River hydrology studies in Southeast Xizang:Progress and challenges
Southeast Xizang is one of the areas with the highest precipitation,runoff production,sediment yield,and transport on the Qinghai-Xizang Plateau and is also one of the areas most sensitive to climate change.The hydrological processes of the rivers in this area are not only affected by regional climate change.However,they are also closely related to changes in the underlying surface conditions,such as vegetation,glaciers,snow,seasonally frozen ground/permafrost,glacial lakes,and occasional natural geo-hazards,such as landslides and debris flows.An in-depth understanding of regional river hydrological characteristics,runoff production,sediment yield,and transport mechanisms,the main influencing factors,and clarification of runoff/sediment regime changes and future trends in the context of global climate change have important scientific value and practical significance for the rational development and utilization of flow energy and water resources in Southeast Xizang,(hydro-)engineering safety and long-term use,as well as scientific mitigation and control of natural geo-hazards(e.g.,debris flow and mountain torrents).Since the 1950s(especially during the past 50 years),comprehensive field scientific investigations,such as the Qinghai-Xizang Plateau scientific expeditions and the unremitting efforts of several generations of scientific researchers,relatively deep explorations,and studies on climate,hydrology,physical geography,geology,and geomorphology in southeastern Xizang have been conducted.This review systematically summarizes the main progress in river hydrology research in Southeast Xizang,including regional climate change and its impact,runoff,sediment characteristics,and changes.It discusses also the main challenges of current research and certain aspects that should be strengthened in the future.Over the past 70 years,the temperature in southeastern Xizang has increased significantly.Although there is no unified conclusion on the interannual variation trend of precipitation,analysis based on the observational data of surface weather stations indicates a weak,increasing trend.The glaciers in southeastern Xizang retreat at an accelerated rate,and the shrinkage is much greater than in other Qinghai-Xizang Plateau areas.The area and number of glacial lakes in this region have increased,increasing the potential risk of glacial lake outbursts.Southeast Xizang has high runoff and sediment production on the Qinghai-Xizang Plateau.Approximately 2/3(～125 billion m3)of the runoff and more than 90％ of the sediment(suspended sediment～190 million tons)at the Pasighat in the lower reaches of the Yarlung Tsangpo River originate from this area(especially the bend area of the Yarlung Tsangpo Great Canyon centered around the Namcha Barwa and Gyala Peri peaks),the runoff and sediment dynamics in this area hence determine the water and sediment flow in the lower reaches of the Yarlung Tsangpo River to a considerable extent.Runoff in South Xizang is generally dominated by precipitation,but the proportion of meltwater is high(up to ～70％)in glacial-intensive watersheds,such as the main tributaries,Parlung Tsangpo and Yigong Tsangpo.Over the past 60 years,runoff in Southeast Xizang has shown an increasing trend but with significant fluctuations.Under future climate change scenarios,the runoff in this region is estimated to show an increasing trend,and sediment production and transport trends are similar but with large uncertainties.The meteorological and hydrological monitoring in Southeast Xizang is still weak,and flow and sediment data are scarce,which is a basic problem that future regional river hydrological research should strive to overcome and solve.
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