bluuebunny/crossref_metadata_2025 huggingface dataset:
Applying the boundary layer concept to model transport of dissolved chemicals in preferential flow paths
Authors: Wallach R. , Parlange J.‐Y.
Published: February 2004

The exchange of dissolved chemicals between preferential flow paths and the surrounding matrix has a major effect on the concentration at the flow path outlet and, on a larger scale, on contaminant transport in structured soils and fractured rock systems. A model for dissolved chemical transport in a discrete fracture accompanied by simultaneous exchange with the surrounding porous matrix is presented and solved analytically. The fracture transport is modeled by a kinematic wave equation which ignores longitudinal dispersion, and the lateral solute transport in the matrix, where fluid is stagnant, is modeled by a diffusion equation. The exchange of dissolved chemicals between the fracture and matrix is assumed to take place through a boundary layer along the fracture‐matrix interface. The analytical solution to the equations is a convergent series, even for large values of the characteristic dimensionless parameter ϵ = kb2/(Dθ2), (where k is mass transfer coefficient, b is half the fracture width, and D and θ are the matrix diffusion coefficient and porosity, respectively) the column length, and long time periods. The number of terms in the series depends on the parameter values and desired accuracy. The model was successfully verified by fitting its output to breakthrough curves measured for highly fractured clay‐till soils. The role of the boundary layer (also known as film resistance) was studied by comparing breakthrough curves predicted by the current model (boundary layer (BL) model) and by a model that assumes a uniform concentration across the fracture‐matrix interface (local equilibrium (LE) model). Analyses of the model equations and simulations revealed that solute displacement from a fracture surrounded by a matrix with immobile solute can be divided into two stages with respect to the boundary layer's effect on breakthrough curve shape. Soon after the initiation of displacement, the boundary layer controls the solute flux to the matrix and the rate of concentration increase at the fracture outlet. The BL model predicts lower fluxes into the matrix than the LE model, and use of the latter for this stage underestimates the displaced solute mass at the fracture outlet. The duration of this stage and the deviation between breakthrough curves predicted by the current and LE models depend on ϵ, flow velocity, and column length. During the following stage, the boundary layer's effect on the rate‐limited exchange steadily diminishes, and the solute flux is controlled by matrix diffusion. At this latter stage, both models predict similar breakthrough curves at the fracture outlet.
Numerical modeling for investigating the physical meaning of the relationship between relative dielectric permittivity and water content of soils
Authors: Tabbagh Alain , Camerlynck Christian , Cosenza Philippe
Published: February 2004

The relative permittivity of soils is related to the volumetric liquid water content by a monotonous and general relationship. Among others, Topp et al. [1980] have proposed a simple empirical function to allow a direct determination of the water content. The moment method is applied here to calculate the apparent permittivity of a given volume of soil resulting from the electrical coupling between its different constituents and to attempt to reproduce the empirical function. This is not possible when considering randomly isotropically distributed elementary volumes of water, solid, and air. To reproduce the empirical function, it is necessary to consider that as the water content increases, the elementary volumes of water become more elongated since the liquid phase is continuous.
Efficient method for simulating gravity‐dominated water flow in unsaturated soils
Authors: Zhang Xiaoxian , Ewen John
Published: February 2004

Numerical convergence difficulties associated with nonlinearity in the hydraulic property functions are often encountered when simulating gravity‐dominated water flow in unsaturated soils using Richards' equation. A new approach for approximating the gravity term in Richard's equation is developed. Tests on a wide variety of soils show it is superior to existing Picard and Newton approaches. It is more reliable and efficient and allows larger time steps to be used.
Modeling the evolution of the structural state of water pipe networks with brief recorded pipe break histories: Methodology and application
Authors: Mailhot Alain , Pelletier Geneviève , Noël Jean‐François , Villeneuve Jean‐Pierre
Published: February 2004

Water managers are concerned about the poor structural state of their water pipe networks and are confronted with the lack of tools to analyze their data in order to evaluate both the present and future state of their networks. Researchers have developed statistical models with the annual number of pipe breaks as an indicator of the structural state of their networks. Unfortunately, most statistical modeling strategies do not perform well when used in municipalities with short recorded pipe break histories. Pipe breaks must have been recorded since the installation of the first pipes, and this is rarely the case in most municipalities. This paper presents a methodology to estimate calibration parameters of statistical models in municipalities with short recorded pipe break histories. The application of this methodology to a municipality with a century‐old water pipe network and a recorded pipe break history of 21 years is also presented.
Chloroform in a pristine aquifer system: Toward an evidence of biogenic origin
Authors: Laturnus Frank , Lauritsen Frants R. , Grøn; Christian
Published: February 2004

