bluuebunny/crossref_metadata_2025 huggingface dataset:
Sulfide mineral oxidation and subsequent reactive transport of oxidation products in mine tailings impoundments: A numerical model
Authors: Wunderly M. D. , Blowes D. W. , Frind E. O. , Ptacek C. J.
Published: February 2004

A versatile numerical model that couples oxygen diffusion and sulfide‐mineral oxidation (PYROX) has been developed to simulate the oxidation of pyrite in the vadose zone of mine tailings. A shrinking‐core oxidation model and a finite element numerical scheme are used to simulate the transport of oxygen and oxidation of pyrite grains. The rate of pyrite oxidation is assumed to be limited by the transport of oxygen to the reaction site. The model determines the spatially variable bulk diffusion coefficient for oxygen on the basis of moisture content, porosity, and temperature, all of which are variable input parameters. The model PYROX has been coupled to an existing reactive transport model (MINTRAN), which uses a finite element scheme for transport of contaminants and MINTEQA2 to solve for the equilibrium geochemistry. The reactions described by MINTRAN are subject to the local equilibrium assumption. The resulting model, MINTOX, is capable of simulating tailings impoundments where the oxidation of pyrite or pyrrhotite is causing acidic drainage and where acid neutralization and attenuation of dissolved metals can be attributed to equilibrium reactions. Because MINTOX uses realistic boundary conditions and hydrogeological properties, the potential benefits of various remediation schemes, such as moisture‐retaining covers, can be quantitatively evaluated.
Catastrophic rainfall from an upslope thunderstorm in the central Appalachians: The Rapidan Storm of June 27, 1995
Authors: Smith James A. , Baeck Mary Lynn , Steiner Matthias , Miller Andrew J.
Published: February 2004

A storm system near the Blue Ridge Mountains of Virginia produced peak rainfall accumulations exceeding 600 mm in a 6‐hour period during the morning and early afternoon of June 27, 1995. The peak flood discharge of 3,000 m3 s−1 on the Rapidan River at a drainage area of 295 km2 places this event on the envelope curve of flood discharge for the United States east of the Mississippi River. Observations of radar reflectivity factor and Doppler velocity made by the Sterling, Virginia, WSR‐88D (Weather Surveillance Radar–1988 Doppler) are used for analyses of the storm. The temporal and spatial variability of rainfall are examined on a 1‐km grid scale and 6‐min timescale. Like many heavy rainfall events, storm motion played a key role in the production of heavy rainfall for the Rapidan storm. Storm motion and storm evolution for the Rapidan storm were closely linked to topographic features at the scale of the ridges which extend southward from the Blue Ridge and delineate the Rapidan basin. Key elements of the storm environment included strong boundary layer winds directed upslope toward the Blue Ridge, weak upper level winds, high precipitable water values, and a near‐saturated atmospheric column up to 6 km. An important element of storm structure was the low‐reflectivity centroid of the storm. This feature of the storm was related both to the exceptional rainfall rates of the storm and to the underestimation of storm total rainfall by the operational WS‐88D precipitation products. Components of the atmospheric and land surface water budgets are derived. The cumulative discharge from the Rapidan River was 0.87×108 m3 (296 mm over the 295‐km2 catchment). The storm total precipitation for the Rapidan basin was 1.01×108 m3 (344 mm over the catchment). The precipitation efficiency of the storm, that is, the ratio of storm total rainfall to atmospheric water vapor inflow, was approximately 90%.
A Computational Investigation of Some Effects of Temperature on Soil Moisture
Authors: Ferrand Lin A. , Sulayman Jamal A.
Published: February 2004

A pore‐scale lattice model for fluid displacement in porous media is used to investigate the effect of temperature on (1) soil drainage and (2) soil freezing. The model is calibrated for a given soil, that is, input parameters for the lattice model are determined, using a single soil drainage curve measured at a given temperature. The calibrated model may be used to simulate drainage at different temperatures (1) using adjusted fluid parameters or (2) freezing at subzero temperatures. Drainage simulations are used to generate capillary pressure‐saturation or soil water characteristic function curves, while freezing simulations give rise to temperature‐liquid water content or soil freezing characteristic function curves. These curves are compared to experimental data to test the validity of assumptions regarding the pore‐scale mechanisms and processes underlying these continuum‐scale phenomena.
An iterative geostatistical inverse method for steady flow in the vadose zone
Authors: Zhang Jinqi , Yeh T.‐C. Jim
Published: February 2004

An iterative geostatistical inverse approach is developed to estimate conditional effective unsaturated hydraulic conductivity parameters, soil‐water pressure head, and degree of saturation in heterogeneous vadose zones. This approach is similar to the classical cokriging technique, and it uses a linear estimator that depends on covariances and cross covariances of unsaturated hydraulic parameters, soil‐water pressure head, and degree of saturation. The linear estimator is, however, improved successively by solving the governing flow equation and by updating the residual covariance and cross‐covariance functions in an iterative manner. As a result, the nonlinear relationship between unsaturated hydraulic conductivity parameters and head is incorporated in the estimation and the estimated fields are approximate conditional means. The ability of the iterative approach is demonstrated through some numerical examples.
Fast computation of head and velocity covariances in three‐dimensional statistically axisymmetric heterogeneous porous media
Authors: Ezzedine Souheil
Published: February 2004

