bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Van Genuchten–Mualem Relationships to Estimate Unsaturated Hydraulic Conductivity at Low Water Contents
Authors: Khaleel Raziuddin , Relyea John F. , Conca James L.
Published: February 2004

Predicting contaminant migration within the vadose zone, for performance or risk assessment, requires estimates of unsaturated hydraulic conductivity for field soils. Hydraulic conductivities, K, were experimentally determined as a function of volumetric moisture content, θ, for Hanford sediments. The steady state head control method and an ultracentrifuge method were used to measure K(θ) in the laboratory for 22 soil samples. The van Genuchten model was used to fit mathematical functions to the laboratory‐measured moisture retention data. Unsaturated conductivities estimated by the van Genuchten–Mualem predictive model, using the fitted moisture retention curve and measured saturated hydraulic conductivity, Ks, were compared to those obtained by a scaled‐predictive method that uses a single K(θ) measurement as a match point near the dry regime. In general, the measured K values and those predicted from van Genuchten–Mualem relationships showed considerable disagreement. This suggests that the use of laboratory‐measured Ks results in an inadequate characterization of K(θ) for the desired range of moisture content. Deviations between the measured and predicted K were particularly severe at relatively low moisture contents; for some samples, there were differences in excess of 2 orders of magnitude at low θ. However, use of the same moisture retention curve‐fitting parameters and a single steady state head control‐based K(θ) measurement near the dry regime resulted in considerable improvement. In fact, for the coarse‐textured soils considered in this study, results indicate that a K∥θ) measurement near the dry regime must be used to obtain reliable estimates of unsaturated K at low θ. The study provided important insight on application of two different experimental techniques of measuring unsaturated conductivities.
An Interblock Conductivity Scheme for Finite Difference Models of Steady Unsaturated Flow in Heterogeneous Media
Authors: Desbarats A. J.
Published: February 2004

With the increased ability to model subsurface heterogeneity provided by stochastic simulation methods, there is a corresponding increase in the need for numerical flow models to accomodate the spatial variability of hydrodynamic parameters. These trends make it necessary to reexamine methods for calculating finite difference interblock conductivities originally developed for use in homogeneous media. This study describes a new method for calculating interblock conductivities better suited to the numerical modeling of unsaturated flow in heterogeneous media. An expression for interblock conductivity is derived by equating the finite difference flux between adjacent blocks with the exact analytical flux for one‐dimensional steady unsaturated flow, assuming an exponential model for relative conductivity. This expression appears as a function of the saturated conductivities, alpha parameters, and matric potentials of the adjacent blocks and the potential at the interblock boundary. Since this last quantity is not directly featured in the discretized flow equation, it must be evaluated iteratively. The interblock conductivity calculation scheme is implemented within a block‐centered seven‐point finite difference model of steady unsaturated flow where nonlinearity is resolved using a Newton‐Raphson iterative approach. In a series of numerical experiments this scheme is compared with the more familiar geometric averaging method. The proposed interblock conductivity scheme is shown to provide perfect agreement between numerical and analytical solutions for the one‐dimensional case of steady infiltration into a perfectly layered medium. In problems of two‐dimensional unsaturated flow in heterogeneous media, comparisons with the geometric averaging method show that the proposed scheme allows more rapid convergence of Newton‐Raphson iterations and consideration of greater parameter spatial variability before the onset of divergence in these iterations.
Inverting fluid conductivity logs for fracture inflow parameters
Authors: Evans David G.
Published: February 2004

Fluid electrical conductivity logging experiments introduced by Tsang et al. [1990] represent a new technology for studying the hydraulic properties of discrete fractures in open boreholes. These experiments consist of replacing water in the well bore with deionized water and then pumping the well to induce formation water to return to the well bore. During and after the fluid exchange the entire well is logged to measure fluid electrical conductivity (FEC). At locations where formation water enters the well there are abrupt increases in the borehole FEC, thereby revealing the location of hydraulically conductive fractures. As pumping continues, high FEC water moves up the well bore at a rate proportional to the total inflow rate below the observation point. Inflow rates and FECs of formation water associated with each fracture can be inferred by modeling the FEC profiles with the one‐dimensional advective dispersion equation. In this paper I present an inverse model which estimates the inflow rates and formation FEC values that optimally fit observed FEC logs acquired during fluid exchange experiments described by Tsang et al. [1990]. With this inverse model, inflow rates are constrained by the rate at which the well is pumped during the experiment. The forward model is solved numerically using a control volume finite difference scheme with power law upstream weighting and source term linearization. The inverse problem is solved using the Gauss‐Newton iterative method. The rows of the Jacobian matrix, or the sensitivity coefficients, are calculated numerically with the same algorithm that solves the forward problem. Both constrained and unconstrained inverse models are used to interpret fluid logging experiments performed in research wells penetrating Piedmont rocks of North Carolina.
A dynamic approach to sediment‐laden turbulent flows
Authors: Mendoza Cesar , Zhou Donghuo
Published: February 2004

