Differences in soil water content and movement drivers of runoff under climate variations in a high-altitude catchment

The interconnected soil water content and movement processes are ill constrained for changing climates, making estimates of surface runoff relatively unpredictable. Particularly for high-altitude areas, which are characterized by vertical climate variance, changing climatic conditions raise many challenges to runoff estimates and ecosystem management. To address these challenges, soil property investigation, hydrological modelling, and multivariate statistical analysis were integrated to quantify the driving forces of surface runoff in three typical sub-basins (headwater, highland, and subtropical valley) in the Tibetan Plateau under climate variations. The results showed that soil water processes in the subtropical areas with high precipitation levels and in the headwaters with deep freeze-thaw processes were very sensitive to climate variations. For the headwater sub-basin, the soil water content was the highest (<= 90 mm), but the percolation and lateral flow were the lowest (>= 14.3 mm and >= 173.9 mm, respectively) among the sub-basins. During the period 1972-2017, the annual surface runoff of the sub-basins decreased as follows: headwater > highland > subtropical valley, which corresponded to the increase in precipitation and temperature during this time period. Furthermore, the redundancy analysis showed that percolation was the most powerful governing process in the subtropical areas and the headwaters. Although the soil water content contributed less than percolation, it was a substantial driver of surface runoff in the headwater and highland sub-basins due to the freeze-thaw process. Our results suggest that it is critical to prioritize soil water content in freeze-thaw areas and subsurface flow in warmer valley areas to improve soil and water resource regulation. These findings provide vital information from a systematic perspective for hydrological, environmental and ecological management in high-allude areas.