Figure 4. Schematic of the discharge of sediment between
sections when the whole surface in the experiments by Zhang et al.
(2020) is exposed to rainfall. Qs.out is
the amount of sediment discharged from a section during time when the
whole surface is exposed to rainfall.
Figure 4 shows a schematic of the discharge of sediment between sections
when the whole surface in the experiments by Zhang et al. (2020) is
exposed to rainfall. It applies to both the tarp and the screen
experiments in the final 15 mins of each 1 hour rainfall event. It
follows from Meyer and Wischmeier (1969), a paper cited by Zhang et al,
that when the steady state occurs and the sediment discharge from a
section is less than the sediment input into the section from upslope,all the sediment entering the section passes through the
section , and the difference between the amounts of sediment entering
and exiting the section results from erosion in the section .
Conversely, when the sediment discharge is less than the sediment input
from upslope then deposition occurs. In the both the Zhang (2019) and
the Zhang et al. (2020) experiments, onlyQs.out.1 was measured when each new section
upslope of section 1 was exposed during the hour rainfall intensity was
held constant. Even so, it can be assumed in the tarp experiments that
the amount of sediment discharged into section 1 from section 2 in the
15-30 min period is equal to the amount of sediment discharged from
section 1 during the 0-15 min period because there is no water or
sediment entering section 2 from upslope during that 15-30 min period.
Similarly, when section 3 becomes exposed, the amount of sediment
discharged into section 2 from section 3 is equal to the amount of
sediment discharged from section 1 during the 0-15 min period, and when
section 4 is newly exposed, the amount of sediment discharged into
section 3 from section 4 is equal to the amount of sediment discharged
from section 1 during the 0-15 min period. As noted above, when the
steady state occurs and Qs.out ≥Qs.in , all the material entering a
section from upslope is transported through the section so that the
rate soil material is discharged from the whole of eroding area is equal
to the sum of the sediment discharges (g m-1min-1) from erosion in each of the contributing
sections. Table 1 shows the result for 90 mm hr-1 rain
on 18 % slope in the tarp experiments reported by Zhang (2019) assuming
steady state conditions. Data for the Zhang (2019) are used
here as they are more readily available than for the Zhang et al. (2020)
experiments.
Table 1. Estimated sediment discharge (g m-1min-1) from erosion (g m-2min-1 ) each section during the four
15-min periods for 90 mm hr-1 rain on 18 % slope in
the tarp experiments reported by Zhang (2019)assuming steady
state conditions .