bluuebunny/crossref_metadata_2025 huggingface dataset:
Acidification, Buffering, and Salt Effects in the Unsaturated Zone of a Sandy Aquifer, Klosterhede, Denmark
Authors: Hansen Bent Kjær , Postma Dieke
Published: February 2004

Acidification of groundwater in a noncalcareous sandy aquifer at Klosterhede, Denmark, is the result of acid rain deposition. In the 4‐ to 5‐m‐thick unsaturated zone the pH ranges from 4.2 to 4.9 with Al concentrations of up to 0.8 mmol L−1. The groundwater at the top of the saturated zone still has a pH below 5. Deposition of sea salt affects the solute profiles, and its importance varies both spatially from the forest margin to the inner part of the forest and temporally through seasonal variations in infiltration and dry deposition. As a result, pulses of high solute concentrations travel downward through the unsaturated zone. The cation exchange capacity (CEC) of the sediments ranges between 0.2 and 1 meq 100 g−1, and in the acidified zone, base saturation is around 17%. The pore waters are close to equilibrium with gibbsite, supersaturated for kaolinite, and strongly undersaturated for other silicateminerals. Mass balance calculations on increases in dissolved silica over depth suggest that the buffering effect of silicate weathering is small. Buffering processes and solute transport were modeled with the code PHREEQM. Simulation of pre–acid rain weathering indicates that this process operates on a timescale of thousands of years, yielding minimum pH values near 5.2 and a base saturation of greater than 70%. The present leaching of Al3+rich acid water from the soil yields acidification rates of 7 and 10 cm yr−1for weathering of a naturally weathered and a pristine profile, respectively. Simulation of infiltration of sea‐salt pulses indicates that the cation distribution quickly becomes attenuated by the exchanger composition. However, due to coupling of gibbsite equilibrium with ion exchange processes, downward traveling pulses with high solute concentrations will cause pH variations throughout the unsaturated zone by precipitation and dissolution of gibbsite. Accordingly, the general acidification pattern at Klosterhede is overprinted by salts effects in a complicated fashion.
Groundwater–Surface Water Management With Stochastic Surface Water Supplies: A Simulation Optimization Approach
Authors: Reichard Eric G.
Published: February 2004

A new simulation optimization model for groundwater–surface water management identifies efficient strategies for meeting water demand and controlling a regional water quality problem, while accounting for varying availability of surface water supplies. Optimal values of model variables are computed for ranges of surface water availability. Expected groundwater levels are computed using a response function approach, which incorporates a probability for each hydraulic stress. Three objective functions are considered: (1) minimize need for supplemental water, (2) minimize imposed water use reductions, and (3) minimize changes from current pumping patterns. The model imposes constraints on expected groundwater levels, expected hydraulic gradients (surrogates for controlling contaminant transport), capacities of pipeline and recharge facilities, and water demand requirements. Capacity and demand constraints must be met every year for all possible surface water conditions. Application to the Santa Clara–Calleguas Basin in southern California indicates that a large quantity of supplemental water or significant pumpage reductions, particularly in the lower aquifer, may be required to control seawater intrusion with current facilities. Supplemental water would be delivered directly to users through pipelines and artificially recharged. Results also indicate that the current artificial‐recharge program has been valuable and that construction of new artificial‐recharge facilities might be beneficial. It should be noted that local agencies are currently developing several potential sources of supplemental water. Monte Carlo simulations yield estimates of the reliability of gradient constraints in controlling advective transport and the likelihood that water level constraints will be violated.
Slug tests in elastic, unconfined aquifers: Incorporating the free surface
Authors: Belitz Kenneth , Dripps Weston , Fairchild J. E.
Published: February 2004

In this paper we evaluate slug tests in shallow, fully screened wells that completely penetrate homogeneous, anisotropic, elastic, unconfined aquifers. If the free surface is incorporated into the initial boundary value problem, the dimensionless head (HD) is a function of five parameters: dimensionless time (tD), system geometry and anisotropy (η), elastic storativity (S), specific yield (Sy), and the size of the initial perturbation relative to the initial saturated thickness (λ). If the water table is assumed to be a constant head boundary, the dependence on Sy and λ is lost. Numerical solution of the problem indicates that the dimensionless response of the well (HD versus tD) is largely a function of λ, Sy, and η the response is less sensitive to S. Application of type curves derived for wells that are cased above a fixed‐head water table and screened beneath will result in an overestimate of transmissivity. The overestimate will be minimized if λ and Sy are small.
Characterization of alluvial hydrostratigraphy with indicator semivariograms
Authors: Johnson Nicholas M.
Published: February 2004

Current trends in hydrogeology seek to enlist sedimentary concepts in the interpretation of permeability structures. However, depositional models tend to account insufficiently for heterogeneities caused by complex controls on sediment preservation. Conversely, geostatistical methods empirically describe both random and structured attributes. This paper presents comparative indicator semivariograms for proximal, medial, and distal alluvial fan settings in Santa Clara Valley, California. Hydrostratigraphic structures revealed by a binary textural indicator are consistent within each setting yet differ from one setting to another. In the bedrock‐bounded proximal setting, buried channel deposits exhibit a small nugget effect and a zonal anisotropy with correlation ranges up to 900 m. In the distal setting, deposits display relatively weak hydrostratigraphic correlation with a relative nugget effect of >50% and a correlation range of 80 m. In the medial setting, the correlation range is similar to the proximal area but with a large nugget effect similar to the distal area.
Spatial Averaging of Water Content by Time Domain Reflectometry: Implications for Twin Rod Probes with and without Dielectric Coatings
Authors: Ferré P. A. , Rudolph D. L. , Kachanoski R. G.
Published: February 2004

