The RUSLE is limited to erosion resulting from rill and interrill erosion, leaving gullying unconsidered (Blanco & Lal, 2010). To determine where gully erosion may occur, the SPI was calculated for the watershed, similarly to part of a study completed by Arabameri et al. (2018). SPI, the measure of a flow’s erosive power, is an important factor when considering erosion on slopes, as a higher SPI results in higher erosion rates (Arabameri et al., 2018). SPI was calculated using the Relative Stream Power Index tool in WhiteBox GAT. The D8 Flow Accumulation method, a single-flow direction model, was used to derive a Flow Accumulation layer of the specific catchment area. This layer was then inputted into the Relative Stream Power Index tool alongside a raster slope layer, that was derived using the Slope tool on the watershed’s DEM.
Figure 4: Methods and programs used to calculate SPI for the McGregor sub-watershed