Soil Freeze/Thaw Dynamics Strongly Influences Runoff Regime in a Tibetan Permafrost Watershed: Insights from a Process-Based Model

Abstract

The Tibetan Plateau (TP) is a region rich in extensive frozen ground and the source of many major Asian rivers. However, how soil freeze/thaw (F/T) dynamics influence runoff production at the catchment scale in the TP under warming is poorly understood. This study employs a process-based permafrost hydrology model with a new soil parameterization to investigate soil F/T dynamics and their impact on runoff in a permafrost watershed located in the central TP, i.e., the source region of Yangtze River (SRYR). The new parameterization accounts for the influence of soil gravel and organic carbon content, as well as variation in saturated hydraulic conductivity along the soil profile. Validation results demonstrate that the modified model accurately simulates soil temperature (mean RMSE of 1.3 ℃), soil moisture (mean ubRMSE of 0.05 cm3 cm-3), and daily runoff discharge (NSE>0.82). The results reveal a pronounced soil warming trend across the entire SRYR, with different altitudinal patterns of permafrost and seasonally frozen ground (SFG). Warming rates in the SFG area increase monotonously with elevation, while a turning point is observed in the permafrost region around 4800 m. With active layer deepening, deep-soil water storage increases, but the rootzone and the middle part of the active layer become drier. Increased liquid soil moisture resulting from enhanced soil thawing primarily replenishes soil water storage rather than directly contributing to runoff recharge. Runoff production is more affected by soil F/T cycles in the permafrost region than in SFG, especially during the freezing period. Delayed soil thaw onset is associated with a higher spring runoff coefficient, while delayed soil freeze onset is linked to slower runoff recession. A longer freezing zero-curtain period often leads to a discontinuous runoff recession process by affecting the connectivity of groundwater flow channels. These findings uncover the regulatory mechanisms of soil F/T dynamics on runoff production and river discharge characteristics, providing a fundamental basis for predicting permafrost hydrology responses to future climate change in the TP.

Keywords: Runoff regime, Permafrost hydrology, Tibetan Plateau, Runoff recession, Freeze/thaw