Spatiotemporal Evolution Analysis Of Cropland And Populus Euphratica In Weigan River Basin In Recent 60 Years

Impacted by dramatic policy-driven human activities,accompanied by water resources regulation and climate changes,agriculture in the Weigan River Basin developed sharply and the cropland expanded rapidly,in which circumstances natural forests were greatly damaged and occupied,especially the populus euphratica.The ecological security of the region has been seriously threatened.The extent of cropland and populus euphratica in the Weigan River Basin from 1960 s to 2019 were obtained based on the multi-source remote sensing data.Under the combined influence of complex historical background conditions and environmental changes,cropland expanding was closely related to populus euphratica disappearing.At the same time,based on the development and utilization of water resources in arid regions and the necessity of "water-based land" brought by water scarcity,the Weigan River Basin is regarded as a relatively closed water cycle system,on account of the principle of water balance and results of remote sensing information extraction.In addition,the water resource utilization structure in the region was analyzed from multiple perspectives,such as surface runoff,groundwater,ecological water consumption,and domestic water,based on hydrometeorological data such as regional runoff,precipitation,groundwater,and air temperature,and statistical data such as population,GDP,and livestock.Moreover,under the condition of ensuring natural ecological water use of populus euphratica and other areas,the available agricultural water consumption has been estimated and a suitable scale evaluation system of cropland under water resources constraints has been constructed.Thereby,the assessment of cultivated land suitability and its development potential has been realized,and suggestions of suitable cultivated land development and agricultural water conservancy facilities have been given.The main conclusions are as follows:1)Cropland in the Weigan River Basin expanded constantly and distributed from scattered to integrated,while the area of populus euphratica decreased and scattered significantly.Area of cropland increased by 307.79% from 1340.73 km2 to 5467.31 km2,and that of populus euphratica overall decreased 1482.73 km2 in the past 60 years.Meanwhile,aggregation index(AI)of cropland increased by rate of 0.18 per decade while that of populus euphratica decreased by rate of 0.5 per decade.In addition,in the transition from populus euphratica in every period,the cropland was always the target,and the disappeared populus euphratica converted to cropland accounted for more than 90%.The change process was divided into four stages:(1)disafforestation of populus euphratica distributed among the human habitats in the slow period of agriculture development(1960s-1977);(2)logging populus euphratica with restraints in the exploration period of agricultural development(1977-1990);(3)large-scale reduction of populus euphratica distributed along the main stream of the Tarim River in the rapid period of agriculture development(1990-2015);(4)the areas of populus euphratica decreased relatively slightly in the transition period of agriculture development,when equal attention was gradually paid to quantity,quality and the benefit of cropland(2015-2019).Generated by the great pressure from human disturbance caused by population growth,development of the social economy and highly intensive use of resources,populus euphratica wanes while cropland waxes.The natural populus euphratica along the Tarim River are highly vulnerable to damage for developing cropland,which ought to be strictly protected.2)The water in the Weigan River Basin mainly comes from surface runoff and available precipitation.The surface runoff accounts for more than 60% of the incoming water,and the precipitation accounts for about 20%-30%.In addition,the inflow,crop planting structure and water-saving irrigation are the main factors affecting the estimation of the suitable area of cropland.In 1990,the actual area of cultivated land was not much different from its suitable area and reached a relatively saturated state while the actual area of cultivated land had significantly exceeded the limit of suitable area of cropland in 2000.The level of cropland was unstable,and the expansion of cropland affected the ecological environment balance;with the adjustment of agricultural planting structure and the increasing popularity of water-saving irrigation,the actual cropland area in 2010 and 2017 is much smaller than the suitable area of cropland under the current year's water supply.Cropland in the Weigan River Basin has a relatively great development potential.