The Study On The Relationship Between Vegetation Change And The Drive Forces In The Midstream Of Heihe Mainstream Basin

Vegetation evolution characteristics,as an important part of the world's terrestrial ecosystem evolutionary process,get more attention in recent years.The driving mechanism of vegetation change has become an important research subject.Water resources are scarce in the Heihe river basin due to the rare rainfall.Water that is diverted upstream and consumed midstream implies less water downstream.Oasis is mainly located in the midstream between enlarged irrigated agriculture in the upstream and degraded natural ecosystem in the downstream.With the population surge,rapid development of city economic,water consumption is excessive.Therefore,it's necessary to increase the study of hydrological cycle,and reasonable exploitation and utilization of water resources is needed.Great progress has been made in previous works on the response of the vegetation on the basic elements such as precipitation,runoff,groundwater,which are mainly about the interrelationships in time series,but without further detailed analysis in the spatial pattern.In this article,with the Heihe river middle reaches as the study area,the normalized difference vegetation index(NDVI)as the research index,the inter annual variation of vegetation is analyzed.It focuses on the influence of hydrological processes on vegetation change.The vegetation evolution characteristics and driving mechanism of Heihe river middle reaches area in the study area during the recent 15 years are analyzed via GIS technology,landscape ecology method and mathematical statistics method.Main results are as follows:(1)The top three land use areas in the study area are the desert Gobi,farmland and grassland.From 1990 to 2010,the artificial surface area increased by 31.48%.,which is mainly distributed in zhangye city,linze county and the administrative region in Gaotai county;Second,farmland area,which increased by 20.74%,is mainly distributed in the coastal area of Heihe river and its south;Wetland reduced by 22.96%.It is the biggest decline,and the distribution is along the river on both sides.(2)Landscape pattern change can been seen through the landscape pattern index.In terms of landscape,from 1990 to 2010,the landscape developed from the integrity to the fragmentation.As for the type,spatial distribution of landscape elements types tends to be balanced,and the bonding patches of the desert Gobi and farmland was still the most dominant type of patch.The shape of farmland landscape patches changes simply and regularly,which is intensively interfered by human activities.And the shape changes of the artificial surface patches are more complex and scattered.(3)In the past 15 years,the annual average temperature change was little in the study area,and it showed a trend of rise with fluctuations.As for precipitation,it rose with fluctuations before 2007,declined after 2007.The runoff of both yingluoxia and zhengyixia appears rising trend.It remained stable after 2001,and had slight fluctuations and gentle rising after 2006.During the period of 2000-2013,the groundwater level continued to decline,and in the northeast and southeast of the oasis,it decreased significantly,especially in Linze.The decline gradually decreased from southwest to northeast.(4)Through the statistics of the Yearbook Data,the total population and urban population in the past 20 years showed a significant upward trend,the rural population showed a significant downward trend,and the urbanization rate increased sharply.GDP and per capita GDP in general is on the rise.After 2000,groundwater irrigation increased,and the surface water irrigation decreased with volatility.In terms of irrigation amount,surface water irrigation always exceeds the amount of groundwater irrigation.(5)NDVI displayed a rising trend from 2000 to 2015,and the vegetation coverage condition was getting better.From the point of view of spatial distribution,the most significant increase of average annual NDVI in the study area occurred in the north of Heihe River,and it is the region where it curred accounted for 66.82%.(6)In the relationship between vegetation changes and driving factors,temperature,precipitation and runoff have no significant impact on vegetation,but relatively,the response of the natural vegetation on temperature is more sensitive than precipitation.Runoff mainly affects the vegetation on both sides of the river,and it is hardly related to the vegetation coverage.In general,NDVI has a significant negative correlation with groundwater.the,Groundwater recharge increases while water supply capacity gets weak.Near the river,NDVI has significant positive correlation with ground water.The main water supply way of vegetation changed into river supply,where groundwater has little impact.NDVI has a higher correlation with groundwater irrigation than surface water irrigation,and Igroundwater irrigation has a greater influence on natural vegetation.Human factors also play a role that cannot be ignored in the vegetation change.The research on the dynamic change of vegetation in the oasis desert ecosystem and its driving mechanism will provide a theoretical basis for the decision-making of water resources allocation in Heihe River Basin,the optimization of oasis ecosystem and the sustainable development of regional ecosystem.