Quantifying sheet wash erosion rates in a mountainous semi-arid basin using environmental radionuclides and a stream power model

Abstract

Erosion rates and processes define how mountainous landscapes evolve. This study determines the range of erosion rates in a semi-arid landscape over decadal time spans and defines the dominant processes controlling variability in erosion rates. The varying topography and climatic regimes of the Xiying Basin (Qilian Shan Mountains, China) enables us to examine the relative roles of sheet wash versus rainsplash and the influence vegetation on soil erosion and deposition. Soil erosion rates since 1954 were determined using 137Cs along 21 transects at 4 sites with varying gradient, rainfall, and vegetation cover. The mean 137Cs derived soil erosion rate ~0.42 mm/a, consistent with the catchment level erosion rate derived from total sediment yield for a 44 year record. However, there is considerable variability in 137Cs erosion rates both between transects and along transects. Perhaps reflecting variation not only in the effectiveness of individual processes but also in their relative roles. We compare the 137Cs-derived erosion rates with 1-D models for sediment flux that incorporate sheet wash and rainsplash processes, testing them over a previously untested 60 year timescale. The variability in 137Cs erosion rates along transects is best replicated by sheet wash dominated simulations, suggesting that this is the dominant erosion process in this semi-arid landscape. The functional form of the sheetwash model can also explain our observations that 137Cs erosion rates decrease with upslope length (i.e. distance down slope) while its variability increases. However, sparsely vegetated sites, located in slightly drier locations, have higher erosion rates, and are not as accurately modeled as densely vegetated sites, suggesting that patchiness of vegetation introduces fine scale variability in erosion rates on these slopes.