Testing AGNPS for soil erosion and water quality modelling in agriculturalcatchments in Hesse (Germany)

Until recently, no suitable computer model was available to quantify P and N yields of nonpoint sources into surface water in Central German watershed s. The event-based erosion and nutrient transport model AGNPS (Version 3.65 ), developed in the USA, was adapted to Central German landscape conditions and linked to a GIS. Drawing on a two-year program of water quality measur ements and on hydrologic data collected over 33 years, this modified model was tested in two medium-sized agricultural watersheds in the state of Hess e (Germany). With deviations of 1.7% and 3.8%, runoff volumes for all obser ved and simulated flood events showed a high level of agreement in both wat ersheds. Only for one of the two watersheds were sediment and total phospho rus yield computed satisfactorily, with average errors of 5% (sediment) and 6% (total P). However, the model correctly registered the overall dynamics of the system as well as the differences of sediment and total P yield bet ween the two watersheds. The modified AGNPS model can thus be employed by e nvironmental agencies as a useful planning tool. (C) 1999 Elsevier Science Ltd. All rights reserved.