An approach that studies the effects of the dynamic coupling between turbulent flow and sediment in suspension on the mean velocity and the sediment concentration profiles of sediment‐laden open‐channel flows is systematically developed. The analysis is based on the Boussinesq approximation to the governing equations that treats the mixture of the liquid and sediment as a single fluid with changing density over the flow depth. As a result, it permits the direct systematic comparison between the turbulent flow of clear water and that with suspended sediment. By further employing the mixing length approach for the related turbulent closure problem, the coupling effects on mean velocity and concentration profiles are evaluated, and the resulting distributions are compared with available experimental data. The derived mean velocity distribution contains profiles of existing models as special cases and thus unifies the results of existing major models conceptually and quantitatively. Moreover, the analysis clearly demonstrates the importance of the coupling effect on the suspended sediment concentration profile and, consequently, a more generalized formula of concentration profile is obtained. Although the mean velocity and concentration profiles are the main concern of this paper, the approach establishes a framework for further study of the effects of the dynamic interaction on other physical quantities.
Lagrangian Analysis of Transport in Heterogeneous Formations Under Transient Flow Conditions
Authors: Dagan Gedeon , Bellin Alberto , Rubin Yoram
Published: February 2004

The coupled effects of porous media heterogeneity and flow unsteadiness on transport are investigated. We address the problem using a stochastic‐Lagrangian approach. The log condictivity is modeled as a space random function; however, the flow unsteadiness is considered as deterministic, and it affects the magnitude and direction of the mean head gradient. Using a first‐order solution in terms of the log conductivity variance, we developed a three‐dimensional solution that includes the velocity covariances, the macrodispersion coefficients, and the displacement tensor, all as a function of the media heterogeneity and the unsteady mean flow parameters. The model is applied to the case of a periodic variation in the flow direction. We show that the effects of unsteadiness on the longitudinal spread are insignificant, while it affects strongly the transverse spread, in the form of an added constant dispersivity. Results obtained in previous studies are discussed and are shown to be particular cases of the present results.
Differentiating a Finite Element Biodegradation Simulation Model for Optimal Control
Authors: Minsker Barbara S. , Shoemaker Christine A.
Published: February 2004

An optimal control model for improving the design of in situ bioremediation of groundwater has been developed. The model uses a finite element biodegradation simulation model called Bio2D to find optimal pumping strategies. Analytical derivatives of the bioremediation finite element model are derived; these derivatives must be computed for the optimal control algorithm. The derivatives are complex and nonlinear; the bulk of the computational effort in solving the optimal control problem is required to calculate the derivatives. An overview of the optimal control and simulation model formulations is also given.
The Role of Colloidal Kaolinite in the Transport of Cesium through Laboratory Sand Columns
Authors: Saiers James E. , Hornberger George M.
Published: February 2004

Recent laboratory and field studies indicate that colloids are capable of adsorbing and enhancing the transport of a variety of contaminants in groundwater. We examined the influence of kaolinite particles on Cs movement through sand columns. The times for transport of the main part of the 137Cs pulse decreased by over a factor of 2 as the concentration of kaolinite in the column influent increased from 0 to 200 mg/L. Very early breakthrough of a relatively small amount of 137Cs was observed and attributed to the existence of sorption sites on the kaolinite that were kinetically controlled. This early breakthrough was essentially unretarded, whereas the initial breakthrough in the absence of kaolinite showed a retardation of about 15 pore volumes. Comparison of experimental observations with calculations from a transport model demonstrates that kaolinite deposition onto the sand was described best by an irreversible, first‐order kinetics process and that retention of 137Cs by these clay particles was described by linear equations for two‐site solute adsorption. Uptake of 137Cs by the sand was represented by the Langmuir version of the two‐site equations. This work indicates the need to account for facilitated transport of contaminants by inorganic colloids and provides a basis for quantifying the phenomenon.
Reactive Solute Transport in Streams: 1. Development of an Equilibrium‐Based Model
Authors: Runkel Robert L. , Bencala Kenneth E. , Broshears Robert E. , Chapra Steven C.
Published: February 2004

An equilibrium‐based solute transport model is developed for the simulation of trace metal fate and transport in streams. The model is formed by coupling a solute transport model with a chemical equilibrium submodel based on MINTEQ. The solute transport model considers the physical processes of advection, dispersion, lateral inflow, and transient storage, while the equilibrium submodel considers the speciation and complexation of aqueous species, precipitation/dissolution and sorption. Within the model, reactions in the water column may result in the formation of solid phases (precipitates and sorbed species) that are subject to downstream transport and settling processes. Solid phases on the streambed may also interact with the water column through dissolution and sorption/desorption reactions. Consideration of both mobile (water‐borne) and immobile (streambed) solid phases requires a unique set of governing differential equations and solution techniques that are developed herein. The partial differential equations describing physical transport and the algebraic equations describing chemical equilibria are coupled using the sequential iteration approach.
Reactive Solute Transport in Streams: 2. Simulation of a pH Modification Experiment
Authors: Runkel Robert L. , McKnight Diane M. , Bencala Kenneth E. , Chapra Steven C.
Published: February 2004

