How suitable are vegetation indices for estimating the (R)USLE C-factor for croplands? A case study from Southeast Brazil

The cover and management factor (C-factor) of the Universal Soil Loss Equation (USLE) represents the effects of crop cover, weighted by rainfall pattern, on predicted soil erosion rates. This requires an estimate of seasonal rainfall erosivity and soil protection afforded by the crop at different phenological stages, expressed by a soil loss ratio (SLR). However, soil erosion modelers often rely on vegetation-index-based regressions to directly estimate the cover and management factor (C-factor) of the USLE from satellite images. Since this approach is based on a single or very few images, it does not characterize the seasonality of the crop cover or reflect the seasonality of the rainfall erosivity. Here, we evaluated five vegetation indices (NDVI, NDRE, SFDVI, ViGREEN, and MGRVI) in predicting SLRs and the C-factor for a sugarcane plot in Southeast Brazil. We used Sentinel-2 images and orthomosaics obtained by UAV surveys performed at the middle of each phenological stage. We compared the estimates of the C-factor based on the SLRs and rainfall erosivity against direct regressions from the literature. Our results confirmed the expected poor correlation between the C-factor and the vegetation indices. On the other hand, using the proposed vegetation indices proved to be a reliable alternative to predict the SLR in sugarcane areas, especially the NDVI, the NDRE, and MGRVI. In particular, the MGRVI accurately predicted the SLR and classified the UAV-derived orthomosaics.