bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of total electron content, amplitude and phase scintillations in the auroral ionosphere
Authors: Erukhimov L. M. , Myasnikov E. N. , Kosolapenko V. I. , Cheremnyj V. A. , Evstaf'ev O. V.
Published: February 2004

The connection between the spatial gradient of the equivalent vertical total electron content (Ntot) of the ionosphere and scintillation patches are examined. It is shown that in the evening hours under quiet geomagnetic conditions, a steplike gradient of Ntot with latitude develops at the northern edge of the main ionospheric trough. Later in the evening, the above structure moves southward and the latitudinal gradient of Ntot becomes very pronounced and around it a scintillation patch is formed. During magnetic disturbances, the monotonic sharp gradient of Ntot is destroyed. A number of maxima of total electron content which become collocated with many scintillation patches are formed. Further, under disturbed conditions, the power law index of both the amplitude and phase scintillation spectra shows a marked variation with latitude which may be quasiperiodic.
Effects of mutual coupling in the design of high‐performance multifeed satellite antennas
Authors: Newell Peter , Bird Trevor S.
Published: February 2004

Predicted and measured results are presented for the effect of mutual coupling on the performance of shaped beam antennas having feed clusters that are irregular arrays of closely shaped small rectangular horn elements. The effects of mutual coupling on the primary and secondary patterns and on gain at specific points within beams are described for the Orion satellite antennas. Also shown is how excitation coefficients can be modified to overcome the changes in performance caused by mutual coupling.
Image theory for DC problems involving a conducting half‐space above an anisotropic impedance surface
Authors: Lindell I. V. , Flykt M. J.
Published: February 2004

A novel image theory is introduced, applicable to dc problems involving current sources in an isotropic conducting half‐space bounded by a planar, reciprocal, anisotropic impedance surface. The theory is a generalization of dc problems involving an isotropic impedance surface or a perfect anisotropic impedance surface with zero and infinite resistivities in two orthogonal directions on the plane, previously analyzed by these authors. The image for a point current source is a combination of an electric line current source and a transverse quadrupole magnetic line current source. The magnetic current component vanishes for vanishing anisotropy, while the electric current component vanishes for perfect anisotropy, of the impedance surface.
Full wave solutions for rough‐surface bistatic radar cross sections: Comparison with small perturbation, physical optics, numerical, and experimental results
Authors: Bahar Ezekiel , Lee Bom Son
Published: February 2004

In this paper, full wave solutions are derived for the like‐ and cross‐polarized electromagnetic fields diffusely scattered by two‐dimensional rough surfaces. These expressions for the diffuse scattered fields are used to obtain the random rough‐surface cross sections. The rough surface is characterized by a Gaussian joint probability density function for the surface heights and slopes at two points. These full wave results are compared with the associated small‐perturbation results and physical optics results. They are also compared with experimental and numerical results (based on Monte Carlo simulations). The earlier assumption that the surface height and slopes can be considered to be uncorrelated is examined. The impact of using the large‐radii curvature assumption and including self‐shadow is also considered.
Thrust geometries and deep structure of the outer and lesser Himalaya, Kumaon and Garhwal (India): Implications for evolution of the Himalayan fold‐and‐thrust belt
Authors: Srivastava Praveen , Mitra Gautam
Published: February 2004

The Kumaon‐Garhwal region of the Himalaya lies near the center of the Himalayan fold‐and‐thrust belt. We have drawn two balanced cross sections, 100 km apart, through the Outer and Lesser Himalaya. The cross sections incorporate all the surface, well log, and earthquake seismic data currently available from the region. Two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn. The three dimensional deep structure of the Outer and Lesser Himalaya is interpreted based on the balanced cross sections and the branch line maps. Deep structure of the Higher and Tethyan Himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas. A sequential evolutionary model for the Kumaon Himalaya along the eastern (Pindari) section is proposed. According to this model, the Kumaon Himalaya evolved by an overall forelandward progression of thrusting, with some reactivation along the Munsiari thrust (MT), the Main Boundary thrust (MBT), and the Main Central thrust (MCT). We use structural, stratigraphic, and radiometric criteria to place time constraints on the motion of these thrusts. Earliest motion along the MBT may have occurred in Early‐Middle Paleocene, but the main episode probably started in Late Eocene and may still be continuing. Emplacement of the MT had occurred by Middle‐Late Eocene, whereas the MCT shows activity around 20 Ma, thus exhibiting break back thrusting with respect to the MT. Shortening estimates are obtained from the Pindari section. Minimum shortening in the sedimentary thrust sheets of the Outer and Lesser Himalaya is 161 km or 65%. As a first approximation, we have also restored the crystalline sheets in order to obtain shortening estimates for the entire Himalaya. Minimum shortening for the Himalaya after restoring the MCT sheet varies from 354 (76%) to 421 km (79%). These estimates were further combined with published data from the area between the MCT sheet and the Indus‐Tsangpo Suture Zone (ITSZ). Minimum shortening between the Indo‐Ganga foreland and the ITSZ thus obtained lies in the range 687–754 km or 69–72%. We compare our shortening estimates with those available from the Pakistan and Nepal Himalaya.
Crustal structure and tectonics of the southeastern Beaufort Sea continental margin
Authors: Stephenson R. A. , Coflin K. C. , Lane L. S. , Dietrich J. R.
Published: February 2004

