| Due to the limited water resources and the mismatch between resources and demands in spatial and temporal distribution,the ecological environment in the arid and semi-arid regions of northwest China is threatened and gradually degraded.China implements ecological water conveyance projects to rationally allocate and unify the water resources in the basin to alleviate the severe situation of continuous ecological degradation in northwest China.This study takes the Aksu River basin,a typical oasis ecosystem in northwest China,as the study area,and uses the domestic GF-1 WFV high-resolution remote sensing images as the main data source,summarizes the spatial and temporal distribution and evolution characteristics of natural vegetation in the basin,and relies on the implementation of the Aksu River basin ecological water conveyance project to investigate the relationship between ecological water conveyance and Based on the implementation of the Aksu River Basin Ecological Water Transfer Project,we analyzed and discussed the ecological response of ecological water conveyance and changes in groundwater level,natural vegetation and natural lakes.Based on the spatial distribution characteristics of natural vegetation in the basin and its response characteristics with ecological water transfer,an optimization framework of ecological water transfer scheduling in the Aksu River basin is established.The main findings of this study are as follows:(1)Natural vegetation in the Aksu River basin is mainly concentrated in four focus areas of natural vegetation:AiXiMan Lake Wetland,DiYiShi Marginal Poplar Forest Area,Lower Basin’s Poplar Forest Area and WuTuan Marginal Poplar Forest Area,among which AiXiMan Lake Wetland is dominated by shrub distribution,while poplar forests and shrubs are widely distributed in the other three area.The DiYiShi Marginal Poplar Forest Area and Lower Basin’s Poplar Forest Area have the largest area of natural vegetation distribution.(2)The natural vegetation area change and transfer characteristics in the basin differed in two stages before and after 2017,and the annual NDVI values showed an increasing trend.From 2015 to 2017,the area of poplar forest and shrubs in the basin increased significantly,the cover types were mainly transferred from desert to shrubs and shrubs to poplar forest.From 2017 to 2020,the area change of natural vegetation in the basin tends to be stable,and there existed some natural vegetation in degradation,especially in the poplar forest area in the lower basin.The annual NDVI values for the four natural vegetation focus areas from 2015 to 2020 are on an increasing trend,with the most pronounced in the AiXiMan Lake Wetland and the DiYiShi Marginal Poplar Forest Area.(3)Natural vegetation showed a significant increase in the first year of ecological water conveyance,and a small decrease in the vegetation area in the following year of water conveyance compared with the first year,and tended to stabilize year by year.The interannual variation of the NDVI value of vegetation reflects the change of natural vegetation growth.The ecological water conveyance has a significant recovery effect on the poorly grown vegetation in the wetlands of Lake AiXiMan,and the natural vegetation growth in the DiYiShi Marginal Poplar Forest Area maintains a recovery trend with the implementation of the water conveyance.The effect of ecological water conveyance on natural vegetation is influenced by the amount of single water conveyance and the number of days of water conveyance.The larger the amount of single water conveyance,the more obvious the response of natural vegetation;the longer the number of days of water conveyance,the longer the duration of response of natural vegetation.At present,the lag time of natural vegetation response to ecological water conveyance in AiXiMan Lake wetland is about 1 month;the lag time of natural vegetation response to ecological water conveyance in the DiYiShi Marginal Poplar Forest Area is shorter,and the NDVI value of vegetation starts to rise significantly in the month when water conveyance starts.(4)The response of different natural vegetation to ecological water conveyance differed in interannual variation,seasonal variation and spatial variation,and the overall response of poplar forest to water conveyance was more obvious than that of shrubs,with a shorter lag time.At the same time,the closer the distance to the water outlet,the better the growth of shrubs;while poplar forests grew best within 100-500 m from the water outlet.The difference between them is mainly caused by the difference in the way the physiological characteristics of poplar and shrub plants adapt to the environment.(5)Ecological water conveyance has a significant effect on the groundwater level in areas with low average water level,and the higher the amount of water conveyance,the higher the groundwater level increase.The effect of ecological water conveyance on groundwater level and natural vegetation is the result of the interaction between the two.During the vegetation growth period,natural vegetation has a high water demand and strong water transpiration,and the groundwater level is at a low value.At the same time,there is an adaptation process of vegetation to the rise of groundwater level,the increase of groundwater recharge is beneficial to vegetation growth,but there may be a short-term water stress effect leading to vegetation growth retardation.(6)Ecological water conveyance has a significant restoring effect on the natural lake of Lake AiXiMan wetland.With the implementation of ecological water conveyance in Lake Eichmann wetland,the water surface area of Lake AiXiMan expands year by year,and there begins to exist a clear pattern of seasonal changes.The water surface continues to shrink from April to June,and then gradually expands from July to November,when the largest water surface of the year occurs.(7)Based on the spatial and temporal distribution characteristics of natural vegetation in the Aksu River basin and the response characteristics of natural vegetation and natural lakes in the basin to ecological water conveyance,the ecological water conveyance and ecological problems in the basin were fully identified.The natural vegetation and natural lakes within the focus area of natural vegetation in the basin were identified as the main restoration targets,and the ecological water demand of the AiXiMan Lake Wetland,the DiYiShi Marginal Poplar Forest Area,the Lower Basin’s Poplar Forest Area and the WuTuan Marginal Poplar Forest Area under the restoration targets were estimated to be 153 million m3,273 million m3,285 million m3,and 114 million m3,respectively.Based on the seasonal distribution of the ecological water stock in the basin and the seasonal changes in water demand for natural vegetation growth,it was determined that the time to carry out water conveyance in the basin is suitable during May-September,and a single or two water conveyance jobs can be carried out in a year.Finally,the water transmission paths were optimized and adjusted by means of canal network analysis,and a scientific and reasonable framework for optimizing ecological water transmission scheduling in the basin was established as a reference for the implementation of ecological water transmission in the Aksu River basin. |
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