Soil Water Movement Characteristics Under Treated Wastewater Irrigation

Treated waste water（TWW）has become a trend to replace the fresh water（FW）in agricultural irrigation.However,the composition of treated waste water is complex and the water quality is different from fresh water,then the characteristics of its infiltration will be different;long-term irrigation with TWW can cause some soil pollution,such as soil water repellency that affects infiltration characteristics.Previous studies mainly focused on the effects of long-term irrigation with treated waste water on soil physical and chemical properties,hydraulic properties and soil quality indexes,the effects on crop yield and quality,as well as ecological and environmental protection.In order to determine the water quality index of different soil reclaimed water irrigation and provide some guidance for the establishment of its irrigation system,in this article five kinds of TWW and tap water（TW）were chosen to irrigate the wettable and repellency soil in the one-dimensional infiltration experiments and the determination of soil water characteristic curve test,And infiltration test was conducted on soil with different degrees of water repellency generated by long-term irrigation with reclaimed water.The main research conclusions were as follow:（1）The larger the comprehensive water quality index was,the smaller the moisture content was under the same matrix suction condition,that is,the easier the soil moisture was sucked out.There was a negative linear correlation between air entry value and water quality.With the increase of the comprehensive water quality index,the extremely small pores in the water-repellent and hydrophilic clay loam decreased,and the medium and large pores increased.Field capacity,wilting coefficient,effective water,ineffective water and easily available water all decreased with the increase of comprehensive water quality index,but the field capacity and ratio of easily available treated waste water did not decrease significantly.（2）Treated waste water had a great influence on the cumulative infiltration amount and wetting front of water-repellent clay loam and water-loving sandy loam.There was a power function relationship between the soil sorptivety（S）and chemical oxygen demand（COD）,a quadratic polynomial relationship between the ratio of sorptivety（S）and wetting front coefficient（a）and the comprehensive water quality index Z_F,and the minimum value existed in the curve.Soil water repellence inhibited soil infiltration,but when the water quality comprehensive index Z_F was large enough in sandy loam infiltration,the water repulsion inhibits soil infiltration less.（3）The greater the water repellent intensity was,the slower the migration speed of the wet front within the same infiltration time was,and the slower the infiltration rate was.The water repulsion results in the non-uniform migration of the wetting front during the infiltration process,and the degree of non-uniformity increases with the degree of water repulsion.Kostiakov model was more suitable to describe the relationship between soil infiltration rate and time with different degrees of water exclusion.The water content of the soil profile was smaller than that of the hydrophilic soil when the water repellent caused the infiltration to stop.At the same time,in the redistribution stage,the evaporation of soil moisture in the upper layer of soil column was inhibited,and the movement of soil moisture in the lower layer of soil column was also delayed.The greater the degree of water exclusion,the slower the redistribution was,the smaller the change of water content was,and the smaller the wetting volume was under the original humid front.（4）In order to improve the effective depth of soil irrigation,the high frequency and small flow irrigation method should be adopted to reduce the irrigation time.The COD and Z_F of the water used for irrigation in the hydrophilic sandy loam were less than 170 mg L^-1and 0.59,respectively,and the COD and Z_F of the water used for irrigation in the water-repellent sandy loam were less than 140 mg L^-1 and 0.10,respectively.For clay loam,the effects of recycled water quality on ecology,environment,crops and human health should be considered.