bluuebunny/crossref_metadata_2025 huggingface dataset:
Highlights in planetary rings
Authors: Porco Carolyn C.
Published: February 2004

There is a rich phenomenology within the planetary rings surrounding the giant planets, most of it discovered by the Voyagers during their historic tours of t he outer solar system in the 1980s. In the last decade, there have been two detailed IUGG reviews of planetary rings. Cuzzi [1983] covered the time period from 1979–1983 which included the Pioneer 11 encounter with Saturn (1979), the Voyager 1 and 2 encounters with Jupiter (1979) and with Saturn (1980 and 1981). Nicholson and Dones [1991] reviewed the developments in the field between 1984 and 1991, a period of time which included the Voyager 2 Uranus (1986) and Neptune (1989) encounters. (References t o additional reviews of planetary rings and related fields can be found in Nicholson and Dones [1991].) Rather than being comprehensive in nature, this review will concentrate on only those areas of ring research in which particularly promising developments have occurred in the last half decade.
Atmospheric aerosol research in the U.S.: 1991–1994
Authors: Kreidenweis Sonia M.
Published: February 2004

A common theme underlying many aspects of recent atmospheric aerosol research is the role of particulate matter in climate and climate change. Aerosols influence climate directly, by scattering and absorbing solar radiation, and indirectly via their role as cloud condensation nuclei (CCN), by modifying optical properties and lifetimes of clouds. The atmospheric cycles of two radiatively‐important species—sulfate and soot—have been significantly perturbed by anthropogenic activities, and thus it is expected that anthropogenic aerosols will have significant effects on global mean radiative forcing.

There is general agreement that, despite uncertainty in estimates, the averaged global direct effects of aerosols are not negligible and are probably comparable in magnitude but opposite in sign to anthropogenic greenhouse gas forcing. However, all estimates are subject to significant uncertainty, in part because of gaps in understanding of critical factors and relationships. To minimize these uncertainties, a combination of activities must be undertaken (NRC (National Research Council) [1993] and Penner et al. [1993a]): long‐term observations at fixed sites, to establish background conditions and trends; short‐term intensive observations, surface‐based and airborne; laboratory analyses and process studies; model development; and satellite observations to connect local observations to global measurements. There is also a need for new techniques for automated and real‐time measurements of aerosol chemical and physical properties. Components of IGAC (International Global Atmospheric Chemistry Program) and its U.S. component, GTCP (Global Tropospheric Chemistry Program) are focused on obtaining the data needed for quantifying the effects of aerosols on climate (NRC (National Research Council) [1993]), and indeed much of the research reported here reflects this priority.
National water policy: Shifts continue
Authors: Moreau David H.
Published: February 2004

In recent years national water policy has continued to follow a path toward decentralization and greater reliance on nonstructural means for solving problems. Neither the Bush administration nor the Clinton administration have sought fundamental changes to general water policies that evolved at the federal level from the period of rapid change in the 1970's and the less volatile but still active period before 1988. Yet, significant changes have occurred through the federal appropriation process, executive decisions, and innovations at the state and local levels. Important changes have occurred in floodplain management strategies, water transfers and water banking, urban water use efficiency, and the distribution of financial burdens among federal, state and local governments. Major changes in surface and groundwater policies and management of public water supplies could occur in the spate of water and environmental policies that are up for reauthorization in 1995.
Fluid flow in metamorphic environments
Authors: Young Edward D.
Published: February 2004

The study of fluid flow in metamorphic environments progressed over the last quadrennium mainly from a focus on the tenets of continuum mechanics, in which rock properties such as porosity and tortuosity that control flow are assumed to be homogeneous over a specified finite volume. Advances have been afforded by these principles not only from their direct application, but also from tests of their veracity. Assumptions inherent in the continuum approach are based on the scale of observation and, in conjunction with the advent of new analytical technologies, have given new impetus for examining mechanisms of material transfer between fluids and metamorphic rocks at smaller and smaller scales. Thus, even as continuum mechanics has laudably come to be widely utilized in the study of metamorphic fluid flow, the “empirical adequacy” [Oreskes et al., 1994] of the methodology is being assessed through access to previously unattainable scales of observation.
Cool season cyclogenesis and associated mesoscale weather
Authors: Businger Steven
Published: February 2004

The modernization of the National Weather Service (NWS) and a profusion of recent field experiments (Table 1) are bringing an explosion of mesoscale observations to the meteorological community during the 1990's. (The term “mesoscale” used in this paper refers to horizontal dimensions ranging from a few tens of kilometers to a few hundred kilometers. Thus, a Doppler weather radar is a tool that can make observations of mesoscale weather phenomena; for example air‐flow in a thunderstorm.) These new observational resources in combination with unprecedented growth in our computing and data handling capacities, for example, high speed workstations and widespread use of CD ROMs for data dissemination, have facilitated a renaissance in our ability to probe the mesoscale structure and evolution of winter cyclones in recent years. Many recent advances have provided substance for a new generation of texts. For example, textbooks byCarlson[1991] andBluestein[1992] focus on synoptic and mesoscale weather systems, whileHolton[1992] presents an updated treatment of their dynamics.Kraus and Businger[1994]. review the air‐sea interaction in cyclones, and Houze [1993] andEmanuel[1994] focus on the dynamics and structure of precipitating systems.
A review of theoretical and observational studies in cloud and precipitation physics: 1991–1994
Authors: Rasmussen Roy M.
Published: February 2004

It is becoming increasingly evident that clouds and precipitation play a critical role in many of the environmental or so‐called “strategic” issues facing our society today, such as the “ozone hole” problem, climate change, ocean‐atmospheric interactions and acid rain. Clouds and precipitation also play an important role in many applied problems such as aircraft icing and short‐term forecasting. Solutions to these and other problems will require a deeper understanding of how clouds form, evolve and impact their surroundings. Scales associated with cloud formation range from the size of a sub‐micron aerosol particle to the scale of a large hurricane. Scientific studies during the last four years have reaffirmed the notion that in order to adequately understand the role of clouds in these strategic and applied problems, fundamental studies on all scales important to cloud formation and evolution are required.
Seismic studies of the lithosphere and asthenosphere
Authors: Davis P. M.
Published: February 2004

Seismic studies of the lithosphere and asthenosphere over the last quadrennium have taken full advantage of recent advances in sensor and computer technology with concomitant increase in data volume, dynamic range, and resolution. The IRIS (Incorporated Research Institutions for Seismology) consortium has implemented first order improvements in portable array seismic studies of the crust and lithosphere (PASSCAL) and in the global seismic network (GSN). PASSCAL came of age with the development of a national pool of standardized, field‐rugged seismic equipment used in experiments worldwide. The GSN now numbers 62 broadband seismic stations which, along with GEOSCOPE (the French global network 22 stations) provides key data on global variations in the lithosphere and asthenosphere. This report covers some of the recent results from PASSCAL and other portable seismic array experiments, as well as analyses of regional network and global data, that pertain to the lithosphere and asthenosphere.
Main field and secular variation
Authors: Cain Joseph C.
Published: February 2004

The scope of this paper is the observational aspects of the Earth's main field and its secular change as known from historical observations. A few comments are also included in regard to paleomagnetic evidence, and a brief discussion is added on the main fields of other worlds of the Solar System. Such work as has been done in the past four years should eventually lead to a better understanding of the magnetic field of the Earth as well as planetary interiors. If the health of a discipline can be measured by the amount of controversy, this one must be extremely robust.
Preface
Authors: Pielke Roger A.
Published: February 2004

This quadrennial report represents a departure from previous ones by incorporating inter‐and cross‐disciplinary articles in response to reader comments. Examples include volcanos and climates (the topic of a recent Chapman Conference); the atmospheric and hydrologic aspects of precipitation; air‐sea interaction; and surficial properties—a subject of interest to atmospheric scientists, hydrologists, planetologists, and solid earth scientists. These topics cut across two or more of the six major sections identified for the report; the major sections and their Editors are • Properties of the Solid Earth (covering volcanology, geochemistry, and petrology; geomagnetism and paleomagnetism), Ken Verosub; and • Dynamics of the Solid Earth and Other Planets (covering tectonophysics, seismology, and geodesy), Gerald Schubert; • Space Sciences (covering atmospheric studies of extraterrestrial bodies, space physics, and the Earths atmosphere from the mesosphere and above), George Siscoe. • Atmospheric Sciences(covering the Earth's atmosphere below the mesosphere), Steve Nelson; • Hydrology (including all aspects of surface and ground water), Rich Vogel; • Ocean Sciences (covering physical, chemical, and biological oceanography), Robert Weller;

The authors for this volume were requested to structure their contributions into three basic segments: 1. undirected research (i.e., basic research); 2. directed research (i.e., applied research for specific applications ); and 3. development and implementation activities.
The effects of particle size distributions on cross‐spectral phase measurements in spatial interferometry
Authors: Chilson P. B. , Ulbrich C. W. , Larsen M. F. , Palmer R. D. , Fukao S. , Yamamoto M. , Nakamura T.
Published: February 2004

Recent observations made with a 50‐MHz Doppler radar interferometer suggest that the presence of precipitation can introduce a bias when using the cross‐spectral phase in spatial interferometry that results in an overestimation of the horizontal wind. The process is akin to turbulent fading, which produces a temporal decorrelation in the time history of the complex radar voltages. In the case of precipitation it has been proposed that the size distribution of precipitation particles produces a similar effect. This paper examines the supposition by presenting further data obtained with the 50‐MHz Doppler radar. In addition, two simulations, one in the time domain and one in the frequency domain, have been created to test for any biases introduced by an exponential form of the drop size distribution. In the time domain case spectra are generated from simulated time series data, whereas in the frequency domain case the spectra are computed directly. Results from the simulations are given for both the cases of Bragg scatter from turbulent variations in the refractive index and Rayleigh scatter from precipitation particles. This work shows that precipitation does influence the cross‐spectral phase data and suggests that spatial interferometry measurements may provide a means of extracting information related to drop size distribution parameters.
Characteristics of lightning echoes observed with VHF ST radar
Authors: Röttger J. , Liu C. H. , Pan C. J. , Su S. Y.
Published: February 2004

The development of tropospheric convection was observed with the Chung‐Li VHF stratosphere‐troposphere (ST) radar in Taiwan, Republic of China. Deep convection evolved into thunderstorms during which radar echoes from lightning were recorded with a particular high time resolution program. These lightning echoes usually exist for only several tens to a few hundred milliseconds. To investigate the fine structure in the amplitude and phase of the lightning returns, the necessary time resolution has to be in the order of a few milliseconds. Such time resolutions are for the first time applied with VHF ST radar and the initial results are presented in this paper. Rapid jumps in the phase path were occurring together with sudden amplitude changes. This indicates that the scattering regions change their position, which could be on different branches of the lightning stroke. Large radial velocities of the lightning scattering regions up to several tens of meters per second were observed. Also, strong velocity shears were noticed in these lightning echo regions. Power peaks in Doppler spectra corresponding to velocities of about 300 m s−1 were occasionally detected. It is contemplated that these are caused by Bragg scattering from sound waves resulting from the lightning shock wave. Also a periodic velocity and amplitude modulation of a thin sheet of radar reflectivity was observed which one could attribute to infra‐sound with a frequency of about 6–7 Hz. Preliminary conclusions are drawn finally to confirm that our observations are generally consistent with backscatter from lightning.
Observations of polar mesosphere summer echoes at EISCAT during summer 1991
Authors: Bremer J. , Singer W. , Keuer D. , Hoffmann P. , Röttger J. , Cho J. Y. N. , Swartz W. E.
Published: February 2004

Polar mesosphere summer echoes (PMSE) have been observed with the European incoherent scatter (EISCAT) VHF radar operating on 224 MHz during summer 1991. The mean height of PMSE was about 85 km during daytime and about 87 km during nighttime, with a markedly enhanced frequency of occurrence during daytime. The occurrence frequency of PMSE is higher during geomagnetically quiet periods than during disturbed periods. There are indications of a PMSE modulation by vertical velocities, which may be caused by atmospheric gravity waves. Comparisons of PMSE structures detected with the EISCAT VHF radar on 224 MHz and the Cornell University portable radio interferometer, which was operated at the same time on 46.9 MHz, demonstrate some common features when operated collocated but marked differences when operated at a distance of some 126 km.
A numerical sensitivity study on the backscattering at 35.8 GHz from precipitation‐sized hydrometeors
Authors: Sturniolo Orazio , Mugnai Alberto , Prodi Franco
Published: February 2004

In this paper we study the behavior of backscattering quantities that are of primary importance for the interpretation of millimeter‐wavelength multiparameter radar observations (i.e., the backscattering cross sections at horizontal and vertical polarization, the differential reflectivity, and the linear and circular depolarization ratios) for rotationally symmetric nonspherical particles having a conical‐elongated form that can be continuously varied. The scattering calculations at 35.8 GHz have been carried out using a computer code based on the Extended Boundary Condition Method for liquid water and soft‐ice hydrometeors in the equal‐volume sphere size ranges 0.05 ≤ rev ≤ 0.4 cm and 0.1 ≤ rev ≤ 1.0 cm, respectively. The calculated backscattering quantities are examined for the effects of four factors: particle size, conicity, elongation, and orientation with respect to the incident field (i.e., radar elevation and particle canting). Results of this sensitivity study show that the effects of nonsphericity tend to increase as size increases; for hydrometeors smaller than rev ≈ 0.4–0.5 cm, however, oscillations dominate the behavior of the considered backscattering quantities with size. Below such sizes, particle elongation and radar elevation are by far the main factors affecting the backscattering. As size increases, however, particle conicity becomes more and more important; at the largest sizes it is comparable to particle elongation. Our results also show that depending on particle shape, even limited variations in radar elevation or in particle orientation due to hydrometeor canting may significantly affect millimeter radar measurements of precipitating clouds. In any case, generally there is a considerable (often, large) sensitivity to particle parameters; in our view this augurs well for the existence of backscattering “signatures” that could be used to characterize the observed hydrometeors. On the other hand, it also implies that interpretation of millimeter radar measurements should be based on a statistical approach, rather than on a deterministic one, due to the variability of hydrometeor shape and orientation in nature and to their imperfect a priori information.
The effect of radar sensitivity on auroral backscatter threshold parameters
Authors: Timofeev E. E. , Vallinkoski M. K. , Inhester B. , Greenwald R. A.
Published: February 2004

Radar auroral backscatter observed by the Scandinavian Twin Auroral Radar Experiment (STARE) radars is compared with the current density in an eastward electrojet as seen by the Scandinavian Magnetometer Array (SMA) for about half an hour on March 27, 1977. The value of the height‐integrated Hall conductivity, obtained from the ionospheric electric field measured by the STARE radars and from the SMA estimates of the ionospheric current density, is considered as a measure of the mean electron density at the height of the diffuse auroral backscatter. The dependence of the volume cross section for auroral backscatter on the ionospheric electric field and electron density is analyzed, and the following points are shown. (1) For a STARE radar directed approximately perpendicular to the Hall current in the backscatter region and ionospheric electric fields between 15 and 35 mV/m, the turbulence level of 1‐m plasma waves associated with diffuse radar echoes has a linear dependence on the field. For larger values of the electric field, the turbulence level seems to saturate to a constant value. (2) A minimum height‐integrated Hall current density of 82 A/km was required to observe diffuse backscatter by either of the STARE radars. The threshold Hall current density is constant to within 20% for all electric field strenghs. For threshold backscatter the height‐integrated conductivity is inversely proportional to the ionospheric electric field. (3) The threshold current values are determined by the sensitivity of the radar used. The squared threshold Hall current is proportional to the minimum detectable value of the backscatter cross section. The radar sensitivity concept is verified quantitatively for the 144‐MHz STARE radar in Finland and for the 90‐MHz radar of the Polar Geophysical Institute.
Performance evaluation of high‐gain radiometer antennas
Authors: Shen Bing , Stutzman Warren L.
Published: February 2004

Temperature sensitivity is an important measure in the evaluation of radiometers. However, it is not directly determined by traditional methods of radiometer performance evaluation. Instead, beam efficiency is commonly evaluated to determine the accuracy of a radiometer antenna. This paper introduces a new parameter, the residue beam efficiency ratio, which relates directly to the relative error of temperature measurement and overcomes the inaccuracies and difficulties associated with beam efficiency. Also discussed is an optimization method which uses the minimum residue beam efficiency ratio to improve the temperature resolution and to reduce the antenna aperture diameter. Although the concept may be extended, the scope of this paper is limited to large‐reflector antennas for space‐based remote sensing of the Earth.
Anomalous transhorizon propagation and meteorological processes of a multilink path
Authors: Shen X. D. , Vilar E.
Published: February 2004

An event‐based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche). The signal enhancements (events) observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction. The statistical analysis carried out on this event basis gives the correlation between the signal levels of different links and the cross correlation between signal levels and local N index derived from surface meteorological monitoring. The results reveal the fundamental properties of the anomalous propagation regimes. Using case studies of several interesting events, the meteorological processes and resultant impact on transhorizon transmission are also explained. This extensive work contributes to a better understanding of anomalous propagation mechanisms present in transhorizon links.
Synthesis of shaped radiation patterns using an iterative method
Authors: Wu Lixue , Zielinski Adam , Bird John S.
Published: February 2004

Radiation patterns with a prescribed shape are useful for many applications. In this paper an iterative algorithm is presented for obtaining a set of excitation coefficients of a linear array that generates a desired main lobe shape with an arbitrarily suppressed sidelobe level. This technique can be applied to linear arrays with uniform or nonuniform spacing and isotropic or nonisotropic elements. The design examples presented show that the iterative algorithm converges rapidly to the desired solution.
Image domain ray tube integration formula for the shooting and bouncing ray technique
Authors: Bhalla Rajan , Ling Hao
Published: February 2004

A simple image domain ray tube integration formula is presented to efficiently compute the inverse synthetic aperture radar (ISAR) image of a complex target by the shooting and bouncing ray (SBR) technique. Contrary to the conventional approach where the ISAR image is obtained by inverse Fourier transforming the computed scattered field data over frequency and aspect, this image domain formula gives the contribution of each ray to the overall ISAR image directly. Under the small angle approximation and utilizing the bistatic‐monostatic equivalence, the image domain ray tube integration formula is determined in closed form. Simulation results using the SBR‐based code “Xpatch” show that the direct image domain method results in good image quality and superior time performance when compared to the conventional frequency aspect approach.
An integral representation of the wave field in inhomogeneous media in terms of diffracting component waves
Authors: Zernov Nikolay N. , Lundborg Bengt
Published: February 2004

This paper gives a review of the advantages offered by an integral representation using diffracting component waves in solving wave propagation problems, deterministic as well as stochastic, in multiscale media. The component waves are constructed in such a way that they account for the diffraction on local inhomogeneities. As a result, the method presented here describes propagation effects which pertain to nonzero values of the wave parameter of the local inhomogeneities embedded in the smoothly inhomogeneous background medium. The technique involves in a natural way the concepts of complex rays and complex caustics. In random problems the method describes the influence of diffraction by local random inhomogeneities on the field also in the near‐caustic areas. The method given is particularly suitable for solving HF wave propagation problems in the disturbed ionosphere.
VLF wave modes determined by the height gains measured during the Extended Life Balloon‐Borne Observatory test flight
Authors: Barnum Benjamin H. , Holzworth Robert H. , Dowden Richard L.
Published: February 2004

The changes with altitude of the vertical electric field from seven VLF ground‐based transmitters were measured during the ascent of the Extended Life Balloon‐Borne Observatory (ELBBO) balloon test flight from Ft. Sumner, New Mexico. The altitude profiles of the VLF vertical electric field allowed us to determine the propagation modes based on the relative amplitude changes of the VLF signals with height. Three of the VLF transmitters had signals which crossed through the dawn terminator on their paths to the balloon's receiver. The observed height gains from these three stations show that at least two models of sunrise modal interference, those proposed by Crombie [1964] and by Kaiser [1968 ], are occurring. For these three cases a time and altitude‐varying height gain signal (the ratio E(,z)/E(0) for altitude z above the surface z = 0 ) was measured during the balloon's 70‐min ascent to an altitude of 28.5 km. This method of balloon‐measured VLF height gains allows a high level of discrimination between modes, which is shown to be a useful method for studying the phenomena of sunrise and sunset terminator effects on VLF modal interference.
Passive beam forming and spatial diversity in meteor scatter communication systems
Authors: Akram Ammad , Cannon Paul S.
Published: February 2004

The method of passive beam formation using a four‐element Butler matrix to improve the signal availability of meteor scatter communication systems is investigated. Signal availability, defined as the integrated time that the signal‐to‐noise ratio (snr) exceeds some snr threshold, serves as an important indicator of system performance. Butler matrix signal availability is compared with the performance of a single four‐element Yagi reference system using ∼6.5 hours of data from a 720 km north‐south temperate latitude link. The signal availability improvement factor of the Butler matrix is found to range between 1.6–1.8 over the snr threshold range of 20–30 dB in a 300‐Hz bandwidth. Experimental values of the Butler matrix signal availability improvement factor are compared with analytical predictions. The experimental values show an expected snr threshold dependency with a dramatic increase at high snr. A theoretical analysis is developed to describe this increase. The signal availability can be further improved by ∼10–20% in a system employing two four‐element Butler matrices with squinted beams so as to illuminate the sky with eight high‐gain beams. Space diversity is found to increase the signal availability of a single antenna system by ∼10–15%, but the technique has very little advantage in a system already employing passive beam formation.
Implementation of frequency domain interferometry at the SOUSY VHF radar: First results
Authors: Chilson Phillip B. , Schmidt Gerhard
Published: February 2004

Frequency domain interferometry (FDI) provides a means of identifying the altitude and width of atmospheric scattering layers with an increased range resolution over conventional pulsed radar measurements. The technique was first formally introduced in 1987 and has since been used in the study of scattering layers from the lower troposphere to the upper mesosphre. Despite its success, FDI has only been implemented on a few atmospheric radars. We present here the first results of FDI measurements conducted at the sounding system (SOUSY) VHF radar in Germany. The flexibility of the SOUSY radar permits simultaneous sampling of regions of the atmosphere using both single and coded pulses, thus facilitating a comparison of the resulting coherences. We present here FDI data obtained during 14 continuous hours of radar operation. The FDI technique is shown to be capable of consistently predicting the location and thickness of atmospheric scattering layers. Such information is important when studying turbulence, scattering mechanisms, and fine‐scale structures in the atmosphere.
Optimization procedure based on a statistical cooling method applied to scattering by bounded nonlinear objects
Authors: Caorsi S. , Massa A. , Pastorino M.
Published: February 2004

This paper deals with the application of an optimization procedure based on a statistical cooling algorithm to the computation of the electromagnetic scattering by nonlinear dielectric objects. In particular, a numerical approach is developed that is aimed at determining the electromagnetic field distributions inside these bodies. An integral equation formulation for the scattering by two‐ and three‐dimensional, nonlinear, inhomogeneous, isotropic scatterers of arbitrary shapes is considered. The scatterers are illuminated by time periodic, incident, electric field vectors, and the nonlinear effects are taken into account by introducing equivalent sources. A system of integral equations is obtained that includes the internal electric field distribution as an unknown. After discretization the method consists in treating the problem as an optimization problem in which a nonlinear cost functional is to be minimized. Because of the large number of unknowns and the strong nonlinearity, traditional minimization algorithms would fail to find the global minimum (i.e., the solution of the electromagnetic problem). Therefore the present approach resorts to a statistical cooling procedure based on one walker meandering in the solution space. Some preliminary numerical results are reported concerning scatterers that exhibited Kerrlike nonlinearities.
Generalized boundary conditions with applications to submillimeter and optical waveguides
Authors: van Deventer T. E. , Katehi L. P. B.
Published: February 2004

In this paper, generalized impedance boundary conditions (GIBC) are derived at an interface between isotropic, homogeneous dielectric media. The advantage of this technique is that rather than requiring that the tangential components of theandfields be continuous over the interface, an equivalent set of boundary conditions involving the electric field and its space derivatives is applied, reducing the problem dimension and therefore the computational needs significantly. The equivalence between the two sets of boundary conditions is validated for the case of an infinite dielectric slab. Finally, an application of the concept of GIBC to the analysis of dielectric waveguiding structures for high‐frequency applications is described.
Fingering of Dense Nonaqueous Phase Liquids in Porous Media: 1. Experimental Investigation
Authors: Held Rudolf J. , Illangasekare Tissa H.
Published: February 2004

The propagation of dense nonaqueous phase liquids (DNAPLs) in water‐saturated, homogeneous porous media was investigated. The static distribution of DNAPL after gravity‐driven displacement was studied using a number of three‐dimensional spill experiments. Fingering intrinsic to the displacement systems was observed in all experiments. The effects of physical and chemical parameters on flow instability were examined for a range of sand grain sizes (fine, medium, and very coarse) and for different DNAPLs (trichloroethylene, trichloroethane, and dibutyl phthalate). Gravitational, viscous, and capillary forces were observed to have a varied influence on the flow behavior in these experiments. Our observations show that the development of finger patterns is sensitive to the porous media grain size, properties of the DNAPL, and spill conditions. By controlling experimental parameters, results are reproducible and yield insight into finger formation and preferential DNAPL flow in homogeneous aquifer materials. This paper discusses the experimental results qualitatively; a companion paper discusses their quantification with fractal concepts.
A Model for Sulfur Accumulation in Soft Water Lake Sediments
Authors: Matisoff G. , Holdren G. R.
Published: February 2004

A geochemical model for sulfur in soft water lake sediments of the Adirondack Mountains, New York, is developed to gain insight into the timing and magnitude of historical changes in sulfur loading. Surficial sulfur concentrations range from 3500 to 10,000 μg g−1, increase to 6000 to >20,000 μg g−1 at a depth of 2–6 cm, and then decrease downcore to background levels of 2500–7500 μg g−1. Sulfate concentrations are about 50–80 μM in the overlying water and decrease rapidly below the sediment‐water interface. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion of dissolved sulfate from the overlying water with subsequent fixation into the sediment solids. Several scenarios of historical sulfate deposition were examined as model boundary conditions. The observed sediment sulfur profiles are best modeled using boundary conditions showing loading increases of about a factor of 8 since about 1940. Assuming as much as a decade of retention of sulfate within the terrestrial portion of the ecosystem, this suggests that significant increases in sulfate deposition rates began sometime about 1930 in this area.
Nonperiodic Eddy Pulsations
Authors: Rubin D. M. , McDonald R. R.
Published: February 2004

Recirculating flow in lateral separation eddies is typically weaker than main stem flow and provides an effective environment for trapping sediment. Observations of recirculating flow and sedimentary structures demonstrate that eddies pulsate in size and in flow velocity even when main stem flow is steady. Time series measurements of flow velocity and location of the reattachment point indicate that these pulsations are nonperiodic. Nonperiodic flow in the lee of a channel margin constriction is grossly different from the periodic flow in the lee of a cylinder that is isolated in a flow. Our experiments demonstrate that placing a flow‐parallel plate adjacent to a cylinder is sufficient to cause the leeside flow to change from a periodic sequence of vortices to a nonperiodically pulsating lateral separation eddy, even if flow conditions are otherwise unchanged. Two processes cause the leeside flow to become nonperiodic when the plate is added. First, vortices that are shed from the cylinder deform and become irregular as they impact the plate or interfere with remnants of other vortices near the reattachment point. Second, these deformed vortices and other flow structures are recirculated in the lateral separation eddy, thereby influencing the future state (pressure and momentum distribution) of the recirculating flow. The vortex deformation process was confirmed experimentally by documenting spatial differences in leeside flow; vortex shedding that is evident near the separation point is undetectable near the reattachment point. Nonlinear forecasting techniques were used in an attempt to distinguish among several possible kinds of nonperiodic flows. The computational techniques were unable to demonstrate that any of the nonperiodic flows result from low‐dimensional nonlinear processes.
Temporal Moments for Nonergodic Solute Transport in Heterogeneous Aquifers
Authors: Selroos Jan‐Olof
Published: February 2004

The first two temporal moments, i.e., mean value and variance of residence time, are used to characterize nonergodic transport in heterogeneous porous media. Due to the nonergodicity stemming from small initial solute sources relative to the hydraulic conductivity integral scale, only statistical moments of the temporal moments can be obtained. The expected value and variance of the first temporal moment and the expected value of the second temporal moment are derived for both conservative solute and solute subject to sorption governed by linear kinetics. The nonergodic temporal moments incorporating sorption kinetics can be expressed in terms of the corresponding conservative moments and the constant rate parameters. The methodology is exemplified for a two‐dimensional aquifer using first‐order results for the velocity statistics. The analytical results compare reasonably well with the simulations for a variability in the hydraulic conductivity up to σlnK2 = 1. The results indicate that the relative difference between nonergodic and ergodic conditions, expressed through the second temporal moment, clearly is diminished for a kinetically sorbing solute as compared to a conservative one.
A Stochastic Approach to the Characterization of Lithofacies From Surface Seismic and Well Data
Authors: Copty Nadim , Rubin Yoram
Published: February 2004

Heterogeneity of natural geologic formations strongly influences groundwater flow and contaminant migration. This study presents a formal stochastic approach for the integration of surface seismic data and well data into the identification of the spatial arrangement (location, geometry, and interconnectedness) of lithofacies and incorporating this information in the estimation of the log permeability probability density function (pdf). To achieve this goal, normal move‐out (NMO) corrections are applied to the seismic reflection data to estimate interval seismic velocities. Calibration curves constructed from the well logs transform these velocity estimates into a lithology indicator prior probability field. From the well data and the indicator prior probability field, the indicator covariance function and its associated confidence limits are computed. Neighboring lithology logs and the indicator covariance function are then combined to update the indicator probability field. The conditional log permeability pdf is finally computed by weighting the log permeability distribution of each lithology with respect to its indicator probability field. The effectiveness of the proposed procedure is shown to depend on the quality of the seismic data and the degree of correlation between seismic and hydrogeologic soil properties. A synthetic case study is performed which shows that the surface seismic and well data complement each other with optimal results achieved when both types of data are incorporated into a joint identification procedure.
A Simple Surface Water Biogeochemical Model: 1. Description, Sensitivity Analyses, and Idealized Simulations
Authors: Jewell Paul W.
Published: February 2004

Accurate simulation of surface productivity, dissolved nutrient and oxygen concentrations, and particulate carbon flux are critical components of any viable surface water quality model. A linear, three‐component model consisting of a dissolved limiting nutrient, phytoplankton, and zooplankton coupled to a one‐dimensional turbulence closure fluid flow model has been constructed and applied to a generalized surface water setting. The model transfers nutrients from the euphotic zone to the subeuphotic zone using empirical biogenic flux‐depth relationships. The model does not require specification of biogenic particle settling velocity which is known to vary by at least 2 orders of magnitude in natural surface waters and thus constitutes a major source of uncertainty in many biogeochemical models. Using an empirical biogenic flux‐depth function to transfer nutrients through the water column introduces spatial errors which are most extreme in deep water. The model is thus most appropriate for lakes, estuaries, and shallow (<1000 m deep) marine settings. Restricting the model to three components with seven empirical constants permits a high degree of computational efficiency. Sensitivity analyses of the empirical constants show that integrated surface productivity is responsive to the three constants related to light (initial slope of the light‐productivity curve, light attenuation coefficient of the water, and photosynthetically available radiation) while being relatively insensitive to constants which are strictly related to biogeochemical transformations (remineralization rate, nutrient half‐saturation constant, maximum zooplankton growth rate, and Ivlev constant for zooplankton growth). Model simulations using idealized surface temperature boundary conditions produce thermal and biogeochemical characteristics of a typical temperate‐latitude lake. The simulations also suggest that vertical mixing plays an important role in producing the late seasonal, deep chlorophyll maximum commonly observed in many surface water settings.
Analysis of the Winter Low‐Flow Balance of the Semiarid White River, Nebraska and South Dakota
Authors: Ferrick M. G. , Mulherin N. D. , Calkins D. J.
Published: February 2004

Low‐flow studies are needed to quantify the effects of water consumption on streamflow, water quality, groundwater resources, and contaminant transport. The low‐flow water balance of a river in a cold region is simplified in winter because evapotranspiration is negligible, irrigation water withdrawals and diversions are halted, and precipitation occurs largely as snow, minimizing the spatial and temporal variability of runoff. We investigated the monthly low‐flow water balance of White River reaches over seven consecutive winters. Water going into or out of storage as ice or melt, obtained with an air temperature index model, can be a dominant component of the water balance. The point estimate method is used to account for parameter uncertainty and variability, providing the mean, variance, and limits of dependent variables such as water storage as ice and inflow from a subbasin. Negative surface water yield from several‐thousand‐ square‐kilometer subbasins occurred regularly through the period, indicating a significant flow from the river to the alluvial aquifers. The winter water balance results suggest either a perched river or a coupled surface water‐groundwater hydrologic system in particular subbasins, consistent with the field investigations of Rothrock (1942). The winter flow exchange between the surface and subsurface can be used to estimate the annual exchange for both hydrologic conditions.
A Soil Water Hysteresis Model for Fingered Flow Data
Authors: Liu Yaping , Parlange J.‐Yves , Steenhuis Tammo S. , Haverkamp Randel
Published: February 2004

Wetting and drying boundaries of a hysteresis loop were measured for fingered flow in a uniform sand, as well as drying scanning curves within the loop. The data are described by a soil water hysteresis model that uses a Brooks and Corey (1964) type main wetting boundary curve to predict the boundaries and scanning curves. The model requires as input parameters the bulk density, matric potential, water content at the two intersection points between the wetting and drying boundary curves, and one parameter obtained by curve fitting. The analysis is used to explain a fingered flow pattern in an uniform sand with different initial moisture contents.
Mixing Model Approaches to Estimate Storm Flow Sources in an Overland Flow‐Dominated Tropical Rain Forest Catchment
Authors: Elsenbeer Helmut , Lorieri Daniel , Bonell Mike
Published: February 2004

Previous hydrometric studies demonstrated the prevalence of overland flow as a hydrological pathway in the tropical rain forest catchment of South Creek, northeast Queensland. The purpose of this study was to consider this information in a mixing analysis with the aim of identifying sources of, and of estimating their contribution to, storm flow during two events in February 1993. K and acid‐neutralizing capacity (ANC) were used as tracers because they provided the best separation of the potential sources, saturation overland flow, soil water from depths of 0.3, 0.6, and 1.2 m, and hillslope groundwater in a two‐dimensional mixing plot. It was necessary to distinguish between saturation overland flow, generated at the soil surface and following unchanneled pathways, and overland flow in incised pathways. This latter type of overland flow was a mixture of saturation overland flow (event water) with high concentrations of K and a low ANC, soil water (preevent water) with low concentrations of K and a low ANC, and groundwater (preevent water) with low concentrations of K and a high ANC. The same sources explained the streamwater chemistry during the two events with strongly differing rainfall and antecedent moisture conditions. The contribution of saturation overland flow dominated the storm flow during the first, high‐intensity, 178‐mm event, while the contribution of soil water reached 50% during peak flow of the second, low‐intensity, 44‐mm event 5 days later. This latter result is remarkably similar to soil water contributions to storm flow in mountainous forested catchments of the southeastern United States. In terms of event and preevent water the storm flow hydrograph of the high‐intensity event is dominated by event water and that of the low‐intensity event by preevent water. This study highlights the problems of applying mixing analyses to overland flow‐dominated catchments and soil environments with a poorly developed vertical chemical zonation and emphasizes the need for independent hydrometric information for a complete characterization of watershed hydrology and chemistry.
Gravitational Equilibrium Moisture Profiles in Swelling Soils
Authors: Ekanayake Jagath C. , Painter David J.
Published: February 2004

Extension of the work of Philip (1969a, 1972) has shown that the effect of overburden pressure on volume change of swelling soils produces equilibrium moisture profiles entirely different from those predicted without considering the overburden pressure effects. Volume change of swelling soils accompanied by moisture changes also affects the overburden pressure. The total volume change under an overburden pressure occurs in three different shrinkage phases from saturation to oven dry. These volume change characteristics are usually described on the shrinkage surface, a surface drawn as void ratio e (volume of voids/volume of solids) versus moisture ratio Θ (volume of water/volume of solids) for different overburden pressures. A relationship for the moisture gradient is developed by assuming that the overburden potential, a component of the total potential of soil water, is a function of moisture ratio only across a small soil element of a long swelling soil column which consists of large numbers of finite soil elements. The moisture gradient has complex behavior based on the properties of the three shrinkage phases on the shrinkage surface. Distribution of equilibrium moisture paths is then explained by evaluating the moisture gradients at points on an idealized shrinkage surface. It is shown that the soil at great depths could either be saturated or unsaturated at equilibrium. (These depths could be over 250 m for some sodium montmorillonite clays).
An Experimental Investigation of Rate‐Limited Nonaqueous Phase Liquid Volatilization in Unsaturated Porous Media: Steady State Mass Transfer
Authors: Wilkins Mark D. , Abriola Linda M. , Pennell Kurt D.
Published: February 2004

Results of one‐dimensional soil column experiments are presented to evaluate the factors influencing volatilization of entrapped nonaqueous phase liquids (NAPLs) in unsaturated sandy porous media. Three‐phase fluid saturations measured in Ottawa and Wagner sands were found to depend upon porous media grain size and distribution, with residual water and NAPL saturations ranging from 8 to 16% and 4 to 10%, respectively. In general, residual NAPL saturations were 2–3 times less than NAPL entrapped in similar two‐phase (organic‐water) systems. During volatilization of three single‐component NAPLs (styrene, toluene, and tetrachloroethylene), contaminant vapor phase effluent concentrations deviated from local equilibrium values by 10–40% for pore velocities ranging from 0.25 to 1.5 cm/s. In contrast to NAPL dissolution, mass transfer rates were found to decrease with decreasing soil mean grain size. An empirical correlation based on the modified Sherwood number and Peclet number was developed which incorporates the soil mean grain size as a surrogate measure of NAPL distribution. The utility of this model is demonstrated for the prediction of steady state volatilization rates in independent NAPL‐porous media systems.