The averaging of apparent relative dielectric permittivities by time domain reflectometry (TDR) is examined for properties varying along TDR waveguides and in the plane perpendicular to a TDR probe. A square root averaging model with uniform weighting factors describes the measured apparent relative dielectric permittivity for axially varying materials; variations in the transverse plane are described by an inverse averaging model with nonuniform spatial weighting. We define the sensitivity of a TDR probe as the change in the measured travel time for a given change in the average soil water content for rods of a fixed length. For rods with dielectric coatings, this sensitivity increases with the dielectric permittivity of the coating and the rod radius and decreases with the soil dielectric permittivity. Due to the dependence of the sensitivity on the soil water content, coated rods will not measure the actual average water content for conditions of axially variable water contents.
Examining the use of time domain reflectometry for measuring liquid water content in frozen soil
Authors: Spaans Egbert J. A. , Baker John M.
Published: February 2004

Time domain reflectometry (TDR) offers a unique opportunity to measure liquid water content θL in frozen soil, since the permittivity of ice is much lower than that of water. However, calibrations of TDR derived from drying unfrozen soil, where water is replaced by air, may not apply to a freezing soil, where water is replaced by ice, since the permittivity of ice is greater than that of air. We designed a gas dilatometer to calibrate TDR for θL in frozen soil. A soil sample is hermetically sealed in the gas dilatometer; subsequent soil freezing reduces total air space and hence increases pressure inside the gas dilatometer, since ice is less dense than water. The amounts of soil water frozen were computed from measured pressure change as temperature was incrementally decreased. TDR calibrations for two samples of the same soil at different total water contents had identical slopes but different intercepts, supporting our hypothesis that there exists no unique calibration of TDR for θL for frozen soil, but rather a family of calibration curves, each curve corresponding to a different total water content.
An Iterative Stochastic Inverse Method: Conditional Effective Transmissivity and Hydraulic Head Fields
Authors: Yeh T.‐C. Jim , Jin Minghui , Hanna Samuel
Published: February 2004

An iterative stochastic approach is developed to estimate transmissivity and head distributions in heterogeneous aquifers. This approach is similar to the classical cokriging technique; it uses a linear estimator that depends on the covariances of transmissivity and hydraulic head and their cross covariance. The linear estimator is, however, improved successively by solving the governing flow equation and by updating the covariances and cross‐covariance function of transmissivity and hydraulic head fields in an iterative manner. As a result the nonlinear relationship between transmissivity and head is incorporated in the estimation, and the estimated fields are approximate conditional means. The ability of the iterative approach is tested with some deterministic and stochastic inverse problems. The results show that the estimated transmissivity and hydraulic head fields have smaller mean square errors than those obtained by classical cokriging even in the aquifer with variance of transmissivity up to 3.
Infiltration and Solute Transport Experiments in Unsaturated Sand and Gravel, Cape Cod, Massachusetts: Experimental Design and Overview of Results
Authors: Rudolph David L. , Kachanoski R. Gary , Celia Michael A. , LeBlanc Denis R. , Stevens Jonathon H.
Published: February 2004

A series of infiltration and tracer experiments was conducted in unsaturated sand and gravel deposits on Cape Cod, Massachusetts. A network of 112 porous cup lysimeters and 168 time domain reflectometry (TDR) probes was deployed at depths from 0.25 to 2.0 m below ground surface along the centerline of a 2‐m by 10‐m test plot. The test plot was irrigated at rates ranging from 7.9 to 37.0 cm h−1 through a sprinkler system. Transient and steady state water content distributions were monitored with the TDR probes and spatial properties of water content distributions were determined from the TDR data. The spatial variance of the water content tended to increase as the average water content increased. In addition, estimated horizontal correlation length scales for water content were significantly smaller than those estimated by previous investigators for saturated hydraulic conductivity. Under steady state flow conditions at each irrigation rate, a sodium chloride solution was released as a tracer at ground surface and tracked with both the lysimeter and TDR networks. Transect‐averaged breakthrough curves at each monitoring depth were constructed both from solute concentrations measured in the water samples and flux concentrations inferred from the TDR measurements. Transport properties, including apparent solute velocities, dispersion coefficients, and total mass balances, were determined independently from both sets of breakthrough curves. The dispersion coefficients tended to increase with depth, reaching a constant value with the lysimeter data and appearing to increase continually with the TDR data. The variations with depth of the solute transport parameters, along with observations of water and solute mass balance and spatial distributions of water content, provide evidence of significant three‐dimensional flow during the irrigation experiments. The TDR methods are shown to efficiently provide dense spatial and temporal data sets for both flow and solute transport in unsaturated sediments with minimal sediment and flow field disturbance. Combined implementation of lysimeters and TDR probes can enhance data interpretation particularly when three‐dimensional flow conditions are anticipated.
Examination of Solute Transport in an Undisturbed Soil Column Using Electrical Resistance Tomography
Authors: Binley Andrew , Henry‐Poulter Siobhan , Shaw Ben
Published: February 2004

