bluuebunny/crossref_metadata_2025 huggingface dataset:
High Rayleigh number compressible convection in Venus' atmosphere: Penetration, entrainment, and turbulence
Authors: Baker R. David , Schubert Gerald , Jones Philip W.
Published: September 2002

Numerical simulations of two‐dimensional, nonlinear fully compressible convection at the subsolar point in the clouds of Venus are presented. One moderate Rayleigh number case (Raq= 6.8 × 106) and two high Rayleigh number (Raq= 1.1 × 109) cases are considered. Cloud‐level convection is characterized by cold, narrow downwellings that deeply penetrate the underlying stable layer and that entrain warmer air from the overlying stable layer. The convection layer depth, as determined from the horizontally averaged static stability, spans 9 km (47–56 km altitude) and 14 km (46–60 km altitude) for the moderate Rayleigh number and high Rayleigh number simulations, respectively. In the high Rayleigh number cases, convective penetration extends over a scale height from the bottom of the convection layer down to 38 km altitude. Strong convective entrainment completely erodes the overlying stable layer in the highRaqcases and incorporates it into the convection layer. The timescale for entrainment of the overlying stable layer is roughly 1 day and may explain why cellular features are predominantly found near and downwind of the subsolar point. The high Rayleigh number simulations are convectively turbulent and exhibit a spectral energy cascade ofk−3, wherekis the horizontal wavenumber. Our results suggest that cellular features in the subsolar region are observed at the cloud tops because convection may extend to much higher altitudes there. Downward penetrative convection may also be responsible for turbulence observed in Venus' atmosphere at 45 km altitude.
On the possibility of coherent cyclotron emission from extrasolar planets
Authors: Farrell W. M. , Desch M. D. , Zarka P.
Published: September 2002

A model of the coherent cyclotron emission from extrasolar planets is presented. Scaling laws known to operate in our solar system (including scaling laws of planetary magnetic fields and the radiometric Bode's law of radio power generation) are applied to the extrasolar systems. We consider the possibility that each of the extrasolar planets possesses a substantial planetary magnetic field which is in quasi‐continuous interaction with the local stellar wind. Cyclotron emission from extrasolar planets is then driven by the stellar wind/magnetospheric interaction, much like the coherent cyclotron radio emission processes associated with planets in our solar system. Based on the model results, the best candidate for solar‐wind‐driven cyclotron emission is Tau Bootes, with an expected median amplitude of about 2 janskys (1 Jy = 10−26 W m−2 Hz−1) at 28 MHz, an intensity level of about a factor of 100 below the current limit of detectability. However, variations in the local stellar medium could conceivably increase power levels by a factor of 100 for short periods of time. Like the solar planets, the extrasolar planets should radiate episodically, with emission reoccurring at the planetary rotation period. Thus spectral integration techniques could also be applied to improve the likelihood of detectability.
Changes in Meridional Temperature and Salinity Gradients in the North Atlantic Ocean (30°–72°N) during the Last Interglacial Period
Authors: Cortijo Elsa , Lehman Scott , Keigwin Lloyd , Chapman Mark , Paillard Didier , Labeyrie Laurent
Published: September 2002

Eight deep‐sea sediment cores from the North Atlantic Ocean ranging from 31° to 72°N are studied to reconstruct the meridional gradients in surface hydrographic conditions during the interval of minimum ice volume within the last interglacial period. Using benthic foraminiferal δ18O measurements and estimates of Sea Surface Temperature (SST) and Sea Surface Salinity (SSS), we show that summer SSTs and SSSs decreased gradually during the interval of minimum ice volume at high‐latitude sites (52°–72°N) whereas they were stable or increased during the same time period at low‐latitude sites (31°–41°N). This increase in meridional gradients of SSTs and SSSs may have been due to changes in the latitudinal distribution of summer and annual‐average insolation and associated oceanic and atmospheric feedbacks. These trends documented for the Eemian ice volume minimum period are similar to corresponding changes observed during the Holocene and may have had a similar origin.
Diffuse spectral reflectance as a proxy for percent carbonate content in North Atlantic sediments
Authors: Ortiz Joseph , Mix Alan , Harris Sara , O'Connell Suzanne
Published: September 2002

Diffuse reflectance records from Feni Drift in the North Atlantic faithfully record sediment percent carbonate. A high‐resolution, reflectance‐based age model for these sediments derived from an orbitally tuned age model for western equatorial Atlantic, Ceara Rise sediments was generated by spectral frequency mapping. Power spectra of the Feni Drift record indicate statistically significant sub‐Milankovitch cyclicity at 7.6–8.4 and 4.8–6.1 kyr. We infer that these ∼8 and ∼5 kyr cycles document a linkage between North and equatorial Atlantic climate given our ability to correlate these records. These climate cycles influence Atlantic basin carbonate prior to the intensification of Northern Hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics of large ice sheets. The presence of these peaks, which could be related to equatorial clipped precession, implies a possible non‐linear response to Milankovitch forcing.
A 36 kyr geochemical record from the Sea of Japan of organic matter flux variations and changes in intermediate water oxygen concentrations
Authors: Crusius John , Pedersen Thomas F. , Calvert Stephen E. , Cowie Gregory L. , Oba Tadamichi
Published: September 2002

