Responses of runoff and sediment yield to the source-sink-path pattern in slope-gully system in the hill and gully Loess Plateau region, China

A slope-gully system is one of the basic units in the soil/water loss and governance in watersheds. Among them, the runoff and sediment can greatly contribute to the restoration and reconstruction of the ecological environment in soil erosion areas. The runoff and sediment yield can also depend mainly on the pattern of sediment sources, sinks, and transport paths in the slope-gully system. But it is still lacking in this field. In this study, unmanned aerial vehicles (UAV) remote sensing and geographic information systems were used to determine the distribution and quantitative indicators of sediment source-pathway-sinks in different slope-gully systems. The monitoring data of rainfall and runoff was combined with the sediment yield of five slope-gully systems in the Fangta small watershed from 2016 to 2022, including two with no human disturbance (SG1 and SG2) and two with grazing disturbances (SG3 and SG4) of slope-gully systems with restored vegetation and natural vegetation, one slope-gully system with orchard and sloping cultivated land (SG5). A systematic investigation was made to clarify the properties of runoff depth and sediment yield in five slope and gully systems, together with their responses to sediment source-pathway-sink patterns. The results show that the sediment yield modulus in SG1 and SG2 was significantly lower than that of the rest (P<0.05), and the maximum runoff depth and sediment yield were found in SG3 under rainfall events. The average annual runoff depth of SG1 and SG2 was significantly lower than that of SG5 (P<0.05), and the average annual yield was significantly lower than that of the rest (P<0.05). The runoff depth was significantly positively correlated with the maximum patch index both of the sediment source and sink area in the slope-gully system, valley land, and gully land (P<0.05), respectively. In addition, the degree of correlation between the sediment source and sink index and the depth of runoff in the valley were greater than those in the gully land. The runoff depth also showed a significant negative correlation (P<0.05) with the path number density and the bifurcation ratio in the total slope-gully system, and the path gradient both in the valley and the gully land. There was no significant correlation between the sediment yield modulus and each index of source and sink, but there was a significant positive correlation (P<0.05) between the path length density and the path bifurcation ratio in the valley land. In addition, the increasing path complexity also significantly reduced the runoff depth and sediment yield modulus. In short, the appropriately increasing complexity and transport distance of sediment transport paths can be expected to effectively reduce the soil and water loss in slope-gully systems. This finding can provide scientific support for the prevention and control of soil erosion in the slope and gully systems, the ecological protection, and the restoration of watersheds.