Modeling of flow and transport through axisymmetric porous media conditional to log conductivity and hydraulic head measurements is reviewed using stochastic Lagrangian framework. This approach necessitates expression of the covariances of the velocities, the hydraulic head, and the log conductivity, on one hand, and cross covariances between the velocities and hydraulic head and log conductivity. It is recognized that conditional modeling becomes very computationally burdensome for even conditioning on a small number of measurements, particularly for three‐dimensional modeling. The present manuscript provides an integration scheme [Anderson, 1979] that simplifies considerably the computations of the velocity covariances [Rubin and Dagan, 1992] and thereby opens the possibility for a wider range of applications of the stochastic analytical models for solute transport by groundwater.
The components of fractional transport rate
Authors: Wilcock Peter R.
Published: February 2004

Fractional transport rates are defined as the product of spatial grain entrainment, displacement length, and displacement frequency. Grain entrainment is defined relative to the population of grains on the bed surface at an initial time in order to account for partial transport, a condition in which only a portion of the exposed grains of a single size Di are mobilized over the duration of a transport event. A relation for grain displacement frequency is calculated from an assumed relation for displacement length and observed values of fractional transport rate, bed surface size distribution, and spatial entrainment in flume experiments. Fractional transport rates are independent of Di for fully mobilized fractions and decrease rapidly with Di for partially mobile fractions; the latter results from a decrease in both the displacement frequency and the mobilized proportion of each fraction. These two terms, which together determine the fractional entrainment rate, have different relations with flow strength and Di and should be independently included in a general fractional transport model. Grain‐size similarity in fractional transport rates does not hold for conditions of partial transport. Partial transport is a grain‐scale representation of incipient motion and can be directly related to the reference shear stress τri a surrogate for the critical shear stress for incipient motion based on transport rate rather than grain mobility. Fractional mobilization at different flows may be predicted from τri, and partial transport appears to be the general condition at the reference transport rate. Using τri as input, the transport component relations are used to predict the bed mobilization and fractional transport rates for a field case.
Frequency‐Dependent Dielectric Permittivity from 0 to 1 GHz: Time Domain Reflectometry Measurements Compared with Frequency Domain Network Analyzer Measurements
Authors: Heimovaara T. J. , de Winter E. J. G. , van Loon W. K. P. , Esveld D. C.
Published: February 2004

Measurements on a seven‐wire coaxial probe carried out with a cable tester in the time domain are compared with measurements carried out with a network analyzer in the frequency domain. Results are compared in the frequency domain and in the time domain. The frequency domain results of the time domain measurements are less smooth than the direct frequency domain measurements, but similar trends can be observed. The measurements carried out with the cable tester (Tektronix 1502B) clearly have a frequency content well above 3 GHz for measurements in air but with a very low signal‐to‐noise ratio for the higher frequencies. The useful frequency band for measurements carried out with a seven‐wire probe depends on the dielectric properties of the material being measured. The higher the complex dielectric permittivity, the lower the useful frequency band. Methods are presented for calibrating seven‐wire coaxial probes and measuring the frequency‐dependent dielectric properties of soil samples using a combination of frequency and time domain analyses. The approach does not depend on a choice of frequency bandwidth. A Debye relaxation curve is capable of describing the frequency domain dielectric permittivity of sandy soils containing a soil solution with an electrical conductivity of approximately 0.4 S m−1. Results indicate an effective bandwidth of at least 0–1 GHz for sandy soils.
Hydrologic response of a steep, unchanneled valley to natural and applied rainfall
Authors: Montgomery David R. , Dietrich William E. , Torres Raymond , Anderson Suzanne Prestrud , Heffner John T. , Loague Keith
Published: February 2004

Observations from natural rain storms and sprinkling experiments at a steep zero‐order catchment in the Oregon Coast Range demonstrate the importance of flow through near‐surface bedrock on runoff generation and pore pressure development in shallow colluvial soils. Sprinkling experiments, involving irrigation of the entire 860 m2 catchment at average intensities of 1.5 and 3.0 mm/h, permitted detailed observation of runoff and the development of subsurface saturation under controlled conditions. A weir installed to collect flow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi‐steady irrigation. Three key observations demonstrate that a significant proportion of storm runoff flows through near‐surface bedrock and illustrate the importance of shallow bedrock flow in pore pressure development in the overlying colluvial soil: (1) greater discharge recovery during both the experiments and natural rainfall at a weir installed approximately 15 m downslope of the weir at the base of the catchment, (2) spatially discontinuous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associated with flow from weathered rock into the overlying colluvium during high‐intensity rainfall. Data from natural storms also show that smaller storms produce no significant runoff or piezometric response and point to a critical intensity‐duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction between flow in colluvium and near‐surface bedrock in governing patterns of soil saturation, runoff production, and positive pore pressures.
Transient temperature solution for stream flow from a controlled temperature source
Authors: Polehn R. A. , Kinsel W. C.
Published: February 2004

