bluuebunny/crossref_metadata_2025 huggingface dataset:
The significance of topology for modeling the surface hydrology of fluvial landscapes
Authors: Dawes Warrick R. , Short David
Published: February 2004

The realistic representation of terrain in hydrological models of fluvial landscapes poses problems particularly for representing convergence and divergence of lateral flows. In this work we consider the topology of contour representation of surfaces. The networks of ridge and drainage lines are linked at critical points: peaks, saddles, confluences, and simple ridge junctions. The properties of these points are given, and from these, three topologic rules are derived. These are used to develop an automatic procedure for generating a catchment boundary and a modified flow net. Use of the flow net increases speed by 2 orders of magnitude over earlier contour‐based topographic analyses, and the use of topology increases speed, accuracy, and sophistication over a recent flow net technique. Topology is used to evaluate simplifications of natural surfaces and, in particular, shows the unsuitability of the common ridge‐to‐stream rectangular flow strip. Finally, implications for the structure of distributed catchment models and for designing and interpreting field experiments are considered.
A forced‐gradient experiment on solute transport in the Borden aquifer: 1. Experimental methods and moment analyses of results
Authors: Mackay Douglas M. , Bianchi‐Mosquera Gino , Kopania Andrew A. , Kianjah Homayoon , Thorbjarnarson Kathryn W.
Published: February 2004

A forced‐gradient test of solute transport was conducted adjacent to the site of a previous natural gradient test to allow comparison of results obtained from the two types of field tests. In the forced‐gradient test, one inorganic tracer and four organic solutes (carbon tetrachloride, trichloroethylene, tetrachloroethylene, and hexachloroethane) were injected for 48 hours. The migration of the dissolved chemicals toward the extraction well was monitored by multilevel samplers and a partially penetrating monitoring well. Moment analyses of the solute breakthrough curves at the multilevel points provide model‐independent insight into the variation of hydraulic conductivity and organic solute retardation in the aquifer. The estimated retardation factors fall within the range estimated in the previous natural gradient test. Sorption distribution coefficients inferred from the transport data indicate spatial variability of sorption for all solutes, with mean values and ranges similar to those observed in laboratory studies of core or bulk samples. Some evidence was obtained for spatially variable in situ transformation of hexachloroethane.
A forced‐gradient experiment on solute transport in the Borden aquifer: 3. Nonequilibrium transport of the sorbing organic compounds
Authors: Thorbjarnarson Kathryn W. , Mackay Douglas M.
Published: February 2004

Transport of four organic solutes was monitored in 15 multilevel points and a partially penetrating well during a forced‐gradient test in the Borden aquifer. Simulations assuming equilibrium sorption were unable to match the observed breakthrough curves, particularly for the more highly sorbing solutes. The data were better fit by first‐order physical nonequilibrium simulations including mass transfer between mobile and immobile water regions. Assuming 86% of total porosity is mobile, as estimated previously, curve fits yielded estimates of Kd (sorption distribution coefficient), ƒ (fraction of sorption sites within mobile water regions), and α (first‐order mass transfer coefficient), Kd and ƒ estimates compare well with those from previous laboratory studies and simulations of the natural gradient test previously conducted nearby. The estimated a values compare reasonably well with those derived from laboratory tests but are considerably higher than the value utilized in simulations of the natural gradient test.
An experimental study of complete dissolution of a nonaqueous phase liquid in saturated porous media
Authors: Imhoff Paul T. , Jaffé Peter R. , Pinder George F.
Published: February 2004

The attenuation of gamma radiation was utilized to measure changing residual trichloroethylene (TCE) saturation in an otherwise water‐saturated porous medium as clean water was flushed through the medium. A front over which dissolution actively occurred was observed. Once developed, this front varied in length from ≈11 mm to ≈21 mm, lengthening as it moved through the porous medium. Gamma attenuation measurements and analyses of effluent water samples indicate that there was minimal if any transport of TCE as colloidal droplets. Even as trapped TCE ganglia decreased in size due to dissolution, there is no evidence that they became mobile and advected downgradient. An extraction of the porous medium at the completion of one experiment indicated that less than 0.002% of the original TCE mass remained, suggesting that minimal amounts of separate phase TCE remained trapped within the medium after flushing with 290 pore volumes. Mass transfer rate coefficients were computed and are shown to be a function of Darcy flux, TCE volumetric content, and distance into the region of residual TCE.
Hydraulic conductivity estimation for soils with heterogeneous pore structure
Authors: Durner Wolfgang
Published: February 2004

The hydraulic conductivity function, which is required to solve the Richards equation, is difficult to measure. Therefore prediction methods are frequently used where the shape of the conductivity function is estimated from the more easily measured water retention characteristic. Errors in conductivity estimations can arise either from an invalidity of the prediction model for a given soil, or from an incorrect description of the retention data. This second error source is particularly important for soils with heterogeneous pore systems that cannot be adequately described by the usually used retention functions. To describe the retention characteristics of such soils, a flexible θ(ψ) function was formed by superimposing unimodal retention curves of the van Genuchten (1980) type. By combining this retention model with the conductivity prediction model of Mualem (1976), conductivity estimations for soils with heterogeneous pore systems are obtained. Estimated conductivities by this model and the classical van Genuchten‐Mualem method can differ by orders of magnitude. Thus reported disagreements between measured and estimated conductivities may in some cases be due to an inadequate description of the retention data rather than due to a failure of the prediction model.
Effect of drop energy and soil and water chemistry on infiltration and erosion
Authors: Agassi M. , Bloem D. , Ben‐Hur M.
Published: February 2004

