bluuebunny/crossref_metadata_2025 huggingface dataset:
Transient radiation from thin, half‐wave, orthogonal dipoles
Authors: Mokole E. L. , Choudhury A. K. , Samaddar S. N.
Published: February 2004

The intent of this work is to ascertain through analytical means whether a circularly polarized radiated field is possible for some antenna configuration with an ultrawideband (UWB) source. Consequently, the behavior of the radiated electric field from identical, thin, half‐wave, electromagnetically uncoupled, orthogonal dipoles in free space that are excited by a finite‐duration UWB voltage is studied. The zero‐order approximate solution for the current distribution along a dipole is used to permit easy calculation and interpretation of the time domain current and radiated far field. The far field is derived, and this result is applied to a finite‐cycle‐sine input (Lcycles) to each dipole, where the carrier frequency ƒ0corresponds to the dipoles' common length and the inputs are separated in time by 0.25/ƒ0. When the dipoles and the feed network are perfectly matched and the observation direction is broadside, the time domain field is extended in time to (L+ 0.75)/ƒ0with respect to the duration of the sourceL/ƒ0, and circular polarization occurs for (L‐ 0.75)/ƒ0. For the UWB case (L= 1), the radiated field's duration is 1.75 times the duration of the source, and circular polarization occurs only for one quarter of the period of the source. When the dipoles and the feed network are not matched exactly, the field's duration is infinite, and circular polarization never occurs in the strict sense. However, if the feed network and the dipoles are approximately matched, the field is negligible after a time, and nearly circular polarization occurs.
Analytic solution for low‐frequency electric induction in an equatorially stratified sphere
Authors: Dawson Trevor W.
Published: February 2004

This paper considers the analytical solution to the problem of low‐frequency induction, by quasi‐uniform electric fields, in an equatorially stratified sphere having the particular conductivity distribution σ(ϕ) = σ0e−λcos(pϕ) where p ∈ {1,2} is a periodicity factor, σ0 is a conductivity amplitude factor, and λ > 0 is a dimensionless conductivity contrast parameter. This distribution has p conductivity maxima and minima as a function of equatorial angle. The resulting induced electric and current density fields are fully three‐dimensional and exhibit interesting, yet physically reasonable, behavior. Most noticeable are the deviations from the straight current paths that would be present in the absence of any conductivity gradient. This solution for electric excitation is supplementary to ones previously published for the quasi‐static magnetic excitation and so completes the solution for induction in the sphere by quasi‐uniform electromagnetic sources. The nature of the solution is illustrated for the case of both singly and doubly periodic conductivity distributions, for the three canonical source directions.
Over‐the‐horizon radar target localization using a hidden Markov model estimated from ionosonde data
Authors: Anderson Richard H. , Krolik Jeffrey L.
Published: February 2004

Uncertainty about the downrange ionospheric conditions is a well‐known source of localization errors in over‐the‐horizon radar. Statistical modeling of ionospheric parameters has recently been proposed in order to derive a maximum likelihood (ML) localization method which is more robust to ionospheric variability. Maximum likelihood coordinate registration consists of determining the most likely target ground coordinates over an ensemble of ionospheric conditions consistent with the data. For greater computational efficiency the likelihood function is approximated by a hidden Markov model (HMM) for the probability of a sequence of observed slant coordinates given a hypothesized target location. In previous work, estimation of the HMM parameters was achieved assuming that the statistics of the underlying ionosphere were known precisely. This paper addresses the problem of estimating the parameters of the HMM from contemporaneous ionospheric sounder measurements. The approach taken here is to treat the plasma frequency profile as a homogeneous random process over the region around the midpoint between the radar and the dwell illumination region. In particular, spatial sampling of a three‐dimensional (3‐D) ionospheric model, fitted to ionosonde measurements, is used to generate quasi 2‐D plasma frequency profile realizations. Estimates of the hidden Markov model parameters are then obtained by using smoothed bootstrap Monte Carlo resampling. A comparison of ML localization and conventional methods, using full 3‐D ionospheric modeling and 2‐D ray tracing, are given using real data from a known target at a ground range of 2192 km. Results for over 250 radar dwells indicate that the ML localization technique achieves better than a factor of 2 improvement over conventional methods.
A VHF boundary layer radar: First results
Authors: Vincent R. A. , Dullaway S. , MacKinnon A. , Reid I. M. , Zink F. , May P. T. , Johnson B. H.
Published: February 2004