In an effort to determine the internal spatial characteristics of solute transport in naturally heterogeneous soils, recent applications of electrical resistance tomography to soil core tracing are reported. Using a high‐speed, biomedical type data acquisition system, migration of an electrolytic tracer in an undisturbed soil column was monitored. Quantitative images produced from the collected data display an interesting sequence of patterns of internal tracer breakthrough at cross sections of the soil column. Analysis of pixel breakthrough behavior reveals spatial variation of transport characteristics throughout the soil column.
Effect of Vegetation Density on the Parameterization of Scalar Roughness to Estimate Spatially Distributed Sensible Heat Fluxes
Authors: Qualls Russell J. , Brutsaert Wilfried
Published: February 2004

The First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) was initiated in part to explore methods to improve our ability to estimate spatially distributed heat fluxes over hilly prairie. In this research, spatially distributed, remotely sensed surface temperatures and surface‐based measurements of leaf area index, wind speed, and air temperature were used in a Monin‐Obukhov type similarity formulation to calculate sensible heat fluxes (Hc). Use of Monin‐Obukhov type similarity required that a parameterization for scalar roughness of sensible heat be developed. Several methods were examined including a linearized version, with respect to leaf area index, of an earlier theoretical expression of scalar roughness for grass‐like vegetation. Comparison between measured and parameterized scalar roughness values gave a correlation of r=0.828. This parameterization was used with data measured on July 11, 1987, under conditions of strong solar heating, high winds, and fairly uniform soil moisture, to calculate Hc values at an array of 10 surface flux stations. A spatial comparison between Hc and measured sensible heat fluxes yielded good agreement with a correlation coefficient of 0.878 and a root‐mean‐square error of 31.1 Wm−2.
Organic Solute Uptake from Aqueous Solutions by Soil: A New Diffusion Model
Authors: Yiacoumi Sotira , Rao Anupa V.
Published: February 2004

A new diffusion model is proposed to describe the uptake of organic solutes from aqueous solutions by soil. The model considers that the uptake process is controlled by the pore diffusion of solutes within the soil pellets, the adsorption of solutes by the mineral fraction of soil, the surface diffusion of solutes along the mineral fraction surfaces, the partition of solutes into the soil organic matter, and the diffusion of solutes within the soil organic matter. An expression, in the Laplace domain, of the solute uptake by a soil pellet is obtained and with appropriate macroscopic conservation equations and standard numerical Laplace transform inversion techniques can be used to describe sorption and desorption as well as transport and distribution of organic solutes under a variety of conditions. On the basis of the uptake expression, a number of calculations are carried out with the objective to identify under which conditions the effect of diffusion into the soil organic matter, as well as the effects of dispersion and mineral adsorption, becomes important. Some comparisons of model predictions with experimental results are also provided to demonstrate the application of the model.
Reactive Solute Transport in a Single Fracture
Authors: Berkowitz Brian , Zhou Jieying
Published: February 2004

The transport of reactive and nonreactive solutes under the action of fully developed laminar flow in a parallel plate fracture has been analyzed. Three types of surface reactions are examined: (1) irreversible first‐order kinetic, (2) instantaneous reversible, and (3) reversible first‐order kinetic. Approximate analytical models for the dispersion of nonreactive and reactive solutes with an irreversible first‐order reaction are developed, and numerical “exact” solutions for the three surface reaction types are obtained. The conditions under which the approximate solutions hold are determined by comparison with the numerical solutions. The solute migration patterns and the extent of retardation due to surface reactions are then analyzed and compared with the nonreactive case. Dimensionless reaction coefficients incorporated into the solutions are estimated from data available in the literature. It is found that the approximate solutions are consistent with the numerical solutions; that the criterion for applicability of a local equilibrium assumption is based on the values of dimensionless time and a dimensionless number representing the ratio of the rate at which the surface reaction reaches equilibrium to the rate of the molecular diffusion in the transverse direction; and that the effect of surface reaction can be described by a retardation factor Rf = 1 + Kd* for the case in which the reaction can be assumed at equilibrium, with small Peclet number,small dimensionless distribution coefficient Kd*, and large dimensionless time. Conditions are also found under which reactive solute transport can be well approximated by nonreactive solute transport and reversible reactions can be treated as irreversible. These results and the dimensionless analysis method employed herein may serve as a basis for development of more complex models of reactive solute transport in saturated fractured media.
Theory of Dipole Flow in Uniform Anisotropic Aquifers
Authors: Zlotnik Vitaly , Ledder Glenn
Published: February 2004

