Sediment delivery ratios: a misaligned approach to determining sediment delivery from hillslopes

Abstract

*Correspondence to: P. I. A. Kinnell, School of Resource, Environmental and Heritage Sciences, University of Canberra, Canberra, ACT 2601, Australia. E-mail: peter.kinnell@canberra.edu.au Considerable interest exists in the impact of land management on water quality throughout the world. A large number of non-point source (NPS) pollution or water quality models exist. In many cases, these models use the universal soil loss equation (USLE; Wischmeier and Smith, 1978) or the revised version of the USLE (RUSLE; Renard et al., 1997) to model erosion on hillslopes in conjunction with sediment delivery ratios (SDRs) to determine the sediment delivered from the hillslope to water bodies. The SDR can be defined as the ratio of the erosion upslope of a point in the landscape to the sediment delivered from that point. Erosion involves detachment, transport and deposition processes that operate all the time. The net loss or gain of soil material depends on the balance between the rates of particle uplift and deposition. In the context of modelling sediment delivery on hillslopes with respect to water quality, erosion refers to the situation where there is a net loss of soil material from the soil surface, and deposition to the situation where there is a net gain. The use of SDRs owes its origin to the observation that using erosion predicted by the USLE overestimates the amount of sediment delivered from hillslopes, because sediment deposition often occurs on hillslopes and the USLE does not account for deposition. The USLE was developed on planar surfaces and was not designed to apply to situations where there is a net gain of soil material on the area being considered. However, SDRs are essentially ‘performance’ factors that simply relate observed or modelled amounts at the plot scale to observed amounts at a larger scale. Using them to predict sediment delivery from hillslopes can produce erroneous results. One of the features of the USLE/RUSLE–SDR approach is that the predicted amount of sediment delivered from the hillslopes varies directly with the predicted erosion for any given hillslope profile. Halving the erosion results in a halving of the amount of sediment delivered from the hillslope. However, the deposition effect results from sediment in or entering the flow exceeding the transport capacity of that flow at a point on the hillslope. As a result, sediment delivery is directly related to erosion if the transport capacity of the flow is not exceeded (i.e. SDR = 1·0), but it is limited by the transport capacity if erosion exceeds the capacity of the flow to transport the eroded material (i.e. SDR < 1·0). This concept is encapsulated in Figure 1 and in so-called process-based erosion models like WEPP