EFFECTS OF TOPOGRAPHIC FEEDBACK ON EROSION AND DEPOSITION PREDICTION

Soil erosion and depositional processes result in changes in topographic and soil profile properties over time. In spite of this, few current soil erosion models account for these changes. To begin to address this deficiency, a spatially distributed version of RUSLE2 was developed and used in combination with the tillage erosion model TELEM. These models were applied to a 6.6 ha research watershed near Treynor, Iowa, where runoff and sediment yield were measured from 1975 to 2002. The watershed contained a grassed waterway, and similar to 1 m wide grass hedges were established in 1991, with 15.4 m intervals between the grass hedges. Beginning in 1996, no-till management replaced conventional tillage management in the watershed. Using a 3 m raster DEM, concentrated flow channels that ended RUSLE2 hillslope profiles were delineated wherever contributing areas exceeded 600 m(2). Average monthly runoff and sediment delivery to concentrated flow areas were compared with measured monthly runoff and sediment delivery at the watershed outlet. For conventional tillage, monthly runoff patterns reasonably matched observations, but measured watershed sediment yield was lower than RUSLE2-predicted hillslope sediment delivery, suggesting that sediment was deposited in the grassed waterway. Grass hedges reduced sediment yield similarly in the RUSLE2 estimates and the measured watershed sediment yield. Conversion to no-till had a much greater impact in reducing RUSLE2 estimates of hillslope erosion than it did on measured watershed sediment yield, suggesting that ephemeral gully erosion remained an important sediment source in this watershed under no-till conditions. To assess the potential influence of distributed patterns of soil erosion and deposition behind the hedges on future erosion estimates, the terrain elevation was modified by summing estimates of water and tillage erosion and deposition that would be expected from 50 years of conventional tillage. As expected, reduced slope gradients generally reduced future erosion. The lack of consideration of erosion or deposition in concentrated flow channels limited the ability of the modeling scheme to reinforce existing ephemeral gully channels.