bluuebunny/crossref_metadata_2025 huggingface dataset:
The Uncertainty of Simple Spatial Averages Using Rain Gauge Networks
Authors: Morrissey Mark L. , Maliekal Jose A. , Greene John Scott , Wang Jianmin
Published: February 2004

A technique is presented whereby the standard error of a simple spatial average can be computed given any spatial configuration of measuring sites. The method should prove useful to those wishing to assess the time‐averaged performance of a network of rain gauges where the site locations are fixed within the averaging area. It is demonstrated that in addition to the site density and the spatial structure of the measured variable, the spatial geometry of the network with respect to the averaging area must be accounted for in the standard error expression. By incorporating estimates of the longterm population mean in the method, bias and random error in the sample statistics are minimized.
Reactive Transport of Petroleum Hydrocarbon Constituents in a Shallow Aquifer: Modeling Geochemical Interactions Between Organic and Inorganic Species
Authors: McNab W. W. , Narasimhan T. N.
Published: February 2004

Dissolved organic contaminants such as petroleum hydrocarbon constituents are often observed to degrade in groundwater environments through biologically mediated transformation reactions into carbon dioxide, methane, or intermediate organic compounds. Such transformations are closely tied to local geochemical conditions. Favorable degradation pathways depend upon local redox conditions through thermodynamic constraints and the availability of appropriate mediating microbial populations. Conversely, the progress of the degradation reactions may affect the chemical composition of groundwater through changes in electron donor/acceptor speciation and pH, possibly inducing mineral precipitation/dissolution reactions. Transport of reactive organic and inorganic aqueous species through open systems may enhance the reaction process by mixing unlike waters and producing a state of general thermodynamic disequilibrium. In this study, field data from an aquifer contaminated by petroleum hydrocarbons have been analyzed using a mathematical model which dynamically couples equilibrium geochemistry of inorganic constituents, kinetically dominated sequential degradation of organic compounds, and advective‐dispersive chemical transport. Simulation results indicate that coupled geochemical processes inferred from field data, such as organic biodegradation, iron reduction and dissolution, and methanogenesis, can be successfully modeled using a partial‐redox‐disequilibrium approach. The results of this study also suggest how the modeling approach can be used to study system sensitivity to various physical and chemical parameters, such as the effect of dispersion on the position of chemical fronts and the impact of alternative buffering mineral phases (e.g., goethite versus amorphous Fe(OH)3) on water chemistry.
Steam Stripping of Contaminated Unsaturated Zone of Subsoils: Theoretical Model of Start‐up Phase
Authors: Brouwers H. J. H. , Li Shuanhu
Published: February 2004

The present paper addresses the unsteady process of steam stripping of the unsaturated zone of soils contaminated with volatile nonaqueous phase liquids. First, on the basis of Darcy's law and the conservation laws of mass and energy, a one‐dimensional model is derived for the propagation of the steam front in the start‐up phase. It is shown that this process is governed by one dimensionless group y. Subsequently, the evaporation mechanism behind, the transport to, and the condensation at the front of volatile contaminants are considered, taking the view that nonequilibrium exists between liquid and vapor phases. The model leads to a moving boundary problem which is of special mathematical interest. By a suitable transformation of the governing partial differential equations, the problem is brought into a fixed domain and solved numerically. For a broad range of the governing dimensionless numbers, computation results are presented. The results obtained in this paper make clear the role of the prevailing principal physical phenomena during the start‐up phase of the process.
Nonlocal Reactive Transport with Physical and Chemical Heterogeneity: Linear Nonequilibrium Sorption with Random Kd
Authors: Hu Bill X. , Deng Fei‐Wen , Cushman John H.
Published: February 2004

A nonlocal, first‐order, Eulerian stochastic theory was developed by Deng et al. (1993) for the mean concentration of a conservative tracer. Here that result is extended to account for linear nonequilibrium sorption with random partition coefficient. The resultant theory is nonlocal in space and time. An important observation is that unlike Deng et al. (1993), nonlocality is manifest not just in the dispersive flux, but in an effective convective flux and in sources and sinks as well. The fully nonlocal theory is solved exactly in Fourier‐Laplace space and converted to a real‐space solution via fast Fourier transform in the spirit of Deng et al. (1993). Where possible, comparisons are made with Bellin et al. (1993) and Dagan and Cvetkovic (1993). Positive, negative, and uncorrelated models relating the fluctuating partition coefficient to the fluctuating log conductivity are used to examine the evolution of mean concentration via contours and spatial moments up to the third. The initial sorbed concentration and the deterministic reaction rate can have a significant effect on the moments, especially the second longitudinal moment. The first moment is relatively insensitive to the various correlation structures, but the second and third may exhibit a sensitivity. In the long time asymptotic limit the first two moments are consistent with Fickian theory; however, in the preasymptotic regime the process is nonlocal and non‐Fickian.
Plume‐Scale Dependent Dispersion in Aquifers with a Wide Range of Scales of Heterogeneity
Authors: Rajaram Harihar , Gelhar Lynn W.
Published: February 2004