A theory of dipole flow is developed to model flow induced by a vertical circulation well consisting of injection and extraction chambers in a single borehole. Included in the theory are an analytical description of the kinematic flow structure around a vertical circulation well and the drawdown in the well chambers. Using Stokes' stream function, simple criteria are derived to determine the region of intensive recirculation. This region extends (from the dipole center) approximately five distances between chamber centers in the radial direction and two distances between chamber centers in both vertical directions. The vertical scale does not depend on anisotropy of hydraulic conductivity, and the radial scale is corrected for anisotropic aquifers. Based on these estimates, criteria are given for the selection of the appropriate aquifer model to employ in five settings, including infinite, semi‐infinite confined, semi‐infinite unconfined, finite confined, and finite unconfined aquifers. Applications of dipole flow theory are given for analytical estimation of the capture zone for recirculation wells and for simultaneous measurement of horizontal and vertical hydraulic conductivity in uniform anisotropic aquifers using steady state measurements of drawdown in the well chambers.
The effect of temperature change of the warm surface waters of the oceans on atmospheric CO2
Authors: Bacastow Robert B.
Published: February 2004

It is widely believed that the concentration of atmospheric CO2 is largely controlled by the high‐latitude oceans, because these waters can be viewed as an outcropping of oceanic deep water. If this tenet is correct, cooling of the low‐latitude surface oceans could contribute only marginally to an explanation of the observed decrease in atmospheric CO2 concentration of about 80 ppm during glacial periods. This tenet is examined with three‐dimensional ocean chemical models based on the circulation fields from 10‐level and 15‐level ocean general circulation models developed at the Max‐Planck‐Institut für Meteorologie, Germany. The three‐dimensional models predict atmospheric CO2 changes that are much larger than those predicted by outcrop models, thereby casting doubt on the use of outcrop models to model atmospheric CO2 changes. The atmospheric response time to a change in sea surface temperature is about 120 years in the 15‐level model, the more realistic of the two three‐dimensional models. This response is fast enough, and the predicted amplitude is of the right size, for one to explain the CO2 increase associated with the Dansgaard‐Oeschger warming events observed in ice core measurements as the effect of a warming of the surface oceans.
Climate and nitrogen controls on the geography and timescales of terrestrial biogeochemical cycling
Authors: Schimel D. S. , Braswell B. H. , McKeown R. , Ojima D. S. , Parton W. J. , Pulliam W.
Published: February 2004

We used the terrestrial ecosystem model “Century” to evaluate the relative roles of water and nitrogen limitation of net primary productivity, spatially and in response to climate variability. Within ecology, there has been considerable confusion and controversy over the large‐scale significance of limitation of net primary production (NPP) by nutrients versus biophysical quantities (e.g., heat, water, and sunlight) with considerable evidence supporting both views. The Century model, run to a quasi‐steady state condition, predicts “equilibration” of water with nutrient limitation, because carbon fixation and nitrogen fluxes (inputs and losses) are controlled by water fluxes, and the capture of nitrogen into organic matter is governed by carbon fixation. Patterns in the coupled water, nitrogen, and carbon cycles are modified substantially by ecosystem type or species‐specific controls over resource use efficiency (water and nitrogen used per unit NPP), detrital chemistry, and soil water holding capacity. We also examined the coupling between water and nutrients during several temperature perturbation experiments. Model experiments forced by satellite‐observed temperatures suggest that climate anomalies can result in significant changes to terrestrial carbon dynamics. The cooling associated with the Mount Pinatubo eruption aerosol injection may have transiently increased terrestrial carbon storage. However, because processes in the water, carbon, and nitrogen cycles have different response times, model behavior during the return to steady state following perturbation was complex and extended for decades after 1‐ to 5‐year perturbations. Thus consequences of climate anomalies are influenced by the climatic conditions of the preceding years, and climate‐carbon correlations may not be simple to interpret.
Storage of terrestrial carbon in boreal lake sediments and evasion to the atmosphere
Authors: Molot Lewis A. , Dillon Peter J.
Published: February 2004

Carbon mass balance studies of 20 small, forested catchments and seven lakes on the Precambrian Shield in central Ontario during a 12‐year period have shown that most carbon in the study lakes is derived from terrestrial sources, primarily peatlands, and that carbon retained by lakes (total inputs less loss via outflow from the lake) is partitioned between lake sediments and the atmosphere. The partitioning of retained carbon is a function of lake alkalinity: the ratio of evaded/sediment carbon increases with decreasing alkalinity. These carbon flux relationships were applied to the global boreal forest biome to evaluate the role of aquatic systems with respect to carbon fluxes and pools within the biome. We calculate that approximately 66 Tg of organic and inorganic carbon are exported annually from the boreal forest biome to adjacent surface waters of which 14 to 36 Tg reach coastal waters. The remainder is either evaded to the atmosphere (12 to 21 Tg yr−1) or stored in lake sediments (18 to 31 Tg yr−1). Approximately 120 Pg of carbon may be stored in boreal lake sediments, a conservative estimate based on an accumulation period of 5,000 years and a size comparable to recent boreal pool estimates of 419 Pg for peatlands and 64 Pg for plant biomass. Hence the amount of total carbon stored in the boreal forest biome may be significantly larger because of storage in lake sediments.
Biomass of termites and their emissions of methane and carbon dioxide: A global database
Authors: Sanderson M. G.
Published: February 2004