The development of a novel VHF radar designed to measure winds and temperatures in the planetary boundary layer is described. The radar operates at 54.1 MHz and is compact and easily transportable. The antenna system consists of 12 Yagis grouped into three subarrays arranged in the form of an equilateral triangle. Transmission takes place on the whole array, and reception takes place on the three subarrays, with winds measured by the spaced antenna technique over a height range between 300 and 3000 m. Results from field trials conducted in southern Australia in a variety of meteorological conditions are presented. Comparisons with high‐resolution radiosondes launched from the radar site show excellent agreement, with rms differences between radiosonde and radar wind components being about 1.5 m s−1. Observations carried out in rain show that echoes from precipitation are clearly distinguishable from clear‐air echoes. Unlike UHF radars, this means that vertical air velocities can be measured during precipitation, and the evolution of drop‐size distributions can be studied down to low altitudes. It is shown that temperatures derived from a radio acoustic sounding system are measured up to heights near 2 km, depending on background wind conditions.
Over‐the‐horizon radar target registration improvement by terrain feature localization
Authors: Barnum James R. , Simpson Erik E.
Published: February 2004

This paper addresses the problem of over‐the‐horizon radar (OTHR) target position registration in the presence of ill‐defined ionospheric structures. At present, OTHR is subject to frequent position uncertainties due to ionospheric abnormalities, such as tilts and ambiguous multipath propagation modes, that are difficult to model adequately during radar operation. The detection and registration of discrete terrain features in parallel with routine radar operations were studied with the goal of significantly reducing radar target position errors under most circumstances. The Wide Aperture Research Facility (WARF) experimental OTH radar testbed was modified to enable the automatic detection, processing, display, and registration of terrain features and HF repeater (beacon) echoes in parallel with aircraft detection and tracking operations. Many distinct terrain feature locations were studied in significant detail, including cities, mountain peaks, and an island, and the automatically determined position correction offsets were statistically compared with collocated beacons that served as ground truth. It was found that 100% of the offsets had expected errors of less than 5.7 nautical miles (1 nautical mile equals 1.852 km); and 93% of the offsets (with better ionospheric propagation) had expected errors of less than 3.2 nautical miles. We conclude from the research that terrain features can be used to provide coordinate registration benchmarks over an OTHR coverage area in the same way that beacons can be used.
A unified approach to detection, classification, and correction of ionospheric distortion in HF sky wave radar systems
Authors: Anderson S. J. , Abramovich Y. I.
Published: February 2004

HF sky wave radar signals are subjected to a wide variety of distortion and contamination effects as they propagate through the ionosphere. Signal processing techniques designed to remove or at least mitigate these effects have been in use for some years, but they do not address the problem of simultaneous occurrence of multiple distortion mechanisms, which may reduce drastically the efficacy of the ameliorative processing. In this paper we present an algorithmic structure for a global optimum distortion correction scheme which can function under arbitrary combinations of signal corruption phenomena. The scheme is based on multiply recursive estimation of distortion parameters. Results from a prototype implementation of this scheme show substantial gains over traditional processing sequences. The new approach is expected to enhance the remote sensing capabilities of HF sky wave radars significantly.
Three‐dimensional nonlinear inversion in cross‐well electrode logging
Authors: Abubakar Aria , van den Berg Peter M.
Published: February 2004

