bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for diagnostic analysis of estuarine geochemistry
Authors: Wood Tamara M. , Baptista António M.
Published: February 2004

A new numerical model for the fate and transport of nonconservative metals in an estuarine environment is introduced. ELAmet solves the depth‐averaged advection/dispersion/transformation equation on a two‐dimensional grid using a finite element formulation in an Eulerian‐Lagrangian framework. The model incorporates aqueous speciation and adsorption/desorption. Adsorption to any number of solid types can take place through linear (partitioning) reactions or nonlinear (complexation) reactions. The model accommodates chemical equilibria and kinetics simultaneously. Rate constants can span any range of time scales, as the computational time step for solving the chemical transformation equations has been decoupled from that dictated by the circulation. Verification of the model is included for a one‐dimensional channel and up to 15 chemical species. Preliminary applications are included which illustrate the concept of diagnostic modeling for a synthetic estuarine system in which the effects of source location, chemical kinetics, and sediment deposition on the mixing plot are examined.
Neural network‐based screening for groundwater reclamation under uncertainty
Authors: Ranjithan S. , Eheart J. W. , Garrett J. H.
Published: February 2004

Uncertainty due to spatial variability of hydraulic conductivity is an important issue in the design of reliable groundwater remediation strategies. Using groundwater management models based on a stochastic approach to groundwater flow, where the log‐hydraulic conductivity is represented as a random field, is a frequently studied technique for the design of aquifer remediation in the presence of uncertainty. Such an approach employs the solution of a management model for a large set of equally probable realizations of the hydraulic conductivity. However, only a few out of the large set of realizations are critical to the final outcome of the design. The spatial distribution of the hydraulic conductivity values in a realization, and the degree of variation of the hydraulic conductivity values within a realization are identified as two important features that determine the level of criticalness of a realization. The association between the hydraulic conductivity pattern and the level of criticalness is not known explicitly and needs to be captured for efficient screening. The screening approach presented here utilizes the pattern classification capability of a neural network and its ability to learn from examples. It is shown that incorporation of only a few critical realizations in a groundwater management model can yield highly reliable remediation designs. The application of the screening tool in a pump‐and‐treat design problem is illustrated via two examples.
Aquifer remediation design under uncertainty using a new chance constrained programming technique
Authors: Morgan David R. , Eheart J. Wayland , Valocchi Albert J.
Published: February 2004

A new technique, called the mixed‐integer‐chance‐constrained programming (MICCP) method is developed in this research. This technique considers uncertainty in all linear programming constraint coefficients and does not require a priori knowledge of the distribution. A groundwater remediation problem serves as an example. The method is developed to find the globally optimal trade‐off curve for maximum reliability versus a minimum pumping objective. As the fields became more heterogeneous, the pumping rate of a reliable solution increases. Four simple “rule of thumb” methods are compared to the MICCP technique. In general, the performance of such methods decreases as the heterogeneity of the hydraulic conductivity field increases.
Spring climate and salinity in the San Francisco Bay Estuary
Authors: Cayan Daniel R. , Peterson David H.
Published: February 2004

Salinity in the San Francisco Bay Estuary almost always experiences its yearly maximum during late summer, but climate variability produces marked interannual variations. The atmospheric circulation pattern impacts the estuary primarily through variations of runoff from rainfall and snowmelt from the Sierra Nevada and, secondarily, through variations in the near‐surface salinity in the coastal ocean. While winter precipitation is the primary influence upon salinity in the estuary, spring climate variations also contribute importantly to salinity fluctuations. Spring atmospheric circulation influences both the magnitude and the timing of freshwater flows, through anomalies of precipitation and temperature. To help discriminate between the effects of these two influences, the record is divided into subsets according to whether spring conditions in the region are cool and wet, warm and wet, cool and dry, or warm and dry. Warm springs promote early snowmelt‐driven flows, and cool springs result in delayed flows. In addition to effects of winter and spring climate variability operating on the watershed, there are more subtle effects that are transmitted into the estuary from the coastal ocean. These influences are most pronounced in cool and dry springs with high surface salinity (SS) in the coastal ocean versus cool and wet springs with low SS in the coastal ocean. A transect of SS records at stations from the mouth to the head of the bay suggests that the coastal ocean anomaly signal is attenuated from the mouth to the interior of the estuary. In contrast, a delayed, postsummer signal caused by winter and spring runoff variations from the upstream watershed are most pronounced at the head of the estuary and attenuate toward the mouth.
An efficient numerical solution of the transient storage equations for solute transport in small streams
Authors: Runkel Robert L. , Chapra Steven C.
Published: February 2004

