bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of the atmospheric water vapor on Mars by the Imager for Mars Pathfinder
Authors: Titov D. V. , Markiewicz W. J. , Thomas N. , Keller H. U. , Sablotny R. M. , Tomasko M. G. , Lemmon M. T. , Smith P. H.
Published: September 2002

The Imager for Mars Pathfinder (IMP) was the first instrument to measure the atmospheric water on Mars from its surface. It took the images of the Sun through the Martian atmosphere in five narrowband filters, two in the 0.94 μm H2O band and three in the continuum around it. The observations were carried out in the mornings and in the evenings when the Sun was between 3° and 8° above the horizon. The absorption due to the atmospheric water vapor did not exceed 2%. An average column density of 6±4 precipitated microns (pr μm) was derived from the IMP data. The dependence of the observed H2O transmittance on Sun elevation tentatively implies that the water vapor is not uniformly mixed in the atmosphere but is rather confined to a layer 1–3 km thick near the surface. IMP observations also indicate a horizontal inhomogeneity of the layer but show no significant morning‐to‐evening variations of the water vapor amount.
Orbitally‐Tuned Sr Isotope Chemostratigraphy for the Late Middle to Late Miocene
Authors: Martin E. E. , Shackleton N. J. , Zachos J. C. , Flower B. P.
Published: September 2002

We present a Sr chemostratigraphic reference section for the late middle to late Miocene (14–5 Ma) from Ocean Drilling Program site 926 on the Ceara Rise. This site combines a precise, orbitally tuned timescale with a high sedimentation rate (15 m/m.y.), continuous deposition, and excellent biostratigraphic control. The Sr isotope curve is based on measurements of cleaned, planktonic foraminifera at 100–200 kyr sample intervals and it illustrates periods of rapid change in 87Sr/86Sr alternating with periods of little change. Chemostratigraphically‐defined ages for these intervals can be determined within +/−0.8 m.y. and +/−1.6 m.y. respectively. There is excellent correlation with the published curve for site 588 [Hodell and Woodruff, 1994]; however the curve for site 747 [Oslick et al., 1994] exhibits less structure, which may be due to small errors in age estimates related to slow sedimentation rates, high‐latitude fauna and an interval of complicated magnetics. Late Miocene data compare favorably with data from site 758 [Farrell et al., 1995].
Volcanic eruptions and climate
Authors: Robock Alan
Published: September 2002

Volcanic eruptions are an important natural cause of climate change on many timescales. A new capability to predict the climatic response to a large tropical eruption for the succeeding 2 years will prove valuable to society. In addition, to detect and attribute anthropogenic influences on climate, including effects of greenhouse gases, aerosols, and ozone‐depleting chemicals, it is crucial to quantify the natural fluctuations so as to separate them from anthropogenic fluctuations in the climate record. Studying the responses of climate to volcanic eruptions also helps us to better understand important radiative and dynamical processes that respond in the climate system to both natural and anthropogenic forcings. Furthermore, modeling the effects of volcanic eruptions helps us to improve climate models that are needed to study anthropogenic effects. Large volcanic eruptions inject sulfur gases into the stratosphere, which convert to sulfate aerosols with ane‐folding residence time of about 1 year. Large ash particles fall out much quicker. The radiative and chemical effects of this aerosol cloud produce responses in the climate system. By scattering some solar radiation back to space, the aerosols cool the surface, but by absorbing both solar and terrestrial radiation, the aerosol layer heats the stratosphere. For a tropical eruption this heating is larger in the tropics than in the high latitudes, producing an enhanced pole‐to‐equator temperature gradient, especially in winter. In the Northern Hemisphere winter this enhanced gradient produces a stronger polar vortex, and this stronger jet stream produces a characteristic stationary wave pattern of tropospheric circulation, resulting in winter warming of Northern Hemisphere continents. This indirect advective effect on temperature is stronger than the radiative cooling effect that dominates at lower latitudes and in the summer. The volcanic aerosols also serve as surfaces for heterogeneous chemical reactions that destroy stratospheric ozone, which lowers ultraviolet absorption and reduces the radiative heating in the lower stratosphere, but the net effect is still heating. Because this chemical effect depends on the presence of anthropogenic chlorine, it has only become important in recent decades. For a few days after an eruption the amplitude of the diurnal cycle of surface air temperature is reduced under the cloud. On a much longer timescale, volcanic effects played a large role in interdecadal climate change of the Little Ice Age. There is no perfect index of past volcanism, but more ice cores from Greenland and Antarctica will improve the record. There is no evidence that volcanic eruptions produce El Niño events, but the climatic effects of El Niño and volcanic eruptions must be separated to understand the climatic response to each.
Wideband analysis of propagation along radiating cables in tunnels
Authors: Liénard Martine , Degauque Pierre
Published: September 2002