Electrode logging as known in the oil industry is a method for determining the electrical conductivity distribution around a borehole or between two boreholes from the static field (dc) measurements in the borehole. In this paper we discuss the reconstruction of the conductivity in a three‐dimensional domain between two boreholes. The electric current density integral equation will be taken as point of departure to develop a nonlinear inversion scheme based on a new inversion method, the so‐called contrast source inversion method (CSI). The CSI method considers the inverse scattering problem as an inverse source problem in which the unknown contrast sources in the object domain are reconstructed by minimizing the object and data error using a conjugate gradient step, after which the contrast is updated by minimizing only the error in the object. The robustness and the abilities of this full‐vector three‐dimensional inversion method have been demonstrated with a number of numerical examples where the synthetic “measured” data used were generated by solving a forward scattering problem using the conjugate gradient fast Fourier transform method.
Inferring rough surface parameters from average scattering data using approximate scattering models: 2. Pierson‐Moskowitz spectrum
Authors: Marchand Roger T. , Brown Gary S.
Published: February 2004

The quasi‐specular model and a two‐scale model are used to extract the wind speed from essentially exact numerical calculations of the average radar cross section of one‐dimensional surfaces with ocean‐like height spectra. These surfaces contain variations in their height on spatial scales which are smaller than the incident electromagnetic wavelength. It is shown that using the quasi‐specular model, good wind speed estimates can be obtained if the quasi‐specular model is modified to use only a portion of the surface spectrum below some cutoff value. However, the wind speed estimates are highly sensitive to this cutoff, and the optimal value depends on the polarization, electromagnetic wavelength, and wind speed. For the two‐scale model, wind speed estimates tend to be slightly low, but in all cases examined herein the exact wind speed was within or very near the 95% confidence limits of the estimated value. Wind speed estimates are also generally observed to be slightly better at an electromagnetic wavelength of 30 cm than at 3 cm. It is also shown that when the so‐called separation wavenumber varies between roughly ko/2.5 and ko6.0, it has little effect on the wind speed estimates.
An F region height change produced by gravity waves
Authors: MacDougall J. W. , Hall G. E.
Published: February 2004

Polar cap F region digital ionosonde height measurements occasionally show a type of height variation that we call a “downward rippling” variation because of its appearance. The variation typically extends over about 2 hours, and the overall height change is of the order of 100 km. We can simulate similar appearing height variations from vertical perturbations caused by gravity waves if the gravity waves have limited vertical wavelengths (a few hundred kilometers) and if there is high‐speed convective flow.
Drift‐parallax determination of the altitude of traveling ionospheric disturbances observed with the Los Alamos radio‐beacon interferometer
Authors: Kirkland Matt W. , Jacobson Abram R.
Published: February 2004

From 1993 to 1997, the Los Alamos very long baseline interferometer was routinely employed to detect traveling ionospheric disturbances (TID) and inner plasmaspheric irregularities by measuring the change in the electrical phase of several satellite beacon signals that backlit the inner plasmasphere and ionosphere from geosynchronous orbit. The fortuitous placement of two satellite beacons nearly in the Los Alamos geographic meridian, in late 1995, permits us to infer the nominal altitude of each TID event by employing a novel parallax ranging technique. In the context of this paper, the nominal altitude of a TID refers to the altitude at which the slant‐path averaged, amplitude‐weighted phase perturbation existed along the lines of sight from the interferometer to the satellite. In this paper, we outline the method and present validation results.
Introduction to special section: Science and technology of over‐the‐horizon radar
Authors: Berkey F. Tom
Published: February 2004

The rationale for the development of over‐the‐horizon (OTH) radar systems operating at frequencies in the HF band arose out of a perceived need for an early‐warning defense network. That need changed with the end of the Cold War; however, today OTH radars play a major role in the CounterDrug Program for the interdiction of drug‐smuggling aircraft. This special section of Radio Science is devoted to a review of OTH radar technology, with emphasis on contemporary developments in this field. The collection of papers presented here has evolved largely from research and development efforts directed to improving the performance of OTH radar systems deployed both in the United States and in Australia.
Simultaneous observations of polar cap patches and Sun‐aligned arcs during transitions of the IMF
Authors: Valladares C. E. , Fukui K. , Sheehan R. , Carlson H. C. , Bullett T.
Published: February 2004

