Estimation And Characteristic Analysis Of Vegetation And Lake Ecological Water Demand In Aksu River Irrigation District

For a long time,the utilization of water resources in arid areas in northwestern China has been dominated by industrial and agricultural production water and domestic water,ignoring the amount of water required by the ecological environment.The Aksu river irrigation area,located to the west of the main stream of Aksu river,is a typical arid oasis agricultural area with superior agricultural production conditions and highly developed irrigation agriculture.The agricultural water consumption in the irrigation area accounts for 98.22%of the total water consumption in the irrigation area.Agricultural water is seriously squeezing ecological water,environmental problems such as vegetation degradation and groundwater level decline have emerged.Therefore,it is of great significance to rationally define the ecological water demand and its spatial and temporal distribution characteristics in irrigated areas,to provide scientific basis for the rational allocation of water resources and the allocation of ecological water demand in the irrigated areas,and to maintain and improve the ecological environment in the irrigated areas.Taking the Aksu river irrigation area as the research area,based on remote sensing image data,meteorological data and groundwater level data.Using ArcGIS software as a platform to analyze the spatial-temporal changes and characteristics of the land use type structure,the water area of Aiximan Lake and the groundwater level during the past two decades,the ecological water demand classification and composition objects were determined based on above analysis.Using area quota method,submerged evaporation method,vegetation evapotranspiration method and water balance method to reasonably define the ecological water demand of vegetation and lakes,and analyzing the spatial and temporal distribution characteristics of the current annual ecological water demand.The main conclusions are as follows:(1)From 1996 to 2018,the changes in land use types in the Aksu River Irrigation District were relatively complicated overall.In general,the area of cultivated land,construction land and other land increased,while the area of forest land,grassland and water area decreased.Cultivated land has the largest increase in area,mainly converted into forest land,grassland and other land.The area of forest land decreased the most,mainly transferred to cultivated land and grassland.The grassland area changed little,mainly in the transfer of forest land and other land,and the main transfer out is cultivated land and other land.The conversion of land use types in irrigation areas was strongly interfered by human activities.The area of cultivated land had expanded significantly.Natural forest land and grassland had shown a tendency to degrade,and the degradation of forest land was particularly serious.Since the "Twelfth Five-Year Plan",under the influence of the ecological protection policies and ecological restoration projects in the Aksu region,the grassland area has been restored.(2)From 1996 to 2018,the water area of Lake Aiximan shrank seriously,and the water area shrank from 61.57km2 to 20.86km2.The changes in the water area of Lake Aiximan experienced three stages of rapid decline,slow decline and slow recovery.From 1996 to 2005,the rate of lake shrinkage was the fastest,the intensity of lake change was-6.57%;from 2005 to 2015,the rate of lake shrinkage slowed down,the intensity of lake change was-1.11%;from 2015 to 2018,the lake water area has recovered,but it is still in a state of severe atrophy.The water area changes of Lake Aiximan showed obvious spatial differences.The changes in the south,southwest,southeast,and north directions were the most dramatic,other directions are relatively slow.(3)From 2000 to 2018,the groundwater level in the Aksu river irrigation area showed a significant downward trend,with the average groundwater depth falling from 2.56m to 3.80m.The groundwater depth in the southeast,southern and central regions of the irrigation area was relatively shallow,the water level changes were small,and the groundwater level was stable;in the west and north,the groundwater depth was deep,and the groundwater level drops significantly.The spatial difference of groundwater depth was significant,showing the spatial differentiation of southeast shallow and northwest deep.(4)The objects of ecological water demand in the Aksu river irrigation area are vegetation and lakes in the natural oasis ecosystem.The current annual vegetation ecological water demand of the Aksu river irrigation area is 3.441-4.012×108m3,and the minimum lake ecological water demand is 0.225 ×108m3;under the restoration target,the vegetation ecological water demand is 4.838-5.724×108m3,and the minimum lake ecological water demand is 0.666×108m3.The current annual ecological water demand shows obvious characteristics of spatial and temporal differentiation,and the ecological water demand of vegetation and lake is concentrated from April to September,with the highest in June and July,seasonal summer>spring>autumn>winter,spring and summer are the seasons with the largest gaps in ecological water demand;The high-value areas of vegetation ecological water demand are concentrated in the southern,southwest and southeastern regions of the irrigation area,and the vegetation ecological water demand is most concentrated within the range of 0-2km along the river,and shows a trend of decreasing with increasing distance from the river,the vegetation ecological water demand within 6km along the banks of the rivers is as follows:Kashgar River>Yeerqiang River>Hotan River>Aksu River.The lake ecological water demand of Aiximan Lake is concentrated in the north,northwest,southeast and southwest directions.