116. Application of the Agricultural Non-Point Source Pollution (AGNPS) Model for Sediment Yield and Nutrient Loss Prediction in the Dumpul Subwater shed, Central Java, Indonesia

Soil erosion is one of Indenesia's most serious environmental degradation problems. Reliable measurements of erosion rates, however, remain uncommon, and estimates of soil productivity are even more rare. Assessing the extent and seriousness of erosion therefore remains a difficult task. Nevertheless, identification and assessment of erosion problems play an important role in bringing about better land-use and conservation practices. The Agricultural Non-Point Sources Pollution Model (AGNPS) model can be used for developing drainage basin management plans. The model was used to identify critical areas within the drainage basin where land treatment should be focused for optimal results. Furthermore, land and water conservation measures can be implemented in critical areas, while taking into consideration the bio-geophysical conditions in the drainage basin. Application of the AGNPS model to the Dumpul sub-drainage basin in central Java resulted in a coefficient of determination of 0.94 between observations and AGNPS results when the model was used for computing sedimentation rates. The relationship between modelled vs measured nutrient and phosphorus loss and chemical oxygen demand has a coefficient of determination of 0.98. Sediment yield, nutrient loss, phosphorus loss, and chemical oxygen demand calculated by the AGNPS model have values of 420, 6.8, 4.3 and 0.1 kg ha^–1, respectively. The modelling result for the Dumpul sub-drainage basin indicates that changing land use to housing areas would increase peak runoff volume, peak runoff flow rate, sediment yield, nitrogen loss, phosphorus loss, and chemical oxygen demand by 37, 40, 118, 87, 91 and 110%, respectively. Conversely, implementing land and water conservation in the form of contouring of the entire drainage basin would decrease these variables by 33, 30, 57, 46, 47 and 41%, respectively. AGNPS results indicate that a combination of contouring and terracing would decrease these losses by 52, 51, 77, 68, 69 and 72%, respectively.