bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure and early kinematic history of the Great Glen Fault Zone, Scotland
Authors: Stewart M. , Strachan R. A. , Holdsworth R. E.
Published: September 2002

The Great Glen Fault Zone (GGFZ) is a major, reactivated strike‐slip fault within the lower Paleozoic Caledonian orogenic belt of the British Isles. The Late Devonian to Tertiary reactivation history is well documented, but the early history is poorly known and controversial. New analysis of early structures and fabrics shows that the GGFZ comprises a 3‐km‐wide belt of fault rocks ranging from rare mylonite and quartz blastomylonite to common cataclasite, hydrated cataclasite, and phyllonite. Kinematic indicators demonstrate a sinistral sense of displacement with a southeasterly component of downthrow. Microstructural analysis indicates that presently exposed levels of the GGFZ developed initially at depths of about 9–16 km, within the frictional‐viscous creep transition zone. Deformation continued during exhumation and was associated with pervasive syntectonic infiltration of hydrous fluids which resulted in widespread retrogression along the GGFZ. Strong protolith mineral phases were progressively replaced by secondary fine‐grained aggregates of phyllosilicates leading to a permanent weakening of the fault zone. Relationships between fault zone structures, dated igneous intrusions, and posttectonic (Devonian/Old Red Sandstone) sedimentary rocks constrain sinistral displacement to a late stage in the Caledonian orogeny, between about 428 Ma and 390 Ma. Early displacements probably overlapped crustal‐scale, foreland‐directed thrusting which resulted from the oblique collision of the Scottish segment of Laurentia with Baltica. There is no geological evidence that these late Caledonian displacements involved reactivation of an older structure at the current exposure level. However, the Caledonian structural architecture of the GGFZ exerted a fundamental influence on the upper crustal Late Devonian to Tertiary reactivation history during which microfracturing and development of discrete fault planes were localized either within or along the boundaries of preexisting belts of cataclasite and phyllonite.
A Study on the pH and conductivity of rural rainfall employing two collocated samplers
Authors: Sequeira R. , Lai C. C. , Peart M. R.
Published: September 2002

A set of about 100 daily rainfall samples were collected over a period of about one year during the 1995–1996 period using two collocated, automated samplers placed ∼4 m apart at the rural Kadoorie Agricultural Research Centre (KARC) in Hong Kong. The p H and conductivity of the rainwater were measured immediately after sample collection. There is a strong correlation between the two free hydrogen ion concentrations (R2 ≈ 0.92) and an even stronger one between the conductivities (R2 ≈ 0.99). Statistically, there is no difference at the 0.05 level of significance between the means of either the two free hydrogen ion concentrations or the two conductivities. The conductivity results suggest that the total dissolved solids in the two samplers is probably quite similar in magnitude. No relationship is observed between the free acid content and daily rainfall volume in either sampler, a result similar to that obtained in previous studies involving bulk fall at the KARC and wet fall in urban Hong Kong as a whole. A weak hyperbolic relationship exists between the rainfall volume and the conductivity, and their log‐log plot indicates only a somewhat weak inverse linear relationship, with correlation coefficients of −0.60 and −0.61 for the two samplers, considered individually. Finally, the unbiased estimates of the product of rainfall volume and conductivity for the collocated samples suggest that the microscale variability (≳4 m) of the mean wet mass flux of total dissolved material in rural Hong Kong rainfall is negligible.
Investigation of flow in water‐saturated rock fractures using nuclear magnetic resonance imaging (NMRI)
Authors: Dijk Peter , Berkowitz Brian , Bendel Peter
Published: September 2002

The application of nuclear magnetic resonance imaging (NMRI) to the direct three‐dimensional measurement of flow in rough‐walled water‐saturated rock fractures is presented for the first time. The study demonstrates the abilities of NMRI to noninvasively measure rock‐water interfaces and water flow velocities in these fractures and investigates the effects of wall morphology on flow patterns inside a typical rock fracture. Two‐ and three‐dimensional flow‐encoded spin‐echo pulse sequences were applied. The stability and reproducibility of the water flow patterns were confirmed by analyzing two‐dimensional velocity images. A variety of geometrical and hydraulic features were determined from three‐dimensional velocity images, including the rock‐water interfaces, the fracture aperture distribution, and the critical aperture path; velocity profiles and volumetric flow rates; flow and stagnant regions; and the critical velocity path. In particular, the effects of a sharp step discontinuity of the fracture walls and the applicability of the cubic law were examined. As a result of the complex three‐dimensional geometry, velocity profiles are generally parabolic but often highly asymmetric, with respect to the fracture walls. These asymmetric velocity profiles are clustered together, with significant correlations; they are not just local random phenomena. However, theoretical considerations indicate that the effects of the measured asymmetry on volumetric flow rates and hydraulic conductivities are insignificant, in that the overall flow inside rough fractures still obeys the cubic law. The features discussed in this study emphasize the strong heterogeneity and the highly three‐dimensional nature of the flow patterns in natural rock fractures and consequently the need for three‐dimensional flow analysis.
Floods, channel change, and the hyporheic zone
Authors: Wondzell Steven M. , Swanson Frederick J.
Published: September 2002