Ambient air, soil air, and groundwater were monitored for volatile halogenated organic compounds in a pristine spruce forest at Klosterhede, Denmark. Although this location is remote from industrial areas and free of any point sources of either soil or groundwater contamination, several volatile chlorinated hydrocarbons were detected. The concentration profiles of chloroform indicated a formation of this compound in the soil. Low concentrations were found in ambient air (0.02 ng L−1), higher concentrations in the upper soil layers (9.6 ng L−1 at 0.75 m below the surface), and a decrease with increasing depth (down to 1.5 ng L−1 just above the groundwater table at 7.5 m below the surface). For the other identified chlorinated compounds the concentration profiles were different from those of chloroform and exhibited less systematic variation between ambient air and soil air and with increasing depth. In the phreatic groundwater, high concentrations (up to 1.6 μg L−1) of chloroform and low concentrations (<0.01 μg L−1) of the other chlorinated compounds identified in air were found. Of the seven volatile brominated and iodinated compounds analyzed in ambient air, soil air, and groundwater, only bromoform was found and only in the upper soil layer. For chloroform a formation in the spruce forest soil is suggested, leading to concentrations in the groundwater in the micrograms per liter range.
Particle shape effects on the effective permittivity of anisotropic or isotropic media consisting of aligned or randomly oriented ellipsoidal particles
Authors: Jones Scott B. , Friedman Shmulik P.
Published: February 2004

The effective permittivity (dielectric constant) of anisotropic or isotropic porous media is affected by the shape of particles composing the mixture. Directional permittivities are influenced by extreme aspect ratio particles, often found aligned with the bedding plane of rock or soil. Our objectives were to determine the effects of particle shape and preferential orientation on the effective permittivity of porous media. Confocal spheroids (ellipsoids of revolution) were used to mathematically describe a range of particle shapes from disks to spheres to needles. Dielectric mixing models which account for the polarization due to inclusion shape and axial alignment were used to estimate the shape effect. Permittivity measurements in an anisotropic packing of disk‐shaped mica particles using time domain reflectometry showed an alteration of the permittivity due to the shape effect. Two‐ and three‐phase predictions based on Maxwell‐Garnett [1904] showed trends similar to measurements in anisotropic packings of mica. Particle shape effects can be a significant factor in dielectric permittivity measurements and should be a consideration especially where particle aspect ratio deviates by more than an order of magnitude from that of a sphere (unity). As the particle shape is less spherical, the resulting effective permittivity of the mixture is more similar to the inclusion permittivity and differs more from the permittivity of the background. Ellipsoid size and surface area provide an estimate of the combined effects of bound water and particle shape on the effective mixture permittivity. For high aspect ratio particles, shape effects on the effective permittivity appear to be comparable in magnitude to those of bound water prevalent in clay‐sized media.
Cost‐effective control of nitrogen loadings in Long Island Sound
Authors: Bennett Lynne L. , Thorpe Steven G. , Guse A. Joseph
Published: February 2004

Long Island Sound is plagued by conditions of severe hypoxia (low levels of dissolved oxygen) during the summer months because of the existence of excessive amounts of nitrogen. A new proposal that would allow sewage treatment plants to buy or sell nitrogen discharge credits is currently being evaluated by the states of Connecticut and New York. Existing theory suggests that a trading program for nitrogen emissions would be a cost‐effective means of addressing the problem. We estimate the costs associated with several trading scenarios and find that the potential for cost savings is substantial and that cost savings rise as the scope of trading expands.
A fast algorithm for the estimation of the equivalent hydraulic conductivity of heterogeneous media
Authors: Renard Philippe , Le Loc'h Gaëlle , Ledoux Emmanuel , de Marsily Ghislain , Mackay Rae
Published: February 2004

Fast upscaling of hydraulic conductivity is a recurrent problem in modeling flow through heterogeneous porous media. We propose a new renormalization technique. It is based on the iterative application of the Cardwell and Parsons [1945] bounds on elementary groups of cells. The combination of the bounds with a heuristic formula allows anisotropy to be taken into account. The new technique is tested and compared with other fast techniques. Among the tested techniques the two most reliable ones are the tensorial renormalization and the new simplified renormalization. The numerical efficiency of the simplified renormalization leads us to recommend it when a diagonal tensor of equivalent conductivity is sufficient.
Development and evaluation of a mathematical model for the transport and oxidation‐reduction of CoEDTA
Authors: Saiers James E. , Guha Hillol , Jardine Phil , Brooks Scott
Published: February 2004

Oxidation‐reduction reactions influence the subsurface mobility of a wide variety of toxic contaminants. In the work reported here, we develop a new model for coupled transport and oxidation‐reduction, and we test the model against published data on the movement of CoEDTA (cobalt ethylenediaminetetraacetic acid) through columns packed with β‐MnO2‐coated sand. The model solves equations for the advective‐dispersive transport of three aqueous species: Co(II)EDTA, Co(III)EDTA, and dissolved oxygen. These transport equations are linked with nonlinear kinetics expressions that describe (1) oxidation of Co(II)EDTA to Co)III)EDTA by β‐MnO2, (2) inhibition of Co(II)EDTA oxidation due to precipitation of Mn2O3, an insoluble reaction product that occludes the β‐MnO2 surface, and (3) regeneration of the redox‐reactive β‐MnO2 surface through oxidation of the Mn2O3 precipitate by dissolved oxygen. Comparison of experimental and calculated results demonstrates that the model describes the response of the coupled hydrological and geochemical processes to changes in flow rate, influent concentrations of Co(II)EDTA, and β‐MnO2 surface coverages.
Temporal behavior of a solute cloud in a heterogeneous porous medium: 2. Spatially extended injection
Authors: Dentz M. , Kinzelbach H. , Attinger S. , Kinzelbach W.
Published: February 2004

