The Research On Oasis Ecological Evolution Of Shule River Basin

The research on remote sensing monitoring and driving mechanism of ecological evolution in arid and semi-arid areas has always been the key contents of globe change research.Shule River Basin,which located in the westernmost part of Hexi corridor,is the border area between Northwest China Arid and Semi-arid Area and Qinghai-Tibet Alpine and cold region.Low precipitation but high evaporation of Shule River Basin result in sparse vegetation and fragile eco-environment.All the factors make it a sensitive area responding to globe climate change.Meanwhile,this area has been a vital Commodity Grain Base of Gansu province.Since 1980s,more than 100 thousand ecological immigrations have moved here by Gansu Provincial Government,which leads to rapid expansion of cropland and agricultural irrigation water consumption,aggravated conflict between human society and natural ecosystem,as well as a series of ecological environment problems.Nevertheless,the studies on Shule River Basin are weaker compare to Shiyang River Basin and Heihe River Basin,which are belongs to Hexi corridor region.Therefore,it is critical to strengthen the research on ecological evolution of Shule River Basin.Given the facts above,we se-lect this basin as study region.Based on the product data from remote sensing image(such as Landsat,MODIS,SPOT,NOAA)and statistical information of natural and social economy-related,this article give an analysis on the spatio-temporal character-istics of LUCC and Vegetation index variation in Shule River Basin since 1980s.The driving mechanisms of ecological evolution are also discussed by quantitative and qualitative analysis from natural and humanistic aspects and main conclusions are as follows.From 1987 to 2015,in the middle and downstream of shule River Basin,the area of cropland,construction land and shrubland had a net increase,while high coverage grassland(HCG),medium coverage grassland(MCG),low coverage grassland(LCG),wetland and non-vegetation land all presented a net decrease.The rate of land-use and land-cover change(LUCC)in the period of 2000?2006 and 2006?2010 were relative higher,but the rate of LUCC in other periods were much lower.The change intensity of MCG and HCG are in the first place,the next are LCG,shrubland and wetland,Construction land and cropland are in the third place,non-vegetation land was the last one.The pattern of regional LUCC was generally stable,except for cropland loss,the gain/loss change of other land-use/land-cover types were always in an active state.The main sources of cropland expansion were HCG,MCG,LCG and shrubland,the main target that cropland transferred out was construction land.The main sources of LCG increase were MCG,HCG and wetland,the main target that LCG transferred out were MCG,HCG,shrubland,and cropland.The main sources of MCG increase and the main target that MCG transferred out were HCG,LCG and shrubland.The main sources of HCG increase and the main target that HCG transferred out were MCG,shrubland and wetland.The main sources of shrubland and wetland increase and the main target that shrubland and wetland transferred out were HCG,MCG and LCG.The main sources of non-vegetation land increase were LCG and wetland,the main target that non-vegetation land transferred out was LCG.The main source of con-struction land expansion was cropland.The areas that land use types had changed ac-counted for 38.18%of study region,more than 10%of which were moderate and high intensity conversion since 1987.As for spatial distribution,there were few changes in the old irrigated area within the oasis.The regions where the land use and land cover had changed mainly concentrated in the oasis fringe area,natural vegetation cover ar-ea and emigrant arrangement regions.As revealed by comparative analysis,there is a strong consistency among Land-sat,MODIS and NOAA data,while SPOT data is poor for comparability;therefor SPOT data is not suitable for Shule River Basin.In recent 30 years,there was a gen-erally improving trend for vegetation growth in Shule River Basin,while natural veg-etation showed degradation trend during 1980s?mid-2000s,and it was worst in the 1990s,an improving trend appeared after the mid-2000s.The NDVI of cropland in the middle and downstream of Shule River Basin exhibited a rapid improving trend dur-ing 1980s?mid-1990s and mid-2000s?mid-2010s,while presented a dramatic degra-dation from mid-1990s to mid-2000s.The degraded areas in the middle and down-stream of Shule River Basin were mainly distributed in the surrounding area of Qiaozi village.In addition,Beishihe area,which located in the east side of the basin,and Dunhuang East Lake,North Lake and West lake,which located in the west side of the basin,all showed scattered distributed vegetation degradation as well.The distribution of degraded areas in the upstream was relatively sporadically.Vegetation improving areas were widely distributed in the basin and relatively concentrated in the surround-ing area of emigrant arrangement regions in Huahai,Changma,and Shuangta irrigated area.The trajectory of vegetation change in the upstream was consistent,which exihi-ted a fluctuant rising trend with discontinuity that occured in the period of 1993?1995 and 2000?2001,during which NDVI decreased firstly and then increased rapidly.The trajectory of vegetation change in the middle and downstream was relatively complex,while most areas experienced vegetation improving in early stage and vegetation deg-radation in later stage of the research period.What was different is that the starting and ending time of vegetation degradation in different regions were not consistent.Vegetation degradation in the western side of the basin mainly occurred in the period of mid-1990s?mid-2000s,while for the eastern regions,vegetation degradation hap-pened during 1990s?mid-2000s.The vegetation in the surrounding area of Qiaozi vil-lage was always in the continuous degradation state.From 1980 to 2015,the surface temperature of Shule River Basin presented a rising tendency,and the warming amplitude in the upstream was slightly lower than that in the middle and downstream.The warming in the upstream mainly occurred in autumn and winter,while that