Is raindrop-induced erosion controlled by the combined effects of soil internal and external forces?

Soil internal forces, including electrostatic, hydration, and Van der Waals forces, are underlying mechanisms responsible for aggregate breakdown and subsequent soil erosion. However, whether these forces consistently initiate raindrop-induced erosion remains unclear. This study aims to elucidate the influence of soil internal forces on raindrop-induced erosion by conducting aggregate stability and simulated rainfall experiments using two soils (Heilu soil and Aeolian sandy soil) from the Loess Plateau. Electrolyte solutions with different concentrations (from 10^–5 to 1 mol L^–1) were employed as soaking solutions and raindrop materials to quantitatively regulate soil internal forces. Our results indicate that an electrolyte concentration of 10^–2 mol L^–1 was the critical point for net pressure (NP), which varied greatly when the electrolyte concentration reduced from 1 to 10^–2 mol L^–1. In this case, the aggregate stability, splash erosion mass (SEM), and cumulative loss mass (CLM) of Heilu soil increased rapidly with decreasing electrolyte concentrations. A significant correlation was also observed between NP and mean weight diameter (MWD) (r = –0.909), SEM (r = 0.821), and CLM (r = 0.806), respectively (p<0.01). However, the MWD, SEM and CLM of Aeolian sandy soil remained unchanged with varying electrolyte concentrations, as it lacks an effective aggregate structure and rarely underwent the “breakdown” process during wetting. Therefore, except structureless soils, raindrop-induced erosion typically is initiated by soil internal forces and then driven by raindrop impact and overland flow. This study enhances our understanding of the driving mechanism of rainfall-induced erosion, providing a theoretical foundation for developing targeted soil erosion prevention strategies.