Macroscopic and multi-scale estimation of karst underground soil loss: toward a subsurface USLE

Underground soil loss (USL), the net removal of eroded sediment from the land surface to underground space, is a difficult-to-measure and puzzling phenomenon that typically occurs in karst areas. Compared to surface soil loss (SSL), USL has been largely ignored in soil loss assessments utilizing Universal Soil Loss Equation (USLE)-type models due to the absence of credible prediction methods. Here, we propose a framework to estimate USL locally, regionally, and globally as a first step toward developing a subsurface USLE, namely, the Underground Soil Loss Equation, as a counterpart to the surface USLE. We find that USL is a relatively minor but indispensable component of soil loss in karst areas, and its contribution increases with spatial scale and large-scale pathways. Globally, USL accounts for 5% of total soil loss and 8% of erosion-induced carbon fluxes, corresponding to an annual financial loss of $21 billion. In karst areas, the annual average soil loss and its resulting carbon fluxes would be underestimated by approximately 25% when the USL is excluded. Using a USL-included framework, we identify soil loss threats among countries with karst landscapes. Low-income countries suffer high SSL rates and should be wary of aggravated USL with insufficient conservation measures. High-income countries tend to have higher USL rates and are more likely to underestimate soil loss if USL is ignored. Middle-income countries, which account for over 3/5 of karst area and 3/4 of USL amount, should be cautious of higher SSL and USL rates as well as significant risks of underestimating soil loss. Our findings emphasize the importance of incorporating USL into soil loss-relevant modeling and assessments to more accurately identify pertinent threats and offer more strategic approaches in monitoring and conservation.