A global database describing the geographical distribution of the biomass of termites and their emissions of methane and carbon dioxide has been constructed. Termite biomasses were assigned to various ecosystems using published measurements and a recent high‐resolution (10′ × 10′) database of vegetation categories. The assigned biomasses were then combined with literature measurements of fluxes of methane and carbon dioxide from termites and extrapolated to give global emission estimates for each gas. The global emissions of methane and carbon dioxide are 19.7 ± 1.5 and 3500 ± 700 Mt yr−1, respectively (1 Mt = 1012 g). These emissions contribute approximately 4% and 2%, respectively, to the total global fluxes of these gases. This database gives an accurate distribution of the biomasses and gaseous emissions by termites and may be incorporated into global models of the atmosphere.
BIOME3: An equilibrium terrestrial biosphere model based on ecophysiological constraints, resource availability, and competition among plant functional types
Authors: Haxeltine Alex , Prentice I. Colin
Published: February 2004

The equilibrium terrestrial biosphere model BIOME3 simulates vegetation distribution and biogeochemistry, and couples vegetation distribution directly to biogeochemistry. Model inputs consist of latitude, soil texture class, and monthly climate (temperature, precipitation, and sunshine) data on a 0.5° grid. Ecophysiological constraints determine which plant functional types (PFTs) may potentially occur. A coupled carbon and water flux model is then used to calculate, for each PFT, the leaf area index (LAI) that maximizes net primary production (NPP), subject to the constraint that NPP must be sufficient to maintain this LAI. Competition between PFTs is simulated by using the optimal NPP of each PFT as an index of competitiveness, with additional rules to approximate the dynamic equilibrium between natural disturbance and succession driven by light competition. Model output consists of a quantitative vegetation state description in terms of the dominant PFT, secondary PFTs present, and the total LAI and NPP for the ecosystem. Canopy conductance is treated as a function of the calculated optimal photosynthetic rate and water stress. Regional evapotranspiration is calculated as a function of canopy conductance, equilibrium evapotranspiration rate, and soil moisture using a simple planetary boundary layer parameterization. This scheme results in a two‐way coupling of the carbon and water fluxes through canopy conductance, allowing simulation of the response of photosynthesis, stomatal conductance, and leaf area to environmental factors including atmospheric CO2. Comparison with the mapped distribution of global vegetation shows that the model successfully reproduces the broad‐scale patterns in potential natural vegetation distribution. Comparison with NPP measurements, and with an FPAR (fractional absorbed photosynthetically active radiation) climatology based on remotely sensed greenness measurements, provides further checks on the model's internal logic. The model is envisaged as a tool for integrated analysis of the impacts of changes in climate and CO2 on ecosystem structure and function.
Evidence for an additional source of atmospheric N2O
Authors: McElroy Michael B. , Jones Dylan B. A.
Published: February 2004

Kim and Craig [1993] have shown that N2O in the stratosphere is enriched in both 18O and 15N relative to tropospheric N2O. It is difficult to account for this result in light of the measurements of Selwyn and Johnston [1981] and Johnston et al. [1995] indicating that isotopic fractionation associated with the known stratospheric sinks, photolysis and reaction with O(1D), is minimal. It is suggested that the puzzle could be resolved by invoking an additional source of isotopically heavy N2O formed perhaps by reaction of N2 with CO3 produced by recombination of O(1D) and CO2. This source would require a significant reduction in estimates for net, mainly biological, emission of N2O at the surface.
CH4 and N2O fluxes in the Colorado shortgrass steppe: 2. Long‐term impact of land use change
Authors: Mosier A. R. , Parton W. J. , Valentine D. W. , Ojima D. S. , Schimel D. S. , Heinemeyer O.
Published: February 2004

As part of a weekly, year‐round program to measure the soil‐atmosphere exchange of nitrous oxide (N2O) and methane (CH4) in a shortgrass steppe, we examined the impact of land use change on these fluxes from 1992 through 1995. We found that conversion of grassland to croplands typically decreased the soil consumption of atmospheric CH4 and increased the emission of N2O. Mean annual CH4 consumption and N2O efflux over 3 years in native grasslands were 35 µg C m−2 hr−1 and 1.9 µg N m−2 hr−1, respectively. Immediately after tilling a native grassland site, CH4 consumption decreased by about 35% and remained at these lower rates for the next 3 years. Although N2O fluxes were about 8 times higher for 18 months following plowing, the relative rates declined to 25–50% higher than the native site after 3 years. Grasslands converted to a winter wheat‐fallow production system about 70 years ago consumed about 25% less CH4 than a newly plowed site, while N2O emissions 2 years after plowing were similar to the wheat fields. During the fallow periods when soils were typically wetter and mineralized N accumulated, CH4 uptake rates were lower and N2O emissions were higher than the correspondingly active wheat fields. A wheat field that was reverted back to grassland in 1987 through the conservation reserve program (CRP) continued to exhibit annual CH4 uptake and N2O emission rates similar to the wheat fields. Winter N2O emissions were, however, much higher in the CRP because of greater snow accumulation and winter denitrification events. Another wheat field that was returned to grassland in 1939 exhibited the same CH4 and N2O flux rates as comparable native pastures.
An integrated modeling approach to global carbon and nitrogen cycles: Balancing their budgets
Authors: den Elzen Michel G. J. , Beusen Arthur H. W. , Rotmans Jan
Published: February 2004