Intervals of organic C‐ and carbonate‐rich laminated sediments occur in the Sea of Japan with roughly the same frequency as temperature changes observed in Greenland ice cores, providing clear evidence of rapid oceanographic change during the past 36 kyr. Planktonic foraminiferal δ18O data suggest that only the laminated sediments deposited during the Last Glacial Maximum (LGM), and perhaps one other interval formed during a period of increased water column stratification. Sedimentary Re and Mo data are consistent with bottom waters that were sulfidic during the LGM and suboxic during other laminated intervals. Results of a numerical model of Corg and Re burial are consistent with a mechanism whereby an increased Corg flux to the seafloor drove oxygen concentrations toward depletion during times of deposition of the suboxic laminated intervals. Such a process could have resulted from increased upwelling driven either by increased deep water formation due to colder and/or more saline surface waters or by stronger northeasterly monsoonal winds.
Surface and deep ocean variability in the northern Sargasso Sea during marine isotope stage 3
Authors: Keigwin Lloyd D. , Boyle Edward A.
Published: September 2002

New geochemical proxy data from Bermuda Rise piston cores reveal ocean and climate conditions in the northern Sargasso Sea during marine isotope stage 3. Using δ18O on the planktonic foraminifer Globigerinoides ruber, we can correlate explicitly with every stadial/interstadial change in Greenland ice between ∼32 and 58 ka. These data suggest repetitive changes of ∼4°C in the annual average sea surface temperature (SST), with maximum cooling comparable to or greater than SST during glacial maximum conditions. The extent of SST depression is about the same for typical stadial events and for Heinrich events 4 and 5, which we have identified on the Bermuda Rise by traces of ice rafting. Benthic foraminiferal δ13C decreases during every stadial event, consistent with reduced production of the deepest component of North Atlantic Deep Water and shoaling of its interface with Antarctic Bottom Water. This interpretation is supported by benthic Cd/Ca data from the climate cycle associated with interstadial 8.
The integral equation method for electromagnetic scattering from the ocean: Beyond a perfect conductor model
Authors: Nedlin G. M. , Chubb S. R. , Cooper A. L.
Published: September 2002

A version of the integral equation method is developed, which applies to scattering from a surface of a substance with a large dielectric constant ε. An “impedance” boundary condition for the tangential components of electric and magnetic fields on the scattering surface is used, and the integral equation for the tangential components of the total magnetic field on a surface is formulated. This equation is applied to the problem of electromagnetic scattering from a slightly rough surface. It is demonstrated that the commonly used perfect conductor approximation (PCA) (ε = ∞) can adequately describe scattering in the case of extremely large ε only. The greatest sensitivity in scattering occurs when the incident and/or scattered waves are vertically polarized. For such cases the PCA does not work until ε > 104. In the particular case of scattering from the ocean (∣ε∣ ∼ 65), the PCA fails to provide an adequate description of the phenomena for either horizontally or vertically polarized waves, for practically all incidence and scattering angles.
Spatial correlation analysis revisited: Theory, and application to “radar backscatter model” data
Authors: Holdsworth David A.
Published: September 2002

Three novel correlation techniques for the estimation of atmospheric wind velocities using spaced antenna (SA) radars are introduced. These techniques apply the concepts of spatial correlation analysis, providing an alternative to the conventional full correlation analysis (FCA). The techniques are implemented using simulated data. The first technique is applicable to three or more noncolinear antennas. This technique is shown to produce velocities in excellent agreement with the FCA “true” velocity and is also subject to the “triangle size effect” (TSE), whereby the velocity decreases below the actual wind velocity with decreasing antenna spacing. The remaining two techniques are applicable to four antennas arranged in either a square or a “Y” configuration. These two techniques are shown to be independent of the TSE and may therefore provide a valuable alternative to the FCA in situations where the FCA may underestimate the actual wind velocity.
A statistical framework for space‐based EUV ionospheric tomography
Authors: Kamalabadi Farzad , Karl William C. , Semeter Joshua L. , Cotton Daniel M. , Cook Timothy A. , Chakrabarti Supriya
Published: September 2002

We present a statistical reconstruction framework for space‐based extreme ultraviolet (EUV) ionospheric tomography. The EUV technique offers a means to invert the nighttime F region electron density on global scales from a single spaceborne spectrograph, using prominent optically thin emissions produced by radiative recombination of O+. Since the EUV technique does not rely on ground receivers to make the measurements, the observations do not suffer from limitations on the viewing angles. The EUV tomography is an ill‐conditioned inverse problem in the sense that its solution is sensitive to perturbations of the measured data. With large condition numbers of a typical projection matrix, simple least squares inversion techniques yield unacceptable results in the presence of noise. This reflects the fact that more degrees of freedom are being sought than are supported by the noisy data. To overcome this limitation, we cast the tomographic inverse problem in a stochastic framework and incorporate a statistical prior model. In doing so we also obtain measures of estimation uncertainty for the solutions. Through simulations, we demonstrate the applicability of these techniques in the context of a space mission designed for EUV ionospheric tomography, namely, the Tomographic Experiment Using Radiative Recombinative Ionospheric EUV and Radio Sources (TERRIERS). The simulations show promising results for EUV tomography as a viable ionospheric remote sensing technique.
Series expansions for the electric and magnetic fields produced by a line or sheet current source above a layered Earth
Authors: Pirjola Risto , Viljanen Ari , Boteler David
Published: September 2002

