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.