We investigate the temporal behavior of transport coefficients in a stochastic model for transport of a solute through a spatially heterogeneous saturated aquifer. While the first of these two companion papers [Dentz et al., this issue] investigated a situation characterized by a point‐like solute injection, we now focus on the case of spatially extended solute sources. The analysis of the finite time behavior of the transport coefficients makes it necessary to distinguish between two fundamentally different quantities characterizing the solute dispersion. We define an “effective” dispersion coefficient which is derived from the average over the centered second moments of the spatial concentration distributions in every realization and an “ensemble” dispersion coefficient which follows from the second moment of the ensemble‐averaged concentration distribution. While the two quantities are equivalent in the asymptotic limit of infinite times or infinitely extended sources, they are qualitatively and quantitatively different for the more realistic situation of finite times and finite source extent. We demonstrate that in this case the ensemble quantity, used more or less implicitly in most of the previous studies, overestimates the true dispersion of the plume. Using a second‐order perturbation theory approach, we derive explicit solutions for the temporal behavior of the dispersion coefficients for various types of isotropic and anisotropic initial conditions. We identify the relevant timescales which separate regimes of different temporal behavior and apply our formulae to the Borden experiment data. We find a good agreement between theory and experiment if we compare the observed dispersion with the appropriate effective dispersion coefficient (including the leading effects of the local dispersion), whereas the ensemble dispersion coefficient commonly used in the literature to analyze these data overestimates the experimental results considerably.
Using multiple experimental methods to determine fracture/matrix interactions and dispersion of nonreactive solutes in saturated volcanic tuff
Authors: Callahan Timothy J. , Reimus Paul W. , Bowman Robert S. , Haga Marc J.
Published: February 2004

The objective of this research was to investigate the effects of matrix diffusion on solute transport in fractured volcanic tuff. Two tuff cores were studied, one with a matrix porosity of 0.27 and the other with a porosity of 0.14. The matrix permeabilities of the cores were 4.7×10−15 and 7.8×10−19 m2, 5 and 9 orders of magnitude less than the respective fracture permeabilities. This suggested that the cores could be modeled as dual‐porosity systems with no flow in the matrix but significant solute storage capacity. Two types of tracer tests were conducted in each fractured core: (1) iodide was injected in separate experiments at different flow rates and (2) two tracers of different matrix diffusion coefficients (bromide and pentafluorobenzoate (PFBA)) were injected in another test. A difference in the maximum concentrations of the solutes and the extended tailing of the breakthrough curves were assumed to be indicative of diffusive mass transfer between the fracture and the porous matrix of the cores. Interpreting the results from both methods allowed the identification of matrix diffusion and dispersion effects within the fracture by simultaneously fitting the data sets (with known constraints) using a relatively simple conceptual model. Estimates of mass transfer coefficients for the fractured cores were also obtained.
Unsaturated‐zone wedge beneath a large, natural lake
Authors: Rosenberry Donald O.
Published: February 2004

Lake Belle Taine (480 ha) in north central Minnesota receives on average 76,000 m3 d−1 of water from Little Sand Creek but has no outlet. Water seeps out of the lake, flows through glacial outwash, and discharges into three nearby lakes with stages 13–14 m lower than Belle Taine. Seepage‐meter data indicate water is seeping out of Belle Taine at velocities up to 263 cm d−1 . Hydraulic‐head measurements made in the lake bed indicate the sediments are unsaturated beneath a portion of the lake edge, and a wedge of unsaturated sediments extends beneath the lake bed as much as 20 m from the shoreline. At the shoreline the water table is as much as 6.7 m below the lake surface. Modeling results of a similar hypothetical setting indicate that the horizontal extent of an unsaturated zone beneath a lake depends on (1) the permeability contrast between the outwash and the lake bed, (2) anisotropy, (3) lake bed slope, and (4) thickness of the lower‐permeability lake bed sediments. Although rarely documented, unsaturated sediments beneath a lake may not be extremely uncommon. Similar, much smaller unsaturated‐zone areas also have been observed beneath two other lakes in Minnesota.
A linear model of the effects of disturbance on dissolved nitrogen leakage from forested watersheds
Authors: Eshleman Keith N.
Published: February 2004

