Study On The Characteristics And Simulation Of Soil Water And Salt Transportation Under Controlling Subsurface Drainage In Hetao Irrigation District

Soil salinity and drought and water shortage are important factors restricting the sustainable development of agriculture in irrigation areas.By raising the height of drainage outlet of dark pipe,controlling drainage can reduce field drainage output and improve water use efficiency.Therefore,it is of great significance to control soil salinity and relieve drought pressure by subsurface control drainage.Based on the above research background,this thesis selects the subsurface drainage test area,carries on the control drainage on the subsurface drainage foundation,carries on the test monitoring for two years,study on the distribution and variation of soil water and salt and its influence on the regulation of water level at the outlet of the subsurface pipe and the subsurface drain spacing;analysis of spatial variation of soil water and salt affected by drainage;discuss the effect of water saving and salt control on drainage.By using the HYDRUS-2D model,the influence of different outlet control depth and engineering parameters on soil water and salt migration characteristics is compared,which provides a theoretical and scientific basis for the implementation of subsurface control drainage in arid and semi-arid irrigation areas in the future.The main research contents and conclusions are as follows:(1)Controlling subsurface drainage has a better effect of preserving soil water content.Compared with the free drainage,controlled drainage increased the soil relative water content of 8.27%,enhanced the soil water increase rate of 53.64%during the irrigation stage,and reduced the soil water decrease rate of 8.98% during the intermittent stage.At the same time,the controlled drainage reduced the soil salt content of 36.73% in soil profile,enhanced the desalination area of 29.17% during the irrigation stage and reduced the salt accumulation area of 14.29% during the intermittent stage.After reducing the drain spacing of control drainage,the soil water increase rate was reduced during the irrigation stage,and the soil water decrease rate was increased during the intermittent stage,the effect of water preservation is weakened,and the effect of desalination is more obvious.(2)The salinity homogeneity of the controlled drainage on the horizontal surface of different soil layers is strong.Compared with free drainage,the coefficient of variation of salt level of controlled drainage soil decreased 29.47% and 47.11%before and after irrigation,which was helpful to improve the uniformity of washing salt and reduce the risk of soil salinization to a certain extent.The soil water and salt condition of tillage layer can directly affect the quality of crop production.The soil salinity in the tillage layer was leached by spring irrigation,the soil desalination rate was 10.11% by free drainage and the soil desalination rate was 14.80% by controlled drainage.Compared with the free drainage,the distribution of soil water and salt under controlled drainage is more uniform,which is beneficial to crop growth and ensures stable yield.(3)The effect of water saving and salt control in controlled drainage is better.Compared with the free drainage,the soil storage capacity of controlled drainage is15.19% higher.The soil conductivity of each layer of controlled drainage is lower,the difference between soil conductivity layers is small.After two years of drainage experiment,the conductivity of soil in the field continues to decrease,and the subsurface drainage can make the soil desalinate obviously,and the average soil desalination rate under controlled drainage was higher,and the controlled drainage is carried out on the basis of the subsurface drainage,and the desalination effect is better.Compared with free drainage,the groundwater depth of controlled drainage is smaller,the salt content in groundwater is higher.Compared with free drainage,corn yield increased 7.17% and sunflower yield increased 9.94%.(4)HYDRUS-2D model can simulate the migration characteristics of soil water and salt under subsurface drainage.Compared with the variation of soil water and salt in typical soil layer under different subsurface pipe spacing,it is found that the smaller the subsurface pipe spacing,the lower the soil moisture content and the smaller the soil conductivity.The closer the horizontal distance to the subsurface pipe,the greater the variation of soil moisture content and conductivity.Compared with the variation of soil water and salt in typical soil layer under different control height of subsurface outlet,it is found that the higher the control height of subsurface outlet,the higher the soil water content and the greater the soil conductivity.Compared with the water discharge of different subsurface pipe spacing and subsurface pipe outlet control height,it is found that the smaller the subsurface pipe spacing,the greater the cumulative water discharge,and the higher the subsurface pipe outlet control height,the smaller the cumulative water discharge.According to the different soil conditions in different regions,field experiments and model simulations should be carried out.At the same time,according to the salt tolerance limit of crops,combined with the growth stage and water demand requirements of crops,the suitable control of drainage spacing and export control height should be explored.Combining the soil water salt and drainage volume at the different subsurface pipe spacing and drainage outlet height,the optimal combination is that subsurface pipe spacing is 50 m and subsurface pipe drainage outlet height is 40 cm.