Dispersivity computations are developed for heterogeneous aquifers which exhibit structured variability across a wide range of scales. Multiscale, fractional Gaussian and self‐similar random field models are used to describe the statistical structure of hydraulic conductivity variations. The concept of a relative dispersivity (Aijr) is introduced, following a similar concept introduced earlier in the context of turbulent diffusion. The relative dispersivity depends on the plume size and is influenced only by velocity variations at scales smaller than the plume scale. In contrast, the dispersivity associated with the ensemble average concentration (Aij) is influenced by all scales of velocity variations, including very large scales. In fractional Gaussian media, previous analyses predict that Aij grows as a power function of the plume displacement. In contrast, the longitudinal relative dispersivity approaches a constant value, and the transverse components gradually approach zero. In fractional Gaussian and self‐similar media the longitudinal dispersivity is shown to be a power function of the transverse second moment of the plume. The initial plume dimensions have a strong influence on dispersivity values for plumes in media with structured variability across a wide range of scales. A generalization of self‐similar random field models is presented to accommodate the widely observed feature of statistical anisotropy in ln K variability. A practical illustration of the results is presented based on ln K variability data at the Cape Cod site. Two‐scale exponential and self‐similar models are fit to the ln K variograms, and dispersivity computations are developed for two plumes with very different initial dimensions. At a displacement of about 1 km, a longitudinal relative dispersivity lpar;A11r) value of 30 m is estimated for the sewage plume with initial dimensions of 500 × 500 × 10 m, while the estimate corresponding to the tracer test plume with initial dimensions of 5 × 5 × 1 m is about 1–2 m. The corresponding ensemble longitudinal dispersivity (A11) is about 90 m. Relative dispersivities are more appropriate for characterizing the dilution and spreading at individual heterogeneous aquifers. Dispersivities associated with the ensemble average concentration will tend to overestimate the degree of dilution and spreading in an aquifer, and this error can be very large in media with a wide range of scales of heterogeneity.
Artificial Neural Network Modeling of the Rainfall‐Runoff Process
Authors: Hsu Kuo‐lin , Gupta Hoshin Vijai , Sorooshian Soroosh
Published: February 2004

An artificial neural network (ANN) is a flexible mathematical structure which is capable of identifying complex nonlinear relationships between input and output data sets. ANN models have been found useful and efficient, particularly in problems for which the characteristics of the processes are difficult to describe using physical equations. This study presents a new procedure (entitled linear least squares simplex, or LLSSIM) for identifying the structure and parameters of three‐layer feed forward ANN models and demonstrates the potential of such models for simulating the nonlinear hydrologic behavior of watersheds. The nonlinear ANN model approach is shown to provide a better representation of the rainfall‐runoff relationship of the medium‐size Leaf River basin near Collins, Mississippi, than the linear ARMAX (autoregressive moving average with exogenous inputs) time series approach or the conceptual SAC‐SMA (Sacramento soil moisture accounting) model. Because the ANN approach presented here does not provide models that have physically realistic components and parameters, it is by no means a substitute for conceptual watershed modeling. However, the ANN approach does provide a viable and effective alternative to the ARMAX time series approach for developing input‐output simulation and forecasting models in situations that do not require modeling of the internal structure of the watershed.
Analytical Solutions of the Nonlinear Groundwater Flow Equation in Unconfined Aquifers and the Effect of Heterogeneity
Authors: Serrano Sergio E.
Published: February 2004

Using the method of decomposition, new analytical solutions of the nonlinear Boussinesq flow equation and of the exact two‐dimensional groundwater flow equation subject to a nonlinear free‐surface boundary condition are presented and tested with respect to the linearized Boussinesq equation. It is found that for mild regional gradients and for the range of recharge values usually encountered in the field, the extensively used linearized equation with Dupuit assumptions is a reasonable approximation to the exact solution for the hydraulic heads and the regional flow velocities. Discrepancies occur in the presence of high regional hydraulic gradients, unusually high recharge rates, or regions of low hydraulic conductivity. With this result a new nonperturbation solution of the two‐dimensional plan view groundwater flow equation in a heterogeneous (heterogeneity represented as a spatial random field) aquifer is presented, and expressions for the statistical properties of the longitudinal and transverse velocities are derived. The results suggest that scale dependency, or spatial variability, in the flow velocity arises naturally as a result of recharge from rainfall and aquifer heterogeneity and may help explain the scale dependency of aquifer dispersion parameters.
Biogeochemical Influences on the Determination of Water Chemistry in a Temperate Forest Basin: Factors Determining the pH value
Authors: Ohte Nobuhito , Tokuchi Naoko , Suzuki Masakazu
Published: February 2004