The effect of water salinity, soil sodicity, and raindrop energy on final infiltration rate, wash erosion, and soil particle splash of two kaolinitic soils (ustochrepts and paleudalf) was studied. Final infiltration rate decreased with the increase in soil sodicity and drop energy and increased with the increase of water salinity. Wash erosion and splash increased with the increase in soil sodicity and drop energy and decreased with the increase in water salinity. Infiltration was very sensitive to relatively low drop energy. The major decrease in final infiltration rate was already achieved at drop energies lower than 10 J mm−1 m−2. Wash erosion increased exponentially with the increase in drop energy compared to a linear increase of the splash. Soil splash was approximately 2–3 times higher than wash erosion for any given drop energy. The threshold energy for soil splash initiation was 4–6 J mm−1 m−2, while for wash erosion (low exchangeable sodium percentage (ESP)) it was 2–4 J mm−1 m−2, and for the high ESP it was beyond the lowest energy we used (2.1 J mm−1 m−2).
Efficiency of air inlet wells in vapor extraction systems
Authors: Ross Benjamin , Lu Ning
Published: February 2004

Vapor extraction is often used to remove volatile organic chemicals from unsaturated soils. Air inlet wells, which are left open to atmospheric pressure, are commonly installed around the extraction well to accelerate the removal of volatile contaminants. We employ the method of images in radial coordinates and the principle of superposition to develop an analytical solution for calculating the amount of air drawn in by inlet wells evenly spaced around an extraction well. Field tests of proposed vapor extraction systems can be compared with this solution to help evaluate whether gas flow will permeate the entire soil or will be concentrated in localized channels.
Design of multiple contaminant remediation: Sensitivity to rate‐limited mass transfer
Authors: Haggerty Roy , Gorelick Steven M.
Published: February 2004

Optimal extraction well locations and pumping rates are compared, based on a limited number of deterministic simulations, for multiple contaminant plumes exhibiting rate‐limited, or nonequilibrium, mass transfer from a mobile to an immobile phase. This rate‐limited mass transfer results in what we define as rate‐limited, or nonequilibrium, transport. Two‐dimensional, nonequilibrium solute transport simulation and optimization are used to study the simultaneous remediation of carbon tetrachloride, 1,2‐dichloroethane and tetrahydrofuran that have been chromatographically separated during transport by groundwater. Minimum total pumping necessary for cleanup is compared for five different well geometries over remediation periods of 3 and 15 years. The sensitivity of these designs to first‐order mass transfer rates is examined for equilibrium and for three levels of nonequilibrium transport. The key results of this particular study are that (1) classic contaminant capture at downgradient wells is a poor design for contaminant cleanup; (2) for effective design of multiple plume remediation, the transport characteristics of each contaminant must be considered; (3) if solute transport is limited by mass transfer from an immobile to a mobile phase, there is a minimum remediation time which cannot be reduced by adjustment of pumping rates or changing well locations; (4) the optimal locations and pumping rates of extraction wells are less sensitive to nonequilibrium transport for long‐term remediation than for short‐term remediation; and (5) competition between pumping wells may significantly affect the feasibility and efficiency of specific multiple‐well cleanup designs. In addition, a method and a measurement are presented for evaluating the importance to remediation of rate‐limited mass transfer.
An evaluation of atmospheric corrections of radiometric surface temperatures for a semiarid rangeland watershed
Authors: Perry Eileen M. , Moran M. Susan
Published: February 2004

As part of an interdisciplinary effort to describe the hydrologic and surface energy balance of a semiarid rangeland watershed, remote and ground radiometric surface temperature measurements were collected at the Walnut Gulch experimental watershed near Tombstone, Arizona. To correct for atmospheric effects on the remotely sensed data, radiosondes were launched on and off the watershed, and the LOWTRAN7 radiative transfer model was used to evaluate these observed as well as two standard atmospheric profiles. Comparison of the results for five atmospheric profiles, for each of three days, showed no correlation between radiosonde location or launch time and resulting temperature corrections. Comparison of corrected temperatures using five different atmospheric profiles on each of two days with the corresponding ground‐based radiometric temperatures demonstrated significant errors (greater than 2.0°C) as the atmospheric profiles generally yielded underestimates of ground radiometric temperatures.
Bidirectional properties and utilizations of high‐resolution spectra from a semiarid watershed
Authors: Qi J. , Huete A. R. , Cabot F. , Chehbouni A.
Published: February 2004