We present an application of an equilibrium‐based solute transport model to a pH‐modification experiment conducted on the Snake River, an acidic, metal‐rich stream located in the Rocky Mountains of Colorado. During the experiment, instream pH decreased from 4.2 to 3.2, causing a marked increase in dissolved iron concentrations. Model application requires specification of several parameters that are estimated using tracer techniques, mass balance calculations, and geochemical data. Two basic questions are addressed through model application: (1) What are the processes responsible for the observed increase in dissolved iron concentrations? (2) Can the identified processes be represented within the equilibrium‐based transport model? Simulation results indicate that the increase in iron was due to the dissolution of hydrous iron oxides and the photoreduction of ferric iron. Dissolution from the streambed is represented by considering a trace compartment consisting of freshly precipitated hydrous iron oxide and an abundant compartment consisting of aged precipitates that are less soluble. Spatial variability in the solubility of hydrous iron oxide is attributed to heterogeneity in the streambed sediments, temperature effects, and/or variability in the effects of photoreduction. Solubility products estimated via simulation fall within a narrow range (pKsp from 40.2 to 40.8) relative to the 6 order of magnitude variation reported for laboratory experiments (pKsp from 37.3 to 43.3). Results also support the use of an equilibrium‐based transport model as the predominate features of the iron and pH profiles are reproduced. The model provides a valuable tool for quantifying the nature and extent of pH‐dependent processes within the context of hydrologic transport.
Solute Transport in Nonstationary Velocity Fields
Authors: Indelman Peter , Rubin Yoram
Published: February 2004

The paper addresses the problem of transport of inert solutes in random nonstationary velocity fields. General equations for the first two moments of the particle trajectory are derived. It is shown that they can be solved analytically in quadratures for the case of quasi‐unidirectional mean flows. The solution generalizes the previous results obtained for transport in velocity fields which are uniform in the average. The theory is applied to the case of transport in media displaying a linear trend in the mean logconductivity. The dynamics of particle displacements is discussed for the case of two‐dimensional media with isotropic fluctuations of the logconductivity.
Regional Hydrogeochemistry of a Modern Coastal Mixing Zone
Authors: Wicks Carol M. , Herman Janet S.
Published: February 2004

In west central Florida, groundwater samples were collected along flow paths in the unconfined upper Floridan aquifer that cross the inland, freshwater recharge area and the coastal discharge area. A groundwater flow and solute transport model was used to evaluate groundwater flow and mixing of fresh and saline groundwater along a cross section of the unconfined upper Floridan aquifer. Results show that between 8% and 15% of the fresh and 30–31% of the saline groundwater penetrates to the depth in the flow system where contact with and dissolution of gypsum is likely. The deeply circulating fresh and saline groundwater returns to the near‐surface environment discharging CaSO4‐rich water to the coastal area where it mixes with fresh CaHCO3 groundwater, resulting in a prediction of calcite precipitation in the modern mixing zone.
Sediment Inclusion Events During Needle Ice Growth: A Laboratory Investigation of the Role of Soil Moisture and Temperature Fluctuations
Authors: Branson J. , Lawler D. M. , Glen J. W.
Published: February 2004

This paper reports on a series of laboratory experiments designed to determine the impact of imposed temperature and moisture variations on the inclusion of sediment within growing needle ice. The results demonstrate that needle‐ice growth reflects coupled heat and water fluxes, and thus provide experimental evidence to support theoretical studies by other authors. Sediment is shown to be incorporated into the ice as layers or dispersed concentrations when there is an imbalance of these two fluxes, that is, restrictions on moisture supply and/or over rapid heat loss from the surface, which causes the freezing front to descend into the soil profile.
Modeling Reservoir Density Underflow And Interflow From A Chemical Spill
Authors: Gu Ruochuan , McCutcheon Steven C. , Wang Pei‐Fang
Published: February 2004

An integral simulation model has been developed for understanding and simulating the process of a density current and the transport of spilled chemicals in a stratified reservoir. The model is capable of describing flow behavior and mixing mechanisms in different flow regimes (plunging flow, underflow, and interflow). It computes flow rate, velocity, flow thickness, mixing parameterized by entrainment and dilution, depths of plunging, separation and intrusion, and time of travel. The model was applied to the Shasta Reservoir in northern California during the July 1991 Sacramento River chemical spill. The simulations were used to assist in the emergency response, confirm remediation measures, and guide data collection. Spill data that were available after the emergency response are used to conduct a postaudit of the model results. Predicted flow parameters are presented and compared with observed interflow intrusion depth, travel time, and measured concentrations of spilled chemicals. In the reservoir, temperature difference between incoming river flow and ambient lake water played a dominant role during the processes of flow plunging, separation, and intrusion. With the integral approach, the gross flow behavior can be adequately described and information useful in the analysis of contaminated flow in a reservoir after a spill is provided.
An Analytical Solution of Macrodispersivity for Adsorbing Solute Transport in Unsaturated Soils
Authors: Yang Jinzhong , Zhang Renduo , Wu Jinquan
Published: February 2004