This paper presents the first observations of simultaneous polar cap patches and polar cap arcs in a single common 1000‐km field of view, and identifies a model that explains the interplanetary magnetic field (IMF) dependencies of the observed phenomenology. To study the characteristics of the polar cap optical emissions in the 630.0 nm line during transitions of the IMF Bz, we have scanned images taken at Qaanaaq, Greeland, between 1989 and 1994. We found that on a few occasions, when Bz changed from a south to a north orientation, a particular pattern of polar cap patches and Sun‐aligned arcs coexisted. No similar pattern of coexisting arcs and patches was found during north‐to‐south IMF transitions. The detailed analyses of three of these events are presented here in which patches and polar cap arcs are clearly identified to reside simultaneously within the Qaanaaq imager field of view. The digisonde located also at Qaanaaq is used to confirm that the optical patches correspond to enhancements in the number density and a simultaneous decrease of the hmF2 value. These two factors increase the capability of the imager to differentiate between patches and the background airglow. Data collected by the DMSP F8 satellite during one of the events reaffirm the appearance of polar cap precipitation during the Bz positive period. The J4 sensor on board DMSP F8 detected typical electron fluxes commonly associated with polar cap arcs. The coexistence of patches and arcs is due to a slower response of the patches in exiting the polar cap, and then the relatively sudden appearance of polar cap arcs presumably driven by dayside reconnection between the IMF and open flux drawn initially equatorward toward the cusp. This model, of dayside reconnection switching from equatorward of the cusp for Bz south to poleward of the cusp for Bz north, likewise explains why arcs and patches are seen by the imager to coexist for rapid Bz reversals only from south to north and not from north to south.
A statistical comparison of horizontal winds obtained by a variety of spaced antenna techniques using the Jicamarca VHF radar
Authors: Chau Jorge L. , Balsley Ben B.
Published: February 2004

We have used the Jicamarca radar system to measure horizontal winds by a variety of time domain and frequency domain spaced antenna techniques. Horizontal wind comparisons for two orthogonal components are carried out in a statistical sense, i.e., using correlation coefficients, mean differences, standard deviation of differences, and percentage of failures. We find that a simple technique that assumes horizontally isotropic scattering compares very closely with the more complicated full correlation analysis techniques. Techniques that attempt to correct for turbulence appear to be very sensitive to the way we estimate the width of the cross‐correlation/spectra functions. This is particularly true above 15 km, where discrepancies in the wind speed are apparent. In general, all techniques give essentially the same wind direction.
Multisource volumetric tomography for over‐the‐horizon radar
Authors: Biswas Chaitali , Na Helen
Published: February 2004

This paper presents a new technique for the mapping of ionospheric electron density distributions for determining ambient HF radio propagation characteristics necessary for applications such as over‐the‐horizon‐radar (OTHR). The multisource volumetric inversion technique (MSVIT) combines information from conventional ionospheric probes used for OTHR, such as quasi‐vertical incidence and oblique backscatter ionosondes, with total electron content data obtained by computerized ionospheric tomography (CIT), to form three‐dimensional maps of ionospheric electron density distribution. MSVIT is an iterative reconstruction technique that draws on the wide‐area coverage of VHF‐UHF data from a CIT imaging system colocated with an OTHR system to enhance the reliability of sector‐wise localized predictions of HF propagation conditions. A detailed analysis of the MSVIT algorithm together with simulations using the international reference ionosphere model of 1990 and experimental data from recent CIT campaigns shows that MSVIT has considerable potential for improving the quality of OTHR operations.
Impedance matrix localization based fast multipole acceleration
Authors: Han Sang‐Kyun , Michielssen Eric , Shanker Balasubramaniam , Chew Weng Cho
Published: February 2004