The electric and magnetic fields that an overhead, infinitely long line current produces at the surface of a uniform Earth have rapidly converging series expansions, derived earlier. In this paper we make an extension by letting the Earth have a layered conductivity structure characterized by a surface impedance. By investigating the behavior of the surface impedance as a function of the wave number and using an analogy with the uniform‐Earth case, approximate series expansions can be derived for the electric and magnetic fields at the Earth's surface. The accuracy of the expansions is considered by numerical examples, in which the line current simulates an auroral electrojet. An increase in the period of oscillation, as well as an increase of the Earth's resistivity, tends to decrease the agreement between field values calculated from the series expansions and from exact integral expressions. However, the inaccuracies are insignificant compared with uncertainties involved in the simplified geophysical model. A sheet current having a finite width is a more appropriate model of an auroral electrojet than a line current. We therefore also present the formulas for the electric and magnetic fields due to a sheet current obtained by integrating the line current expansions.
Synchronous and velocity‐partitioned thrusting and thrust polarity reversal in experimentally produced, doubly‐vergent thrust wedges: Implications for natural orogens
Authors: Storti Fabrizio , Salvini Francesco , McClay Ken
Published: September 2002

A sandbox analog modeling research program was used to study the detailed evolution of doubly‐vergent thrust wedges. High‐resolution multilayers of thin alternating sand and mica layers were used in contractional Coulomb wedge experiments to simulate deformation of anisotropic, brittle upper crustal strata in doubly‐vergent orogens. Experiments incorporating syntectonic sedimentation in foreland basins and in piggyback basins were also carried out. Our laboratory models evolved in two main stages: (1) initial high‐velocity thrusting in the retrowedge and high‐frequency together with low displacement folding and thrusting in the prowedge; and (2) low‐frequency, high‐displacement synchronous thrusting in the prowedge and low‐velocity thrusting in the retrowedge. Transition from stage I to stage II occurred when the growing wedges reached the critical height at which they behaved as a backstop for further prowedge accretion. Addition of syntectonic sediments increased the persistence of stage I and triggered out‐of‐sequence thrusting in the axial zone of the experimental orogens. Thrust motion was stick‐slip. Retrovergent thrusting occurred along a long‐lived ramp whose lower tip was located at the subduction slot. Provergent kink bands nucleated at the subduction slot in stage I. In contrast, during the second stage of wedge evolution, kink bands nucleated in a piggyback fashion in the foreland far from the subduction slot and then evolved into high‐displacement faults that remained active up until the end of the experiments, at progressively decreasing rates of thrusting. The axial zones of the model wedges were characterized by fast uplift rates during stage I due to backward translation of the belt along the retrovergent, long‐lived ramp and due to the localization of deformation close to the subduction slot. Outward migration of the deformation front in the prowedge region during stage II caused the progressive decrease in the rate of the wedge uplift, until it eventually stopped. Analytical models to quantify the Coulomb behavior in the wedges validated the reversal of thrust polarity with increasing shortening, triggered by the buildup of the topographic load during deformation. This thrust polarity reversal highlights problems with the classic concept of back thrusting as a reflection of a significant change in the deformation regime. Our results compare well with the kinematic evolution of natural accretionary prisms and of thrust‐and‐fold belts such as the Lesser Antilles Arc System, the Mediterranean Ridge, and the Pyrenees.
West‐northwest directed obduction of the Batain Group on the eastern Oman continental margin at the Cretaceous‐Tertiary boundary
Authors: Schreurs Guido , Immenhauser Adrian
Published: September 2002

The Batain coast area in eastern Oman is dominated by allochthonous Permian to Late Maastrichtian sedimentary and volcanic rocks (Batain Group), unconformably overlain by neoautochthonous Tertiary sediments. The allochthonous rocks of the Batain coast were previously attributed to the Hawasina complex, the Permian to Coniacian/Santonian sedimentary infill of the neo‐Tethyan Hawasina basin off northern Oman. Previous structural interpretations suggested that the Batain Group, along with the Hawasina complex and the Semail ophiolite, was obducted in the Coniacian to Campanian from NE to SW onto the northern Oman continental margin. Results of our work in the Batain area differ from previous interpretations, with most significant differences concerning timing and direction of obduction. Our results show that WNW directed tectonic movements formed a fold‐and‐thrust belt and led to the obduction of allochthonous rocks onto the east Oman continental margin during latest Maastrichtian/earliest Paleocene times. This is coeval with emplacement of ophiolitic fragments along the eastern coast of Oman (eastern ophiolite belt) but is about 15–20 Myr later than emplacement of Hawasina complex and Semail ophiolite in northern Oman. Postemplacement structural evolution during the Tertiary involved intraplate extension, possibly reflecting the Red Sea/Gulf of Aden opening, and late Tertiary shortening related to convergence between Arabia and Eurasia. Late Tertiary contractional deformation resulted in refolding of the Batain nappes and in folding of the overlying Tertiary sediments. A palinspastic reconstruction of the Batain area indicates that the Permian to Upper Cretaceous sediments were formerly deposited in the Batain basin, a part of the proto‐Indian Ocean, along the present‐day eastern Oman margin. This leads us to propose that Permian breakup of Gondwanaland created both continental margins of Oman and led to the opening of two major basins: the neo‐Tethyan Hawasina basin in the north and the proto‐Indian Ocean Batain basin in the east, the latter separating Arabia from greater India.
Development of analytical solutions for multispecies transport with serial and parallel reactions
Authors: Sun Y. , Petersen J. N. , Clement T. P. , Skeen R. S.
Published: September 2002

