Hydrological Modeling And Calibration In High-Mountain Cryospheric Regions

Snow and glacier melting and accumulation are important processes of the hydrological cycle in the cryosphere,e.g.,high-mountain areas.High-mountain areas have significant and rapid hydrological response to global climate change.The Tibetan Plateau,known as the Earth's Third Pole and Asian's water towers,is a typical highmountain cryospheric region,characterized by complex terrain and lack of observational networks that have largely limited our understanding of hydrological processes over this region.This study uses the Yarlung Zangbo River basin in the southern Tibetan Plateau as an example.First,a snow and glacier melt module based on a degree day factor is coupled with a distributed hydrological model(CREST)to simulate hydrological processes of the study basin.Second,multisource remote sensing data in combination with in situ flow data is used to force and calibrate the developed model e.g.,precipitation and land surface temperatures.Third,a progressive two-stage calibration strategy is developed to derive model parameters,i.e.,(1)snow melting processes(stage I)and(2)glacier melting and runoff generation and routing using multisource data(stage II).StageI calibration is performed using the MODIS snow cover area(SCA)product and a blending snow water equivalent(SWE)product combined with partial in situ measurements.Stage-II calibration is based on Gravity Recovery and Climate Experiment(GRACE)satellite-derived total water storage(TWS)changes and discharge observed at a gauging station of the lower reach of the basin.Results indicate that the developed two-stage calibration method provides more reliable streamflow,snow(both SCA and SWE),and TWS change simulations against corresponding observations than commonly used methods based on streamflow and/or SCA performance.The simulated TWS time series shows high consistency with GRACE counterparts for the study period 2003-2014,and overestimated melting rates and contributions of glacier meltwater to runoff in previous studies are improved to some degree by the developed model and calibration strategy.Snow and glacier runoff contributes 10.6% and 9.9% of the total runoff,and the depletion rate of glacier mass is ~-10.0 mm/a over the study basin.These results correct for the overestimation of melting rates of ice and snow in previous studies over the Yarlung Zangbo River basin.This study is valuable in examining the impacts of climate change on hydrological processes of cryospheric regions and providing an improved approach for simulating more reliable hydrological variables over the Yarlung Tsangpo basin and potentially similar regions globally.