Experimental Study On Regulation And Utilization Of Water And Salt Transport In Unsaturated Zones In Arid Areas

This paper is based on the field in-situ test site of the typical oasis irrigation area in the Shiyang River Basin.The main data source is the water and salt transport profile in the unsaturated zone and the corresponding system automatic monitoring device.After sampling,laboratory analysis and statistical analysis of the monitoring data and systematic comparative analysis,the laws and main controlling factors of water and salt migration in the unsaturated zone are clarified,and the driving influence mechanism of the water and salt migration in the unsaturated zone is revealed.Furthermore,according to the experimental results of water and salt transport regulation,a model for efficient use of groundwater is proposed.The main conclusions are as follows:(1)The factors affecting the distribution and migration of water and salt in the unsaturated zone include meteorological conditions(precipitation and evaporation),lithology of the unsaturated zone,groundwater depth and irrigation.Among them,the effect of evaporation on soil salt accumulation is much stronger than that of precipitation on soil leaching salt.The factors that control salt and are easy to control are groundwater table depth and farmland irrigation.(2)At different depths,the soil water content and salt content are different: the shallow part(20~60cm)is the water dissipation layer and the salt accumulation layer.Due to the strong evaporation,the closer the soil to the surface,the lower the soil water content and the higher the salt content.The deep part(150cm)is a water-saturated layer and a salt deposit layer,which is mainly affected by groundwater,and its water content and salt content are significantly higher than that of shallow soil.The middle part(90~120cm)is the water and salt change layer,during the irrigation and pumping process,affected by the adjacent two layers,the soil water content and salt content decreased or increased significantly.(3)After fresh water irrigation,the water content of shallow soil(within 90 cm)decreased by 0.1% to 0.3%,and the salt content decreased by 3% to 5%;after salt water irrigation,the water content and salt content of shallow soil both showed a growth trend,with increases of 3% to 4% and 10% to 40%,respectively.Experiments show that fresh water irrigation can effectively transfer shallow soil salt to the deep part,producing a certain leaching effect;salt water irrigation will significantly increase the soil salt content,which has a certain effect on the soil water and salt balance.(4)Lowering the groundwater level can achieve the effect of reducing evaporation and suppressing salt,which is beneficial to reduce soil salinization.When the water level burial depth is reduced from 1.5m to 2m,the shallow soil water content after fresh water irrigation is higher than 1%~3% before irrigation,but the salt content is reduced by 3.5%~7.5%.The soil water content and salt content after saline water irrigation are still higher than before,but the increase is lower than when the water level is shallow.The energy driving of water and salt transport in the unsaturated zone is mainly the soil water potential.After the precipitation recharge,the salt infiltration process is dominated by gravity,while in the diving evaporation process,the water overcomes itself under the action of evapotranspiration,capillary force and adsorption force.In addition to natural and geological factors,gravity moves upward,and human activities are also important driving forces for water and salt movement.(5)During the agricultural irrigation period from May to September,under the two modes of water level regulation—fresh water irrigation and water level regulation—salt and light mixed irrigation,the crop output is around 350 kg/mu,water level regulation-mixed irrigation mode increased groundwater utilization efficiency by 13.3%.Therefore,under the conditions of water level regulation,freshwater irrigation is used in the early stage of crop growth,and shallow shallow groundwater is used in the later stage of crop growth to carry out salt water and fresh water rotation or mixed irrigation.The groundwater utilization efficiency and salinization control can be achieved during the entire growth period of the crop The double effect.