An exact solution representing the idealized thermal response of a flowing stream is presented and analyzed. The flowing stream, issuing from a controlled temperature water source such as a large lake or reservoir, is modeled as a one‐dimensional region subject to sinusoidal solar heating and convective heat exchange with air, whose temperature also varies sinusoidally. The maximum downstream diurnal temperature fluctuation is found to occur at 12 hours of travel, at the average water flow speed, from the water source. For an isothermal water source the peak daily fluctuation at this location has a magnitude approximately 50% larger than the temperature fluctuation far downstream from the source.
Simulation, Bayes, and bootstrap in statistical hydrology
Authors: Fortin Vincent , Bernier Jacques , Bobée Bernard
Published: February 2004

Statistical simulation in hydrology is discussed from a Bayesian perspective. The inherent difficulties in both parametric simulation, based on a parent distribution, and classical nonparametric simulation, based on the bootstrap, are discussed. As an alternative to these procedures, a nonparametric Bayesian simulation methodology, Pólya resampling, is introduced. It consists of simulating from a nonparametric predictive distribution obtained from the analysis of a reference sample, and it is asymptotically equivalent to the bootstrap. The method is generalized to take into account a prior hypothesis on the parametric distribution of a variable. A hybrid simulation model is then obtained that includes parametric and nonparametric simulation as particular cases. An extensive application is presented in a related paper [Fortin et al., 1997], where Pólya resampling is used to compare statistical models for flood frequency analysis. In this paper an example is used to demonstrate how Pólya resampling can help assess the influence of a distribution hypothesis on simulation results.
Combined effects of dissolution kinetics, secondary mineral precipitation, and preferential flow on copper leaching from mining waste rock
Authors: Eriksson Nils , Destouni Georgia
Published: February 2004

A probabilistic Lagrangian approach to reactive subsurface transport is used to investigate possible processes that may explain reported discrepancies between modelled and observed copper concentrations at the Aitik waste rock heaps in northern Sweden and to quantify the implications of these processes for the long‐term dynamics of copper leaching. The presented Lagrangian transport system couples two different types of primary dissolution kinetics with flow heterogeneity and pH‐dependent equilibrium precipitation/dissolution of secondary copper‐bearing minerals. Of the investigated processes, which we chose to study based on previous geochemical studies of the site, only flow heterogeneity in the form of preferential flow paths can provide a possible explanation for the low drainage water concentrations of copper currently measured in the field. The investigated pH‐dependent precipitation‐dissolution of secondary copper bearing minerals provides an explanation for even lower copper concentrations observed in laboratory column experiments in fresh waste rock; these experiments correspond to the initial part of the weathering phase in the field. If both preferential flow and secondary copper precipitation/dissolution occur at the Aitik site, the assumed prevailing type of primary dissolution kinetics does not influence the modelled expected long‐term copper leaching nearly as much as it does under homogeneous flow conditions without formation of secondary minerals. The present sensitivity analysis does not provide a comprehensive prediction model for the Aitik site, since not all possible hydrological and chemical processes have been included in the investigation. This study rather exemplifies the application of the probabilistic Lagrangian approach as an investigation methodology for field‐scale geochemical problems by using and extending results of detailed geochemical modelling.
Observation and simulation of non‐Darcian flow transients in fractured rock
Authors: Kohl T. , Evans K. F. , Hopkirk R. J. , Jung R. , Rybach L.
Published: February 2004

Two independent multirate flow experiments were conducted in 1994 in the open hole depth interval of a well bore at the hot dry rock (HDR) test site Soultz. The steady state and transient downhole pressure records gave clear indications of non‐Darcian flow. A numerical model has been set up to evaluate these two measurements. An excellent fit of the transient pressure responses of the two flow tests could be achieved by assuming one model of simple geometry. The model predicts fluid transport along a conduit with substantial surface area in which fully turbulent flow is occurring. All the parameters required by our best‐fit simulation fall into a physically reasonable range. Sensitivity analysis demonstrates a non‐Darcian flow regime along highly conductive features. The existence of high capacity far‐field faults as postulated in our model confirms earlier characterizations of the Soultz test site.
Aquifer remediation by pumping: A model for stochastic‐advective transport with nonaqueous phase liquid dissolution
Authors: Berglund Sten
Published: February 2004

The coupled effects of advection and rate‐limited mass transfer to the aqueous phase from residual dense nonaqueous phase liquid (DNAPL) in the saturated zone are investigated within an analytical stochastic‐advective framework. The transport domain is assumed to consist of a source zone where mass transfer takes place and a down‐gradient plume zone where transport is by advection only. The mass flux in the aquifer is evaluated from an analytical solution to coupled advection‐reaction and nonlinear mass transfer equations. The results emphasize the importance of the parameters associated with the source zone: its length in the mean flow direction, the residual DNAPL saturation, and the mass transfer rate coefficient. The sensitivity to the hydraulic conductivity variance is low for significantly rate‐limited mass transfer. It is found that advection in the plume zone has small effect on mass fluxes and cleanup times for the considered lengths of the source and plume regions.
Electrical properties of partially saturated Topopah Spring Tuff: Water distribution as a function of saturation
Authors: Roberts Jeffery J. , Lin Wunan
Published: February 2004

