A document by P.I.A. Kinnell. Click on the document to view its contents.

15 16 Wang et al (2022) undertook an evaluation of the Water Erosion Prediction Project (WEPP) 17 model on 134 USLE runoff and soil loss plots. Wang et al did not compare the capacities of 18 WEPP and USLE based models to predict soil loss. The importance of doing that on bare 19 fallow plots is illustrated here. Data from comparisons of WEPP, RUSLE2, and the USLE-M 20 undertaken by Kinnell (2017) demonstrated that both RUSLE2 and the USLE-M predicted 21 event soil losses on 4 historic bare fallow USLE plots in the USA better than WEPP. It is 22 apparent that because WEPP is a steady state model designed to model event soil loss for 23 ridged tillage cultivation, WEPP is in not well suited to predicting event soil losses from bare 24 fallow plots that are planar with rills occurring in some storms but not all storms.. Given that 25 calibrated WEPP does not model event soil losses on bare fallow USLE plots better than 26 either RUSLE2 or the USLE-M, the fundamental ability of WEPP to model event erosion 27 under natural rainfall must be questioned at this time. 28 29 30 Keywords: WEPP; RUSLE2; USLE-M; calibration; natural rainfall 31 32 33 2

Traditionally, so-called physically based models of erosion by rain-impacted flow focus on the conservation of mass. Raindrop induced saltation and rolling produce random intermittent movement of soil particles and it seems that the traditional approach to modelling sediment transport in rain-impacted flows does not account for the effects these transport processes well. Arguably, the traditional approach needs to be replaced by one that is better able the deal with the stochastic nature of the transport of coarse material in rain-impacted flows.

Zhang (2109) presented results from experiments where 4 sections in 1.8 m long flumes were sequentially exposed to rainfall during 15 minute periods during an hour where rainfall intensity and slope gradient remained constant. The upslope areas were protected by a tarp in one treatment, and a screen placed 5 cm above the soil surface to provide sheet flow protected to some degree from raindrop impact in another treatment. Zhang presented two equations, one for the screen experiments, one for the tarp experiments, for estimating the soil loss rates in the sections under steady state conditions. The equation used for the screen experiments is shown here to produce results that do not conform to well known long established rules that apply to the determination of erosion in a section. In terms of modelling sediment discharge, Zhang applied an equation developed by Zhang and Wang (2017). It is shown here that that equation was not well suited to predicting the discharges in the screen treatment. Zhang also observed sediment discharges were well correlated to stream power even though sediment concentrations were influenced by rainfall intensity. Considerable insights exists in respect to the detachment and transport mechanisms that operate in rain-impacted flows a few millimetres deep but the closeness of the flow surface to the soil surface has a major impact on sediment transport by saltation and rolling. Further study of sediment transport by very shallow rain-impacted flows is warranted

Zhang et al (2020) measured soil splashed and washed by rain-impacted flows on surfaces where 4 sections were exposed in sequence during a 1 hour rainfall event. In one treatment, upslope areas were protected by a tarp, and in the other, protected by a gauze screen. Comparison between the amounts splashed and eroded by the rain-impacted flow were used to determine if detachment by surface water flows occurred. In this comment, the conclusion 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 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.