An analytical solution of macrodispersivity was derived for the adsorbing solute transport in physically and chemically heterogeneous unsaturated soils under the condition of gravity‐dominated flow. The unsaturated hydraulic conductivity and water content were treated as spatial random functions. The chemical adsorption was described by a linear equilibrium isotherm, and the adsorption coefficient was represented as a spatial random function with specified correlation structures. The closed‐form expression for macrodispersivity was expressed as a function of the hydraulic and adsorbing statistical properties of the unsaturated soil. For a short solute travel distance the macrodispersivity linearly increased with the increase of the solute plume travel distance. The macrodispersivity approached an asymptotic value when the center of the solute plume moved a long distance. The general solution quantified the effects of various factors, such as variabilities of the water content and unsaturated hydraulic conductivity, mean water content, and the retardation factor, on the macrodispersivity.
Effects of Thermal Vapor Diffusion on Seasonal Dynamics of Water in the Unsaturated Zone
Authors: Milly P. C. D.
Published: February 2004

The response of water in the unsaturated zone to seasonal changes of temperature (T) is determined analytically using the theory of nonisothermal water transport in porous media, and the solutions are tested against field observations of moisture potential and bomb fallout isotopic (36Cl and 3H) concentrations. Seasonally varying land surface temperatures and the resulting subsurface temperature gradients induce thermal vapor diffusion. The annual mean vertical temperature gradient is close to zero; however, the annual mean thermal vapor flux is downward, because the temperature‐dependent vapor diffusion coefficient is larger, on average, during downward diffusion (occurring at high T) than during upward diffusion (low T). The annual mean thermal vapor flux is shown to decay exponentially with depth; the depth (about 1 m) at which it decays to e−1 of its surface value is one half of the corresponding decay depth for the amplitude of seasonal temperature changes. This depth‐dependent annual mean flux is effectively a source of water, which must be balanced by a flux divergence associated with other transport processes. In a relatively humid environment the liquid fluxes greatly exceed the thermal vapor fluxes, so such a balance is readily achieved without measurable effect on the dynamics of water in the unsaturated zone. However, if the mean vertical water flux through the unsaturated zone is very small (<1 mm y−1), as it may be at many locations in a desert landscape, the thermal vapor flux must be balanced mostly by a matric‐potential‐induced upward flux of water. This return flux may include both vapor and liquid components. Below any near‐surface zone of weather‐related fluctuations of matric potential, maintenance of this upward flux requires an increase with depth in the annual mean matric potential; this theoretical prediction is supported by long‐term field measurements in the Chihuahuan Desert. The analysis also makes predictions, confirmed by the field observations, regarding the seasonal variations of matric potential at a given depth. The conceptual model of unsaturated zone water transport developed here implies the possibility of near‐surface trapping of any aqueous constituent introduced at the surface.
An Analytical Evaluation for Airflow to Inlet Wells in Vapor Extraction Systems Under Leaky Conditions
Authors: Ge Shemin , Liao Minru
Published: February 2004

Air inlet wells have been employed to enhance the fresh air flushing through the target‐contaminated soils in many soil vapor extraction systems. The analytical solution presented here evaluates the efficiency of the airflow to these inlet wells under the conditions of leaky surface boundary, anisotropic soils, and partially screened extraction. Using the analytical solution, the efficiency of air inlet wells is illustrated through sensitivity studies by varying system parameters (number of inlet wells, well spacing, well radius, and screen condition) and soil parameters (leaky factor and anisotropy).
Probability Distribution of Solute Travel Time for Convective Transport in Field‐Scale Soils Under Unsteady and Nonuniform Flows
Authors: Kavyas M. L. , Chen Z.‐Q. , Govindaraju R. S. , Rolston D. E. , Koos T. , Karakas A. , Or D. , Jones S. , Biggar J.
Published: February 2004

This study addresses the development of probability distributions of travel times for one‐dimensional (vertical) solute transport in soils. The field‐scale soils are considered heterogeneous, with stationary fluctuations of soil hydraulic properties in the horizontal direction but nonstationary fluctuations of these properties in the vertical direction due to layering of the soil, which induces nonstationary heterogeneity. Approximate ensemble probability distribution functions of conservative solute travel time for vertical convective solute transport were derived directly from the convective transport stochastic partial differential equation, under both deterministic and stochastic soil surface water flux (infiltration rate) and under unsteady and nonuniform soil water flows. General depth‐varying initial and time‐varying boundary conditions were used in these derivations. The magnitude of the approximation in the theoretical probability distribution functions of travel time is quantified mathematically. Utilizing the soil water content data from a University of California, Davis, field site, it is shown that the mathematical condition for this approximation is satisfied for this field. The spatial heterogeneity is represented through a nonstationary soil water content random field which covaries both in time and in space. Dispersion emerges naturally in the derived ensemble probability distribution functions of solute travel time, owing to the stochasticity of soil water content at field scale. Then the theoretical expression for mean solute concentration over a field is derived, by means of the theoretical solute travel time distribution, as a function of time and soil depth, under vertical transport with rectangular pulse solute loading for the upper boundary condition. Comparisons of theoretical probability density functions of solute travel time against their empirical counterparts, obtained from field experimental observations under steady but nonuniform soil water flow, show good agreement. Comparisons of theoretical mean solute concentrations, as they evolve with time and soil depth, against field experimental observations also show good agreement. However, further field experiments under unsteady flow conditions are required for the comprehensive validation of the developed theory.
The Radial Diffusion Method: 2. A semianalytical model for the determination of effective diffusion coefficients, porosity, and adsorption
Authors: Novakowski K. S. , van der Kamp G.
Published: February 2004

