Figure 3: Schematic of the uplift of soil material into the air and into the flow by a drop impact.
It is well known that not all materials detached by raindrop impact become airborne. Figure 3 shows a schematic of the uplift of soil material into the air and into the flow by a drop impact. Theoretically, the mass lifted into the air by a drop impact is given by
MM.A = a MM (2)
and the mass that remains in the flow
MM.F = b MM (3)
where a + b = 1.0. It has been well demonstrated by Moss and Green (1983) that the ratio of material splashed and material transported by raindrop induced saltation decreases greatly as flow depth increases. Although a tends towards 1.0 as flow depth decreases. its value is not known to be 1.0 in the shallow flows that occur in the Zhang et al. (2020) experiments. Also, when rain falls and produces runoff on any inclined plane, the value of a will vary spatially as flow depths vary down along the plane. Consequently, the amount of material splashed into the slot is not a reliable measure of the amount of material detached by the raindrops that impact the flow during the Zhang et al experiments.