A direct method for transforming multiple solute transport equations, coupled by linear, series, and/or parallel first‐order, irreversible reactions, into a series of simple transport equations having known solutions is developed. Using this method, previously published analytical solutions to single‐species transport problems, in which the transported species reacts with first‐order kinetics, can be used to derive analytical solutions to multispecies transport systems with parallel, serial, and combined reaction networks. This new method overcomes many of the limitations that were implicit in previously published methods. In particular, the number of species that can be described is unlimited, and the reaction stoichiometry does not have to be unimolar. To illustrate the method, an analytical solution is derived for a five‐species serial‐parallel reactive transport system. The analytical solution obtained for this problem is compared with a numerical solution obtained with a previously developed code. This analytical method is applicable to the verification of new numerical codes.
A simple scaling model for extreme rainfall
Authors: Menabde Merab , Seed Alan , Pegram Geoff
Published: September 2002

The simple scaling hypothesis is applied to the intensity‐duration‐frequency (IDF) description of rainfall. It is shown that the cumulative distribution function for the annual maximum series of mean rainfall intensity has a simple scaling property over the range 30 min to 24 hours and in some instances to 48 hours. This behavior is demonstrated through an examination of the scaling properties of the moments and the scaling of the parameters of an extreme value distribution fitted to the data. A simple analytical formula for the IDF relationship is proposed, which embodies the scaling behavior. Once the scaling parameter has been obtained for a gauge or set of gauges in a region, this formula enables the calculation of rainfall amounts, of a chosen return period and duration shorter than a day, directly from the information obtained from the analysis of daily data.
A comparison of simplified methods for routing topographically driven subsurface flow
Authors: Wigmosta Mark S. , Lettenmaier Dennis P.
Published: September 2002

The relative performance of an explicit grid cell by grid cell approach and a statistical‐dynamical method widely used in Topmodel for modeling topographically driven subsurface flow was evaluated using a series of numerical experiments. Both approaches were compared with analytical solutions to the kinematic wave equation for flow down an inclined plane of constant slope. The hillslope discharge and water table profiles simulated by the explicit method were in good agreement with the analytical solution in all test cases. The statistical‐dynamical method converged to the correct steady state solution but failed to reproduce accurately transient conditions. The two algorithms were also compared using topography and observed hourly precipitation for the U.S. Department of Agriculture Mahantango, Pennsylvania, research catchment. The percent root‐mean‐ square‐difference in hourly discharge between the two methods for a 1 year simulation ranged from 20% to several hundred percent. The agreement in discharge between the two methods was best for deep soils, high surface conductivity, and large values of the power law exponent describing the decay in vertical hydraulic conductivity with depth. “Calibration” of the statistical‐dynamical model to discharge simulated by the explicit method was most effective for soils with low power law exponents. However, even in the calibrated cases, there were large discrepancies between local water table depths simulated by the two models.
Quantification of precipitation measurement discontinuity induced by wind shields on national gauges
Authors: Yang Daqing , Goodison Barry E , Metcalfe John R , Louie Paul , Leavesley George , Emerson Douglas , Hanson Clayton L. , Golubev Valentin S. , Elomaa Esko , Gunther Thilo , Pangburn Timothy , Kang Ersi , Milkovic Janja
Published: September 2002

Various combinations of wind shields and national precipitation gauges commonly used in countries of the northern hemisphere have been studied in this paper, using the combined intercomparison data collected at 14 sites during the World Meteorological Organization's (WMO) Solid Precipitation Measurement Intercomparison Project. The results show that wind shields improve gauge catch of precipitation, particularly for snow. Shielded gauges, on average, measure 20–70% more snow than unshielded gauges. Without a doubt, the use of wind shields on precipitation gauges has introduced a significant discontinuity into precipitation records, particularly in cold and windy regions. This discontinuity is not constant and it varies with wind speed, temperature, and precipitation type. Adjustment for this discontinuity is necessary to obtain homogenous precipitation data for climate change and hydrological studies. The relation of the relative catch ratio (RCR, ratio of measurements of shielded gauge to unshielded gauge) versus wind speed and temperature has been developed for Alter and Tretyakov wind shields. Strong linear relations between measurements of shielded gauge and unshielded gauge have also been found for different precipitation types. The linear relation does not fully take into account the varying effect of wind and temperature on gauge catch. Overadjustment by the linear relation may occur at those sites with lower wind speeds, and underadjustment may occur at those stations with higher wind speeds. The RCR technique is anticipated to be more applicable in a wide range of climate conditions. The RCR technique and the linear relation have been tested at selected WMO intercomparison stations, and reasonable agreement between the adjusted amounts and the shielded gauge measurements was obtained at most of the sites. Test application of the developed methodologies to a regional or national network is therefore recommended to further evaluate their applicability in different climate conditions. Significant increase of precipitation is expected due to the adjustment particularly in high latitudes and other cold regions. This will have a meaningful impact on climate variation and change analyses.
Evidence for a nonzero tansport threshold in porous media
Authors: van der Marck Steven C.
Published: September 2002

