The Benefit Evaluation Of Artificial Ecological Water Conveyance In Recovery Of Degraded Grassland Ecosystems In The Lower Reaches Of Tarim River

The lower reaches of Tarim River,climate of which is extremely dry,belong to warm temperate zone.Because of the arid desert climate,the zonal vegetation is warm shrubs and semi-shrubs desert in the lower reaches of Tarim River.Due to the supply of river water and groundwater,there is a large area of low-level meadow vegetation,which was dominated by weeds,leguminous grass and intermediate trees,on both sides of the river.Dominant species include Phragmites communis,Alhagi sparsifolia,Kareliniacaspia,Halogeton glomeratus and so on.In addition,there are many species of trees and shrubs such as Tamarix ramosissima and Populus euphratica.The construction of Daxihaizi reservoir leaded to the disconnection of river way in 1972.The degeneration of herbaceous plants was very serious,such as the decrease in species diversity and coverage.The growth of the trees and shrubs was also declined.The country began to implement the ecological treatment project of Tarim River Basin in 2000,and ecological water conveyance in the lower reaches of Tarim River was carried out.However,it is difficult to evaluate the ecological water conveyance efficiency of degraded grassland system because of lack of long-term monitoring and accumulation of grassland and tree-shrub species,which lead to a lack of reference for water resources allocation and water conveyance in subsequent water conveyance.Therefore,the evaluation of ecological water conveyance efficiency of degraded grassland system has important scientific guiding significance for water resources management and rational utilization in the lower reaches of Tarim River.Based on the above,the research object of this paper was focuses on the herbaceous plants in the lower reaches of Tarim River,and based on long-term field monitoring data for herbaceous plants,trees and shrubs,combining with data of remote sensing,the changing of ecological characteristics of herbaceous plants as the main line of research,and some methods and models such as Diversity index methods,Biomass methods,Calculus methods,GAM,Principal component analysis were used,the changes of plant community ecosystem service value based on grassland ecosystem has been analyzed since the ecological water conveyance,and what’s more,the influence of artificial water conveyance on local ecological economy was discussed.The results as follows:After 15^thecological water conveyance,the recharge of river water to groundwater was about 20.45×10⁸m³,which was about 43.98%of river water;the recharge of river water to soil water was about 40.18%.The water of herbaceous plants mainly comes from soil water,and availability of groundwater was very low;the water of T.ramosissima and P.euphratica mainly come from groundwater.The key factor affecting the plant ecological characteristics in the lower reaches of Tarim River is groundwater depth.Herbaceous plants showed the most significant response to the changes of groundwater depth.The coverage of herbaceous plants decreased gradually,and the species diversity index increased first and then decreased with the increase of groundwater depth.Species diversity of the herbaceous plant community is significantly reduced when the groundwater depth is more than 6 m.The biomass of T.ramosissima and P.euphratica decreased with the increase of groundwater depth.Ecological water transport promotes the growth of herbaceous plants,and the growth of T.ramosissima and P.euphratica were also improved.The average coverage of herbaceous plants increased by 204.51%,and species diversity index and evenness index increased significantly.The coverage and crown breadth of T.ramosissima increased by 38.45%and 164.56%respectively.The canopy density and crown breadth of P.euphratica increased by 191.3%and 336.5%respectively,and the under-branch height decreased by 7.44%.The response of herbaceous biomass to ecological water delivery is the most significant,and the responses of T.ramosissima and P.euphratica biomass to ecological water transport were slow relatively.From 2000 to 2016,increment of ecological service value of herbaceous plants in the lower reaches of Tarim River was0.611×10⁸yuan,and the increase rate was 144.73%;Increment of ecological service value of T.ramosissima was 3.93×10⁸yuan,and the increase rate was 14.76%;Increment of ecological service value of P.euphratica was 3.93×10⁸yuan,and the increase rate was 73.21%;Maintenance value of biodiversity was 0.856×10⁸yuan.From 2000 to 2015,on Kaogan section,the ratio between input value of ecological water and output value of herbaceous plant communities was 1:5.63.In2016,in the lower reaches of Tarim river,potential distribution area of herbaceous plants was 9 158.17 km²,and the potential service value was 6.10×10⁸yuan;Potential distribution area of T.ramosissima was 6 243.61 km²,and the potential service value was 156.96×10⁸yuan;Potential distribution area of P.euphratica was 234.51 km²,and the potential service value was 3.88×10⁸yuan.From 2000 to 2016,the ratio between input value of ecological water and output value of vegetation restoration was 1:3.3.From 2000 to 2016,the ecosystem service value of the lower reaches of Tarim River was increased by 69.57%;The ecological environment index was increased by40.67%;Ecosystem stability of herbaceous plants raised gradually,but it was still in a state of instability until 2016;The index of ecological economic benefit increased by246.88%.Ecological water conveyance has promoted the increase of groundwater depth in the lower reaches of Tarim River.The coverage and diversity index of herbaceous plants increased,so as the area of grassland.The recruitment in ecological service value of herbaceous plants is the highest,and the service value of T.ramosissima and P.euphratica increased to some extent.The value of regional ecosystem services has been increased by ecological water conveyance,and the ecological environment has improved,the stability of herbaceous ecosystem has increased,and the ecological economy has also increased.