We have measured the dielectric constant and electrical resistivity of Topopah Spring tuff as a function of saturation at 23° and 40°C. Dry and saturated resistivities can differ by more than 4 orders of magnitude. Frequency‐dependent impedance measurements indicate that the overall electrical response of the rock arises from at least three conduction mechanisms, which we tentatively identify as conduction through adsorbed water on solid surfaces, conduction through isolated pockets of water, and conduction through continuous (bulk) water. The relative contribution of each mechanism varies with saturation. A continuous (bulk) water film is indicated at saturations as low as 35%. Reasonable agreement was found between predicted permeability based on electrical measurements and microstructural parameters using the Kozeny‐Carmen relation and experimentally determined permeabilities. Similarly, good agreement was found between estimates of cation exchange capacity (CEC) based on electrical properties through the Waxman and Smits [1968] equation and published CEC values for tuff.
Stochastic modeling of pool‐to‐pool structure in small Nevada rangeland streams
Authors: Myers Thomas J. , Swanson Sherman
Published: February 2004

We developed, calibrated, and verified a compound Poisson process model of pool‐to‐pool spacing and size using an exponential distribution for spacing and gamma distributions for length and width on 12 rangeland streams in Nevada. Neither distribution parameter varied with simple stream morphologic or vegetation characteristics. We verified the model by comparing the first three moments and distributions of stream simulations with observed streams using two transect‐based sampling schemes. Very small errors stemmed from an inability to reproduce autocorrelation of width at short distances, pool cyclicity, and additional density at the tails. We conclude that the presented model is accurate for small, Nevada, rangeland streams and for pools located randomly on small streams with forced pool‐riffle or step‐pool sequences and regularly on larger, pool‐riffle systems. Simulated streams may be used for testing stream survey procedures and hypotheses regarding pool habitat, spacing, and length.
Effects of field drainage on soil parameters related to methane production and emission from rice paddies
Authors: Sigren L. K. , Lewis S. T. , Fisher F. M. , Sass R. L.
Published: February 2004

Field drainage has been reported to effect a reduction in the amount of methane emitted from rice paddies. In this study we monitored the redox state, soil acetate concentration, and methane emissions in two field sites and an outdoor potted rice experiment. The rate of methane production was determined by in vitro soil incubations. We found field drainage to be an effective method of mitigating methane emissions from rice paddies, with a 64% reduction in seasonal emissions reported after a single midseason drainage at the Richmond, Texas, site. The decreased methane emissions were in response to soil aeration, as indicated by changes in the Eh and ferrous ion concentration of the soil. The mechanism for this response appears to be a reduction in the rate of methane production, as indicated by significantly reduced soil acetate concentrations and decreased methane production rates in anaerobic soil incubations. Evidence of drainage‐induced methanogenic substrate depletion is presented. In addition, the duration and timing of field drainage required for effective mitigation are discussed. We found no evidence that field drainage adversely affected grain yield.
Timescales in atmospheric chemistry: CH3Br, the ocean, and ozone depletion potentials
Authors: Prather Michael J.
Published: February 2004

Methyl bromide (CH3Br) supplies about half of the chemically active bromine (Bry) in the stratosphere. Efforts to control Bry‐catalyzed ozone depletion by phasing out, for example, agricultural use of CH3Br may be thwarted by a lack of understanding of how the varied biogeochemical processes interact as a coupled system: in addition to the chemical industry, large natural sources come from the ocean; and losses occur in the atmosphere, ocean, and soils. A simplified one‐dimensional stratosphere‐troposphere‐ocean model for {CH3Br, Bry} that fits current understanding of sources and sinks is analyzed in terms of natural modes. Surface and ocean sources have effectively different steady state lifetimes (1.0 and 0.5 years, respectively), but the natural‐mode decay times of the system (1.8 years for CH3Br and 4.5 years for stratospheric Bry) do not depend on the location of sources. The cumulative ozone depletion resulting from a single atmospheric release of CH3Br integrates over the consequent slow rise and fall of Bry in the lower stratosphere. Thus, in spite of the 1‐year lifetime of CH3Br, only half of the anticipated ozone recovery occurs in the first 7 years.
A timescale for dissolved organic carbon production in equatorial Pacific surface waters
Authors: Archer David , Peltzer Edward T. , Kirchman David L.
Published: February 2004

