Effect Of Changes In Groundwater Level On The Ecological Environment In Hetao Irrigation Area Using Remote Sensing Method

Hetao irrigation district is located in the west of Innermongolia.It has scarce rainfall but strong evaporation so that its ecological environment is fragile.Hetao irrigation district mainly depends on irrigation from Yellow River where groundwater depth is directly related to soil salinization,vegetation growth,lakes,bare land and other ecological elements.In recent years,groundwater level in Hetao irrgation district was apparently declined due to the decrease of unified allocation of water resources from the Yellow River,and water saving measures,especially,irrigation canal lining.Study on the effect of decline in groundwater level on the ecological environment in Hetao irrigation area,is able to provide support for determination of the ecological groundwater level in Hetao irrigation area and quantitative analysis of reasonable water saving potential.In this paper,the relationships between ecological factors and groundwater depth in Hetao irrigation area were analyzed by means of laboratory experiment,remote sensing and field investigation.The main work of this paper was as follows:(1)the different degrees of saline soil laboratory evaporation experiment was conducted and we obtained the spectra data of saline soil under different water content.Besides,we used EPO algorithm to eliminate the effect of soil moisture on soil salinity in spectra.(2)Based on the Landsat satellite remote sensing data and the results of laboratory experiments,we extracted the information of land use types such as lake and saline soils area and analyzed the effects of the changes in groundwater depth from the time series.(3)We carried out field investigation of poplar forest.From the space,poplar forest with different groundwater depths was investigated about local soil moisture,soil salinity and poplar growth(density,height,DBH,crown width,rings,etc).We established the relation curves between these indicators and groundwater depth.From the time,we established the time series between normalized differential vegetation index(NDVI)of poplar and groundwater depth,and analyzed it using remote sensing method.Through the above work,the main conclusions were as follows:(1)soil moisture can significantly affect the soil spectral characteristics.The EPO method could effectively filter the mask of soil moisture to the soil spectral information,so as to improve the inversion precision of soil salinity.(2)The amount of water in lakes come from the natural discharge of groundwater in the low-lying area,and the decline in groundwater level leads to the decrease of the lake area.But at the same time,it could be seen that the decline of groundwater level also had a positive impact on the improvement of regional soil salinization.(3)Combined the results of field investigation in 2014 and the NDVI values of historical remote sensing image,the most suitable groundwater level for the growth of poplar in Hetao Irrigation District is 1.6?2.0 m.In other words,groundwater level whether it is below or above the line is not conducive to the growth of poplar.