No abstract

Saturated hydraulic conductivity is a measure of the ability of a soil to transmit water and is one of the most important soil parameters. New single‐ring infiltrometer methods that use a generalized solution to measure the field saturated hydraulic conductivity (Ks) were developed and tested in this study. The Ks values can be calculated either from the whole cumulative infiltration curve (Method 1) or from the steady‐state part of the cumulative infiltration curve by using a correction factor (Method 2). Numerical evaluation showed that the Ks values calculated from the simulated infiltration curves of representative soil textural types were in the range of 87 to 130% of the real Ks values. Field infiltration tests were conducted on an Arlington fine sandy loam (coarse‐loamy, mixed, thermic, Haplic Durixeralfs). The geometric means of the Ks values calculated from the field‐measured infiltration curves by Method 1 and Method 2 were not significantly different. The geometric mean of the Ks calculated from the detached core samples, however, was about twice that of the Ks calculated from the infiltration curves, which was consistent with earlier findings. Unlike the earlier approaches, Method 1 calculates Ks values from the whole infiltration curve without assuming a fixed relationship between saturated hydraulic conductivity and matric flux potential ϕm

This paper presents a new approach to solve Richards' equation. It introduces a nonlinear transformed pressure, Pt, as the dependent variable with the modified Picard method. The new approach was compared to, and contrasted with, two efficient existing methods: the o-based transformation method (Kirkland et at., 1992), and the h-based modified Picard method (Celia et at., 1990). A total of 12 different one-dimensional cases were considered (saturated, unsaturated, layered and uniform soil profiles, with pressure and flux type boundary conditions). The results show that the new method offers excellent CPU et•ciency and, unlike the h-based method, is numerically robust for all cases of variabty saturated, heterogeneous media, and first or second type boundary conditions. The method does not require difficult numerical coding, and its CPU efficiency is not affected by complicated heterogeneous and hysteretic media. The Pt transformation is easy to incorporate into existing h-based codes.where 0 and h are volumetric water content and soil water pressure head, respectively; K is the hydraulic conductivity, which is assumed constant for saturated soils but varies strongly with 0 or h in unsaturated soils; z denotes the vertical dimension; and t is time. To solve (1), the soil water retention function is introduced to eliminate one of the two dependent variables. This results in either an "h-based" or a "O-based" form of (1). Because of the highly nonlinear K-h and O-h relationships, analytical solutions to (1) are impossible, except for special cases. However, there exist many numerical methods.Numerical models to solve the h-based form of (1) seem to be the most common. They can be used for saturated and unsaturated soils, as well as for layered soils. However, these models suffer from poor mass balances for unsaturated soils, and from unacceptable time step limitations, or poor CPU efficiency especially for very dry initial conditions. Milly [1985] presented a mass-conserving solution procedure that uses a modified definition of the capacity term to force global mass balance. Allen and Murphy [1985, 1986] Bouloutas [1989] and Celia et al. [1990] greatly improved the performance of h-based models by using a mixed form of Richards' equation. Their numerical algorithms are denoted as "quasi-Newton" or "modified Picard," but probably should be called "h-based quasi-Newton" and "h-based modified Picard" because pressure head is still the dependent variable. The strategy used by Celia et al. [1990] is to evaluate the water content change over one time step directly from the change of the pressured head. This results in much improved mass balances. However, as shown by Kirkland et al. [1992], CPU efficiency is still a problem with h-based models for very dry initial conditions.

No abstract

Abstract. A commonly used numerical scheme to simulate solute transport in the subsurface is the random walk particle method (RWPM). While significant progress has been made in developing random walk particle algorithms for porous media, an RWPM for fractured porous media, represented by dual continua, has not yet been developed in the literature. A key issue in developing an RWPM for fractured porous media is the determination of particle transfer between the two continua. In this study a relatively simple, yet physically based scheme to determine the particle transfer is developed. The usefulness of the scheme is demonstrated by comparing solute transport results determined from the proposed scheme with (1) an analytical solution for solute transport in a fractured porous medium with parallel fractures and (2) a numerical simulation obtained using the TOUGH2 code, a simulator for multiphase and multicomponent flow and transport in fractured porous media.

No abstract

No abstract

No abstract