The structure of the southeast margin of the Canada Basin is synthesized from seismic reflection and refraction profiles in the southern Beaufort Sea and Mackenzie Delta, interpreted in conjunction with potential field data and the exploration seismic data base. The present margin was formed in the Jura‐Cretaceous and comprises a complex pattern of rifted and transform faulted crustal segments. Thinning in the upper crust is bounded by the Eskimo Lakes Fault Zone (ELFZ), a series of extensional listric normal faults, and is controlled by preexisting structures. Lower crustal thinning and the transition to oceanic crust occurs outboard of the ELFZ. A thick (up to 16 km) Late Jurassic and younger synrift and postrift sedimentary succession overlies oceanic crust in the eastern part of the Canadian Beaufort Sea and thinned continental crust between the Mackenzie Delta and Alaska. Tertiary faulting in the sedimentary basin appears to be related to the crustal structure. Present‐day seismicity in the southern Beaufort Sea is essentially limited to the area underlain by oceanic crust. Abrupt along‐strike changes in crustal affinity and degree of thinning allow the recognition of a NW‐trending transform fault. Regional gravity data, dominated by a series of coastline parallel highs, are used to extrapolate crustal features to the northeast along the Canadian polar continental margin. It is inferred that the Canadian polar margin consists of a number of 250‐to‐350‐km‐long stretched crustal segments separated by possible fracture zones. The orientation of the analogous tranform fault identified in the southeastern Beaufort Sea offers the possibility of kinematic constraints on models of ocean floor development within Canada Basin.
A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massachusetts
Authors: Goldstein Arthur G.
Published: February 2004

The area around Worcester, Massachusetts, has been used to determine which deformational and metamorphic features are due to the Alleghanian (Permian) orogeny and which are pre‐Alleghanian. Isolated, fault‐bounded inliers of Carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages. The first cleavage formed synchronously with metamorphism. Pre‐Carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages. The initial cleavage formed during the first metamorphism and the second cleavage formed during the second, retrogressive metamorphism. Thus the second two cleavages and the second metamorphism in pre‐Carboniferous rocks are interpreted as Alleghanian. Normal displacements on two distinct faults are bracketed by the formation of the first and second Alleghanian cleavages. The initial faulting, along the newly defined Wachusett mylonite zone (WMZ), formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest. The second faulting, along the Clinton‐Newbury fault (CNF) occurred along a steeply inclined plane which cuts the WMZ mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west. Alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined. This zone is interpreted as a ductile shear zone which moved twice, forming the first and second Alleghanian cleavages. Early‐formed, pre‐Alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates, and new hydrous minerals formed in the shear zone, indicating that it was a pathway for fluid flow. The initial motion was left‐lateral with a thrust component. The second cleavage formed during top‐to‐the‐northwest normal displacements. Thus the history of Alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones. This suggests that the strain history included two distinct episodes, with the second experiencing slightly different strain orientations at different times, resulting in first WMZ normal motion, then CNF normal motion and finally normal displacements related to the second Alleghanian cleavages. This history agrees well with other work on the nature of the Alleghanian orogeny in the northeast United States.
Cache Creek terrane entrapment: Oroclinal paradox within the Canadian Cordillera
Authors: Mihalynuk Mitch G. , Nelson JoAnne , Diakow Larry J.
Published: February 2004

Exotic and far‐traveled oceanic crustal rocks of the Cache Creek terrane (CC) are bordered by less exotic Quesnel (QN) and Stikine (ST) arc terranes to the east, north, and west. All of these terranes are enveloped by an arcuate belt of displaced continental margin rocks; the Kootenay (KO), Nisling (NS), and parts of the Yukon‐Tanana (YTT) terranes, that have indirect ties to ancestral North America (NA). Initial 87Sr/86Sr isopleths conform to this arcuate pattern. Such a pattern of concentric belts presents a geological conundrum: How did the QN, ST, and CC come to be virtually enveloped by terranes with ties to NA? Past and current models that explain assembly of the Canadian Cordillera are deficient in their treatment of this problem. We propose that Early Mesozoic QN and ST were joined through their northern ends as two adjacent arc festoons that faced south toward the Cache Creek ocean (Panthalassa?). Oceanic plateau remnants within the CC today were transported from the Tethyan realm and collided with these arcs during subduction of the Cache Creek ocean. Counterclockwise oroclinal rotation of ST and NS terranes in the Late Triassic to Early Jurassic caused enclosure of the CC. Rotation continued until these terranes collided with QN in the Middle Jurassic. Paleomagnetic declination data provide support for this model in the form of large average anticlockwise rotations for Permian to Early Jurassic sites in ST but moderate clockwise rotations for sites in QN. Specific modern analogues for the Cordilleran orocline include the Yap trench, where the Caroline rise is colliding end‐on with the Mariana Arc and the Banda Arc, located on the southeastern “tail” of the Asian plate, which is being deformed into a tight loop by interactions with the Australian and Pacific plates.
The analysis of constant drawdown wells using instantaneous source functions
Authors: Murdoch Lawrence C. , Franco John
Published: February 2004