The transport properties of porous media at low porosities are analyzed. In particular, the electrical conductivity and the permeability are considered. Several experimental results in the literature indicate that conductivity and permeability decrease significantly in the region 5–10% porosity. Recent progress in the theoretical understanding of simple porous media lead to the conclusion that transport ceases at a threshold value of 3% porosity. By modifying the Archie equation and the Kozeny‐Carman equation accordingly, one can account for the low‐porosity behavior of both conductivity and permeability.
Solving the Boussinesq equation using solutions of the Blasius equation
Authors: Hogarth W. L. , Parlange J. Y.
Published: September 2002

The response of a water table to a sudden drawdown is examined assuming that it can be described by the Boussinesq equation. An approximate analytical solution of this equation is given. This solution is based on significant improvements to previous equations obtained by Heaslet and Alksne [1961]. In comparison with an “exact” numerical solution the new approximate solution gives a maximum error of 0.02%. Such an analytical result is not only of theoretical interest but could be used as a standard reference, for instance, to validate other analytical or numerical schemes.
Nitrogen chemistry of subsurface storm runoff on forested Canadian Shield hillslopes
Authors: Hill A. R. , Kemp W. A. , Buttle J. M. , Goodyear D.
Published: September 2002

The nitrogen dynamics of storm runoff was studied using throughfall trenches on slopes with thin soils in a white pine forest catchment near Dorset, Ontario. Hydrologic data were combined with analysis of isotopic signatures and nitrogen chemistry in throughfall, soil water, and hillslope runoff. Two hypotheses were tested: (1) macropore preferential flow pathways are a source of nitrate flushing in storm runoff, and (2) the nitrogen chemistry of subsurface storm flow is controlled by the mixing of event water fluxes via macropores with preevent soil water. Most flow occurred at the soil‐bedrock interface on the slopes, and the use of 18O indicated that a considerable fraction of event water moved vertically to bedrock via preferential flow paths. Despite high levels of inorganic N in throughfall, subsurface runoff N losses during autumn storms were dominated by dissolved organic nitrogen, and little nitrate flushing occurred via preferential flow paths. Comparisons of observed NO3‐ and NH4+ concentrations versus concentrations predicted from the mixture of event and preevent water in subsurface flow did not support hypothesis 2 and instead indicated depletion of inorganic N. Low rates of N mineralization and negligible nitrification in surface 0–0.1 m soil during June–October suggested high biological utilization of a limited soil N supply. Laboratory experiments in which soil cores were leached with solutions containing NO3‐ and bromide confirmed that the organic Ae horizon was a sink for NO3‐. These data suggest that the biogeochemistry of the organic horizon can regulate patterns of inorganic N loss in subsurface runoff moving by preferential flow pathways in forest soils.
Effect of dynamic competitive sorption on the transport of volatile organic chemicals through dry porous media
Authors: Thoma Greg , Swofford Jason , Popov Valentin , Soerens Thomas
Published: September 2002

Dynamic (time‐varying) competitive sorption, modulated through fluctuating relative humidity, is shown to significantly affect the diffusive flux of 1,2,4‐trichlorobenzene through soil columns. The flux varied by a factor of up to 2 during a 24 hour cycle in the experimental system when subjected to humidity fluctuations of ±40%. This effect was observed despite conditions in which sufficient water coverage would have been present to significantly depress volatile organic chemical (VOC sorption). Many vadose zone chemical transport models do not incorporate vapor‐solid sorption, which is known to be important in low‐moisture soils. The models that do incorporate vapor sorption do not account for dynamic variations of the partitioning behavior associated with variable moisture content. We present and validate a mathematical model for VOC transport through soils that accounts for dynamic nonlinear competitive sorption on volatile species transport. Simulations are presented which show that the effect of a thin dry zone at the soil‐air interface can result in large variations in the instantaneous chemical release rate. This implies that for some conditions in the field, VOC sampling should be conducted over longer periods to avoid bias due to short‐term spikes in the chemical release rate.
An anaerobic field injection experiment in a landfill leachate plume, Grindsted, Denmark: 1. Experimental setup, tracer movement, and fate of aromatic and chlorinated compounds
Authors: Rügge Kirsten , Bjerg Poul L. , Pedersen Jørn K. , Mosbæk Hans , Christensen Thomas H.
Published: September 2002

