Study On Soil Water Salt Transport Law In Unsaturated Zone Under Saline Water Point Source Infiltration

The phenomenon of regional water shortage is obvious,especially in the arid and semi-arid areas of Northwest China.The surface fresh water resources are very limited,which is difficult to meet the demand of large amount of water for agriculture.Therefore,the salinized shallow groundwater resources are gradually developed and utilized as supplementary water sources.In the 1980s,Xinjiang began to use the method of"film mulching drip irrigation"for agricultural irrigation.Water and salt entered the soil evenly,regularly and quantitatively through the dripper to achieve the purpose of water saving.In arid agricultural oasis area of Northwest China,from piedmont to plain area,groundwater mineralization gradually increases and soil permeability gradually decreases.In order to ensure soil moisture,drip irrigation（emitter spacing,flow rate and time）should be suitable for different places.Therefore,it is urgent to further study the law of soil water salt transport in unsaturated zone under the condition of point source infiltration with different salinity and permeability media,so as to provide technical support for saline water irrigation in northwest arid oasis.In this paper,the law of soil water salt transport under the condition of point source infiltration with different salinity and permeability media was studied by means of field saline fresh water irrigation experiment,indoor physical experiment and numerical simulation.Firstly,the indoor physical experiments were carried out in a sand model box to analyze the water salt transport law of sand under the condition of 0.18g/L（fresh water）,3g/L（brackish water）,7g/L and 10g/L（saline water）point source infiltration of different salinity water.Combined with the field saline water and fresh water irrigation experimental data in 2018,the influence of point source infiltration of different salinity water on the water salt transport of the same soil was analyzed.Secondly,the hydrus numerical simulation method is used to further compare the water salt transport laws of sandy soil,loamy sandy soil and sandy loam under the condition of salinized water point source infiltration,and analyze the influence of different permeable media on water salt transport.Finally,the distribution and migration distance of point source infiltration water and salt with different soil mass and mineralization degree were comprehensively analyzed to provide the basis for drip irrigation system in northwest arid oasis.The main conclusions are as follows:（1）The influence of different salinity on water salt transport of sandy soil:the change rate of water content in salinized water is significantly greater than that in fresh water,and the water salt transport distance from salinized water is greater than that of fresh water,and the greater the salinity is,the farther the water salt transport distance is.The vertical migration range of sandy brackish water（1.70g/L）is30-40cm,and that of fresh water（0.68g/L）is lower than that of brackish water in the range of 0-30cm.At the edge of sand wet body expansion,the change rate of water content is 10g/L>7g/L>3g/L>0.18g/L,the change rate of salt and the salt migration distance are 3g/L<7g/L<10g/L,and the vertical migration rate is higher than the horizontal migration rate.Under the condition of water infiltration with different salinity,obvious salt accumulation occurs near the vertical distance of 40 cm,and the maximum values of salt concentration increase△C_max are 5g/L,11g/L and 25g/L,respectively.（2）The numerical simulation results show that with the increase of salinity of infiltration water,soil salt concentration increases,and salt accumulation occurs in the wetting front.The salinity of salt accumulation area increases with the increase of salinity of infiltration water,which are 4～4.5g/L,8～10g/L and 14～15g/L respectively.Under the same salinity water infiltration condition,the vertical migration distance of soil water is sand>loamy sand>sandy loam,and the horizontal migration distance is sandy loam>loamy sand>sandy soil.（3）Based on the experimental data and numerical simulation results,the influence of salinity and medium permeability on water salt migration in vadose zone is quantitatively analyzed,and the following linear equation is obtained.Sand in test area:The relationship between horizontal migration distance S_L and mineralization degree TDS is S_L=0.0180TDS+0.2730;the relationship between vertical migration distance S_T and mineralization degree TDS is S_T=0.0154TDS+0.3202.The horizontal migration distance of water salt decreases with the increase of saturated permeability coefficient K_S,and the vertical migration distance increases with the increase of saturated permeability coefficient K_S.（4）Based on the characteristics that the permeability of vadose zone media from upstream to downstream gradually weakens and the groundwater mineralization gradually increases in northwest arid oasis area,in order to ensure soil moisture,the drip irrigation scheme with small emitter spacing,small flow and long time in upstream area is recommended,and the drip irrigation scheme with large emitter spacing,large flow and short time in middle and downstream area is recommended.