Wells are commonly held at constant drawdown to maximize their discharge or to recover nonaqueous phase liquids, but only the simplest cases of constant drawdown wells have been treated analytically. In this paper the method of instantaneous source functions, which was originally limited to cases where the discharges of wells are specified, is extended to predict transient discharge and drawdown in the vicinity of wells held at constant drawdown. The analysis requires applying a Laplace transform to the Duhamel integral, rearranging the resulting algebraic expression, and then numerically inverting to obtain the discharge function that will result in a constant drawdown at a particular location. This approach allows flow problems with a wide range of geometries to be analyzed, and it also allows superposition to be used for transient problems without violating the condition of constant drawdown at the well.
Steady infiltration in unsaturated soil from a buried circular cylinder: The separate contributions from top and bottom halves
Authors: Mandal A. C. , Waechter R. T.
Published: February 2004

Waechter and Philip (1985) obtained the asymptotic expansion of the mean infiltration rate for large s from a buried circular cylinder using a scattering analog. Here s(= αl/2) is defined as the ratio of the characteristic length l of the water supply surface (in fact, its radius) to the sorptive length 2α−1 of the soil and a satisfies the relationship K(ψ) = K(0) eαψ, where K is the hydraulic conductivity, and ψ is the moisture potential. This exact solution cannot be used directly to obtain the separate contributions to the mean infiltration rate from the top and the bottom halves of the cylinder; our analysis is based on a new class of special functions derived from the modified Bessel equation with a forcing term. In this paper, we obtain the separate asymptotics for the two halves for large s to make a comparison with the results of the trench problem (Waechter and Mandal, 1993). The asymptotic expansions for top and bottom halves are (2/π)(0.69553s−2/3) and (2/π)(1+0.30066s−2/3), respectively, whereas for a semicircular trench, the mean infiltration rate is given by (2/π)(1+0.30066s−2/3).
Hyperfiltration‐induced fractionation of lithium isotopes: Ramifications relating to representativeness of aquifer sampling
Authors: Fritz Steven J. , Whitworth Thomas M.
Published: February 2004

A 10‐mM LiCl solution was hydraulically forced through a 0.3‐cm‐thick smectite membrane at a constant solution flux in order to depict the chemical and isotopic evolution of analogous hyper filtration cells in the subsurface. At steady state (∼19 days) there was, relative to the input solution, a measured 12% buildup of solute within the entire cell and a calculated 248% solute increase at the clay/solution interface. The maximum depletion of 7Li (about 13‰ relative to the input solution) was calculated to occur about 1 cm away from the membrane out into the high‐pressure reservoir. When scaled to realistic solution fluxes in natural systems, the position of maximum fractionation would occur meters away from a shale/aquifer contact. A steady state hyperfiltration model indicates that a solute's heavy isotopic species will always be depleted on the high‐pressure side of a clay membrane, a prediction corroborated not only by the Li experiment but also by previous hyperfiltration studies involving stable isotopes of C and Cl. The extent of fractionation increases with higher values of the membrane's reflection coefficient, σ. In the experiment a σ of 0.55 for the 90% porosity clay membrane yielded these results. Because shales have substantially lower porosities with attendant higher values of σ, we conclude that the hyperfiltration phenomenon in the subsurface may result in sampling of waters that are not chemically and isotopically representative of the formation penetrated by the well.
Efficient simulation of single species and multispecies transport in groundwater with local adaptive grid refinement
Authors: Wolfsberg Andrew V. , Freyberg David L.
Published: February 2004

The computational burden associated with multidimensional, multicomponent, numerical solute transport models can be prohibitive. To solve these CPU intensive problems efficiently and accurately, we have implemented and extended a local adaptive grid refinement method of Berger and Oliger (1984) to track error‐prone regions and supply high‐resolution subgrids where they are locally needed while maintaining relatively few nodes elsewhere on a coarse, base grid. Novel features include a unique method for detecting a priori where the numerical error is unacceptable, variable time step control which allows smaller time steps on subgrids than on the base grid, and a modular framework which allows easy exchange of partial differential equation solvers to accommodate different problem formulations. Three examples show how the subgrids track the error‐prone regions, how multiple subgrids are used for geometrically complex front shapes, and how increased resolution is provided for self‐sharpening fronts when multiple reacting species are considered. For all three examples, solutions with accuracies comparable to those achieved with a uniform fine grid are obtained at between 20 and 30% of the computational cost.
An Eulerian‐Lagrangian approach with an adaptively corrected method of characteristics to simulate variably saturated water flow
Authors: Huang K. , Zhang R. , van Genuchten M. T.
Published: February 2004