We investigated the influence of flood‐induced channel changes on the hyporheic zone of 4th‐ and 5th‐order reaches of a mountain stream network. Preflood versus postflood comparisons were made in three study reaches from well networks established before and reestablished after a major flood. Flood effects were scale dependent and varied with channel constraint and the dominant channel forming process. Large changes were observed in unconstrained stream reaches where channel incision drove large changes in subsurface flow paths and the extent of the hyporheic zone. However, subreach scale differences were apparent. In the lower portion of the studied reach, channel incision lowered the water table, leading to abandonment of secondary channels, and decreased the extent of the hyporheic zone that previously extended more than 30 m into the floodplain. In contrast, the extent of the hyporheic zone increased at the head of the studied reach where channel incision steepened head gradients through a meander bend. In another unconstrained reach, lateral channel jumps dramatically altered exchange flow paths. However, the extensive hyporheic zone was maintained throughout the reach. Less change was observed in the constrained stream reach where both the depth and area of sediment available to be reworked by the flood were limited by bedrock constraining the width of the valley floor. This flood dramatically changed the hyporheic zone at the three study sites and these physical changes are expected to be biologically important, considering the role of the hyporheic zone in stream ecosystem processes.
Mass transfer correlations for nonaqueous phase liquid pool dissolution in saturated porous media
Authors: Kim Tae‐Joon , Chrysikopoulos Constantinos V.
Published: September 2002

Correlations describing the rate of interface mass transfer from singlecomponent nonaqueous phase liquid (NAPL) pools in saturated subsurface formations are developed. A three‐dimensional contaminant transport model is employed to obtain overall mass transfer coefficients computed from concentration gradients at the NAPLwater interface. The model assumes that the porous medium is homogeneous, the interstitial fluid velocity is steady, and the dissolved solute may sorb under local equilibrium conditions. Furthermore, it is assumed that the dissolved concentration along the NAPL‐water interface is equal to the solubility concentration. Power law correlations relate the overall Sherwood number to the appropriate overall Peclet numbers. Both rectangular and elliptic/circular source geometries are considered. The proposed relationships are fitted to numerically determined mass transfer coefficients, and the correlation coefficients are determined by nonlinear least squares regression. Good agreement between predicted and available experimentally determined overall mass transfer coefficients is observed.
Transport of polydisperse colloid suspensions in a single fracture
Authors: James Scott C. , Chrysikopoulos Constantinos V.
Published: September 2002

The transport of variably sized colloids (polydisperse) in a fracture with uniform aperture is investigated by a particle‐tracking model that treats colloids as discrete particles with unique transport properties while accounting for either matrix diffusion or irreversible colloid deposition. For the special case of a monodisperse colloid suspension the particle‐tracking model is in perfect agreement with predictions based on an existing analytical solution. It is shown that lognormal colloid size distributions exhibit greater spreading than monodisperse suspensions. Increasing the fracture porosity of the solid matrix leads to higher matrix diffusion, which in turn delays particle breakthrough for both the monodisperse and variably sized colloid suspensions. The smallest particles of a distribution are more greatly affected by matrix diffusion whereas the largest particles are transported faster and further along a fracture. Both perfect sink and kinetic colloid deposition onto fracture surfaces are examined. Kinetic deposition accounts for colloid surface exclusion by either a linear or nonlinear blocking function. For both cases the smallest colloid particles tend to preferentially deposit onto the fracture wall. Both matrix diffusion and surface deposition tend to discretize colloid distributions according to particle size so that larger particles are least retarded and smaller particles are more slowly transported. Furthermore, it is shown that the rate of colloid deposition is inversely proportional to the fracture aperture.
Can we distinguish Richards' and Boussinesq's equations for hillslopes?: The Coweeta Experiment revisited
Authors: Steenhuis T. S. , Parlange J.‐Y. , Sanford W. E. , Heilig A. , Stagnitti F. , Walter M. F.
Published: September 2002

Previous analyses of subsurface flow down sloping, shallow soil hillsides are further simplified. Surprisingly, whether the starting point is Richards' or Boussinesq's equation, the derived analytical expressions for cumulative outflow are identical, and they agree very well with experimental results from the Coweeta Hydrological Laboratory [Hewlett and Hibbert, 1963]. We show that Boussinesq's model can be improved from a theoretical point of view at a very small cost. We also solved Boussinesq's equation numerically. However, this improved model did not fit the experimental data any better, while the numerical solution only showed a marginal agreement with the observed data.
Seasonality of streamflow: The Upper Mississippi River
Authors: Baldwin Connely K. , Lall Upmanu
Published: September 2002

