Finger Flow Development Experimental Research In Water-repellent Soils

Water-repellent soils exist extensively in the world,and it could make negative effects on the environment directly or indirectly,through aggravating soil erosion and runoff on slope,causing finger flow and other preferential flows which leading solute transporting and underground water contamination.Therefore,Soil Water Repellency(SWR)is negative to the sustainable agriculture development.To research the SWR major mechanism and its influence factors,with referring previous concerning researches,three measurement methods,including the Water Droplet Penetration Time(WDPT),the Molarity of Ethanol(MED)and the Contact Angle(denoted as ? below)methods were applied to obtain different SWR indices for the tested soils collected in Manas River basin,Xinjiang,China.The height and the weight methods were conducted to measure ? values.WDPT values of different soil samples were compared to investigate whether WDPT values were affected by oven-dry or different bulk densities,the relationships among three SWR indices(WDPT,MED and ?)were compared,and the major soil physical chemical properties which influence SWR were analyzed to discuss the intrinsic mechanism that causes SWR.On the basis of comparing different SWR indicies,water movement law was researched in the water-repellent soils.The method of adding water-repellent chemical into wettable soils was conducted.Through soil chamber infiltration experiments,the water moving behaviors in different levels of water-repellent homogeneous and layered soil were analyzed and the finger flow indicies were calculated in the treatments which finger flow occurred.The experiments result showed that:(1)There were correlations between the three SWR indices of WDPT,MED and ?,but MED was insignificantly related to ?,which showed the differences among various SWR indices although they were related.WDPT values of high bulk density were larger than those of low bulk density;and WDPT values of oven-dried soils were larger than those of weather-dried soils.When using the Height and the Weight methods measure ? values of soil-water,? values obtained using the Height method were larger than values obtained using the Weight method,and the differences of ? for the three replications with the Height method was smaller than with the Weight method.When using Octane as the reference liquid for measuring ? values of soil-water,its differences of the three replications were small,being lower than ethyl alcohol.Among the studied multi-physical chemical properties,clay contentaffected WDPT and ? values more significantly than other soil physical chemical properties.Concentrations of K+ and Na+ were both positively correlated to ? values;while the contents of montmorillonite were negatively correlated to WDPT and MED values.Among the three SWR measurement methods,the WDPT method is quite simple in operation but is susceptible;the MED method shortens the infiltration time but is time-consuming in operation;the contact angle method is complicated in operation but the results are relatively accurate,and when using octane as the reference liquid for the Height method,it is more reliable than using the other liquids.(2)No mater homogeneous and layered soil,SWR could impede water infiltration,longing the infiltration time.In the condition of homogeneous soils,infiltration time in Lou Soil was higher than Wei River Sand.It means that water moving velocity in clay loam was slower than sandy loam.Using Kostiakov infiltration model to fit the water movement in homogeneous soils,the result showed that the cumulative infiltration(CI)in the five levels(wettable,slightly water-repellent,strongly water-repellent,severely water-repellent and extremely water-repellent)of water-repellent Lou Soil decreased with the time.However,in water-repellent Wei River Sand,except the wettable treatment,CI of other four ones increased with the time,which was contrary to Lou Soil.This means the finger flow in water-repellent soil could cause water moving quickly.(3)In homogeneous soils,with the water-repellent level increased,the shape of wetting front changed from almost parallel lines to non-uniform curves and non-uniform and finger flow occurred.At the same level of SWR,non-uniform wetting front preferred to occur in Wei River Sand rather than Lou Soil.This showed that soil texture and SWR could effect non-uniform flow and finger flow comprehensively.Coarser soil texture was more easily to cause the non-uniform flow and finger flow.In homogeneous soils,cumulative wetting area(WA)and CI had a good positive linear relationship.But in the treatments of SL3~SL5(homogeneous Wei River Sand with water-repellent level ranging from strongly to extremely),this relationship was power functional type.It means that finger flow could bring quite large amount of water through the soil.The finger flow indices were calculated in the treatments of SL3~SL5.SWR could accelerate water moving in homogeneous soil.The value of finger top velocity(Fv)increased with SWR level increasing.But it could decreased with the SWR level further increasing.In the homogeneous Wei River Soil,the shape of finger flow changed from thin,long to wide and short.SWR could make an intensive unevenly effect on soil water content(?v).However,there were different influences of ?v among different water-repellent levels.In Lou Soil,the coefficient of variation(Cv)of LL5 had the largest value,which performed the most uneven ?v distribution.In homogeneous Wei RiverSoil,the Cv of SL3 had the largest value.And the Cv of Wei River Soil was higher than Lou Soil,which means that the sandy loam preferred to have uneven ?v distribution rather than clay loam.(4)In layered water-repellent soils,water movement was different with the wettable water-repellent soils.The temporal variations of CI decreased with the increase of the water-repellent level.CI was generally larger when the top layer soil was sandy loam rather than clay loam;therefore,the top layer soil texture controlled CI more than the sub-layer.For the wettable treatments,finger flow clearly and uniformly were generated in layered soils with a sub-layer of heavy gravel rather than sand.But for water-repellent layered treatments,fingers developed irregularly with the water-repellent levels.Finger length(FL),width(FWh),finger top and bottom velocity(Fv and Bv),finger shape index(SI)and finger distributing index(DI)varied with the water-repellent levels.The maximum value of SI and DI appeared in the GAL3(strongly water-repellent clay loam/sand)and GCL5(extremely water-repellent clay loam/sand).This showed the condition of water-repellent level and soil layers,where finger flow most easy occur.Under different water-repellent conditions,finger flow parameters changed randomly.But among water-repellent and wettable treatments,differences were still obvious.There were good linear correlations between CI and WA,and between CI and FL.?v in the top 10 cm layer was higher than sub-layer and generally decreased with the increase of water-repellent levels.The finger flow development in layered water-repellent soils were generally irregular and showed large extent of complexity.