第三极地区冰川径流研究进展

A review of glacier runoff studies in the Third Pole region

青藏高原是地球上除南北极之外冰川面积最大的区域,被称为地球“第三极”。全球变暖导致该地区冰川普遍退缩,融水释放成为冰川径流,使得下游河川径流发生重大变化,给下游流域水资源利用与管理带来挑战。然而由于第三极地区特殊的地形和复杂的气候,加上冰川水文过程内在的复杂性,使得冰川径流的研究十分困难。本文总结了目前关于冰川径流研究的几类主要方法:直接观测法、遥感观测法、水量平衡法、水化学示踪法和冰川水文模型法,其中冰川水文模型法使用最为广泛。在第三极地区,前人利用这些方法对于冰川径流的研究结果表明,自20世纪90年代以来,冰川径流普遍呈现上升趋势,但是其对于总径流的贡献同时受气候条件和流域内冰储量的影响,存在显著的空间差异;总体来看,位于西风控制区的流域的冰川径流贡献普遍大于季风控制区的流域。未来变化方面,除部分冰储量较大的西风区流域(塔里木河、印度河)外,第三极地区大多数流域冰川径流将在本世纪中叶前达到峰值。但是目前由于观测不足、模型物理机制简化等制约,对于第三极地区冰川径流的研究存在很大的不确定性,未来需要开展更多观测、开发更先进的冰川水文模型以提高第三极地区冰川径流研究的准确性,进而为该地区水资源利用与管理和防洪减灾工作提供科学依据。

The Tibetan Plateau and its surrounding mountains contain the largest amount of glaciers outside the polar regions and therefore is called the“Third Pole”(TP). The TP is very sensitive to climate change. In the context of elevation-dependent warming over the mountainous TP,river runoff has changed dramatically due to the accelerated glacier melting,bringing grand challenges to integrated water resources management over the TP and its downstream regions. However,it is very difficult to quantify glacier runoff owing to very sparse in-situ glacier-hydrology observations over the TP and the inherent complexity of glacio-hydrological processes.This paper reviewed the major research progresses and problems of glacier runoff studies in the TP region.Comprehensive literatures indicate that the research methods of glacier runoff can be roughly classified into five categories:direct discharge measurement,remote sensing monitoring,water balance calculation,hydrochemical tracers,and glacio-hydrological modeling. The glacio-hydrological modeling can be further divided into temperature-index and energy-balance methods. The modeling approach is the most widely used since it can systematically and quantitatively describe the glacio-hydrological processes and project future changes.Previous studies on glacier runoff based on these methods showed that glacier runoff has generally increased since the1990 s induced by atmospheric warming over the TP,but the contribution of glacier runoff to total river runoff is influenced by both climate and glacier storage in the basin. The results of the contribution of glacier runoff to total river runoff have significant uncertainties among different studies mainly due to the following reasons,e. g.,different methods used,distinct definition of glacier runoff,different meteorological inputs(especially the precipitation),and different glacier data(glacier extent,volume,and debris cover). In general,studies suggested that the contribution of glacier runoff to total river runoff in the westerlies-dominated region was greater than that in the monsoon-dominated region.As for the projections for future changes in the glacier runoff over the TP,the results varied widely among previous studies based on different climate change projection scenarios. The latest research indicates that glacier runoff in most basins will reach the peak before2050 and decrease after the peak except for some basins in the westerlies-dominated region with large glacier storage(e. g.,Tarim and upper Indus),based on the climate change projections in the CMIP6(Coupled Model Intercomparison Project Phase6)climate model ensemble.There are considerable uncertainties among the studies or literatures in the estimates of glacier runoff over the TP,mainly caused by sparse in-situ observations and highly-simplified models. Limited by available groundbased observations,most studies have used the temperature-index glacio-hydrological models for the TP basins,while the energy-balance models were applied to small basins. In addition,even the state-of-the-art energy-balance glacio-hydrological models do not fully consider some factors that affect glacier ablation,e. g. the glacier dynamics,debris covers,and black carbon. These factors may bring significant uncertainties to the simulated results. Therefore,we suggest that comprehensive observations and new advanced glacio-hydrological models need to be constructed and developed in the future studies;and,on the other hand,process-based multi-variable calibration/verification should be adopted in the glacio-hydrological modeling studies to improve the accuracy of glacier runoff studies over the TP. Doing so will provide a scientific basis for water resources utilization and management,flood prevention and mitigation in the downstream of TP.