| The influence of groundwater on the ecological pattern of terrestrial vegetation is the main research content of Eco-hydrogeology.Wu shenzhao Basin is located in the arid and semi-arid areas of Northwest China,where the ecological environment is strongly dependent on groundwater.Therefore,it is great significant for ecological protection of this area to study the relationship among vegetation,groundwater and evapotranspiration.In this paper,the relationship among groundwater depth,the Enhanced Vegetation Index and land-surface evapotranspiration(ET)is studied by building statistical model,and the dependence of vegetation coverage on of groundwater depth in Wu shenzhao Basin is determined.For collecting the groundwater depth data comprehensively,261 wells data were measured in May 2017,and 457 observation wells data were collected from previous maps and boreholes.Based on these observation data,the contours of groundwater depth was built by Kriging interpolation method and applied to construct 250-m resolution raster data.The data shows that the range of groundwater depth is large,ranging from groundwater spillage to lake and the groundwater depth more than 50 m.MODIS EVI data in mid-late August of 2000-2017 were collected to estimate the relative vegetation coverage with 250-m resolution.The spatial distribution characteristics of vegetation coverage and the variation characteristics of vegetation index from 2000 to 2017 were obtained by pretreatment.The results show that the distribution of vegetation coverage is characterized by zoning,plaque and randomness in spatial and the increase rate of 0.0018/a in time.The 2000-2014 monthly scale 1 kmresolution MOD16-ET data was collected and adjusted by water balance method at the catchment scale.It indicates the spatial distribution of growing season ET is high in the East area and low in the west of the study area.A statistical model of EVI and groundwater depth was established,and it is found that the data points is highly dispersed in the shallow depth of groundwater.Data points were divided into several zones according to an interval of 1 m of groundwater depth to calculate the statistical values of EVI(P=50%,P=98%)in different zones.The P=50% and P=90% probability curves of EVI replace the mean and large values of EVI,respectively.Both curves show that the critical groundwater depth affecting significantly vegetation coverage is 10 m.When groundwater depth is less than 10 m,EVI increases first and then decreases with the increase of groundwater depth.When the groundwater depth is more than 10 m,EVI fluctuates slightly with the groundwater depth increasing.The statistical models of ET and vegetation coverage,ET and groundwater depth were established respectively.The results show that ET increases approximately linearly with the increase of vegetation coverage.The P=50% and P=90% probability curves of ET were calculated with data points of different zones divided with 1m groundwater depth interval.The average ET represented by P = 50% curve showed a piecewise change characteristic.In the range of 0~7.5 m,with the increase of groundwater depth,the variation of average ET is: when the groundwater depth is 0~1.5 m,ET decreases in an approximately linear form;when the groundwater depth is 1.5~3.5 m,ET increases in a linear form;when the groundwater depth is 3.5~7.5 m,ET decreases in an exponential form;when the groundwater depth is greater than 7.5 m,the ET is basically unchanged.From the statistical characteristics,it is preliminarily believed that the critical groundwater depth affecting ET is 7.5 m.The results of this paper provide a new understanding of the vegetation ecological pattern under the action of groundwater as the main controlling factor in Wushenzhao Basin. |
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