摘要:

This paper concerns the three‐dimensional block hydraulic conductivity of blocks of limited size in cross‐bedded sediments of meandering rivers. Based on a simplified sedimentological model of a single point bar, a stochastic model is proposed to simulate the spatial distribution of core scale conductivities within point bar deposits. The required parameters for these simulations are obtained from actual hydraulic conductivity measurements and sedimentological field data. The simulations are then used to calculate the three‐dimensional block conductivity tensor with a numerical upscaling procedure. The stochastic simulation model is used to investigate the influence of slope and aspect angle of the cross‐beds and the core scale anisotropy ratio on the block hydraulic conductivity. It is expected that the off‐diagonal elements of the block conductivity tensor generally cannot be neglected. The core scale anisotropy ratio, which reflects the small scale layering within the cross‐beds, can potentially have a significant impact on the elements of the block conducti

ISSN:0043-1397    

DOI:10.1029/94WR01049

年代:1994

数据来源: WILEY

摘要:

To describe the flow of water in unsaturated soils, the constitutive relationships of capillary pressure and relative permeability are necessary. To measure these relationships using traditional methods may require different experimental apparatuses and can be time‐consuming, particularly for multiple drainage and imbibition relationships for systems containing surfactants at various concentrations. Thus a critical need exists for an apparatus that can make rapid measurements. This paper outlines the design of an automated apparatus that can produce both capillary pressure and relative permeability relationships simultaneously and rapidly. The rapidity of the measurements is based on the use of a thin soil sample, a highly conductive capillary barrier, and stripper tensiometers. The capillary pressure is changed externally, with the change within the sample monitored through the use of tensiometers. Saturation is determined by monitoring the cumulative effluent, with the accuracy of the mass balance enhanced through the active removal of air bubbles from beneath the capillary barrier. A comparison of capillary pressure relationships for a fine‐grained sandy porous medium produced with this apparatus with those produced with a Tempe cell shows that this apparatus yields comparable measurements in about 2% of the time. Thus the burden of making multiple transport property measurements can be reduced considerably through the use of this appara

ISSN:0043-1397    

DOI:10.1029/94WR01495

年代:1994

数据来源: WILEY

摘要:

Soil vapor extraction (SVE) is a technique that is commonly used to remove volatile organic compounds from the vadose zone. Recent research has demonstrated that rate‐limited sorption and desorption of these compounds can have a profound impact on the concentration reductions achievable by SVE. In this note, one‐dimensional equations presented by Brusseau (1991), which describe rate‐limited transport of sorbing organic compounds in the vadose zone, are modified to model a SVE remediation at an idealized site and analytically solved. The analytical model presented herein describes transport of a sorbing organic contaminant in a converging radial flow field in the vadose zone, with sorption rate limitations described by first‐order rate expressions. The model equations are solved in the Laplace domain and numerically inverted to simulate contaminant concentrations at an extraction well. A Laplace domain solution for the total contaminant mass remaining in the vadose zone is also derived. It is shown that under certain conditions, rate‐limited sorption can have a significant impact upon SVE remediation in the vadose zone. The solutions presented in this note may be useful in verifying numerical codes which are being developed to model organic transport in the vadose zone under conditions of rate‐limit

ISSN:0043-1397    

DOI:10.1029/94WR01775

年代:1994

数据来源: WILEY

摘要:

The random, two‐dimensional, and steady in time Eulerian velocity field is assumed to be given. It pertains to flow through heterogeneous porous media of spatially variable permeability. Advective transport of a passive scalar is modeled by the Lagrangian approach by seeking the statistical moments of the trajectory of a tagged particle. The trajectories' variances and the associated dispersion coefficients are evaluated by a perturbation expansion in the velocity variances and by Corrsin's conjecture. First‐order results obtained in the past show that for flow through porous formations of log‐permeability, two‐point covariance of finite integral scale, the transverse dispersion coefficient tends asymptotically to zero. In contrast, Corrsin's conjecture leads to a finite dispersion coefficient, quadratic in the velocity variance. To investigate this effect, an exact nonlinear correction to the transverse dispersion coefficient is derived for a normal velocity field. It is shown that this term tends to zero asymptotically, in contrast with the result based on Corrsin's conjecture. Furthermore, an illustrative computation shows the transport‐related nonlinear correction to be small at

ISSN:0043-1397    

DOI:10.1029/94WR00904

年代:1994

数据来源: WILEY

摘要:

A numerical study of a negatively buoyant plume intruding into a neutrally stratified porous medium has been undertaken using finite different methods. Of particular interest has been to ascertain whether the experimentally observed gravitational instabilities that form along the lower edge of the plume are reproduced in the numerical model. The model has been found to faithfully reproduce the mean flow as well as the gravitational instabilities in the intruding plume. A linear stability analysis has confirmed the fact that the negatively buoyant plume is in fact gravitationally unstable and that the stability depends on two parameters: a concentration Rayleigh number and a characteristic length scale which is dependent on the transverse dispersivity.

