| Wetlands in arid regions possess ecological functions such as carbon storage,water purification,maintenance of biodiversity,and inhibition of oasis degradation and desertification.Maintaining and preserving wetlands in arid areas is of critical significance to the social stability and economic development of arid areas.Ecological water conveyance is one of the main ecological restoration methods to restore degraded ecosystems,which has been widely used in many inland river basins and wetlands in northwest China and has achieved good ecological restoration effects.Groundwater is the main water source for the growth of wetland vegetation in arid areas,and the change in groundwater level determines the process of vegetation restoration.Topographic relief and human intervention lead to strong spatiotemporal variability of groundwater level depth under ecological water conveyance,which correspondingly leads to significant spatiotemporal variability of vegetation restoration.Clarifying the co-evolution mechanism between groundwater and vegetation ecosystem in the case of ecological water conveyance,and accurately depicting the corresponding coupling process of groundwater and vegetation ecosystem through model are the keys to the maintenance of the health of restored wetland ecosystem and the efficient utilization of water resources in the basin.However,the existing research focuses on qualitative or quantitative analysis at the point scale or regional average perspective and lacks a discussion on spatialtemporal variability.Moreover,there is still no report on the spatiotemporal fine description of the coupling process between groundwater depth and vegetation ecosystem using numerical models.Under the above background,this study selects Qingtu Lake wetland in Shiyang River Basin as the research area,a multi-element integrated dynamic monitoring network of the groundwater-wetland ecosystem was constructed,the spatiotemporal evolution characteristics of wetland water,salt,and vegetation growth were analyzed,and the co-evolution mechanism of groundwater and vegetation ecosystem under the ecological water conveyance condition was revealed.A numerical model of wetland groundwater flow that can reflect lake dynamics was constructed and simulated.And the relationship model between groundwater depth and vegetation growth was raised.Combined with the numerical model,the coupling model of wetland groundwater depth and vegetation growth was established.Based on the verified model,the responses of wetland under different water intake levels were simulated,and the established comprehensive evaluation method was employed to evaluate the comprehensive benefits under different water intake levels.The main results are as follows:1.Temporal and spatial evolution characteristics of water body,soil salinity,and vegetation growth in Qingtu Lake wetland areaBased on the interpretation data of satellite remote sensing images and monitoring data in the wetland,statistical analysis methods and spatial analysis methods including Semivariogram,Moran’s I Index,and Structural Similarity were applied.And the spatial and temporal distribution characteristics of surface water,surface soil salinity,and NDVI as well as the spatial interrelationship between them was analyzed and calculated.And the correlation between groundwater depth,soil electrical conductivity,and NDVI of multiple monitoring points was also discussed.The results demonstrated that the spatial distribution of surface water altered over time.From the center of the wetland to the periphery of the wetland,the variation range of groundwater level decreased after ecological water conveyance to the next year before ecological water conveyance.In addition to the ecological water conveyance,the distribution of surface water was comprehensively affected by factors including the direct water conveyance area,the distance from the water conveyance channel,and the surface elevation.The regional average NDVI generally revealed an upward trend over time,while the regional average surface soil salinity generally showed a decreasing trend.The distribution range of the high NDVI area and low surface soil salinity area was highly coincident,and the growth and development of surface vegetation in the area were affected by the distribution of soil salinity;The area not covered by surface water was highly coincident with the distribution area of high surface soil salinity.The area submerged by surface water accounts for the main proportion of the area with low surface soil salinity.The effect of ecological water conveyance on reducing the surface soil salinity in most areas is significant by forming the form of surface water flooding the surface.The high NDVI area in the Qingtu Lake area was mainly distributed in and around the lake.The surface water coverage may inhibit the growth and development of vegetation,but its area proportion was limited;The salinity of the surface soil also greatly inhibited the growth and development of vegetation when the groundwater level was moderate.The low salinity zones with good vegetation growth were located near the center of the lake and the water conveyance channel,that is,there was a sufficient water supply.The growth and development of vegetation in the previous year and the distribution of groundwater depth in the growing season jointly affected the growth and development of vegetation.2.Co-evolution mechanism between wetland area groundwater and vegetation ecosystem under ecological water conveyanceBased on the obtained satellite and monitored data,the influencing factors of the formation of the Spatiotemporal distribution characteristics of surface water and groundwater in Qingtu Lake wetland and their impacts,the correlation mechanism between the groundwater depth and the Spatiotemporal distribution characteristics of soil water content and salinity,and the response of the vegetation ecosystem of Qingtu Lake wetland to the distribution of soil water content and salinity were analyzed and discussed.The spatial and temporal distribution pattern of surface water and groundwater in wetlands was controlled by ecological water conveyance,evapotranspiration,water conveyance area under human control,distance from the water conveyance channel,and surface elevation in different ways.The spatial distribution of the highest groundwater level after the ecological water conveyance generally affected the spatial distribution of soil water content and its subsequent dynamic change characteristics,as well as the soil freezing depth and freezing water volume.The water level,capillary rise characteristics,and regional location after freezing and thawing generally affected the time when soil water content starts to decrease and the depth range of soil water content reduction.The distribution of salinity in the surface soil of Qingtu Lake wetland was greatly affected by the distribution of salinity before water conveyance.The influence of ecological water conveyance