Several investigators have proposed solute transport models that incorporate the effects of transient storage. Transient storage occurs in small streams when portions of the transported solute become isolated in zones of water that are immobile relative to water in the main channel (e.g., pools, gravel beds). Transient storage is modeled by adding a storage term to the advection‐dispersion equation describing conservation of mass for the main channel. In addition, a separate mass balance equation is written for the storage zone. Although numerous applications of the transient storage equations may be found in the literature, little attention has been paid to the numerical aspects of the approach. Of particular interest is the coupled nature of the equations describing mass conservation for the main channel and the storage zone. In the work described herein, an implicit finite difference technique is developed that allows for a decoupling of the governing differential equations. This decoupling method may be applied to other sets of coupled equations such as those describing sediment‐water interactions for toxic contaminants. For the case at hand, decoupling leads to a 50% reduction in simulation run time. Computational costs may be further reduced through efficient application of the Thomas algorithm. These techniques may be easily incorporated into existing codes and new applications in which simulation run time is of concern.
Modeling of carbon dioxide transport and production in soil: 1. Model development
Authors: Šimůnek Jiří , Suarez Donald L.
Published: February 2004

Knowledge of the CO2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process‐oriented relationships. The model includes one‐dimensional water flow and multiphase transport of CO2 utilizing the Richards and the convection‐dispersion equations, respectively, as well as heat flow and a CO2 production model. The transport of CO2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.
Stochastic modeling of unsaturated flow in heterogeneous soils with water uptake by plant roots: The Parallel Columns Model
Authors: Rubin Yoram , Or Dani
Published: February 2004

The effects of spatial variations in soil hydraulic properties and in water uptake by plant roots on the spatial distribution of matric potential (ψ) and saturation (S) were investigated. A stochastic‐analytic model for one‐dimensional steady state unsaturated flow including water uptake by plant roots was developed and tested. The model is based on the small‐perturbation approximation in two soil parameters characterizing the unsaturated hydraulic conductivity and water retention, and a plantrooting depth parameter. The soil is considered as made of decoupled soil columns with different properties. The model provides closed‐form expressions for the spatial moments of soil water matric potential and saturation using statistical information on the soil properties and plant attributes as input. The proposed model favorably compared with Monte Carlo simulations using synthetically generated random fields of soil properties and plant‐rooting depth. The results indicate that (1) there is a region above the water table where the variances of ψ and S undergo a sharp decrease from their stationary values to zero at the water table; (2) for a small enough steady surface flux this transitional region can occupy the entire unsaturated zone; (3) water uptake by plant roots skews both the means and the variance profiles of ψ and S such that they may never attain stationarity; and (4) the saturation variance profile is extremely sensitive to variations in ψ due to the nonlinear relationship between ψ and S.
A new model for bed load sampler calibration to replace the probability‐matching method
Authors: Thomas Robert B. , Lewis Jack
Published: February 2004

In 1977 extensive data were collected to calibrate six Helley‐Smith bed load samplers with four sediment particle sizes in a flume at the St. Anthony Falls Hydraulic Laboratory at the University of Minnesota. Because sampler data cannot be collected at the same time and place as “true” trap measurements, the “probability‐matching method” was used to derive surrogate pairs for calibration analysis. The method is invalid since it implicitly assumes that sampler and trap data have no sampling or measurement errors, it gives biased and highly variable results, and it does not contain information enabling model specification. A new calibration model was developed that regresses transformed individual sampler measurements on daily means of transformed trap data and incorporates within‐day variation in trap rates to explain part of the sampler variation. Three small‐nozzle samplers performed more uniformly than three large‐nozzle samplers did. There is evidence that samplers with higher nozzle ratios collect more bed load in most particle size classes tested. However, between the two small‐nozzle samplers with ratios of 3.22 and 1.40, significant differences could be detected for only one particle size. The standard sampler with a 76 × 76 mm nozzle trapped sediment less efficiently than a similar sampler with a 152×152 mm nozzle in three of four particle sizes tested. Limitations in the data restricted more definitive statements about the samplers, but the results of this study can be used to design a more rigorous calibration experiment.
The vapor diffusion coefficient for snow
Authors: Colbeck S. C.
Published: February 2004

The coefficients of diffusion for water vapor in snow and other porous media are fundamental parameters of great importance. While they are generally thought to be greater than the coefficient for air, there is much uncertainty about what value to use. Attempts to determine the diffusion coefficient in snow experimentally have led to contradictory results, but Yosida's experiments seem to give reasonable results. Yosida's idea of hand‐to‐hand diffusion is quantified here in a particle‐to‐particle model which shows that the diffusion coefficient depends on the mean pore size and that the coefficient in snow is four to seven times greater than in air.
Eulerian‐Lagrangian Theory of transport in space‐time nonstationary velocity fields: Exact nonlocal formalism by conditional moments and weak approximation
Authors: Neuman Shlomo P.
Published: February 2004

