Impact of soil freeze-thaw mechanism on the runoff dynamics of two Tibetan rivers

Soil freeze-thaw is typically not fully considered in quantifying the hydrology of seasonally frozen catchments located, for instance, on the Third Pole. We investigate the role of soil water content and freeze-thaw state on the runoff dynamics of the headwaters of the Yellow and Yangtze rivers, both situated on the eastern Tibetan Plateau. A version of augmented Noah land surface model (LSM) allowing reliable simulation of key hydrometeorological processes over the Tibetan Plateau is employed and further validated using measured monthly discharge records. From measurements supported by the Noah LSM simulations for more than thirty years (1979-2010), we deduce an annual hysteresis loop, viz. a time lag between measured/simulated runoff and precipitation for both catchments. Our simulation results with the augmented Noah LSM further demonstrate that annual anticlockwise (or clockwise) hysteresis loops are also observed for the liquid soil water (or soil ice). We infer from the LSM simulations that the amount of water stored in the soil is the factor driving the hysteresis between runoff and precipitation, whereby the state of the stored water plays a crucial role in the seasonality of the runoff regime. Further analyses illustrate that inclusion of soil freeze-thaw model physics effectively increases the thermal inertia of the soil column that dampens large variations of soil temperature and turbulent heat fluxes. These findings highlight the importance of soil freeze-thaw for the hydrology and runoff regime across the High Asia's rivers as well as the need for a thorough understanding of this process to generate reliable projections.