A relatively simple method of characteristics is developed to simulate one‐dimensional variably saturated water flow. The method uses the Eulerian‐Lagrangian approach to separate the governing flow equation into “convection” and “diffusion” parts, which are solved with the method of characteristics and the conventional finite element method, respectively. The method of characteristics combines a single‐step reverse particle tracking technique with a correction strategy to ensure accurate mass balances. The correction process is implemented by weighing the calculated convective contribution to the pressure head at each node with the pressure head values of two upstream nodes, using an adaptive weighing factor λ. The value of λ is automatically adjusted by considering the global mass balance at each time step. Numerical experiments for ponded infiltration are presented to illustrate the scheme's performance for situations involving highly nonlinear soil hydraulic properties and extremely dry initial conditions. Results indicate that the proposed method is mass‐conservative, virtually oscillation‐free, and computationally quite efficient. The method is especially effective for simulating highly nonlinear flow scenarios for which traditional finite difference and finite element numerical methods often fail to converge.
Modeling the acid‐base chemistry of organic solutes in Adirondack, New York, lakes
Authors: Driscoll Charles T. , Lehtinen Michael D. , Sullivan Timothy J.
Published: February 2004

Data from the large and diverse Adirondack Lake Survey were used to calibrate four simple organic acid analog models in an effort to quantify the influence of naturally occurring organic acids on lake water pH and acid‐neutralizing capacity (ANC). The organic acid analog models were calibrated to observations of pH, dissolved organic carbon (DOC), and organic anion (An−) concentrations from a reduced data set representing 1128 individual lake samples, expressed as 41 observations of mean pH, in intervals of 0.1 pH units from pH 3.9 to 7.0. Of the four organic analog approaches examined, including the Oliver et al. (1983) model, as well as monoprotic, diprotic, and triprotic representations, the triprotic analog model yielded the best fit (r2 = 0.92) to the observed data. Moreover, the triprotic model was qualitatively consistent with observed patterns of change in organic solute charge density as a function of pH. A low calibrated value for the first H+ dissociation constant (pKal = 2.62) and the observation that organic anion concentrations were significant even at very low pH (<4) indicate that naturally occurring organic solutes in these waters have strongly acidic functional groups. Inclusion of organic acidity in model calculations resulted in good agreement between measured and predicted values of lake water pH and ANC. Assessments to project the response of surface waters to future changes in atmospheric deposition, through the use of acidification models, will need to include representations of organic acids in model structure to make accurate predictions of pH and ANC.
Spatial variability of residual nitrate‐nitrogen under two tillage systems in central Iowa: A composite three‐dimensional resistant and exploratory approach
Authors: Mohanty B. P. , Kanwar R. S.
Published: February 2004

Soil nitrate‐nitrogen (NO3‐N) data arranged on a three‐dimensional grid were analyzed to compare tillage effect on the spatial distribution of residual NO3‐N in the soil profile of agricultural plots drained by subsurface tiles. A three‐dimensional median‐based resistant (to outlier(s)) approach was developed to polish the spatially located data on soil NO3‐N affected by directional trends (nonstationarity in the mean) in three major directions (row, column, and depth) and along the horizontal diagonal directions of the grid. Effect of preferential or nonpreferential path of transport of NO3‐N in the vertical direction defined as sample hole effect was also removed to make the data trend‐free across holes. Composite three‐dimensional semivariogram models (along horizontal and vertical directions) were used to describe the spatial structure of residual soil nitrate distribution. Two plots in the same field, one under each tillage system (conventional tillage and no tillage), were studied. In each plot, soil samples were collected at five depths (30, 60, 90, 120, and 150 cm) from 35 sites (holes) arranged on a 7×5 regular grid of 7.6×7.6 m. In the conventional tillage plot, residual NO3‐N concentrations decreased gradually to a depth of 90 cm and increased beyond this depth. The coefficient of variation, however, became gradually smaller, showing more uniform distribution for greater depths. In the no‐tillage plot, trends were similar to those in the conventional tillage system, but were spatially more stable across the profile. Structural analyses indicated that under conventional tillage, the semivariogram of residual soil nitrate distribution was linear in the horizontal and vertical directions. In contrast, the semivariograms for no‐tillage showed nugget‐type behavior, indicating a lack of spatial structure in the residual soil nitrate.
Three‐parameter lognormal distribution model for soil water retention
Authors: Kosugi Ken'ichirou
Published: February 2004

Many models for soil water retention have been proposed. However, most of these models are curve‐fitting equations and do not emphasize the physical significance of their empirical parameters. A new retention model that exhibits increased flexibility was developed by applying three‐parameter lognormal distribution laws to the pore radius distribution function ƒ(r) and to the water capacity function, which was taken to be the pore capillary pressure distribution function ƒ(ψ). This model contains three parameters that are closely related to the statistics of ƒ(ψ): the bubbling pressure ψc, the mode ψ0 of ƒ(ψ) and the standard deviation σ of transformed ƒ(ψ). By comparison of this model with three existing models (the van Genuchten model, the Brooks‐Corey model, and the modified Tani model), it was shown that ψc, ψ0, and σ are all essential for a general retention model.
Infiltration‐advance equation for radial spreading
Authors: Rasmussen William O.
Published: February 2004