In order to clarify the mechanism of pH determination in a temperate forest watershed in Japan, intensive hydrochemical observations that included in situ measurement of dissolved pCO2 were carried out in 1991 and 1992. From the variations of observed pCO2 and pH and estimated alkalinity associated with the hydrological process, the factors determining pH were described. There were two hydrological processes which have different determining hydrochemical processes: (1) rainfall and throughfall to infiltration in the soil layer to stable groundwater and (2) stable groundwater to spring water to stream water. In the first process, pH is influenced by infiltration from the low pCO2 layer to the high CO2 layer and by an increase of alkalinity, which is mainly caused by an exchange reaction and chemical weathering. In the shallow soil layer the protons for alkalinity generation are supplied by acid deposits from rainfall and throughfall, microbial acid production, and CO2 dissolution reaction. In the deeper layer an increase of alkalinity caused by Na+ generation becomes remarkable as depth increases. This process is strongly controlled by chemical weathering. In the second process, pH increases with CO2 degassing around the spring point. The alkalinity is kept at the same level as that of the stable groundwater. These results suggests that the biochemically supplied CO2 in soil not only directly controls the pH determination, but also has influences on the alkalinity generation as another determining factor of pH.
Scaling of Input Data for Macroscale Hydrologic Modeling
Authors: Kite G. W.
Published: February 2004

Hydrologic models provide the land‐phase link between atmospheric models and oceanographic models within the global water cycle. They provide independent validation of the outputs from atmospheric models and may also provide a mechanism to examine the implications of climatic change on water resources. To form a component of a global model, hydrologic models must be applicable at macroscale and continental scale. The sources and scales of input data are critical to the development of such models. Results from the application of a distributed hydrologic model to Canadian watersheds from 500 km2 to 1.6 million km2 in area are used to compare errors found using input data at different scales. Data considered include land cover, vegetation index, and snow water equivalent from satellite sensors and distributed climate data from a general circulation model and from numerical weather prediction models. Analysis of the results allows a consideration of appropriate data scaling in the development of macroscale hydrologic models.
Controls on High Sulfate Concentrations in the Upper Floridan Aquifer in Southwest Florida
Authors: Sacks Laura A. , Herman Janet S. , Kauffman Shelley J.
Published: February 2004

Chemical profiles of groundwater in the Upper Floridan aquifer in southwest Florida vary between an inland and a coastal site. At the inland site, sulfate concentrations increase with depth, and chloride concentrations remain low. At the coastal site, sulfate concentrations are high and constant, but chloride concentrations increase with depth. The sulfate source at the inland site is gypsum dissolution, whereas at the coastal site, the sulfate source is predominantly gypsum dissolution overprinted by saltwater mixing. Mass transfer modeling in conjunction with isotopic data was used to evaluate hypothetical flow paths within the aquifer. Dedolomitization reactions (gypsum and dolomite dissolution with calcite precipitation) dominate along flow paths where sulfate concentrations increase. The distribution of gypsum in the aquifer indicates that a deeply circulating flow path controls the chemical evolution of groundwater between the two sites. Fresh groundwater moves into deep zones in the aquifer where gypsum is found. Farther along the flow path in the discharge area, this groundwater moves upward to shallower depths prior to the saltwater mixing zone. Saltwater mixing becomes more pronounced with both proximity to the coast and with increasing depth. These results underscore the importance of evaluating vertical, as well as lateral, flow paths when quantifying geochemical reactions in a chemically stratified aquifer.
Hydrologic and Geochemical Effects on Oxygen Uptake in Bottom Sediments of an Effluent‐Dominated River
Authors: McMahon P. B. , Tindall J. A. , Collins J. A. , Lull K. J. , Nuttle J. R.
Published: February 2004

More than 95% of the water in the South Platte River downstream from the largest wastewater treatment plant serving the metropolitan Denver, Colorado, area consists of treated effluent during some periods of low flow. Fluctuations in effluent‐discharge rates caused daily changes in river stage that promoted exchange of water between the river and bottom sediments. Groundwater discharge measurements indicated fluxes of water across the sediment‐water interface as high as 18 m3 s−1 km−1. Laboratory experiments indicated that downward movement of surface water through bottom sediments at velocities comparable to those measured in the field (median rate ≈0.005 cm s−1) substantially increased dissolved oxygen uptake rates in bottom sediments (maximum rate 212 ± 10 μmol O2 L−1 h−1) compared with rates obtained when no vertical advective flux was generated (maximum rate 25 ± 8.8 μmol O2 L−1 h−1). Additions of dissolved ammonium to surface waters generally increased dissolved oxygen uptake rates relative to rates measured in experiments without ammonium. However, the magnitude of the advective flux through bottom sediments had a greater effect on dissolved oxygen uptake rates than did the availability of ammonium. Results from this study indicated that efforts to improve dissolved oxygen dynamics in effluent‐dominated rivers might include stabilizing daily fluctuations in river stage.
The Value of Hydrologic Information in Stochastic Dynamic Programming Models of a Multireservoir System
Authors: Tejada‐Guibert J. Alberto , Johnson Sharon A. , Stedinger Jery R.
Published: February 2004