A unified Eulerian‐Lagrangian theory is presented for the transport of a conservative solute in a random velocity field that satisfies locally ∇ · v(x, t) = f(x, t), where f(x, t) is a random function including sources and/or the time derivative of head. Solute concentration satisfies locally the Eulerian equation ∂c(x, t)/∂t + ∇ · J(x, t) = g(x, t), where J(x, t) is advective solute flux and g(x, t) is a random source independent of f(x, t). We consider the prediction of c(x, t) and J(x, t) by means of their unbiased ensemble moments 〈c(x, t)〉ν and 〈J(x, t)〉ν conditioned (as implied by the subscript) on local hydraulic measurements through the use of the latter in obtaining a relatively smooth unbiased estimate ν(x, t) of v(x, t). These predictors satisfy ∂〈c(x, t)〉v/∂t + ∇ · 〈J(x, t)〉ν = 〈g(x, t)〉ν, where 〈J(x, t)〉ν = ν(x, t)〈c(x, t)〉ν + Qν(x, t) and Qν(x, t) is a dispersive flux. We show that Qν, is given exactly by three space‐time convolution integrals of conditional Lagrangian kernels αν with ∇·Qν, βν with ∇〈c〉ν, and γν with 〈c〉ν for a broad class of v(x, t) fields, including fractals. This implies that Qν(x, t) is generally nonlocal and non‐Fickian, rendering 〈c(x, t)〉ν non‐Gaussian. The direct contribution of random variations in f to Qν depends on 〈c〉ν rather than on ∇〈c〉ν,. We elucidate the nature of the above kernels; discuss conditions under which the convolution of βν and ∇〈c〉 becomes pseudo‐Fickian, with a Lagrangian dispersion tensor similar to that derived in 1921 by Taylor; recall a 1952 result by Batchelor which yields an exact expression for 〈c〉ν at early time; use the latter to conclude that linearizations which predict that 〈c〉ν bifurcates at early time when the probability density function of v is unimodal cannot be correct; propose instead a weak approximation which leads to a nonlinear integro‐differential equation for 〈c〉ν due to an instantaneous point source and which improves with the quantity and quality of hydraulic data; demonstrate that the weak approximation is analogous to the “direct interaction” closure of turbulence theory; offer non‐Fickian and pseudo‐Fickian weak approximations for the second conditional moment of the concentration prediction error; demonstrate that it yields the so‐called “two‐particle covariance” as a special case; conclude that the (conditional) variance of c does not become infinite merely as a consequence of disregarding local dispersion; and discuss how to estimate explicitly the cumulative release of a contaminant across a “compliance surface” together with the associated estimation error.
Analysis of the mineral acid‐base components of acid‐neutralizing capacity in Adirondack Lakes
Authors: Munson R. K. , Gherini S. A.
Published: February 2004

Mineral acids and bases influence pH largely through their effects on acid‐neutralizing capacity (ANC). This influence becomes particularly significant as ANC approaches zero. Analysis of data collected by the Adirondack Lakes Survey Corporation (ALSC) from 1469 lakes throughout the Adirondack region indicates that variations in ANC in these lakes correlate well with base cation concentrations (CB), but not with the sum of mineral acid anion concentrations (CA). This is because (CA) is relatively constant across the Adirondacks, whereas CB varies widely. Processes that supply base cations to solution are ion‐specific. Sodium and silica concentrations are well correlated, indicating a common source, mineral weathering. Calcium and magnesium also covary but do not correlate well with silica. This indicates that ion exchange is a significant source of these cations in the absence of carbonate minerals. Iron and manganese concentrations are elevated in the lower waters of some lakes due to reducing conditions. This leads to an ephemeral increase in CB and ANC. When the lakes mix and oxic conditions are restored, these ions largely precipitate from solution. Sulfate is the dominant mineral acid anion in ALSC lakes. Sulfate concentrations are lowest in seepage lakes, commonly about 40 μeq/L less than in drainage lakes. This is due in part to the longer hydraulic detention time in seepage lakes, which allows slow sulfate reduction reactions more time to decrease lake sulfate concentration. Nitrate typically influences ANC during events such as snowmelt. Chloride concentrations are generally low, except in lakes impacted by road salt.
Influence of organic acids on the pH and acid‐neutralizing capacity of Adirondack Lakes
Authors: Munson R. K. , Gherini S. A.
Published: February 2004

