QUANTIFYING RATES OF AGRICULTURALLY-INDUCED SOIL EROSION IN THE MIDWESTERN U.S

Soil erosion in agricultural landscapes is a major concern for crop production, social stability, and environmental quality. Although it has been recognized for a century that agricultural practices in the midwestern U.S. have degraded soils, the rate that soil is being lost is poorly constrained. Although most of the midwestern U.S has been converted to farmland, native prairie remnants, which record the pre-settlement topography are scattered throughout the landscape and are often adjacent to agricultural fields. Accelerated erosion rates in fields creates an erosional escarpment between the field and prairie, whereby the prairie becomes perched above the field. We surveyed the boundary between 15 native prairies and adjacent agricultural fields in Iowa, South Dakota, Nebraska, and Kansas using a real-time kinematic GPS to measure the elevation difference between the perched prairies and the plowed fields. Elevation differences between the prairies and fields ranges from 2 cm for flat topography to 50 cm in areas with rolling, hummocky topography. Assuming the initiation of erosion coincided with European settlement of the region, approximately 150 years ago, erosion rates in the agricultural fields ranges from 0.1 - 3 mm/yr. The mean erosion rate at 7 of the fields is greater than the soil loss tolerance value defined by the USDA. We also find that the highest erosion rates occur when the boundary between the field and prairie is located on convex topography, suggesting that the erosion rate is dependent on topographic curvature. The association between erosion rate and topographic curvature is consistent with erosion occurring primarily by tillage, which is not currently incorporated in soil erosion models used for conservation planning in the U.S.