The understanding of seasonal variations in streamflow is important for water resource management. The dynamics of streamflow are often dominated by annual and intra‐annual variations, and the global warming debate has also generated an interest in potential changes in the seasonal cycle of hydroclimatic variables. Thus there are mechanistic as well as policy motivations for an empirical analysis of the historical seasonal variations in streamflow. The seasonality of the upper Mississippi River streamflow is investigated in this paper using a 123‐year record of daily flow. This long streamflow series provides an interesting look at the high‐ and low‐flow seasons within the year, their interannual variation, and within‐season attributes. Evidence for changes in the timing and amplitude of these seasons and the annual flow extremes is presented. Connections to similar trends in regional climate variables are noted. The upper Mississippi River streamflow exhibits bimodal probability distributions for monthly averages and for specific seasons. Transitions across the high‐ and low‐flow regimes corresponding to these modes exhibit memory across seasons and over years. Thus an empirical basis for seasonal or longer prediction is provided. Needs for developing a mechanistic explanation of the empirical observations offered are also indicated.
Optimal spatial sampling design for the estimation of the variogram based on a least squares approach
Authors: Bogaert Patrick , Russo David
Published: September 2002

A methodology for the choice of optimal spatial sampling designs is proposed. The problem is to find an optimal finite set of space locations where a random field has to be sampled, in order to minimize the variability of the parametric variogram estimator. Under the hypothesis that the random field is Gaussian second‐order stationary and using a first‐order approximation for the linearization of the parametric variogram with respect to its parameters, we are able to express the covariance matrix of the parameter estimators as a function of the sampling design. An optimization algorithm based on a generalized least squares approach is proposed in order to reduce the variability of these estimators through the minimization of the determinant of their covariance matrix. In order to validate this approach, a practical case study is conducted for two different variogram models. It shows that compared to random sampling designs, the benefit of the optimization procedure is somewhat limited for a variogram model without nugget effect. For a variogram model with nugget effect, random sampling designs are associated with much higher variability on the parameter estimators than optimized designs because of the typical lack of information offered by random sampling designs for small distances between locations. The performance of random sampling designs compared to optimized or alternative regular designs is shown to be poor with respect to parameter estimation, especially when a nugget effect is included in the variogram model. The parameters that benefit the most from the optimization procedure are the variance and the nugget effect, whereas the improvement for the range parameter estimation is limited. The optimization algorithm provides yardstick results, yielding reference values for the selection of an alternative regular design. The applicability of the algorithm is very wide, and it can greatly help the user to understand the way the parameters are influenced by the choice of a set of sampling locations instead of another.
Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions
Authors: Lahvis Matthew A. , Baehr Arthur L. , Baker Ronald J.
Published: September 2002

Aerobic biodegradation and volatilization near the water table constitute a coupled pathway that contributes significantly to the natural attenuation of hydrocarbons at gasoline spill sites. Rates of hydrocarbon biodegradation and volatilization were quantified by analyzing vapor transport in the unsaturated zone at a gasoline spill site in Beaufort, South Carolina. Aerobic biodgradation rates decreased with distance above the water table, ranging from 0.20 to 1.5 g m−3 d−1 for toluene, from 0.24 to 0.38 g m−3 d−1 for xylene, from 0.09 to 0.24 g m−3 d−1 for cyclohexene, from 0.05 to 0.22 g m−3 d−1 for ethylbenzene, and from 0.02 to 0.08 g m−3 d−1 for benzene. Rates were highest in the capillary zone, where 68% of the total hydrocarbon mass that volatilized from the water table was estimated to have been biodegraded. Hydrocarbons were nearly completely degraded within 1m above the water table. This large loss underscores the importance of aerobic biodradation in limiting the transport of hydrocarbon vapors in the unsaturated zone and implies that vapor‐plume migration to basements and other points of contact may only be significant if a source of free product is present. Furthermore, because transport of the hydrocarbon in the unsaturated zone can be limited relative to that of oxygen and carbon dioxide, soil‐gas surveys conducted at hydrocarbon‐spill sites would benefit by the inclusion of oxygen‐ and carbon‐dioxide‐gas concentration measurements. Aerobic degradation kinetics in the unsaturated zone were approximately first‐order. First‐order rate constants near the water table were highest for cyclohexene (0.21–0.65 d−1) and nearly equivalent for ethylbenzene (0.11–0.31 d−1), xylenes (0.10–0.31 d−1), toluene (0.09–0.30 d−1), and benzene (0.07–0.31 d−1). Hydrocarbon mass loss rates at the water table resulting from the coupled aerobic biodgradation and volatilization process were determined by extrapolating gas transport rates through the capillary zone. Mass loss rates from groundwater were highest for toluene (0.20–0.84 g m−2 d−1), followed by xylenes (0.12–0.69 g m−2 d−1), cyclohexene (0.05–0.15 g m−2 d−1), ethylbenzene (0.02–0.12 g m−2 d−1), and benzene (0.01–0.04 g m−2 d−1). These rates exceed predicted rates of solubilization to groundwater, demonstrating the effectiveness of aerobic biodgradation and volatilization as a combined natural attenuation pathway.
Fine bed material in pools of natural gravel bed channels
Authors: Lisle Thomas E. , Hilton Sue
Published: September 2002

