| The overarching objective of this research was to provide an improved understanding of the roles of land use and associated management practices on long-term, water-driven, soil erosion in a small agricultural watershed (South Amana Watershed, SAW) by coupling the established, physically based, distributed parameter Water Erosion Prediction Project (WEPP) model with long-term hydrologic, land use, and soil data. Considering the importance of event-based nonpoint source pollution simulations and the critical role of antecedent soil/land conditions on the erosional impact of single storm events, the auxiliary goal of this study was set as investigating the event-based erosion rates within the SAW and the associated controlling factors. For model calibration, physical ranges of the governing parameters were obtained via critical literature review and in-situ observations within the SAW. Verification of the calibrated-model predictions was conducted by using observed precipitation data and field measurements of water and sediment discharge at the SAW outlet and also by comparing the results with data reported in the literature for other Iowa watersheds. Once the continuous WEPP simulation was verified, "thought experiments" were conducted to test our hypothesis that land use and associated management practices are the major controls of long-term erosion in small agricultural watersheds. Results of the continuous simulation showed that the magnitude of the expected correspondence between erosion rates and high magnitude precipitation events was strongly affected by the crop rotation characteristics. Tillage type affected the surface roughness and resulted in increases of the rill/interrill erodibilities up to 35% and 300%, respectively. Furthermore, the timing of tillage operations affected the "life-time" of residue cover and consequently the degree of protection that residue cover offers against the water action on the soil surface. The cumulative effects of tillage type and timing in conjunction with canopy/residue cover led to the conclusion that land management practices can significantly amplify or dampen the impact of precipitation on long-term soil erosion in small agricultural watersheds. Finally, event-based erosion rates were found to be controlled primarily by residue cover, root density and rainfall characteristics. |
