冰雪的融化和积累过程是寒区水文过程中重要的一环，在全球气候变化背景下, 寒区具有显著的水文响应特征。有“世界第三极”和“亚洲水塔”之称的青藏高原作为高原寒区的典型代表，其复杂的地形条件和观测资料的匮乏，限制了人类对该地区独特水文过程的研究。本研究以青藏高原南部的雅鲁藏布江流域为研究区，针对高原寒区水文特点，为分布式水文模型CREST添加冰川和积雪冻融模块，扩大CREST模型的应用范围，使其可以在高原寒区等复杂水文条件下，进行水文过程模拟，探究在气候变化条件下，青藏高原南部雅鲁藏布江地区的水文响应机制。然后进一步联合采用遥感和实地观测资料，解决研究区内缺少降水、温度等实地观测数据对水文模拟的限制，为缺资料地区的水文模拟研究，提供一种以多源遥感数据为基础的高精度解决方案。在综合利用遥感数据的基础上，本研究提出了一种新型多源分步式水文模型率定方案：算法的第一步采用遥感反演的积雪面积和雪水当量数据，为融雪模型的率定过程中的多目标优化提供参考数据；算法的第二步采用重力卫星GRACE反演的总水储量数据和径流数据，为冰川冻融过程和汇流过程提供率定依据。研究结果显示，分步式率定方法的第一步中，积雪面积和雪水当量的模拟效果相互制约，可以在一定程度上减小融雪模型的过度拟合的可能性，使模型模拟的积雪面积和雪水当量表现处于平衡状态，为寒区水文过程模拟提供更加准确的数值模拟结果。分步式率定算法的第二步中，加入GRACE反演的总水储量信息后，在径流模拟效果仅有小幅度降低的条件下，大幅度地提高了冰川冻融的模拟效果。新的模拟结果显示，2003年到2014年间，雅鲁藏布江流域的积雪和冰川融水对于总径流的贡献比例分别为10.6%和9.9%，研究区内的冰川质量亏损速率大约为10.0 mm/年，纠正了之前在同一地区的研究中冰雪融水的高估情况。本研究中所采用的遥感数据具有广泛的空间覆盖，因此文中的研究方法亦可以推广到其他类似缺资料地区，为寒区水文过程模拟和研究水文循环机制提供了一种新的有效方法。
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 high-mountain 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). Stage-I 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.