A continuous, natural‐gradient field injection experiment, involving 18 xenobiotic compounds and bromide as tracers, was performed in the anaerobic region of a leachate plume downgradient from the Grindsted Landfill, Denmark. The injection lasted for 195 days, and within this period a continuous cloud was established. Over a period of 924 days the cloud movement was monitored in approximately 70 discrete sampling points in the central part of the cloud, and the spatial distribution was described by seven cloud snapshots involving 400–700 sampling points. The bromide cloud movement closely followed the varying flow direction predicted by the water table measurements. Moment analysis showed decreasing tracer flow velocities and reduced capture of bromide mass with time, which may be explained by varying flow conditions (direction, hydraulic gradient) and the heterogeneous geological conditions in the sandy aquifer. Naphthalene, having the highest log Kow value, was the most retarded compound, with a retardation of less than 10%. Therefore sorption was not considered to be a significant attenuation process for any of the compounds studied. Transformation under iron‐reducing conditions was observed for toluene, o‐xylene, TeCM, 1,1,1‐TCA, PCE, and TCE, while transformation of benzene and napthalene was not detected in the aquifer within the time frame of this study. First‐order transformation rates were in the range of 0.028–0.039 d−1 and 0.0014–0.0028 d−1 for the aromatic compounds toluene and o‐xylene, respectively. The rates for the chlorinated aliphatic compounds, tetrachloromethane, 1,1,1‐ trichloroethane, tetrachloroethylene, and trichloroethylene, were >0.7 d−1, 0.0044–0.0054 d−1, 0.0012–0.0038 d−1, and 0.0003–0.001 d−1, respectively. Long lag periods and slow transformation rates were observed for some of the compounds, suggesting that lack of transformation reported in the literature may be attributable to short experimental periods in those studies.
Estimating hysteresis in the soil water retention function from cone permeameter experiments
Authors: Šimůnek Jiří , Kodešová Radka , Gribb Molly M. , van Genuchten Martinus T.
Published: September 2002

Data obtained from modified cone penetrometer experiments were used to estimate the hysteretic soil hydraulic properties with a parameter estimation technique which combined a numerical solution of the Richards equation with Marquardt‐Levenberg optimization. The modified cone penetrometer was designed to inject water into a soil through a cylindrical screen, measure the infiltration rate with time, and track the movement of the wetting front using two tensiometer rings positioned above the screen. After reaching relatively stable tensiometer readings during the experiments, the source of water was cut off and pressure head readings measured while water in the soil profile redistributed. Cumulative inflow and pressure head readings for two experiments with different supply pressures were analyzed to obtain estimates of the soil water retention and hydraulic conductivity functions. Analysis of flow responses obtained during the infiltration period, and of those obtained during the combined infiltration and redistribution phases, demonstrated the importance of hysteresis of the soil hydraulic functions. We found that the redistribution phase could not be described accurately when hysteresis was neglected. Hysteresis in the soil hydraulic functions was modeled using a relatively simple empirical model in which wetting scanning curves are scaled from the main wetting curve and drying scanning curves are scaled from the main drying curve. This model was deemed adequate for our examples. Optimization results for various combinations of unknown soil hydraulic parameters were compared to results of standard laboratory and in situ methods. Estimates of the saturated hydraulic conductivity were well within the range of in situ measurements. The estimated main hysteretic loops of the soil water retention curve were for the most part situated between the wetting and drying curves obtained with standard methods.
Free‐air CO2 enrichment and soil nitrogen effects on energy balance and evapotranspiration of wheat
Authors: Kimball B. A. , LaMorte R. L. , Pinter P. J. , Wall G. W. , Hunsaker D. J. , Adamsen F. J. , Leavitt S. W. , Thompson T. L. , Matthias A. D. , Brooks T. J.
Published: September 2002

In order to determine the likely effects of the increasing atmospheric CO2 concentration on future evapotranspiration, ET, plots of field‐grown wheat were exposed to concentrations of 550 µmol/mol CO2 (or 200 µmol/mol above current ambient levels of about 360 µmol/mol) using a free‐air CO2 enrichment (FACE) facility. Data were collected for four growing seasons at ample water and fertilizer (high N) and for two seasons when soil nitrogen was limited (low N). Measurements were made of net radiation, Rn; soil heat flux; air and soil temperatures; canopy temperature, Ts; and wind speed. Sensible heat flux was calculated from the wind and temperature measurements. ET, that is, latent heat flux, was determined as a residual in the energy balance. The FACE treatment increased daytime Ts about 0.6° and 1.1°C at high and low N, respectively. Daily total Rn was reduced by 1.3% at both levels of N. Daily ET was consistently lower in the FACE plots, by about 6.7% and 19.5% for high and low N, respectively.
Analyzing investments for managing Lake Erie levels under climate change uncertainty
Authors: Venkatesh Boddu N. , Hobbs Benjamin F.
Published: September 2002

Analyses of investments that are irreversible and have uncertain benefits should consider the option of delaying a decision. For instance, the benefits of many water resource projects could change if global warming occurs. The magnitude of that warming is uncertain, and delaying projects until more information is available might be optimal. We examine whether this is true for construction of an outflow control structure for Lake Erie. Using Bayesian Monte Carlo (BMC)–based decision analysis, we find that considering climate uncertainty does make a difference. Climate change beliefs, in the form of prior distributions over transient climate scenarios, can affect the optimal strategy: in particular, climate change makes delaying construction more attractive. The option value of deferring the decision to build is as high as $20 million. Ignoring the possibility of climate warming can inflict an expected penalty as large as 20% of the cost of the control structure. We also compare climate risks to uncertainties in stage‐damage curves and find that they are approximately of equal importance.
Experimental studies of water seepage and intermittent flow in unsaturated, rough‐walled fractures
Authors: Su Grace W. , Geller Jil T. , Pruess Karsten , Wen Feng
Published: September 2002

