requires control for sustainability.