Natural gravel bed channels commonly contain a fine mode of sand and fine gravel that fills voids of the bed framework of coarser gravel. If the supply of fine bed material exceeds the storage capacity of framework voids, excess fine material forms surficial patches, which can be voluminous in pools during low flow. Data collected in 34 natural channels in northern California and southern Oregon indicate the following. (1) Fine material on the bed surface can be readily winnowed and transported at high particle velocities, much of it in intermittent suspension. Fine material can dominate the bed material load in gravel bed channels, but its abundance on the bed surface is limited by its increasing mobility as hiding places among prominent particles are filled. (2) Fine material in pools is typically replaced many times per year. (3) The proportion V* of residual pool volume filled with fine bed material correlates with annual sediment yield in channels whose parent material produces abundant sandy sediment. (4) Temporal and spatial changes in V* appear to correspond to variations in the balance between sediment inputs and water discharge. These results suggest that V* can be used to monitor and evaluate the supply of excess fine material in gravel bed channels and that samples of fine material in pools can characterize the fine, mobile mode of bed material load.
Improving the ability of a water market to efficiently manage drought
Authors: Characklis Gregory W. , Griffin Ronald C. , Bedient Philip B.
Published: September 2002

Some water markets maintain institutional elements that provide allocative advantages to specified water users. In the Lower Rio Grande Valley, water rights are designated as either municipal or agricultural (irrigation), with certain prioritization advantages afforded to municipal accounts. While sales of rights between municipalities and irrigators are allowed, the priority disparity results in a prohibition on leasing between sectors. Concern over meeting future urban demand has led municipalities to purchase rights well in excess of current needs. The inability to lease municipal water to irrigators removes a significant and growing fraction of available water from the market. The additional flexibility provided by leasing provides a valuable tool for managing seasonal drought. In this analysis the justification for prioritized municipal water is investigated. Results indicate that the added security municipalities may derive from higher prioritization during drought is accompanied by economic inefficiencies in regional water allocation. It is argued that eliminating municipal protection and the consequent allowance of intersectoral leasing would contribute to regional well‐being at small cost to municipal water users.
Field investigation of the waste isolation pilot plant (WIPP) site (New Mexico) using a nonstationary stochastic model with a trending hydraulic conductivity field
Authors: Seong Kwanjae , Rubin Yoram
Published: September 2002

Verification of a stochastic model which models the phenomena of flow and transport in a nonstationary conductivity field [Rubin and Seong, 1994] is attempted with field data from the Waste Isolation Pilot Plant (WIPP) site near Carlsbad, New Mexico. The nonstationarity is manifested as a spatial linear trend in the mean log conductivity field. Analysis of the log conductivity and the hydraulic head data shows that the site corresponds to Rubin and Seong's α2 model, where the mean flow direction and the gradient of mean log conductivity are normal to each other. The experimental semivariogram of the log conductivity obtained from field data strongly suggests that it is nonstationary and can be fit with a Gaussian covariance function. The α2 model is successful in predicting the experimental semivariogram of the head obtained from the field data, especially in capturing the finite sill characteristics. The stationary model predicts an unbounded semivariogram at large distances. Solute travel times are also investigated by comparing the travel time cumulative distribution function (CDF) resulting from the stationary and the nonstationary hydraulic conductivity models. The stationary model predicts a higher CDF at early times and hence overestimates the very early travel times to the regulatory plane of compliance than the α2 model.
Spatial and temporal variations in snowmelt runoff chemistry, Northwest Territories, Canada
Authors: Marsh P. , Pomeroy J. W.
Published: September 2002

In order to demonstrate the spatial and temporal variations in meltwater chemistry at both the lysimeter (0.25 m2) and basin scale, field measurements of snowmelt were conducted in northern Canada. These observations show that microscale variations in flow volume are accompanied by variations in meltwater chemistry. For example, the solute concentrations were largest in areas with low flow, while the largest mass flux occurred in the areas with highest flow. The observed variations in both concentration and mass flux can be quantitatively described by the relationships described by Hibberd [1984). The field measurements clearly demonstrate that in order to estimate the average meltwater chemistry, it is necessary to sample the flow field at a scale similar to that required to average the lateral variations in meltwater volume. Variations in meltwater runoff chemistry also occur at the basin scale due to changes in snowcover depth and the resulting differences in the timing of meltwater release. For example, at this site, meltwater release occurs up to a week earlier from the shallow snow covers than for the deeper snow covers. It would be expected that this asynchronous meltwater runoff would result in a smoothing of the ionic pulse at the basin scale, with lower peak values and a more gradual decline in concentration when compared with meltwater at a point.
Hydrodynamic dispersion and mass transfer in unsaturated flow
Authors: Haga D. , Niibori Y. , Chida T.
Published: September 2002

