MODELING SOIL EROSION AND SEDIMENTATION ON SMALL AGRICULTURAL WATERSHEDS

Two mathematical modeling approaches, distributed-system and lumped-system, were used for simulating watershed hydrology and temporal variations of watershed sediment yields. Major upland erosion processes as well as physical characteristics of natural rainfall were reviewed and modeled. Based on the results from upland erosion models, a distributed-system sedimentation model was developed and coupled with the ANSWERS hydrologic model (Beasley and Huggins, 1980). A lumped-system hydrologic model was developed from the general hydrologic system model of Chow and Kulandaiswamy (1971). A lumped-system sediment discharge model was developed from physically-based assumptions for a watershed sedimentation system. These four models were tested with field data from small agricultural watersheds of less than 20 ha in size.The distributed-system sedimentation model was developed so that the parameters may be approximated using factors for the universal soil loss equation. Optimal parameters were less than those determined from the factors. Splash erosion and flow erosion are linearly additive in the model. It also simulates spatial variations of soil erosion within a watershed, which can be useful for determining management practices to reduce soil erosion and sediment loads. The dynamic lumped-system sediment discharge model was developed so that sediment discharge is related to simple hydrologic parameters. Both models simulated actual data very well.Hydrologic simulation results from ANSWERS and the lumped-system hydrologic model were comparable with field data. The ANSWERS simulation results were sensitive to changes in parameters. The lumped-system model was nonlinear since the parameters vary with storm characteristics. Both the models were applicable to small watersheds.