bluuebunny/crossref_metadata_2025 huggingface dataset:
Cation and proton exchange,pH variations, and carbonate reactions in a freshening aquifer
Authors: Appelo C. A. J.
Published: February 2004

Freshening of aquifers is accompanied by sequential elution of the saltwater (seawater) cations from the sediment's exchange complex. The resulting Chromatographic patterns are modeled with a one‐dimensional geochemical transport model that can handle the complex interplay of transport and mineral and ion exchange equilibria. The transport part is based on the mixing cell approach, with different time steps for advective and diffusive transport used when required by small grid size. The chemical reactions are calculated explicitly after each time step with the geochemical model PHREEQE. Ion exchange is included in the form of association half reactions, which allows simulation of the dynamic nature of the exchange process. The variation in the constant for proton association is obviated with an activity coefficient for H‐Xthat is derived from the constant capacitance model. All coefficients for the exchange model are obtained by fitting to literature data to be able to perform the modeling as realistically as possible. The code is applied to a laboratory column experiment, and subsequently used to demonstrate Chromatographic development of solute profiles in a freshening aquifer. Sequential peaks of Mg2+, K+, and Na+along a flow path in the Aquia aquifer in Maryland are modeled, and the results confirm that the variation of water qualities in this aquifer has basically a Chromatographic origin. Proton exchange acts here as a source of acid in NaHCO3water in which calcite dissolves. This explains the Na+to HCO3−ratio and high δ13C observed in these waters.
Feasibility of point‐nonpoint source trading for managing agricultural pollutant loadings to coastal waters
Authors: Crutchfield Stephen R. , Letson David , Malik Arun S.
Published: February 2004

A recent focus of water quality policy discussions has been the trading of pollution abatement between point and nonpoint sources. Point‐nonpoint trading would allow point sources to sponsor nonpoint source controls rather than install further controls of their own. If nonpoint source loadings are significant and the marginal costs of their control are lower than for additional point source controls, water quality goals could be met at lower cost with trading. We isolate difficulties particular to incentive policies such as point‐nonpoint trading and then screen coastal watersheds for those satisfying conditions that play a major role in determining whether trading can improve water quality. We follow the recent Coastal Zone Act Reauthorization Amendments in emphasizing agriculture, the single largest cause of nonpoint source pollution. Our screening analysis provides an initial, empirical assessment of the feasibility of trading for managing agricultural land use to protect coastal water quality. We also illustrate the additional analysis required to quantify the potential for successful trading in those watersheds which meet our screening criteria.
Modeling water infiltration in unsaturated porous media by interacting lattice gas‐cellular automata
Authors: Di Pietro L. B. , Melayah A. , Zaleski S.
Published: February 2004

A two‐dimensional lattice gas‐cellular automaton fluid model with long‐range interactions (Appert and Zaleski, 1990) is used to simulate saturated and unsaturated water infiltration in porous media. Water and gas within the porous medium are simulated by applying the dense and the light phase, respectively, of the cellular automaton fluid. Various wetting properties can be modeled when adjusting the corresponding solid‐liquid interactions. The lattice gas rules include a gravity force step to allow buoyancy‐driven flow. The model handles with ease complex geometries of the solid, and an algorithm for generating random porous media is presented. The results of four types of simulation experiments are presented: (1) We verified Poiseuille's law for steady and saturated flow between two parallel plates. (2) We analyzed transient water infiltration between two parallel plates of varying degrees of saturation and various apertures. (3) Philip's infiltration equation was adequately simulated in an unsaturated porous medium. (4) Infiltration into an aggregated medium containing one vertical parallel crack was simulated. Further applications of this lattice gas method for studying unsaturated flow in porous media are discussed.
Explicit expressions for Green‐Ampt (delta function diffusivity) infiltration rate and cumulative storage
Authors: Salvucci Guido Daniel , Entekhabi Dara
Published: February 2004

The sharp wetting front model of infiltration (Green and Ampt, 1911; Philip, 1954) yields through simple integration an exact solution relating the infiltration rate (i), cumulative infiltration (I), and time (t). The relation, however, is implicit for i or I; i.e., it is of the form t = A[I − B ln (1 + I/B)]. Numerical iteration is required to find the infiltration rate, and furthermore, analytic manipulations are limited using this traditional formulation of the Green‐Ampt infiltration. In this note we present an accurate expression for the infiltration rate in the form of a rapidly converging series in the variable τ = t/(t + χ). Truncating the series at four terms yields a useful expression for i(t). The proposed four‐term expression gives less than 2% error at all times and is readily integrated to yield the cumulative infiltration I(t). In conjunction with the exact expression for time (t) given i or I, the proposed expression is useful in infiltration/runoff calculations that necessitate the time compression approximation (TCA).
Application of multiscale water and energy balance models on a tallgrass prairie
Authors: Famiglietti J. S. , Wood E. F.
Published: February 2004

