Study On Vegetation And Hydrological Response Of Ecological Water Conveyance In Tarim River Basin

Tarim River is the longest inland river in China,with the dual characteristics of rich natural resources and fragile ecosystem.Under the background of global warming and large-scale human water and soil exploitation,the ecological environment of the Tarim River basin is facing important challenges,and ecological water conveyance is an important way to promote the restoration of degraded ecosystems in the arid area basin.Therefore,it is of great significance to quantitatively evaluate the effect of ecological water conveyance on ecological restoration and optimize the ecological water conveyance mode for ecological restoration of the Tarim River.Therefore,based on MODIS data and field survey data,the temporal and spatial variations of key elements(species diversity,groundwater depth,NDVI,TVDI and RSEI)in the Tarim River Basin in the past 22 years were studied,constructs the evaluation system of ecological environment quality in the Tarim River basin,and comprehensively evaluates the restoration status of the damaged desert forest ecosystem and the effect of ecological water conveyance.The main conclusions are as follows:(1)From 2000 to 2021,the drought degree in the Tarim River Basin showed a mitigation trend.The proportion of pixel area with a decreasing trend of drought degree in the basin after ecological water conveyance is 56.2%,which is greater than the proportion of pixel area with an increasing trend of drought degree.The whole basin is still in moderate drought(0.57<TVDI≤0.76),accounting for up to 31.7%.After the ecological water conveyance,the groundwater level shows an overall upward trend.Among them,the groundwater uplift rate of the Kuergan section in the lower reaches of the Tarim River is the largest,which is 0.0348 m/month.The ecological water conveyance greatly alleviated the soil drought degree of the basin and effectively raised the groundwater level on both sides of the river.(2)The ecological water conveyance project is conducive to individual vegetation restoration and community species diversity improvement.Taking the monitoring data of Luopu County in Hotan River Basin as an example,compared with the non-water conveyance area,the crown width of Populus euphratica in the water conveyance area is nearly 500%higher,while the branch height is reduced by 28.3%,and the endangered Populus euphratica is reduced by 52.2%.The density of new Populus euphratica seedlings per100 m~2 in the water conveyance area reaches 8,and no new Populus euphratica seedlings are found in the non-water conveyance area.From the perspective of the whole basin scale,under the normalized ecological water conveyance mode,the Simpson index and Shannon-Wiener index of the water conveyance area have significantly increased compared with the initial stage of the ecological water conveyance,but with the increase of the frequency of water conveyance,the level of diversity began to decline and then tended to stabilize.(3)From 2000 to 2021,the vegetation coverage of the Tarim River basin showed an overall increasing trend.After the ecological water conveyance,80.5%of the pixel area with an overall increase and change trend of vegetation coverage in the basin is larger than the area with a decrease trend of vegetation coverage.And the area with extremely low vegetation coverage is gradually transformed into medium vegetation coverage area and high vegetation coverage area.The surface runoff formed by ecological water conveyance stimulated the germination of soil seed bank,increased the species and quantity,and restored and revitalized the desert riparian forest dominated by Populus euphratica and Tamarix chinensis,so the vegetation coverage was improved to a certain extent.(4)From 2000 to 2021,the ecological environment quality of the Tarim River Basin showed a better trend.After the ecological water conveyance,the proportion of the area where the ecological environment quality of the Tarim River Basin showed an overall upward trend was 59.0%,which was greater than the proportion of the area where the ecological environment quality showed a decreasing trend,and it is shown that the areas with medium and poor ecological environment grade are gradually transformed into areas with excellent and good ecological environment grade.The overall ecological environment quality of the Tarim River basin is medium,and the pixel area with medium ecological quality(0.2<RSEI≤0.4)accounts for the highest proportion,reaching 37.3%.After the ecological water conveyance,the groundwater level in the water conveyance area rose significantly,the degree of soil drought was alleviated,and the vegetation coverage was significantly improved,so the ecological environment quality was significantly improved.(5)The correlation between FVC and groundwater depth,TVDI and RSEI in the Tarim River Basin is significant.FVC and TVDI in the main stream of Tarim River,Aksu River,Yeerqiang River and Hotan River are significantly negatively correlated at the level of p=0.01,with correlation coefficients of-0.725,-0.587,-0.758 and-0.689,respectively.From 2003 to 2020,the annual average of groundwater depth in the Qiala-Yingsu and Yingsu-Taitema Lake sections was significantly positively correlated with FVC at the p=0.01 level,and the correlation coefficients were 0.942 and 0.957,respectively.FVC and RSEI in the main stream of Tarim River,Aksu River,Yeerqiang River and Hotan River are significantly positively correlated at the level of p=0.01,with correlation coefficients of 0.658,0.877,0.738 and 0.573 respectively.