A model for interpreting diffusional transport in porous geological materials is developed. The model is based on a laboratory method described in a companion paper [van der Kamp et al., this issue] by which radial diffusion from or into a cylindrical reservoir in a core‐sized sample is measured. The model accounts for radial diffusion, mass balance in the reservoir, linear adsorption, decay or transformation, and periodic abstraction of samples. The model is derived using the Laplace transform method for both finite and semi‐infinite domains. For conditions where solute concentrations equilibrate (i.e., in finite diameter samples), a simple expression is derived that can be used to interpret the results for effective porosity and a retardation factor. It is demonstrated that the method can provide independent measures of the effective diffusion coefficient, adsorption, and effective porosity when the results are interpreted using the model. Several real and hypothetical diffusion experiments are presented to illustrate the use of the model.
Installing Piezometers in Deepwater Sediments
Authors: Lee David R. , Harvey F. Edwin
Published: February 2004

A new method has been developed for installing piezometers in the sediments of deep harbors and lakes, where it has been difficult to measure hydrogeological parameters and collect pore waters for geochemical analyses. Using an underwater hammer operated from the surface, piezometers have been installed as much as 12 m below the water‐sediment interface. The piezometer screen is held in place by barbs on the drive head and is connected to the water surface via flexible tubing. These piezometers have been installed from boats and from ice cover in water up to 30 m deep. However, installation in water more than 100 m deep should be possible.
Development of a Hydrochemical Model for Seasonally Snow‐Covered Alpine Watersheds: Application to Emerald Lake Watershed, Sierra Nevada, California
Authors: Wolford Ross A. , Bales Roger C. , Sorooshian Soroosh
Published: February 2004

We have developed and tested a model to assess the hydrologic and biogeochemical responses of seasonally snow covered alpine areas to changes in inputs of water, chemicals, and energy. This alpine hydrochemical model (AHM) is capable of incorporating a detailed understanding of watershed processes in order to simulate events critical to biota such as the ionic pulse associated with spring snowmelt, which is only a few days long and may involve only a portion of the catchment. The model computes integrated water and chemical balances for multiple terrestrial, stream, and lake subunits within a watershed, each of which can have a unique and variable snow‐covered area. Two years of data from the Emerald Lake watershed in the southern Sierra Nevada were used for fitting and testing by comparing observations with modeled daily output. To the extent possible, model parameters were set on the basis of independent physical or chemical measurements, leaving only a few fitted parameters. In its current application, model capabilities include (1) tracking of chemical inputs from precipitation, dry deposition, snowmelt, mineral weathering, flows external to the watershed, and user‐defined sources and sinks; (2) tracking surface and subsurface water and chemical movements through vegetation canopy, snowpack, soil litter, multiple soil layers, streamflow, and lakes; (3) calculating chemical speciation, including precipitates, exchange complexes, and acid‐neutralizing capacity; (4) simulating nitrogen reactions; (5) using a snowmelt optimization procedure to aid in matching observed watershed outflows; and (6) modeling riparian areas. Using one year of stream data for parameter estimation and a second for evaluation, the agreement between model and data was judged to be quite good. AHM is a flexible, precise algorithm for simulating watershed hydrochemistry and can readily be adapted to other alpine catchments using the Emerald results as a guide. Application of AHM to forested catchments should also be feasible.
Impacts of Physical and Chemical Heterogeneity on Cocontaminant Transport in a Sandy Porous Medium
Authors: Tompson Andrew F. B. , Schafer Annette L. , Smith Robert W.
Published: February 2004

A simplified numerical study of the transport of a uranyl‐citric acid mixture through a nonuniform and reactive sandy porous medium is presented. The study seeks to identify the more important impacts of medium heterogeneity, as embodied in spatially variable physical and chemical properties, on the migration and dilution rates of a model cocontaminant mixture, as well as on the overall partitioning among the aqueous and solid species formed from complexation and sorption reactions. Solid phase reactions are considered to occur on hydrous‐ferric oxide (goethite) coatings on the sand and are controlled by the abundance of the oxide as a function of the specific sand surface area and larger‐scale patterns of oxide deposition. The simulations involve calculation of fluid flow and chemical migration within highly resolved, two‐ and three‐dimensional regions with synthetic material properties that approximate observed conditions in a sandy coastal aquifer. Model simulations in this system indicate that (1) the impact of correlation between reactive surface area and hydraulic conductivity, although evident, seems much less significant than the overall abundance and distribution of the reactive area, such as the kind of banded goethite patterns observed in a coastal sand body; (2) strong multicomponent interactions clearly reinforce the need to treat the mixture as a coupled system, as opposed to a series of independently reactive compounds; (3) simplifications can be made in extremely dilute problems that allow retardation effects to become concentration independent; and (4) for nonlinear reaction problems, three‐dimensional models will be more appropriate than two‐dimensional models to the extent that dispersion in the added dimension accelerates chemical dilution rates.
Aircraft measurements of the stable carbon isotopic ratio of atmospheric methane over Siberia
Authors: Sugawara Satoshi , Nakazawa Takakiyo , Inoue Gen , Machida Toshinobu , Mukai Hitoshi , Vinnichenko Nikolay K. , Khattatov Vyachaslav U.
Published: February 2004