The leakage of dissolved nitrogen (N), primarily in the form of nitrate, from forested watersheds in the mid‐Appalachian region has important water quality ramifications for small acid‐sensitive streams and for downstream receiving waters such as Chesapeake Bay. Dissolved N leakage is a common, well‐documented response of forested watersheds to forest management practices such as clear‐cutting. Patterns of N leakage from mid‐Appalachian watersheds during the late 1980s and early 1990s have also been shown to display considerable temporal and spatial synchrony with outbreaks of defoliation by the gypsy moth (Lymantria dispar) larva (a nonnative forest insect pest). This evidence suggests that forest disturbance may be an important contributor to N leakage in the mid‐Appalachian region. A logical first step in testing this hypothesis is evaluating the ability of a simple, unit nitrogen export response function (UNERF) model to explain temporal changes in annual N export from gaged forested watersheds in the years following disturbance. Annual N export data from seven such watersheds were analyzed as part of the study: two small (<0.5 km2) watersheds subjected to deforestation and five larger (1.6–12.6 km2) watersheds subjected to repetitive defoliation by the gypsy moth larva. Several forms of linear UNERF models, parameterized by deconvolution of annual time series of N export using linear programming or by a least squares method, were generally found to be minimally biased and to explain high percentages (38–98%) of the total variation in annual N export. Despite their neglect of spatial and temporal ecosystem nonlinearities these linear models appear reasonably robust, making them at least as useful as their more complex nonlinear brethren for purposes of regionalization.
Saturated hydraulic conductivity and soil water retention properties across a soil‐slope transition
Authors: Mohanty Binayak P. , Mousli Zak
Published: February 2004

The hydraulic properties of soil and their spatial structures are important for understanding soil moisture dynamics, land surface and subsurface hydrology, and contaminant transport. We investigated whether landscape features, including relative position on a slope, contribute to the variability of soil hydraulic properties in a complex terrain of a glacial till material. Using 396 undisturbed soil cores collected along two orthogonal transects, we measured saturated hydraulic conductivity (Ksat) and soil water retention functions at two (15 and 30 cm) depths across a glacial till landscape in central Iowa that encompassed two soil types (Nicollet loam with 1–3% slope on the hilltop position and Clarion loam with 2–5% slope on the shoulder position). The van Genuchten‐Mualem model was fitted to the experimental data using the RETC optimization computer code. At the 15 cm depth a statistical comparison indicated significant differences in Ksat, saturated water content (θs), water content at permanent wilting point (θ15,000) and van Genuchten fitting parameters (α and n) between soil types and landscape positions. At the 30 cm depth, θs, θ15,000, and residual water content (θr) were found to be significantly different across the soil‐slope transition. Available water content (θ333–15,000) did not show any significant difference across the soil‐slope transition for either depth. No clear directional trend was observed, with some exceptions for Ksat, θs, and α on specific transect limbs and depths. Drifts in the soil hydraulic parameters due to soil‐slope transition were removed using a mean‐polishing approach. Geostatistical analyses of these parameters showed several important characteristics including the following: (1) The spatial correlation lengths and semivariogram patterns of the independently measured (or estimated) loge Ksat and θs at 30‐cm depth matched extremely well; (2) better spatial structures with large correlation lengths were observed for (macro and micro) porosity‐related loge Ksat, θs, and loge α than for texture‐related loge θ333–15,000, loge θ15,000, θr and loge n at 30‐cm depth; and (3) a higher nugget effect at 15‐cm depth was evident for most soil hydraulic parameters, indicating tillage and other surface disturbances. These novel findings may prove to be critical for modeling and interpreting field‐scale or larger‐scale soil moisture dynamics, surface and subsurface flow, and solute transport.
Numerical modeling of kinetic interphase mass transfer during air sparging using a dual‐media approach
Authors: Falta Ronald W.
Published: February 2004

A dual‐media multiphase flow approach is proposed for modeling the local interphase mass transfer that occurs during in situ air sparging. The method is applied to two‐ or three‐phase flow in porous media to simulate the small gas channels that form during air sparging, allowing resolution of the local diffusive mass transfer of contaminants between the flowing gas phase and nearby stagnant liquid‐filled zones. This approach provides a good match with laboratory column experiments in which dissolved trichloroethylene (TCE) is removed by air sparging, and it is shown that the simulation results are very sensitive to the nature of the local mass transfer regime. The numerical model is then applied to hypothetical field air‐sparging cases involving either a dissolved plume of TCE or a TCE nonaqueous phase liquid source. In these simulations the local mass transfer appears to play a much smaller role than in the laboratory‐scale tests, and significant deviations from local equilibrium simulations only occur when the mass transfer rates are reduced below the calibrated laboratory‐scale values.
A flux‐correct green element model of quasi three‐dimensional multiaquifer flow
Authors: Taigbenu Akpofure E. , Onyejekwe Okey O.
Published: February 2004