This paper describes the main electromagnetic characteristics of a radiating cable installed in a gallery. The emphasis is on the effect of the cable environment on the coherence bandwidth and on the delay spread of the radio channel. The direction of arrival of the waves inside the tunnel has also been deduced from the complex impulse response in order to optimize a method of localization of a mobile moving in the gallery.
New integral equation formulation of the measured equation of invariance and the extension to analyze two‐dimensional cylinders with impedance boundary conditions
Authors: Hirose Masanobu , Miyake Masayasu , Takada Jun‐ichi , Arai Ikuo
Published: September 2002

We have derived a new form of the integral equation formulation of the measured equation of invariance (IE‐MEI). The new formulation clarifies the existence of a relationship between scattered electric and magnetic fields at consecutive nodes in the IE‐MEI and indicates that the relationship in a problem for a perfect electric conductor (PEC) holds for a problem with arbitrary materials. In a scattering problem of a two‐dimensional cylinder with an impedance boundary condition (IBC), every matrix in the IE‐MEI is a band‐like sparse matrix. That is, the solution process in the IE‐MEI with an IBC is the same as that for a PEC. Therefore the IE‐MEI with an IBC has the same merits of the IE‐MEI for a PEC: The more efficient computation can be achieved with the smaller memory than those of the method of moments (MOM). The IE‐MEI with an IBC is validated by numerical examples for a circular cylinder and a square cylinder by comparison with a combined field MOM that satisfies exact boundary conditions. Numerical examples show that the IE‐MEI with an IBC is applicable to the case where the generalized skin depth is less than half the width of a scatterer.
New narrow‐beam meteor radar results at Christmas Island: Implications for diurnal wind estimation
Authors: Chang J. L. , Avery S. K. , Vincent R. A.
Published: September 2002

Solar heating of the atmosphere is responsible for most of the diurnal oscillations in the neutral wind velocities and temperatures of the mesosphere and lower thermosphere. When these oscillations are global‐scale waves, they are called atmospheric tides. Excited in the lower atmosphere, tides can propagate up into the upper atmosphere where they can break and deposit considerable energy and momentum into the mean flow. Ground‐based estimates of the diurnal winds over Christmas Island have been made using a narrow‐beam meteor radar system. Previously, data collected using the three‐beam antenna configuration of the meteor radar were processed under the assumption that the vertical wind component was at least 2 orders of magnitude smaller than the horizontal component for all temporal scales of motion. The addition of two oblique beams to the radar configuration in late 1993 made it possible to estimate the horizontal wind field without applying the negligible vertical wind assumption, by using a coplanar analysis technique. Not only did the diurnal fits of the horizontal coplanar winds agree better with the results of the collocated medium‐frequency radar and model predictions, but also the Christmas Island meteor radar appears to be measuring a significant vertical velocity. This velocity has a diurnal amplitude of 10–15 m s−1 and maximizes at midnight across all heights. Under the assumption that this strong vertical motion is produced by geophysical phenomena, two hypotheses to explain this velocity are presented: the vertical motion associated with gravity wave breaking and the influence of strong electric fields on the ionized meteor trails.
Exact ULF/ELF dipole field strengths in the Earth‐ionosphere cavity over the Schumann resonance region: Idealized boundaries
Authors: Barrick Donald E.
Published: September 2002

The recent advent of ULF/ELF signal generation by high‐latitude HF ionospheric interaction facilities increases the need for quick estimates of radiated field strength distributions worldwide. This paper is a first step in that direction, cataloguing the exact spherical harmonic basis function solutions for all field component combinations excited by the four source dipole possibilities. Ideally conducting spherical boundaries are assumed here as the lowest‐order representation of the problem. Emphasis is directed toward simple algorithmic computation of field strengths within and below the Schumann resonance region. A method to speed the problematic numerical convergence of the summations is presented and demonstrated. Field calculations are accurate both near and far from the source. They reveal that only the vertical electric and horizontal magnetic dipoles produce appreciable field strength at great distances, as the other two dipole pairs cannot excite the far‐traveling cavity‐waveguide mode when the boundaries are perfectly conducting. Plots show complex distributions of excited fields around the Earth. The methods form the basis for later extensions that incorporate a representative ionospheric refractive index profile and the terrestrial magnetic field.
Study of polarization dependent scattering anomalies with application to oceanic scattering
Authors: Sei Alain , Bruno Oscar , Caponi Maria
Published: September 2002

An analysis of anomalous scattering behavior is performed on various corrugated surfaces relevant to oceanic scattering. In particular, it is shown that HH to VV ratios larger than 1 are produced as a result of polarization anomalies in the scattering returns. It is also shown that HH returns larger than those predicted by first‐order perturbation theory do occur. These calculations thus provide the first evidence that some of the characteristics associated with “sea spikes,” found in backscatter measurements from the ocean, can arise as a result of scattering from modulated wave trains. A qualitative and quantitative correlation is observed between our analysis and the water waves experiments of Lee et al. [1995].
Diffraction analysis of a double‐shielded antenna in the Fraunhofer and Fresnel regimes: Model predictions
Authors: Tello C. , Villela T. , Wuensche C. A. , Figueiredo N. , Torres S. , Bersanelli M. , Bensadoun M. , De Amici G. , Smoot G. F.
Published: September 2002