The models presented in the previous paper (Famiglietti and Wood, this issue) are applied at their appropriate scales for evapotranspiration modeling at the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) site. The local soil‐vegetation‐atmospheric transfer scheme is applied at five flux measurement stations in the northwest quadrant of the FIFE site. Simulations were performed for three of the four FIFE “golden (cloud‐free) days” with good results. The spatially distributed model was applied at the 11.7‐km2 King's Creek catchment, also located in the northwest quadrant of the FIFE site, during FIFE Intensive Field Campaigns (IFCs) 1–4. Simulated catchment average evapotranspiration was compared to an average of observations made at the five aforementioned measurement stations with good results. The macroscale formulation was applied to both the King's Creek catchment and the entire 15‐km FIFE site for evapotranspiration simulations. Macroscale model simulations for King's Creek were nearly identical to the spatially distributed results, implying that at this location and at this scale, the assumptions invoked in the development of the macroscale formulation are reasonable. The macroscale model was also employed to simulate evapotranspiration from the entire 15‐km site for the four golden days. Simulated evapotranspiration rates show reasonably good agreement with the 22‐station average of observations. However, it is suggested that at 15‐km and larger scales, simulation error may arise as a result of the macroscale assumptions of areally averaged atmospheric forcing, vegetation parameters, soil parameters, and the methods by which these data and other flux observations are aggregated. A methodology to combat these problems at larger scales is reviewed.
Slug tests in partially penetrating wells
Authors: Hyder Zafar , Butler James J. , McElwee Carl D. , Liu Wenzhi
Published: February 2004

A semianalytical solution is presented to a mathematical model describing the flow of groundwater in response to a slug test in a confined or unconfined porous formation. The model incorporates the effects of partial penetration, anisotropy, finiteradius well skins, and upper and lower boundaries of either a constant‐head or an impermeable form. This model is employed to investigate the error that is introduced into hydraulic conductivity estimates through use of currently accepted practices (i.e., Hvorslev, 1951; Cooper et al., 1967) for the analysis of slug‐test response data. The magnitude of the error arising in a variety of commonly faced field configurations is the basis for practical guidelines for the analysis of slug‐test data that can be utilized by field practitioners.
An Analytic Approach to Modeling Land‐Atmosphere Interaction: 1. Construct and Equilibrium Behavior
Authors: Brubaker Kaye L. , Entekhabi Dara
Published: February 2004

A four‐variable land‐atmosphere model is developed to investigate the coupled exchanges of water and energy between the land surface and atmosphere and the role of these exchanges in the statistical behavior of continental climates. The land‐atmosphere system is substantially simplified and formulated as a set of ordinary differential equations that, with the addition of random noise, are suitable for analysis in the form of the multivariate Îto equation. The model treats the soil layer and the near‐surface atmosphere as reservoirs with storage capacities for heat and water. The transfers between these reservoirs are regulated by four states: soil saturation, soil temperature, air specific humidity, and air potential temperature. The atmospheric reservoir is treated as a turbulently mixed boundary layer of fixed depth. Heat and moisture advection, precipitation, and layer‐top air entrainment are parameterized. The system is forced externally by solar radiation and the lateral advection of air and water mass. The remaining energy and water mass exchanges are expressed in terms of the state variables. The model development and equilibrium solutions are presented. Although comparisons between observed data and steady state model results re inexact, the model appears to do a reasonable job of partitioning net radiation into sensible and latent heat flux in appropriate proportions for bare‐soil midlatitude summer conditions. Subsequent work will introduce randomness into the forcing terms to investigate the effect of water‐energy coupling and land‐atmosphere interaction on variability and persistence in the climatic system.
Comparison of two general circulation models to downscale temperature and precipitation under climate change
Authors: Matyasovszky Istvan , Bogardi Istvan , Duckstein Lucien
Published: February 2004

A semiempirical approach for downscaling general circulation model (GCM) based daily atmospheric circulation patterns (CP) and predicting local climatological variables under climate change is developed. Specifically, the daily 500‐hPa surface outputs of the Canadian Climate Center (CCC) and Max Planck Institute (MPI) (Germany) GCMs are linked stochastically, using a split sampling approach, to local temperature and precipitation in Nebraska. Three series of data are analyzed: historical data, 1 × CO2 GCM results and 2 × CO2 GCM results. Between these three data sets, no significant difference can be detected in either CP typology (constructed by principal component analysis and k means method) or stochastic properties of daily time series (Markov matrix). On the other hand, the average geopotential height of the 500‐hPa pressure field exhibits significant change, labeled the ΔCO2 effect, between the 1 × CO2 and 2 × CO2 cases. Accordingly, climate change is assumed to be represented by the historical average geopotential height augmented by the ΔCO2 increment. It is found that both the CCC and MPI GCMs lead to predicting a winter temperature increase of 3°–6°C, a smaller but significant increase in spring and fall temperatures, and no increase in summer. The probability of precipitation occurrence is found to remain almost unchanged, as well as the dry period duration. The estimates of local response to climate change depend upon the location and the GCM used for downscaling the CP. The MPI GCM, which includes an ocean‐atmosphere coupling, appears to yield smaller downscaled changes than the purely atmosphere‐based CCC GCM.
On the Velocity Covariance and Transport Modeling in Heterogeneous Anisotropic Porous Formations: 2. Unsaturated Flow
Authors: Russo David
Published: February 2004