Concentrations of dissolved organic carbon (DOC) in the equatorial Pacific Ocean are high (roughly 80 μM) in sunlit surface waters and low (∼40 μM) in subsurface waters. Concentrations in recently upwelled waters in the central equatorial region and in the coastal upwelling zone near Peru are intermediate between the surface water and deep water values (∼65 μM). The data appear to be consistent with the existence of a short‐lived “semilabile” DOC component, generated in surface waters, superimposed on a long‐lived “refractory” DOC concentration of ∼40 μM. We assume that the oligotrophic 80 μM end‐member is close to a biochemical steady state where production is balanced by consumption, and the lower concentrations in recently upwelled water represent a transient approach to steady state. The steady state assumption for oligotrophic surface waters can be used to couple the kinetic rate constants of DOC production and consumption, so that the only tunable parameter in the model is the e‐folding timescale for the approach to steady state. We tested and assessed the implications of this model by embedding surface ocean production and concentration‐dependent consumption of DOC into the three‐dimensional flow field of a high resolution equatorial circulation model. The distribution of DOC near the equator can be reproduced using a grow‐in timescale of 30–120 days, consistent with inferred values from field data and with direct field measurements. Implied gross DOC production rates are roughly half of measured particulate primary production rates [Barber et al., 1996]; this proportion applied globally would imply a global rate of DOC production of 50–100 Gton C yr−1, an order of magnitude higher than previous estimates [Siegenthaler and Sarmiento, 1993]. The DOC export production rate predicted by the model from the equatorial region (5°N to 5°S and 90°W to the dateline) is 0.2–0.4 Gton C yr−1, which is equivalent to 20–40% of the non‐El Niño 15NO3− uptake based new production estimate from the equatorial region [McCarthy et al., 1996; Murray et al., 1997].
Long‐term nitrogen dynamics of Coweeta Forested Watersheds in the southeastern United States of America
Authors: Swank Wayne T. , Vose James M.
Published: February 2004

We analyzed long‐term (23 years) data of inorganic N deposition and loss for an extensive network of mature mixed hardwood covered watersheds in the southern Appalachians of North Carolina to assess trends and dynamics of N in baseline ecosystems. We also assessed watershed N saturation in the context of altered N cycles and stream inorganic N responses associated with management practices (cutting prescriptions, species replacement, and prescribed burning) and with natural disturbances (drought and wet years, insect infestations, hurricane damage, and ozone events) on reference watersheds. Reference watersheds were characterized as highly conservative of inorganic N with deposition < 9.0 kg ha−1 yr−1 and stream water exports below 0.25 kg ha−1 yr−1. However, reference watersheds appeared to be in a transition phase between stage 0 and stage 1 of watershed N saturation as evidenced by significant time trend increases in annual flow‐weighted concentrations of NO3− in stream water and increases in the seasonal amplitude and duration of NO3 concentrations during 1972–1994. These stream water chemistry trends were partially attributed to significant increases in NO3− and NH4+ concentrations in bulk precipitation over the same period and/or reduced biological demand due to forest maturation. Levels and annual patterns of stream NO3− concentrations and intra‐annual seasonal patterns characteristic of latter phases of stages 1 and 2 of watershed N saturation were found for low‐elevation and high‐elevation clear‐cut watersheds, respectively, and were related to the dynamics of microbial transformations of N and vegetation uptake. Evidence for stage 3 of N saturation, where the watershed is a net source of N rather than an N sink, was found for the most distributed watershed at Coweeta (hardwood converted to grass, fertilized, limed, treated with herbicide, and subsequently characterized by successional vegetation). Compared to other intensive management practices, prescribed burning had little effect on stream water NO3− concentrations, and stream NO3− losses associated with natural disturbances are small and short‐lived.
Testing global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration
Authors: Stephens Britton B. , Keeling Ralph F. , Heimann Martin , Six Katharina D. , Murnane Richard , Caldeira Ken
Published: February 2004

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O2 and CO2 concentration. We combine these measurements to define a tracer, atmospheric potential oxygen (APO ≈ O2 + CO2), which is conservative with respect to terrestrial photosynthesis and respiration. We then compare observations of APO to the simulations of an atmospheric transport model which uses ocean‐model air‐sea fluxes and fossil fuel combustion estimates as lower boundary conditions. We present observations of the annual‐average concentrations of CO2, O2, and APO at 10 stations in a north‐south transect. The observations of APO show a significant interhemispheric gradient decreasing towards the north. We use air‐sea CO2, O2, and N2 fluxes from the Princeton ocean biogeochemistry model, the Hamburg model of the ocean carbon cycle, and the Lawrence Livermore ocean biogeochemistry model to drive the TM2 atmospheric transport model. The latitudinal variations in annual‐average APO predicted by the combined models are distinctly different from the observations. All three models significantly underestimate the interhemispheric difference in APO, suggesting that they underestimate the net southward transport of the sum of O2 and CO2 in the oceans. Uncertainties in the model‐observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the physical and biological components of the ocean models. Potential deficiencies in the physical components of the ocean models, which have previously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the APO observations. These deficiencies include the inadequate parameterization of subgrid‐scale isopycnal eddy mixing, a lack of subgrid‐scale vertical convection, too much Antarctic sea‐ice formation, and an overestimation of vertical diffusivities in the main thermocline.
Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems
Authors: Seitzinger Sybil P. , Kroeze Carolien
Published: February 2004

