A Transient Evaporation Method for Determining Soil Hydraulic Properties at Low Pressure

Abstract

A two-stage laboratory method is presented for rapid estimation of the soil water retention function ψ(θ) and the hydraulic conductivity function K (θ) from near saturation to air dry. The pressure head, h , at the bottom of an initially saturated soil core was stepwise decreased whenever the outflow rate had essentially ceased. After attaining this near-equilibrium at ψ = −160 cm, the bottom boundary was sealed, the soil surface was uncovered, and the evaporation rate at a controlled constant temperature was measured. The ψ(θ) was determined by curve-fitting of the equilibrium outflow and psychrometric data obtained from soil samples after evaporation, while the K (θ) was estimated inversely using cumulative evaporation amounts and the final water content profile. The simulated cumulative evaporation and final water content profiles from the optimized parameter values agreed well with the measured data. The root mean square errors for evaporation and the final water content profile were less than 0.022 cm and 0.011, respectively. In addition to close agreement with the simulation, agreement with independent K (θ) data obtained from a steady-state, direct method confirm the reliability of the method presented here.