Reservoir operating policies can be derived using stochastic dynamic programming (SDP) with different hydrologic state variables. This paper considers several choices for such hydrologic state variables for SDP models of the Shasta‐Trinity system in northern California, for three different benefit functions. We compare how well SDP models predict their policies will perform, as well as how well these policies performed when simulated. For a benefit function stressing energy maximization, all policies did nearly as well, and the choice of the hydrologic state variable mattered very little. For a benefit function with larger water and firm power targets and severe penalties on corresponding shortages, predicted performance significantly overestimated simulated performance, and policies that employed more complete hydrologic information performed significantly better.
Colloid‐facilitated contaminant transport in discretely fractured porous media: 1. Numerical formulation and sensitivity analysis
Authors: Ibaraki M. , Sudicky E. A.
Published: February 2004

A two‐dimensional numerical model is developed that incorporates the mechanism of colloid‐facilitated transport in discretely fractured porous media. The numerical model accounts for aqueous phase contaminant transport in the fractures and the porous matrix, colloid transport in the fractures, and sorption of the solute. Deep‐bed filtration of the colloids is accounted for, and the solute is allowed to sorb on both the mobile and filtered colloids. The numerical formulation allows for either equilibrium or kinetic sorption reactions onto the fracture walls, the matrix solids, and the mobile and filtered colloids according to either a Langmuir or a Freundlich isotherm. The results of a series of simulations involving a system of parallel fractures explore the importance of mobile colloids on contaminant migration and indicate that if sorption onto the colloids is a slow kinetic process, then the mobile colloids may lead to significantly enhanced contaminant migration.
Field Experiments on Erosion by Overland Flow and Their Implication for a Digital Terrain Model of Channel Initiation
Authors: Prosser Ian P. , Dietrich William E.
Published: February 2004

Dietrich et al. (1992, 1993) proposed a digital terrain model for predicting the location of channel heads on the basis of the assumption that they occur where saturation overland flow exerts a boundary shear stress in excess of a critical value. Flume experiments were conducted in the modeled field site to evaluate the threshold hypothesis and to constrain critical shear stress and flow resistance parameters. Under complete grass cover, microtopography and grass stems were found to prevent significant sediment transport at all but the highest flows. When the grass stems were cut close to the ground, flow resistance and critical shear stress for significant sediment transport were reduced by up to an order of magnitude, but the remaining dense root mat prevented deep scour. These field experiments support the threshold assumption and the model estimations of the critical shear stress if local topographic convergence of flow is taken into account. The experiments also support the interpretation that significant degradation of vegetation cover is required for channel incision.
Potentials and Their Role in Transport in Porous Media
Authors: Nitao John J. , Bear Jacob
Published: February 2004

The concept of “capillary,” or “matric,” potentials is commonly used in soil physics to describe water movement in unsaturated soils. The rigorous definition of these and other potentials is presented from fundamental thermodynamic principles at the microscopic level and extended to the macroscopic level by averaging over a representative elementary volume. Of special interest is the treatment of adsorptive surface forces and their associated potentials. Porous medium potentials are extended to a domain containing multiple fluid phases and multiple components. A macroscopic motion equation for a fluid phase (Darcy's law) is derived, incorporating the effect of potentials and surface forces. It relates advective fluxes to gradients of macroscopic chemical potentials and temperature. It reduces to the usual form of Darcy's law only when the aqueous phase is sufficiently dilute and temperatures are uniform. Kelvin's law, which relates relative humidity to matric potential, is extended to the case of multiple multicomponent fluid phases in a porous medium domain. The concept of “irreducible” (or “residual”) wetting fluid saturation and its relationship to capillary pressure, surface forces, and the Gibbs chemical potential, are discussed. Common methods for determining the matric potential are reexamined in light of this work.
A Finite Volume Eulerian‐Lagrangian Localized Adjoint Method for Solution of the Contaminant Transport Equations in Two‐Dimensional Multiphase flow Systems
Authors: Binning Philip , Celia Michael A.
Published: February 2004

An Eulerian‐Lagrangian localized adjoint method (ELLAM) numerical solution is developed for the multiphase contaminant transport equations in two dimensions. The ELLAM uses finite volume test functions in the space‐time domain defined by the characteristics of the hyperbolic part of the governing equation. The use of the characteristics results in an approximation that allows large time steps while still maintaining accurate solutions. This greatly reduces the computational effort required to find solutions to the advection‐dispersion equation. Combination of the finite volume test functions with the conservative form of the governing equation results in a local conservation of mass property. For problems with a constant saturation such as that of contaminant transport in the saturated zone this property is highly advantageous as the resulting numerical approximation conserves mass both globally and locally. For problems with a variable saturation the requirement of local conservation of mass is too stringent, particularly around first‐type boundaries where oscillations can occur because of errors inherent in the numerical determination of fluid velocities and in the backtracking routine. A combined conservative/nonconservative ELLAM is developed with an ELLAM formulation based on the nonconservative form of the governing equation being applied to subdomains intersecting first‐type boundaries and a conservative ELLAM being used for all other subdomains. The combined conservative/nonconservative ELLAM is compared to a Galerkin finite element scheme and is found to have greatly superior performance, requiring far fewer time steps to obtain a solution of equivalent accuracy.
Contaminant Displacement in Aquifers: Coupled Effects of Flow Heterogeneity and Nonlinear Sorption
Authors: Berglund Sten , Cvetkovic Vladimir
Published: February 2004

