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Soil hydraulic properties are basic inputs for simulating water and solute transport in the vadose zone. Many methods have been developed to directly measure or indirectly estimate soil hydraulic properties. However, low-cost, simple and robust methods are still scarce even in a laboratory. Here we present a new analytical method, based on an approximate solution to the one-dimensional vertical infiltration problem that can be used to determine Brooks-Corey model parameters. Measurements include cumulative infiltration, wetting front advance with time, and initial and final upper boundary water contents, during a one-dimensional vertical infiltration experiment. Infiltration experiments with twelve disturbed soils were performed to test the new method. The results indicated that the estimated saturated hydraulic conductivities from transient infiltration processes were consistent with those measured with a constant-head method. The estimated soil water retention curves (SWRCs) by the analytical method for the twelve soils were in agreement with those measured by a direct method in which soil water potentials were measured through tensiometers. The root mean square error (RMSE) of the estimated soil water contents by the new method were less than 0.045 cm3 cm-3 for the twelve soils. Compared to most direct methods, the new method is inexpensive, rapid and easy to use, and thus can be a substitute for direct methods in a laboratory. Although how to monitor representative wetting front remains unsolved, the new method has a potential for use in the future in determining in situ soil hydraulic properties where lateral flows are negligible. [ABSTRACT FROM AUTHOR]

The Green--Ampt model (GAM) is a widely used water infiltration model. However, it lacks a reliable theoretical basis to independently and simultaneously determine the GAM parameters of effective hydraulic conductivity (Ke ) and the average matric pressure head at the wetting front (Hf ) from those commonly measured soil hydraulic properties. In this paper, we derived an approximate analytical solution similar to GAM for one-dimensional vertical infiltration into soils with initially uniform soil moisture distribution, with Ke and Hf being simultaneously related to the Brooks--Corey (BC) model parameters. The new relationships are not restricted to the piston-type moisture profile or delta-type water diffusivity like in the traditional GAM (TGAM). Infiltration experiments on three soils of various textures were conducted to validate the new approximate analytical solution and the corresponding GAM parameters. It was demonstrated that the predicted cumulative infiltrations and soil moisture profiles by the GAM with Ke and Hf calculated by the new expressions (new GAM, NGAM) generally agreed well with the numerical solutions, especially for initially dry soils. In contrast, cumulative infiltrations were considerably over-predicted by the TGAM using Ke = Ks and Hf calculated by Neuman's equation. Our theoretical analysis further indicated that even without air entrapped during infiltration, the effective hydraulic conductivity can still be less than the saturated hydraulic conductivity Ks . The relative effective hydraulic conductivity ζ (Ke/Ks ) and the relative average matric pressure head at the wetting front η (-Hf/hd where hd is absolute value of the air-entry suction) show a dependency only on soil pore structure index (n) and effective initial soil-water saturation (Si ). The values of ζ increased but η decreased with Si and n. The theoretical ranges of ζ and η were from 0.5 and 2 for dry heavy-texture soils to 1 and 0 for saturated soils, respectively, which are consistent with the experimental values reported in literatures. [ABSTRACT FROM AUTHOR]

The Green-Ampt equation is often used to estimate infiltration in hydrologic modeling. Only a few of the currently used simulation models, however, can simulate infiltration into a heterogeneous soil with nonuniform initial soil water content by the Green-Ampt equation during nonsteady rainfall. In this study, we derived an infiltration model for layered soils under nonsteady infiltration based on the Green-Ampt method (Green-Ampt for Layered Soils, or GALS). In the GALS model, a formula for estimating average suction (5) was derived to include initial soil water effect, and the soil profile was divided into several layers to consider both soil texture and initial soil moisture effects on infiltration. The new model can predict infiltration capacity, ponding time, and cumulative infiltration. Three infiltration experiments, steady-state infiltration into a layered soil with nonuniform initial soil water content, nonsteady infiltration into a layered soil with uniform soil water content; and nonsteady-state infiltration into a layered soil with nonuniform soil water content, were conducted for model testing. Comparison showed that the GALS model was in good agreement with another Green-Ampt-based model and the measured data. [ABSTRACT FROM AUTHOR]