摘要:

A model is presented which relates farm economics, pollutant delivery, fisheries habitat quality and reliability, and spatial optimization. The model identifies the most profitable cropping practices which provide an acceptable level of risk of impairment to the physical and chemical fish habitat. The potential of the model as a watershed management aid is demonstrated with an application to salmonid habitat in a Lake Michigan tributary.

ISSN:0043-1397    

DOI:10.1029/WR025i012p02399

年代:1989

数据来源: WILEY

摘要:

Mathematical studies of solute transport in porous media have often utilized “equivalent” models of the transport process to remove undesired variability in the transport coefficients at the space and time scales of direct interest. Both deterministic and stochastic approaches in this genre produce an “effective” convection‐dispersion equation with time‐dependent coefficients. This type of equation in one spatial dimension is investigated mathematically in the present paper. A closed‐form solution of the solute transport equation is derived for a semi‐infinite spatial domain with arbitrary initial and boundary flux conditions. It is shown that the solution reduces to well‐known results for special forms of the time‐dependent coefficients. In general, however, a Volterra integral equation of the second kind must be solved to evaluate the analytical solution of the transport equation. We present a stable and convergent numerical scheme, utilizing a trapezoidal quadrature rule, for the solution of the Volterra equation. The method of solution developed should be applicable to a broad variety of solute transport problems, including particularly those in heterog

ISSN:0043-1397    

DOI:10.1029/WR025i012p02407

年代:1989

数据来源: WILEY

摘要:

This paper describes the development and application of a new mathematical technique to include stochasticity in environmental models. The technique, named the probability density function/moment (PDF/M) technique by the authors, is based on a two‐tiered process. First, the basic governing equations are expanded to include stochastic terms. Stochastic terms are separated from nonfluctuating terms, and the resulting set of equations is solved simultaneously. Solutions are used to calculate the moments of the output variables. Second, the moments are used in conjunction with the Fokker‐Planck equation to produce an analytical solution for the probability density functions of the dependent variables. Because the approach produces analytical solutions, it offers greater flexibility than a Monte Carlo approach in treating complex environmental situations. Unlike the stochastic differential equation approach, it is not necessary to assume Gaussian distributions in the solution technique, complex random functions of time and space may be included, and solutions are possible for higher‐dimensioned problems and cases with stochastic variations in stream velocity. The PDF/M technique represents a new and potentially powerful tool for extending the capabilities of computer models in management and decision analysis by providing analytical solutions for the probability density functions and associated moments of important environmental vari

ISSN:0043-1397    

DOI:10.1029/WR025i012p02417

年代:1989

数据来源: WILEY

摘要:

A physically based heat and mass flow model is presented and compared with experimental data measured on a bare soil (27.2% clay, 61.7% silt, 11.0% sand) under field conditions. Both liquid and vapor phases were taken into account, and soil temperature and water pressure head were the descriptive variables. The model was directly driven by soil surface temperature and water pressure head (derived from moisture content), which were used as boundary conditions. Coupled equations were solved using a numerical finite element method, from the soil surface to 1‐m depth. The experiment was conducted on a bare soil (0.1 ha) for a 7‐day period. The period was dry for 5 days (calibration phase) and then rainy (validation phase). Soil water balance was determined from gravimetric water content, neutron probe profiles, and tensiometer measurements. The unsaturated hydraulic conductivity/volumetric water content relationship was measured under field conditions, and the apparent thermal conductivity/water content relationship was derived from the thermal profile analysis. Results showed that the proposed model described soil temperature and water content variations versus time quite well: After a calibration phase, differences between the measured and calculated temperatures and volumetric water contents were below 1.5° and 0.03 m3m−3, respectively. Analysis of the errors involved in both initial and soil surface boundary conditions showed that these errors induced moderate effects on actual evaporation calculations. Although the vapor phase contributed largely to the total water fluxes, differences between coupled heat and water transport or isothermal liquid phase models were very small in regard to the actual evaporation or infiltration estimates. This was explained by the use of soil surface moisture content as boundary conditions which induced increasing soil surface pressure head gradients when the simplified isothermal liquid phase water flow model wa

ISSN:0043-1397    

DOI:10.1029/WR025i012p02423

年代:1989

数据来源: WILEY

摘要:

A previously calibrated soil/atmosphere heat and water transport model was used to analyze (1) errors involved in soil water balance estimates under drying periods when soil surface moisture contents derived from microwave measurements were used as soil surface boundary conditions, and (2∥ consequences of time periods separating two consecutive soil surface boundary conditions under evaporation or infiltration conditions on soil water balance calculations. Soil moisture and dry bulk density measurements were performed on a 0.4‐ha bare field (27.2% clay, 61.7% fine and coarse loam) with simultaneous measurements of backscattering coefficients (5.3 GHz, HH polarization, 15° incidence angle). Regression lines between backscattering coefficient and volumetric water content were calculated taking into account different soil depths (0‐1, 0‐2, 0‐3, 0‐4, 0‐5, 0‐6, 0‐7, and 0‐10 cm). Two methods for estimating soil surface moisture contents were proposed. In the first one, moisture contents for an arbitrary soil depth were directly extracted from a single “backscattering coefficient/water content” calibration line. The second method combined several calibration lines relative to several arbitrary soil depths. Results showed that (1) under drying periods (5 days) the methods led generally to moderately biased or unbiased water balances, (2) both errors due to the backscattering coefficient measurements and calibration line parameters had little effect on water balance estimations (≤10%), and (3) under evaporation conditions (5 days) or combined evaporation and infiltration phases (15 days), numerous soil surface boundary conditions versus time should be available to avoid wrong water balance estimates. Under evaporation conditions, one soil surface moisture content per day appeared satisfactory when it corresponded to the mean daily soil surface moisture content. Under combined evaporation and infiltration conditions, results depended strongly on the precise position of water content boundary conditions versus time, connect

ISSN:0043-1397    

DOI:10.1029/WR025i012p02437

年代:1989

数据来源: WILEY

摘要:

This paper presents a two‐phase flow model based on a three‐dimensional, finite‐difference formulation. As three‐dimensional simulations can require substantial computer effort, a numerical technique that takes advantage of vector and parallel processing computer architecture is developed. The model is posed in terms of water saturation and nonwetting fluid pressure. It uses three‐phase capillary pressure and relative permeability relationships to permit simulation within or below the unsaturated zone. A modified formulation of slice successive overtaxation (an iterative matrix solution technique) is introduced. This technique is designed to use parallel processing capabilities of new computers. The model is applied to immiscible fluid flow at two chemical waste landfills near Niagara Falls, New York. At both sites, denser than water, nonaqueous liquids (NAPLs) are present in the groundwater regimes in relatively large quantities. The model applications address several technical concerns at the two sites, including the effectiveness of clay as a geologic barrier to NAPL migration owing to capillary pressure forces, the three‐dimensional aspects of dense NAPL flow, and the sensitivity of NAPL recovery in pumping wells due to various hydrogeologic and fluid properties. The results of the applications show that (1) even under a downward hydraulic gradient, natural differences in capillary pressure relationships for different lithologies can prevent downward migration of NAPL, (2) without any lithologic‐capillary barrier, an upward hydraulic gradient induced by a de watering system can prevent downward migration of NAPL, (3) NAPL recovery at wells is sensitive to relative permeability, a relationship that requires field calibration in many settings, and (4) the three‐dimensional aspects of two‐phase flow and hydrogeologic heterogeneity require explicit treatment

ISSN:0043-1397    

DOI:10.1029/WR025i012p02449

年代:1989

数据来源: WILEY

摘要:

Soil and climatic variability contribute in an unknown manner to the leaching of pesticides below the surface soil zone where degradation occurs at maximum levels. In this paper we couple the climatic variability model of Eagleson (1978) to the soil variability transport model of Jury (1982) to produce a probability density distribution of residual mass fraction (RMF) remaining after leaching below the surface degradation zone. Estimates of the RMF distribution are shown to be much more sensitive to soil variability than climatic variability, except when the residence time of the chemical is shorter than one year. When soil variability dominates climatic variability, the applied water distribution may be replaced by a constant average water application rate without serious error. Simulations of leaching are run with 10 pesticides in two climates and in two representative soil types with a range of soil variability. Variability in soil or climate act to produce a nonnegligible probability of survival of a small value of residual mass even for relatively immobile compounds which are predicted to degrade completely by a simple model which neglects variability. However, the simpler model may still be useful for screening pesticides for groundwater pollution potential if somewhat larger residual masses of a given compound are tolerated. Monte Carlo simulations of the RMF distribution agreed well with model predictions over a wide range of pesticide properties.