This study examines N2O emissions from aquatic environments globally, particularly as they are affected by anthropogenic activity. The global distribution of N2O production in rivers and estuaries was modeled as a function of nitrification and denitrification rates, which were related to external nitrogen (N) inputs. N loading rates were estimated as a function of environmental parameters in the watersheds using two existing models that we adapted for global databases. Model estimated export of dissolved inorganic nitrogen (DIN) by world rivers to estuaries in 1990 is 20.8 Tg N yr−1; approximately 75% is estimated to be anthropogenic. DIN export to the Atlantic and Indian Oceans is similar (5.4 Tg N yr−1 and 4.6 Tg N yr−1, respectively); inputs to the Pacific are approximately 50% greater. China and southeast Asia account for over 50% of DIN export by world rivers. Globally, anthropogenic DIN export is predominately attributed to fertilizer N, followed by sewage and atmospheric deposition. About 8% of the total N inputs to the terrestrial environment can be accounted for as DIN export by rivers. Worldwide N2O emissions from rivers (55%), estuaries (11%), and continental shelves (33%) are calculated to be 1.9 Tg N yr−1. For rivers and estuaries, approximately 90% of N2O emissions are in the northern hemisphere in line with the regional distribution of DIN export by rivers. China and India account for about 50% of N2O emissions from rivers and estuaries. About 1% of the N input from fertilizers, atmospheric deposition, and sewage to watersheds is lost as N2O in rivers and estuaries. Globally, rivers and estuaries could account for approximately 20% of the current global anthropogenic N2O emissions and are similar in magnitude to a number of previously identified sources including direct emissions of N2O from soils induced by anthropogenic N inputs.
Anthropogenic CO2 in the Atlantic Ocean
Authors: Gruber Nicolas
Published: February 2004

The anthropogenic CO2 in the Atlantic Ocean is separated from the large natural variability of dissolved inorganic carbon using the method developed by Gruber et al. [1996]. Surface concentrations of anthropogenic CO2 are found to be highest in the tropical to subtropical regions and to decrease toward the high latitudes. They are very close to what is expected from thermodynamic considerations assuming that the surface ocean followed the atmospheric CO2 perturbation. Highest specific inventories (inventory per square meter) of anthropogenic CO2 occur in the subtropical convergence zones. Large differences exist between the North and South Atlantic high latitudes: In the North Atlantic, anthropogenic CO2 has already invaded deeply into the interior; north of 50°N it has even reached the bottom. By contrast, waters south of 50°S contain relatively little anthropogenic CO2, and hence specific inventories are very low. An anthropogenic CO2 inventory of about 22 ± 5 Gt C is estimated for the Atlantic north of the equator for 1982, and 18 ± 4 Gt C is estimated for the Atlantic south of the equator for 1989. The Princeton ocean biogeochemistry model predicts anthropogenic CO2 inventories of 20.0 Gt C (North Atlantic, 1982) and 17.7 Gt C (South Atlantic, 1989) for the same regions in good agreement with the observed inventories. Important differences exist on a more regional scale, associated with known deficiencies of the model.
A model of regional vegetation dynamics and its application to the study of Northeast China Transect (NECT) responses to global change
Authors: Gao Qiong , Yu Mei
Published: February 2004

We developed a dynamic regional vegetation model to address problems of responses of regional vegetation to elevated ambient CO2 and climatic change. The model takes into consideration both local ecosystem processes within a patch or grid cell, such as plant growth and death, and mass and energy flow, such as plant migration, across adjacent grid cells. The model is able to couple vegetation structure dynamics and primary production processes. The normalized differential vegetation index from meteorological satellite AVHRR was used to parameterize the model. Plant migration rates were derived based on effective seedling distribution around parent plants. The model was applied to Northeast China Transect at a spatial resolution of 10 min latitude by 10 min longitude per grid cell and a temporal resolution of 1 month. The results indicated that with doubled CO2 concentration, a 20% increase in precipitation and a 4°C increase in temperature, the model predicted that net primary productivity (NPP) of Larix forests, conifer‐broadleaf mixed forests, Aneurolepidium chinense steppes, Stipa grandis steppes, and wetland and salty meadows would decrease by 15% to 20%. However, NPP of deciduous broadleaf forests, woodland and shrubs, Stipa baicalensis meadow steppes, and desert grasslands would increase by 20% to 115%, as predicted by the model for the same climatic scenario. The average NPP of natural vegetation over the whole transect would decrease slightly, largely because of the compensation between the positive effects of increased CO2 and precipitation and the negative effect of increased evapotranspiration induced by increased temperature.
Solar cycle and the Pacific ‘centers of action'
Authors: Christoforou P. , Hameed S.
Published: February 2004

The sun's integrated irradiance varies by about 0.1% during its 11 year cycle, a change that is apparently too small to directly alter the mean climate of the earth. Yet, large correlations between the sunspot cycle and local climate variables have been noted. In this study the variations of the semi‐permanent pressure systems the Aleutian Low and the Hawaiian High are investigated. It is found that extremes in solar variability as measured by the mean annual sunspot numbers, correlate highly with the locations of these centers of action. The Aleutian Low migrates eastward during minimum solar activity and the Hawaiian High migrates southward. Effects on the intensity of the Centers of Action were also significant and were in general agreement with recent theoretical results. Because shifts in the Centers of Action change storm trajectories, large anomalies in regional climatic conditions can be generated in the extreme phases of the solar cycle.
The impact of permafrost thawing on the carbon dynamics of tundra
Authors: Waelbroeck Claire , Monfray P. , Oechel W. C. , Hastings S. , Vourlitis G.
Published: February 2004