Air samples collected using aircraft during the Siberian Terrestrial Ecosystem‐Atmosphere‐Cryosphere Experiments (STEACE) in the summer of 1993 and 1994 were analyzed for the carbon isotopic ratio, δ13C, of atmospheric CH4 as well as for the CH4 concentration. The CH4 concentrations and δ13C values observed in the lower troposphere over wetlands in the West Siberian Lowland varied considerably, showing a clear negative correlation between the two components. From the relationships between measured values of the CH4 concentration and δ13C, values of δ13C of CH4 released from wetlands into the atmosphere were estimated to be −75 to −67‰. The results observed over oil wells and pipelines showed isotopic evidence for leakage of natural gas. Mean values of δ13C measured in the middle and upper troposphere over Siberia in the summer season were −47.9±0.3 and −47.8±0.2‰ for 1993 and 1994, respectively, which are quite similar to each other.
The 13C Suess Effect in the world surface oceans and its implications for oceanic uptake of CO2: Analysis of observations at Bermuda
Authors: Bacastow Robert B. , Keeling Charles D. , Lueker Timothy J. , Wahlen Martin , Mook Willem G.
Published: February 2004

Surface ocean water δ13C measurements near Bermuda are examined in an attempt to find the annual decrease caused by the addition of anthropogenic CO2 to the atmosphere. We refer to this trend as the surface ocean 13C Suess effect. Interannual variability, which may be related to the El Niño ‐ Southern Oscillation in the Atlantic Ocean, is apparent. We try to correct the data for this variability so as to better determine the trend. The trend has implications for the uptake of anthropogenic CO2 by the oceans. We employ a three‐dimensional model of ocean chemistry to relate the trend at Bermuda to the average ocean trend, then use the average ocean trend to estimate the vertical diffusivity K in a one‐dimensional ocean model, and finally use this model to calculate the oceanic uptake of CO2. Uncertainties associated with the estimation of the Suess effect at Bermuda and in the analysis procedure preclude a firm estimate of the oceanic uptake of CO2. Results are, in general, consistent with the low side of the Intergovernmental Panel on Climate Control estimation of 2.0 ± 0.8 GtC yr−1. With a longer record at Bermuda and δ13C observations at additional locations, we believe this approach will lead to a useful estimate of oceanic uptake.
Cross‐correlation analysis of the dynamics of methane emissions from a boreal peatland
Authors: Kettunen Ann , Kaitala Veijo , Alm Jukka , Silvola Jouko , Nykänen Hannu , Martikainen Pertti J.
Published: February 2004

The effects of temperature, water table, and precipitation on the methane fluxes from a boreal low‐sedge Sphagnum papillosum pine fen were analyzed with statistical cross correlations of daily data. The six measurement sites represented different vegetation surfaces of the mire (hummocks, lawns, and flarks) with increasing moisture. The dynamics were analyzed separately for the early summer (May‐July) and the late summer (August‐October) periods in addition to the whole summer (May‐October) period. Methane emissions increased with increasing peat temperature. During the late summer period, changes in peat temperatures at depths of 20 and 50 cm were reflected in methane emissions within 2 days. The persistently high water tables during the measurement period probably did not reveal the dynamics between water table fluctuations and methane emissions very clearly. Methane emission levels correlated negatively with depths of the water tables, that is, high methane emissions were associated with low water tables and vice versa. The suppression of methane emissions by filling the unsaturated gas space during precipitation and the increased release rate caused by a declining water table could explain the result. Methane emissions correlated positively with changes in water tables, that is, a rise in water table increased methane emissions during the early and whole summer periods. Precipitation increased emissions with a lag from zero to several days throughout the summer. Generally, the estimated responses of methane fluxes to precipitation and changes in water table indicated similar time lags. Methane flux from the flark surfaces seemed to respond rapidly to rainfall and changes in water table with a lag of zero or 1 day. In the lawn‐low hummock, the lawn and one hummock site, methane flux showed a slow response with several days lag. This study strongly indicates that temperature, water table, and precipitation affect methane emissions with complex interactions.
Global net carbon exchange and intra‐annual atmospheric CO2 concentrations predicted by an ecosystem process model and three‐dimensional atmospheric transport model
Authors: Hunt E. Raymond , Piper Stephen C. , Nemani Ramakrishna , Keeling Charles D. , Otto Ralf D. , Running Steven W.
Published: February 2004