Past approaches for evaluating the effects of organic acids on the acid‐base characteristics of surface waters have typically treated them solely as weak acids. Analysis of data collected by the Adirondack Lakes Survey Corporation (ALSC) from 1469 lakes throughout the Adirondack region shows that this approach is not valid. While the data indicate that natural organics contain a continuum of acid functional groups, many of which display weak acid characteristics, a significant fraction of the organic acid is strong (pKa < 3). Dissolved organic carbon (DOC) contributes 4.5–5 μeq/mg DOC of strong acid to solution. The associated anions make a negative contribution to Gran acid‐neutralizing capacity (ANC). Because organic anions can produce negative Gran ANC values, the common practice of considering negative values of Gran ANC evidence of acidification solely by mineral acids is not valid. The strength of organic acids also influences the observed deviation between Gran ANC values and ANC values calculated as the difference between base cation and mineral acid anion concentrations (CB – CA). Ninety percent of the deviation is due to the presence of strong organics while the remaining 10% is due to DOC‐induced curvature in the F1 Gran function. Organic acids can also strongly influence pH. Their largest effects were found in the 0–50 μeq/L Gran ANC range where they depressed pH by up to 1.5 units. In addition, a method for predicting changes in pH in response to changes in mineral acidity, DOC, or both without having to rely on inferred thermodynamic constants and the uncertainties associated with them has been developed. Using the predictive method, the response of representative lakes from four sensitive lake classes to a 15‐μeq/L decrease in mineral acidity ranged from +0.17 to +0.38 pH units. If concurrent increases in DOC are considered, the pH changes would be even smaller.
A detailed model for simulation of catchment scale subsurface hydrologic processes
Authors: Paniconi Claudio , Wood Eric F.
Published: February 2004

A catchment scale numerical model is developed based on the three‐dimensional transient Richards equation describing fluid flow in variably saturated porous media. The model is designed to take advantage of digital elevation data bases and of information extracted from these data bases by topographic analysis. The practical application of the model is demonstrated in simulations of a small subcatchment of the Konza Prairie reserve near Manhattan, Kansas. In a preliminary investigation of computational issues related to model resolution, we obtain satisfactory numerical results using large aspect ratios, suggesting that horizontal grid dimensions may not be unreasonably constrained by the typically much smaller vertical length scale of a catchment and by vertical discretization requirements. Additional tests are needed to examine the effects of numerical constraints and parameter heterogeneity in determining acceptable grid aspect ratios. In other simulations we attempt to match the observed streamflow response of the catchment, and we point out the small contribution of the streamflow component to the overall water balance of the catchment.
Cloud effect in the estimation of instantaneous downward longwave radiation
Authors: Sugita Michiaki , Brutsaert Wilfried
Published: February 2004

The data set obtained during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) allowed an assessment of several expressions originally proposed to estimate daily downward longwave radiation Rld under clear sky. These expressions with the original values of the constants were found to yield instantaneous Rld values with a standard error of estimate of the order of 10–20 W m−2, but with some bias, when applied to the instantaneous values of the input data set obtained under clear sky. Calibration of the constants for the FIFE area and for the instantaneous Rld estimation under clear sky improved the results with a standard error smaller than 10 W m−2 and practically without bias. Inclusion of the cloudiness and cloud type information into the formulation allowed the expansion of its usage for both clear and cloudy sky conditions; the resulting standard error of the Rld estimate was found to be around 15–17 W m−2 if only the cloudiness data were utilized, and around 12 W m−2 if both cloudiness and cloud types were considered. These results were some 5 and 10 W m−2, respectively, better than the error in the estimation of Rld without cloudiness information.
Export of nutrients and major ionic solutes from a rain forest catchment in the Central Amazon Basin
Authors: Lesack Lance F. W.
Published: February 2004

The relative roles of base flow runoff versus storm flow runoff versus subsurface outflow in controlling total export of solutes from a 23.4‐ha catchment of undisturbed rain forest in the central Amazon Basin were evaluated from water and solute flux measurements performed over a 1 year period. Solutes exported via 173 storms during the study were estimated from stream water samples collected during base flow conditions and during eight storms, and by utilizing a hydrograph separation technique in combination with a mixing model to partition storm flow from base flow fluxes. Solutes exported by subsurface outflow were estimated from groundwater samples from three nests of piezometers installed into the streambed, and concurrent measurements of hydraulic conductivity and hydraulic head gradients. Base flow discharge represented 92% of water outflow from the basin and was the dominant pathway of solute export. Although storm flow discharge represented only 5% of total water outflow, storm flow solute fluxes represented up to 25% of the total annual export flux, though for many solutes the portion was less. Subsurface outflow represented only 2.5% of total water outflow, and subsurface solute fluxes never represented more than 5% of the total annual export flux. Measurement errors were relatively high for storm flow and subsurface outflow fluxes, but cumulative measurement errors associated with the total solute fluxes exported from the catchment, in most cases, ranged from only ±7% to 14% because base flow fluxes were measured relatively well. The export fluxes of most solutes are substantially less than previously reported for comparable small catchments in the Amazon basin, and these differences cannot be reconciled by the fact that storm flow and subsurface outflows were not appropriately measured in previous studies.
Compound random field models of multiple scale hydraulic conductivity
Authors: Brannan James R. , Haselow John S.
Published: February 2004