Velocity covariances, and the resultant macrodispersion coefficient tensor, derived by Russo (this issue) for saturated flow conditions, are applied for unsaturated flow conditions, employing the assumption that for a given mean capillary pressure head, water saturation is a deterministic constant and log conductivity is a multivariate normal, stationary random space function. The applicability of the approach for modeling flow and transport in the vadose zone was evaluated by the use of the stochastic theory of Yeh et al. (1985a, b) for steady, unsaturated flow. Results of the analyses suggest that the approach may be applicable to vadose zone flow and transport, as long as the scale of heterogeneity in the direction of the mean flow is smaller than approximately one tenth of the characteristic length of unsaturated flow. For porous formation of given statistics, the magnitude of macrodispersion in unsaturated flow is larger than that in saturated flow, and increases as water saturation decreases. For a given water saturation, transport in unsaturated flow may approach asymptotic Fickian behavior more slowly than in saturated flow, when the two formation properties log Ks and α are positively cross‐correlated and when the correlation scale of α is relatively large as compared with the correlation scale of log Ks.
Convective transport of acids and bases in porous media
Authors: Scheidegger André , Bürgisser Christa S. , Borkovec Michal , Sticher Hans , Meeussen Hans , van Riemsdijk Willem
Published: February 2004

We present a convective description of acid‐base transport in porous media which is based on classical one‐component nonlinear Chromatographic theory applied to the acidity of the system. In the mobile phase the solution acidity is given by c = [H+]t ‐ [OH−]t, where [H+]t and [OH−]t are the total solution concentrations of H+ and OH−, respectively. In the stationary phase the surface acidity corresponds to the charge density σ, which is commonly presented as a function of pH of the solution. The response of a Chromatographic column to a step pH change at column inlet results in a pH breakthrough curve which consists of a combined Chromatographic front. This combined front begins with a diffuse subfront and ends with a self‐sharpening subfront. The present Chromatographic theory is used to predict experimentally observed pH breakthrough curves for columns filled with materials of known pH‐dependent charging behavior. Good agreement between theoretical predictions and experimental results is observed. Essentially, the same Chromatographic theory is used to explain pH breakthrough curves when the salinity of the input solution is changed. This situation leads usually to an additional nonretarded front where a change in pH occurs.
Development of an optimal control system for maintaining minimum groundwater levels
Authors: Tankersley Claude D. , Graham Wendy D.
Published: February 2004

This paper describes the development and application of an optimal control system designed to reduce the deviation of piezometric head from minimum monthly target levels. The control strategy is generated from a Box‐Jenkins double input‐single output transfer function (SARIMAX) model relating local piezometric head fluctuations at an ecologically vulnerable location to regional rainfall and pumping volumes. The optimal control scheme is developed to minimize head deviations from the minimum target levels while simultaneously satisfying pumping/injection constraints associated with predetermined water resource management preferences. The control action manipulates the control variable (pumping) to compensate for existing and expected deviations of the piezometric head from target levels due to both observed disturbances (i.e., rainfall) and random (i.e., white noise) disturbances. An example applied to a localized region of the Upper Floridan aquifer in northeast Florida shows that the control action reduces the number of deviations below target head levels by 79% and the magnitude of the mean deviation below target by 72%. This improvement was achieved through a 29% reduction in pumping volume. The optimal control system thus provides a useful means of managing groundwater fluctuations at ecologically vulnerable locations by restricting regional pumping to offset regional rainfall deficits.
Scale and directional dependence of macrodispersivities in colonnade networks
Authors: Khaleel Raziuddin
Published: February 2004

