Research Needed to Improve Upland Soil Erosion by Water Process Modeling

Abstract

Erosion by water on agricultural areas has been studied by USDA (SCS, ARS, FS, NRCS) since the 1930‘s, initially consisting of natural runoff plots at numerous locations in the country that provided data that culminated in the development of the Universal Soil Loss Equation (USLE) in 1965 in Agriculture Handbook 282 (Wischmeier and Smith, 1965). Rainfall simulation equipment developed by ARS scientists located at Purdue University during the early 1960‘s was used to supplement the natural rainfall plot data and provided the ability to rapidly study soil erosion processes in more controlled laboratory and field experiments. The USLE update in Agriculture Handbook 537 (Wischmeier and Smith, 1978) contained a soil erodibility nomograph, developed from the results of extensive field rainfall simulation studies across the Midwest U.S. Transitions to process-based soil erosion modeling approaches in the 1980‘s-1990‘s relied upon laboratory and field rainfall simulation experiments to study processes and develop parameterization approaches for sensitive model input parameters. The Water Erosion Prediction Project (WEPP) model solves differential equations down a slope profile, that compute changes in sediment load and detachment/deposition at discrete points (Flanagan and Nearing, 1995; Flanagan et al., 2001). Unfortunately, most data available for validation of process-based hillslope interrill/rill erosion is from natural runoff plots (USLE database) or other natural or simulated rainfall studies where soil (sediment) loss was only measured at the plot end. Newer experimental measurement techniques, equipment, and approaches are needed to allow examination of changes in sediment detachment/deposition/loads with very fine spatial resolution down a hillslope and down a rill channel. The current modeling techniques also assume either uniformly spaced rill channels down a slope profile (default is 1-m spacing), or alternately simulations can be conducted where all surface flow is assumed to be distributed at a uniform shallow depth across the entire flow width. However, in real world situations the soil conditions, landscape complexities and increases in slope lengths can result in situations where no evident rilling occurs, rill spacings change with increasing slope lengths, or surface flows converge or diverge which may result in channelized rill flows changing to broader sheet flows. These complex surface flow situations need to be accounted for in any new water erosion prediction technologies, as they can greatly affect soil detachment and deposition processes. Additionally, subsurface hydrology effects also need to be better accounted for in any new technologies. Current erosion prediction models poorly account for water moving within the soil, and how saturation conditions can weaken the soil surface and make it more susceptible to detachment. A comprehensive understanding of how subsurface water moves and can converge in fields (and may emerge to the surface) is needed in new modeling approaches that can also be linked to initiation and development of ephemeral gullies. Additionally, better understanding and modeling of the effects of subsurface drainage and controlled drainage systems on hydrology and soil erosion is required. This also highlights the needs to move to erosion prediction systems that truly represent the landscape and fields as 2-D/3-D surfaces. Current prediction technologies are largely limited to estimating soil erosion processes at points down a line to represent a rectangular area. Long-term temporal effects of various practices on soil health and soil hydrology and erosion also should be accounted for in new erosion models. Currently, baseline soil erodibility is assumed to be a constant value for a particular soil and cropping system as a function of soil texture, organic matter, and other properties. However, fields that are in long-term pasture, or long-term no-till row crop management systems along with the use of cover crops may substantially change soil organic matter, soil structure, infiltration characteristics, and erodibilities that are not currently reflected in USDA models. This presentation will explore these and other areas of research needed.