Tracer response experiments were carried out using a packed bed of glass beads, where fluid flowed as either a single phase (water) or two phases (water and gas) under steady unsaturated conditions. The results depended on the water saturation and could be divided into two regions at the critical water saturation, that is, the saturation at which the gas phase began to be immobile. To determine the hydrodynamic dispersion coefficients, the experimental responses were compared with the numerical results of a one‐dimensional advection‐dispersion equation. These results demonstrated that the hydrodynamic dispersion coefficients increased with a decrease in water saturation. Furthermore, the rates of the increase in the dispersion coefficients above the critical saturation were different from those below it. The following mathematical relations between the Peclet number, Pe, and the water saturation, Sw, were obtained from the results of the two unsaturated flow experiments: Pe=0.8Sw1.2 for single‐phase flow and Pe=0.9Sw3.1 for two‐phase flow. When Sw=1 in the relation for single‐phase flow, the value of Pe is in a range that has been reported previously. The relation for two‐phase flow was used to analyze the mass transport of the two‐phase tracer (ammonia‐water solution) through unsaturated beds.
Solutions to equations incorporating the effect of rate‐limited contaminant mass transfer on vadose zone remediation by soil vapor extraction
Authors: Huang Junqi , Goltz Mark N.
Published: September 2002

Soil vapor extraction (SVE) is a technique that is frequently used to remediate unsaturated soils in the vadose zone that are contaminated with volatile organic compounds. Laboratory research and field experience have demonstrated that rate‐limited mass transfer of a contaminant from aquifer solids and soil water to the gas phase significantly impacts the efficacy and speed of the remediation. An exact analytical solution to equations describing SVE controlled by rate‐limited mass transfer is presented. Processes modeled include contaminant advective‐dispersive transport in a gas phase converging on an extraction well and rate‐limited mass transfer of dissolved and sorbed contaminants into the gas phase, with the rate limitation modeled assuming first‐order kinetics. In addition to the exact solution an approximate solution, as well as an exact solution for plug flow (no dispersion), is presented. These analytical solutions may be useful in verifying numerical codes that are being developed to model SVE of volatile organic compounds.
A new multistage groundwater transport inverse method: presentation, evaluation, and implications
Authors: Anderman Evan R. , Hill Mary C.
Published: September 2002

More computationally efficient methods of using concentration data are needed to estimate groundwater flow and transport parameters. This work introduces and evaluates a three‐stage nonlinear‐regression‐based iterative procedure in which trial advective‐front locations link decoupled flow and transport models. Method accuracy and efficiency are evaluated by comparing results to those obtained when flow‐ and transport‐model parameters are estimated simultaneously. The new method is evaluated as conclusively as possible by using a simple test case that includes distinct flow and transport parameters, but does not include any approximations that are problem dependent. The test case is analytical; the only flow parameter is a constant velocity, and the transport parameters are longitudinal and transverse dispersivity. Any difficulties detected using the new method in this ideal situation are likely to be exacerbated in practical problems. Monte‐Carlo analysis of observation error ensures that no specific error realization obscures the results. Results indicate that, while this, and probably other, multistage methods do not always produce optimal parameter estimates, the computational advantage may make them useful in some circumstances, perhaps as a precursor to using a simultaneous method.
Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef
Authors: Langdon Chris , Takahashi Taro , Sweeney Colm , Chipman Dave , Goddard John , Marubini Francesca , Aceves Heather , Barnett Heidi , Atkinson Marlin J.
Published: September 2002

The concentration of CO2 in the atmosphere is projected to reach twice the preindustrial level by the middle of the 21st century. This increase will reduce the concentration of CO32− of the surface ocean by 30% relative to the preindustrial level and will reduce the calcium carbonate saturation state of the surface ocean by an equal percentage. Using the large 2650 m3 coral reef mesocosm at the BIOSPHERE‐2 facility near Tucson, Arizona, we investigated the effect of the projected changes in seawater carbonate chemistry on the calcification of coral reef organisms at the community scale. Our experimental design was to obtain a long (3.8 years) time series of the net calcification of the complete system and all relevant physical and chemical variables (temperature, salinity, light, nutrients, Ca2+,pCO2, TCO2, and total alkalinity). Periodic additions of NaHCO3, Na2CO3, and/or CaCl2 were made to change the calcium carbonate saturation state of the water. We found that there were consistent and reproducible changes in the rate of calcification in response to our manipulations of the saturation state. We show that the net community calcification rate responds to manipulations in the concentrations of both Ca2+ and CO32− and that the rate is well described as a linear function of the ion concentration product, [Ca2+]0.69[CO32−]. This suggests that saturation state or a closely related quantity is a primary environmental factor that influences calcification on coral reefs at the ecosystem level. We compare the sensitivity of calcification to short‐term (days) and long‐term (months to years) changes in saturation state and found that the response was not significantly different. This indicates that coral reef organisms do not seem to be able to acclimate to changing saturation state. The predicted decrease in coral reef calcification between the years 1880 and 2065 A.D. based on our long‐term results is 40%. Previous small‐scale, short‐term organismal studies predicted a calcification reduction of 14‐30%. This much longer, community‐scale study suggests that the impact on coral reefs may be greater than previously suspected. In the next century coral reefs will be less able to cope with rising sea level and other anthropogenic stresses.
Geographical variation of the acid buffering of vegetated catchments: Factors determining the bicarbonate leaching
Authors: Ohte Nobuhito , Tokuchi Naoko
Published: September 2002