We analytically investigate the use of a wire mesh ground screen (fence) and a halo of extension panels around a helically fed parabolic reflector in order to estimate the ground contribution to the antenna noise temperature in an experiment aimed at surveying the sky at decimeter wavelengths. We use geometric diffraction theory to model the effect of these screening and blocking shields when scanning in azimuth at tilt angles from zenith in the range 0° ≥ Z ≥ 45°. We report estimates based on existing formulas for monofilar axial‐mode helical antennas with expected low‐level sidelobes in the direction of the halo region. As long as there is no significant coupling between the near‐field patterns of both the feed and the diffracting halo, estimates using the Fraunhofer approximation agree with those calculated with the Fresnel approach at a tilt angle Zeq, which increases with the proximity of the diffracting edge from the near‐/far‐field boundary of the feed pattern. Our estimates show that for a fence of some 10‐dB attenuation and high enough to level out the horizon profile at the prime focus of the antenna, the diffracted components dominate the contribution for tilt angles Z ≲ 35°. The fence is the main diffractor when Z ≳ 20°, but for Z ≳ 25° its contribution becomes insensitive to the presence of the halo. On the other hand, if the attenuation is low (<1 dB), the increase in ground solid angle with tilt angle makes the contribution due to transmission and ground exposure the dominant one.
The Stokes matrix in conical scattering from a one‐dimensional perfectly conducting randomly rough surface
Authors: Novikov I. V. , Maradudin A. A.
Published: September 2002

We calculate the elements of the Stokes matrix for the scattering of light from a one‐dimensional, perfectly conducting, randomly rough surface in the general case when the plane of incidence is not perpendicular to the generators of the surface. By using Green's second integral identity we obtain a pair of inhomogeneous integral equations for the surface values of the components of the total electric and magnetic fields in the system parallel to the generators of the surface. All components of the scattered field can be obtained from these two components, and from them the elements of the Stokes matrix can be calculated. These equations are solved numerically for each of 2000 realizations of the surface profile function, and the elements of the Stokes matrix are averaged over this ensemble of realizations of the surface profile function. We find that only four unique elements of the Stokes matrix are nonzero, as in the case in which the plane of incidence is normal to the generators of the surface (in‐plane scattering). Moreover, these nonzero elements in the conical scattering of light of wavelength λ can be obtained from the corresponding elements in the in‐plane scattering of light of an effective wavelength λeff = λ/ cos ф0, where ф0 is the conical angle. The results of the study enable complete information about the diffuse scattering properties of one‐dimensional perfectly conducting random surfaces to be obtained.
Electric field induced in a spherical cell in the human body when exposed to a 50‐ to 60‐Hz electromagnetic field
Authors: King Ronold W. P.
Published: September 2002

A 50‐ to 60‐Hz electromagnetic field Eincz, Bincy incident on a human body induces in it, respectively, an axial current I3z(z) with associated electric field E3z(z) and a circulating current I3θ(y, z) with associated electric field E3θ(y, z). Some experimental, clinical, and epidemiological studies suggest that these may be hazardous at the cellular level. The induced currents flow primarily in the conducting saline fluid that makes up most of the body. Individual cells embedded in this medium are enclosed in a thin membrane that has very low conductivity. In this study, the electric fields and currents in a spherical cell and its membrane are determined rigorously in terms of the electric field induced in the body. This, in turn, is quantitatively related to the incident electric field in the air outside the body. The electric fields and currents induced by the magnetic field are not determined since they are negligibly small.
Signal level statistics and case studies for an over‐the‐horizon mid‐Atlantic coastal link operating at C‐band
Authors: Goldhirsh Julius , Musiani Bert H.
Published: September 2002

We examine the results of 1 year of near‐continuous measurements for a 128‐km over‐the‐horizon C‐band coastal propagation link. The link extends between Dam Neck, Virginia (16 km south of Virginia Beach), and Wallops Island, Virginia (approximately 150 km southeast of Washington, D. C.). The objectives of this effort are to explore the different mechanisms of propagation through an analysis of several case studies and to assess the statistical connectivity over the 1‐year period. Case studies involving the linking of environmental information and measured signal levels are analyzed. Propagation factor levels due to evaporation ducts, surface ducts, and scattering from irregularities of the refractive index in the common volume are determined. Cumulative distributions of the measured propagation factor for the annual, fall–winter, and spring–summer periods are presented. Conditional and absolute distributions of propagation factor time durations are also presented and analyzed. It is demonstrated that during the spring–summer period, received signal levels were consistent with ducting and not with troposcatter. The fall–winter levels may be due to troposcatter from irregularities of the refractive index. The months giving the smallest and largest propagation factors were January and June, respectively.
What does an instrument measure? Empirical spatial weighting functions calculated from permeability data sets measured on multiple sample supports
Authors: Tidwell Vincent C. , Gutjahr Allan L. , Wilson John L.
Published: September 2002

With the aid of linear filter theory we analyze 13,824 permeability measurements to empirically address the question, What does an instrument measure? By measure we mean the sample support or sample volume associated with an instrument, as well as how the instrument spatially weights the heterogeneities comprising that sample support. Although the theoretical aspects of linear filter analysis are well documented, physical data for testing the filtering behavior of an instrument, particularly in the context of porous media flow, are rare to nonexistent. Our exploration makes use of permeability data measured with a minipermeameter on a block of Berea sandstone. Data were collected according to a uniform grid that was resampled with tip seals of increasing size (i.e., increasing sample support). Spatial weighting (filter) functions characterizing the minipermeameter measurements were then calculated directly from the permeability data sets. In this paper we limit our presentation to one of the six rock faces, consisting of 2304 measurements, as the general results for each rock face are similar. We found that the empirical weighting functions are consistent with the basic physics of the minipermeameter measurement. They decay as a nonlinear function of radial distance from the center of the tip seal, consistent with the divergent flow geometry imposed by the minipermeameter. The magnitude of the weighting function decreases while its breadth increases with increasing tip seal size, reflecting the increasing sample support. We further demonstrate, both empirically and theoretically, that nonadditive properties like permeability are amenable to linear filter analysis under certain limiting conditions (i.e., small variances). Specifically, the weighting function is independent of the power average employed in its calculation (e.g., arithmetic versus harmonic average). Finally, we examine the implications of these results for other instruments commonly employed in hydraulic testing (e.g., slug and pump tests).
Streamflow record extension using power transformations and application to sediment transport
Authors: Moog Douglas B. , Whiting Peter J. , Thomas Robert B.
Published: September 2002

To obtain a representative set of flow rates for a stream, it is often desirable to fill in missing data or extend measurements to a longer time period by correlation to a nearby gage with a longer record. Linear least squares regression of the logarithms of the flows is a traditional and still common technique. However, its purpose is to generate optimal estimates of each day's discharge, rather than the population of discharges, for which it tends to underestimate variance. Maintenance‐of‐variance‐extension (MOVE) equations [Hirsch, 1982] were developed to correct this bias. This study replaces the logarithmic transformation by the more general Box‐Cox scaled power transformation, generating a more linear, constant‐variance relationship for the MOVE extension. Combining the Box‐Cox transformation with the MOVE extension is shown to improve accuracy in estimating order statistics of flow rate, particularly for the nonextreme discharges which generally govern cumulative transport over time. This advantage is illustrated by prediction of cumulative fractions of total bed load transport.
Nonideal transport of reactive solutes in heterogeneous porous media: 4. Analysis of the Cape Cod Natural‐Gradient Field Experiment
Authors: Brusseau Mark L. , Srivastava Rajesh
Published: September 2002

One of the largest field studies of reactive‐solute transport is the natural‐gradient experiment conducted at Cape Cod from 1985 to 1988. Major findings regarding the transport behavior of the reactive solute (lithium) were that the rate of plume displacement decreased with time (temporal increase in effective retardation), the degree of longitudinal spreading was much greater than that observed for bromide for an equivalent travel distance, and the plume was asymmetric, with maximum concentrations located near the leading edges. The objective of our work was to quantitatively analyze the transport of lithium and to attempt to identify the factor or factors that contributed significantly to its observed nonideal transport. We used a mathematical model that accounted for several transport factors, including spatially variable hydraulic conductivity and spatially variable, nonlinear, rate‐limited sorption, with all parameter values obtained independently. The transport behavior observed during the first 250 days, corresponding to a transport distance of 60 m, was predicted reasonably well by the simulation that incorporated spatially variable hydraulic conductivity; nonlinear, rate‐limited, spatially variable sorption; and uniform water chemistry. However, the larger degree of deceleration observed during the latter stage of the experiment (the filial 20 m) was not. The larger deceleration was successfully simulated by increasing 3‐fold the mean sorption capacity of the latter portion of the transport domain. Such a change in sorption capacity is consistent with the potential impact on lithium sorption of measured changes in water chemistry (e.g.,pH increase, reduction in resident Zn)at occur in the zone through which the lithium plume traversed. The results of the analyses suggest that nonlinear sorption and variable water chemistry may have btors responsible for the nonuniform displacement of the lithium plume, with rate‐limited sorption/desorption having minimal impact. In addition, the asymmetry of the plume appears to have been caused primarily by nonlinear sorption, whereas the enhanced longitudinal spreading appears to have been caused by the combined influences of spatially variable hydraulic conductivity and sorption, nonlinear sorption, and rate‐limited sorption/desorption. A comparison of the results of this analysis to those we obtained from an analysis of the Borden natural‐gradient study reveals several similarities regarding the transport of reactive contaminants at the field scale.
Correlative velocity fluctuations over a gravel river bed
Authors: Dinehart Randal L.
Published: September 2002

Velocity fluctuations in a steep, coarse‐bedded river were measured in flow depths ranging from 0.8 to 2.2 m, with mean velocities at middepth from 1.1 to 3.1 m s−1. Analyses of synchronous velocity records for two and three points in the vertical showed a broad range of high coherence for wave periods from 10 to 100 s, centering around 10–30 s. Streamwise correlations over distances of 9 and 14 m showed convection velocities near mean velocity for the same wave periods. The range of coherent wave periods was a small multiple of predicted “boil” periods. Correlative fluctuations in synchronous velocity records in the vertical direction suggested the blending of short pulses into longer wave periods. The highest spectral densities were measured beyond the range of coherent wave periods and were probably induced by migration of low‐relief bed forms.
Effect of fracture aperture variations on the dispersion of contaminants
Authors: Keller Arturo A. , Roberts Paul V. , Blunt Martin J.
Published: September 2002

Dispersion of dissolved contaminants in fractured rocks is strongly dependent on the variability of fracture aperture. Large fracture aperture regions result in significant channeling of the fluid flow, accelerating the movement of solutes in a particular direction, which may differ locally with respect to the main flow direction from high to low hydraulic potential. This may result in very early breakthrough of the contaminants, significantly earlier than in the conventional “parallel plate” simplification of fluid flow in fractured systems. In addition, flow and transport in the low aperture regions can be orders of magnitude smaller, which has major implications for remediating contaminated fractured media. In this study we observed the transport of a nonsorbing solute in a naturally fractured rock which was characterized at very high resolution using computed tomography scanning to determine the fracture aperture distribution. We compared the observed breakthrough curve of the solute at various flow rates with the predicted breakthrough curve using the fracture's geostatistics and small perturbation analysis, as well as direct numerical simulation of flow through the fracture using the measured fracture aperture distribution. The experimentally determined fracture transmissivity and dispersivity are within a factor of 2 of those predicted using either the small perturbation approach (using the fracture geostatistics) or explicit numerical modeling of breakthrough, which differ by around 20%, indicating that the geostatistics of the fracture aperture can be used to predict the approximate shape of the breakthrough curve for a fracture.
Prediction of relative and absolute permeabilities for gas and water from soil water retention curves using a pore‐scale network model
Authors: Fischer Ulrich , Celia Michael A.
Published: September 2002

Functional relationships for unsaturated flow in soils, including those between capillary pressure, saturation, and relative permeabilities, are often described using analytical models based on the bundle‐of‐tubes concept. These models are often limited by, for example, inherent difficulties in prediction of absolute permeabilities, and in incorporation of a discontinuous nonwetting phase. To overcome these difficulties, an alternative approach may be formulated using pore‐scale network models. In this approach, the pore space of the network model is adjusted to match retention data, and absolute and relative permeabilities are then calculated. A new approach that allows more general assignments of pore sizes within the network model provides for greater flexibility to match measured data. This additional flexibility is especially important for simultaneous modeling of main imbibition and drainage branches. Through comparisons between the network model results, analytical model results, and measured data for a variety of both undisturbed and repacked soils, the network model is seen to match capillary pressure–saturation data nearly as well as the analytical model, to predict water phase relative permeabilities equally well, and to predict gas phase relative permeabilities significantly better than the analytical model. The network model also provides very good estimates for intrinsic permeability and thus for absolute permeabilities. Both the network model and the analytical model lost accuracy in predicting relative water permeabilities for soils characterized by a van Genuchten exponent n≲3. Overall, the computational results indicate that reliable predictions of both relative and absolute permeabilities are obtained with the network model when the model matches the capillary pressure–saturation data well. The results also indicate that measured imbibition data are crucial to good predictions of the complete hysteresis loop.
Farmers' willingness to pay for groundwater protection
Authors: Lichtenberg Erik , Zimmerman Rae
Published: September 2002

The effectiveness of current groundwater protection policies depends largely on farmers' voluntary compliance with leaching reduction measures, an important component of which is their willingness to adopt costlier production practices in order to prevent leaching of chemicals. Data from an original survey of 1611 corn and soybean growers in the mid‐Atlantic region were used to estimate farmers' willingness to pay to prevent leaching of pesticides into groundwater. The results indicate that farmers are willing to pay more for leaching prevention than nonfarm groundwater consumers, both absolutely and relative to total income. The primary motivation appears to be concern for overall environmental quality rather than protection of drinking water or the health and safety of themselves and their families. Hobby farmers are willing to pay more than farmers with commercial activity. Certified pesticide applicators are willing to pay less than farmers without certification.
Evaluation of simultaneous reduction and transport of selenium in saturated soil columns
Authors: Guo Lei , Frankenberger William T. , Jury William A.
Published: September 2002

Speciation plays an important role in determining the overall leachability of selenium in soil. In this study we present a mathematical model and results of miscible displacement experiments that were conducted to evaluate simultaneous reduction and transport of selenate in saturated soil columns. The experiments were carried out in organic amended (compost manure or gluten) or unamended soil, with O2‐sparged or nonsparged influent solution. In all columns, reduction of selenate was fast enough to produce selenite flux in the effluent and elemental Se in the soil profile during a mean residence time of ∼30 hours. Reduction was accelerated in the presence of organic amendments and under low O2 concentrations, resulting in an increased retardation of selenium transport as a whole. The results of our experiments show that although selenate does not sorb to solid surfaces during transport, it reduces rapidly to forms that are strongly retarded. On the basis of simulation with the consecutive reaction and transport model using parameters derived from this study, selenium is expected to be retained near the soil surface, even under extreme leaching conditions.
Turbulence structure in an ice‐covered, sand‐bed river
Authors: Sukhodolov Alexander , Thiele Michael , Bungartz Heinz , Engelhardt Christof
Published: September 2002

Although many high‐latitude rivers are characterized by seasonal ice cover, few empirical studies have examined the effect of surface ice on the turbulent properties of the underlying flowing water. This study presents three‐dimensional turbulence measurements obtained with an acoustic doppler velocimeter (ADV) in a straight reach of an ice‐covered, sand‐bed river. The statistical properties of three‐dimensional turbulence were analyzed. It is shown that the spatial structure of the flow was nonuniform due to nonuniformity of the ice cover in the cross‐stream direction and due to the riverbed geometry. These factors are responsible for the complex spatial structure of the flow field which limits the predictability of available physical models. Observed distributions over depth of Reynolds stresses and mean streamwise velocities generally conform with distributions derived from piecewise solutions of a two‐dimensional flow model at least in the central part of the flow, but observed and expected values of some parameters differ substantially. We have hypothetically addressed these differences to the effect of flow stratification, and the values of a stratification parameter have been estimated. The velocity measurements allowed to analyze the coherent structures near the bed and beneath the ice cover. Their spatial scales were evaluated to be as large as the river depth for ejections and 0.7–0.8 times the river depth for sweeps, whereas low‐frequency variations of the streamwise velocity were observed to have scales of two to four river depths.
Modeling the rainfall‐runoff response of a headwater wetland
Authors: McKillop Robert , Kouwen Nick , Soulis Eric D.
Published: September 2002

In the eastern temperate region of North America, treed headwater swamps are a familiar watershed feature. These low‐gradient wetlands commonly exist at groundwater discharge sites and represent a link between the underlying groundwater system and the surface drainage system. In contrast to the extensive literature pertaining to the hydrologic modeling of agricultural and forest land classes, little attention has been focused on the development and testing of numerical simulation models for predicting the hourly stormflow response from headwater wetland sites. If required to predict the rate of outflow from a wetland‐dominated catchment, the hydrologist or engineer has few numerical tools and little data available to assist in the prediction. The objective of this research was to investigate the feasibility of applying a numerical model to simulate the rainfall‐runoff response from a treed headwater wetland site. The wetland model utilizes a hydrology model coupled to a hydraulic stream‐routing model. A depth‐averaged laminar flow model is used to simulate the horizontal movement of stormwater both through and over the wetland sediments. The development and testing of the wetland model were completed in conjunction with a data collection program in which hydrometric and meteorologic data were obtained at a 400‐ha first‐order headwater swamp located within the Teeswater River watershed in southern Ontario, Canada. An analysis revealed that the simulated wetland streamflows were sensitive to the antecedent saturation of the wetland sediments, the storage and flow transport characteristics of the wetland sediments, and the conveyance capabilities of the wetland channel system.
Conditional stochastic averaging of steady state unsaturated flow by means of Kirchhoff Transformation
Authors: Tartakovsky Daniel M. , Neuman Shlomo P. , Lu Zhiming
Published: September 2002

We consider the effect of measuring randomly varying soil hydraulic properties on one's ability to predict steady state unsaturated flow subject to random sources and/or initial and boundary conditions. Our aim is to allow optimum unbiased prediction of system states (pressure head, water content) and fluxes deterministically, without upscaling and without linearizing the constitutive characteristics of the soil. It has been shown byNeuman et al. [1999] that such prediction is possible by means of first ensemble moments of system states and fluxes, conditioned on measured values of soil properties, when the latter scale in a linearly separable fashion as proposed by Vogel et al. [1991]; the uncertainty associated with such predictions can be quantified by means of the corresponding conditional second moments. The derivation of moment equations for soils whose properties do not scale in the above manner requires linearizing the corresponding constitutive relations, which may lead to major inaccuracies when these relations are highly nonlinear, as is often the case in nature. When the scaling parameter of pressure head is a random variable independent of location, the steady state unsaturated flow equation can be linearized by means of the Kirchhoff transformation for gravity‐free flow. Linearization is also possible in the presence of gravity when hydraulic conductivity varies exponentially with pressure head. For the latter case we develop exact conditional first‐ and second‐moment equations which are nonlocal and therefore non‐Darcian. We solve these equations analytically by perturbation for unconditional vertical infiltration and compare our solution with the results of numerical Monte Carlo simulations. Our analytical solution demonstrates in a rigorous manner that the concept of effective hydraulic conductivity does not apply to ensemble‐averaged unsaturated flow except when gravity is the sole driving force.
Optimal decomposition of covariance matrices for multivariate stochastic models in hydrology
Authors: Koutsoyiannis Demetris
Published: September 2002

A new method is proposed for decomposing covariance matrices that appear in the parameter estimation phase of all multivariate stochastic models in hydrology. This method applies not only to positive definite covariance matrices (as do the typical methods of the literature) but to indefinite matrices, too, that often appear in stochastic hydrology. It is also appropriate for preserving the skewness coefficients of the model variables as it accounts for the resulting coefficients of skewness of the auxiliary (noise) variables used by the stochastic model, given that the latter coefficients are controlled by the decomposed matrix. The method is formulated in an optimization framework with the objective function being composed of three components aiming at (1) complete preservation of the variances of variables, (2) optimal approximation of the covariances of variables, in the case that complete preservation is not feasible due to inconsistent (i.e., not positive definite) structure of the covariance matrix, and (3) preservation of the skewness coefficients of the model variables by keeping the skewness of the auxiliary variables as low as possible. Analytical expressions of the derivatives of this objective function are derived, which allow the development of an effective nonlinear optimization algorithm using the steepest descent or the conjugate gradient methods. The method is illustrated and explored through a real world application, which indicates a very satisfactory performance of the method.
Stochastic analysis of solute transport in heterogeneous aquifers subject to spatiotemporal random recharge
Authors: Li Liyong , Graham Wendy D.
Published: September 2002

This paper extends the work of Li and Graham [1998] to deal with the unconditional moments of head, velocity, and concentration under transient flow conditions, which are assumed to be caused by a spatiotemporally random recharge. Semianalytical solutions are derived for the unconditional covariances for transient velocity with a constant mean recharge using a Fourier transform approach. Results demonstrate that the velocity covariance derived for the steady state random recharge field is a limiting case of the spatiotemporally variable velocity covariance with an infinite temporal correlation scale. Another limiting case indicates that introduction of temporally random but spatially uniform recharge has no effect on the velocity covariances (over that induced by the mean recharge on the mean head gradient). Thus for this limiting case there is no increased effect on the ensemble mean concentration plume spreading or the concentration prediction uncertainty. Following Deng et al. [1993], the equations for mean concentration and macrodispersive flux under zero mean transient recharge are decoupled in the Laplace‐Fourier domain and solved using a fast Fourier transform algorithm, which significantly reduces the computational demand. The first‐order concentration variance is solved using three different approximate techniques: an approximate fast Fourier transform technique, a finite element method, and a direct numerical integration. The simulation results show that introduction of a spatiotemporally random recharge enhances both longitudinal and lateral mean concentration plume spreading compared to the no recharge case. However, transient recharge produces less spreading and less concentration prediction uncertainty than the steady state spatially random recharge case.
A mechanistic model for water seepage through thick unsaturated zones in fractured rocks of low matrix permeability
Authors: Pruess K.
Published: September 2002

The theme developed in this paper is that water seepage in thick unsaturated zones in fractured rocks of low matrix permeability cannot be understood by considering spatial and temporal averages. We suggest that most such seepage proceeds in an unsteady, episodic manner in localized preferential pathways. Mechanisms are proposed and explored which permit water to flow relatively freely in networks of interconnected fractures in the presence of strong suction pressures from an unsaturated rock matrix. Imbibition into the rock matrix is reduced by limited wetted area, the episodic nature of flow, and possibly also by the presence of mineral coatings of low permeability on the fracture walls. These effects and conditions combine to limit total imbibition rates to values less than saturated matrix hydraulic conductivity, leaving the rock matrix in partially saturated conditions even as water is flowing freely in portions of the fracture network. The proposed conceptual model is elaborated and substantiated with analytical estimates and numerical simulation studies. Mechanisms are demonstrated that can funnel distributed seepage into spatially localized preferential paths. Subsequent horizontal broadening from dispersive mechanisms is found to be a slow process for fractures of high intrinsic permeability. The presence of an unsaturated rock matrix provides constraints on fracture‐matrix interface areas for imbibition and on the frequency of episodic deep seepage events. Further studies are needed to test the proposed model.
Effects of solute concentration–dependent surface tension on unsaturated flow: Laboratory sand column experiments
Authors: Smith James E. , Gillham Robert W.
Published: September 2002

Laboratory and numerical modeling studies were conducted to investigate the effects of solute concentration–dependent surface tension on unsaturated flow and transport of a dissolved organic compound. The laboratory experiments were conducted in a 2 m long unsaturated/saturated sand column with a water table at the bottom and butanol as the solute. Pressure head was measured with tensiometers connected to pressure transducers, and water content was measured using time domain reflectometry probes. The results show that changes in surface tension caused by changes in concentration of a dissolved organic compound can have significant effects on unsaturated flow. A highly localized large flow perturbation was shown to be associated with the solute front. Relatively good agreement was found between the numerical model and experimental results. The results suggest that in many cases where dissolved organic solutes are present in the vadose zone their effects on surface tension should be included in conceptual and numerical models.
Antecedent precipitation as a methodological concern in attitude surveys on water conservation
Authors: Trumbo Craig W. , Markee Nancy L. , O'Keefe Garrett J. , Park Eunkyung
Published: September 2002

Are precipitation cycles a factor in analyses of water conservation attitudes? This article reports a post hoc analysis of two independently generated data sets examining water conservation attitudes in Reno, Nevada, in 1994 and 1997 (dry and wet years, respectively). Six attitudinal and behavior variables are contrasted across time (attitude toward water conservation, self‐efficacy for water conservation, perceived social normative pressure, reported conservation behavior, percentage of yard in grass, and perceived value of landscaping), controlling for three demographic variables that shifted significantly across the time span (income, single family home occupancy, and home value). Results confirm expectations of more positive attitudes toward water conservation and less variance in those attitudes during the dry period, as well as a stronger attitude‐behavior correlation then. While unmeasured variables could be at play, results suggest that precipitation could be an important factor to consider when investigating attitude‐behavior associations for water conservation.
The substitution of high‐resolution terrestrial biosphere models and carbon sequestration in response to changing CO2 and climate
Authors: Meyer Robert , Joos Fortunat , Esser G. , Heimann M. , Hooss G. , Kohlmaier G. , Sauf W. , Voss R. , Wittenberg U.
Published: September 2002

Strategies are developed to analyze and represent spatially resolved biosphere models for carbon sequestration in response to changes in atmospheric CO2 and climate by reduced‐form, substitute models. We explore the High‐Resolution Terrestrial Biosphere Model as implemented in the Community Terrestrial Biosphere Model (HRBM/CTBM), the Frankfurt Biosphere Model (FBM), and the box‐type biosphere of the Bern model. Storage by CO2 fertilization is described by combining analytical representations of (1) net primary productivity (NPP) as a function of atmospheric CO2 and (2) a decay impulse response function to characterize the timescales of biospheric carbon turnover. Storage in response to global warming is investigated for the HRBM/CTBM. The relation between the evolution of radiative forcing and climate change is expressed by a combination of impulse response functions and empirical orthogonal functions extracted from results of the European Center/Hamburg (ECHAM3) coupled atmosphere‐ocean general circulation model. A box‐type, differential‐analogue substitute model is developed to represent global carbon storage of the HRBM/CTBM in response to regional changes in Temperature, Precepitation and cloud cover. The substitute models represent the spatially resolved models accurately and cost‐efficiently for carbon sequestration in response to changes in CO2 or in CO2 and climate and for simulations of the global isotopic signals. Deviations in carbon uptake simulated by the spatially resolved models and their substitutes are less than a few percent.
Rainwater dissolved organic carbon: Concentrations and global flux
Authors: Willey Joan D. , Kieber Robert J. , Eyman Mary S. , Avery G. Brooks
Published: September 2002

Dissolved organic carbon (DOC) is a major component of both marine (23 μM) and continental (161 μM) rain, present in concentrations greater than nitric and sulfuric acids combined. Rain is a significant source of DOC to surface seawater (90 × 1012 g C yr−1), equivalent to the magnitude of river input of DOC to the open ocean and half the magnitude of carbon buried in marine sediments per year on a global scale. Current models of global carbon cycling focus primarily on inorganic forms of carbon and are unable to account for approximately 20% of the global carbon dioxide, suggesting a significant missing carbon sink. Quantification of the average DOC concentration in marine rain allows calculation of the global rainwater flux of DOC of 430 ± 150 × 1012 g C yr−1. When inorganic carbon is included, this rainwater carbon flux becomes 510 ± 170 × 1012 g C yr−1, which, although not the same carbon, is equivalent in magnitude to over one third of the missing carbon sink.