Figure 2: Schematic of spatial variations in the detachment and transport mechanisms operation to move coarse material on surfaces eroding as the result of detachment by raindrop impact and surface water flow. RD = Raindrop detachment, ST = transport aerially by splash, RIS = raindrop induced saltation, RIR = raindrop induced rolling, FDS = flow driven saltation, FDR = flow driven rolling, FD= flow detachment.
For soil erosion to occur, particles must be extracted from being held within the surface of the soil matrix by cohesion and inter-particle friction and then subsequently transported away from the site of the extraction. Detachment is the term used to the process of extraction of particles from the surface of the soil matrix. It is well known that both raindrop impact and surface water flows can cause detachment. However, detachment by flow only occurs once flows possess sufficient energy to cause detachment. Often detachment by flow is considered to occur once the flow has a shear stress that exceeds a critical value (τc(bound) ) that is soil dependent. As shown in Figure 1, raindrop detachment can cause erosion when raindrops impact flows that have shear stresses below that critical value. Consequently, as shown in Figure 2, various detachment and transport mechanisms may operate to detach and transport soil material on surfaces like those used by the Zhang et al. The video provided by Zhang at al shows fine material moving in suspension as expected from Figure 1 while the coarse material moved intermittently in direct association with the rainfall pulses as would be expected when situation B or C occurred. If flow detachment occurred in section 1 (situation E), then coarse particles would be discharged in a continuous stream. However, it seems that Zhang et al concluded from comparing wash and splash rates that the situation depicted in E occurred in all the screen experiments and in all the tarp experiments on the 27 % slope and the tarp experiment with 120 mm h-1 rain on the 18 % slope. In this comment, this conclusion is shown to be unsound for two reasons. Firstly, the conclusion is unsound because it is well known that not all the material mobilized by raindrop impact is lifted into that air and transported aerially over the surface in raindrop splash. Secondly, the conclusion is unsound because the equation used to determine apparent soil loss rates in the screen experiments does not estimate the contribution of erosion in the sections to the sediment discharged from the eroding surfaces.