Macrodispersion in discrete colonnade network models is investigated using a series of Monte Carlo type numerical experiments. The numerical simulations consider fluid flow and advective transport through a square flow region of a two‐dimensional network of hexagonal colonnades. Macrodispersivities are calculated as a function of the scale and orientation of the square flow region within the larger, parent geometry. Particle tracking, with flux‐weighted tracer injection and monitoring, is used to generate experimental residence time distributions (RTDs). The method of moments is used to characterize the longitudinal tracer spreading. The simulated RTDs are utilized to examine the macrodispersive behavior in colonnade networks (column diameter, 1 m) with lognormally distributed fracture apertures (b, in millimeters). The network models are assumed to consist of open fractures with μln b = −1.945 and σln b = 0.896; this translates to an equivalent continuum log‐conductivity variance (σln K2) of 3.67. Based on an ensemble average of 100 realizations, the slope for spatial variance versus scale of observation ranges from 3.15 to 3.36 m for varying orientations of the hydraulic gradient. For ensemble‐averaged data, the varying orientations appear to have little effect on the macrodispersive behavior. For single‐realization experiments, the computed macrodispersivities are directionally dependent at a length scale as large as 30 times the column diameter (and probably much beyond). The computed asymptotic and preasymptotic macrodispersivities are compared with available stochastic solutions for two‐dimensional isotropic heterogeneity in the horizontal plane. The ensemble‐based numerical data are in excellent agreement with the solutions of Dagan (1984, 1988) and Gelhar and Axness (1983). However, for individual realizations, nonergodic behavior is clearly apparent in the near‐source, evolving region of transport, and the numerical data are quite variable between realizations. The study provides important insight on applicability of stochastic continuum theories to discrete colonnade network models having σln K2 much greater than 1.
Analytical expressions to estimate the free product recovery in oil‐contaminated aquifers
Authors: Corapcioglu M. Yavuz , Tuncay Kagan , Lingarn Rajasekhar , Kambham Kiran K. R.
Published: February 2004

Petroleum products, such as gasoline, leaked from an underground storage tank can be recovered successfully by two‐pump operations. The success of the recovery effort depends on the accurate placement of the recovery well at the spill site. An effective recovery operation can minimize the remaining contamination mass in the subsurface. Therefore, a careful evaluation and determination has to be made as to where to locate the recovery well. The location of the well can be decided based on an estimation of the extent and thickness of free product on the water table. Such an estimation should be based on analysis of governing mechanisms. In this study we present analytical solutions to estimate the recovery of oil from an established oil lens. These solutions are obtained by applying the Laplace transformation to averaged linear partial differential equations governing the phenomenon. The governing equation for the free product thickness is derived by averaging the oil phase mass balance equation along the free product thickness and substituting the boundary conditions at the oil/water interface and oil surface. The analytical solutions estimate the temporal and spatial distribution of free product thickness on the water table for a number of recovery scenarios. Results are presented for the temporal and spatial variation of the free product thickness, temporal variation of the free product volume recovered, and recovery efficiency based on the readings at the monitoring wells. Since they can be utilized without a great deal of data, analytical solutions are quite attractive as screening tools in two‐pump free product recovery operations.
Transient Behavior of Mixing Induced by a Random Velocity Field
Authors: Zhang Qiang
Published: February 2004

We present a new analysis for Corrsin's hypothesis. We show this hypothesis is the leading term contribution in a renormalization group perturbation expansion. We present the exact results of a nonlinear dynamic equation obtained from Corrsin's hypothesis. The macroscopic dispersion induced by a random velocity field on all timescales is studied. Due to the nonlinearity of the governing equation, the transient growth rate of the mixing layer is nonfractal even for a fractal velocity field. For a velocity field with an exponentially decaying correlation function, the comparisons of the exact results under Corrsin's approximation with the results from the linear and quasi‐linear theories are presented. The results from Corrsin's hypothesis show that for an exponentially decaying correlation function with variance σ, the longitudinal asymptotic dispersion coefficient is proportional to σ2for small values of σ and proportional to σ for large values of σ. This result is further confirmed by a mean field approximation.
A Note on Horizontal Redistribution with Capillary Hysteresis
Authors: Raats P. A. C. , van Duijn C. J.
Published: February 2004

In a recent paper, Philip analyzed horizontal redistribution with capillary hysteresis. He considered the special case for which one semi‐infinite half of a column has an initial condition corresponding to the point where the boundary wetting curve joins the boundary drying curve and the other semi‐infinite half of the column has an initial condition corresponding to the point where the boundary drying curve joins the boundary wetting curve. The similarity solution of this problem developed by Philip can be generalized to cases involving any uniform initial conditions of the semi‐infinite halves of the column, corresponding to arbitrary histories of the pressure head and the water content. For any such pair of initial conditions for the semi‐infinite halves, one of the following three possibilities will apply: (1) no flow between the two halves; (2) conventional flow from the wet half to the dry half; or (3) nonconventional flow from the dry half to the wet half.
Calculation of Internodal Transmissivities in Finite Difference Models of Flow in Heterogeneous Porous Media
Authors: Romeu R. K. , Noetinger B.
Published: February 2004

We present an analytical and numerical investigation of the finite difference computation of the equivalent conductivity of heterogeneous porous media. The customary harmonic scheme to evaluate finite difference internodal transmissivities produces a systematic bias in the numerical results unless an extremely fine grid is used. In order to quantify such effects, we have developed an analytical approach in the form of a series expansion of the equivalent numerical conductivity in powers of the conductivity variance. This leads to an expression of the numerical answer as a function of the grid block size. The calculation confirms the existence of a strong bias and of a very slow convergence. We propose a simple method to correct it, which is well suited for upscaling. Numerical experiments performed with more contrasting heterogeneous media show similar results. This allows the use of coarser gridding and consequently an appreciable speedup in the numerical simulation approach.
LMoment and Probability Plot Correlation Coefficient Goodness‐of‐fit Tests for the Gumbel Distribution and Impact of Autocorrelation
Authors: Fill Heinz D. , Stedinger Jery R.
Published: February 2004