Transient flow in multiply layered aquifers, separated by connecting layers of aquitards which provide hydraulic interactions between the aquifers, is solved by the Green element method (GEM) in a manner that reveals one of its strengths of being able to correctly model the leakage flux without resorting to adjusting the grid representing the one‐dimensional (1‐D) flow in the aquitards, as done in the finite element method (FEM). The hydraulic approach of approximating the flow is adopted so that flow in the aquifer takes place in two dimensions and that in the aquitards takes place in the one‐dimensional vertical direction. The 1‐D Green element (GE) model earlier developed for transient diffusion and referred to as the transient GE (TGE) formulation [Taigbenu and Onyejekwe, 1999] is used in modeling the flow in the aquitards, while the 2‐D GE model developed for linear and nonlinear transient diffusion [Taigbenu and Onyejekwe, 1998] is used for calculating the flows in the confined and unconfined aquifers. Both models are coupled to solve regional flow problems in multiaquifer systems of arbitrary geometry which receive point and distributed recharge of arbitrary strengths. The solution procedure, which is iterative, provides information on the hydraulic heads and fluxes in the aquifers and aquitards at various specified times. Because GEM is founded on the singular integral theory, singularities that arise from water abstractions at wells (point recharge) are naturally captured in the singular Green's function, thereby making it possible to use a more coarse grid for problems in which there exist active wells. Furthermore, the implementation procedure of GEM achieves sparsity of the coefficient matrix so that less amount of computing resources is required for its decomposition. The superiority of the current approach over FEM in predicting the leakage flux through the aquitards and achieving comparable accuracy for well problems with coarser grid is demonstrated.
Modeling the influence of variable pH on the transport of zinc in a contaminated aquifer using semiempirical surface complexation models
Authors: Kent D. B. , Abrams R. H. , Davis J. A. , Coston J. A. , LeBlanc D. R.
Published: February 2004

Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH‐dependent adsorption; the Zn extended 15 m into the 30‐m‐thick sewage plume within approximately 100 m of the source but only 2–4 m into the plume between 100 and 400 m downgradient. A two‐dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59‐year simulation with a one‐site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage‐contaminated zone. Simulation with a two‐site SCM describes both the sharpness and approximate location of the leading edge of the Zn‐contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms.
Equivalence of bicontinuum and second‐order transport in heterogeneous soils and aquifers
Authors: Montas Hubert J. , Shirmohammadi Adel , Haghighi Kamyar , Engel Bernie
Published: February 2004

The mathematical equivalence of a closed second‐order transport equation for reactive solutes in saturated heterogeneous porous media (e.g., soils and aquifers) and a two‐region, mobile‐mobile, formulation is demonstrated in two ways: (1) by averaging the bicontinuum equations and (2) by transforming the second‐order equation to canonical form. The derivation of this equivalence is limited to media with heterogeneities in a plane orthogonal to the flow direction which represents a bidirectional generalization of the stratified case. Results indicate how the parameters of the bicontinuum formulation can be obtained from statistical descriptions of this heterogeneity and establish a stochastic interpretation for the bicontinuum equations. One‐dimensional analytical solutions of the equivalent transport equations are compared to a three‐dimensional numerical simulation of transport in a spatially autocorrelated lognormal random conductivity field. Results indicate that the upscaled equations provide good approximations of the dynamics of the mean resident concentration, the flux‐averaged concentration, and their variances.
Evolution of soil moisture spatial structure in a mixed vegetation pixel during the Southern Great Plains 1997 (SGP97) Hydrology Experiment
Authors: Mohanty B. P. , Famiglietti J. S. , Skaggs T. H.
Published: February 2004

Different factors contribute to soil moisture variability at different space scales and timescales, including soil properties, topography, vegetation, land management, and atmospheric forcings, such as precipitation and temperature. Field experiments supported by adaptive geostatistical and exploratory analysis, including categorical elimination of different governing factors, are needed to bring new insight to this important hydrologic problem. During the Southern Great Plains 1997 (SGP97) Hydrology Experiment in Oklahoma, we investigated the within‐season (intraseasonal) spatiotemporal variability of surface (0–6 cm depth) soil moisture in a quarter section (800 m×800 m) possessing relatively uniform topography and soil texture but variable land cover. Daily soil moisture measurements were made between June 22 and July 16 using portable impedance probes in a regular 7×7 square grid with 100‐m spacings. Initially, the land cover was split between grass and wheat stubble; row tilling on June 27 converted the wheat stubble to bare ground. Geostatistical and median polishing schemes were used to analyze the within‐season evolution of the spatial structure of soil moisture. The effects of daily precipitation, variable land cover, land management, vegetation growth, and microheterogeneity including subgrid‐scale variability were all visible in the analysis. Isotropic spatial correlation range for soil moisture varied between <100 m (for nugget and subgrid‐scale variability) and >428 m (for spherical and Gaussian models) within the 4‐week‐long SGP97 experiment. The findings will be useful for assessing remotely sensed soil moisture data collected during the SGP97 Hydrology Experiment in mixed vegetation pixels.
Observations of an intense anticyclonic warm eddy in the Newfoundland Basin
Authors: Caniaux Guy , Giordani Hervé , Prieur Louis , Hernandez Fabrice
Published: February 2004

An intense anticyclonic warm eddy was sampled in February 1997 in the Newfoundland Basin. Its presence, detected during a hydrographic survey, was further corroborated through ERS/TOPEX sea surface height anomaly and satellite tracked drifters. This 120 km wide eddy was very homogeneous in temperature and salinity down to 800 m and its transport (15–1800 dbar) reached 27 Sv. Its location, 49°N 40°W, in the warm and saline waters of the Northwest Corner, the northernmost position reached by the North Atlantic Current, is clearly exceptional. Although intense anticyclonic eddies have previously been observed with satellite tracked buoys and RAFOS floats, this is the first eddy (to our knowledge) ever documented at such a high latitude in the North Atlantic Current system. Its formation is discussed in term of thermohaline processes.
Why the properties of El Niño changed during the late 1970s
Authors: Wang Bin , An Soon‐Il
Published: February 2004

