How do the distribution patterns of exposed roots affect the rainfall-runoff processes of sloped land under simulated multi-rainfall conditions in karst region?

Exposed root system is used as a reference of quantifying the erosion rate on slopes, but its influence mechanism of exposed root on rainfall-runoff processes during rainfall is still unclear. This study aimed to investigate the effects of exposed root distribution patterns on the rainfall-runoff processes of sloped land. Specifically, we achieved it by conducting eight multi-rainfall events under rainfall intensity of 75- and 50-mm h-1 and fixed slope gradient of 25 on the flume (1.0 m long, 0.5 m wide and 0.3 m deep) packed with yellow soils with different exposed root distribution patterns (vertical slope arrangement, parallel slope arrangement, transverse slope arrangement) on sloped land. The results revealed that during the 1st rainfall, the initial runoff time of surface, subsurface and underground runoff was shorter on parallel and vertical slope arrangement compared to transverse slope arrangement. During subsequent rainfall, surface and subsurface runoff was reduced by 5-15 times in comparison to the 1st rainfall. Furthermore, the amount of surface runoff increased in the order of parallel slope arrangement > vertical slope arrangement > transverse slope arrangement from the 1st to 4th rainfalls, while the amounts of subsurface and underground runoff decreased. On average, the amounts of surface, subsurface and underground runoff for parallel slope arrangement accounted for 61.77 %, 29.33 % and 8.90 % of the total runoff amount, respectively, whereas for vertical slope arrangement, it accounted for 57.27 %, 26.81 % and 15.92 %, and for transverse slope arrangement, it accounted for 55.78 %, 29.71 % and 14.52 % of the total runoff amount. Our findings suggested that the distribution patterns of exposed root had a significant impact on rainfall-runoff processes. Specifically, a vertical slope arrangement was conducive to rainfall infiltration, a parallel slope arrangement resulted in more surface runoff, and a transverse slope arrangement could reduce water loss, although its effect decreased with increasing rainfall frequency. In this way, the presence of exposed roots in topsoil created a rough surface that could affect the onset of flow during the initial stages of rainfall and modify the flow direction and distribution once it commenced. The results of this study would be critical in enhancing our understanding of the relationship between plant roots and hydrological processes in karst region.