Study On Groundwater Evolution And Soil Salinization Risk In Yongji Irrigation Area Under Water Saving Transformation

Groundwater is an important water resource in China,and it is also the main environmental factor affecting soil salinization.With the implementation of large-scale water-saving reconstruction project in Hetao irrigation area,the groundwater environment is bound to change.The thesis focuse d on the impact of large-scale water-saving renovation projects in the Hetao irrigation district on the groundwater environment,taking Yongji irrigation area as the research object,the evolution of groundwater was explored from the perspective of temporal and spatial probability distribution i n Yongji irrigation area of water-saving transformation,predicted the depth of groundwater in different periods,clarified the source of groundwater chemical substances and the cause of salinization,explored the critical depth and salinity of groundwater in the prevention and control of soil salinization in different periods.The main conclusions are as follows:(1)The water-saving transformation effectively increased the groundwater depth.In the late stage of the water-saving renovation(2013?2018),the average groundwater depth increased by 0.36m compared with that before the water-saving renovation(1998?2000).The area with groundwater depth?2.0m expanded by 33%,and was affected by urbanization,the depth of groundwater in the central andsouthern regions has increased significantly.Apply the CAR model to simulate the groundwater depth in different periods,the groundwater depth of Yongji irrigation area was 2.67m,2.42m and 2.16m in freeze-thaw period,growth period and autumn irriga tion period.(2)The average groundwater salinity in the later stage of the water-saving reconstruction increased by 1.37 g·L^-1compared with that before the water-saving reconstruction.The area of groundwater salinity<2.5 g·L^-1and?3.0g·L^-1expanded by 17%and 4%,respectively,the groundwater in the southcentral region tended to be desalinized,and the north and the east and west edges tended to be mineralized.In other words,themineralized areas were mostly concentrated near the drains,and it wasrecom mended to further improve the drainage system.Na⁺+K⁺and Cl^-were the main variables determining groundwater salinization,and the groundwater chemical types were mainly Cl-Na,HCO₃-Na and HCO₃·Cl·SO₄-Na.The leaching effect of agricultural irrigation on soil salinity and strong transpiration and evaporation were important reasons for the salinization of water environment.(3)Before spring irrigation,49%of the Yongji irrigation area was the area with high risk of soil salinization(mild saline soil),35%was the area with high probability of shallow groundwater(groundwater depth?2.2m),and 17%was the area with high risk of groundwater mineralization(groundwater mineralization?3.0g·L^-1).In the growth period,32%of the areas are high risk areas of soil salinization,59%of the areas are high probability areas of shallow groundwater,and 30%of the areas are high risk areas of groundwater mineralization.Before spring irrigation,compared with the growth period,the area with high risk o f soil salinization increased,the area with high probability of shallow groundwater decreased,and the area with high risk of groundwater mineralization decreased.(4)Before spring irrigation,the critical depth of groundwater under mild and moderate sal inization was 2.6m and 2.2m,and the critical salinity was2.0g·L^-1and 2.5g·L^-1.In the growth period,the critical depth was 2.2m and 1.8m,and the critical salinity was 2.5g·L^-1 and 3.0g·L^-1.The northern,southeastern and central areas of the study area were high-probability areas where the groundwater was less than the critical depth and greater than the critical salinity,and was a high-risk area for soil salinization.It is suggested to further improve the drainage system to reduce the groundwater dep th and salinity in the local salinization risk areas.