An Integrated Systems Approach to Modeling Sediment Yield from Rangeland Watersheds

Soil degradation in arid and semiarid ecosystems is a significant problem. Knowledge of sediment yield at the outlet of watersheds is insufficient to interpret internal watershed erosion and deposition processes. An integrated system is under development to incorporate databases containing channel cross section information, a sediment yield model (APOINT) that is applicable to individual cross sections along alluvial channels, and interpreted model output. The complexity of sediment transport processes is expressed in the APOINT simulation model. The APOINT sediment transport model was calibrated and validated at small semiarid watersheds within the USDA-ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona. The APOINT model explained about 85 percent of the variation in observed sediment discharge from a 3.68 ha subwatershed located in the upper end of a 43.7 ha watershed. Results of model validation at the same watershed indicate that APOINT is able to explain 98 percent of the variation in observed sediment discharge. APOINT was further calibrated at the outlet of the 43.7 ha watershed by comparing model results with observed sediment accumulation in the pond located at the watershed outlet. APOINT was able to explain 77 percent of the variation in sediment accumulation during 7 time periods from 1962–1996. Following the establishment of APOINT as a viable model for simulating sediment discharge through individual channel cross sections, efforts were focused on integrating the model with a database containing field data collected at several points along the main channel within the 43.7 ha watershed. An integrated system is under development to link the model and the database, provide a user friendly interface, and interpret model results with respect to zones of erosion, transport, and deposition within the watershed. The result is a scientifically defensible, readily accessible, easy to use tool for simulating sediment yield from small semi-arid watersheds.