A modeling attempt is presented to describe the global carbon and nitrogen cycle: where it originates, where it stays, and how the global carbon and nitrogen budgets can be balanced. We used integrated but simple box models: a coupled global carbon‐nitrogen cycle and climate model. The global carbon‐nitrogen cycle model describes on a global scale the exchange fluxes between the compartments and their main internal processes and different biogeochemical feedbacks (e.g., for CO2 and N fertilization and for temperature). The coupled model also includes an energy balance climate model, using as input the induced radiative forcing of greenhouse gases and sulfate aerosols. Simulation experiments performed with the coupled model showed that by incorporating only the CO2 fertilization and temperature‐related feedbacks a balanced past carbon budget could not be created. Therefore the N fertilization of the anthropogenic nitrogen depositions for temperate forests, which depends on the fate of the anthropogenic nitrogen inputs, must be included. By varying the N fluxes by river transport, and deposition and denitrification, a balanced past nitrogen budget was obtained. This resulted in an anthropogenic deposition of 40 Tg N yr−1 for the temperate forests and a N fertilization which constitutes 20–50% of the missing carbon sink. Finally, the future fate of anthropogenic emissions of the C and mobilization of the N compounds were projected given the condition of balanced carbon and nitrogen budgets.
Paleomagnetic field intensity recorded in submarine basaltic glass from the East Pacific Rise, the last 69 KA
Authors: Mejia V. , Opdyke N. D. , Perfit M. R.
Published: February 2004

Paleointensity determinations were obtained from basaltic glass recovered from the northern East Pacific Rise using the Thelliers' (1959) method as applied by Pick and Tauxe (1993a). The samples are up to a69 Ka old based on their distance from the ridge axis and U‐series disequilibria measurements. The results show a general increase in paleointensity of the field from 69 Ka to the present with values ranging from 50% higher than the present field to as low as 26% of the present dipole field. The average dipole moment is 8.32 × 1022 Am² which is similar to the present axial dipole moment. The results are in general agreement with other absolute and relative paleointensity records.
Seismic reflections from the near‐vertical San Andreas Fault
Authors: Hole J. A. , Thybo H. , Klemperer S. L.
Published: February 2004

Major near‐vertical strike‐slip faults should reflect seismic energy due to velocity contrasts within and across the fault zone, analogous to sub‐horizontal faults commonly imaged by both multichannel and wide‐angle reflection surveys. An increase of velocity with depth in the upper crust allows such reflections to be detected in surface seismic surveys. We have observed fault reflection events from both sides of the San Andreas Fault in long‐offset common‐receiver gathers acquired near San Francisco. The events can be traced to the known surface position of the fault and provide constraints on the subsurface position to 3–7 km depth. Such reflections can be used to find the geometry of major near‐vertical faults, even in areas without active seismicity, providing information crucial to the understanding of the tectonics and seismic hazards of strike‐slip faulting.
ATMOS measurements of H2O+2CH4 and total reactive nitrogen in the November 1994 Antarctic stratosphere: Dehydration and denitrification in the vortex
Authors: Rinsland C. P. , Gunson M. R. , Salawitch R. J. , Newchurch M. J. , Zander R. , Abbas M. M. , Abrams M. C. , Manney G. L. , Michelsen H. A. , Chang A. Y. , Goldman A.
Published: February 2004

Simultaneous stratospheric volume mixing ratios (VMRs) measured inside and outside the Antarctic vortex by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument in November 1994 reveal previously unobserved features in the distributions of total reactive nitrogen (NOy) and total hydrogen (H2O+2CH4). Maximum removal of NOydue to sedimentation of polar stratospheric clouds (PSCs) inside the vortex occurred at a potential temperature (Θ) of 500–525 K (∼20 km), where values were 5 times smaller than measurements outside. Maximum loss of H2O+2CH4due to PSCs occurred in the vortex at 425–450 K, ∼3 km lower than the peak NOyloss. At that level, H2O+2CH4 VMRs inside the vortex were ∼70% of corresponding values outside. The Antarctic and April 1993 Arctic measurements by ATMOS show no significant differences in H2O+2CH4 VMRs outside the vortices in the two hemispheres. Elevated NOy VMRs were measured inside the vortex near 700 K. Recent model calculations indicate that this feature results from downward transport of elevated NOy produced in the thermosphere and mesosphere.
An independent check of VAN's criteria for signal recognition
Authors: Nagao T. , Uyeshima M. , Uyeda S.
Published: February 2004

In order to examine the generality of VAN's stated criteria for recognition of seismic electric signals (SES), an independent check of the original VAN geoelectric potential records has been made. (1) 338 anomalous changes were identified in three month of records (July 19–Oct. 18, 1988) of the Ioannina (IOA) station. After applying VAN's criteria, only 8 changes survived as possible SES. 7 of them coincided with anomalies used by the VAN group to issue successful predictions during that period. (2) A similar check of the IOA data was conducted for the 28.5 month period from April 30, 1988 to Sept. 15, 1990. 41 changes were selected as possible SES out of more than 3,000 anomalous changes. During this period, the VAN group issued 18 predictions. 14 of the SES used by the VAN group were identical to the SES we selected, while 4 were judged as noise. SESs common to both sets were generally stronger ones.
Results of 2‐ and 3‐D High‐Resolution seismic reflection surveying of surficial sediments
Authors: Lanz Eva , Pugin André , Green Alan , Horstmeyer Heinrich
Published: February 2004

