Effects And Effectiveness Evaluation Of Saline Soil Improvement By Subsurface Drainage In Hetao Irrigation District

Hetao irrigation area,located in the inland of Northwest China,is an important grain production base in China.The area of saline alkali land in the irrigation area is large,the degree of salinization is heavy,and the area of severe salinization is more than 9.544×108hm²,which seriously affects the ecological,agricultural,economic and social development of the irrigation area.Therefore,improving saline alkali land and reasonably developing saline soil are of great significance to the irrigation area.Underground drainage technology plays a significant role in crop growth and agricultural yield increase.It is a widely used measure for improving saline alkali land.In this thesis,the modern underground drainage technology demonstration area in wulateqianqi banner with serious salinization is selected as the background to carry out the underground drainage and salt drainage experiment for three consecutive years.The temporal and spatial variation law of soil water and salt,salt ion transport characteristics and influence mechanism,water and salt balance law in the experimental area,groundwater buried depth Based on the mechanistic research on the variation law of groundwater salinity,this thesis evaluates the improvement effect of"underground pipe drainage and salt drainage"technology in the experimental area,and provides reliable data conclusions.The main results are as follows:（1）From 2019 to 2021,the coefficient of variation of soil surface（0-40cm）water content in concealed pipe drainage area is 10.191%-25.077%,which belongs to medium variability,and the coefficient of variation of deep（40-100cm）water content is 3.268%-10.462%,which belongs to weak variability.The variability of surface soil is medium,and the variability of deep soil is weak.From 2019 to 2021,the variation range of soil salt surface block base ratio in concealed pipe drainage area is 0.052%～9.073%,0.44 6%～11.501%and 1.482%～12.801%.It can be seen that the surface layer is gradually affected by structure and random factors.The variation range of soil salinity（EC value）block basis ratio in the open ditch drainage area from 2019 to 2021 is 1.894%～14.439%,3.568%～15.230%,2.711%～15.270%,with strong spatial autocorrelation.Compared with the open ditch drainage area,the block basis ratio of soil salt（EC value）in the concealed pipe drainage area is smaller as a whole,indicating that the concealed pipe drainage area is greatly affected by spatial structure factors.（2）The order of anion desalination rate before and after leaching is Cl^->SO₄^2->HCO₃^-,the order of cation desalination rate is Na⁺+K⁺>Mg²⁺>Ca²⁺,the annual average desalination rate of Cl^-is 55.4%,and the annual average desalination rate of Na⁺+K⁺is51.9%.From 2019 to 2021,the overall content of ions decreased year by year before spring irrigation.Anions Cl^-,HCO₃^-,SO₄^2-decreased by 24.82%,6.03%and 17.50%respectively,and cations Na⁺+K⁺,Mg²⁺,Ca²⁺decreased by 39.01%,9.14%and 6.6%respectively.Before spring irrigation,the overall content of ions in the open ditch drainage area decreased by 1.77%,3.05%and 2.18%respectively,and the cations Na⁺+K⁺,Mg²⁺and Ca²⁺decreased by 3%,5.6%and 4.2%respectively.Compared with the concealed pipe drainage area,the main ions in the open ditch drainage area changed less.It shows that the underground drainage measures have an obvious effect on the reduction of salt ion content.（3）The order of anion concentration in drainage is Cl^->SO₄^2->HCO₃^->CO₃^2-,and the order of cation concentration is K⁺+Na⁺>Mg²⁺>Ca²⁺.Among them,the anion with the largest proportion in the drainage is Cl^-,the variation range of ion concentration in each drainage is 0.98～2.4g/l,accounting for 47%of the total anion,the cation with the largest proportion is K⁺+Na⁺,the variation range of ion concentration in each drainage is1.05～2.01g/l,accounting for 55%of the total cation,SO₄^2-accounts for 30.58%of the total anion,and the drainage mainly discharges Na⁺,Cl^-,SO₄^2-and other salt substances.（4）From 2019 to 2021,the salt desalination rate of 0-20cm soil layer before spring irrigation is 17.13%,and that of 40-100cm is 16.86%.With the increase of soil depth,the desalination rate decreases gradually,and the desalination effect of surface soil is the most obvious.The desalination rates of 0-20cm,20-40cm and 40-100cm soil layers in the open ditch drainage area before spring irrigation from 2019 to 2021 are 9.1%,4.1%and 2.6%respectively.The desalination effect of the concealed pipe area is significantly better than that of the open ditch drainage area.After the improvement of the concealed pipe technology,the salt content in the test area is significantly reduced and the desalination effect is better.（5）Over the past three years,the area of mild and moderate salinized soil has increased by 3.92%and 5.88%respectively,and the area of severe salinized soil has decreased by 21.57%from 31.37%.Although the area of saline soil has not changed,the average salt content of saline soil has decreased year by year.After the improvement,the soil p H value decreased,0-20cm,20-40cm and 40-100cm decreased by-2.35%,-2.72%and-3.42%respectively,showing a trend that the change rate of p H value gradually increases with the depth of soil layer,indicating that the laying of concealed pipe has a significant effect on reducing soil p H value.（6）From 2019 to 2021,the salt desalination rate of 0-20cm soil layer is 69.13%,that of 20-40cm soil layer is 48.6%,and that of 40-100cm soil layer is 43.61%.With the increase of soil depth,the desalination rate decreases gradually,and the desalination effect of surface soil（0-20cm）is the most obvious.The desalination rates of 0-20cm,20-40cm and 40-100cm soil layers in the concealed pipe area are 2.19,2.48 and 2.71 times higher than those in the open ditch drainage area respectively,and the desalination effect is significantly better than that in the open ditch drainage area.Over the past three years,the area of mild and moderate salinized soil has increased by 3.92%and 5.88%respectively,and the area of severe salinized soil has decreased by 21.57%from 31.37%.Although the area of saline soil has not changed,the average salt content of saline soil has decreased year by year.After the improvement,the soil p H value decreased,0-20cm,20-40cm and40-100cm decreased by-2.35%,-2.72%and-3.42%respectively,showing a trend that the change rate of p H value gradually increased with the increase of soil depth,indicating that the laying of concealed pipe has a significant effect on reducing soil p H value.（7）The yield of fresh soybean in 2019 is almost 30.44%lower than that in conventional saline alkali area,which is 531～715 hm²/kg.After two years of improvement,crops in areas with severe salinization will be changed to chaomu No.1,Hunan millet and Sesbania.The yield of chaomu No.1（fresh weight）is 36171.92 kg/hm²,that of Hunan millet（fresh weight）is 49281.04 kg/hm²,and that of Sesbania（fresh weight）is 25782.12kg/hm²,which is basically consistent with the conventional yield.The yields of wheat,corn and sunflower before improvement were 7159,10589 and 3251.5kg/hm² respectively.After three years of improvement,the yields were 7856,11355 and 3562.1kg/hm²respectively,increasing by 9.74%,7.23%and 9.55%respectively.