| Water is the indispensable material base for human survival and social economic development.The lake is an important form of surface water,accounting for more than 90%of the liquid fresh water resources on the earth's surface.Its spatial distribution reflects the regional difference and utilization of land surface water to some extent.On the other hand,the temporal and spatial changes of water resources also reflect the impacts of climate change,land surface processes and human activities on water cycle,material migration and ecosystem change.Global warming,coupled with strong human activity disturbances in recent years have result in the shrinking of many lakes,the decline of water level,the intensification of eutrophication,the reduction of biodiversity,the serious pollution,the deterioration of ecosystem function,and even the extinction of the ecosystem.Central Asia is located in the middle latitudes of the northern hemisphere and the hinterland of Eurasia,which is a typical arid and semi-arid region with scarce precipitation and strong evaporation in the world.Lakes play an important role in maintaining the balance of the ecosystem and promoting the development of regional economy.Under the background of global warming and with the rapid development of industrialization and urbanization in Central Asia,the development of human high-intensity water and soil resources has led to great changes in the ecological environment,such as water levels in rivers drop,lakes shrink or even disappear.And because of a series of disputes over water resources,it has created frictions between neighboring countries and regions,and has also posed a threat to regional stability and security.Therefore,the study of changes of lake water resources in Central Asia,the correct assessment of the impact of climate change and human activities on the lake,can provide scientific basis for the protection of water resources and the sustainable development of the ecological environment in arid areas.In this paper,30 lakes with larger area in Central Asia are selected,and the long time series monitoring data of lake level,lake area and lake volume are systematically studied,combined with temperature and precipitation data,the water level,water area,water volume of each lake and the response of land water reserves to the climate change in Central Asia were analyzed.The main content of this paper is as follows:(1)From 1992 to 2015,water level changes of 30 lakes in Central Asia were monitored using three kinds of radar altimetry data from Topex/Poseidon,Envisat/RA-2,and Jason-2.On the whole,water level changes of each lake are different in Central Asia during the period of 1992-2015.The water level of 17 lakes(accounted for about 57%)decreases in different degree,the water level of Aral-south Sea shows the most significant decrease(-0.447 m/a).The water level of 13 lakes(accounted for about 43%)increases in different degree,the water level of Ayakkum Lake shows the most significant increase(0.281 m/a).There are 9 terminal lakes located in the plain area exhibit different characteristics,some of them continue to decline,some of them continue to rise,and most remain stable.The seasonal variation of water level in 8 closed lakes in the alpine plateau region is obvious,but varies greatly during the year,while the inter-annual variation is small.The water level of the 13 exorheic lakes shows a general decline.The annual and inter-annual variations are very significant,and the extent of each lake decrease is also different.The water level change of lakes in Central Asia has obvious tendency from west to east and from big to small.The water level of most lakes in the central region in Central Asia has increased,and the water level of most lakes in the eastern and western regions has declined.(2)From 1992 to 2015,water area changes of 30 lakes in Central Asia were monitored using optical satellite Landsat TM/ETM+/OLI images as data sources.On the whole,the area of lakes is also shrinking,with a total decrease of 27494.872 km2,a decrease of 5.868%compared with the area in 1992.The water area of 15 lakes decreases in different degree,the water area of Aral-south Sea shows the most significant decrease.The water area of 15 lakes increases in different degree,the water area of Aral-north Sea shows the most significant increase.The water area of terminal lakes changes significantly,the area of Aral-north Sea has the largest increased proportion(17.368%),while the area of Aral-south Sea has the largest reduced proportion(-81.069%).The area of closed lakes in the alpine plateau region varies little and is relatively stable.There is no consistent regularity of the area change of the exorheic lakes,and the area of most lakes shows a trend of decline,both the number and the extent of lakes increased is small.The water area of most lakes in the central region in Central Asia has increased,and the water area of most lakes in the eastern and western regions has declined.(3)According to the change of water level and area of the lakes,the total variation of water volume of 30 lakes in Central Asia was calculated on the basis of empirical formula from 1992 to 2015.And combined with meteorological station data and human activity factors,the reasons of lake volume change were analyzed.From 1992 to 2015,the water volume of lakes in Central Asia decreased,with the total amount of 499.846 km3 and an average decrease of 21.732 km3 per year.The decrease of lake volume is increasing year by year,and more and more serious.Water volume change of terminal lakes is relatively great,in which the water volume of Sarykamyshskoye increases the most with the amount of 24.273 km.Water volume change of closed lakes is not great,most lakes remain relatively stable except the Ayakkum Lake which water volume changes the most with the amount of 10.479 km3.From the spatial distribution,the water volume of most lakes in the central region in Central Asia has increased,and the water volume of most lakes in the eastern and western regions has decreased.(4)From 2002 to 2015,changes of terrestrial water storage in Central Asia were estimated by using GRACE time-variable gravity and compared with the results of lake volume change.On the whole,the change of terrestrial water storage in Central Asia is decreasing,and the rate of decline is-0.318 cm/a.It shows a trend of increasing gradually from east to west,and has obvious seasonal variation regularity.TWS experienced a decreasing trend at the border of southern Tarim Basin and Tibetan Plateau,and it had an obvious increasing trend at parts of Junggar Basin at southwest of the Altai Mountains and the whole Aral Sea Basin,with the maximum decrease rate is-5.271 cm/a.(5)Water volume change of lakes in Central Asia is related to climate change and human activities.The increase of temperature will cause the increase of lake volume in the region with glacial distribution,such as Balkhash Lake near Tianshan Mountains.The water volume of lakes that without glaciers in the basin will decrease as the temperature rises,evapotranspiration increases,and the amount of precipitation decreases,such as some lakes on the Mongolia Plateau.The water volume of lakes in the region with dams and hydroelectric power stations is affected by human activities greatly,such as Hyargas Lake and Bosten Lake.Terrestrial water storage in Central Asia has negative correlation with temperature,and has positive correlation with precipitation.The increase of temperature caused the melting of glaciers in the northern part of the Tibetan Plateau,coupled with the increase of precipitation here,which eventually led to the increase of terrestrial water storage in the surrounding areas.The reduction of terrestrial water storage in the Tianshan Mountains is due to the rise in temperature and the decrease in precipitation.The reduction of terrestrial water storage in the Aral Sea basin is due to the excessive exploitation of water resources by large-scale agricultural development in the basin.(6)From 1992 to 2015,Balkhash Lake experienced the process of water level continues to rise and then decreased in a fluctuation way.Before 2006,the rise in water levels was greater than the decline in water levels after 2006,so the water level generally showed an upward trend with a total increase of 1.290 m in 24 years.The causes of water level change in Balkhash Lake are related to climate change and human activities,and the contribution of climate change is greater than that of human activities.From 1992 to 2015,Bosten Lake experienced the process of water level continues to rise and then continues to decline.The decline range of water levels is larger than the rise range of water levels,so the water level generally showed a decline trend with a total decrease of 1.912 m in 24 years.The causes of water level change in Bosten Lake are related to climate change and human activities.The increase of water level and volume are more related to climate change,the decline in water levels and the continued decrease in water volume are inextricably linked to the excessive use of water resources by mankind. |
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