There is debate on the potential release of the tundra's immense carbon stocks into the atmosphere in response to global warming. We present here results obtained with a model of CO2 exchanges, coupled to a model of the soil thermal and hydrological regime in the tundra. We show that, because of the partial thawing of permafrost and subsequent increase in nutrient availability, the ecosystem's response to warming may be a long‐lasting increase in C accumulation, following a temporary increase in CO2 emissions. Our study also provides a consistent picture of CO2 exchanges in tundra ecosystems, reconciling the short‐term experimental response to warming, recent field measurements, and Holocene C accumulation estimates.
Back arcs basins and P‐wave crustal velocity in the Ionian and Aegean regions
Authors: Alessandrini Bruno , Beranzoli Laura , Drakatos George , Falcone Claudia , Karantonis George , Mele Francesco M. , Stavrakakis George
Published: February 2004

Tomographic images of the lower crust and uppermost mantle beneath the Central Mediterranean have been obtained from arrival times of regional earthquakes. We find a low velocity belt at about 25 km beneath the Hellenic Arc. A low velocity anomaly at a depth of 40 km extends under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian Sea close to the Calabrian and Aegean arcs and under the Aegean Sea. The results suggest a close relation between P wave crustal velocity and the subduction systems of the studied area.
Pick‐up ions and associated wave energy transport at comet P/Halley: A case study
Authors: Söding Anne , Glassmeier Karl‐Heinz , Johnstone Alan , Neubauer Fritz M.
Published: February 2004

During the flyby of the spacecraft Giotto at comet p/Halley Poynting vectors and Elsässer variables have been determined to study wave propagation directions. The observed wave properties are compared with the theoretically predicted RH−, LH+ and LH− wave modes. Between 10:36 and 19:11 SCET on March 13, 1986 the predicted dominating energy propagation direction mostly corresponds with the observed one. Only between 12:11 and 13:44 SCET there is no agreement. In this region the excitation of waves is dominated by pick‐up ions which are not locally implanted ions in the solar wind but have been picked‐up outside the plasma regime studied, and whose free energy is not already used up.
IMF induced changes to the nightside magnetotail: A comparison between WIND/Geotail/IMP 8 observations and modeling
Authors: Winglee R. M. , Skoug R. M. , Elsen R. K. , Wilber M. , Lin R. P. , Lepping R. L. , Mukai T. , Kokubun S. , Reme H. , Sanderson T.
Published: February 2004

A 3‐D global fluid simulation is used to investigate the changing magnetic field topology of the magnetotail as observed by Geotail and IMP 8. The event studied is of particular interest as the solar wind density and speed as observed by WIND were approximately constant so that the influence of the interplanetary magnetic field (IMF) can be isolated. Loading of the tail fields during southward IMF is seen at high latitudes with IMP 8 moving from the sheath into the magnetosphere while at low latitudes Geotail moves from the plasma sheet into the lobes. The reverse is true for northward turnings. The tail cross‐section is shown to be elliptical during southward IMF with an eccentricity of about 0.2 and that this eccentricity is slowly eroded over a period of about an hour during northward IMF.
Isopycnal RAFOS floats as roving hydrographers in the North Atlantic Current Region
Authors: Carr Mary‐Elena , Kearns Edward J. , Rossby H. Thomas
Published: February 2004

The properties measured by isopycnal floats can be compiled to provide hydrographic information about the sampled surfaces. The height and temperature of two specific volume anomaly surfaces reported by 84 RAFOS floats deployed in the North Atlantic Current (NAC' region are compared here with the climatological hydrographic database. The mean pathway of the NAC and other prominent features of the region are similar in float‐derived and climatological surfaces. The most striking difference between floats and climatology, an extended trough in the NAC path at 44°N in the float‐derived surface, reflects the sampling time scales. The baroclinic transport of the NAC estimated from RAFOS floats using the observed climatological relationship between the potential energy anomaly and the height of the specific volume anomaly surfaces was 42 Sv at 42°N and 24 Sv at 47°N, in reasonable agreement with climatological estimates.
Implications of the October 1995 Colima‐Jalisco Mexico earthquakes on the Rivera—North America Euler vector
Authors: Bandy W. , Kostoglodov V. , Singh S. K. , Pardo M. , Pacheco J. , Urrutia‐Fucugauchi J.
Published: February 2004

