新疆水循环变化机理与水资源调蓄

MECHANISM OF WATER CYCLE CHANGES AND IMPLICATIONS ON WATER RESOURCES REGULATION IN XINJIANG UYGUR AUTONOMOUS REGION

干旱区水资源紧缺,研究气候变化与人类活动双重营力作用下水循环变化机理,制定合理有效的调蓄对策,是应对全球变化的重要任务。新疆作为全球极端干旱区之一,近期水循环发生了显著变化。作为中国最西端及中亚地区的水资源发源地,天山发挥了中亚水塔的作用,其周边干旱区广泛分布,人类生存与生态系统受水资源变化影响十分强烈。近十年来,笔者在天山山区及其周边的塔里木盆地、准噶尔盆地等地开展了大气-地表-地下的多尺度水循环研究。本文从降水形成条件、地表水变化、地下水对地表水变化的响应等不同视角对水循环进行系统分析,探讨在气候变化与人类活动影响下水循环发生变化的机理及其水资源效应。在此基础上,提出干旱区水资源的调蓄途径。研究表明,青藏高原和天山隆升在新疆产生雨影效应,新疆大气降水的水气来源以西风带水气为主,本地再循环水气在降水构成中仅占8%,区域造雨能力低下。双重因素长期作用导致新疆地区干旱化。天山山区与盆地相比,更有利于形成降水,加上冰雪储备丰厚,因此在区域水循环中占主导地位。在气候变化的影响下,气温升高、降水增加、冰川融化加速、主要河流出山径流平均增加约10%,对于气候变化敏感的一些冰川河,增加可达40%以上,造成洪水灾害。下游荒漠中的绿洲,基本上没有降水,水循环由地表水主导,河道断流导致地下水位下降,引发生态退化。塔里木河调水,在流域尺度上证明了这一点。天山山前地带的绿洲农业,是区域用水最多的环节,因而,也是调蓄水资源的有利位置。利用山前戈壁带有利的地质结构和快速循环的地下水,地下水库工程符合干旱区水循环基本规律,相对于平原水库具有明显优势。天山南北符合条件的储水构造很多,可调蓄水量可观。这一地表水地下水联合调蓄的模式,将成为干旱区应对环境变化带来的水资源问题的重要途径。

Water cycle changes and impacts on water resources in arid regions is a central issue as environmental change and adaptation is concerned.Using Xinjiang Uygur Autonomous Region in western China,one of the driest regions in the world as an example,we have investigated this issue in the past decade.The study area included major catchments,e.g.Tarim and Jungar basins surrounding the Tianshan Mountains.The investigation was at the catchment scale.Methodologies have included precipitation monitoring,analysis of hydrological observation data,as well as Hydrogeological testing.Results show that,caused by the rain shadow effect by the rising of Himalayan and Tianshan mountains,moisture source of precipitation comes solely from the westerlies; recycled moisture takes up only 8％ in the local precipitation,incapable of forming large rainfall; the dryness of the region has been caused by both effects above.Tianshan Mountains dominate the regional water cycle,due to its higher precipitation,rich ice and snow storage in the glaciers.Under climatic changes,both of air temperature and precipitation have increased and glacier melting has speeded.As a result,since 1996 as compared to the mean of previous 40 years,runoff from the mountains has increased by 10％ on the average and up to 40％ for some rivers sensitive to climatic change,causing heavy flooding.Water cycle in the lower reaches of the rivers,on the other hand,is controlled by surface water,due to the fact that there is essentially no rainfall there.Groundwater-dependent ecosystem has deteriorated seriously as a consequence of dried out river channels.The water diversion project in Tarim River was just a unique groundwater recharge test at a very large scale,serving as a good proof of such mechanisms,evidenced clearly by Tritium and stable isotopes.Oasis at the mountain front,where water resource is heavily utilized,is the most suitable location for water resources regulation as an adaptive measure towards environmental changes.Making use of the 10％ increase in mountain runoff,groundwater recharge to aquifers in the foreland basins can be enhanced through infiltration ponds,and the aquifers can serve as reservoirs to store and supply water for agriculture when needed.Drawing from the wisdom of ancient Karez systems,the groundwater reservoir built in Tailan river basin is a successful pilot.Studies show that the groundwater there has secured modern recharge.Water relocation at the catchment scale is necessary to ensure the stability of ecosystems in the lower reaches of the rivers,and the Tarim River case has indicated that,the range of affected groundwater is limited within a few hundreds of meters from the riverbank,though the usefulness of the water diversion project was positively assessed.