Comment on ‘Investigating ponding depth and soil detachability for a mechanistic erosion model using a simple experiment’ by Gao, B., et al., 2003. Journal of Hydrology 277, 116–124

Gao et al. [J. Hydrol. 277 (2003) 166] analyzed the results of a simple experiment to test the theoretical relationships for two factors in a mechanistic model. They concluded that the theoretical values for the parameter a, the parameter that accounts for the effect of water depth on the amount of material detached and lifted into the water by raindrop impact, and the exponent of rainfall intensity were appropriate. Previous work on the mechanics of erosion by rain-impacted flow supports the concept that there is a direct relationship between rainfall intensity and sediment discharge by rain-impacted flow, as do the experiments of Gao et al. Fundamentally, an exponent value of one indicates that variations in the time period between successive drop impacts do not significantly affect of the ability of a drop impact to detach and transport soil material in rain-impacted flows. However, theoretically, the form of the relationship between the amount of material in the water layer and water depth observed by Gao et al. may be produced by an a to water depth relationship that differs from the one used in the mechanistic model. Some of the response relationships in the mechanistic model are based on experiments that were not designed to determine the nature of those particular relationships. Because of this, these relationships are conceptual and have not properly verified by experiment. Gao et al. strived to overcome this deficiency using a simple experiment. This work is a step in the right direction and is to be encouraged.