Flow visualization experiments were conducted on a transparent replica of a natural, rough‐walled rock fracture from the Stripa Mine, Sweden, for inlet conditions of constant pressure and flow rate over a range of angles of inclination. The experiments demonstrated that infiltrating water proceeds through unsaturated rock fractures along nonuniform, localized preferential flow paths. Even though constant inlet conditions were maintained, pervasive unsteady or intermittent flow was also observed in these experiments, where portions of the flow channel underwent cycles of snapping and reforming. Experiments conducted on parallel plates with a sequence of apertures progressing from small to large to small reproduced intermittent flow. Measurements of the frequency of intermittent flow events and the volume of water metered between events were obtained from the fracture replica and parallel plate experiments and related to the Bond and capillary numbers to generalize the results. The frequency data from the fracture replica experiments did not follow the same trend as the data from the parallel plate experiments for similar Bond and capillary numbers, but the volume of water metered data was consistent in these experiments.
Impact of the circulation on Sapropel Formation in the eastern Mediterranean
Authors: Stratford Kevin , Williams Richard G. , Myers Paul G.
Published: September 2002

The role of the thermohaline circulation in controlling export production, oxygenation of deep waters, and hence possible sapropel formation in the eastern Mediterranean is examined using a simple nutrient‐cycling model. The model is driven by velocity fields from a general circulation model and receives fluxes of nutrient from river run‐off and atmospheric deposition. The model is used to study three scenarios: a strong anti‐estuarine circulation, a weakened anti‐estuarine circulation, and a weak estuarine circulation. Nutrient transports, ventilation of oxygen, and deposition of organic matter are investigated in each case. With a present‐day circulation the model provides reasonable agreement with observed phosphate and oxygen profiles and for export production. With the weakened anti‐estuarine circulation, consistent with surface salinity reconstructions for the most recent sapropel S1, there is a modest increase in export production and reduced ventilation leading to anoxia in intermediate and deep waters. Sapropel formation is possible near the coastal margins, particularly if there is enhanced river run‐off. With an estuarine circulation, there is significant increase in export production in addition to anoxia below a shallow winter mixed layer. While both the latter circulations allow sapropel formation in the model, the estuarine case is distinguished by higher organic carbon deposition and anoxia in the near‐surface waters.
The potential for calcium depletion in forest ecosystems of southeastern United States: Review and analysis
Authors: Huntington Thomas G.
Published: September 2002

Biogeochemical mass balance assessments of calcium status in southeastern forests indicate that losses through harvesting and soil leaching often exceed inputs from atmospheric deposition and weathering. Many forest soils of the southeastern United States are particularly sensitive because these soils and the underlying saprolite from which these soils are derived are largely depleted of weatherable calcium. At most of the intensively studied sites in the southeastern United States, it is estimated that calcium depletion has already reduced or will likely reduce exchangeable soil calcium reserves to less than the estimated requirement for a merchantable forest stand in 150 years or less. At most sites, calcium uptake into merchantable wood equals or exceeds soil leaching losses. Chronic atmospheric deposition of sulfate and nitrate and declining atmospheric deposition of calcium are likely to accelerate calcium depletion. The southeastern U.S. regional distribution of soil calcium pools and calcium fluxes (deposition and uptake in merchantable wood) indicates that the depletion status of the intensively studied sites is representative of a substantially larger area. Where weathering inputs are insufficient to replace leaching and uptake losses, there is a potential for a regional problem in forest nutrition over the long term.
A climatic record from l4C‐dated wood fragments from southwestern Colorado
Authors: Epstein Samuel , Xu Xiaomei , Carrara Paul
Published: September 2002

Deuterium concentrations in trees are related to the climatic temperature at which the trees grew. Deuterium analyses were made on all available (39) 14C‐dated (all 14C dates cited are uncorrected) wood fragments collected from Lake Emma sediments. The 14C dates range from 9600 to 5400 “B.P.”. Tree line was above Lake Emma at 9600 “B.P.”, was at Lake Emma at about 5000 “B.P.”, and is 80 m below Lake Emma at the present time. The isotopic records at the various intervals of time coincide very well with this history. The range of δD values is maximum at 9600 “B.P.” and is minimum at about 5400 “B.P.”. These data allow us to estimate the temperature range for the area between tree line and Lake Emma between these times. These results confirm previously observed cooling trends from several sources in the Western Hemisphere.
Wintertime CO2 efflux from Arctic soils: Implications for annual carbon budgets
Authors: Fahnestock Jace T. , Jones Michael H. , Welker Jeffrey M.
Published: September 2002