An analytical solution is developed for displacement of a nonlinearly sorbing solute in a three‐dimensional heterogeneous aquifer. Nonlinear sorption is modeled by means of different isotherm equations including the commonly used Langmuir and Freundlich isotherms, but also the Langmuir‐Freundlich, Tóth, and two‐site Langmuir isotherms. Evaluation of cleanup times, defined as the times required to reduce the concentration to a given fraction of its initial value, for different isotherms and combinations of input parameters demonstrate the effects of the properties of the isotherms. In particular, the asymptotic behavior as the aqueous concentration approaches zero is important for transport predictions, where Freundlich sorption leads to infinitely increasing cleanup times as the initial plume concentration decreases. The effect of the choice of sorption isotherm, evaluated by using “best‐fit”parameters for four different isotherms, is found to be large and dependent on heterogeneity for the particular data set considered.
On the Transport Properties of Anisotropic Networks of Capillaries
Authors: Friedman Shmulik P. , Seaton Nigel A.
Published: February 2004

Porous media such as sedimentary rocks and soils are often anisotropic in their resistance to flow of fluids, solutes, heat, and electrical current. Lattice‐based calculations of the transport properties of pore networks are of considerable interest, both because of their tractability and because methods exist for the characterization of porous solids in terms of lattice model parameters. We have investigated the dependence of the anisotropy factor for different diffusive and convective transport mechanisms, on the direction‐dependent connectivities and pore size distributions, using a uniaxial, anisotropic simple cubic lattice. While connectivity‐induced anisotropy, due to different coordination numbers in directions parallel and perpendicular to the bedding plane, significantly affects transport coefficients in both directions, anisotropy induced by pore size distribution mainly affects the perpendicular transport coefficients.
Calculation of Temperature Effects on Wetting Coefficients of Porous Solids and Their Capillary Pressure Functions
Authors: Grant Steven A. , Salehzadeh Amir
Published: February 2004

We explored the notion that changes in wetting coefficients of porous solids contributed to the temperature sensitivities of capillary pressure functions (CPFs). A chemical‐thermodynamic explanation for these contributions was developed. If the temperature sensitivities of CPFs were due to capillarity (i.e., due to temperature‐induced changes in liquid‐gas interfacial tensions or wetting coefficients), then for a given degree of saturation the ratios of capillary pressures to their temperature derivatives should have been linear functions of thermodynamic temperature with slopes equal to 1. This indeed was the case for samples of both synthetic and natural porous media. Further, the estimated intercepts of these linear functions indicated that changes with temperature of these porous materials' wetting coefficients had pronounced effects on temperature sensitivities of their CPFs. A simple model for temperature effects on CPFs, which was derived from the linear relationship between temperature and the ratio of capillary pressure to its temperature derivative, could be fitted precisely by nonlinear regression to CPFs of two soils determined at four temperatures.
An Improved Genetic Algorithm for Pipe Network Optimization
Authors: Dandy Graeme C. , Simpson Angus R. , Murphy Laurence J.
Published: February 2004

An improved genetic algorithm (GA) formulation for pipe network optimization has been developed. The new GA uses variable power scaling of the fitness function. The exponent introduced into the fitness function is increased in magnitude as the GA computer run proceeds. In addition to the more commonly used bitwise mutation operator, an adjacency or creeping mutation operator is introduced. Finally, Gray codes rather than binary codes are used to represent the set of decision variables which make up the pipe network design. Results are presented comparing the performance of the traditional or simple GA formulation and the improved GA formulation for the New York City tunnels problem. The case study results indicate the improved GA performs significantly better than the simple GA. In addition, the improved GA performs better than previously used traditional optimization methods such as linear, dynamic, and nonlinear programming methods and an enumerative search method. The improved GA found a solution for the New York tunnels problem which is the lowest‐cost feasible discrete size solution yet presented in the literature.
El Niño and the Natural Variability in the Flow of the Nile River
Authors: Eltahir Elfatih A. B.
Published: February 2004

Natural variability in the annual flow of the Nile River has been the subject of great interest to the civilizations that have historically occupied the banks of that river. Here we report results from analysis on two extensive data sets describing sea surface temperature of the Pacific Ocean, and the flow of water in the Nile River. The analysis suggests that 25% of the natural variability in the annual flow of the Nile is associated with El Niño oscillations. A procedure is developed for using this observed correlation to improve the predictability of the Nile flood. A simple hypothesis is presented to explain physically the occurrence of the Hurst phenomenon in the Nile flow.
Estimator Bias in a Lake Phosphorus Model with Observation Error
Authors: Stow Craig A. , Reckhow Kenneth H.
Published: February 2004

