An Innovative Morphometric Model—The Soil Loss Model Utilizing Watershed Hierarchy and the RUSLE Framework: The Buzău River Basin, Romania

Soil loss due to erosion is a form of land degradation, having significant effects on agricultural productivity, water resource quality, and food security. The morphometric analysis of the drainage network is based on mathematical relationships and geometric progressions. In previous studies, several morphometric parameters have been analyzed in relation to stream order. While numerous studies have assessed soil loss at the watershed level, its spatial variability at the sub-watershed level remains less understood, even though it can highlight sectors that disproportionately contribute to soil loss. Thus, this study aims to analyze whether the RUSLE-estimated soil loss values follow the same morphometric relationship and geometric progressions by examining the hierarchical distribution of soil loss in the 39 sub-watersheds of the Buzău River Basin. The methodology consisted of extracting the mean and summed values of soil loss from the RUSLE2015 raster for all 9369 sub-watersheds, which were hierarchized into seven orders of streams. We created two laws of summed and mean soil loss, and the law of the number of sub-watersheds corresponds to the law of the number of segments. The three data series exhibit geometric progression patterns for which ratios have been calculated. Internal variations in the estimated soil loss have been observed. The sub-Carpathian sector is the most susceptible to the estimated mean (66%) and total (73%) soil loss. Sub-watersheds whose area is smaller or comparable with that of other sub-watersheds in the Carpathian sector exhibited much higher soil loss values (SW 11- 0.16 t ha−1 yr−1 vs. SW 33- 3.36 t ha−1 yr−1). The Slănic sub-Watershed loses a total of 202,325.48 t yr−1, while Bâsca loses only 50,573.86 t yr−1, even though its area is much larger (78,300 ha). The highest values of mean soil loss are recorded in Oleșești, at 9.19 t ha−1 yr−1. These results can support authorities in identifying and prioritizing sub-watersheds that may require soil conservation measures and can be reproduced in watersheds with different characteristics.