The Effect Of Different Water Repellent Degree And Water Repellent Depths To Characteristics Of Soil Infiltration

Soil water repellency has a great influence on water infiltration and migration and affects agricultural water management.For studying the effect of water repellent degree and depths of repellent layers on soil infiltration characteristics,the stable water repellent soils were obtained by adding different doses of octadecylamine to the soil.The treatments with repellent thickness of 5 cm for single layer and 10cm for double layer were designed,and the indoor one-dimensional vertical infiltration tests were conducted to study its infiltration characteristics.HYDRUS-1D was used to simulate water movement in the water repellent soil at the depth of 0-10 cm.Treatments C0-C4(5 cm)and V0-V4(10 cm)were conducted to calibrate and validate the parameters of van Genuchten model,respectively.Treatments S1-S10 were selected to simulate tilled scenarios.RRMSE(relative root mean square error)and CRM(coefficient of residual mass)were used to evaluate accuracy.At present,there is no modified infiltration model applicable to water repellent soil.Under the assumption that the saturated and unsaturated zone had the same thickness,the Green-Ampt model is modified by combining computational equations of suction head and harmonic mean of saturated hydraulic conductivity.Our objectives is to validate simulation effect of modified Green-Ampt model on cumulative infiltration(CI),distance of wetting front(Z_f)and infiltration rate of repellent soil.The main research conclusions were as follow:(1)Variation of infiltration characteristics of layered water-repellent soil.The results showed that soil saturated water conductivity decreases with the water repellent degree increased.The cumulative infiltration curves converted from concave to convex.However,the late infiltration time of treatments with double repellent layers was close to that of the single repellent layer when the surface layers were same repellent degree.Moreover,the infiltration time of high-degree water repellent treatments with a thickness of 5 cm were much longer than that of low-degree treatments with a depth of 10 cm.This reflected that the influence of the water repellent degree on the infiltration is greater than the repellent thickness.When the wetting front passed through the contact layer between water repellent layer and the hydrophilic layer,infiltration rate showed sudden change and increased slowly in the water repellent layer,and reached the maximum value in the contact layer at this stage,and then gradually decreased to the stable value.The water repellent degree had a great influence on moisture content of soil column profile,the water content of severely repellent and extremely repellent layers decreased greatly.(2)HYDRUS-1D was used to simulate water transport in layered water-repellent soils.When Z_f reached 40 cm,the measured CI andθ_vwere compared with the simulated data.For the treatments of calibration and validation,the values of RRMSE were smaller than 0.251(extremely water repellent soil)and 0.101,respectively;and the values of CRM ranged from-0.009 to 0.136(extremely water repellent soil)and-0.019 to 0.089,respectively.Furthermore,for single layered and double layered water repellent soil,the fitting accuracy ofθ_v,CI,Z_f decreased with the increase of repellent degree and thickness.Treatments at the repellent depth of 10 cm had higher fitting accuracy than that at 5 cm when the topsoil had the same water repellency.(3)A modified Green-Ampt model for water-repellent soil infiltration was established.The results showed that as the repellent degree increased,the soil suction head decreased,and the relationship between the value of soil suction head and the degree of water repellent soil was exponential.In addition,the simulated values of the modified CI equation highly fitted the measured values of all treatments.The modified Green-Ampt-VG model had the best simulation effect to infiltration rate and Z_f.Results indicated that the suction head values obtained from parameters of VG model were closest to the actual values.The results can provide reference for the computation of suction head and water infiltration into water repellent soil in the future.