We investigated observation‐error‐induced parameter bias using least squares and Stein‐corrected least squares estimators in a model for predicting lake phosphorus. The Stein‐corrected estimator performed better than the uncorrected estimator from bias and “closeness” perspectives, though the corrected estimator was still biased. Examination of the model structure revealed that parameter bias is strongly related to both the parameter space and sample space. Additionally, the model is robust to parameter bias over a large portion of the sample and parameter space, indicating that this model may be particularly useful for estimation and prediction. Analogous structure in other models could be an important consideration for model selection.
Estimates of Evapotranspiration or Effective Moisture in Rocky Mountain Watersheds from Chloride Ion Concentrations in Stream Baseflow
Authors: Claassen Hans C. , Halm Douglas R.
Published: February 2004

The principle that atmospherically derived chloride is a conservative tracer in many watersheds can be used to calculate average annual evapotranspiration or effective moisture if estimates are available for (1) the average annual chloride input to the watershed, (2) the average annual precipitation, and (3) the baseflow chloride concentration are known. The method assumes that no long‐term storage of chloride occurs and there is no lithologic source of chloride, or that such source releases only insignificant amounts to groundwater compared to the atmospheric source. National Atmospheric Deposition Program estimates of chloride wet deposition, watershed precipitation records or hyetal map estimates of precipitation input to watersheds, and a single sample of chloride concentration in base flow were used to calculate evapotranspiration for diverse Rocky Mountain watersheds. This estimate was compared to evapotranspiration determined by subtracting mean discharge from precipitation. Of the 19 watersheds used to test the method, 13 agreed within 10%, 2 appear to have not met the lithology criterion, 1 appears to have not met the flow criterion, and 1 neither criterion. The method's greatest strength is the minimal data requirements and its greatest weakness is that for some watersheds it may be difficult to obtain reliable estimates of precipitation and chloride deposition. If reliable discharge data are available, the method may be used to estimate watershed‐average precipitation; this is especially useful in high‐altitude mountain watersheds where little or no precipitation data are available.
A Kinetic Approach to Modeling Mobile Bacteria‐Facilitated Groundwater Contaminant Transport
Authors: Kim Seunghyun , Corapcioglu M. Yavuz
Published: February 2004

Mobile bacterial particles can act as carriers and enhance the transport of hydrophobic contaminants in groundwater by reducing retardation effects. Because of their colloidal size and favorable surface conditions, bacteria act as efficient contaminant carriers. When such carriers exist in a water‐saturated porous medium, the system can be thought of as three phases: an aqueous phase, a carrier phase, and a stationary solid matrix phase. Contaminant may be present in either or all of these phases. In this study, a mathematical model based on mass balances is developed to describe the simultaneous transport and fate of a biodegradable contaminant and bacteria in a porous medium. The Monod model describes the relation between the growth rate of bacteria and concentration of the contaminant which serves as a growth substrate. The bacterial decay is assumed to be a first‐order reaction. The mechanisms of bacterial mass transfer between aqueous and solid matrix phases, and contaminant mass transfer between aqueous and bacterial phases are represented by kinetic models. Governing equations are nondimensionalized and solved to analyze the bacteria‐facilitated contaminant transport. The model is compared with a similar model obtained with local equilibrium assumption to understand the characteristics of kinetic and local equilibrium descriptions. Results show that the contaminant transport as well as bacterial transport can be described by local equilibrium assumption when Damköhler numbers are larger than 10. Significant sensitivities to model parameters, particularly bacterial growth rate and influent bacterial concentration, were observed. The numerical results of the bacteria carrier effect match favorably with experimental data reported in the literature.
Direct Simulation of Groundwater Age
Authors: Goode Daniel J.
Published: February 2004

A new method is proposed to simulate groundwater age directly, by use of an advection‐dispersion transport equation with a distributed zero‐order source of unit (1) strength, corresponding to the rate of aging. The dependent variable in the governing equation is the mean age, a mass‐weighted average age. The governing equation is derived from residence‐time‐distribution concepts for the case of steady flow. For the more general case of transient flow, a transient governing equation for age is derived from mass‐conservation principles applied to conceptual “age mass.” The age mass is the product of the water mass and its age, and age mass is assumed to be conserved during mixing. Boundary conditions include zero age mass flux across all noflow and inflow boundaries and no age mass dispersive flux across outflow boundaries. For transient‐flow conditions, the initial distribution of age must be known. The solution of the governing transport equation yields the spatial distribution of the mean groundwater age and includes diffusion, dispersion, mixing, and exchange processes that typically are considered only through tracer‐specific solute transport simulation. Traditional methods have relied on advective transport to predict point values of groundwater travel time and age. The proposed method retains the simplicity and tracer‐independence of advection‐only models, but incorporates the effects of dispersion and mixing on volume‐averaged age. Example simulations of age in two idealized regional aquifer systems, one homogeneous and the other layered, demonstrate the agreement between the proposed method and traditional particle‐tracking approaches and illustrate use of the proposed method to determine the effects of diffusion, dispersion, and mixing on groundwater age.
Peak Flow Responses to Clear‐Cutting and Roads in Small and Large Basins, Western Cascades, Oregon
Authors: Jones J. A. , Grant G. E.
Published: February 2004