Enormous amounts of hydrologic data are required to accurately simulate subsurface contaminant transport. Effectively supplementing measurements of hydrologic parameters such as permeability and porosity with “soft” information obtained from the interpretation of geologic cores and geophysical logs can improve the simulation of contaminant transport while reducing the measured data that are required. A method is presented herein for generating hydraulic conductivity fields comprised of several geological materials with hydraulic conductivities that can range over several orders of magnitude. This method utilizes indicator fields that are designed to allow random variation at the megascopic scale but are constrained by observations inferred from geophysical logs and geologic core data. The statistical description of random hydraulic conductivity values of distinct geological materials at the macroscopic scale may be obtained by conventional parameter estimation techniques. The combined approach can then be used to generate realizations of a hydraulic conductivity field for subsequent use in flow and transport simulations. The structural constraints confine realizations to vary about a nominal model in a physically reasonable manner. The uncertainty may be viewed as arising from two different scales. Variation in hydraulic conductivity at the macroscale may be viewed as an intrinsic statistical property of the particular geologic material but where measurements may be made in order to ascertain the statistical description. On the other hand, uncertainty at the megascale level arises from the fact that the large‐scale system structure is only partially observable. It is the latter area where soft geologic information and geological knowledge can be useful in confining the uncertainty in the model to geologically reasonable limits.
Self‐similar heterogeneity in granular porous media at the representative elementary volume scale
Authors: Ghilardi Paolo , Kai Abdulai Kai , Menduni Giovanni
Published: February 2004

Geometrical measurements of sandstone pore microstructure (e.g., Katz and Thompson, 1985) indicated that both the surface and volume of pores are self‐similar and exhibit the same fractal dimension. This result yields the microscale hydraulic conductivity to be given as a function of the inner and outer cutoffs and fractal dimension. Accordingly, the fractal dimension can be viewed as the imprint of the process responsible for shaping the structure of voids in a porous medium. We show that the same approach can be extended to different porous media, e.g., to granular aggregates, and it can also be used to describe the soil structure at the scale of the representative elementary volume. For this purpose the fractal rock fragmentation model proposed by Turcotte (1986) has been used to derive the particle size distribution for a fractal mixture. Sedimentation of this mixture is then simulated using a ballistic aggregation algorithm, and both the solid phase and the void space geometry of the resulting aggregate are analyzed. The results show that the same fractal dimension embedded in the fragmentation process is preserved by the sedimentation mechanism and it also characterizes the volume of the pore space and the spatial distribution of the particles.
Tidally averaged models for dispersion in shallow water
Authors: Heemink A. W.
Published: February 2004

This paper deals with the advection and diffusion of contaminants in tidal waters. The analysis starts with the well‐known vertically integrated advection‐diffusion equation. In order to derive a tidally averaged model to describe the advection and dispersion processes over long periods and to gain insight into the tidally induced dispersion, a new method to average the tide is introduced. Here it is assumed that both the dispersion coefficient and the residual flow are sufficiently small. The averaging procedure is derived by using a random walk model to describe the dispersion process. This random walk model is shown to be exactly consistent with the advection‐diffusion equation. The averaging procedure results in equations for the averaged advective transport as well as equations for the effective dispersion tensor describing the mixing process that occurs within one tidal cycle. By using a numerical model to determine the tidal movement over one tidal cycle, the tidally averaged advective transport and dispersion tensor in each grid point of the model is approximated numerically. In this way a tidally induced dispersion map of the area under study is obtained.
Evaporation from Lake Kinneret: 2. Estimation of the Horizontal variability using a two‐dimensional numerical mesoscale model
Authors: Mahrer Y. , Assouline S.
Published: February 2004

The horizontal distribution of evaporation over a small lake surrounded by heterogeneous terrain is quite complex. It is therefore usually not possible to provide accurate evaporation estimates for the whole lake based upon a single observational site, even if this site is located offshore. In the present study, a numerical two‐dimensional terrain‐following mesometeorological model is applied to Lake Kinneret (area of 170 km2, located in northern Israel) to obtain appropriate spatial and temporal resolution of the climatic parameters needed to estimate evaporation. The model is verified by comparing its calculated evaporation rates with those measured at an offshore site by the eddy correlation system and by its ability to reproduce well‐established characteristics of the microclimate over the lake. Results indicate that, for the typical meteorological conditions characterizing the experimental period, a concave horizontal distribution of evaporation rate is predicted across the lake. Consequently, the assumption of a uniform evaporation rate over the whole lake equal to that at the measuring site could lead to a misestimation of the amount of water lost through the evaporative process.
Application of energy concepts to groundwater flow: Adaptive modeling of a leaky aquifer
Authors: Seneviratne Asitha , Karney Bryan
Published: February 2004

