Field Experiment And Simulation Of Soil Water And Salt Distribution In Farmland Under The Impact Of Subsurface Drainage

Based on the research background of water resource shortage and soil secondary salinization in Hetao Irrigation District(HID),the YongJi Experimental Base of China Agricultural University in The YongJi irrigation area of HID was selected as a typical research area,and conducts research on the relationship between water and salinity distribution of Sunflower growth in farmland under the impact of subsurface drainage.By analyzing the field experiment data in the research area,the main influencing factors of soil water and salinity distribution,salt ion composition and crop growth under the condition of subsurface drainage were analyzed.On this basis,RZWQM2(Root-Zone-Water-Quality-Model 2)is used for relevant numerical simulation,which provided theoretical basis for the optimization of farmland subsurface drainage pipe layout and preventing soil salinization in the study area.The main conclusions are as follows:(1)Within the 0-1 m vertical section of each experimental plot,the dynamic change trends of soil moisture and salinity were similar,and the soil moisture generally fluctuated and decreased with time.The soil salinity is mainly distributed in a "Inverted Triangle" pattern with high surface layer and low deep layer,which fluctuates and rises with time.The type of soil salt composition is mainly sodium salt,and the crop growth period changes from soda type to sulfate type.During the growing period of crops,the soil was seriously affected by salinization,less affected by alkalization,and the overall degree of comprehensive influence by salinization increased fluctuatingly.As soil depth increases,soil salt ion mobility decreases.The soil salinity of each layer in the area without drainage measures was higher than that in the underground drainage area,and the soil salt storage increased greatly.During the growth period of 2021,the lateral seepage of the HeJi Branch of the YongJi Canal will supply groundwater of about 5 921.27 m3,accounting for about 27.34%of the total water entering the study area in that year.(2)The variation trend of groundwater depth in each experimental area is basically the same with seasonal variation,and the variation trend of shallow groundwater salinity is the same with short-term fluctuation and overall slow rise.Groundwater quality is mainly fresh water and brackish water,showing weak alkalinity.The SO42-,Cl-,Mg2+,K++Na+ions are closely related between groundwater and soil.The salinity increment of groundwater without drainage measures is greater than that of subsurface drainage,and irrigation,the salinity of groundwater during the growth period is greater than that of subsurface drainage.(3)The water quality of the subsurface drainage is mainly brackish water,showing weak alkalinity.The salt discharge is closely related to SO42-,Cl-,K++Na+,Mg2+ions in the soil,and HCO3-,Cl-,SO42-,K++Na+ions in the groundwater.And the salinity of groundwater during the growth period is greater than that of subsurface drainage.(4)The growth indexes of sunflower crops in 2021 decreased compared with that in 2020.And the average yield decreased by about 21.21%year-on-year.Compared with the treatment without drainage measures,the growth of sunflower under subsurface drainage treatment was better,with an average increase of about 8.06%.(5)After calibration and verification of field test data,RZWQM2 can be used to simulate different layout schemes of subsurface drainage.The simulation results show that the layout with buried depth of 1.4 m and spacing of 45 m is better than the existing layout of subsurface drainage pipe.Considering the distribution of soil moisture and salinity as well as crop yield,it is more appropriate to set 1.5 m in the rainy season and 1.2 m in the non-rainy season to control the depth of the drainage gate under the existing arrangement of drainage pipes.