This study quantified long‐term changes in streamflows associated with clear‐cutting and road construction and examined alternative hydrologic mechanisms to explain stream hydrograph changes in the Cascades Range, western Oregon. We examined differences in paired peak discharges for 150 to 375 storm events for five basin pairs, using 34‐year records from two pairs of 60‐to‐101‐ha experimental basins in the H. J. Andrews Experimental Forest, and 50‐to‐55‐year records from three pairs of adjacent basins ranging from 60 to 600 km2. Forest harvesting has increased peak discharges by as much as 50% in small basins and 100% in large basins over the past 50 years. These increases are attributable to changes both in flow routing due to roads and in water balance due to treatment effects and vegetation succession.
Spring Snowpack Anomaly Patterns and Winter Climatic Variability, British Columbia, Canada
Authors: Moore R. D. , McKendry I. G.
Published: February 2004

The objective of this study was to describe the spatial and temporal structure of spring snowpack anomalies in British Columbia, Canada, and to relate the anomaly patterns to climatic fluctuations. Cluster analysis was used to identify relatively homogeneous groups of snow course sites, based on the April 1 snowpack measurements, for the periotd 1966–1992. Time series of cluster‐averaged anomalies were then computed. Synoptic‐scale circulation types were defined by applying a correlation‐based map classification technique to gridded data sets of surface pressure and 500‐hPa height levels. Frequencies of the circulation types were calculated for each winter (November to March). Total precipitation and mean temperature for each winter were calculated from monthly data from 16 climate stations. Snowpack anomalies from 1966 to 1976 were dominated by two patterns: one characterized by generally heavier‐than‐average snowpacks over most of the province and the other by heavier‐than‐average snowpacks in the south and lighter‐than‐average in the north. From 1977 to 1992, snowpack conditions were generally either lighter than average over the whole province or were heavier than average in the north and lighter than average in the south. Snowpack and precipitation were generally correlated, although at some stations it appears that high winter temperatures, likely causing more midwinter melt and more rain, can act to reduce the snowpack. The differences in anomaly patterns and frequencies of synoptic types between the 1966–1976 and 1977–1992 periods accord with decadal‐scale shifts in sea surface temperatures and atmospheric circulation patterns over the North Pacific, as reported in the literature. The shift in snowpack anomaly patterns following 1976 is consistent with reported shifts in glacier mass balance and rates of retreat and with streamflow fluctuations.
Evaluation of Sampling in the Subsurface
Authors: Andričević Roko
Published: February 2004

The evaluation of sampling is presented in the form of a sampling error defined as a difference between the true average and the sampled average of the measured process over the sampling domain. The sampling error is quantified as a product between the process point variance (defined at the scale of a sampling device's support volume) and the sampling variance function, which quantifies the reduction of uncertainty due to the sampling activity. The sampling variance function is expressed analytically as a function of the scale of fluctuation of the measured process. For a single sampling point the sampling variance function defines the reduction in the process point variance as a result of sampling over a certain support volume. The presented evaluation of the sampling performance can be used for designing the monitoring activity and for measurement conditioning of stochastic theories. The presented examples demonstrate the use of sampling evaluation for the purpose of conditioning on the concentration measurements.
Effect of water management on methane emission from a Japanese rice paddy field: Automated methane monitoring
Authors: Yagi Kazuyuki , Tsuruta Haruo , Kanda Ken‐ichi , Minami Katsuyuki
Published: February 2004

The effect of differing water management schemes on the emission of methane (CH4) from rice paddies to the atmosphere was studied in a Japanese paddy field. Using an automated sampling and analyzing system, the test site was divided into two plots: a continuously flooded plot which was maintained flooded by constant irrigation from May to August, and an intermittently drained plot in which short‐term draining practices were performed several times during the flooding period . The draining practice had a strong effect on CH4 emission. A large flush of CH4 emission was observed in the intermittently drained plot immediately after each drainage. It was followed by a rapid decrease in CH4 flux in most of the cases. A large flush of CH4 was observed after the final drainage at the end of August in the continuously flooded plot, accounting for about 7% of the total CH4 emitted in the plot. Total emission rates of CH4 during the cultivation period were 14.8 and 8.63 g m−2 for 1991 and 9.49 and 5.18 g m−2 for 1993 in the continuously flooded and intermittently drained plots, respectively. Companion N2O flux measurements showed that almost no N2O was emitted from either plot until the final drainage. These results indicate that short‐term draining practices strongly reduce CH4 emission from rice paddy fields, and that improvement in water management can be one of the most important mitigation strategies for CH4 emission from rice paddy fields.
An improved method for detecting anthropogenic CO2 in the oceans
Authors: Gruber Nicolas , Sarmiento Jorge L. , Stocker Thomas F.
Published: February 2004