Volume balance and flow rate considerations were used to develop an equation for the radial rate of advance of irrigation water. The equation is applicable to radial flow from bubblers, sources near the edge of a basin, or other situations in which the primary flow direction is radial from the source. Examples are given for the use of the equation in designing the spatial and temporal characteristics of an applied slug of water. A technique is presented to allow equations associated with previous strip irrigation analyses to be transformed for use in radial irrigation analyses.
Chloride migration in heterogeneous soil: 2. Stochastic modeling
Authors: Destouni Georgia , Sassner Mona , Jensen Karsten H.
Published: February 2004

The observed statistics of chloride breakthrough presented by Sassner et al. (this issue) were compared with predictions of a stochastic‐advective modeling approach. The stochastic‐advective model based on the observed spatial distribution of flow rates and on transport parameters consistent with parameter values obtained from local breakthrough curves (BTCs) agreed well with the observations. Alternative models that agreed well with the local BTCs failed to predict the large‐scale BTC with realistic parameter values. The results support the assumption that compared to advection variability, local dispersion within the mobile water will often have a second‐order effect on field scale solute transport. The stochastic‐advective model is robust with regard to the rate of mass transfer between mobile and immobile water zones. This robustness implies that order of magnitude estimates may be sufficient for providing useful predictions of both field scale solute transport and the associated prediction uncertainty. In contrast, accurate estimation of the statistics of solute advection at the scale of interest for the transport problem is necessary.
Comparison of simple versus complex distributed runoff models on a midsized semiarid watershed
Authors: Michaud Jene , Sorooshian Soroosh
Published: February 2004

The increasing availability of distributed rainfall data and computational resources is providing the opportunity to use distributed models for rainfall‐runoff forecasting or other applications. This paper compares the accuracy of simulations from a complex distributed model (KINEROS), a simple distributed model (based on the Soil Conservation Service (SCS) method), and a simple lumped model (SCS method). The 150 km2, semiarid Walnut Gulch experimental watershed was the test site; models were validated using 24 severe thunderstorms and rain gauge densities similar to those found at flash flood warning sites (one gauge per 20 km2). Under these circumstances, none of the models were able to accurately simulate peak flows or runoff volumes from individual events. Models showed somewhat more skill in predicting time to peak and the ratio of peak flow to volume. When calibration was performed, the accuracy of the complex distributed model was similar to that of the simple distributed model. Without calibration, the complex distributed model was more accurate than the simple distributed model. The spatially lumped model performed very poorly. The complex distributed model was validated under real‐time forecasting conditions; forecasts based on observed rainfall had lead times of 30—75 min.
Infiltration of water into snow
Authors: Conway H. , Benedict R.
Published: February 2004

Measurements from a rectangular grid of thermistors set in a maritime snowpack are used to study the infiltration of water during two midwinter rain on snow events. The progress of wetting is tracked in real time by monitoring changes in the position of the zero‐degree isotherm. Rates and patterns of infiltration are calculated for each event. Infiltration was not uniform, and water penetrated through localized channels that often occupied less than 50% of the total volume of the snowpack. The evolution of wetting was strongly influenced by the snow stratigraphy. In one case the snowpack contained multiple ice layers, and vertical flow was impeded and diverted laterally for several hours at each layer. In the other case the snowpack was more homogeneous, and water concentrated in channels and penetrated to depth more rapidly. The measurements of temperature are also used to calculate the components of heat transfer within the snow during each rain event. Heat transfer in dry snow occurs primarily by conduction, and rates are relatively slow. However, introduction of liquid water results in the release of latent heat when water freezes on contact with subfreezing snow at the wetting front. The release of latent heat dominates heat transfer and has the potential to warm the snowpack rapidly. Rates of freezing needed to satisfy the heat equation are calculated. In both cases studied, less than 4% of the total influx of rainwater needed to change phase. Most of the rain remained liquid and wet the snow or drained through the snowpack.
Estimation of mountain front recharge to regional aquifers: 2. A Maximum Likelihood Approach incorporating prior information
Authors: Chavez Adolfo , Sorooshian Soroosh , Davis Stanley N.
Published: February 2004

In this two‐part series a stochastic estimation procedure applicable to the analytic streamflow model derived in the companion is introduced. The parameter estimation problem is posed in the framework of maximum likelihood theory, where prior information about the model parameters and a suitable weighting scheme for the error terms in the estimation criterion are included. Various optimization methods are combined for parameter estimation. The issues of model and parameter identifiability, uniqueness, and stability are addressed, and strategies to mitigate identifiability problems in our modeling are discussed. The seasonal streamflow model is applied to a mountainous watershed in southern Arizona, and maximum likelihood estimates of mountain front recharge and other model and statistical parameters are obtained. The analysis of estimation errors is performed in both the eigenspace and the original space of the parameters.
Two‐dimensional modeling of aquifer remediation influenced by sorption nonequilibrium and hydraulic conductivity heterogeneity
Authors: Rabideau Alan J. , Miller Cass T.
Published: February 2004