A generalized terrestrial ecosystem process model, BIOME‐BGC (for BIOME BioGeoChemical Cycles), was used to simulate the global fluxes of CO2 resulting from photosynthesis, autotrophic respiration, and heterotrophic respiration. Daily meteorological data for the year 1987, gridded to 1° by 1°, were used to drive the model simulations. From the maximum value of the normalized difference vegetation index (NDVI) for 1987, the leaf area index for each grid cell was computed. Global NPP was estimated to be 52 Pg C, and global Rh was estimated to be 66 Pg C. Model predictions of the stable carbon isotopic ratio 13C/12C for C3 and C4 vegetation were in accord with values published in the literature, suggesting that our computations of total net photosynthesis, and thus NPP, are more reliable than Rh. For each grid cell, daily Rh was adjusted so that the annual total was equal to annual NPP, and the resulting net carbon fluxes were used as inputs to a three‐dimensional atmospheric transport model (TM2) using wind data from 1987. We compared the spatial and seasonal patterns of NPP with a diagnostic NDVI model, where NPP was derived from biweekly NDVI data and Rh was tuned to fit atmospheric CO2 observations from three northern stations. To an encouraging degree, predictions from the BIOME‐BGC model agreed in phase and amplitude with observed atmospheric CO2 concentrations for 20° to 55°N, the zone in which the most complete data on ecosystem processes and meteorological input data are available. However, in the tropics and high northern latitudes, disagreements between simulated and measured CO2 concentrations indicated areas where the model could be improved. We present here a methodology by which terrestrial ecosystem models can be tested globally, not by comparisons to homogeneous‐plot data, but by seasonal and spatial consistency with a diagnostic NDVI model and atmospheric CO2 observations.
Carbon isotope discrimination during photosynthesis and the isotope ratio of respired CO2in boreal forest ecosystems
Authors: Flanagan Lawrence B. , Brooks J. Renee , Varney Gregory T. , Berry Stephanie C. , Ehleringer James R.
Published: February 2004

Our objective was to measure the carbon isotope ratio of CO2released by respiration (δr) within forest canopies at different times during the growing season and to use this information to estimate forest ecosystem carbon isotope discrimination. We made measurements in the three major forest types (black spruce, jack pine, and aspen) at the southern and northern ends of the boreal forest in central Canada. This research was part of a larger study, the Boreal Ecosystem‐Atmosphere Study (BOREAS). The δrvalues, calculated from measurements of change in the concentration and carbon isotope ratio of atmospheric CO2in air samples collected at night, ranged from −28.1‰ to −25.9‰ with an average (± s.d.) of −26.‰ ± 0.5‰. There was good correlation between calculated δr values and measurements of (1) the carbon isotope ratio of CO2released directly from the soil and (2) the δ13C values of foliage collected from the dominant tree species at each site. Carbon isotope discrimination during photosynthetic gas exchange (ΔA) by each forest ecosystem was estimated as the difference between the carbon isotope ratio of atmospheric CO2at the top of the canopy (δa) and the isotopic composition of respired CO2: ΔA= δa− δr. All three of the major forest types had similar values of ΔA, with an average (± s.d.) of 19.1‰ ± 0.5‰. However, a seasonal change in forest discrimination was observed for aspen forests in both the northern and southern study areas, with an increase in ΔAoccurring between the middle and end of the growing season. In contrast, the evergreen conifer canopies exhibited relatively constant discrimination values throughout the active growing season.
Introduction to Special Section: Global Analysis, Interpretation and Modelling—Toward the Integration of Global Biogeochemical Systems
Authors: Sahagian Dork L. , Moore Berrien
Published: February 2004

The following special section consists of papers that were presented at the First Science Conference of the Global Analysis, Interpretation and Modelling Task Force (GAIM) that was held September 24–29, 1995, in Garmisch‐Partenkirchen, Germany. The goal of GAIM is to advance the study of the coupled dynamics of the Earth system using as tools both data and models. The goal of the science conference was to provide a venue for the dissemination of scientific findings that would be steps toward developing prognostic biogeochemical models which could, in time, be coupled to models of the physical climate system.

The science conference focused on papers in the areas of global data analysis and assessment, modeling of biogeochemical systems and their relationship to physical climate and hydrologic systems, and interpretation of current trends as indicated by global databases and model results for extrapolation with regard to future global change. Oral and poster session topics were grouped by time periods, including “Paleo” (<20 kyr), “Historical” (<2 kyr), “Contemporary” (<20 years), and “Future,” with an additional session concerned with global systems integration. The new and continued research directions stemming from these contributions should eventually lead to answers regarding the measurement, causes, and consequences of natural and anthropogenic global change.
Consistent fractionation of13C in nature and in the laboratory: Growth‐rate effects in some haptophyte algae
Authors: Bidigare Robert R. , Fluegge Arnim , Freeman Katherine H. , Hanson Kristi L. , Hayes John M. , Hollander David , Jasper John P. , King Linda L. , Laws Edward A. , Milder Jeffrey , Millero Frank J. , Pancost Richard , Popp Brian N. , Steinberg Paul A. , Wakeham Stuart G.
Published: February 2004

