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.outQs.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 .