This paper analyzes the power of twoLmoment and the probability plot correlation coefficient (PPCC) goodness‐of‐fit tests for the Gumbel distribution and the impact of autocorrelation. The twoLmoment tests are the kappa test suggested by Hosking et al. (1985) using biased PWM estimators, and theL‐Cstest suggested by Chowdhury et al. (1991) using unbiased PWM estimators. The generalized extreme value (GEV) distribution with various values of the shape parameterκwas used as the parent distribution. Results show that theLmoment‐based tests outperform the PPCC test for independent data, or data with small autocorrelations (ρ ≤ 0.4). For high autocorrelation (ρ = 0.8), all tests are invalid because the type 1 error probability is larger than the target value. An example demonstrates consistency problems with scale and shape parameters estimated using the biased PWM estimators; these cause us to advise against their use and to recommend instead unbiased PWM estimators that employ a sample's order statistics. Overall, this paper provides another endorsement of the use of unbiasedLmoment estimators for goodness‐of‐fit tests and distribution selection, as well as a recommendation for parameter estimation.
Analysis of Data From Pumping Tests in Anisotropic Aquifers: Equations and Graphical Solutions
Authors: Grimestad Garry
Published: February 2004

Traditional methods for characterizing horizontally anisotropic groundwater aquifers seek solutions in terms of a fictitious directional hydraulic parameter called directional transmissivity. But all common pumping test drawdown models respond to such conditions as though transmissivity were insensitive to direction and instead an easily identified class of ordinary hydraulic coefficients were the exclusive carriers of directional information. The failure of traditional techniques to fully exploit this universal phenomenon has resulted in the development of inflexible model‐specific anisotropy methods which are needlessly complicated, excessively prone to failure, and consequently seldom used. An alternative method presented here is based directly on these characteristic responses. It enables for the first time a choice of analytical techniques ranging from preliminary back‐of‐the‐envelope graphical anisotropy assays and manual solutions (emphasized here) to rigorous least squared error full characterizations using simple variations of just one basic approach. Unlike traditional methods which typically fail to yield meaningful information when no formal anisotropy solution is possible, the proposed method always provides a qualitative indication of horizontal anisotropy, isotropy, nonhomogeneity, or lack of sufficient data, because diagnostic data plots are constructed prior to formal analysis.
Precipitation Trends and Water Consumption Related to Population in the Southwestern United States: A Reassessment
Authors: Diaz Henry F. , Anderson Craig A.
Published: February 2004

Water consumption figures for the southwest United States are compared for the last four decades. Past trends in consumption are evaluated in the context of precipitation variability in the region and with regard to Colorado River streamflow changes. The study represents a follow‐up look at a previous assessment of water consumption, regional precipitation, and demographic trends in Arizona, California, Colorado, Nevada, New Mexico, and Utah, which account for much of the annual depletions of Colorado River water. The previous study was completed during a wet spell in the West, and trends in all major categories of water consumption were consistently upward. This study indicates that a decline or reversal has taken place in water use in many of the western states. The greater water efficiency (reduced per capita water use) is particularly noteworthy in California, which alone accounts for the lion's share of water depletions from the Colorado River Basin. The years since the mid‐1980's have been predominantly dry in much of the West. At the same time, population in the six‐state region has ncreased at about the same pace it had grown during prior decades. A shift from irrigation‐related uses to civil consumption is evident in the 1980's. Taking into consideration a situation where multiyear dry spells are a normal part of the climate of the region, it appears that irrigation depletions may have peaked in the West. In the future, allocations for civil supply, recreation, and other in‐stream uses as well as for hydropower generation may heighten the competition for available water supplies, put pressure on existing pricing policies, and force users toward greater conservation efforts and improved efficiencies.
Transport of Bromide, Simazine, and MS‐2 Coliphage in a Lysimeter Containing Undisturbed, Unsaturated Soil
Authors: Poletika Nicholas N. , Jury William A. , Yates Marylynn V.
Published: February 2004

