Subsurface erosion by soil piping: significance and research needs

Soil erosion is not only a geomorphological, but also a land degradation process that may cause environmental damage affecting people's lives. This process is caused both by overland and subsurface flow. Over the last decades, most studies on soil erosion by water have focused on surface processes, such as sheet (interrill), rill and gully erosion, although subsurface erosion by soil piping has been reported to be a significant and widespread process. This paper presents a state of art regarding research on soil piping and addresses the main research gaps. Recent studies indicate that this process (1) occurs in almost all climatic zones and in the majority of soil types, (2) impacts landscape evolution by changing slope hydrology, slope stability and slope-channel coupling, (3) is controlled by various factors including climate and weather, soil properties, topography, land use and land management. These issues are illustrated with various case studies from around the world. However, the majority of the reviewed studies used surface methods for soil pipe detection, although soil piping is a subsurface process. Surface methods, such as geomorphological mapping, may underestimate the piping-affected area by 50%. Moreover, most studies are limited to few case studies without presenting thresholds for soil pipe development in different environments. Subsurface erosion by soil piping is not represented in currently used soil erosion models. Therefore more research is needed to better understand the morphology and connectivity of soil pipes, their subsurface catchments, as well as soil erosion rates by piping in different environments. Knowledge of thresholds that induce erosion in pipes and subsequent initiation of gullies may help to improve models of hillslope hydrology and soil erosion that include pipeflow and piping erosion. The investigation of soil piping also requires improved methods that allow to better predict pipe development and collapses, and thus to detect piping-affected areas. Studies dealing with effective prevention and control measures of soil piping are scarce. Addressing these research gaps will help to improve our insights into subsurface erosion by soil piping, and thus help to better understand landscape evolution and hillslope hydrology, as well as to develop and improve effective piping erosion control techniques and strategies.