Following the abrupt North Pacific climate shift in the mid‐1970s, the period, amplitude, spatial structure, and temporal evolution of the El Niño notably changed. Theory is needed to explain why the coherent changes in several El Niño characteristics are nearly synchronized with the decadal climate shift. While the decadal variation in the equatorial thermocline could potentially change El Nino behavior, observation indicates that from the pre‐shift (1961–1975) to the post‐shift (1981–1995) period the change of equatorial eastern Pacific thermocline is insignificant. Our numerical experiments with a coupled atmosphere‐ocean model illustrate that the observed changes in ENSO properties may be attributed to decadal changes in the surface winds and associated ocean surface layer dynamics without changes in the mean thermocline. A theoretical analysis is presented to elucidate the mechanisms by which the decadal variations in winds and upwelling modify the structure and propagation of the El Niño and amplify and prolong the El Niño‐La Niña cycle.
3‐D crustal velocity structure at the rift tip in the Western Woodlark Basin
Authors: Zelt Barry C. , Taylor Brain , Goodliffe Andrew M.
Published: February 2004

We present a three‐dimensional (3‐D) crustal velocity model for part of the western Woodlark Basin in the SW Pacific, where rifting of a continental arc transitions to seafloor spreading. Velocities to ∼14 km depth are constrained by regularized inversion of first arrival traveltimes recorded at 14 ocean bottom instruments from a network of airgun profiles within an 80 km × 80 km area. The 3‐D velocity model images the axis and flanks of the rift basin adjacent to the westernmost spreading segment. Velocity contours that dip north at ∼10° parallel seismic reflectors from within a thick ophiolitic basement. High velocities (7.5 km/s) at the base of our model and preliminary analysis of earthquake P‐wave arrival times are consistent with an average crustal thickness of 15–20 km. Anomalously shallow upper mantle (8 km/s) velocities occur 6 km south of the spreading center at a depth of 3 km below the seafloor, and in the western part of the model, ∼30 km east of the D'Entrecasteaux Islands, at a depth of 6 km below the seafloor.
Laboratory results indicating complex and potentially unstable frictional behavior of smectite clay
Authors: Saffer Demian M. , Frye Kevin M. , Marone Chris , Mair Karen
Published: February 2004

A central problem in explaining the apparent weakness of the San Andreas and other plate boundary faults has been identifying candidate fault zone materials that are both weak and capable of hosting earthquake‐like unstable rupture. Our results demonstrate that smectite clay can be both weak and velocity weakening at low normal stress (<30 MPa). Our data are consistent with previous work, which has focused on higher normal stress conditions (50 MPa and greater) and found only velocity strengthening. If natural fault zones contain significant smectite, one key implication of our results is that localized zones of high pore pressure, which reduce effective normal stress, could be important in controlling potential sites of earthquake nucleation. Our experiments indicate that friction of smectite is complex, and depends upon both sliding velocity and normal stress. This complexity highlights the need for detailed experiments that reflect in‐situ conditions for fault gouges.
Global temperature change and its uncertainties since 1861
Authors: Folland C. K. , Rayner N. A. , Brown S. J. , Smith T. M. , Shen S. S. P. , Parker D. E. , Macadam I. , Jones P. D. , Jones R. N. , Nicholls N. , Sexton D. M. H.
Published: February 2004

We present the first analysis of global and hemispheric surface warming trends that attempts to quantify the major sources of uncertainty. We calculate global and hemispheric annual temperature anomalies by combining land surface air temperature and sea surface temperature (SST) through an optimal averaging technique. The technique allows estimation of uncertainties in the annual anomalies resulting from data gaps and random errors. We add independent uncertainties due to urbanisation, changing land‐based observing practices and SST bias corrections. We test the accuracy of the SST bias corrections, which represent the largest source of uncertainty in the data, through a suite of climate model simulations. These indicate that the corrections are likely to be fairly accurate on an annual average and on large space scales. Allowing for serial correlation and annual uncertainties, the best linear fit to annual global surface temperature gives an increase of 0.61±0.16°C between 1861 and 2000.
Changes of Enso stability due to greenhouse warming
Authors: Timmermann Axel
Published: February 2004

Based on a Coupled General Circulation Model (CGCM) simulation we study the influence of anthropogenic greenhouse warming on the stability of the El Niño‐Southern Oscillation phenomenon (ENSO). The linear stability of such a complex model cannot be assessed directly, hence we will derive empirical low order models for ENSO from the CGCM simulation under consideration. These models capture essential features of ENSO and are sensitive also to temporal changes in ENSO statistics. An eigenvalue analysis of these reduced models reveals that as greenhouse warming progresses a transition takes place from a stable oscillatory behavior to an unstable oscillation. This transition coincides with an abrupt change in simulated ENSO activity and can be explained in terms of changing ocean dynamics.
Repeat meridional survey of the Western North Pacific Subtropical Gyre by a VOS ADCP during 1997 to 1998
Authors: Kaneko Arata , Yuan Zhou , Gohda Noriaki , Arai Masazumi , Nakajima Hideo , Zheng Hong , Sugimoto Takashige
Published: February 2004