Estimates of annual carbon loss from arctic tundra ecosystems are based nearly entirely on measurements taken during the growing season in part because of methodological limitations but also reflecting the assumption that respiration during winter is near zero. Measurements of CO2 flux during winter, however, indicate significant amounts of carbon loss from tundra ecosystems throughout the 240‐day nongrowing season. In our study during the 1996 and 1997 nongrowing seasons, winter carbon losses ranged from 2.0 g CO2 m−2 season−1 in moist dwarf shrub communities to 97 g CO2 m−2 season−1 in natural snowdrift communities, with an average wintertime CO2 efflux of 45 g CO2 m−2 for all Low Arctic tundra communities (0.14 Pg CO2 yr−1 worldwide). These measurements indicate that current estimates of annual carbon loss from tundra ecosystems are low. Inclusion of wintertime losses of CO2 into annual carbon budgets increases the annual carbon efflux of arctic tundra ecosystems by 17% and changes some ecosystems from net annual sinks to net sources of CO2 to the atmosphere.
Hierarchical control on nitrous oxide emission in forest ecosystems
Authors: Brumme R. , Borken W. , Finke S.
Published: September 2002

While much is known about process level control on N2O production by nitrification and denitrification, knowledge of the environmental controls responsible for site variation in annual N2O fluxes on ecosystem level is low. Our goal was to improve existing concepts of controls on N2O fluxes. We measured N2O emission weekly or biweekly during 1 year in 11 temperate forest ecosystems using closed chambers. We identified three types of forest with different temporal emission patterns: forest with seasonal, event‐based and background emission patterns. Comparison of annual data sets from literature showed that most temperate forests had low N2O emissions throughout the year (background emission pattern) with mean annual fluxes of 0.39 ±0.27 kg N ha−1 yr−1 (n = 21). Event‐based emission patterns were observed during frost/thaw periods and after rewetting. Highest fluxes up to 72 kg N ha−1 were emitted from a drained alder forest with organic soil in 46 weeks, followed by well drained tropical and temperate forests with seasonal emission patterns and fluxes between 2 − 6 (n = 3) and 1 − 5 kg N ha−1 yr−1 (n = 4), respectively. Seasonal emission patterns were explained by combined effect of high annual precipitations; broad leave trees; amount and structure of organic upper horizon; high mineral bulk densities; and plant community. These state variables reduce gas diffusivity so that oxygen demand by microorganism and roots exceeded oxygen supply during wet and warm periods (>10° C). The resultant upper mean level was about 100 μg N2O−N m−2 h−1 in both temperate and tropical forests. Annual N2O losses of the seasonal emission type were controlled by both duration and upper mean level of the periods with high emissions. We conclude that “short‐term controls” of climate determine the duration of high emissions, whereas “long‐term controls” by state variables determine the difference between background and seasonal emission types.
Constraints on N2O sinks inferred from observed tracer correlations in the lower stratosphere
Authors: Nevison C. D. , Keim E. R. , Solomon S. , Fahey D. W. , Elkins J. W. , Loewenstein M. , Podolske J. R.
Published: September 2002

Recent isotopic studies have suggested that the trace gas N2O has a missing stratospheric sink of potentially major significance. While these studies have raised interesting questions, the constraints on N2O photochemistry imposed by correlations between N2O, total reactive nitrogen, and other tracers measured in situ in the lower stratosphere also should be considered. Measured tracer correlations, when compared to the results of models using standard photochemistry, provide evidence in support of conventional N2O sinks. Stratospheric tracer correlations, however, cannot be used to preclude a new atmospheric source of N2O in the troposphere or to argue against an undiscovered stratospheric sink that contributes less than −20% of the total sink.
Silicon‐nitrogen coupling in the equatorial Pacific upwelling zone
Authors: Dunne John P. , Murray James W. , Aufdenkampe Anthony K. , Blain Stephane , Rodier Martine
Published: September 2002

We describe the role of diatoms on nitrogen and silicon cycling in the equatorial Pacific upwelling zone (EUZ) using water column nutrient data from 19 equatorial cruises and particle concentration, new production, and sediment trap data from the U.S. Joint Global Ocean Flux Study (JGOFS) equatorial Pacific (EqPac), France JGOFS fluxes in the Pacific (FLUPAC), and U.S. Zonal Flux cruises. Our results suggest that production and sinking of diatoms dominate particulate nitrogen export at silicate concentrations above 4 μM. Below this level, silicate is preferentially retained; while inorganic nitrogen is completely utilized, silicate remains at concentrations of 1–2 μMand is completely exhausted only under nonsteady state conditions. This lower nutrient condition accounts for a majority of particulate nitrogen export in the EUZ with minor loss of particulate silicon. Retention of silicon relative to nitrogen appears due to a combination of new production by nondiatoms, dissolution of silica frustules after grazing, iron limitation, and steady state upwelling. This synthesis supports the argument that diatom production was tightly coupled to new production during the U.S. JGOFS EqPac survey II cruise [Dugdale and Wilkerson, 1998]. However, this compilation suggests EqPac survey II cruise took place during a period of atypically high subsurface nutrients. We conclude that silicon and nitrogen are tightly coupled only at periods of very high nutrient concentration and nonsteady state. In addition, nutrient cycling in the EUZ is consistent at all times with a mechanism of combined iron and grazing control of phytoplankton size classes [Landry et al., 1997].