科研之友
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摘要
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 the 1990s 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 before 2050 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 Phase 6）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 ground-based 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.
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