WEPP Simulation of Observed Winter Runoff and Erosion in the U.S. Pacific Northwest

Abstract

The Palouse area of the Northwestern Wheat and Range Region suffers high erosion throughout the winter season. The excessive soil loss is a result of a combination of winter precipitation, intermittent freezing and thawing of soils, steep land slopes, and improper management practices. Soil strength is typically decreased by the cyclic freeze and thaw, particularly during the period of thawing. When precipitation occurs during these freeze–thaw cycles, soil is easily detached and moved downslope. This study was aimed at improving the knowledge of winter hydrology and erosion in the Pacific Northwest (PNW) through combined field experimentation and mathematical modeling. Surface runoff and sediment were collected for three paired field plots under conventional tillage and no‐till, respectively. Additionally, transient soil moisture and temperature at various depths were continuously monitored for two selected plots. These data were used to assess the suitability and performance of the USDA's Water Erosion Prediction Project (WEPP), a physically based erosion model, under the PNW winter conditions. Field observations revealed that minimal erosion was generated on the no‐till plots, whereas erosion from the conventionally tilled plots largely exceeded the tolerable rates recommended by the Natural Resources Conservation Service. The WEPP model could reasonably reproduce certain winter processes (e.g., snow and thaw depths and runoff) after code modification and parameter adjustment. Yet it is not able to represent all the complicated processes of winter erosion as observed in the field. Continued field and laboratory investigation of dynamic winter runoff and erosion mechanisms are necessary so that these processes can be properly represented by physically based erosion models.