primarily appeared in spring and summer in the middle and downstream.Annual high temperature days showed an increasing tendency,while spring low temperature days exhibited a decreasing trend.Annual precipitation gener-ally presented a fluctuant increasing tendency,while less precipitation was seen dur-ing 1980s to 1990s,and the precipitation increased gradually since 2000,meanwhile the amplitude of variation was also increased.The increasing amplitude of annual precipitation in the upstream was larger than that in the middle and downstream.The increase of annual precipitation in the upstream mainly occurred in summer and au-tumn,while that primarily appeared in autumn in the middle and downstream.Sun-shine hours in the upstream exhibited a decreasing tendency,while that in the middle and downstream presented a fluctuant increasing trend.Mean wind speed,high wind days and annual air relative humidity all presented a decreasing tendency.Annual evaporation generally exhibited an increasing trend,while decreased during 1980?1993 and increased during 1994?2015.Annual runoff from upstream generally presented an increasing tendency.From 1980 to 1994,the annual runoff of Changmabao hydrometric station showed a decreaing tendency while Dangchengwan presented an increasing trend.On the contrary,from 1994 to 2015,Changmabao ex-hibited an increasing tendency while Dangchengwan showed a decreaing trend.Since 1986,the agricultural population in the middle and downstream of Shule River Basin grew rapidly,with a net increase of 78.90 thousand people,of which Guazhou county and Yumen city exhibited the largest growth.The non-agricultural population in the middle and downstream increased slowly,with a net gain of 14.10 thousand people,of which Yumen city showed a continuous decreasing tendency since 1995,with a net loss of 66.6 thousand people.The cropland area expanded dramatically from 2005 to 2014,with a net increase of 474.4 thousand mu,which mainly distributed in Guazhou county and Yumen city.Agricultural planting structure developed towards the trend of water saving,the planting proportion of grain Crops decreased continuously,while that of economic crops increased constantly.The confined farming of livestock such as cattle and sheep has gradually replaced traditional grazing,which basically released the negative influences that caused by Livestock husbandry development on natural vegetation.Agricultural development policies,ecological policies,migration policies,watershed management and urban and rural development policies all played important effects on land use change and ecological evolution in the study region.From 1996 to 2006,the irrigation water consumption of Shuangta,Changma and Huahai irrigated area cumulatively increased by 5.82 billion m;at the same time,the annual runoff of the Changmabao hydrometric station cumulatively increased by 7.22 billion m3.Therefore,compared with 1990s,the Shule River agricultural development project did not put addional irrigational pressure on the local eco-environment.From 1987 to 2015,the surface soil moisture in the middle and downstream primarily exihibited a decreasing trend and the reduced area mainly concentrated in the natural vegetation areas,which were far from the cultivation area,of which the surrounding area of the Qiaozi village and Dunhuang East Lake were most siginificant.In the last 30 years,the vegetation change in the upstream of Shule River Basin was mainly affected by annual mean surface temperature,annual precipitation,high temperature days,strong precipitation days and high wind days.Among them,the first three played a positive role,the latter two play a negative role,and the annual surface temperature and annual precipitation had the strongest influence,and almost at the same level.The driving factors of natural vegetation change in the middle and down-stream were not the same in different counties,of which the cropland area,annual precipitation and strong precipitation days were the global driving factors,while the cropland area had the largest and positive effect,and this indicated that human activi-ties had more influence on local vegetation growth than natural factors.In addition,the annual precipitation and strong precipitation days also promoted local vegetation growth and the role of strong precipitation days was more significant than annual pre-cipitation in some local areas.Although annual mean surface temperature and annual accumulated temperature had great contribution values,they only played a role in lo-cal areas.The shrinking of wetlands,the decline of groundwater level and the degra-dation of vegetation in the surrounding areas of Qiaozi village were mainly caused by the river closure and water storage after the completion of Changma reservoir.It can be concluded from our analysis that the vegetation changes in the upstream of Shule River Basin are mainly controlled by natural factors such as temperature and precipitation,while the changes in the middle and downstream are mainly affected by human activities and precipitation,and human activities have greater influence.Bene-fited from the continuous increase of precipitation and runoff from the upstream of Shule River Basin,the newly developed cropland with a mount of more than 400 thousand mu in the middle and downstream did not exert globally negative influence on the natural vegetation growth,as vegetation degradation appeared only in local ar-ea.It should be noted that the runoff of Shule River will return to normal state once the climate change bonus disappeared,then the large scale irrigation will be difficult to maintain,and natural vegetation will be inevitably confronted with the risk of deg-radation.Therefore,the local Watershed Management Authority should seize the op-portunity that Shule River is in high-flow period,steer the transfer of agricultural population actively,hold the quantity controlling of cropland restrictively,promte the industrial transformation,optimize the allocation of water resources,improve the ag-ricultural water use efficiency,strengthen the watershed management,carry out eco-logical construction vigorously,and ultimately realize the harmony between human and nature as well as the sustainable development of society and economy.