Traditionally, so-called physically based models of erosion by rain-impacted flow focus on the conservation of mass. Raindrop induced saltation and rolling produce random intermittent movement of soil particles and it seems that the traditional approach to modelling sediment transport in rain-impacted flows does not account for the effects these transport processes well. Arguably, the traditional approach needs to be replaced by one that is better able the deal with the stochastic nature of the transport of coarse material in rain-impacted flows.