A computational model was developed to simulate aquifer remediation by pump and treat for a confined, perfectly stratified aquifer. A split‐operator finite element numerical technique was utilized to incorporate flow field heterogeneity and nonequilibrium sorption into a two‐dimensional, radially symmetric advective‐dispersive transport model. Simulations reproduced the tailing behavior commonly observed at remedial sites. Predicted remediation times were observed to increase as the degree of hydraulic conductivity heterogeneity and/or sorption nonequilibrium increased. Dimensional analysis was used to facilitate the general application of simulation results to a range of aquifer/contaminant systems. The effects of sorption nonlinearity, sorption capacity heterogeneity, and intermittent pumping were also analyzed.
Laboratory measurement of capillary pressure‐saturation relationships in a rock fracture
Authors: Reitsma Stanley , Kueper Bernard H.
Published: February 2004

A laboratory technique has been developed to measure the hysteretic relationship between capillary pressure and fluid saturation for a rough‐walled rock fracture under different states of normal stress. The technique incorporates a loading system and a uniquely designed capillary barrier to the nonwetting phase which allows for fracture closure and provides excellent hydraulic connection of the wetting phase in the fracture with external pressure control. The method is applied to a single, rough‐walled fracture in limestone using oil and water as the nonwetting and wetting phase, respectively. The measured capillary pressure curves were found to be well represented by a Brooks‐Corey porous media capillary pressure function. A distinct entry pressure, giving rise to initial nonwetting phase invasion, was observed in each test. It was found that the inferred aperture distributions became less skewed with increased normal load. These aperture distributions were inferred using one of three conceptual models presented for nonwetting phase invasion of a rough‐walled fracture under conditions of main drainage.
A three dimensional time‐dependent approach to calibrating sediment trap fluxes
Authors: Buesseler Ken O. , Michaels Anthony F. , Siegel David A. , Knap Anthony H.
Published: February 2004

We conducted an experiment to test explicitly the accuracy of upper ocean sediment trap fluxes using the particle‐reactive radionuclide 234Th (t1/2 = 24.1 days). Two independent VERTEX‐style multitrap arrays were used for collection of sinking particles at 95 m and 97 m depths over a four‐day period in May 1992 at the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time‐series Study (BATS) site. Samples for total 234Th were collected every 8 m between the surface and 96 m and immediately combined for analysis to obtain the vertically integrated activity of 234Th. We collected a total of 27 samples over the four‐day period. The234Th samples were collected daily at each of the two traps and every other day on a 6 × 6 km grid to characterize the entire source region for particles collected in the traps. In situ flow sensors at one trap array indicated low horizontal shear at the trap mouth (5‐10 cm/s) compared to normal values at BATS. The predicted 234Th flux from the watercolumn profiles was not significantly different from zero (−30 ± 140 disintegrations per minute/m2/d). The measured trap 234Th flux at both arrays was significantly higher (290 ± 15 dpm/m2/d). We hypothesize that upper ocean traps at Bermuda may overcollect during low‐flux periods and undercollect during high‐flux periods, thus recording a biased signal of the true particle flux.
The separate and combined effects of temperature, soilpCO2, and organic acidity on silicate weathering in the soil environment: Formulation of a model and results
Authors: Gwiazda Roberto H. , Broecker Wallace S.
Published: February 2004

A model is used to evaluate the relative importance of temperature, soil pCO2, and organic acidity on silicate weathering rates in an average soil of warm temperate climate. The model has a structure similar to the Model of Acidification of Groundwater in Catchments, (Cosby et al., 1985a, b) used to predict catchment responses to acid deposition, but it is modified to account for the effects of temperature, the partial pressure of carbon dioxide in the soil, and organic acidity on dissolution of silicate minerals. The model is run with a mean soil temperature of 15°C, and when equilibrium is achieved it is raised to 19°C. Soil pCO2 and organic acids adjust accordingly. It is found that temperature is the main control of the weathering rate; the other two factors have a very minor forcing effect. The transient after the step function disturbance occurs on a century to millennial timescale. Consequently, it is not possible to study weathering rates through either purposeful soil manipulations lasting several years or by examining the effect on soils of land use changes because steady state may have not been achieved yet.
Methane emissions from rice fields: Effect of soil properties
Authors: Sass R. L. , Fisher F. M. , Lewis S. T. , Jund M. F. , Turner F. T.
Published: February 2004

Flooded rice fields emit methane and are important contributors to the increasing atmospheric methane concentration. Various estimates of global release rates of methane from rice paddies range from a low of 20 Tg per year to a high of 200 Tg per year. Global estimates of methane emissions from rice fields depend upon obtaining reliable data from a variety of soil types. We have compared a variety of methane emission data sets obtained over a four‐year period from three different soil types found at the Texas Agricultural Experiment Station near Beaumont, Texas, with several physical and chemical properties of the soils. We find that seasonal methane emissions directly correlate with the percent sand in the soils. Along a transect with soil sand content ranging from 18.8% to 32.5%, seasonal methane emissions ranged from 15.1 g m−2to 36.3 g m−2.
Simple Global Carbon Model: The atmosphere‐terrestrial biosphere‐ocean interaction
Authors: Kwon O‐Yul , Schnoor Jerald L.
Published: February 2004

