Simulation of Long-Term Water Erosion in Rainfed Croplands of Eastern Washington

Abstract

<b>Abstract</b> <p /> Water erosion is an ongoing problem in eastern Washington due to its hilly terrain, highly erodible silt loam soils, rainy winters with frequent freeze and thaw cycles that weaken the soil, and the prevalence of conventional tillage. The region is characterized by a Mediterranean-type climate with warm, dry summers and cool, wet winters comprising three distinct precipitation zones: high (>460 mm), intermediate (300–460 mm), and low (<300 mm), which dictate the area‘s crop rotations. A unique 43-year (1940–1982) dataset of winter erosion measured on multiple agricultural fields in Whitman County, eastern Washington (by Verle Kaiser, USDA Soil Conservation Service agronomist) showed annual erosion rates averaging 53.8 t ha^−1, far exceeding the Natural Resources Conservation Service tolerable limit of 11 t ha^−1 yr^-1 for the soils in the area. This dataset allowed the comparison of field-measured erosion rates with those simulated by the WEPP (Water Erosion Prediction Project) model. Anthropogenic factors, such as tillage and crop rotation, change with time. Conservation tillage, including reduced- and no-till, has been increasingly adopted in eastern Washington since the mid-1980s. Assessment of long-term erosion trends will help elucidate the effect of the climate-management interaction on water erosion. <fig><graphic xlink:href=23031_files/23031-00.jpg id=ID_202d0f3c-20b0-41e7-9ffb-9a82f9470b31></graphic></fig> The specific objectives of this study were to (i) apply the WEPP model to simulate soil erosion in eastern Washington and compare the results with the 43-yr historical erosion data, and (ii) elucidate the long-term soil erosion trend, and the interactive effects of climate and management on water erosion in the study area. The WEPPcloud interface was used to delineate a representative subwatershed within each precipitation zone of the study area (Figure 1). Climate inputs were divided into two periods: past (1939–1982) and present (1983–2020). Water erosion was simulated for the three subwatersheds and the two periods. WEPP-simulated average annual erosion for 1940–1982 was 51.7 t ha^−1as compared to the field-measured 53.8 t ha^−1. The year-to-year trends from WEPP simulations were generally agreeable with those of the historical erosion data. Erosion has noticeably decreased from the past to the present, with WEPP model simulated annual erosion averaging 13.4, 34.0, and 51.7 t ha^−1 for the past, and 9.5, 14.1, and 15.5 t ha^−1 for the present, in the low-, intermediate-, and high-precipitation zones, respectively. The decreasing trend was primarily due to the increased implementation of conservation tillage and crop rotation as well as the decrease in the number of high-intensity winter precipitation events in the present climate.