Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2013
Publication Date: 3/30/2013
Citation: Dabney, S.M., Vieira, D.A., Yoder, D.C. 2013. Effects of topographic feedback on erosion and deposition prediction. Transactions of the ASABE. 56(2):727-736. Interpretive Summary: This article describes the effects of soil movement due water erosion and tillage operations on changes in agricultural landscapes and runoff flow patterns. The study compares model predictions with runoff and sediment delivery measurements made from 1975 to 2002 on a research watershed near Treynor, Iowa. Narrow grass strips called “hedges” were planted in 1991. The results show that over a number of years, soil slopes would change and flow patterns would be altered that would result in lower future erosion than would occur from the same rain storms on the topography that existed at the time the hedges were planted. The study also illustrates some limitations of the current erosion models, and outlines some steps to overcome those limitations. The conclusion is that there is a need for a more comprehensive modeling system that includes a quantitative accounting of soil erosion caused by concentrated runoff on landscape evolution. Such a system would improve the accuracy and reliability of conservation planning and practice impact assessment.
Technical Abstract: 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 distributed version of RUSLE2 has been developed and used in combination with the tillage erosion model TELEM (Vieira and Dabney, 2010). These models were applied to a 6.6 ha research watershed near Treynor, IA, where runoff and sediment yield were measured on a daily basis from 1975 through 2002. The watershed contained a grassed waterway and, beginning in 1991, ~1-m wide grass hedges were established with 15.4 m interval 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 areas ending RUSLE2 hillslope profiles were defined where contributing areas were > 600 m2. Average monthly runoff and sediment delivery to concentrated flow areas for a representative RUSLE2 hillslope profile were compared with measured monthly runoff and sediment delivery at the watershed outlet. For conventional tillage management, monthly runoff patterns reasonably matched observations, but watershed sediment yield was considerably lower than that predicted by RUSLE2 sediment delivery. These results suggest that sediment was deposited in the grassed waterway in the footslope area of the watershed. The establishment of grass hedges reduced sediment yield similarly in the RUSLE2 hillslope estimate and the watershed sediment yield. Conversion to no-till had a much greater impact in reducing RUSLE2 sheet and rill erosion than it did on watershed sediment yield, suggesting that ephemeral gully erosion remained an important sediment source. Distributed patterns of soil erosion appeared reasonable and the impact of grass hedges on average soil loss was similar to effects on observed sediment yield. To assess the potential influence of topographic feedback on future erosion estimates, terrain elevation was modified by summing estimates of water and tillage erosion that would be expected from 50 years of conventional tillage. As expected, reduced slope gradients generally reduced future erosion. The 3x3 kernel was modified to correctly determine local slope steepness downslope of aggrading grass hedges. The lack of consideration of erosion or deposition or concentrated flow channels limited the ability of the modeling scheme to reinforce existing ephemeral gully channels. A more sophisticated and complete modeling system that will include an ephemeral gully model and should update terrain elevation, slope steepness, and flow vectors on at least an annual basis when assessing conservation impacts of land management systems that result in the development of numerous permanent slope breaks.