Hydrological vs. mechanical impacts of soil water repellency on erosion

Soil erosion is a major concern for both agricultural and natural resources. Soil water repellency (SWR) is known to hinder wetting of soils, decreasing infiltration of water and thus increasing overland flow—the driving force for erosion. These hydrological impacts of SWR on erosion, are quite well established. In contrast, the mechanical impacts of SWR, namely on the resistance to erosion, are poorly understood. Here, we provide a critical review of the current understanding of both the hydrological and mechanical impacts of SWR on erosion. Analysis of compiled experimental data provides contradictory evidence: an increase in erosion with increasing SWR in some cases, versus a decrease in others, with a strong dependency on the mechanism (weather, fire, or pollution-induced SWR). We offer a plausible explanation for this contradiction—that the net erosional impacts of SWR depend on the balance between its hydrological and mechanical effects on erosion, and exemplify this in a simple 1D slope model. Our simulations illustrate the dual nature of SWR's influence on soil erosion, and explain the diversity of published data. Finally, we identify research gaps and suggest ways to address them.