During 1997 to 1998, near‐surface currents in the western North Pacific subtropical gyre have been measured repeatedly by an ADCP mounted on the hull of the volunteer observing ship (VOS) operated mainly between Japan and Australia. The Kuroshio Current, the Kuroshio Recirculation and the North Equatorial Current are clearly identified as two‐year mean currents in the latitude belt 10–34.5° N. Current variability was dominated by baroclinic Rossby waves or mesoscale eddies over the 10–30° N belt except in the latitude belt 13–17° N where the north‐to‐south shifts of the North Equatorial Current stream axis may dominate current variability. The energy of the mesoscale variability was concentrated to form large peaks around 18.5°N and 23°N.
The influence of short‐term wind variability on air‐sea CO2 exchange
Authors: Bates Nicholas R. , Merlivat Liliane
Published: February 2004

Quantifying the regional and global exchange of CO2 between the ocean and atmosphere requires knowledge of the factors that affect CO2 gas transfer (e.g., wind speed) and the air‐sea difference in partial pressure of CO2 (pCO2). A major uncertainty is the effect of short‐term variability on air‐sea CO2 flux. Using high sampling frequency wind speed and pCO2 data collected during deployments of the autonomous CARbon Interface OCean Atmosphere (CARIOCA) buoy, we compare CO2 fluxes at different sampling frequency of wind speed (i.e., hourly versus daily averaged). Air‐sea CO2 flux was up to three times greater if high frequency wind data was used rather than daily average values. This difference arises from the non‐linear relationship between wind speed and CO2 gas transfer coefficient, and a better representation of wind distribution at a higher frequency (i.e., hourly) of sampling. This finding has significant implications for determining regional and global air‐sea CO2 fluxes, and understanding of the global carbon cycle.
Carbon flow to acetate and C1 compounds in northern wetlands
Authors: Hines Mark E. , Duddleston Khrystyne N. , Kiene Roland P.
Published: February 2004

High latitude wetlands are significant sources of atmospheric methane, with emission rates that are susceptible to effects of climate change. Our data demonstrate that unlike mid‐latitude wetlands, methane in northern peatlands is not derived from acetate or C1 compounds. These latter compounds accumulate to high levels with acetate as the primary organic end product of anaerobic decomposition. Acetate is ultimately degraded aerobically to carbon dioxide after diffusion into oxic regions of peat. Therefore, organic precursors destined for methane in mid‐latitude wetlands are degraded to carbon dioxide in northern wetlands. A warming‐induced initiation of acetoclastic methanogenesis could substantially increase methane production.
Intensive or extensive use of soil moisture: Plant strategies to cope with stochastic water availability
Authors: Rodriguez‐Iturbe Ignacio , Porporato Amilcare , Laio Francesco , Ridolfi Luca
Published: February 2004

Some plants rely on a dependable winter recharge, as opposed to others that quickly respond to the intermittent and uncertain rainfall during the growing season. Using a stochastic model for the soil moisture process and a quantitative measure of plant water stress, we find climate, soil, and vegetation characteristics leading to the dominance or possible coexistence of these two strategies of water use.
Stress drop with constant, scale independent seismic efficiency and overshoot
Authors: Beeler N. M.
Published: February 2004

To model dissipated and radiated energy during earthquake stress drop, I calculate dynamic fault slip using a single degree of freedom spring‐slider block and a laboratory‐based static/kinetic fault strength relation with a dynamic stress drop proportional to effective normal stress. The model is scaled to earthquake size assuming a circular rupture; stiffness varies inversely with rupture radius, and rupture duration is proportional to radius. Calculated seismic efficiency, the ratio of radiated to total energy expended during stress drop, is in good agreement with laboratory and field observations. Predicted overshoot, a measure of how much the static stress drop exceeds the dynamic stress drop, is higher than previously published laboratory and seismic observations and fully elasto‐dynamic calculations. Seismic efficiency and overshoot are constant, independent of normal stress and scale. Calculated variation of apparent stress with seismic moment resembles the observational constraints of McGarr [1999].
A non‐isostatic global sea level response to barometric pressure near 5 days
Authors: Hirose Naoki , Fukumori Ichiro , Ponte Rui M.
Published: February 2004

Changes in surface atmospheric pressure usually induce an isostatic response in the ocean characterized by an adjustment in sea level at the rate of approximately −1 cm/hPa. Nonisostatic signals are, however, observed at many tropical tide gauges at periods near 5 days. Our analysis of satellite observations reveals the global nature of these signals, involving nonisostatic sea level fluctuations with nearly uniform phase within separate basins and an out‐of‐phase oscillation between the Atlantic and Pacific Oceans. Comparisons with a barotropic model further link the observed signals to forcing by a large scale pressure wave in the atmosphere and show that constrictions between basins prevent an isostatic response from being established. The apparent forced, nonresonant nature of the nonisostatic response contrasts with the predominantly resonant ocean dynamics at higher frequencies and may be relevant to explain the behavior of the long period tides.
Joint variations of temperature and water vapor over the midlatitude continents
Authors: Cripe Douglas G. , Randall David A.
Published: February 2004