The net work and energy flux at the boundaries of an aquifer change its internal energy and overcome its resistance to flow. In saturated porous media, the change in internal (strain) energy is stored in the elastic soil matrix and in pore water compression. In unsaturated media, an additional term accounts for changes in gravitational potential energy. The energy approach complements conventional insight by allowing spatially distributed processes to be integrated into energy and work terms which characterize a system's response to a set of excitations. Specifically, a technique is developed in this paper to interpret the dynamic behavior of a one‐dimensional leaky aquifer in terms of its composite energy functions. In particular, the work interaction at the leaky boundary is used as an index of the significance of the leakage: when the work parameter indicates a relatively small leakage, the flow components of the multiaquifer can be isolated and modeled separately with a controllable loss of accuracy.
Emplacement of nonaqueous liquids in the vadose zone
Authors: Pantazidou Marina , Sitar Nicholas
Published: February 2004

The results of experimental and analytical investigations of the movement of lighter than water organic liquids in the vadose zone are presented. The experiments consisted of physical model tests simulating nonaqueous phase liquid (NAPL) spills in unsaturated, two‐dimensional domain above the water table. The evolution of the plume was observed through the transparent side of a tank containing sand, and the contaminant front was traced at appropriate intervals. Visual observations were supplemented with pressure and saturation measurements suitable for quantitative analysis of the process. The experiments show that during infiltration the front advances at constant speed (area/time). After spreading comes to a halt, the bulk of the contaminant is contained within a pancake‐shaped lens situated on top of the capillary fringe. Fluctuations of the phreatic surface result in trapping the NAPL below the water table and in spreading the contaminant over a larger area. In layered samples, the behavior is largely geometry dependent, although heterogeneities invariably result in extensive horizontal spreading. Simplified analytical relationships were developed to estimate the spreading rate of the contaminant during infiltration and the thickness of the final immobilized oil lens observed during the experiments. These relationships are based on capillary pressure considerations, which have to be included in order to model the immobilization of the plume at saturations higher than the residual values, which typically occur in vadose zone, when the NAPL encounters the capillary fringe in its downward migration.
Boundary integral solutions to the unsaturated moisture flow equation
Authors: Khanbilvardi Reza M. , Ahmed Shabbir , Sadegh Ali M.
Published: February 2004

The boundary integral equation method is formulated for problems concerning unsaturated moisture flow in porous media. The boundary integral formulation presented here is derived using the time‐dependent fundamental solution of the governing partial differential equation. Two boundary integral approaches are presented. The first approach solves the unsaturated moisture flow problem by directly applying the time‐dependent Green's function. The second method is based on the expansion of moisture content into a perturbation series. The governing equation is decomposed into a moisture flow equation without a known gravity term on the right‐hand side. The perturbation equations are then solved in succession by evaluating the gravity term of the unsaturated leachate flow equation from the preceding solution level. The two boundary integral approaches, the direct Green's function and the perturbation Green's function, are closely related because the same time‐dependent fundamental solution is used to formulate the boundary integral expressions. The direct Green's function formulation solves the boundary integral expression in one step. However, the perturbation Green's function approach requires solution of the integral expression for homogeneous boundary conditions. The solutions obtained by both methods are compared with the exact solution. A close agreement of the internal fluxes between boundary integral methods and the exact solution, for an unsaturated portion of a landfill, shows the stability of the boundary integral methods to compute accurate recharge from the unsaturated zone to the saturated leachate mound.
Solute transport through large uniform and layered soil columns
Authors: Porro I. , Wierenga P. J. , Hills R. G.
Published: February 2004

Solute transport experiments are often conducted with homogeneous soils, whereas transport in real situations takes place in heterogeneous soils. An experiment was conducted to compare unsaturated solute transport through uniform and layered soils. Pulse inputs of tritiated water, bromide and chloride were applied under steady flow conditions to the tops of two large (0.95 m diameter by 6 m deep) soil columns. One column was uniformly filled with loamy fine sand and the other filled with alternating 20‐cm‐thick layers of loamy fine sand and silty clay loam. Soil solution samples were collected during the experiment with suction candles installed at various depths in the columns. Solute transport parameters were estimated by fitting the convection‐dispersion equation to the observed breakthrough curves for each solute at various depths in each column. The match between the resulting calibrated curves and the experiment was better for the layered soil column than for the uniform soil column. The results displayed no clear relationship between the dispersion coefficients and depth for any of the tracers for either column. However, dispersivities were greater in the uniform column (3.5 cm) than in the layered column (1.2 cm), while retardation factors for bromide and chloride were similar (0.8 and 0.83, respectively, for the uniform and layered columns). A retardation factor less than one is attributed to anion exclusion. There was evidence of preferential flow in the uniform soil column. The peak concentrations at 5 m depth were greater than those observed at 4 m. Such behavior is inconsistent with one‐dimensional flow. Similar results were observed in an experiment performed 3.5 years earlier using the same soil column and approximately the same flow rates, but using a different tracer and associated chemical analysis, different soil saturation prior to the execution of the experiment, and different experimental personnel. This supports the thesis that the anomalous behavior is due to some form of preferential flow rather than due to experimental error.
Southern oscillation effects on daily precipitation in the southwestern United States
Authors: Woolhiser D. A. , Keefer T. O. , Redmond K. T.
Published: February 2004