A simple global carbon model has been developed for scenario analysis, and research needs prioritization. CO2 fertilization and temperature effects are included in the terrestrial biosphere compartment, and the ocean compartment includes inorganic chemistry which, with ocean water circulation, enables the calculation of time‐variable oceanic carbon uptake. Model‐derived Q10 values (the increasing rate for every 10°C increase of temperature) are 1.37 for land biota photosynthesis, 1.89 for land biota respiration, and 1.95 for soil respiration, and feedback temperature is set at 0.01°C/ppm of CO2. These could be the important parameters controlling the carbon cycle in potential global warming scenarios. Scenario analysis, together with sensitivity analysis of temperature feedback, suggests that if CO2 emissions from fossil fuel combustion continue at the present increasing rate of ∼1.5% per year, a CO2 doubling (to 560 ppm) will appear in year 2060. Global warming would be responsible for 40 Gt as carbon (Gt C) accumulation in the land biota, 88 Gt C depletion from the soil carbon, a 7 Gt C accumulation in the oceans, and a 19 ppm increase in atmospheric CO2. The ocean buffering capacity to take up the excess CO2 will decrease with the increasing atmospheric CO2 concentration.
Climatic, edaphic, and biotic controls over storage and turnover of carbon in soils
Authors: Schimel David S. , Braswell B. H. , Holland Elisabeth A. , McKeown Rebecca , Ojima D. S. , Painter Thomas H. , Parton William J. , Townsend Alan R.
Published: February 2004

Soil carbon, a major component of the global carbon inventory, has significant potential for change with changing climate and human land use. We applied the Century ecosystem model to a series of forest and grassland sites distributed globally to examine large‐scale controls over soil carbon. Key site‐specific parameters influencing soil carbon dynamics are soil texture and foliar lignin content; accordingly, we perturbed these variables at each site to establish a range of carbon concentrations and turnover times. We examined the simulated soil carbon stores, turnover times, and C:N ratios for correlations with patterns of independent variables. Results showed that soil carbon is related linearly to soil texture, increasing as clay content increases, that soil carbon stores and turnover time are related to mean annual temperature by negative exponential functions, and that heterotrophic respiration originates from recent detritus (∼50%), microbial turnover (∼30%), and soil organic matter (∼20%) with modest variations between forest and grassland ecosystems. The effect of changing temperature on soil organic carbon (SOC) estimated by Century is dSOC/dT= 183e−0.034T. Global extrapolation of this relationship leads to an estimated sensitivity of soil C storage to a temperature of −11.1 Pg° C−1, excluding extreme arid and organic soils. In Century, net primary production (NPP) and soil carbon are closely coupled through the N cycle, so that as temperatures increase, accelerated N release first results in fertilization responses, increasing C inputs. The Century‐predicted effect of temperature on carbon storage is modified by as much as 100% by the N cycle feedback. Century‐estimated soil C sensitivity (−11.1 Pg° C−1) is similar to losses predicted with a simple data‐based calculation (−14.1 Pg° C−1). Inclusion of the N cycle is important for even first‐order predictions of terrestrial carbon balance. If the NPP‐SOC feedback is disrupted by land use or other disturbances, then SOC sensitivity can greatly exceed that estimated in our simulations. Century results further suggest that if climate change results in drying of organic soils (peats), soil carbon loss rates can be high.
Modeling the Global Carbon Cycle: Nitrogen fertilization of the terrestrial biosphere and the “missing” CO2sink
Authors: Hudson Robert J. M. , Gherini Steven A. , Goldstein Robert A.
Published: February 2004

The discrepancy between estimates of net terrestrial CO2emissions derived from (1) inverse modeling of the ocean/atmosphere system and (2) modeling of land use change, better known as the “missing” CO2sink, suggests that some changing environmental factor, such as CO2, anthropogenic N emissions, or climate, has fertilized terrestrial ecosystems. To address this question, we herein describe and apply GLOCO, a global carbon cycle model. GLOCO's ocean submodel combines a box diffusion model with representations of chemical equilibria and biological processes to simulate the distributions and cycling of inorganic and organic carbon, phosphate, and alkalinity. The terrestrial submodel divides the biosphere into seven natural biomes with dynamic carbon and nitrogen cycling in both vegetation and soils. Anthropogenic influences on the functioning of the carbon and nitrogen cycles, such as fossil fuel combustion, forestry, and agricultural development, are also incorporated in the model. Our analysis confirms previous suggestions that because temperate and boreal forests are N limited, CO2fertilization of these forests is less than predicted by short‐term CO2response factors. Modeling of temperate/boreal forest fertilization by anthropogenic N deposition suggests that CO2is initially sequestered at a C:N ratio of ∼100, rather than the steady state value for the ecosystem of ∼30. If N deposition is to account for the 40–70% of the fertilization of the terrestrial biosphere not explainable by CO2fertilization and temperature increases, then we estimate that 26‐30 Tg N yr−1of anthropogenic deposition in the temperate and boreal zones would be required. Recent anthropogenic NOxand NH3deposition fluxes at northern temperate latitudes have been estimated to be 20–28 Tg N yr−1. Thus fertilization by anthropogenic N emissions likely constitutes a significant portion of the missing CO2sink.
Nitrogenous fertilizers: Global distribution of consumption and associated emissions of nitrous oxide and ammonia
Authors: Matthews Elaine
Published: February 2004

