Parent materials regulate the effects of rainfall intensity and slope gradient on interrill erosion processes in the mountainous canyon regions of Southwest China

Abstract

Soil erosion constitutes a critical threat to the sustainability of soil and water resources. The mountainous canyon regions of Southwest China are typified by diverse parent material types, pronounced slope gradients, and hydrologically complex rainfall regimes, however, the regulatory mechanisms by which parent materials modulate rainfall intensity and slope gradient effects on erosion remain poorly elucidated. In this study, rainfall simulation experiments on the microplots were conducted on three typical parent materials (sandstone, basalt, and limestone) under two rainfall intensities (45 and 90 mm h⁻¹) and three slope gradients (10°, 20°, and 30°). The runoff coefficient (Rc), sediment concentration (Cs), sediment yield rate (Sy) and effective size characteristics were determined to unravel the interrill erosion processes in the mountainous canyon regions of Southwest China. A three-way ANOVA with categorical factors (parent material, rainfall intensity) and slope gradient as an ordinal variable was applied to quantify the main and interactive effects. The Rc, Sy, and sediment particle size were primarily influenced by rainfall intensity (F = 75.18–391.98, p < 0.001), followed by parent material type (F = 59.90–265.40, p < 0.001), and least by slope gradient (F = 8.67–29.44, p < 0.001). Rainfall intensity exerted a significant positive effect: higher rainfall intensity increased the Rc, Cs, and Sy, with the magnitude of these effects decreasing in the order of sandstone > basalt > limestone. Sediment transport for sandstone was dominated by suspension/saltation (< 0.25 mm), whereas for basalt and limestone, both suspension/saltation (0.25–1 mm) and rolling mechanisms (> 1 mm) were observed. Clay particles, cation exchange capacity (CEC), and exchangeable cations (mainly exK⁺ and exCa²⁺) were strongly correlated with erosion processes under varying rainfall intensities, suggesting that parent materials influence interrill erosion primarily through aggregate stability.