Publication Date



AGNPS, C factor, erosion, GIS, Iowa, T value, water quality modeling


Eroded soil from overland is one of the major nonpoint pollution sources in many watersheds. The subsequent sediment not only reduces conveyance capacity of streams and usable storage volume of reservoirs but it also adsorbs and transports pollutants into and impairs its receiving water bodies. These negative environmental impacts may be alleviated by reducing sediment loading, which is positively associated with soil erosion rate. Targeted to critical erosion areas, which have a soil erosion rate higher than the tolerable level (T value) of 4536 kg/ac-y (5 tons/ac-y), limited funds may be more efficiently used to control sediment. With this regard, it is necessary to identify these areas in a watershed using an efficient tool such as an ArcView GIS based AGNPS (AGriculture Non-Point Source) model. The objective of this study was to use the GIS- AGNPS model to identify erosion-source areas within the 7075-ha Lake Icaria watershed, located in the Adams County, Iowa. The simulation results indicated that under current conventional cultivation practices, approximately 20% of the watershed in size was incurring a soil erosion rate above the T value. However, iterative simulation results revealed that the erosion rates in more than 63% of these identified critical areas could be reduced to a magnitude less than the T value, provided that the cropping (C) factors corresponding to the conventional cultivation practices would be adjusted down by 25%.