on soil salinity is related to its distribution in the Qingtu Lake wetland.The spatial distribution of soil salinity in Qingtu Lake wetland is the cumulative result of soil salinity changes under the influence of Spatio-temporal changes in water level after water conveyance over the years.Formation of surface water is an important way to reduce the salt content of surface soil in Qingtu Lake wetland.The shallow groundwater depth with no surface water formed may lead to the accumulation of surface soil salt.With the increase of the groundwater depth,the species diversity of the surface vegetation amplified,the coenotype transformed from the Phragmites australis group to the Haloxylon ammodendron-Nitraria tangutorum group,root water uptake depth increased,the root length density peak depth in the shallow layer moved down,the main distribution range of the root system enlarged,the root length density generally increased first and then decreased,and the root stratification phenomenon arose and became more noticeable with the increase of groundwater depth.With the increase of groundwater depth,the photosynthetic capacity and water use efficiency of the same vegetation decreased,and with the transformation of vegetation types,the net photosynthetic rate was decreasing,the transpiration rate was increasing,and the water use efficiency was decreasing.With the increase in soil salinity,the growth and development of plants except halophytes were obviously inhibited,and the coenotype gradually transformed to Kalidium foliatum group.The comprehensive performance of the above internal response process is that NDVI increases first and then decreases with the increase of groundwater depth.The spatial distribution of groundwater level,soil salinity,and vegetation in Qingtu Lake wetland can be generally divided into three kinds of areas.The area near the wetland center and with low surface elevation has sufficient soil moisture,low soil salt,and the best vegetation growth condition during the vegetation growth season.In the area with high surface elevation in the wetland,the area with shallow groundwater level is seriously salinized,which is suitable for halophyte growth,while the area with deep groundwater level in the vegetation growing season is less salinized,and the vegetation growth is poor.The impact of ecological water conveyance on the area outside the wetland is limited,and the vegetation growth in the area is poor.3.Construction of the numerical model of groundwater flow in Qingtu Lake wetland area.On the basis of inductive analysis of the co-evolution mechanism between wetland groundwater and vegetation ecosystem,the conceptual model of Qingtu Lake wetland was constructed.Combined with high-precision digital elevation data of UAV and remote sensing data,the numerical groundwater flow model of Qingtu Lake wetland that could reflect the dynamic change of distribution range and water level of surface water was constructed.After the model identification of parameters and sensitivity analysis process with monitored groundwater depth data,the temporal and spatial distribution of the groundwater depth and the water balance in the case of ecological water conveyance were calculated.The simulation results demonstrated that the groundwater level is most sensitive to the phreatic evaporation intensity and lake water evaporation intensity.The influence of ecological water conveyance on the groundwater level was mainly concentrated in the range of Qingtu Lake and its surrounding areas.The annual evapotranspiration in the Qingtu Lake area and the annual lateral discharge of groundwater to the outer area increased with the ecological water conveyance volume.4.Construction of groundwater depth and vegetation growth coupling model(GPCM)in the arid wetland.On the basis of the previous chapters,the relationship between the average groundwater depth from March to July and the summer surface soil salinity and summer NDVI was analyzed.A spatial quantitative relationship model between groundwater depth and surface soil salinity was formed with the machine learning method.And the spatial quantitative calculation method of NDVI was constructed by adding salt salinity limiting factor to the fixed environmental carrying capacity model combined with cellular automata.Coupled with the coupling numerical model of groundwater flow,the coupling model of wetland groundwater depth and vegetation growth was constructed.The calculation results of the constructed model fit well with the results obtained from remote sensing interpretation,indicating that the constructed method can be applied to the calculation and prediction of the spatial distribution of surface soil salinity and NDVI of wetland affected by salinization in arid areas.5.Benefit evaluation of ecological water conveyance volume flowing into the lake based on the GPCM modelModel scenarios with ecological water conveyance volume flowing into the lake as a variable were set up and the response of wetland ecosystems corresponding to different ecological water conveyance volumes was simulated and calculated based on the constructed model.The comprehensive response evaluation system which coalesces Analytic Hierarchy Process,Entropy Value method,and TOPSIS method was constructed,and the response characteristics of wetland under different ecological water conveyance volumes were comprehensively evaluated and analyzed.The results exhibited that with the increase of ecological water conveyance volume,the groundwater level generally increased,the lake area generally increased,the surface soil salinity generally decreased,and the regional NDVI generally increased,but the vegetation in some areas was inhibited by the high water level.The comprehensive evaluation results indicated that when the ecological water conveyance volume flowing into the lake is between 1.925×107m3 to 2.4×107m3,the comprehensive benefit of water inflow and the utilization efficiency of water conveyance are high,and the overall comprehensive benefit of the actual ecological water conveyance in the last several years of Qingtu Lake wetland is relatively high.Based on multi-source data and spatial analysis,this study revealed the co-evolution mechanism of wetland groundwater and vegetation ecosystem in arid areas with spacetime variability under ecological water conveyance conditions.Combining the optimized groundwater flow numerical model,which can reflect the drastic dynamic changes of water bodies,and the relationship model between groundwater depth and vegetation growth for wetland affected by salinization in arid areas,a groundwater depth and vegetation growth coupling model was constructed,which can accurately depict the groundwater depth and vegetation growth in wetlands in arid areas under the ecological water conveyance conditions,and provides important support for ecological water transport regulation and efficient utilization of water resources. |
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