Comment on “Plot-based experimental study of raindrop detachment, in interrill wash and erosion-limiting degree on a clayey loessal soil”

Abstract

Zhang et al. (2019) reported results from experiments where 80 cm long clayey lossal soil surfaces eroded by rain-impacted flow produced by artificial rainfalls. The target was surrounded by a 30 cm buffer area and slots along the sides of the target enabled splashed material to be collected during the experiments that lasted about 40 mins. Both splashed material and material discharged by the rain-impacted flow were collected in order to produce data on splash and sediment discharge rates for each rainfall event. Slope gradients ranging from 7 ° to 25 ° and rainfall intensities varying from 42 mm hr−1 to 90 mm hr−1 were used in the experiments. Splash rates initially decreased rapidly before reaching relatively steady values. Sediment discharge rates for 90 mm hr−1 rainfalls showed a similar pattern. However, with 42 mm hr−1 and 60 mm hr−1 rainfalls, there was an initial increase in sediment discharge rate followed by a peak before sediment discharges declined towards steady values which were not achieved with 42 mm hr−1 rainfalls. Given current understanding of sediment transport in flows deeper than 1–2 mm, it is suggested here that particles of different sizes and densities travel at different rates in the very shallow flows that operated in these experiment and, because the steady state discharge is not achieved until the slowest moving particle detached at the top of the slope is discharged, that the effects of slope gradient, rainfall intensity and time on the sediment discharge rates observed result from the numerous time scales that operate to transport eroded material over the surface. Also, because results using other soils in the same apparatus by others show temporal variations in sediment discharges that differ between different soils, it is suggested that additional work is required beyond experiments such as those reported by Zhang et al. (2019) before the processes that operate in very shallow flows are well understood.