To investigate the potential of seismic reflection methods for studying surficial sediments, 2‐ and 3‐D high‐resolution surveys have been conducted across a plot of land adjacent to a landfill in northern Switzerland. Results of the reflection profiling demonstrated that closely spaced common‐reflection points (≤ 2.5 m) and high‐fold (≥ 12) data were prerequisites for producing clear images at this site. For the 3‐D survey, an area of 115×160 m² was covered with common‐reflection points every 2.5×2.5 m². Three‐dimensional migration of the resultant 18‐ to 70‐fold data proved to be a critical step in correctly locating out‐of‐plane reflections and collapsing diffractions that were prominent features in the 2‐D profile data. Within the 3‐D data volume a conspicuous reflective zone and a marked decrease in reflection amplitudes at ∼200 m depth delineates the bedrock surface. Immediately above this level are continuous reflections from flat‐lying lacustrine sediments. These are in turn overlain by regions of strong reflected energy interpreted as either large erratic blocks, anomalous compaction zones, or gravel channels/lenses.
Retrieval of aerosol optical thickness over land using the ATSR‐2 Dual‐Look Satellite Radiometer
Authors: Flowerdew Roland J. , Haigh Joanna D.
Published: February 2004

We present a new method of retrieving aerosol optical thickness over land that takes advantage of the dual look capability of the ATSR‐2 instrument. Unlike some methods, the approach does not require dense dark vegetation to be located, nor is it dependent upon invariant targets, nor does it assume that the underlying surface reflectance is purely Lambertian. It makes use of a new surface approximation that exploits the property of the surface which is least variable with wavelength to allow atmospheric parameters to be retrieved and, consequently, is applicable over virtually any land surface. The method is tested using simulated ATSR‐2 measurements based upon a radiative transfer model coupled with a surface bidirectional model that includes the hot‐spot effect, and is shown to be robust to the effects of differing underlying canopy types. At high optical depths, the retrieval process achieves a mean error in optical thickness of less than 2.5%, depending on surface type, with a maximum absolute error (for the same data sets) of 5% while at low optical depths errors are of the order of 5%. The effects of systematic error in the simulated radiances are also shown. The method may be readily extended to other multiple look satellite radiometers.
Airborne gas chromatograph for in situ measurements of long‐lived species in the upper troposphere and lower stratosphere
Authors: Elkins J. W. , Fahey D. W. , Gilligan J. M. , Dutton G. S. , Baring T. J. , Volk C. M. , Dunn R. E. , Myers R. C. , Montzka S. A. , Wamsley P. R. , Hayden A. H. , Butler J. H. , Thompson T. M. , Swanson T. H. , Dlugokencky E. J. , Novelli P. C. , Hurst D. F. , Lobert J. M. , Ciciora S. J. , McLaughlin R. J. , Thompson T. L. , Winkler R. H. , Fraser P. J. , Steele L. P. , Lucarelli M. P.
Published: February 2004

A new instrument, the Airborne Chromatograph for Atmospheric Trace Species IV (ACATS‐IV), for measuring long‐lived species in the upper troposphere and lower stratosphere is described. Using an advanced approach to gas chromatography and electron capture detection, the instrument can detect low levels of CFC‐11 (CCl3F), CFC‐12 (CCl2F2), CFC‐113 (CCl2F‐CClF2), methyl chloroform (CH3CCl3), carbon tetrachloride (CCl4), nitrous oxide (N2O), sulfur hexafluoride (SF6), Halon‐1211 (CBrClF2), hydrogen (H2), and methane (CH4) acquired in ambient samples every 180 or 360 s. The instrument operates fully‐automated onboard the NASA ER‐2 high‐altitude aircraft on flights lasting up to 8 hours or more in duration. Recent measurements include 24 successful flights covering a broad latitude range (70°S–61°N) during the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign in 1994.
An estimate of the surface shortwave cloud forcing over the western Pacific during TOGA COARE
Authors: Waliser Duane E. , Collins William D. , Anderson Steven P.
Published: February 2004

Estimates of the shortwave cloud forcing at the surface have been computed for the TOGA COARE Intensive Observation Period (IOP) using the IMET buoy surface insolation measurements. Two different methods have been employed to calculate the clear‐sky surface insolation. The first is based on an empirical approximation using the buoy observations. The second incorporates both modeling and empirical procedures. The net surface shortwave cloud forcing values derived from these two methods are −103 and −107 Wm−2;, respectively. These values indicate that for the COARE IOP, the surface cooling effect from shortwave cloud forcing is comparable to that from latent heat flux (−108.5 Wm−2). Comparing IOP values of OLR and MSU rain rate to their climatological values indicates that the mean cloudiness during the IOP corresponds closely to climatology, suggesting that these estimates of shortwave cloud forcing may be fairly representative of the climatological value.
Relativistic electron beam propagation in the Earth's atmosphere: Modeling results
Authors: Neubert T. , Gilchrist B. , Wilderman S. , Habash L. , Wang H. J.
Published: February 2004

