A Proposed Modification to the WEPP Erosion Process Model Concept

In the Water Erosion Prediction Project (WEPP) model, erosion and deposition are calculated based on apredefined value of sediment transport capacity (Tc), estimated from flow hydraulics, slope and sedimentproperties. Under this model concept, net erosion or deposition is estimated by the difference between sedimentload (qs) and Tc, i.e., erosion when qs < Tc and deposition when qs > Tc. Separate erosion and depositionequations are derived and an accurate assessment of erosion or deposition depends on how the Tc is estimated.Unfortunately, Tc is rarely measured experimentally, making it more a hypothetical concept rather than ameasurable quantity. Recent laboratory results from experiments conducted on a multiple-box system, underseepage and drainage conditions that changed soil erodibility for the same soil, challenged the Tc concept.Experimental observations suggest that erosion and deposition processes are occurring simultaneously anddifferent surface, flow and rainfall conditions may trigger the dominance of one process over the other.Therefore, we propose an alternative, single sediment mass balance equation, which contains both erosion anddeposition terms, to model sediment transport. As a first approximation, the erosion process is still modeled bya first-order rate process similar to the current WEPP erosion model, and the deposition process is estimated tobe proportional to qs. Separate rate constants are used for erosion and deposition processes. An analyticsolution for the proposed erosion equation is derived and the solution is examined for conditions similar to thelaboratory multiple box experiments. The analytic solution reproduced experimental data trends, in otherwords, it was able to simulate sediment mass balance scenarios ranging from deposition- to detachment- andtransport-dominated regimes. This proposed modification may lead to a better understanding of erosionprocesses, and consequently, an improved erosion prediction model.