Introduction
Zhang et al. (2020) report results from an experimental design that was
modified from the design reported by Zhang (2019). The experimental
design involved the use of two flumes (1.8 x 0.5 x 0.1 m) set side by
side with a 2-5 cm gap between them. The flumes were packed with soil
having 20.8 % sand, 58.6 % silt, and 20.6 % clay. Rain was applied
for one hour using a nozzle type simulator whose intensity was
controlled by the frequency of nozzle oscillation. 3 rainfall
intensities (60, 90, 120 mm h-1) and 3 slope gradients
( 9, 18, and 27%) were used. Splashed material was collected in the
slot while sediment discharged in runoff was collected at the bottom of
the flumes. The soil was the same as used by Zhang (2019) but packed to
a bulk density of 1.4 g cm-3 as opposed to 1.2 g
cm-3 used previously. The 1.4 g cm-3bulk density was chosen after trying several alternatives because the
authors perceived that it created a detachment-limited regime under high
intensities and slopes but a transport-limited regime under lower
intensities and slopes.
Two treatments were used in association with the variations 3 variations
in rainfall intensity and 3 variations in slope gradients. In one
treatment, a screen was suspended 5 cm above the soil surface. In the
other, a tarp was placed over the surface to prevent upslope runoff from
flowing over the exposed surface. In the first 15 minutes of experiments
lasting 1 hour, the bottom quarter of the flumes was exposed to rainfall
produced by the rainfall simulator. In the 2nd 15-minute period, erosion
by the rainfall occurred on the bottom half of the flumes.
Three-quarters of the flume area was exposed in the
3rd 15-minute period with the whole area being exposed
in the 4th 15-minute period. In the first hour, 60 mm
hr-1 rainfall intensity was used. In the
2nd hour, the exposure sequence was repeated with 90
mm hr-1 rainfall intensity, and 120 mm
hr-1 rainfall intensity in the 3rdhour. The screen experiments were designed so that surface water flow
conditions did not vary in the bottom section of the 1.8 m long surface
as the number of exposed sections varied. Consequently, the transport
capacity of the rain-impacted flows in the bottom section remained
constant during each 1 hour rainfall event. In contrast, the tarp
experiments were designed so that the transport capacity of
rain-impacted flows in the bottom section varied as the number of
exposed sections varied. The two treatments were applied on 9 %, 18 %
and 27 % slopes. Newly prepared surfaces were used for a sequence of 3
one hour rainfalls varying in intensity from 60 mm
hr-1 to 120 mm hr-1 and totalling
270 mm on for each slope gradient. Sediment-laden runoff was collected
every 3 minutes
In analysing the data for splash in these experiments, Zhang et al
assumed that following Zhang (2019), splashed material per unit area of
the slot was equal to the detached and/or previously detached soil
material per unit area of the plot. In order to make direct comparisons
between event total wash loss and splash, the total splash amounts were
upscaled according to ratios of respective area of the flumes and the
slot and those values reflected the total loose material available on
the soil surface during the entire event. Given that, Zhang at al
assumed that if these measured splash amounts exceeded the amounts
washed, then transport-limiting conditions existed. Conversely, if the
washed amounts exceeded the measured splash amounts, then flow
detachment had occurred. Based on this assumption, they concluded that
flow detachment 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.