Linear accelerators (linacs), capable of producing 5 MeV energy electron beams at 80mA currents, are now down to a size that allow them to be flown on sounding rockets or balloons. This opens up new opportunities for atmospheric/ionospheric modification experiments where the mesosphere and thermosphere regions of the atmosphere can be perturbed down to 40 km altitude. In this paper beam propagation and atmospheric perturbation effects are studied by Monte Carlo simulations and by analytical means. It is shown that the earth' magnetic field severely limits the radial expansion of the beam otherwise induced by electron‐neutral collisions. It is also shown that the so‐called “envelope‐equations” from high‐energy laboratory physics adequately describe beam propagation in the upper atmosphere. The plasma density and electric conductivity modifications to the atmosphere are calculated from the Monte Carlo simulations. Inside the beam the conductivity in the 40–50 km altitude region is enhanced more than one order of magnitude by a 10 µs‐duration pulse. Some ideas for future scientific investigations are discussed, including the generation of electrical discharges by beams injected over thunderstorm regions.
Chemical and isotopic variations in throughfall in a Mediterranean Context
Authors: Pichon Agneès , Travi Yves , Marc Vincent
Published: February 2004

The chemical and isotopic influence of interception is obtained by comparing the composition of incident rain, throughfall and stemflow, at the plot scale in the Réal Collobrier catchment (southern France). Measurements made from March to Nov. 1993, on 450 samples, show that enrichment ratios vary significantly from storm to storm and from one chemical to the other. However, the mode ratio (NO3− and K+ excepted) remains around 2.5 or 3.5 and appears representative of Mediterranean climate. The Na‐Cl relationship in incident rain preserved in throughfall, and the specific evolution of K+ and NO3− under vegetation are of practical hydrological interest. The observed isotopic changes range from −0.5 to +1.4‰ and are in good agreement with theo‐retical values calculated in other mediterranean contexts. The enrichment by isotopic fractionation due to evaporation can be enhanced or reduced by an active rain‐selection canopy process.
Prediction of geomagnetic storms from solar wind data using Elman Recurrent Neural Networks
Authors: Wu Jian‐Guo , Lundstedt Henrik
Published: February 2004

In order to accurately predict geomagnetic storms, we exploit Elman recurrent neural networks to predict the Dst index one hour in advance only from solar wind data. The input parameters are the interplanetary magnetic field z‐component Bz (GSM), the solar wind plasma number density n and the solar wind velocity V. The solar wind data and the geomagnetic index Dst are selected from observations during the period 1963 to 1987, covering 8620h and containing 97 storms and 10 quiet periods. These data are grouped into three data sets; a training set 4877h, a validation set 1978h and a test set 1765h. It is found that different strengths of the geomagnetic storms are accurately predicted, and so are all phases of the storms. As an average for the out‐of‐sample performance, the correlation coefficient between the predicted and the observed Dst is 0.91. The predicted average relative variance is 0.17, i.e. 83 percent of the observed Dst variance is predictable by the solar wind. The predicted root‐mean‐square error is 16 nT.
Molecular dynamics simulation of structures, bulk moduli, and volume thermal expansivities of silicate liquids in the system CaO‐MgO‐Al2O3‐SiO2
Authors: Matsui Masanori
Published: February 2004

Materials in the system CaO‐MgO‐Al2O3‐SiO2 are important constituents of the Earth's lower crust and mantle. Silicate liquids in this geophysically important system have been studied using molecular dynamics(MD) simulation with an empirical interatomic potential (CMAS94). MD simulations are quite satisfactory in reproducing well the observed structure and pressure‐volume‐temperature equation‐of‐state parameters of molten enstatite (MgSiO3), wollastonite (CaSiO3), diopside (CaMgSi2O6), and anorthite (CaAl2Si2O8) at 1900 K and 0 GPa. However, the MD simulated bulk modulus of molten forsterite (Mg2SiO4) at 2300 K and 0 GPa, K0=18.0(6) GPa, is found to be much smaller than the value, K0=∼60 GPa at similar temperature and pressure conditions, estimated previously based on melting curve analyses of forsterite. In an attempt to investigate the possible occurrence of the density inversion between magmatic liquids and residual crystals in the upper mantle conditions, as proposed by Stolper et al. [1981], we have further applied the MD technique with the CMAS94 potential to the diopside system at 1900 K as an example, and have found that the density inversion between crystal and liquid in this system actually occurs at approximately 11 GPa.
A 30,000 year record of extreme floods in tropical Australia from relict plunge‐pool deposits: Implications for future climate change
Authors: Nott Jonathan F. , Price David M. , Bryant Edward A.
Published: February 2004

Relict plunge‐pool sedimentary sequences provide much longer paleoflood records than normally provided by slackwater deposits and other previously reported paleoflood sedimentary signatures. The only two relict plunge‐pool sedimentary sequences so far reported lie 300 km apart in tropical northern Australia and provide a record of extreme floods for the last 30,000 years. Each of these sequences identify the early to mid‐Holocene and the period immediately prior to the Last Glacial Maximum as the two periods of greatest flood magnitudes of the late Quaternary. Flood discharges at these times were up to five times greater than any floods experienced over the last 4,000 years. Most climate models predict that the magnitude and frequency of storms and rainfall events will increase under a warmer ‘greenhouse’ climate. The plunge‐pool sedimentary sequences, however, show that periods of greatly enhanced discharge compared to present can be associated with both warm, wet and cool, wet phases of climate.