Study On Soil Pore Structure Under Reclaimed Water Irrigation Based On Fractal Theory

Reclaimed water irrigation is beneficial to alleviate the shortage of agricultural water in China.However,the composition of reclaimed water is complex,and it will affect the soil properties if it is used for irrigation.In this study,the indoor soil column simulation test was conducted,and tap water（W4）was taken as the control group.Three kinds of water quality were set,namely domestic sewage（W1）,reclaimed water 1（W2）and reclaimed water 2（W3）.The data of soil salinity,soil moisture characteristic curve,soil particle distribution and soil pore structure were obtained after irrigation for 1 year and 2 years respectively.Firstly,the relationship among soil salinity,soil pore structure and soil hydraulic properties under reclaimed water irrigation was studied.The second is to quantify the spatial heterogeneity and complexity of soil pore structure through fractal theory,explore the relationship among fractal parameters,soil pore structure and hydraulic properties,and study whether fractal parameters can be used as potential indicators to reflect the changes of soil physical properties after reclaimed water irrigation,and finally provide some theoretical basis for the safe and efficient utilization of reclaimed water.The conclusions of this study are as follows:（1）Irrigation with reclaimed water and domestic wastewater alters soil salinity and hydraulic properties.For soil salinity,soil acidity（pH）,electrical conductivity（EC）and sodium adsorption ratio（SAR）increased under W1-W3 treatment compared to W4 treatment as follows:after 1 year of irrigation,soil pH,EC and SAR were the largest under W1 treatment and showed significant differences between W1-W3 treatment and W4 treatment（P<0.05）.After 2 years of irrigation,soil pH,EC and SAR were the smallest under the W4 treatment and there was no significant difference between the W1-W3 and W4 treatments.The mean increase in EC was greater under treatments W2 and W3 compared to 1 year of irrigation,at91.10%and 96.48%respectively,meaning that soil salinisation may occur with long-term reclaimed water irrigation.For soil hydraulic properties,soil air intake value（1/α）,field water holding capacity,wilting coefficient and effective soil water content increased in the W1-W3treatments compared to the W4 treatment after 1 year of irrigation,and the initial soil drainage difficulty,lower soil water limit and water retention properties increased,with the W1treatment having the greatest soil field water holding capacity,wilting coefficient and effective soil water content,with an average increase compared to the W4 treatment of 11.55%,7.66%and 19.81%.The W1-W3 treatments 1/αand effective soil water content increased after 2years of irrigation,and the initial soil drainage difficulty and water retention properties increased.Compared to 1 year of irrigation,soil 1/α,field water holding capacity,wilting coefficient and effective soil water content increased in the W2 and W3 treatments after 2years of irrigation and decreased under the W1 treatment.（2）During the test period,the T₂ spectrum of soil under irrigation with four kinds of water quality is mainly short relaxation peaks,showing two or three peaks,and the sum of"mesopores+macropores"of soil is less than the sum of"micropores+ultramicropores",so the water retention capacity of the test soil is greater than the drainage capacity,which is easy to cause water accumulation.Compared with W4 treatment,after one year of irrigation,the proportion of micropores and mesopores in 0-10cm soil layer increased under W1-W3treatment,while the proportion of ultrafine pores decreased.In 10-20cm soil layer,the proportion of micropores increased,while the proportion of mesopores and macropores decreased.After two years of irrigation,compared with W4 treatment,the proportion of ultra-micro pores in W1-W3 treatment increased,while the proportion of micro-pores and middle pores decreased.Compared with one year’s irrigation,after two years’irrigation,the proportion of ultramicro pores in W1-W3 treatment increased,while the proportion of micro pores decreased.W4 treatment reduced the proportion of ultra-micro pores,increased the proportion of micro-pores,middle pores and macro-pores,and W1-W3 treatment would reduce the larger pores in soil.（3）The soil pore distribution under the four water quality treatments showed multifractal characteristics.After one year of irrigation,compared with W4 treatment,the multifractal parametersΔD、Δα（q）andΔf（α（q））of soil pore distribution in W1-W3 treatment increased,that is,the complexity,inhomogeneity and asymmetry of soil pore distribution increased in the initial stage of reclaimed water and sewage irrigation.On the contrary,after two years of irrigation,the multifractal parameters of soil in W1-W3 treatments decreased,and the pore structure of soil tended to be single.In addition,the multifractal parameters of soil pores are controlled by the relative contents of soil ultramicro-pores,micro-pores and mesopores,and are significantly positively correlated with available water and K_s（P<0.05）.Therefore,multifractal parameters can indirectly characterize soil pore structure and hydraulic properties,so they can be used as potential indicators to reflect the changes of soil physical properties after reclaimed water irrigation.