The effect of rate‐limited adsorption on transport of environmental contaminants is difficult to characterize at the field scale. This study investigated transport, during unsaturated water flow, of pulse inputs of bromide, simazine (2‐chloro‐4,6‐ bis(ethylamino)‐s‐triazine), and MS‐2 coliphage in a field lysimeter (0.8 m × 0.8 m square) containing undisturbed Tujunga loamy sand (mixed, thermic, Typic Xeropsamment). Sixty‐four fiberglass wick soil solution samplers collected drainage fractions from the exit surface (30 cm depth) following daily 2‐cm water inputs applied at 0.5 cm h−1. After 19.7 cm of cumulative drainage, the soil above 10 of the 64 locations was sampled to determine final depth distributions of simazine and virus. Most of the bromide was leached from the transport volume, while the sorbing pesticide and virus remained in the soil. Variance analysis indicated that local dispersion processes contributed more to the observed bromide spreading than did differences in local water velocities. A linear, first‐order, kinetic adsorption submodel was incorporated into a generalized linear transport model relating the bromide flux concentrations to the simazine and virus final resident concentrations. Least squares fitting showed that area‐averaged bromide transport could be described reasonably well by the two‐parameter convection‐dispersion model (CDM), although the mobile‐immobile water model provided a slightly better representation of effluent tailing. The CDM parameters fitted to the bromide data were then held constant while the two parameters of the adsorption submodel were varied to fit the pesticide soil concentrations at the end of the experiment at 10 days. A good fit was obtained for simazine, and the fitted value 0.54 d−1 of the rate coefficient was in the range characterizing nonequilibrium adsorption. A batch adsorption/desorption experiment produced Freundlich isotherms describing nonlinear adsorption (exponent m = 0.85) and hysteresis in desorption. There was poor agreement between the retardation factor (R) estimated from a linearized batch distribution coefficient Kd and the R fitted to lysimeter data. Virus concentrations fitted to the model yielded coefficients implying strong adsorption (R = 254) and rapid inactivation (inactivation rate coefficient of 1.64 d−1), whereas the laboratory sorption study implied that the virus should be very mobile in soil. The difference in field and laboratory sorption may be due to air‐water interfacial forces in the unsaturated field experiments.
Comparison of Moments for Classical‐, Quasi‐, and Convolution‐Fickian Dispersion of a Conservative Tracer
Authors: Deng F.‐W. , Cushman J. H.
Published: February 2004

Transport moments up to fourth order for convolution‐, quasi‐, and classical‐Fickian dispersion of a conservative tracer are presented and compared. Convolution‐Fickian yields a non‐Gaussian distribution while quasi‐ and classical‐Fickian result in Gaussian type distributions regardless of the distribution of a stationary velocity field. The first and second central spatial moments are the same for convolution‐ and quasi‐Fickian dispersion, while the third and fourth central moments differ.
On Higher‐Order Corrections to the Flow Velocity Covariance Tensor
Authors: Deng F. W. , Cushman J. H.
Published: February 2004

Second‐order log fluctuating conductivity variance (σƒ2) corrections to the head and velocity covariance functions are derived for a lognormal, stationary hydraulic conductivity field. The Fourier transform method proposed by Deng et al. (1993) is used extensively to obtain numerical estimates of these functions for an exponential log fluctuating conductivity covariance. It is shown that the velocity covariance is insensitive to second‐order corrections in the head field. The velocity covariance, on the other hand, is highly sensitive to second‐order corrections in the velocity when the log fluctuating conductivity variance approaches unity. A closed expression is derived for a second‐order correction to the velocity variance when there is no second‐order correction to the head field. The longitudinal second‐order correction to the velocity variance is 0.4σƒ2 different from the first‐order approximation in isotropic media, 1.5σƒ2 different in a highly stratified formation, and no different when the ratio of vertical to horizontal integral scales approaches infinity. The second‐order corrections to the horizontal and vertical transverse velocity variances are 2σƒ2 different from the first‐order approximations for both isotropic and anisotropic systems.
Observations of Downstream Fining on the North Fork Toutle River Near Mount St. Helens, Washington
Authors: Seal Rebecca , Paola Chris
Published: February 2004

We carried out a series of measurements to provide data for testing models of downstream fining in an actively braiding gravel bed river, in particular, the model developed in the previous paper. We sampled a series of nine cross sections for bulk grain size distributions over a downstream distance of 5.4 km on the North Fork Toutle River, near Mount St. Helens, Washington. In addition, we sampled two of the nine sections for detailed analysis of surface grain size distributions in the cross‐stream direction by Wolman counting within areas of distinct mean grain size (“patches”). Our theory predicts that the rate of downstream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain size patches. Application of the theory to the study area shows that most of the observed downstream fining can be explained using patchiness as the dominant mechanism.
Flow in Heterogeneous Media Displaying a Linear Trend in the Log Conductivity
Authors: Indelman Peter , Rubin Yoram
Published: February 2004

The purpose of this study is to derive a general solution for the problem of flow in nonstationary geological formations where the nonstationarity manifests itself in the form of a spatial trend in the mean log conductivity. A stochastic frame of reference is adopted to account for the spatial variability of the hydraulic conductivity. For a complete stochastic description we derive the expected values and spatial covariances of the hydraulic head and the fluid flux vector, as well as a relation between the expected values of the head and the fluxes. These expressions are obtained using a perturbation expansion of the log conductivity about its nonstationary mean, and they are correct to the first order in the variance of the log conductivity. The expressions we derive are applicable for any space dimensionality and for arbitrary orientation of the trend in space. A general methodology is outlined for derivation of these expressions for any type of spatial covariance; and for demonstration, explicit results are obtained for a Gaussian isotropic covariance.
A Transformed Pressure Head‐Based Approach to Solve Richards' Equation for Variably Saturated Soils
Authors: Pan Lehua , Wierenga Peter J.
Published: February 2004