ISSN:0043-1397    

DOI:10.1029/WR025i012p02465

年代:1989

数据来源: WILEY

摘要:

The effect of physicochemical interactions between the soil solution and the soil matrix on the spatial variability of soil properties pertinent to the transport of mixed Na/Ca−Cl salts in the unsaturated zone was analyzed. These properties were the soil hydraulic conductivity and the soil water retention functions and the retardation and the elution factors which account for Na/Ca exchange and chloride exclusion. On the local scale, effects of the soil solution concentration and composition (in terms of the chloride concentrationCand the sodium adsorption ratio SAR, respectively) on these soil properties were derived using a theoretical approach which combined the mixed‐ion diffuse double layer theory, the structure of the clay particles, the soil's pore size distribution, and hydrodynamic principles. On the field scale the effect of the soil solutionCand SAR on these soil properties was analyzed by coupling the theoretical approach with measured spatial distributions of the soil hydraulic properties at a reference “inert” state as well as of the soil cation exchange capacity and the soil specific surface area. The effect of the soil solution‐soil matrix interactions on the spatial variability of the soil hydraulic properties and the retardation and the elution factors was quantified in terms of mean values and coefficients of variation CV, expressed as functions of the soil solutionCand SAR, and the degree of effective saturation Θ. Results of the analyses suggested that the spatial variability (relative to the inherent field variability in the inert reference state) of both the hydraulic conductivityKand the water contentθincreased as both the SAR and Θ increased and asCdecreased but decreased asCwas further decreased (whenC<≈ 15 meq L−1). The mean value ofKandθrelative to the inert reference state increased and decreased, respectively, asCincreased and as both SAR and Θ decreased. The relative variability of both the retardation factor for Na,RfNa, and the elution factor for Cl,Eƒ, increased as the SAR increased and as bothCand Θ decreased. The mean value ofRfNaincreased and the mean value ofCƒdecreased as bothCand Θ decreased an

ISSN:0043-1397    

DOI:10.1029/WR025i012p02475

年代:1989

数据来源: WILEY

摘要:

The effect of physicochemical interactions between the soil solution and the soil matrix on the spatial variability of the soil‐dependent variables during transient transport of mixed Na/Ca−Cl salts through the unsaturated zone of a large field‐scale soil is analyzed. The spatial variability of the soil water and solute transport properties evaluated in part I were used as inputs to a conceptual stochastic model describing one‐dimensional vertical transport of water and soil‐interacting solutes through the unsaturated zone of a spatially variable soil, viewed as a collection of vertically homogeneous and independent soil columns. Using data of the spatial distributions of pertinent soil properties from the Bet Dagan field, the transport process was simulated for boundary and initial conditions pertinent to the application of low‐salinity and low‐alkalinity waters (rain) to the surface of a saline‐alkaline soil, considering effects of the soil solution concentration and composition on the soil water and solute transport properties. The spatial variability (in terms of field averages and coefficients of variation CV) of the profiles of the soil water contentθ, the chloride concentrationC, and the sodium adsorption ratio SAR of the soil solution and the soil hydraulic conductivityKat given elapsed timestwere presented and compared with those evaluated from simulations of the same boundary value problem where the effect of the soil solution concentration and composition on the water and solute transport properties was disregarded. Results of the analyses suggested that because of soil solution‐soil matrix interactions the field‐average movement of both the water and the solutes may be retarded and their spatial variability may be increased relative to the case where these interactions had not been considered. In the Bet Dagan field, aftert= 5 hours of continuous infiltration, the field averages of the positions of the wetting front and the chloride front were retarded by 10 and 15%, respectively; the relative variabilities of the positions of these fronts were increased by 16 and 38%, respectively; and the equivalent effective dispersivity was increased by 18%, relative to the

ISSN:0043-1397    

DOI:10.1029/WR025i012p02487

年代:1989

数据来源: WILEY

摘要:

The convective transport of a dissolved chemical species in a pipe is studied both for an injection and pulse problem. The entire solutions of the equations of the second‐order dispersion model for the averaged mass fraction of the solute are derived in the case of no molecular diffusion. Although the exact representation ofv′v′ω′¯,xandv′ω′,ii¯deviate from corresponding constitutive equations proposed by A. F. B. Tompson and W. G. Gray (1986a), they do not reflect the influence of diffusion in the overall dispersion processes considered in A. F. B. Tompson and W. G. Gray's (1986b) validation exercises. The dispersion coefficient of the injection problem considered clearly exhibits

ISSN:0043-1397    

DOI:10.1029/WR025i012p02497

年代:1989

数据来源: WILEY