ISSN:0043-1397    

DOI:10.1029/94WR01180

年代:1994

数据来源: WILEY

摘要:

The Southern Oscillation index (SOI) can yield information about subsequent streamflows on the South Island of New Zealand; however, the relationship between streamflow and SOI may be nonlinear and heteroscedastic. To deal with such difficulties, the conditional probability of streamflow's being below a critical magnitude, given a prior observation of SOI, can be considered a random variable, with its probability density function (pdf) estimated by Bayesian analysis of existing observations of streamflow and SOI. The conditional pdf can yield a probabilistic forecast of critically low streamflows given a precursor value of SOI. In the Clutha River basin of New Zealand, average austral summer inflow to the headwater lakes has an unconditional probability of nonexceedance of 360 m3/s of approximately 17%. However, during a moderate La Niña, average austral spring SOI = 12, the conditional probability of nonexceedance has an expected value of 55%. For an El Niño with a spring SOI of −12, the expected probability of nonexceedance is 18%; for a neutral SOI of 0, the expected probability is 10%. Thus, the probability of nonexceedance of seasonal streamflow can be seen to vary by a factor of more than 5 as a function of the

ISSN:0043-1397    

DOI:10.1029/94WR01308

年代:1994

数据来源: WILEY

摘要:

The cross‐correlation function (CCF) and autocorrelation function (ACF) for Great Lakes annual net basin supplies are derived under the assumptions that annual lake outflows and water levels are autoregressive lag 1 processes and that each lake behaves as a linear reservoir. Except for the pairing between Lakes Superior and Michigan‐Huron, there is reasonable agreement between the sample and predicted CCFs, especially among net basin supplies to the small lower basin lakes. The derived ACFs reduce to an expression identical in form to the ACF for an ARMA(1, 1) process at all the Great Lakes except Lake Michigan‐Huron. At the upper basin lakes, sample and predicted ACFs drop to zero rapidly. At the lower basin lakes the ACFs exhibit a much more gradual decay, suggesting the presence of long‐term persistence. Prominent tails in the ACFs of the annual net basin supplies have been attributed to historical shifts in the precipitation regime at the lower basin lakes. Results from this study show that the residual method currently used to estimate net basin supplies can also induce a similar artificial long tail in the ACF. This observation has important ramifications in efforts to simulate Great Lakes water levels, since simulation results are quite sensitive to the covariance structure of the annual net basin s

ISSN:0043-1397    

DOI:10.1029/94WR01215

年代:1994

数据来源: WILEY

摘要:

The definition of preevent soil moisture profile and time compression analysis are critical components in water balance models that are based on realistic infiltration/exfiltration relations and include profile redistribution of vadose zone moisture. In this paper, detailed analysis of these two fundamental components of water balance modeling is presented. Numerical integration of the governing equations for liquid moisture flow in the unsaturated zone is used in simulations designed to illustrate the role of temporal variability in the system. The simulations consist of forcing the surface of a one‐dimensional soil column, bounded at its base by a fixed water table, with the output of a stochastic event‐based model of precipitation and potential evaporation. The simulations are run until (and beyond when) an equilibrium condition is reached between the long‐term mean values of surface and bottom fluxes. For situations with a deep water table, two distinct zones develop: a near‐surface, highly unsteady zone, and a deeper, quasi‐steady zone. The equivalent steady moisture profile, i.e., the steady profile corresponding to the mean of the simulated column flow, is found to reasonably approximate the temporal mean, mean prestorm, and mean poststorm moisture profiles, particularly in the deeper zone. In the upper zone the equivalent steady profile forms a biased estimate of the temporal mean. The bias is shown, through perturbation analysis, to lead to overestimation of the wetness of the mean moisture profile. For shallow water tables the distinction between the two zones collapses. In this case both the equivalent steady profile and the mean profile are close to the hydrostatic profile. The simulations are also used to test the utility of the time compression approximation (TCA) in modeling surface fluxes under temporally variable initial conditions. We demonstrate that the use of the mean prestorm and poststorm event moisture profiles as initial conditions for the infiltration and exfiltration flux capacities does not produce major bias in TCA‐predicted surface fluxes. In this sense the TCA behaves as a linear operator. Furthermore, we demonstrate that the estimation of these mean initial condition profiles by the equivalent steady state soil moisture profile is an adequate approximation for determining mean land surface response to event‐based atmosp

ISSN:0043-1397    

DOI:10.1029/94WR00948

年代:1994

数据来源: WILEY

摘要:

With minor modification an existing analytic model employing a simple statistical description of soil moisture dynamics is found to give accurate estimates of the mean water balance partitioning; comparisons are made to a Monte Carlo numerical simulation of unsaturated moisture flow in a soil column. In the statistical‐dynamical model the complex temporal and spatial characteristics of the soil moisture profile are approximated by a single effective soil moisture: the equilibrium soil moisture (S0) for which the modeled long‐term mean fluxes are in balance. The relation between this equilibrium soil moisture and the equivalent steady state moisture profile (the steady profile which transmits the long‐term mean flow) is discussed. The test numerical integrations of the flow and conservation equations are designed to represent a deep one‐dimensional soil column. Soil surface boundary conditions are defined by the interaction of the surface soil moisture with a stochastic event‐based atmospheric forcing model, allowing for state dependent switching from flux boundary conditions (rainfall intensity and potential evaporation) to concentration conditions (ponded and dry surface), thus simulating infiltration‐excess runoff production and soil‐limited evaporation. The ability of the equilibrium model to estimate the mean water balance under a large range of soil textures (from clay to sand loam) and climate forcings (humid to arid) implies that the soil moisture profile may, for purposes of climate water balance, be represented in a s

ISSN:0043-1397    

DOI:10.1029/94WR00949

年代:1994

数据来源: WILEY

摘要:

Subsurface flow from a hillslope can be described by hydraulic groundwater theory, as formulated by the Boussinesq equation. Solution of its linear version brings out some of the essential features of this phenomenon. Arbitrary inputs can be accommodated by simple convolution of the sudden drawdown problem. The relative magnitude of the driving mechanisms, namely the streamwise pressure gradient, resulting in diffusion and gravity (i.e., slope), resulting in advection, can be determined by a simple dimensionless parameter.

ISSN:0043-1397    

DOI:10.1029/94WR01396

年代:1994

数据来源: WILEY