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 ϕ‐based transformation method (Kirkland et al., 1992), and the h‐based modified Picard method (Celia et al., 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 efficiency and, unlike the h‐based method, is numerically robust for all cases of variably 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.
An Evaluation of the Bouwer and Rice Method of Slug Test Analysis
Authors: Brown David L. , Narasimhan T. N. , Demir Z.
Published: February 2004

The method of Bouwer and Rice (1976) for analyzing slug test data is widely used to estimate hydraulic conductivity (K). Based on steady state flow assumptions, this method is specifically intended to be applicable to unconfined aquifers. Therefore it is of practical value to investigate the limits of accuracy of the K estimates obtained with this method. Accordingly, using a numerical model for transient flow, we evaluate the method from two perspectives. First, we apply the method to synthetic slug test data and study the error in estimated values of K. Second, we analyze the logical basis of the method. Parametric studies helped assess the role of the effective radius parameter, specific storage, screen length, and well radius on the estimated values of K. The difference between unconfined and confined systems was studied via conditions on the upper boundary of the flow domain. For the cases studied, the Bouwer and Rice analysis was found to give good estimates of K, with errors ranging from 10% to 100%. We found that the estimates of K were consistently superior to those obtained with Hvorslev's (1951) basic time lag method. In general, the Bouwer and Rice method tends to underestimate K, the greatest errors occurring in the presence of a damaged zone around the well or when the top of the screen is close to the water table. When the top of the screen is far removed from the upper boundary of the system, no difference is manifest between confined and unconfined conditions. It is reasonable to infer from the simulated results that when the screen is close to the upper boundary, the results of the Bouwer and Rice method agree more closely with a “confined” idealization than an “unconfined” idealization. In effect, this method treats the aquifer system as an equivalent radial flow permeameter with an effective radius, Re, which is a function of the flow geometry. Our transient simulations suggest that Re varies with time and specific storage. Thus the effective radius may be reasonably viewed as a time‐averaged mean value. The fact that the method provides reasonable estimates of hydraulic conductivity suggests that the empirical, electric analog experiments of Bouwer and Rice have yielded shape factors that are better than the shape factors implicit in the Hvorslev method.
Multicriterion Validation of a Semidistributed Conceptual Model of the Water Cycle in the Fecht Catchment (Vosges Massif, France)
Authors: Ambroise B. , Perrin J. L. , Reutenauer D.
Published: February 2004

Model validation is still a crucial issue in hydrology. It is usually limited to comparing simulated and measured streamflows, while many other fluxes and storages are also simulated. This is especially true for spatially distributed models, for which multivariable and multiscale validation procedures are needed. Such an approach has been tested on the Fecht research basin (Vosges Massif, France) over a 5‐year period, using the semidistributed conceptual MC model, whose physical soundness has been improved. The catchment discretization into land units was based on simple physiographic criteria, which were statistically validated. Water balance parameters were derived from measured soil water retention, without any calibration. Catchment streamflow was used to calibrate water transfer parameters over a single year and validate the model. Two subcatchments were used for the internal validation, and two neighboring nested catchments were used for the regional transposition of both the model and its distributed parameterization. At the mesh scale (250 to 1000 m), snowpack and soil water storages and contribution to streamflow were validated by using measurements from two small catchments. Despite the very limited calibration, the results were generally satisfactory. Nash‐Sutclifife efficiencies on daily streamflow varied from 0.92 in calibration to 0.89 in validation and 0.71 in transposition. The results of the simulated patterns of fluxes and storages can therefore be considered with some confidence.
The isotopic climatic records in the Alleröd‐Bølling‐Younger Dryas and Post‐Younger Dryas events
Authors: Epstein Samuel
Published: February 2004

The combined isotopic records in the Greenland ice cores, in modern and ancient wood samples, and in marine foraminifera provide a climatic record for the Alleröd‐Bølling‐Younger Dryas and post‐Younger Dryas Periods that is not compatible with the usual climatic interpretation of the 2H/1H ratios in the Greenland ice cores. For example, the Younger Dryas was not solely a North Atlantic event because evidence for it is also found in the 2H/1H record in wood samples which grew on the north slope of Alaska. The Younger Dryas was not necessarily a time of cooling over the ice caps. Moreover, it might have been a warming period in the temperate zones of the northern hemisphere continents. A better understanding of the climatic scenarios during these periods of time could be ascertained from additional isotopic data in plant remains which grew during these intervals and which cover a wider climatic range on the continents.
Terrigenous Fe input and biogenic sedimentation in the glacial and interglacial equatorial Pacific Ocean
Authors: Murray R. W. , Leinen M. , Murray D. W. , Mix A. C. , Knowlton C. W.
Published: February 2004