The global distribution of nitrogen input via application of chemical nitrogenous fertilizers to agricultural ecosystems is presented. The suite of 1° (latitude/longitude) resolution data bases includes primary data on fertilizer consumption, as well as supporting data sets defining the distribution and intensity of agriculture associated with fertilizer use. The data were developed from a variety of sources and reflect conditions for the mid‐1980s. East Asia, where fertilizer use is increasing at ∼10%/year, accounted for ∼37% of the total, while North America and western Europe, where fertilizer use is leveling off, accounted for 17% and 14% of global use, respectively. Former centrally planned economies of Europe consumed one fifth of the 1984 total, but rapid increases in the 1980s are slowing, and consumption trends are variable. The most widely used chemical nitrogenous fertilizer is urea which accounted for 40% of the world's total in the mid‐1980s. While almost every country consumes urea, ∼75% of the large East Asian fertilizer use is supplied by this one fertilizer. Ammonium nitrate, used primarily in the former centrally planned economies of Europe, in West Asia, and in Africa, accounted for about one quarter of global consumption. These data were used to estimate distributions of the annual emission of nitrous oxide (N2O) and of ammonia (NH3) associated with the use of fertilizers. Applying published ranges of emission coefficients for fertilizer types in the data base yields a median emission of 0.1 Tg N2O‐N, with lower and upper values of 0.03 and 2.0 Tg N2O‐N in 1984. This equals <1% to ∼3% of the total nitrogen applied via commercial fertilizers and represents <1% to 15% of the annual emission of N2O from terrestrial sources. Assuming that the ∼4% annual increase in consumption of nitrogenous fertilizers during the 1980s corresponds to a ∼4% rise in the release of N2O‐N, yearly increases in emissions from fertilizer use are <0.01 to 0.08 Tg N2O‐N equal to <1% to 3% of the current growth of atmospheric nitrous oxide. However, since no measurements of fertilizer‐derived nitrous oxide emissions are available for agricultural environments in the tropics/subtropics, where ∼40% of fertilizer N is consumed and where consumption is increasing rapidly, relative contributions of climatic regions to current and future emissions remain uncertain. Ammonia emission coefficients for simple groups of fertilizer types were applied to derive the global distribution of ammonia volatilization associated with nitrogenous fertilizer consumption. The 1984 total of ∼5–7 Tg NH3‐N, about 10–15% of the annual ammonia source, is concentrated overwhelmingly in subtropical Asia owing to the dominant use of urea with high rates of volatilization. However, the paucity of measurements in representative ecological and management environments suggests that the magnitude and distribution of current and future ammonia emission from fertilizers is still poorly known.
Tropospheric budget of reactive chlorine
Authors: Graedel T. E. , Keene W. C.
Published: February 2004

Reactive chlorine in the lower atmosphere (as distinguished from chlorofluorocarbon‐derived chlorine in the stratosphere) is important to considerations of precipitation acidity, corrosion, foliar damage, and chemistry of the marine boundary layer. Many of the chlorine‐containing gases are difficult to measure, and natural sources appear to dominate anthropogenic sources for some chemical species. As a consequence, no satisfactory budget for reactive chlorine in the lower atmosphere is available. We have reviewed information on sources; source strengths; measurements in gas, aqueous, and aerosol phases; and chemical processes and from those data derive global budgets for nine reactive chlorine species and for reactive chlorine as a whole. The typical background abundance of reactive chlorine in the lower tropospheric is about 1.5 ppbv. The nine species, CH3Cl, CH3CCl3, HCl, CHClF2, Cl2* (thought to be HOCl and/or Cl2), CCl2= CCl2, CH2Cl2, COCl2, and CHCl3, each contribute at least a few percent to that total. The tropospheric reactive chlorine burden of approximately 8.3 Tg Cl is dominated by CH3Cl (≈45 %) and CH3CCl3(≈25 %) and appears to be increasing by several percent per year. By far the most vigorous chlorine cycling appears to occur among seasalt aerosol, HCl, and Cl2*. The principal sources of reactive chlorine are volatilization from seasalt (enhanced by anthropogenically generated reactants), marine algae, volcanoes, and coal combustion (natural sources being thus quite important to the budget). It is anticipated that the concentrations of tropospheric reactive chlorine will continue to increase in the next several decades, particularly near urban areas in the rapidly developing countries.