The Experiment On Water And Salt Transport Of Unsaturated Zone And The Study On Dynamic Regulation Of Groundwater Depth In Soil Secondary Salinization Area

The prevention and treatment of the soil secondary salinization is an unavoidable issue for the sustainable agricultural development and eco-environmental protection in the Hetao plain. The informed research of groundwater level critical depth mainly depend on qualitative analysis or Capillary height calculation, which have produced many problems, such as Strong subjective,lower accuracy and difficult to be applied etc. In the paper, an in-situ testing field for the water and salt migration has been set up in the typical saline region and the water and salt controlling tests have been conducted in different lithologies,different ground water depths and different crop planting condition. The Influence Mechanism of unsaturated zone water and salt migration has been discussed.Based on the field experiment, a coupling model for the water and salt migration in the saturated-unsaturated zone has been built and the quantitative research for the groundwater depth control has been carried out to decide the critical value of the dynamic control for the groundwater depth in different lithologies and different stages. Then the prevention and treatment measures for the saline soil are put forward. The main conclusions of this paper are as follows:(1) the groundwater depth,unsaturated zone lithology and irrigation are the leading control factors that influenced the water and salt migration in the Hetao plain。Since the unsaturated zone lithology can't be controlled, we mainly take measures to regulate the groundwater depths and control the intensity and frequency of and irrigation based on the differences of different unsaturated zone lithologies.(2) The groundwater depth exerts a controlling influence on the water and salt distribution and migration in the unsaturated zone. When the groundwater table is lesser than 1.0m, the surface of the unsaturated zone is in salt accumulation stage with strong evaporation at most time of the year, which leads to severe salinization; When the groundwater table is 2.0m, the capillary can't rise to the shallow part of the unsaturated zone, the evaporation now becomes noticeably weak, which brings an obvious decrease in the salt accumulation degree on the surface of the unsaturated zone; When the groundwater table is 3.0m, the groundwater basically doesn't participate in the salt accumulation on the surface of the unsaturated zone. The surface salt will gradually decrease due to the washing of rainfall or irrigation and will border on the state of total desalination.(3) Different lithological capillary rising heights and speed lead to different groundwater evaporation degrees and different levels of soil surface salt accumulation. The capillary rising height for the sandy loam and the clay loam is about the same, namely 1.5-1.7m, while the height for the silt sand is about 1.0-1.2m. In terms of the speed, the silt sand is the fastest, the sandy loam is the second, and the clay loam is the slowest. The groundwater evaporation tests in different testing fields indicate:when the water table is 1.0m, the silt sand has the largest evaporation capacity, the sandy loam is the second, and the clay loam has the smallest. When the water table is 2.0m, the evaporation capacity in the three testing fields all significantly declines. The sandy loam has much larger evaporation capacity than the silt sand and the clay loam, while the evaporation in the silt sand is a bit larger than that in the clay loam. When there is strong precipitation or irrigation, all the three types show obvious decrease in the salt content within the range of 0-50cm, but with different decreasing degrees. The clay loam has the lowest decrease, the sandy loam ranks the second, and the s silty sand has the highest decrease. After the precipitation or irrigation, the salt content in the shallow part of the unsaturated zone begins to gradually increase due to the evaporation. When the water table is deep, the sandy loam has the largest salt increase, the silt sand ranks the second, and the clay loam has the smallest. When the water table is shallow, the silt sand has the largest increase, the sandy loam is the second, and the clay loam has the smallest.(4) For the surface of the unsaturated zone, the crop plantation can effectively alleviate the soil evaporation, thus reducing the salt accumulation on the surface. In the root zone, owing to the water absorption of the root, the salt accumulation is larger than the bare land at the same depth. The irrigation within the period of growth boasts great influence on the water and salt migration. After irrigation, the salt in the shallow part will significantly decline as the water infiltrating. The change of salt content in the deeper part is closely related with the intensity of the irrigation.(5) The coupling model for the water and salt migration in the saturated-unsaturated zone has been established. By employing the HYDRUS-1D software, the quantitative research for the groundwater depth control has been carried out to decide and calculate the irrigation ration for washing salt as well as the critical value of the dynamic control for the groundwater depth in different lithologies and different stages. For the clay sand area, form March to June, the irrigation ration shouldn't be lower than 160 m3/mu, and the water depth should be deeper than 2.4m. From early July to early September, the irrigation ration shouldn't be lower than 80 m3/mu, and the water depth should be deeper than 1.8m. From mid September to mid November, the irrigation ration shouldn't be lower than 110 m.3/mu, and the water depth should be deeper than 1.5m. From late November to early March of the next year, the water depth should be deeper than 2.0-2.2m before the freeze-up. For the sand clay area, form March to June, the irrigation ration shouldn't be lower than 190 m3/mu, and the water depth should be deeper than 2.0m. From early July to early September, the irrigation ration shouldn't be lower than 80 m3/mu, and the water depth should be deeper than 1.7m. From mid September to mid November, the irrigation ration shouldn't be lower than 110 m3/mu, and the water depth should be deeper than 1.4m. From late November to early March of the next year, the water depth should be deeper than 1.7-2.0m before the freeze-up. For the silt sand area, form March to June, the irrigation ration shouldn't be lower than 100 m3/mu, and the water depth should be deeper than 2.3m. From early July to early September, the irrigation ration shouldn't be lower than 70 m3/mu, and the water depth should be deeper than 2.0m. From mid September to mid November, the irrigation ration shouldn't be lower than 60 m3/mu, and the water depth should be deeper than 1.7m. From late November to early March of the next year, the water depth should be deeper than 1.5-1.7m before the freeze-up.The characteristic of this paper lies in the following respects:this paper has gathered the soil samples of clay sand, sand clay and silt sand at different depths, and has regularized their key parameters of the water and salt migration, which have improved the accuracy of the model prediction; The water and salt migration field tests in different lithologies, different water tables, and different crop-coverage have been conducted by in-situ test pool which controlled the groundwater level depth and unsaturated zone lithology;The quantitative research for the groundwater depth control has been carried out to calculate the critical value of the dynamic control for the groundwater depth in different lithologies and different stages, which throws light on the prevention and treatment measures concerning salinization.