The effect of the Southern Oscillation on daily precipitation in the southwestern United States is examined by using the Southern Oscillation Index (SOI) to perturb parameters of a stochastic daily precipitation model. Daily precipitation is modeled with a Markov chain‐mixed exponential model and seasonal variability of model parameters is described by Fourier series. The hypothesized linkage between the SOI and the model parameters is of the form Gi(N, t) = Gi(t) + biS(N, t − τi) where Gi(N, t) is the perturbed parameter i for day t of year N, Gi(t) is the annually periodic parameter i for day t, bi is a coefficient, S is the SOI, and τi is a lag in days. Daily precipitation data for 27 stations in California, Nevada, Arizona, and New Mexico were analyzed. Perturbations of the logits of the dry‐dry transition probabilities resulted in statistically significant improvements in the log likelihood function for 23 stations and perturbations of the mean daily rainfall resulted in significant increases for 18 stations. The most common lag identified was 90 days, suggesting the possibility of conditional simulations of daily precipitation. Seasonal effects were detected, confirming the results of previous analysis with groups of stations.
Testing for nonzero skew in maximum discharge runoff data
Authors: Whitley Robert , Hromadka T. V.
Published: February 2004

Water Resource Council Bulletin 17B (Interagency Advisory Committee on Water Data, 1982) recommends finding T‐year flood values by fitting stream gauge data with a log Pearson III distribution. A problem arising in using this method is that, as is well known, the estimator for the skew of this distribution is highly variable and the computed T‐year values are sensitive to the value of skew used. The variability in estimating the skew is studied in this note by computing operating characteristic curves which display the probability that a sample from a site with a nonzero skew will actually fall in a specific interval. In rough summary it is not usually possible to reject, at a high level of significance the hypothesis that the skew at a site is zero by using only the data available for that site.
Estimating the benefits of shore erosion protection in Ohio's Lake Erie Housing Market
Authors: Kriesel Warren , Randall Alan , Lichtkoppler Frank
Published: February 2004

Lakeshore erosion, characterized as a reduction in distance between property improvements and the lake, will eventually threaten improvements. An option price model suggests that erosion‐prone lakeshore property will be discounted and this discount may be measured by hedonic price analysis. We discuss the conditions that permit benefit estimation from the hedonic regression. Analysis of mail survey returns estimates willingness to pay for erosion protection, as measured by the number of years remaining until property improvements are endangered.
The use of nuclear magnetic resonance for studying and detecting hydrocarbon contaminants in porous rocks
Authors: Hedberg Stephen A. , Knight Rosemary J. , MacKay Alex L. , Whittall Kenneth P.
Published: February 2004

Proton nuclear magnetic resonance (NMR) measurements have been made to assess the use of NMR in quantifying the pore‐scale behavior of hydrocarbon contaminants in porous rocks. NMR measurements were made on bulk water, bulk gasoline (the selected hydrocarbon contaminant), and a sample of Berea sandstone, saturated to various levels with the two fluids. When the sandstone contained either water or gasoline, decreasing the level of fluid saturation resulted in a linear decrease in the total integral of the signal amplitude and in the relaxation time T1. The first effect allows for the determination of the level of fluid saturation from the NMR data; the second effect indicates the preferential drainage of large pores as saturation decreases for both fluids. A significant difference in liquid/rock interaction, which was quantified by calculating the surface sink parameter of the two systems, resulted in separation between the response of the gasoline and the water when the sandstone sample was saturated with a mixture of the two liquids. NMR laboratory measurements proved to be a useful means of detecting and estimating the quantity of a contaminant present in a porous rock.
Mass transfer from nonaqueous phase organic liquids in water‐saturated porous media
Authors: Geller J. T. , Hunt J. R.
Published: February 2004

