Terrain analysis: Integration into the agricultural nonpoint source (AGNPS) pollution model

Grid- and contour-based digital elevation models are readily available. Terrain analysis methods use these data to estimate the topographic attributes of a catchment, many of which are essential parameters in hydrologic and water quality models. Integration of these techniques into hydrologic and water quality models permits better representation of the effects of three dimensional terrain on runoff and erosion processes and improves the efficiency of topographic data input. Terrain analysis methods were interfaced with the agricultural nonpoint source (AGNPS) pollution model to demonstrate how this integration can be achieved. Two terrain-enhanced versions of the AGNPS model were developed: AGNPS-C, a contour-based version, and AGNPS-G, a grid-based version. These terrain-enhanced models automatically generate the cell network, the cell connectivity, and the required topographic parameters. Peak discharge predicted by the three versions of the AGNPS model for five storms on a small catchment near Treynor, Iowa, were similar, but sediment yield predictions exhibited some differences. Sensitivity analysis shows that flow path lengths and contributing areas computed by the terrain analysis depend on cell size.