Sediment, nutrients and pollutants discharged from sheet and interrill erosion areas by rain-impacted flows may influence water quality in streams and rivers. The depth of water on the soil surface influences the capacity of raindrop impacts to detach soil material underlying rain-impacted flows, and a number of so-called process-based and mechanistic models erroneously use equations on the basis of the effect of water depth on splash erosion to account for this effect. Also, a number of these models require complex mathematical solutions to make them operate and can only predict sediment composition and discharges well if many of their parameters are calibrated specifically to the situations where they are being applied. Experiments with rainimpacted flows, where flow depth and velocity over eroding surfaces have been controlled, have been reported in the literature and provide more appropriate equations to account for the drop size ? flow depth interactions that affect detachment and transport of particles in rain-impacted flows. There is a need to develop modeling approaches that rely on relevant data obtained under well-controlled flow conditions where flow depths and velocities are known.