Erosion experiments were performed using 3.7 mm raindrops impacting flows over various sized sands. The results from these experiments, when combined with data from previous experiments using other drop sizes, enabled the effects of flow and rain to be isolated from the effect of the eroding surface. Although the rate of decrease in erosion rate with flow depth was observed to vary with drop size when drops impact flows deeper than about 4 mm, raindrop size may not have a significant influence on the erosion rate when shallower flows are impacted by medium-to-large raindrops. However, this is not the case for smaller drops and for medium-to-large drops traveling at much less than their terminal velocities.
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Kinnell et al. (1992) studied this question.