A ground‐ and air‐based high spectral resolution data set was collected during the summer Monsoon '90 experiment at the Walnut Gulch experimental watershed in southeastern Arizona for the purpose of (1) characterizing solar and view angle interactions on dry and wet season canopy spectra, and (2) exploring the use of multidirectional measurements to infer vegetation properties for semiarid watershed studies. Bidirectional reflectance factors were measured up to 40° off nadir with a spectroradiometer over a semidesert grassland site. High‐spectral resolution aircraft data were collected over grass and desert shrub sites in order to investigate scaling effects. In this study, solar and view angle effects and interactions on canopy spectra varied with spectral wavelength as well as between dry and wet seasons. The solar zenith angle modified the view angle behavior of the bidirectional reflectance factors. In general, view angle influences and spectral signature contrasts were greatest at the larger solar zenith angles and similarly, Sun angle influences were more apparent at the larger view zenith angle. The scale dependency of the data was relatively minor. The bidirectional measurements were sufficiently characterized by a physically based bidirectional reflectance distribution function (BRDF) model. The parameters retrieved from the inversion of the BRDF model corresponded with observed vegetation variations.
Suitability of simplified overland flow equations
Authors: Richardson J. R. , Julien P. Y.
Published: February 2004

A dimensionless formulation of the acceleration terms of the Saint‐Venant equations is presented for one‐dimensional overland flows under either laminar or turbulent conditions. For stationary storms over a plane surface of uniform roughness, dimensionless analytical expressions are derived during the rising limb for the local acceleration al*, and during equilibrium for the convective acceleration ac* and the pressure gradient ap* ((13), (14), and (15), respectively). In terms of the order of magnitude, the three acceleration terms are inversely proportional to the kinematic flow number K. At equilibrium, the pressure gradient ap* is also inversely proportional to the square of the Froude number Fr. The relative magnitude of the acceleration terms for supercritical overland flow (al* > ac* > ap*) differs from subcritical overland flow (ap* > al* > ac*), which in all cases contrasts with open‐channel flows (ap* > ac* > al*). The kinematic wave approximation is therefore only suitable when both K and Fr are large. Improvements using the diffusive wave approximation are only possible for subcritical overland flow. Both the diffusive wave and the quasi‐steady dynamic wave approximations are not suitable for supercritical overland flow. The analysis of moving storms corroborates these findings in that the local acceleration exceeds the convective acceleration. These effects are particularly pronounced during the rising limb of overland flow hydrographs for downstream moving rainstorms.
Building flood control rule curves for multipurpose multireservoir systems using controllability conditions
Authors: Marien J. L. , Damázio J. M. , Costa F. S.
Published: February 2004

Building flood control rule curves (FCRC) for multipurpose multireservoir systems involves treating simultaneously the stochasticity of future inflows and the multivariate aspect of the problem. The study demonstrates how theoretical findings concerning flood control multireservoir systems regulation obtained by Marien (1984) (the so‐called controllability conditions) can be used to build generalized FCRC in multipurpose multireservoir systems. These FCRC are generalized FCRC in the sense that they consist of a time varying constraint set on the needed empty space provisions in the different reservoirs of the system. The applicability of the proposed technique is enhanced by the derivation of new controllability conditions for systems with more than one flood control section and mandatory releases. By means of several examples, it is shown how the proposed methodology is superior to the approach which consists of lumping all the reservoirs of the system into a single imaginary reservoir. A practical example with an eight‐reservoir hydropower system in Brazil demonstrates the applicability of the methodology.
Measuring distributions of hydraulic conductivity and specific storativity by the double flowmeter test
Authors: Kabala Z. J.
Published: February 2004

The flowmeter test, when modified, can be applied long before the well response reaches a quasi‐steady state. In many practical applications the traditional flowmeter test provides only an order of magnitude estimate of the hydraulic conductivity distribution. Conducting the double flowmeter test, i.e., running the flowmeter test on the same borehole twice (at appropriate times), improves the accuracy of the estimated downhole distribution of the hydraulic conductivity and, in addition, provides the downhole distribution of the specific storativity. Formulae for the hydraulic conductivity and specific storativity of each layer are derived for a case of a layered aquifer. The accuracy of the estimated parameter distributions is evaluated via a first‐order sensitivity analysis. A formula is also found for the equivalent specific storativity of a layered aquifer. In addition, it is shown that accounting for well bore storage in the flowmeter test leads to an inconsistent model, and hence the flowmeter test should be conducted only after well bore storage effects disappear.
Forest floor frost dynamics during spring snowmelt in a boreal forested basin
Authors: Stein Jean , Proulx Serge , Lévesque Denis
Published: February 2004

A previous study (Prévost et al., 1990) has shown that a lowering in the soil infiltrability index improves hydrograph snowmelt runoff peaks simulation of the Lac Laflamme basin. The hypothesis that explains this phenomenon is as follows: when the snowpack becomes discontinuous or thin during the spring, cold air temperatures during the night (<−10°C) freeze the water at the soil surface. To test this hypothesis, four plots were instrumented to follow the evolution of water table fluctuations, snow cover area, soil and air temperatures, and liquid and total water contents at different levels in the soil. Transect lines were also established to monitor snow depth, percentage of soil free of snow, and presence of basal ice and different types of soil frost (porous or concrete). Our results show that the minimum nightly air temperature of −5°C during snowmelt did not produce soil ice. It was not possible to validate or invalidate the hypothesis. The results of the survey at the basin scale show that concrete frost is prevalent throughout the basin, is associated with forest floors dominated by sphagnum or hypnobryale mosses, and originates from winter meteorological events.
A model for real‐time quantitative rainfall forecasting using remote sensing: 1. Formulation
Authors: French Mark N. , Krajewski Witold F.
Published: February 2004

