Modification of AGNPS for Agricultural Land and Climate Conditions in Central Germany

Until recently, no suitable computer model was available to quantify P and N yields of nonpoint sources into surface water in central German watersheds. 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 2‐yr program of water quality measurements and on hydrologic data collected during 33 yr, this modified model was tested in two medium‐sized agricultural watersheds in the state of Hesse, Germany. With deviations of 1.7 and 3.8%, runoff volumes for all observed and simulated flood events showed a high level of agreement in both watersheds. Peak flows showed larger errors averaging 14.8 and 24%. Only for one of the two watersheds were sediment and total P yield computed satisfactorily, with average errors of 5% (sediment) and 6% (total P). Deviations for the second watershed were considerably greater. The same is true for the dissolved P and N yields. In the case of small events, AGNPS consistently overestimated the particulate P yield in both watersheds. However, the model correctly registered the overall dynamics of the system as well as the differences of sediment and total P yield between the two watersheds. The modified AGNPS model can thus be employed by environmental agencies as a useful planning tool.