Many ocean regions important to the global carbon budget, including the equatorial Pacific Ocean, have low chlorophyll concentrations despite high levels of conventional nutrients. Iron may instead be the limiting nutrient, and elevated input of terrigenous Fe during windy glacial episodes has been hypothesized to stimulate oceanic productivity through time and thus regulate the oceanic and atmospheric CO2 balance. To test whether particulate Fe input is related to the accumulation of biogenic matter in one important low chlorophyll‐high nutrient area, that is, the equatorial Pacific Ocean, we present results from a suite of sediment cores that collectively record biogenic deposition through the last six glacial‐interglacial cycles (∼600,000 years). Our data set includes new chemical data on total Fe, terrigenous, and biogenic components in three cores as well as previously published mineralogic records of eolian input to the region. Chemical, spectral, and stratigraphic analysis indicates that (1) terrigenous input to the region shows no consistent pattern of either glacial or interglacial maxima, (2) the accumulation of particulate Fe is closely related to the accumulation of terrigenous matter (linear r2 = 0.81–0.98), (3) there are no coherent spectral relationships between Fe input and glacial periodicity (i.e., δ18O) in any of the orbital frequency bands, (4) the linear and cross‐spectral correlations between Fe or eolian input and CaCO3 concentration are most commonly the strongest observed relationships between Fe and any biogenic component, yet indicate a largely inverse pattern, with higher Fe being associated with low CaCO3, (5) there is no consistent linear r2 correlation or spectral coherence between the accumulation of Fe and that of CaCO3, Corg, or opal. Thus in total there is no relationship between terrigenous Fe input and sedimentary sequestering of carbon. Additionally, although we cannot specifically address the potential for changes in solubility of the terrigenous fraction that may be driven by a terrigenous compositional change, the Fe/Ti ratio (which monitors first‐order mineralogic changes) records only slight variations that also are linearly and spectrally unrelated to glacial periodicity, the bulk Fe flux, and the accumulation of any biogenic component. Finally, we find that the paleoceanographic flux of Fe is several order‐of‐magnitudes larger than modern observations of eolian Fe input, suggesting that the long‐term importance of Fe input by dust storms (which deliver Fe on the order of the sedimentary burial) may be underestimated. The removal of particulate terrigenous Fe from the recently discovered source within the Equatorial Undercurrent, however, remains unquantified and may also prove significant.
Characteristics of the ion pressure tensor in the Earth's magnetosheath
Authors: Li Xinlin , Lewis H. R. , LaBelle J. , Phan T.‐D. , Treumann R. A.
Published: February 2004

AMPTE/IRM satellite data are used to examine characteristics of the ion pressure tensor in the earth's magnetosheath. The eigenvalues and principal axes of the pressure tensor are computed, and the directions of the principal axes are compared to the direction of the independently measured magnetic field B. When the pressure tensor is anisotropic, as is usually the case in the magnetosheath, one of its eigenvalues is observed to be distinguishable from the other two, which are about equal to one another. Thus, the eigenvector associated with the distinguishable eigenvalue defines an axis of symmetry of the pressure tensor. The symmetry axis is generally not parallel to B. New features of the plasma distribution function are revealed by using the actual eigenvalues of the pressure tensor rather than the usual p⟂ and p∥, where ⟂ and ∥ denote directions perpendicular and parallel to B.
Vertical structure of the midlatitude temperature from stratosphere to mesopause (30–105 km)
Authors: She C. Y. , Yu J. R. , Krueger David A. , Roble Raymond , Keckhut Philippe , Hauchecorne Alain , Chanin Marie‐Lise
Published: February 2004

Data sets from Rayleigh lidar (1979–93) at the Observatory of Haute‐Provence (44°N, 6°E) and at Biscarrosse (44°N, 1°W) of southern France, and from narrowband Na lidar (1991–4) at Fort Collins, Colorado (41°N, 105°W) are used to deduce the vertical temperature structure of the midlatitude middle atmosphere from 30–105 km. Nightly averaged temperatures measured across the Atlantic at comparable latitudes between 81 and 90 km showed seasonal variations tracking one another. Harmonic analyses of the measured profiles at these sites have been compared to the reference atmosphere, CIRA 1986. Considerable differences are noted in the annual mean temperatures in the mesopause region. Two temperature minima at 86 and 99 km are observed, suggesting heating and cooling mechanisms not previously incorporated into the reference atmosphere. Using the new 3D TIME‐GCM of Roble and Ridley which includes both dynamical and chemical processes, we account for this anomalous behavior by model simulation for the first time. Except the existence of strong semi‐annual oscillations, the observed annual and semi‐annual temperature variations are compatible with CIRA 1986. Observed annual temperature and solar flux forcing are out‐of‐phase between 65 and 99 km, reflecting a dynamics dominated atmosphere at these altitudes.