In order to understand the determining factors of the geographical variation of buffering in the world's vegetated catchments, this study examined the HCO3− leaching of 107 experimental watersheds selected from regions ranging from tropical rain forests to humid continental or subpolar forests including several temperate grassland. With consideration of the soil development and the factors controlling it, the influences of geological, climatological, and topographical conditions on discharge HCO3− concentrations and fluxes are examined. The HCO3− discharge fluxes from catchments were correlated with climatological indexes. The increase of annual mean temperature reflected the increase in fluxes, although this tendency was not that significant in the precipitation‐flux relationships. Notable differences in these correlations were not found among three major groups of bedrock types; igneous, metamorphic, and sedimentary groups, with exceptions being noted at the sites having materially HCO3−‐rich calcite, dolomite and volcanic ash. There were, however, exceptionally high flux sites whose fluxes could not be predicted by the empirical model when considering only temperature and precipitation controls. These sites belong to the mountainous part of the Japanese archipelago that is orogenically active and have in common steep hillslopes. Positive correlations were found between relief ratio, which was calculated by dividing elevation difference between highest and lowest points in the catchment by the distance between those two points, and fluxes in igneous and sedimentary groups including these high‐ HCO3− flux sites. These facts suggest that it is a fundamental requirement to consider the status of the soil forming when discussing the global variations in the buffering of watersheds. The differences in geomorphologic conditions controlling soil development are also just as important as the climatological effects.
Combining satellite data and biogeochemical models to estimate global effects of human‐induced land cover change on carbon emissions and primary productivity
Authors: DeFries R. S. , Field C. B. , Fung I. , Collatz G. J. , Bounoua L.
Published: September 2002

This study uses a global terrestrial carbon cycle model (the Carnegie‐Ames‐Stanford Approach (CASA) model), a satellite‐derived map of existing vegetation, and global maps of natural vegetation to estimate the effects of human‐induced land cover change on carbon emissions to the atmosphere and net primary production. We derived two maps approximating global land cover that would exist for current climate in the absence of human disturbance of the landscape, using a procedure that minimizes disagreements between maps of existing and natural vegetation that represent artifacts in the data. Similarly, we simulated monthly fields of the Normalized Difference Vegetation Index, required as input to CASA, for the undisturbed land cover case. Model results estimate total carbon losses from human‐induced land cover changes of 182 and 199 Pg for the two simulations, compared with an estimate of 124 Pg for total flux between 1850 and 1990 [Houghton, 1999], suggesting that land cover change prior to 1850 accounted for approximately one‐third of total carbon emissions from land use change. Estimates of global carbon loss from the two independent methods, the modeling approach used in this paper and the accounting approach of Houghton [1999], are comparable taking into account carbon losses from agricultural expansion prior to 1850 estimated at 48–57 Pg. However, estimates of regional carbon losses vary considerably, notably in temperate midlatitudes where our estimates indicate higher cumulative carbon loss. Overall, land cover changes reduced global annual net primary productivity (NPP) by approximately 5%, with large regional variations. High‐input agriculture in North America and Europe display higher annual NPP than the natural vegetation that would exist in the absence of cropland. However, NPP has been depleted in localized areas in South Asia and Africa by up to 90%. These results provide initial crude estimates, limited by the spatial resolution of the data sets used as input to the model and by the lack of information about transient changes in land cover. The results suggest that a modeling approach can be used to estimate spatially‐explicit effects of land cover change on biosphere‐atmosphere interactions.
Controls on the carbon isotopic composition of southern ocean phytoplankton
Authors: Popp Brian N. , Trull Tom , Kenig Fabien , Wakeham Stuart G. , Rust Terri M. , Tilbrook Bronte , Griffiths Brian , Wright Simon W. , Marchant Harvey J. , Bidigare Robert R. , Laws Edward A.
Published: September 2002

Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north‐south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica). Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq). This relationship does not hold along the PET transect. Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the δ13C of Southern Ocean suspended particulate organic matter. Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role. It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (EP) on the reciprocal of CO2(aq). An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply. Changes in microalgal growth rates appear to have a minor effect on EP. Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter. These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation.
Estimating historical changes in global land cover: Croplands from 1700 to 1992
Authors: Ramankutty Navin , Foley Jonathan A.
Published: September 2002

Human activities over the last three centuries have significantly transformed the Earth's environment, primarily through the conversion of natural ecosystems to agriculture. This study presents a simple approach to derive geographically explicit changes in global croplands from 1700 to 1992. By calibrating a remotely sensed land cover classification data set against cropland inventory data, we derived a global representation of permanent croplands in 1992, at 5 min spatial resolution [Ramankutty and Foley, 1998]. To reconstruct historical croplands, we first compile an extensive database of historical cropland inventory data, at the national and subnational level, from a variety of sources. Then we use our 1992 cropland data within a simple land cover change model, along with the historical inventory data, to reconstruct global 5 min resolution data on permanent cropland areas from 1992 back to 1700. The reconstructed changes in historical croplands are consistent with the history of human settlement and patterns of economic development. By overlaying our historical cropland data set over a newly derived potential vegetation data set, we analyze our results in terms of the extent to which different natural vegetation types have been converted for agriculture. We further examine the extent to which croplands have been abandoned in different parts of the world. Our data sets could be used within global climate models and global ecosystem models to understand the impacts of land cover change on climate and on the cycling of carbon and water. Such an analysis is a crucial aid to sharpen our thinking about a sustainable future.
A laboratory investigation of lower hybrid wave interactions with a field‐aligned density depletion
Authors: Rosenberg S. , Gekelman W.
Published: September 2002

Spatially localized bursts of intense lower hybrid wave radiation are often encountered in ionospheric observations. These electric field enhancements have been correlated with depletions in plasma density. This laboratory investigation was undertaken to provide the kind of insight into the nature of the interaction which is difficult to obtain from ionospheric observations alone. The experiments were performed in the LArge Plasma Device (LaPD) at UCLA. The dependence of the interaction on wave frequency, depletion shape and size and density gradient is investigated in three spatial dimensions and in time. This letter concentrates on the dependence of the phenomenon on the density gradient. The main result is that the lower hybrid waves are observed to have the largest electric field in the regions of steepest density gradient.
Shallow throughflow variability in the outflow Straits of Indonesia
Authors: Chong J. C. , Sprintall J. , Hautala S. , Morawitz W. L. , Bray N. A. , Pandoe W.
Published: September 2002

Since December 1995, the Indonesian throughflow has been monitored in five major passages as it flows from the Indonesian interior seas to the Indian Ocean. Pressure differences across the straits enable us to infer the geostrophic surface flow, and so provide the first simultaneous time‐series measurements of surface geostrophic flow through these passages. Intraseasonal signals (30'90 day) are a ubiquitous feature in the surface flow, and are consistent with wind‐forced Kelvin waves from both the eastern equatorial Indian Ocean and the south coast of Java. Using a relationship with surface velocity from three contemporaneous ADCP surveys, we approximate surface (0–100 m) volume transport fluctuations through four of the main exit passages. The amplitude of the total surface transport variation through these passages is fairly uniform, ranging from 10–15 Sv.
Global adjustment of the thermocline in response to deepwater formation
Authors: Huang Rui X. , Cane Mark A. , Naik Naomi , Goodman Paul
Published: September 2002

The global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta‐plane. The signal is carried from ocean to ocean by Kelvin waves, which travel equatorward along western boundaries, eastward across the equator, poleward at the eastern boundaries, and then eastward around the southern tip of continents into the next ocean basin. The interior is filled by Rossby waves emanating from eastern boundaries. Stronger (weaker) deepwater formation induces an upward (downward) motion of the main thermocline in the world oceans. The adjustment is completed on centennial time scales.
The Three Gorges Dam: Reducing the upwelling and thus productivity in the East China Sea
Authors: Chen Chen‐Tung Arthur
Published: September 2002

It has been well documented that since 1850 the amount of phosphorus as well as nitrogen input to coastal seas around the world has increased by a factor of ten, thus raising concern over eutrophication. Here, evidence is provided to show that despite a large riverine input of nutrients to the East China Sea (ECS) only a small fraction (7% for P and 33% for N) of the external nutrient supply supporting new production is provided by the total riverine input. The major nutrient supply in fact originates from the on‐shore advection of the subsurface Kuroshio waters. Whether the ECS will become eutrophicated, therefore, actually depends on the amount of phosphorus supply supported by on‐shore advection. This supply, though, will very likely be reduced after the completion of the Three Gorges Dam, and this will lead to a diminished productivity in the ECS.
Temperature variability within Makassar Strait
Authors: Ffield Amy , Vranes Kevin , Gordon Arnold L. , Dwi Susanto R. , Garzoli Silvia L.
Published: September 2002

