An assessment of sediment loading into an agricultural reservoir in a semi-arid region of Kenya

Abstract

The 95.7 version of Water Erosion Prediction Project (WEPP 95.7) model was applied to an approximately 2.7 km2 agricultural catchment to estimate the amount ofsediment loading into the reservoir ofNdaragwiti Dam, Kenya. The reservoir had I .9 ha surface area, 1.2 m mean depth and atotal volume of 34000 m3, and was fed by the catchment in a semi-arid region ofLakipia District ofKenya. The catchment was divided into three subwatersheds based on existing three channels feeding the reservoir. The sub-watersheds were subdivided into 22 hillslopes (plots) according to the slope orientation. Annual sediment that entered the reservoir from each of the sub-watersheds, as well as the sediment that left the reservoir and that retained in the reservoir were generated by the WEPP (95.7) model based on the constructed data sets for the period of 1996. These estimated values were compared with the measured sediment to assess the performance of the WEPP (95.7) model. The estimated annual amount ofsediment from each sub-watershed into the reservoir, sediment out ofthe reservoir through the spillway and the sediment retained in the reservoir ranged between 71 % and 750/0 ofthe measured values. The WEPP (95.7) model estimated an annual total sediment yield ofabout 2206 t corresponding to an average sedimentation rate of 817 t km-2 yr-I . This estimate is about 71% of the measured rate. The study constitutes a fairly test ofWEPP (95.7)' s capabilities since no parameters were calibrated. The findings indicate that the WEPP (95.7) model can reasonably estimate sediment loading when executed without calibration on a catchment devoid of roads in semi-arid region