A hybrid mechanism for the initiation and expansion of loess caves across the Chinese Loess Plateau

Loess caves are a distinctive and widespread landform on the Chinese Loess Plateau (CLP). Their formation contributes considerably to the soil erosion and land degradation; however, quantitative studies that improve the mechanistic understanding of their formation are still lacking. We aimed to explore the formation mechanism of loess caves and selected nine study sites across the CLP with different degrees of cave development and varied environmental conditions. We surveyed the study sites (similar to 9.6 km(2)) using an unmanned aerial vehicle (UAV). We mapped the loess caves using high-resolution UAV images and quantified their degree of development (i.e., density and area). We then analyzed the topographic characteristics of loess caves at individual sites and examined the environmental controls (i.e., climatic factors and soil properties) on the degree of loess cave development at different sites. The results show that loess caves are predominantly distributed in gullies and high topographic wetness index areas with clear topographic preferences, which suggests that cave initiation is accomplished by mechanical scouring. The average area correlated negatively with density but positively with saturated hydraulic conductivity, indicating loess cave expansion is a dissolution process caused by infiltrated water. Finally, we proposed a hybrid mechanism for loess cave development, where mechanical scouring dominates the initiation and dissolution expansion drives their expansion. Both initiation and expansion need, but compete for, surface runoff, and hydrologically relevant processes dominate loess cave development. Based on the non-linear relationship between loess cave development and precipitation (NDVI), we proposed that the impact of vegetation rehabilitation on loess cave development needs to be assessed on a sub-regional basis.