Defining lateral subsurface flow and identifying its water sources in an alpine-permafrost hillslope

Abstract

Climate change has resulted in significant changes to subsurface hydrological processes in permafrost regions. Lateral subsurface flow (LSF) represents the dominant flow path in hillslope runoff generation. However, the contributions of runoff components to LSF, such as precipitation, soil water, and ground ice, remain unclear. This study aimed to characterize LSF generation processes in an alpine permafrost hillslope of Northeastern Tibetan Plateau, using stable isotopes and total dissolved solids (TDS) as tracers. Samples of precipitation and soil water [including mobile soil water and supra-permafrost groundwater (SPG)], LSF, and ground ice samples were collected from different thaw depths of the active layer in 2021. The results showed that LSF came directly from SPG in the active layer. Two-source partitioning using δ2H or TDS suggested that the dominant source of LSF gradually shifted from ground ice during the initial thaw period to precipitation with increasing thaw depths. The contributions of ground ice to LSF were 70 % and 30 % at thaw depths of 0–30 cm and >30 cm, respectively. The results of three-source partitioning indicated ground ice, precipitation, and SPG to be the dominant sources of LSF at thaw depths of 0–30 cm, 30–150 cm, and >150 cm, respectively. SPG largely regulates hillslope hydrologic processes at thaw depths ≥250 cm. Therefore, with continuing climate warming, SPG will play an increasing role in hydrological processes of alpine meadow permafrost hillslopes.