Evaluating the effect of vegetation type and topography on infiltration process in an arid mountainous area: Insights from continuous soil moisture monitoring network

Infiltration processes in mountainous areas are complex and play a crucial role in runoff generation and ecological services in drylands. However, the combined effects of vegetation type and topography on infiltration processes are poorly understood due to the difficulties in monitoring. In this study, based on a long-term (7 years) continuous soil moisture monitoring network in the Qilian Mountains of China, we analyzed the characteristics of infiltration indicators in barren, grassland, shrub and forestland across three slope gradients (gentle, moderate and steep slope gradients), explored the relationship between wetting front depth/velocity and cumulative infiltration, and identified the key environmental factors on infiltration indicators. Results showed that matrix flow events accounted for the largest proportion of events (49 %), followed by no response events (33 %), with preferential flow events being the least (18 %). In barren and grassland, wetting front depth, wetting front velocity, and cumulative infiltration are greatest at moderate slope gradient, gradually decreasing with increasing slope gradient in shrub, with little difference in forestland. In both matrix and preferential flow events, the wetting front depth exhibited a linear positive relationship with cumulative infiltration, whereas the wetting front velocity showed a nonlinear relationship with cumulative infiltration. In addition, preferential flow contributed about 24 % to the cumulative infiltration. Soil properties are the most important factors influencing the overall infiltration process across slope gradients. Our findings highlighted the important role of preferential flow in infiltration processes despite their small proportion and demonstrated the need for incorporating preferential flow in the simulation of hydrological processes in arid mountainous areas.