Cryospheric-hydrologic modeling and prediction of a mountainous catchment in the northeast Tibet Plateau

Increased attention directed at cryosphere hydrology is prompted by climate change. In spite of an increasing number of field measurements and modeling studies, the impact of cryospheric change, especially of frozen soil, on hydrological processes at the catchment scale is still largely unclear. Traditional frozen soil hydrology models have mostly been developed based on a “bottom-up” approach, i.e. by aggregating prior knowledge at point scale, which is an approach notoriously suffering from equifinality and uncertainty. Therefore, in this study, we explore the impact of frozen soil at catchment-scale, following a “top-down” approach, implying: expert-driven data analysis -> qualitative perceptual model -> quantitative conceptual model -> testing of model realism -> future prediction. The complex mountainous Hulu catchment, northeast of the Tibet Plateau (TP), was selected as the study site. Firstly, we diagnosed the impact of frozen soil on catchment hydrology, based on multi-source field observations, model discrepancy, and our expert knowledge. Two new typical hydrograph properties were identified: the low runoff in the early thawing season (LRET) and the discontinuous baseflow recession (DBR). Secondly, we developed a perceptual frozen soil hydrological module, to describe the LRET and DBR properties. Thirdly, based on the perceptual model and a landscape-based modeling framework (FLEX-Topo), a semi-distributed conceptual cryosphere-hydrologic model (FLEX-Cryo) has been developed, considering all cryospheric factors, including glacier and snow accumulation/ablation, and soil freeze/thaw processes. The results demonstrate that the FLEX-Cryo model can represent the effect of soil freeze/thaw processes on hydrologic connectivity and groundwater discharge and significantly improve hydrograph simulation. Furthermore, its realism has been confirmed by alternative multi-source and multi-scale observations, particularly the freezing and thawing front in the soil, the lower limit of permafrost, and the trends in groundwater level variation. In the end, we used the FLEX- Cryo model to predict the impacts of future climate change on hydrology, including the glacier retreat, the decreasing snow cover area, and permafrost degradation. The FLEX- Cryo model is a novel conceptual cryosphere-hydrologic model, which represents these complex processes and has potential for wider use in the vast TP and other cold mountainous regions.