Recent mooring observations of ocean temperature provide the first high resolution, long term record of temperature variability in the Makassar Strait of the Indonesian Seas. The mooring observations span the entire cycle of the strong 1997/1998 El Niño. A high correlation (r = 0.67) is found between variability in the average thermocline temperature, to variability in the southward Makassar volume transport: during high (low) volume transport, the average temperature of the thermocline is also high (low). In addition, from nearly 15 years of XBT data, the Makassar thermocline temperature is shown to be highly correlated (r = 0.77) to SOI. This reveals that the Makassar temperature field–when coupled with the throughflow–transmits the equatorial Pacific El Niño and La Niña temperature fluctuations into the Indian Ocean. The ENSO variability in the internal energy transport is calculated: 0.63 PW during the La Niña months of December 1996 through February 1997, and 0.39 PW during the El Niño months of December 1997 through February 1998.
Evidence of electron impact ionization in the magnetic pileup boundary of Mars
Authors: Crider D. , Cloutier P. , Law C. , Walker P. , Chen Y. , Acuña M. , Connerney J. , Mitchell D. , Lin R. , Anderson K. , Carlson C. , McFadden J. , Rème H. , Mazelle C. , d'Uston C. , Sauvaud J. , Vignes D. , Brain D. , Ness N.
Published: September 2002

A sharp decline in electron fluxes is observed in the Mars Global Surveyor Electron Reflectometer data in conjunction with the magnetic pileup boundary. We examine the characteristics of the evolution of the electron distribution function for one orbit. We determine that the spectra can best be explained by electron impact ionization of oxygen and hydrogen. To reproduce the observed spectral evolution, we construct a model of the effects of electron impact ionization on the electron distribution function as a flow element encounters the neutral atmosphere. Using the observed post‐shock electron distribution function, we are able to reproduce the observed flux attenuation. We conclude that electron impact ionization is the physical mechanism responsible for the spectral feature.
Bursty energetic oxygen events in the dayside magnetosheath: GEOTAIL observations
Authors: Zong Q. ‐G. , Wilken B.
Published: September 2002

Two energetic oxygen ion enhancements with a clear anti‐sunward flow were observed in the dayside magnetosheath when GEOTAIL was located at (4.3, 16.3, 0.7) RE on 26 January 1994. The oxygen ions peaked at energies of about 300 keV. The commencement of the two oxygen enhancements at 1405 and 1440 UT were in coincidence with two substorm injections at 1349 and 1422UT. The oxygen events are characterized by strong impulsive increases in the flux whereas energetic protons and helium ions were essentially absent. The magnetosheath field projection in the x‐y plane (GSE) was almost constant and pointed into the dusk/tail sector forming an angle of 135° to the Sun‐Earth axis. The field magnitude was 35 nT with rather little fluctuations and the Bz component was negative. These observations show that energetic oxygen ions can leak out of the magnetosphere into the magnetosheath during isolated substorm activity (storm conditions are not required). Enhanced solar wind dynamic pressure compressed the magnetopause inward and may have helped to observe energetic oxygen ions to leave from the magnetopause current sheet thus escaping into the magnetosheath.
Possible evidence of damped cavity mode oscillations stimulated by the January, 1997 magnetic cloud event
Authors: Goldstein J. , Hudson M. K. , Lotko W.
Published: September 2002

A sudden drastic change in the solar wind dynamic pressure is one proposed mechanism for the excitation of cavity mode resonances in the plasmasphere. To study the possibility of cavity mode occurrence following a 180 cm−3 solar wind density pulse on January 11, 1997, a numerical MHD simulation was run on a dipole grid. The simulated spectra recorded by a virtual satellite are compared with those produced from the Polar magnetic time signals during the interval 0228–0304 UT. In both sets of spectra, power is concentrated between 2 and 5 mHz; observed FLR‐like spectral peaks in the range 6–20 mHz are in rough agreement with simulated plasmaspheric FLRs between 7–21 mHz. In the simulation, the fundamental plasmaspheric cavity mode is at 3.5 mHz; phase analysis of observed 3.5 mHz electric and magnetic field signals from the interval 0230–0237 UT supports the possibility of a heavily damped standing wave at this frequency.
Interplanetary acceleration of coronal mass ejections
Authors: Gopalswamy N. , Lara A. , Lepping R. P. , Kaiser M. L. , Berdichevsky D. , St. Cyr O. C.
Published: September 2002

Using an observed relation between speeds of CMEs near the Sun and in the solar wind, we determine an “effective” acceleration acting on the CMEs. We found a linear relation between this effective acceleration and the initial speed of the CMEs. The acceleration is similar to that of the slow solar wind in magnitude. The average solar wind speed naturally divides CMEs into fast and slow ones. Based on the relation between the acceleration and initial speed, we derive an empirical model to predict the arrival of CMEs at 1 AU.