Study on the runoff characteristics of grassland and shrub slope in typical hydrological years in the Amidongsuo sub-watershed of the Qilian Mountains

The runoff generation process on slopes in the permafrost region of alpine areas is jointly regulated by precipitation, freeze-thaw cycles, and the underlying surface structure, exhibiting significant phase-specific and spatial heterogeneity. This study focused on the grassland and shrub slopes in the Amidongsuo sub-watershed of the Babao River Basin in the Qilian Mountains, and based on the monitoring data from September 2023 to August 2024, explored the changing characteristics of runoff generation patterns on alpine slopes during the freeze-thaw alternation under the background of a “normal and slightly abundant year”. The results show that: (1) Frequency analysis of the 41-year precipitation sequence at the Qilian Meteorological Station indicates that the observed annual precipitation is 467.73 mm, with a recurrence period of approximately 4.2 years, which is a common hydrological event. (2) There are significant differences in the runoff generation components between the two slopes. The shrub slope is dominated by interflow (accounting for 96.62 % of the total runoff), while the grassland slope is dominated by surface runoff (accounting for 53.4 % of the total runoff); the total runoff volume of the shrub slope is 38.13 times that of the grassland slope. (3) Runoff generation is restricted during the freezing period, and surface runoff rapidly increases during the thawing period, with the contribution of stratified runoff changing in phases as the freeze-thaw process progresses. (4) Slope runoff generation can be classified into three typical patterns: overland surface runoff (triggered by short-term heavy rain, typical event on June 16-17, 2024), shallow saturated surface runoff (caused by continuous precipitation in the early stage, even weak rain can generate runoff, typical event on August 29-30, 2024), and interflow. The discrimination basis comes from the coupling characteristics of rainfall intensity, soil moisture/temperature time series, and runoff response. The study reveals the slope runoff generation patterns jointly controlled by “precipitation-freeze-thaw-underlying surface type”, which can provide observational basis for the simulation of alpine slope hydrological processes and the assessment of water source conservation in the Qilian Mountains; at the same time, it points out that the conclusions are primarily applicable to the background of a “normal and slightly abundant year”, and further verification and promotion with multi-year sequences and extreme year-type data are still needed.