Seasonal soil water origins and determinants in an alpine hillslope on the northeastern Qinghai-Tibet Plateau

Understanding the seasonal origins of mobile soil water (MSW) is critical for assessing the response of water movement to freeze-thaw, particularly in vulnerable alpine hillslope. However, the dominant factors determining the seasonal origins of MSW across different spatiotemporal scales remain poorly understood. To identify these origins under different freeze-thaw stages and topographic conditions, field samples were collected from the south, north, and valley regions of the northeastern Qinghai-Tibet Plateau from May to September 2022. We analyzed the dominant factors influencing the seasonal origins of MSW across topographies using a random forest regression model and explored causal relationships among factors through a structural equation model. Our results reveal dynamic changes in the seasonal origins of MSW and highlight key influencing factors under varying dry and wet conditions due to topographic heterogeneity. Specifically, (1) uneven seasonal precipitation, combined with substantial summer rainfall, results in summer precipitation accounting for 81 % of MSW replenishment. As melting progresses, the contribution from winter precipitation increases from 11 % in June to 23 % in September, indicating greater winter input at the watershed outlet in later seasons; (2) shallow MSW is replenished by both recent and antecedent summer precipitation, while deep MSW is primarily sustained by antecedent summer precipitation, emphasizing the significant role of summer precipitation in shallow MSW; and (3) under low soil water content (south slope), dynamic climate factors such as relative humidity and precipitation significantly influence precipitation infiltration, making them critical for determining the seasonal origins of MSW. In contrast, under high soil water content conditions (north slope and valley), static topographic heterogeneities influence water pathways, thus playing a dominant role in the seasonal sources of MSW.