Study On Movement Mechanism Of Water And Salt In Vadose Zone Under Saline Water Irrigation

Hengshui area belongs to Heilonggang Basin,and it is a typical area with agriculture as the main economic subject.Wheat is its main crop over there because of the climate.Fresh water resources are one of the important factors for agricultural irrigation.However,the lack of fresh water resources which severely restricts the development of agricultural production in the region.Meanwhile,Hengshui city has rich underground saline water resources.The exploitation and utilization of saline water resources for agricultural irrigation is of great significance to the agricultural development of the region.In this paper,the field test was carried out to monitor the changes of soil water and salt content and discuss the law of soil water and salt migration in the study area.The distribution of salt ions in soil under different salinity irrigation water and different initial salinity of soil was studied by pot experiment.The basic parameters of water and salt transport were obtained by soil column test.Hydrus-1D software was used to simulate the process of water and salt transport by combining the distribution data of soil water and salt under saline water irrigation conditions and the parameters obtained from soil column test.According to the results of the pot experiment,the law of water and salt transport and the effect of salt accumulation in soil under different irrigation modes were simulated.The main conclusions in this paper are as follows:(1)Through field test data analysis,under the high salinity saline water irrigation,the soil salinity in the study area exhibited obvious surface accumulation,with a maximum value of 7.29g/kg.During fresh water irrigation,salt mainly concentrated in the bottom area,and the soil salt content in the tillage layer was less than 1.5g/kg,which was conducive to the growth of crops.(2)According to the pot experiment,under the same initial conditions of irrigation water with different salinity,the soil salt content showed obvious surface accumulation phenomenon.Under the condition of irrigation water concentration of 7g/L,the soil surface salt content reached 20g/kg.The salt content of subsoil gradually increased with the increase of the salinity of irrigation water,and the maximum value reached 4.35g/kg.When irrigation water of the same salinity degree was used with different initial soil salinity,the maximum soil salinity reached 16.79g/kg except for the accumulation phenomenon in the surface layer.The other layers of soil all have different degree of washing salt effect,with the increase of soil initial salt content,the better the washing salt effect.(3)Hydrus-1D software was used to simulate the soil water and salt transport process under saline water irrigation conditions,and the reliability of the model was verified.The experimental data obtained from October 2018 to June 2019 were used to calibrate and verify the model.The effect of model fitting was evaluated through three indexes: root-mean-square error mean,relative error,and coincidence index.It was concluded that the fitting effect was acceptable,which could effectively reflect the distribution of soil water and salt in the region,and the model was reliable.(4)According to the growth characteristics of wheat,the models of fresh water irrigation,saline water irrigation and fresh-fresh-saline-saline water irrigation were set.It can be known from the simulation results that the effect of the salinity of irrigation water on soil salinity has a decreasing tendency with the increase of the burial depth.When using both freshwater and fresh-fresh-saline-saline water irrigation modes,the soil surface accumulation phenomenon is significantly better than the full-saline water irrigation mode.The soil salt content at the depth of 0.2m is averaged at 1.08g/kg for all fresh water irrigation.In the fresh-fresh-saline-saline water irrigation mode,the average soil salt content at 0.2m is 1.25g/kg.When irrigated with saline water,the average soil salt content at the depth of 0.2m is 2.57g/kg.Thus,it can be seen that on the premise of saving fresh water resources,fresh-fresh-saline-saline water irrigation mode is more conducive to crop planting.