The carbon isotopic fractionation accompanying formation of biomass by alkenone‐producing algae in natural marine environments varies systematically with the concentration of dissolved phosphate. Specifically, if the fractionation is expressed by єP≈ δe− δp, where δeand δpare the δ13C values for dissolved CO2and for algal biomass (determined by isotopic analysis of C37alkadienones), respectively, and if Ceis the concentration of dissolved CO2, μmol kg−1, thenb= 38 + 160*[PO4], where [PO4] is the concentration of dissolved phosphate, μM, andb= (25 − єp)Ce. The correlation found betweenband [PO4] is due to effects linking nutrient levels to growth rates and cellular carbon budgets for alkenone‐containing algae, most likely by trace‐metal limitations on algal growth. The relationship reported here is characteristic of 39 samples (r2= 0.95) from the Santa Monica Basin (six different times during the annual cycle), the equatorial Pacific (boreal spring and fall cruises as well as during an iron‐enrichment experiment), and the Peru upwelling zone. Points representative of samples from the Sargasso Sea ([PO4] ≤ 0.1 μM) fall above theb=f[PO4] line. Analysis of correlations expected between μ (growth rate), єp, andCeshows that, for our entire data set, most variations in єpresult from variations in μ rather thanCe. Accordingly, before concentrations of dissolved CO2can be estimated from isotopic fractionations, some means of accounting for variations in growth rate must be found, perhaps by drawing on relationships between [PO4] and Cd/Ca ratios in shells of planktonic foraminifera.
A note on isotopic ratios and the global atmospheric methane budget
Authors: Tans Pieter P.
Published: February 2004

The temporal and spatial behavior of the isotopic ratio of atmospheric methane can be quite different from that of total methane. Changes in the isotopic ratio are the result of shifts between isotopically different methane sources, as well as changes in the total amount of methane into which methane from sources is being diluted. As a result, the timescale for isotopic changes is often considerably longer than for changes in total methane, and large‐scale spatial gradients of the isotopic ratio adjust very slowly to a new source configuration, in sharp contrast to total methane.
The use of differencing to eliminate environmental effects in the intercomparison of absolute gravity meters
Authors: Robertson Douglas , Sasagawa Glenn , Klopping Fred , Bilham Roger
Published: February 2004

Conventional intercomparisons between absolute gravity meters are limited in accuracy by the difficulty of removing environmental signals, including effects such as Earth tides, ocean loading, and barometric pressure loading. These error sources can be eliminated completely through the use of exactly simultaneous measurements. Differencing techniques have been developed and tested which are capable of determining both the differential meter bias and the gravity difference between two nearby piers to an accuracy that is limited only by the instrumental noise. Preliminary tests show that a comparison accuracy of 0.3 µgal may be obtained in less than 400 drops, independent of drop interval (1 µgal = 10 nanometer/sec²).
Crack stiffness from crack wave velocities
Authors: Nagano Koji , Niitsuma Hiroaki
Published: February 2004

We estimate crack stiffness by analyzing crack wave velocities. We measured crack waves which propagated along an artificial subsurface fracture in Higashi‐Hachimantai Hot Dry Rock model field, Iwate prefecture, Japan. The artificial subsurface fracture was created at a depth of about 370 m by hydraulic fracturing. We controlled contact conditions of the fracture surfaces by hydraulic pressurization. A time‐frequency representation technique, the pseudo‐Wigner distribution, is applied to determine the travel time of the crack wave. The crack wave shows dispersion; low frequency components below 100 Hz have lower velocities. We compare velocity data obtained in the field crack‐wave measurement to those calculated in a simple fracture model in which dimensionless crack stiffness reflects a degree of contact of the asperity on the fracture surfaces. Slope of the dispersion estimated from the field data agrees with that of the theoretical curves. We estimate the dimensionless crack stiffness is 0.001 to 0.005 from this comparison. This dimensionless crack stiffness, 0.001, means that crack stiffness on propagation path of the crack waves is 74 GPa/m.
Fe‐rich solar energetic particle events during solar minimum
Authors: Mazur J. E. , Mason G. M. , von Rosenvinge T. T.
Published: February 2004

During the first 10 months of WIND observations, we have detected several time periods with energetic particle abundances that are characteristic of impulsive flares: enrichments in the ³He isotope, and in heavy ions compared to the corona. Using the Supra‐Thermal through Energetic Particle sensor on WIND, we find that at ∼100 keV/nucleon these events typically arrive in sequences of multiple events when the spacecraft is magnetically connected to an active region at western solar longitudes, preceding the arrival of a high speed solar wind stream. During recurrent high speed solar wind streams with their associated flux enhancements Fe‐rich events are seldom seen: almost all of the events occur on days with solar wind speeds <450 km/sec. The impulsive events we observe arrive at a rate of ∼30 events/year at solar minimum for energies ≥120 keV/nucleon. This rate is comparable to upper limits placed on the observed rate of higher energy ³He‐rich events in the last solar minimum with measurements from ISEE‐3.