The Numerical Simulation Of Water Movement In Water Repellent Soils

Soil water repellency(SWR)is a soil physical property which was found globally.SWR affects soil water movement significantly.With the popularity of numerical simulation applying in soil water movement,the research of numerical simulation of water movement in water repellent(WR)soils is of guiding significance to fully understand the law of water movement in WR soils.In this paper,the experiments of infiltration and evaporation in homogeneous and layered WR soils and summer maize growing in the WR soils were simulated with HYDRUS-1D under the initial and boundary conditions.The performances were evaluated with R~2,RRMSE and NSE.For infiltration and evaporation experiments,the RETC software was used to obtain the soil water hydraulic parameters.The parameters of residual water content(θ_r),saturated water content(θ_s)and saturated soil water conductivity(K_s)were measured.The soil water movements were simulated after the calibration and validation ofαand n.There were 5experiments(CK,WR1,WR2,WR3 and WR4)in summer maize growth experiments under the rain-free shelter conducted in 2016 and 2017.The soil water repellent levels of Lou soil were wettable and slightly.The WR levels were wettable and slightly.The observed daily LAI during growth period of summer maize in 2016 and 2017 were fitted with a Gaussian-3Parameter function dependent on cumulative temperature.The daily ET_p values,calculated with Penman-Monteith equation were separated to E_p and T_p according to LAI and input to HYDRUS-1D.the soil water hydraulic parameters ofα,n and K_s were inversely obtained with HYDRUS-1D according to the measured soil water content(θ_v)during the summer maize growth period in 2016.The parameters ofθ_r andθ_s were measured.The soil water hydraulic parameters were validated withθ_v in 2017.Finally,the HYDRUS-1D was applied to simulate the law of water movement in WR soils,which could obtain data that was difficult to be directly obtained by experiments and simulate different experiments scenarios.The main results of the simulation of water movement in WR soils are:(1)The HYDRUS-1D was used to simulate the water movement in the different soils with wettable,slightly WR and strongly WR during the horizontal imbibition and vertical infiltration.The soil water hydraulic parameters ofθ_r,θ_s and K_s were measured andαand n were fitted by soil water retention curves with RETC softeware.The soil water hydraulic parameters were calibrated and validated.The performances of HYDRUS-1D for simulating the water movement in the WR soils were generally good.For wettable soils,the range of RRMSE,R~2 and NSE were 2.2%-16.6%,0.93-0.999 and 0.783-0.998,respectively.For slightly WR soils,the range of RRMSE,R~2 and NSE were 1.5%-11.7%,0.942-0.996 and0.796-0.996,respectively.For strongly WR soils,the range of RRMSE,R~2 and NSE were8.4%-21.1%,0.876-0.99 and 0.774-0.989,respectively.For the same soil,the performance decreased as the WR level increased.In general,HYDRUS-1D could be used to simulate soil water dynamics of wettable,slightly WR and strongly WR soils effectively.The simulated CI,Z_f andθ_v for strongly WR soils during vertical infiltration using HYDRUS-1D clearly revealed the differences of soil water dynamics for Lou soil,sand,lime concretion black soil and saline-alkaline soils as ponded depth increased from 4 to 6,8,and 10 cm,respectively.It could provide references for simulating soil water movement and solute transport under different WR levels and soil textures.(2)The soil water hydraulic parameters of Lou soils with wettable,slightly WR,strongly WR and severly WR levels and wettable Sand were calibrated and validated with HYDRUS-1D during the infiltarion in the layered soils.For example,in the Lou soil/sand scenario and sand/Lou soil scenario,R~2 for CI ranged from 0.912 to 0.999 and 0.957 to 0.999,R~2 for Z_f ranged from 0.951 to 0.999 and from 0.976 to 0.999,and R~2 forθ_v ranged from 0.940 to 0.993and from 0.952 to 0.999,respectively.The infiltrations in the silt loam/sand and sand/silt loam scenarios with interlayer positions of 10 and 20 cm and different WR levels of silt loam were simulated by HYDRUS-1D.Overall,the calibration and validation verified the generally good performance of HYDRUS-1D and its reliability to simulate water movement in WR layered soils.The simulated results of the various layered WR soils conditions confirmed that the WR interlayer blocked and backfilled water and reduced the infiltration rate.The layered WR soil conditions greatly affected the soil infiltration characteristics with blocking water and reducing infiltration,especially for the silt loam/sand scenario.In this scenario,when the top layer thickness was 10 cm for all WR levels,the greatest effects were observed.For the sand/silt loam scenario,the infiltration reduction was most pronounced in the treatments with a 10-cm top layer thickness when the silt loam interlayer was wettable.When silt loam interlayers were slightly,strongly and severely WR,the effects were the most pronounced in the treatments with a 5-cm top layer thickness.In general,the WR interlayer position controls the infiltration behavior.The WR level controls infiltration more strongly than the interlayer position,especially for the sand/silt loam scenario.As the WR level increased from wettable to severely WR,θ_v in the top and interlayer soil increased similarly with t,butθ_v in the bottom layer noticeably decreased.HYDRUS-1D provides a powerful help for the research of water movement process in WR soils.