Rivera‐North America Euler vectors fall into three groups based on the predicted motion of the Rivera plate relative to the North America plate along the Middle America Trench north of 18.5°N. Namely, those that predict (1) a slow, more easterly directed motion, (2) a slow, more northerly directed motion, and (3) a faster, more northerly directed motion. The focal mechanism solution, rupture area and seismic moment of the great (Mw=8.0) Colima‐Jalisco, Mexico, earthquake of October 9, 1995 and its foreshocks and aftershocks provide additional constraints on the Rivera‐North America Euler vector. The slip direction (N30°E) of the main event clearly favors those Euler poles which predict a more northerly direction of motion. This direction lies within 5° of those predicted by these poles, whereas, it differs by between 9° and 18° from those poles which predict a more easterly motion. The amount of slip which occurred during the main event suggests a minimum convergence rate of between 4.3 and 6.5 cm/yr. This favors Euler vectors which predict higher convergence rates, however, a firm conclusion cannot be made due to the uncertainty in the recurrence time estimation. A new Rivera‐North America Euler pole, calculated incorporating the information provided by the 1995 events, lies at 21.87°N, 109.58°W. It predicts a N30.3°E directed motion of the Rivera plate relative to North America at the location of the main event; a difference of only 0.3° from the slip direction of the main event.
Global Pc5 geomagnetic pulsations of March 24, 1991, as observed along the American Sector
Authors: Trivedi N. B. , Arora B. R. , Padilha A. L. , Da Costa J. M. , Dutra S. L. G. , Chamalaun F. H. , Rigoti A.
Published: February 2004

Analysis of the globally coherent Pc5 geomagnetic pulsation event of March 24, 1991, from a chain of stations extending from the auroral oval to the equatorial region along the American sector, has shown unequivocal evidence on the enhancement of pulsation amplitude in the narrow equatorial band centered at the dip equator. The fine spatial structure of this equatorial enhancement, documented observationally for the first time, emphasizes the importance of an ionospheric component associated with enhanced Cowling conductivity. The presence of an ionospheric component is also indicated at mid‐latitudes by the sharp increase in the amplitude of pulsations, more dominantly in the Y than in the X component, on the sunward side of the dawn terminator. The sunrise effect also causes a reversal of the sense of rotation of the wave polarization across the dawn terminator. Propagation of the magnetospheric inducted polar electric field to low and equatorial latitudes through the ionosphere is invoked to account for the equatorial enhancement and the sunrise effect seen in these Pc5 pulsations.
Verifying the precision of a new generation absolute gravimeter FG5—Comparison with superconducting gravimeters and detection of oceanic loading tide
Authors: Okubo Shuhei , Yoshida Shigeo , Sato Tadahiro , Tamura Yoshiaki , Imanishi Yuichi
Published: February 2004

We carried out a several day experiment at two sites to verify the precision of the FG5 absolute gravimeter through collocation with superconducting gravimeters. We found 2 to 4 microgal diurnal/semidiurnal signals in the time series of absolute gravity measurements. These signals agreed well with theoretical ocean tide both in amplitude and in phase, demonstrating that the FG5's precision is better than 2 microgal. Furthermore, we found that the discrepancy between the FG5 measurements and the SG measurements is 1.3 to 1.5 microgal. These results verify that the FG5 is precise to 1 to 2 microgal.
Transport in fractured porous solids
Authors: Mattisson Charlotte , Knackstedt Mark A. , Senden Tim J.
Published: February 2004

Laboratory measurements are made of the permeability and the resistivity of sintered porous media with disordered fractures over a wide range of matrix porosity. We discuss the preparation and characterisation of the samples. Approximating the topology of a rough fracture by a single discrete fracture can introduce large errors in the prediction of the permeability. We test the validity of empirical expressions relating permeability, resistivity and porosity for fractured samples. Resistivity correlations with porosity are independent of the presence of fractures. In contrast, permeability correlations show a strong dependence. Attempts to decouple the permeability of the fractured sample as a parallel sum of matrix and fracture permeability leads to large errors. The results indicate that transport in a medium with two distinct families of pathways cannot be described by a single‐valued transport coefficient.
Study of plasmaspheric evolution using geosynchronous observations and global modeling
Authors: Weiss L. A. , Lambour R. L. , Elphic R. C. , Thomsen M. F.
Published: February 2004

We examine the dynamics of the outer plasmasphere during a 3‐day interval in February, 1992 by comparing observations from three widely‐spaced geosynchronous satellites with the predictions of the Magnetospheric Specification and Forecast Model (MSFM'. For this study the MSFM was modified to include a cold, plasmaspheric population subject to the effects of corotation, convection, and ionospheric refilling. The model does a very good job of reproducing the observations; in particular, it clearly shows the formation and westward transport of duskside plasmaspheric plumes, accounting for the progressively earlier local time observations of cold ions following periods of enhanced convection. The MSFM's ability to model dynamically the evolution of the outer plasmasphere under varying solar wind conditions improves upon previous plasmaspheric models and has aided our understanding of spatially and temporally separated observations of plasmaspheric ions at geosynchronous orbit.
Using eigenfunctions of the two‐point correlation function to study convection with multiple phase transformations
Authors: King Scott D. , Balachandar S. , Ita Joel J.
Published: February 2004

We describe and illustrate a new approach for extracting and understanding the pattern of flow in complex, time‐dependent convection. In this approach we calculate the eigenfunctions of the two‐point correlation function and show that the two‐point correlation can be efficiently characterized by the dominant few eigenmodes. We apply this methodology to extract structural information from convection models with two phase transformations.