A physically based rainfall forecasting model for real‐time hydrologic applications is developed with emphasis on utilization of remote sensing observations. Temporal and spatial scales of interest are lead times of the order of hours and areas of the order of 10 km2. The dynamic model is derived from conservation of mass in a cloud column as defined by the continuity equations for air, liquid water, water vapor, and cloud water. Conservation of momentum is modeled using a semi‐Lagrangian frame of reference. The model state is vertically integrated liquid water content in a column of the atmosphere. Additionally, laws of thermodynamics, adiabatic air parcel theory, and cloud microphysics are applied to derive a basic parameterization of the governing equations of model dynamics. The parameterization is in terms of hydrometeorologic observables including radar reflectivity, satellite‐infrared brightness temperature, and ground‐level air temperature, dew point temperature, and pressure. Implementation and application is described by French et al. (this issue) and involves incorporation of uncertainty analysis and a two‐dimensional spatial domain, where the dynamics of the continuous space‐time rainfall process are discretized onto a rectangular grid.
A quasi‐dynamic wetness index for characterizing the spatial distribution of zones of surface saturation and soil water content
Authors: Barling Rowan D. , Moore Ian D. , Grayson Rodger B.
Published: February 2004

A quasi‐dynamic wetness index that accounts for variable drainage times since a prior rainfall event is derived from simple subsurface flow theory. The method is tested through a series of field observations and numerical experiments using a spatially distributed, dynamic hydrologic model. The quasi‐dynamic wetness index is shown to be a useful extension of previously developed static indices for predicting the location of zones of soil saturation and the distribution of soil water (i.e., the soil water content overlying a shallow impermeable or semiimpermeable layer). The new index is not constrained by the steady state assumption that forms the basis of existing indices.
Impact of gypsum application on the methane emission from a wetland rice field
Authors: Denier van der Gon H. A. C. , Neue H. U.
Published: February 2004

Methane emission from Philippine rice paddies was monitored with a closed chamber technique during the 1991 and 1992 wet season. The methane emission from plots amended with 6.66 tons.ha−1 gypsum was reduced by 55–70% compared to non‐amended plots. Although CH4 emission from fields with a high input of fresh organic matter was strongly enhanced, the experiments showed that the relative reduction in CH4 emission upon gypsum application was independent of organic matter addition. The reduced CH4 emission upon gypsum application was most likely due to inhibition of methanogenesis by sulfate‐reducing bacteria. Observed SO42− concentrations in the soil solution of gypsum‐amended plots were well above minimum concentrations reported in the literature for successful competition of sulfate‐reducing bacteria with methanogens. The data provide a base for reducing the estimates of CH4 emissions from rice grown on high‐sulfate containing soils or gypsum‐amended soils.
Air‐sea carbon dioxide exchange in the North Pacific Subtropical Gyre: Implications for the Global Carbon Budget
Authors: Winn Christopher D. , Mackenzie Fred T. , Carrillo Christopher J. , Sabine Christopher L. , Karl David M.
Published: February 2004

The role of the ocean as a sink for anthropogenic carbon dioxide is a subject of intensive investigation and debate. Interest in this process is driven by the need to predict the rate of future increase of atmospheric carbon dioxide and subsequent global climatic change. Although estimates of the magnitude of the oceanic sink for carbon dioxide appear to be converging on a value of ∼2 (Gt) C yr−1 for the 1980s, a detailed understanding of the temporal and spatial variability in the rate of exchange of carbon dioxide between the ocean and the atmosphere is not available. For example, recent modeling work and direct measurements of air‐sea carbon dioxide flux produce very different estimates of the air‐sea flux in the northern hemisphere. As a consequence, it has been suggested that a large unidentified oceanic carbon dioxide sink may exist in the North Pacific. As a part of our time series observations in the North Pacific Subtropical Gyre, we have measured dissolved inorganic carbon and titration alkalinity over a four‐year period. These measurements constitute the most extensive set of observations of carbon system parameters in the surface waters of the central Pacific Ocean. Our results show that the ocean in the vicinity of the time series site is a sink for atmospheric carbon dioxide. On the basis of these observations, we present a mechanism by which the North Pacific Subtropical Gyre can be a potential sink for ∼0.2 Gt C yr−1 of atmospheric carbon dioxide. Although our observations indicate that the North Pacific Subtropical Gyre is a sink for atmospheric carbon dioxide, the magnitude of this oceanic sink is relatively small. Our data and interpretations are therefore consistent with the argument for a relatively large sink during the 1980s in northern hemisphere terrestrial biomass. Another possibility is that the net release of carbon dioxide to the atmosphere owing to land use activities in tropical regions has been overestimated.
Modeling carbon biogeochemistry in agricultural soils
Authors: Li Changsheng , Frolking Steve , Harriss Robert
Published: February 2004

An existing model of C and N dynamics in soils was supplemented with a plant growth submodel and cropping practice routines (fertilization, irrigation, tillage, crop rotation, and manure amendments) to study the biogeochemistry of soil carbon in arable lands. The new model was validated against field results for short‐term (1–9 years) decomposition experiments, the seasonal pattern of soil CO2 respiration, and long‐term (100 years) soil carbon storage dynamics. A series of sensitivity runs investigated the impact of varying agricultural practices on soil organic carbon (SOC) sequestration. The tests were simulated for corn (maize) plots over a range of soil and climate conditions typical of the United States. The largest carbon sequestration occurred with manure additions; the results were very sensitive to soil texture (more clay led to greater sequestration). Increased N fertilization generally enhanced carbon sequestration, but the results were sensitive to soil texture, initial soil carbon content, and annual precipitation. Reduced tillage also generally (but not always) increased SOC content, though the results were very sensitive to soil texture, initial SOC content, and annual precipitation. A series of long‐term simulations investigated the SOC equilibrium for various agricultural practices, soil and climate conditions, and crop rotations. Equilibrium SOC content increased with decreasing temperatures, increasing clay content, enhanced N fertilization, manure amendments, and crops with higher residue yield. Time to equilibrium appears to be one hundred to several hundred years. In all cases, equilibration time was longer for increasing SOC content than for decreasing SOC content. Efforts to enhance carbon sequestration in agricultural soils would do well to focus on those specific areas and agricultural practices with the greatest potential for increasing soil carbon content.
Solubilities of Al, Pb, Cu, and Zn in rain sampled in the marine environment over the North Atlantic Ocean and Mediterranean Sea
Authors: Lim B. , Jickells T. D. , Colin J. L. , Losno R.
Published: February 2004

Chemical processes controlling the dissolved and particulate phase distribution of crustal (Al) and noncrustal metals (Pb, Cu, and Zn) appear to differ in marine precipitation sampled over the North Atlantic Ocean and Mediterranean Sea. Dissolved Al appears to be in equilibrium with a trivalent Al salt at rainwater pH < 5.1, whereas dissolved Pb, Cu, and Zn concentrations are probably controlled by adsorption/desorption processes in which rainwater particulates provide surface‐active sorption sites. In both processes, rainwater pH is a critical parameter. Results suggest that in marine precipitation with pH < 5, > 80% of the total Pb, Cu, and Zn concentrations are delivered to the surface oceans in the dissolved form. For a corresponding pH range, Al solubility varies from <5% to >60%. Over the wider observed pH range (of 3.5 to 6.9), the solubilities of Pb, Cu, Zn, and Al are highly variable. The use of mean trace metal solubilities for the assessment of dissolved atmospheric trace metal wet deposition fluxes, and their effects on surface ocean biogeochemistry should be constrained by taking into account rainwater pH in future estimates in global models.
Field and laboratory studies of methane oxidation in an anoxic marine sediment: Evidence for a methanogen‐sulfate reducer consortium
Authors: Hoehler Tori M. , Alperin Marc J. , Albert Daniel B. , Martens Christopher S.
Published: February 2004

Field and laboratory studies of anoxic sediments from Cape Lookout Bight, North Carolina, suggest that anaerobic methane oxidation is mediated by a consortium of methanogenic and sulfate‐reducing bacteria. A seasonal survey of methane oxidation and CO2 reduction rates indicates that methane production was confined to sulfate‐depleted sediments at all times of year, while methane oxidation occurred in two modes. In the summer, methane oxidation was confined to sulfate‐depleted sediments and occurred at rates lower than those of CO2 reduction. In the winter, net methane oxidation occurred in an interval at the base of the sulfate‐containing zone. Sediment incubation experiments suggest both methanogens and sulfate reducers were responsible for the observed methane oxidation. In one incubation experiment both modes of oxidation were partially inhibited by 2‐bromoethanesulfonic acid (a specific inhibitor of methanogens). This evidence, along with the apparent confinement of methane oxidation to sulfate‐depleted sediments in the summer, indicates that methanogenic bacteria are involved in methane oxidation. In a second incubation experiment, net methane oxidation was induced by adding sulfate to homogenized methanogenic sediments, suggesting that sulfate reducers also play a role in the process. We hypothesize that methanogens oxidize methane and produce hydrogen via a reversal of CO2 reduction. The hydrogen is efficiently removed and maintained at low concentrations by sulfate reducers. Pore water H2 concentrations in the sediment incubation experiments (while net methane oxidation was occurring) were low enough that methanogenic bacteria could derive sufficient energy for growth from the oxidation of methane. The methanogen‐sulfate reducer consortium is consistent not only with the results of this study, but may also be a feasible mechanism for previously documented anaerobic methane oxidation in both freshwater and marine environments.
Thermal skin effect and the air‐sea flux of carbon dioxide: A seasonal high‐resolution estimate
Authors: Van Scoy Kim A. , Morris Kevin P. , Robertson Jane E. , Watson Andrew J.
Published: February 2004

Understanding the role the oceans play in sequestering anthropogenic CO2 is crucial to understanding global climate change. Correct parameterization of air‐sea flux of CO2 is an important challenge to modelers. Recently it has been demonstrated that the thin thermal layer at the surface of the ocean can lead to an underestimate of CO2 solubility (Robertson and Watson, 1992). We re‐evaluate the effect of the cool thermal skin and present a high‐resolution seasonal estimate of its effect on the air‐sea flux of CO2. We compare air‐sea flux estimates derived using both a mean wind field and a more realistic Rayleigh distribution of the wind field. Using the mean monthly wind stress and a linear relationship between wind speed and the gas exchange coefficient of CO2 (Tans et al., 1990), we estimate that excluding the southern ocean, the surface skin correction increases the air‐sea flux of carbon by 0.48 Gt yr‐1. This is 25% lower than the correction suggested by Robertson and Watson (1992) and the difference is attributed to the better temporal and spatial resolution of the present data set. When a more realistic representation of the temporally varying winds is used, the corrected carbon flux decreases to 0.36 Gt yr−1. Conservatively, adding a 10% contribution from the southern ocean, we estimate a mean global increase in CO2 flux due to the skin effect of 0.39 Gt C yr−1. This is 40% lower than the previous estimate of Robertson and Watson (1992). Finally, adopting the gas transfer parameterization of Liss and Merlivat (1984), we estimate a CO2 flux anomaly of only 0.17 Gt C yr−1 which is approximately 50% lower than the analogous estimate using the Tans et al. (1990) formulation and a full 75% lower than the estimate of Robertson and Watson (1992). These results suggest that both a proper representation of the wind speed/flux correlation and a realistic distribution of the wind field is essential in making large‐scale flux estimates. We also examine the seasonal variation of the thermal skin effect. The largest negative temperature gradients (‐0.75°C) are found during the northern hemisphere winter in the regions of the Kuroshio and the Gulf Stream Currents, whereas the central North Pacific has a small positive temperature gradient during the summer months.
Spawning and Shedding Helical Magnetic Fields in the Solar Atmosphere
Authors: Rust D. M.
Published: February 2004

Measurements of the helical fields in interplanetary magnetic clouds have corroborated recent evidence that the magnetic fields in solar filaments in the northern and southern hemispheres of the Sun are preferentially left‐handed and right‐handed heliform, respectively. In 13 cases out of 16, the chirality of magnetic clouds passing Earth about 4 days after a filament eruption or an eruptive solar flare corresponded to the predominant chirality in the hemisphere originating the solar event. Active regions and sunspots also indicate that helicity is segregated by hemisphere. Torque due to an equatorial jet stream beneath the photosphere may twist the fields.
First assessment of GPS‐based reduced dynamic orbit determination on TOPEX/Poseidon
Authors: Yunck T. P. , Bertiger W. I. , Wu S. C. , Bar‐Sever Y. E. , Christensen E. J. , Haines B. J. , Lichten S. M. , Muellerschoen R. J. , Vigue Y. , Willis P.
Published: February 2004

The reduced dynamic GPS tracking technique has been applied for the first time as part of the GPS experiment on TOPEX/Poseidon. This technique employs local geometric position corrections to reduce orbit errors caused by the mismodeling of satellite forces. Results for a 29‐day interval in early 1993 are evaluated through postfit residuals and formal errors, comparison with GPS and laser/DORIS dynamic solutions, comparisons on 6‐hr overlaps of adjacent 30‐hr data arcs, altimetry closure and crossover analysis. Reduced dynamic orbits yield slightly better crossover agreement than other techniques and appear to be accurate in altitude to about 3 cm RMS.
Auroral electrojet activity during isolated substorms at different local times: A statistical study
Authors: Kamide Y. , Kroehl E. W.
Published: February 2004

Magnetic variations processed from the AE observatories and IMP‐8 measurements of the solar wind and interplanetary magnetic field (IMF) are used to study the response of the auroral electrojets at different local times to solar wind and IMF parameters. On the basis of an interpolation technique, magnetic variations at four different local times (0000, 0600, 1200, and 1800 MLT) are estimated, which constitutes our data base for this statistical study. Forty two events in 1979 met our “isolated substorm” criteria. It is found that electrojet variations at different local times are significantly different The substorm expansion current system at midnight appears to be initiated when IMF becomes less southward, and the current systems at dawn and dusk are already active when the substorm expansion current begins and remain active even after the midnight substorm current recovers to its pre‐substorm level. During the initial stage of the recovery phase (in terms of the midnight current), the westward electrojet at 0600 MLT continues to intensify while the 1800 MLT eastward electrojet slowly decays.
Site velocities before and after the Loma Prieta and Gulf of Alaska earthquakes determined from VLBI
Authors: Argus Donald F. , Lyzenga Gregory A.
Published: February 2004

We use geodetic data from VLBI to determine the pre‐ and postseismic velocities of two sites. We then place limits on variations in interseismic strain buildup. The 1987 and 1988 Gulf of Alaska earthquakes (each Ms=7.6) broke the Pacific plate interior. During the earthquakes the Cape Yakataga site moved 78 mm toward southwest. During the 1989 Loma Prieta earthquake (Ms=7.1) the Fort Ord site moved 48 mm toward north. Baselines (a) from Fairbanks to Cape Yakataga and (b) from Mojave to Fort Ord change at nearly the same rate before and after the earthquakes. Postseismic transients, which we determine from differences between post‐ and preseismic rates, are minor: at Cape Yakataga the transient is 3±4 mm in a postseismic interval of 23 months, and at Fort Ord the transient is 6±5 mm in 21 months. The slip beneath the Loma Prieta rupture needed to generate the Fort Ord transient is 0.22±0.19 m, one‐tenth the coseismic slip (2 m). We analyze elastic lithosphere‐viscous asthenosphere models to determine that the characteristic time describing exponential decay in deep fault slip is longer than 6 years. The VLBI measurements are consistent with uniform interseismic strain buildup. They disagree with fast postseismic rates caused by an asthenosphere with very low viscosity.
Atmospheric ammonia emissions from agricultural waste combustion
Authors: Lee David S. , Atkins D. H. F.
Published: February 2004

Measurements of ammonia and ammonium aerosol were made during straw and stubble burning experiments in the field. Factors were determined for the calculation of emissions of ammonia and ammonium ion, from this source, in the United Kingdom between 1981 and 1992. Emissions of NHx from straw burning were calculated to be equivalent to approximately 20 ktonnes N yr−1 in 1981 and have declined to 3.3 ktonnes N yr−1 in 1991 as a result of changes in agricultural practices in response to impending U.K. legislation. The fraction of total plant nitrogen released as NHx was estimated to be between approximately 40 and 80%. Emissions of ammonia from straw and stubble burning over a 6—8 week period over which this typically occurs were calculated to be 27% of the total U.K. emissions over the equivalent period in 1981 and 7% in 1991. We have identified straw and stubble burning as another source of ammonia currently not accounted for in European and North American emission inventories; these focus almost exclusively on emissions from animal sources.
Probability of one or more M ≥7 earthquakes in southern California in 30 years
Authors: Savage J. C.
Published: February 2004

Eight earthquakes of magnitude greater than or equal to seven have occurred in southern California in the past 200 years. If one assumes that such events are the product of a Poisson process, the probability of one or more earthquakes of magnitude seven or larger in southern California within any 30 year interval is 67% ± 23% (95% confidence interval). Because five of the eight M ≥ 7 earthquakes in southern California in the last 200 years occurred off of the San Andreas fault system, the probability of one or more M ≥ 7 earthquakes in southern California but not on the San Andreas fault system occurring within 30 years is 52% ± 27% (95% confidence interval).
Insoluble particles in polar ice: Identification and measurement of the insoluble background aerosol
Authors: Ram Michael , Gayley Robert I.
Published: February 2004

We have completed a new series of measurements of the size distribution of insoluble particles in the radius range 0.050–1.31 μm retrieved from Greenland ice cores. Taken together with our previous measurements, our work demonstrates that an insoluble background aerosol with a very distinctive size distribution exists. We also demonstrate that, in looking for evidence of the background aerosol, size distribution is a better indicator than particle concentration.
Generalized scaling of permeabilities: Validation and effect of support scale
Authors: Neuman Shlomo P.
Published: February 2004

The permeabilities and dispersivities of geologic media are known to vary with the scale of observation. Particularly well documented is the consistent increase in apparent longitudinal dispersivity with the mean travel distance of a tracer. This has been previously interpreted by the author to imply that the permeabilities of many geologic media scale, on the average, according to the power‐law semivariogram γ (s)=c √ s where c is a constant and s is distance. Tracer test data support this conclusion indirectly at least over scales from 10 cm to 3,500 m. The present paper cites evidence for such behavior over scales from 10 cm to 45 km based directly on permeability and transmissivity data. The paper then investigates theoretically the implications of such power‐law behavior on the equivalent permeability of a block of rock having a characteristic length (support scale) L. It predicts that the equivalent Isotropie permeability should generally decrease with L in one‐dimensional media, increase with L in three‐dimensional media, and show no systematic variation with L in two‐dimensional media. This prediction appears to be consistent with observations.
Record low ozone at the South Pole in the spring of 1993
Authors: Hofmann D. J. , Oltmans S. J. , Lathrop J. A. , Harris J. M. , Vömel H.
Published: February 2004

On October 12, 1993, a balloon‐borne ozone detector recorded a total ozone value of 91±5 Dobson Units (DU) at the U.S. Amundsen‐Scott Station at the south pole. This is the lowest value of total ozone ever recorded anywhere, 13% below the previous low of 105 DU at the south pole in October of 1992 [Hofmann and Oltmans, 1993]. A region with a thickness of 5 km, from 14 to 19 km, was totally devoid of ozone as compared to only about half this thickness for the ozone void in 1992. Sub‐100 DU total ozone values were observed on several soundings during 1993 whereas the 105 DU value was observed on only one occasion in 1992. The vertical profile of ozone indicates that the main reason for the record low ozone values in 1993 was an approximately 1 km upward extension of the ozone hole caused by unusual ozone loss in the 18–23 km region. Temperatures in this region were unusually low in September and October. Thus, the extension of the ozone hole may have been the result of the prolonged presence of polar stratospheric clouds at 18–23 km combined with the continued presence of sulfate aerosol from the Pinatubo eruption and, finally, increased chlorine levels. This scenario resulted in elevated ozone loss in a region where the ozone loss process is normally not saturated.
Highlights from a seismic broadband array on Stromboli Volcano
Authors: Neuberg Jügen , Luckett Richard , Ripepe Maurizio , Braun Thomas
Published: February 2004

An array of nine three‐component broadband seismometers was deployed in two different configurations on Stromboli volcano. The analysis of the seismic wavefield related to volcanic explosions revealed some observations which offer a completely new insight into the internal dynamics of a volcano. These new observations are restricted to the low‐frequency range below 1 Hz and underline, therefore, the superiority of broadband recordings over conventional short‐period observations. Surprisingly simple wavelets indicate an initially contracting source mechanism. Gas‐jets, that could not be seen in a short‐period seismic record at all, generate a clear dilatational wavelet in a broadband recording suggesting the same contracting source mechanism. The analysis of particle motion and seismic array techniques permits a location of the seismic source. We find low‐frequency signals of 3s and 6s period that are not related to eruptions and do not share a common source with the eruption‐related events. A video recording of visible volcanic activity at the crater region allows one to correlate precisely eruptive features with seismic signals.
Observations of the vertical distribution of stratospheric OClO
Authors: Pommereau Jean‐Pierre , Piquard Jacques
Published: February 2004

A light and easily operated balloon‐borne UV‐visible spectrometer was designed for investigating ozone chemistry in the Arctic winter. The optical sonde was flown 10 times during EASOE in northern Scandinavia in winter 1991–92 to measure O3 and NO2, the results of which are presented in a companion paper. On two occasions, 13 and 16 January 1992, during the coldest period of the campaign, when PSCs were visible from the ground, OClO mixing ratios of 16 ± 2 and 19 ± 3 ppt (1 s.d.) respectively were detected around 20 km at sunset. These are the first OClO profiles to be reported. They yield integrated columns of 5.8 and 8.9 1012 mol./cm² above 16 km and 17 km respectively. These values are consistent with ground‐based and aircraft total column measurements in the Arctic.
Paleomagnetic evidence for no tectonic rotation of the central Italy Tyrrhenian Margin since Upper Pliocene
Authors: Sagnotti Leonardo , Mattei Massimo , Faccenna Claudio , Funiciello Renato
Published: February 2004

We present paleomagnetic results from 141 samples collected at 12 sites from Plio‐Pleistocene clayey units in the extensional Central Italy Tyrrhenian margin. All samples were subjected to stepwise alternating field and/or thermal demagnetization. Positive fold and reversal tests indicate that the results refer to a primary component of magnetization. Paleomagnetic mean direction (Declination = 358.0°; Inclination = 55.2°; α95 = 4.1°) reflects the expected geocentric axial dipole field for the locality, thus indicating that this sector of the Tyrrhenian margin did not undergo any tectonic rotation during the past 4 million years.
Comparative hypsometric analysis of Earth and Venus
Authors: Rosenblatt P. , Pinet P. C. , Thouvenot E.
Published: February 2004

The comparison between Earth and Venus global hypsometric distributions is reinvestigated on the basis of the high‐resolution data provided by the recent Magellan global topographic coverage of Venus' surface, and the detailed ETOPO‐5 topographic database for the Earth's surface. The study of the cumulative hypsometric curve for both planets reveals that there exists a domain of elevation for which the relationship between the elevation and the square root of cumulative area percentage is linear. This domain covers about 80% of the mapped venusian surface and nearly all the terrestrial oceanic surface (55% of the earth's surface) and corresponds to an elevation range of about 2000 m for both planets. Proposing the upper threshold elevation of this domain as a common geophysical reference level, it is shown that the terrestrial oceanic peak and the venusian peak coincide very closely. In addition, the terrestrial and venusian hypsometric distributions for elevations lower than this reference level are quite similar, in their amplitude, modal distribution, and pronounced symmetry. The correspondence of the venusian peak with the terrestrial oceanic peak is a striking feature, but it does not necessarily imply the occurrence of plate tectonics on Venus. The present observation is consistent with the existence of a similar thermal isostasy acting at the planetary scale. It places an upper bound at 90±10 km for the modal thermal lithosphcric thickness of Venus, if the Venus mantle thermal conditions are earth‐like. This estimate is reduced if hotter conditions occur.
Diffusion and solubility of HCl in ice: preliminary results
Authors: Dominé Florent , Thibert Emmanuel , Van Landeghem Frank , Silvente Eric , Wagnon Patrick
Published: February 2004

The diffusion and solubility of HCl in ice have been measured between −5 and −15°C by exposing large single crystals of ice to low vapor pressures of HCl for several days. The solubility is found to be (1.0±0.15) × 10−4 mol/1 at −15°C under a 5.6×10−4 Pa HCl partial pressure. The diffusion coefficient is of the order of 10−12 cm2s−1 at −8°C, but the determination of the value is affected by small angle boundaries, where HCl motion is much faster. Extrapolation of these results suggests that HCl is not incorporated into stratospheric ice crystals.