We have used warm‐season data from the Atmospheric Radiation Measurement (ARM) Program's Southern Great Plains site to investigate the joint variability of the temperature and moisture soundings, and their relationship to the variability of the generalized convective available potential energy (GCAPE). The actual temperature and humidity soundings vary together in such a way as to produce variations of the GCAPE which are far smaller than those which would occur if the relative humidity varied while the temperature sounding was fixed, or vice versa.
Parametric decay of oblique arc‐polarized Alfvén waves
Authors: Del Zanna Luca
Published: February 2004

The long‐term nonlinear evolution of monochromatic large‐amplitude Alfvén waves in oblique propagation is studied via one‐dimensional magnetohydrodynamic (MHD) numerical simulations. Arc‐polarized waves, which are exact solutions for non‐parallel propagation and are observed in solar wind data, are shown to be subject to parametric decay instabilities just like the well studied case of circularly polarized waves, not only when arc polarization is used as initial condition but also when this state is reached after nonlinear evolution of linearly polarized waves.
Healing of the shallow fault zone from 1994–1998 After the 1992 M7.5 Landers, California, Earthquake
Authors: Li Yong‐Gang , Vidale John E.
Published: February 2004

We conducted seismic surveys at the Johnson Valley fault in 1994, 1996, and 1998. We found that the shear velocity of the fault zone rock increased by ∼1.2% between 1994 and 1996, and increased further by ∼0.7% between 1996 and 1998. This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observed fault‐zone strength recovery is consistent with a decrease of ∼0.03 in the apparent crack density within the fault zone. The ratio of decrease in travel time for P to S waves changed from 0.75 in the earlier two years to 0.65 in the later two years between 1994 and 1998, suggesting that cracks near the fault zone are partially fluid‐filled and have became more fluid saturated with time.
Solar cycle variations in the electron heat flux: Ulysses observations
Authors: Scime Earl E. , Littleton J. E. , Gary S. Peter , Skoug Ruth , Lin Naiguo
Published: February 2004

Solar wind observations by the Ulysses spacecraft now include nearly ten years of continuous ion and electron measurements. In this study, we report detailed measurements of the electron heat flux in the solar wind. In particular, we examine the heat flux measurements for long‐term correlations with wave activity and solar wind speed. We find that the average heat flux, when scaled by R2.9 to account for variations due to distance from the Sun, is constant and independent of heliographic latitude or solar cycle. We find that during both solar maximum and solar minimum, there is no significant correlation between the magnitude of the electron heat flux and the solar wind speed. Comparison of the electron heat flux data with wave activity indicates that the whistler heat flux instability does not play an important role in limiting the solar wind heat flux.
A new strategy for real‐time integrated water vapor determination in WADGPS Networks
Authors: Hernández‐Pajares Manuel , Juan J. Miguel , Sanz Jaume , Colombo Oscar L. , van der Marel Hans
Published: February 2004

A major issue in many applications of GPS is the real‐time estimation of the Zenith Tropospheric Delays (ZTD). Several authors have developed strategies that estimate ZTD, with a latency of one hour or more in order to compute Integrated Water Vapor (IWV), using local measurements of surface pressure, or to assimilate ZTD into Numerical Weather Predicition (NWP) models. These strategies require that data from a regional GPS network be processed in near real‐time, using precise IGS orbits and partial orbit relaxation. Recently it has been shown that in Wide Area Differential GPS (WADGPS) networks of several hundred kilometers across, double‐differenced carrier phase ambiguities can be computed on‐the‐fly, using a real‐time tomographic model of the ionosphere obtained from the same GPS data. In this work we show how ambiguity resolution can help determine in real‐time the ZTD for a WADGPS network user, only 10–20% worse than those of the post‐processed solutions.
Empirical relationship of T‐wave energy and fault parameters of northeast Pacific Ocean earthquakes
Authors: Dziak Robert P.
Published: February 2004

A total of 179 northeast Pacific Ocean earthquakes that occurred between 1992–1998 are used to investigate the relationship of T‐wave acoustic energy and earthquake fault parameters. The moderate‐sized (3.7 ≤ Mw ≤ 6.9) earthquakes were recorded by ocean hydrophones and had their source parameters estimated by land‐based seismic networks. The acoustic energy release at the seafloor‐water interface of each earthquake was estimated by removing the signal attenuation along the propagation path and instrument gain from the hydrophone signal. There appears to be a relationship where, for a given magnitude earthquake, T‐wave energy is lower for normal and reverse fault earthquakes than strike‐slip events. A more detailed comparison indicates acoustic energy of the earthquake will often decrease as the component of dip‐slip motion increases. These observations suggest fault parameter information is contained within the T‐wave signal packet.