The Impact of Management and Other Factors on the Size Distribution of Hillslope Eroded Sediment

Land disturbances cause considerable environmental, economic, and health impacts from the detachment, transport, and deposition of sediment. The Revised Universal Soil Loss Equation-version 2 (RUSLE2) is an erosion and sediment transport model commonly used by regulatory agencies to develop management strategies that mitigate some of these impacts. RUSLE2 uses the eroded sediment size distribution (ESSD) at the point of detachment in mass transport calculations, which are generally assumed to obey particle diameter/fall velocity relationships consistent with Stokes Law. RUSLE2 currently estimates the distribution of eroded sediment by dividing the ESSD at the point of detachment into 5 size classes solely as a function of the matrix soil dispersed clay content.

The present model fails to describe the impacts of some popular sustainable agriculture systems because specific effects of soil management practices (tillage and residue), as they influence the ESSD, were not accounted for when the equations were developed. Additionally, the current relationships only describe ESSDs at the point of detachment, which fails to describe spatially related ESSD differences. Finally, the number of size classes currently used to describe the ESSD does not adequately represent true conditions.

This study revised current ESSD equations through an analysis of published research not accounted for in the current RUSLE2 model, with emphasis on the effects of management and scale. The research found that the proposed revisions to these equations better describe the size distribution of hillslope detached particles for four independent data sets (avg. RMSE = 182.5 micrometer) than the current approach (RMSE = 208.4 micrometer), partially by including previously unaccounted management factors. Spatial relationships as they influence the ESSD were also defined to aid RUSLE2 in modeling difference and correlations between the resulting sheet and rill ESSD as either scale predominates. The resulting RUSLE2 modifications were found to be more accurate and describe a broader range of conditions, so these equations could ultimately be implemented in conjunction with a RUSLE-based field scale model to aid in managing chemical transport as well as sediment yield.