All soil loss prediction models are wrong, some more than others, some are useful

Soil loss prediction models are designed to aid making decisions on land management. To do this, they predict erosion for existing and proposed land management options that are tenable on a given soil on a given topography in a given climate. The Universal Soil Loss Equation (USLE) and subsequent revisions of it provide the most widely used model for this purpose in the world. While the climate input is reportedly based on the product of storm kinetic energy (E) and the maximum 30-minute rainfall intensity (I30), the model ignores the fact that storm kinetic energy per unit of rain varies with the synoptic conditions that dominate the production of rainfall at a particular location. It also ignores the fact that runoff per unit area can vary with slope length and gradient. However, these failures exist for pragmatic reasons. The mathematical structure of the USLE models is based on predicting the long-term soil loss from the unit plot so that any method that enables that to be achieved can be used to predict erosion for situations that do not conform to the unit plot using appropriate USLE equations for factors such as slope length and gradient. WEPP has for a long time been touted as a replacement for USLE models in the USA but does not predict event soil loss on bare fallow areas better than the EI30 index. Also, parameterization of WEPP focussed on areas where crops are grown on ridges rather than planar slopes. Consequently, WEPP is not an appropriate replacement for USLE models in making land management decisions. Other models like APEX break mathematical rules.: