Validation of agricultural non-point source (AGNPS) pollution model in Kori watershed, South Wollo, Ethiopia

Abstract

Soil erosion is a major problem on the Ethiopian highlands. The poor soil management and land use practices are the causes of high soil erosion rate. Despite the extensive soil and water conservation programs launched by the Government of Ethiopia throughout the country, the achievements are far from satisfactory. One of the reasons was the inappropriate method used for assessment of soil erosion processes.This study was conducted to test and validate the agricultural non-point source (AGNPS) model in Kori gauged-watershed, South Wollo zone. Primary and secondary data collection methods were used to derive the spatial and attribute data. The primary data collection involved field survey. The soil conservation research project (SCRP) has collected hydrology and soil erosion data in the catchment since 1981. Some of the input parameters were also determined from digitized slope map using GIS.Ten rainfall events from 1989 and 1993 were used to calibrate the model. Sensitivity analysis conducted on the AGNPS model showed that curve number (CN) and universal soil loss equation (USLE) C factor were the most sensitive parameters and were subjected to calibration.The model calibration resulted in model efficiencies of 0.73, 0.53 and 0.90 for surface runoff, peak runoff rate, and sediment yield, respectively. The correlation coefficients were 0.87, 0.81 and 0.99, respectively, all significant at p≤0.01. Eight rainfall events from 1992 were used to validate the model. Validation results produced model efficiencies of 0.86, 0.65 and 0.88 for surface runoff, peak runoff rate and sediment yield, respectively. Surface runoff and peak flow simulations were improved in the validation stage. The corresponding correlation coefficients were 0.94, 0.90 and 0.98, all significant at p≤0.01, which showed the high linear correlation between measured and predicted values. It can be concluded that the AGNPS model is a useful prediction tool for understanding erosion processes on the Ethiopian highlands and for locating and targeting specific areas within the watershed that have high potential for soil loss, thus helping the conservation planner to design conservation plans.