Results of dissolution experiments with trapped nonaqueous phase liquids (NAPLs) are modeled by a mass transfer analysis. The model represents the NAPL as isolated spheres that shrink with dissolution and uses a mass transfer coefficient correlation reported in the literature for dissolving spherical solids. The model accounts for the reduced permeability of a region of residual NAPL relative to the permeability of the surrounding clean media that causes the flowing water to partially bypass the residual NAPL. The dissolution experiments with toluene alone and a benzene‐toluene mixture were conducted in a water‐saturated column of homogeneous glass beads over a range of Darcy velocities from 0.5 to 10 m d−1. The model could represent the observed effluent concentrations as the NAPL underwent complete dissolution. The changing pressure drop across the column was predicted following an initial period of NAPL reconfiguration. The fitted NAPL sphere diameters of 0.15 to 0.40 cm are consistent with the size of NAPL ganglia observed by others and are the smallest at the largest flow velocity.
Estimation of in situ hydraulic conductivity function from nonlinear filtering theory
Authors: Katul Gabriel G. , Wendroth Ole , Parlange Marc B. , Puente Carlos E. , Folegatti Marcos V. , Nielsen Donald R.
Published: February 2004

A method based on an optimal nonlinear filtering technique is proposed and tested for the determination of the hydraulic conductivity function from a field drainage experiment. Simplifications to Richards's equation lead to a Langevin type differential equation to describe the redistribution of stored water as a function of drainage flux excited by a random initial condition and state forcing. The derived equation is then utilized in an optimal estimation scheme that explicitly accounts for the formulation and observation uncertainty in determining the hydraulic conductivity parameters. A field drainage experiment was carried out to study the usefulness of the proposed method for routine in situ hydraulic conductivity function estimation.
The influence of solute size, pore water velocity, and intraparticle porosity on solute dispersion and transport in soil
Authors: Brusseau Mark L.
Published: February 2004

The purpose of this work was to investigate the effect of solute size, pore water velocity, and intraparticle porosity on dispersion and to delineate conditions under which it is appropriate to use a tracer‐derived dispersivity to represent the dispersivity of other, dissimilar‐sized solutes. This was accomplished by evaluating the physical properties of four porous media and the results of several column experiments performed with nonreactive tracers and these media. Intraparticle porosities of three sandy media were shown to constitute negligible fractions of the total porosities. As a result, intraparticle diffusion did not contribute to dispersive flux. The contribution of intraparticle diffusion, as well as film diffusion, to solute dispersion in fine‐grained media having small intraparticle porosities will be negligible for most conditions. In addition, the importance of axial diffusion decreases as pore water velocity increases. Hydrodynamic dispersion is the predominant source of dispersion under these conditions, and, as such, dispersivity is essentially independent of solute size. Dispersivities determined with a tracer may therefore be used to represent the dispersivity of other, dissimilar‐sized solutes. Under these conditions, the practice of using a nonreactive tracer such as 3H2O to characterize the dispersive properties of a soil column is valid. Solute dispersion in an aggregated soil was shown to be caused by a combination of hydrodynamic dispersion, film diffusion, and intraparticle diffusion. In addition, axial diffusion was shown to be important at low pore water velocities. Hence, the apparent dispersivities obtained by applying the single‐component dispersion equation to transport in structured soils will generally be a function of solute size. The use of a tracer‐derived dispersivity for solutes of different sizes would not be valid in this case.
Detection of nonconductive heat transport in soils using spectral analysis
Authors: Hinkel Kenneth M. , Outcalt Samuel I.
Published: February 2004

Variance spectra were generated for three high‐frequency near‐surface soil temperature time series representative of soil conditions typically encountered in temperate regions: saturation, strong evaporation, and soil frost penetration. For the latter two conditions, a surrogate measure of the soil water ionic concentration was also obtained and variance spectra were calculated. In a purely conductive system of multiday duration, maximum thermal variance should occur at a wavelength of 1 day and should decrease with depth. During ground saturation, when the pores are occupied by water, this pattern is observed. However, a spectral power density peak is observed in the subditirnal and subhourly range coincident with surface evaporation or freezing. In addition, the ionic concentration of soil water at depth is strongly coupled to temperature variations at the surface. These patterns are atypical of conductive systems and suggest the operation of nonconductive heat transfer processes, particularly water advection, phase transformations of water, and high‐velocity internal distillation.
Normative and positive aspects of institutional economics: The implications for water policy
Authors: Livingston Marie Leigh
Published: February 2004

Institutional economics provides an alternative framework for analyzing existing and proposed water policies. This approach differs from conventional economics in both normative and positive aspects. The normative model employs the concept of instrumental value, rather than efficiency. Water policies are judged based on whether they foster the long‐run viability of economic and ecological systems, not on whether they maximize net present value. The positive model is holistic and more inclusive than standard market models. The role of technical circumstances, nonmonetary goals, interest groups, and power are discussed. The implications of both positive and normative models for water policy are discussed. A set of questions useful in understanding or predicting the prospects for change in water policy is provided, and an institutionalist perspective is offered on water markets.