Variation And Simulation Of Hydraulic Characteristics Of Typical Subtropical Soil Irrigated With Reclaimed Water

The utilization of wastewater resources is of great significance to optimize water supply structure,increase water supply,alleviate the contradiction between supply and demand,reduce water pollution and ensure water ecological security.Irrigation and utilization of reclaimed water in subtropical region is an important measure to alleviate seasonal and water quality water shortage in the region.Long term use of reclaimed water with relatively high concentration of various salt substances for irrigation may lead to soil salinization and degradation of soil hydraulic characteristics,which aggravates soil erosion and potential risk of agricultural non-point source pollution in subtropical region.In this paper,red soil,fluvo aquic soil,purple soil,paddy soil and other four kinds of typical soil in subtropical areas were taken as the research objects.The indoor soil column simulation irrigation experiment was carried out,and the secondary treatment effluent of urban sewage treatment plant was selected as the reclaimed water source.The effects of four kinds of reclaimed water quality(reclaimed water stock solution and the reclaimed water stock solution diluted 2,4,6 times)on soil physicochemical and hydraulic characteristics were carried out under single irrigation mode,the influence of reclaimed water quality difference on soil physical and chemical key indicators and hydraulic characteristics was investigated.Considering the unique condition of water and heat synchronization in subtropical region,the change characteristics of soil physicochemical indexes and soil hydraulic parameters under alternate irrigation of reclaimed water and distilled water were revealed.The effects of three-year reclaimed water irrigation on soil physicochemical indexes and hydraulic characteristics were simulated,and the change law of soil hydraulic characteristics was explained.Through correlation analysis,principal component analysis and cluster analysis,the relationship among irrigation water quality,soil physicochemical and soil hydraulic parameters was analyzed,and the statistical model between soil hydraulic characteristics and soil physicochemical indexes was established.The main conclusions are as follows:(1)There is a significant correlation between reclaimed water irrigation water quality and soil salinity and other physicochemical indexes.Reclaimed water irrigation will increase soil EC value.The higher the concentration of reclaimed water,the greater the EC value.The alternative irrigation mode of reclaimed water distilled water will reduce the correlation and reduce the impact of water quality on soil.Compared with distilled water irrigation,EC Value of reclaimed water single irrigation increased by 110.3% on average,and exchangeable cations,especially exchangeable Na+,increased by 238.6%.Alternative irrigation could reduce the increase of EC Value by 6.0%,but the increase of exchangeable Na+ increased by 113.6%,resulting in the increase of ESP and SAR.With the increase of irrigation years,the salt content of red soil tends to accumulate,and the changes of soil salinity and SOM affect the changes of soil pores,resulting in the decrease of micro pores and available pores and the increase of macro pores in red soil,while the other soils are on the contrary.During the three-year irrigation process,reclaimed water stock solution irrigation is conducive to the maintenance and formation of available pores and macro pores in red soil and fluvo aquic soil.(2)With the increase of wet-drying cycles,the infiltration rate of aquic soil,purple soil and paddy soil under single irrigation with reclaimed water decreased by 83.4%,37.4% and84.5% on average,respectively.Alternate irrigation with reclaimed water and distilled water increased the attenuation degree of soil infiltration rate,which increased by 8.2%,37.9%and 5.4% in aquic soil,purple soil and paddy soil,respectively.The soil infiltration rate of reclaimed water irrigation is higher than that of distilled water irrigation.(3)There was a significant correlation between saturated water conductivity and soil salt and pore distribution,especially in red soil,where salt mainly played a inhibiting role.The saturated water conductivity of red soil,aquic soil,purple soil and paddy soil was reduced by 36.8%,38.3%,24.8% and 5.3%,respectively,by single irrigation mode with reclaimed water.The correlation between saturated water conductivity and salt content was decreased in the regenerated water-distillation alternate mode.The saturated water conductivity of red soil,aquic soil,purple soil and paddy soil changed by-37.7%,52.9%,26.7% and-28.2%,respectively.With the increase of irrigation years,the saturated water conductivity of each treatment soil has a tendency of decline.(4)The water holding capacity of red soil increased by about 60.0% on average under single irrigation mode with reclaimed water stock solution,while decreased by about 86.1%under alternate irrigation.the single irrigation mode of reclaimed water stock solution is beneficial to the water holding capacity of fluvo aquic soil,and alternate treatment of reclaimed water stock solution and distilled water reduced the water holding capacity of fluvo aquic soil by about 54.6%.The water holding capacity of purple soil and paddy soil decreased by 43.1% and 40.2% respectively under single irrigation mode with reclaimed water,and increased by 157.0% and 31.8% respectively under alternate irrigation.With the increase of irrigation years,the water holding capacity of red soil and paddy soil decreased.Under the irrigation of reclaimed water stock solution of red soil,the water holding capacity decreased by 62.9%.Under the alternate irrigation of reclaimed water stock solution distilled water,the water holding capacity of paddy soil decreased by 47.1%,while that of fluvo aquic soil and purple soil changed little.The soil water characteristic curve of reclaimed water irrigation was fitted by van genuchte model,and good results were obtained(R2 > 0.98).(5)The water availability of red soil irrigated by reclaimed water changed the largest,the drainable water of red soil increased by 8.5% and the available water decreased by10.7% under single irrigation mode with reclaimed water,and the drainable water increased by 38.5% and the available water decreased by 12.6% under alternate irrigation.With the increase of irrigation years of reclaimed water,the available water of red soil decreased,and the reduction degree of the alternate irrigation of reclaimed water stock solution and distilled water was twice that of the reclaimed water stock solution.(6)Single irrigation mode with reclaimed water alone inhibited the unsaturated water conductivity and water diffusivity of red soil,but promoted the unsaturated water conductivity and water diffusivity of fluvo aquic soil,and had little effect on the rest of the soil.Alternate irrigation with reclaimed water and distilled water will weaken the influence of water quality on soil.with the increase of irrigation years,reclaimed water irrigation can improve the unsaturated hydraulic conductivity of red soil and the unsaturated hydraulic conductivity of fluvo aquic soil,purple soil and paddy soil with lower moisture content(<0.4 cm3/cm3),and reduce the unsaturated hydraulic conductivity of fluvo aquic soil with higher moisture content(>0.4 cm3/cm3);reclaimed water irrigation can increase the soil water diffusivity,especially red soil.(7)Soil saturated hydraulic conductivity,van Genuchten model fitting parameters and soil physicochemical properties were in line with multiple linear regression model.The fitting effect of red soil and fluvo aquic soil was the best under single irrigation mode(R2>0.70).Alternate irrigation would reduce the fitting effect,and the fitting effect of each soil was better under different irrigation years.In conclusion,the reclaimed water irrigation of red soil,fluvo aquic soil,purple soil and paddy soil will cause the increase of salt content and the change of soil hydraulic index.Among them,the red soil is the largest affected,and the better water quality intervention will weaken the influence degree of soil.However,the variation of exchangeable sodium,ESP,SAR and other indicators of red soil,as well as the attenuation degree of saturated water conductivity and infiltration rate,will increase.Considering the water in subtropical region in the same period of heat,special attention should be paid to the irrigation and utilization of reclaimed water in red soil.The research results in this paper are expected to provide theoretical reference and reference for the rational irrigation of reclaimed water in subtropical region and further research.