An improved method has been developed for the separation of the anthropogenic CO2 from the large natural background variability of dissolved inorganic carbon (C) in the ocean. This technique employs a new quasi‐conservative carbon tracer ΔC*, which reflects the uptake of anthropogenic CO2 and the air‐sea disequilibrium when a water parcel loses contact with the atmosphere. The air‐sea disequilibrium component can be discriminated from the anthropogenic signal using either information about the water age or the distribution of ΔC* in regions not affected by the anthropogenic transient. This technique has been applied to data from the North Atlantic sampled during the Transient Tracers in the Ocean North Atlantic (TTO NAS) and Tropical Atlantic study (TTO TAS) cruises in 1981–1983. The highest anthropogenic CO2 concentrations and specific inventories (inventory per square meter) are found in the subtropical convergence zone. In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior of the ocean, north of 50°N it has even reached the bottom. Only waters below 3000 m and south of 30°N are not yet affected. We estimate an anthropogenic CO2 inventory of 20 ± 4 Gt C in the North Atlantic between 10°N and 80°N. The 2.5‐dimensional ocean circulation model of Stocker et al. [1994] and the three‐dimensional ocean general circulation biogeochemistry model of Sarmiento et al. [1995] predict anthropogenic CO2 inventories of 18.7 Gt C and 18.4 Gt C, respectively, in good agreement with the observed inventory. Important differences exist on a more regional scale, associated with known deficiencies of the models.
Interannual variation of carbon exchange fluxes in terrestrial ecosystems
Authors: Kindermann Jürgen , Würth Gudrun , Kohlmaier Gundolf H. , Badeck Franz‐W.
Published: February 2004

A global prognostic physiologically based model of the carbon budget in terrestrial ecosystems, the Frankfurt Biosphere Model (FBM), is applied to simulate the interannual variation of carbon exchange fluxes between the atmosphere and the terrestrial biosphere. The data on climatic forcing are based on Cramer and Leemans climate maps; the interannual variation is introduced according to records of temperature anomalies and precipitation anomalies for the period 1980 to 1993. The calculated net exchange flux between the atmosphere and the terrestrial biosphere is compared to the biospheric signal deduced from 13C measurements. Some intermediate results are presented as well: the contributions of the most important global ecosystems to the biospheric signal, the contributions of different latitudinal belts to the biospheric signal, and the responses of net primary production (NPP) and heterotrophic respiration (Rh). From the simulation results it can be inferred that the complex temperature and precipitation responses of NPP and Rh in different latitudes and different ecosystem types add up to a global CO2 signal contributing substantially to the atmospheric CO2 anomaly on the interannual timescale. The temperature response of NPP was found to be the most important factor determining this signal.
Effects of plankton dynamics on seasonal carbon fluxes in an ocean general circulation model
Authors: Six Katharina D. , Maier‐Reimer Ernst
Published: February 2004

We discuss the effect of embedding a simple plankton model in the Hamburg model of the oceanic carbon cycle (HAMOCC3) [Maier‐Reimer, 1993]. The plankton model consists of five components: phytoplankton, zooplankton, detritus, dissolved organic carbon, and nutrients. Interactions between compartments are described by one global set of parameters. Despite its simplicity the plankton model reproduces regional differences in seasonal oceanic pCO2 and improves the biogeochemical tracer distributions at the depth of the oxygen minimum in the Pacific Ocean. The predicted seasonal turnover of organic material is consistent with recent atmospheric O2 measurements in the remote areas of the Southern Ocean.
Denitrification in marine sediments: A model study
Authors: Middelburg Jack J. , Soetaert Karline , Herman Peter M. J. , Heip Carlo H. R.
Published: February 2004

The rate and factors controlling denitrification in marine sediments have been investigated using a prognostic diagenetic model. The model is forced with observed carbon fluxes, bioturbation and sedimentation rates, and bottom water conditions. It can reproduce rates of aerobic mineralization, denitrification, and fluxes of oxygen, nitrate, and ammonium. The globally integrated rate of denitrification is estimated by this model to be about 230–285 Tg N yr−1, with about 100 Tg N yr−1 occurring in shelf sediments. This estimate is significantly higher than literature estimates (12–89 Tg N yr−1), mainly because of a proposed upward revision of denitrification rates in slope and deep‐sea sediments. Higher sedimentary denitrification estimates require a revision of the marine nitrogen budget and lowering of the oceanic residence time of nitrogen down to about 2×103 years and are consistent with reported low N/P remineralization ratios between 1000 and 3000 m. Rates of benthic denitrification are most sensitive to the flux of labile organic carbon arriving at the sediment‐water interface and bottom water concentrations of nitrate and oxygen. Denitrification always increases when bottom water nitrate increases but may increase or decrease if oxygen in the bottom water increases. Nitrification is by far the most important source of nitrate for denitrification, except for organic‐rich sediments underlying oxygen‐poor and nitrate‐rich water.