Experimental Research On Effect Of Initial Water Content On Repellent Soil Infiltration Characteristics

As a special property of soil,water repellency couid block water wetting soil particles,inhibit water diffusion migration,destroy the stable seepage process,reduce soil infiltration and water retention capacity,generate water runoff and unstable flow,aggravate soil erosion damage,and be not conducive to product agriculture and environment.Water repellent soil distributes widely,involves many types,and has many differences in physical and chemical properties at different spatial scales.Repellency levels is impacted by combination complex factors and various environments.This paper measured water drop penetration time,contact angle of different soil types and repellency levels in evaporating moisture test,studied the soil infiltration laws of different soil types and initial soil moisture contents,established and verified an infiltration model with water ponding infiltration experiments of water repellent sand in different initial soil moisture contents.The conclusions are as follows:(1)With the increase of initial soil moisture content,WDPT was a unimodal curve change trend and expressed repellency peak point that repellent clay loam was in 12%~15% and repellent sand was in 1.5%~2% moisture content scope.With the increase of water repellency degrees,lower threshold moisture content of WDPT decreased and WDPT’s peak values increased gradually.Among them,the initial moisture content decreased which corresponding to WDPT’s peak values of sandy soil,and it was not clear for water repellent clay loam.The evaluation index of contact agle method and WDPT method is different.Contact angle method is the strength levels of water repellency,while WDPT method is the persistence.According to actual situation,we can choose suitable one.(2)Based on results with two-dimensional soil box.For water repellent clay loam,with the initial moisture content increased,the largest wetting front transport distance increased after decreased first,and wide depth ratio decreased.There was the power function relationship between wetting front transport distance and time.Cumulative infiltration amount increased after reduced,and infiltration rate decreased with the increase of time.In infiltration process,there was unstable seepage phenomenon and infiltration rate increased in a short time of higher repellency treatments.For water repellent sand.Because of water repellency,there was some water ponding on the soil surface and wetting front movement distance increased with initial moisture content increasing at the early test stages,and water moved rapidly in vertical direction at the stage of unstable flow.Infiltration rate was mainly affected by water supply intensity and the section area size of seepage channel.There were linear relationships between wetting front movement distance,two-dimensional wet body area and time.Both finger flow length and wetting front speed increased and then decreased,and finger flow width decreased gradually,the values of morphological index presented random fluctuations and the average value increased and then decreased.(3)It was a great influence on soil water distribution by the initial moisture content.For water repellent clay loam.Soil moisture concentrated on the center area and water potential gradient increased significantly at the edge of body with initial moisture content increased,at the moisture content of WDPT’s peak,there was saturation overshoot.With initial moisture content further improved,both soil moisture and water potential gradient had a contrary trend to the moisture content before WDPT’s peak.For water repellent sand.Infiltration status had a great effect on profile moisture content.It was affected by the quantity of soil water ponding during initial period.The more water ponding accumulated,the greater moisture content of shallow profile appeared.It had great influences on infiltration process by the cross-sectional area size of finger seepage channel during late period.The smaller cross-sectional area was,the higher profile moisture appeared.(4)Based on the water ponding infiltration test results of water repellent sand with a one-dimensional hollow pillar.Through the parameters relevance from Green-Ampt model and Philip model,this paper established a model about average moisture content increment and time,and solved parameters and verified its applicability.There was a linear growth relationship between wetting front movement distance and time.In infiltration process,matrix suction,water gravity and water repellency worked together and changed all the time,and soil permeability coefficient could not reach saturated hydraulic conductivity.This paper only studied rules of water repellent soil aimed at initial moisture content in two kinds soils.Less factors and soil types makes the results have certain limitations.