Estimation of Usle K-Values With a Process-Based Approach.

The susceptibility of a soil to detachment by erosive agents is commonly referred to as the soil erodibility. In the Universal Soil Loss Equation (USLE), the K – erodibility factor is defined as the rate of soil loss per unit of the erosion index (EI) from a continuously tilled fallow plot 72.6 ft long on a 9% slope (unit plot). A soil erodibility nomograph, which allows rapid estimation of K values for different soils, was developed from both long-term natural runoff plots, as well as from rainfall simulation studies. However, the nomograph does not always provide good estimates for K, and this has been noted for soils with high clay contents. In this study, we  utilize a combination of laboratory measurements and process-based WEPP erosion model simulations to back-calculate USLE/RUSLE K factors for two soils, a Miami clay loam soil from Indiana, and an Opal clay soil from South Dakota. Experiments included: 1) rainfall experiments to estimate WEPP interrill erodibility; 2) mini-flume experiments to estimate WEPP rill erodibility and critical shear stress. Erosion measurements were made under three soil moisture conditions: initially dry, pre-wetted then drained, and pre-wetted and saturated. Preliminary results show average K_i for the Miami silt loam was 6.44×10⁶ kg·s·m^-4 under initially dry soil conditions, 3.73×10⁶ kg·s·m^-4 under pre-wetted then drained conditions, and 3.80×10⁶ kg·s·m^-4 under saturated conditions. This soil was particularly vulnerable to soil loss when the dry soil aggregates disintegrated under rainfall and rapid wetting. For the Opal silty clay, average K_i values were 3.01×10⁶ kg·s·m^-4, 1.23×10⁶ kg·s·m^-4, and 4.00×10⁶ kg·s·m^-4 for the dry, drained, and saturated soil conditions, respectively, indicating that this South Dakota soil was most erodible under soil seepage conditions. Mini-flume studies and WEPP model simulations are in progress.