A technique for applying the impedance matrix localization (IML) method in conjunction with the fast multipole method (FMM), termed the hybrid IML‐FMM, is presented. This technique significantly reduces the matrix‐vector multiplication time in the iterative solution of the method of moment equations arising from the integral‐equation‐based analysis of two‐dimensional electromagnetic scattering phenomena. The reduction in computational time is accomplished through an IML‐based sparsification of the FMM far‐field transformation matrices. Numerical experiments illustrate the effect of various IML‐FMM truncation parameters on the sparsity obtainable, the corresponding reduction in matrix‐vector multiplication time, and the solution accuracy.
A statistical comparison of VHF techniques to study clear‐air vertical velocities in the lower atmosphere using the Jicamarca radar
Authors: Chau Jorge L. , Balsley Ben B.
Published: February 2004

We compare two types of VHF vertical velocity measurement techniques using a special configuration of the Jicamarca 50‐MHz radar antenna. Zenith (vertically directed single‐beam) techniques and spaced antenna (SA) techniques are compared using a 54‐hour concurrent data set. We compare two different zenith techniques using different antenna beam widths (0.85° and 3.5°). In addition, we compare vertical velocities measured by a “quasi” Doppler beam swinging (QDBS) technique and a SA approach that incorporate either “inferred” or “corrected” vertical velocities. All of the results are compared in terms of statistical basis. Results are presented for both quiet (low wind, low variability) and active (high wind, high variability) conditions. We use the results of the narrow‐beam zenith technique as a reference profile and discuss its validity as a reference. Our comparisons show that relative to the reference, (1) the use of corrected values (i.e., correcting for horizontal leakage) gives better results than the wide‐beam zenith technique, although mean differences of ∼−3 and ∼4 cm s−1 are still observed below 12 and above 15 km, respectively, (2) QDBS results give the closest mean value agreement of all the techniques, and (3) most of the vertical velocity discrepancies are seen as variations in the mean values.
Deformation pattern around the conjoining strike‐slip fault systems in the Basin and Range, southeast Nevada: The role of strike‐slip faulting in basin formation and inversion
Authors: Çakir Mehmet , Aydin Atilla , Campagna David J.
Published: February 2004

Within the extensional regime of the Basin and Range, strike‐slip faults create a regional pattern of opposing sense of fault systems. The relationship between these faults and other deformational features nearby is enigmatic. This paper addresses a diverse assemblage of contractional and extensional structures reflecting local uplift and subsidence, respectively, at the junction of two large Neogene strike‐slip faults in southeastern Nevada, the right‐lateral Las Vegas Valley shear zone and the left‐lateral Bitter Spring Valley fault of the Lake Mead fault system. First, a middle Miocene lacustrine carbonate basin, the Bitter Ridge‐Lovell Wash carbonate basin, formed north of the strike‐slip faults. Second, the lacustrine basin inverted locally, while sediments accumulated south of the strike‐slip faults. Third, the study area was deformed by north‐northeast trending, high‐angle oblique faults with normal and left‐slip components. The results, both from field observations and numerical modeling of the intersecting strike‐slip faults, show that the Las Vegas Valley shear zone and the Bitter Spring Valley fault may have produced the basin in the north and its intense contractional deformation as well as the southward shift of deposition during the inversion event. We conclude that conjoining strike‐slip fault systems can promote localized vertical tectonics and lead to basin formation and uplift right next to each other. Subsequent inversion of the earlier basinal deposits, however, requires a reversal in the sense of slip across the Las Vegas Valley shear zone and a change in the regional stress system.
Ophiolitic basement to a forearc basin and implications for continental growth: The Coast Range/Great Valley ophiolite, California
Authors: Godfrey Nicola J. , Klemperer Simon L.
Published: February 2004

We present a compilation of 18 published models from the length of the Great Valley forearc basin, California, based on seismic reflection, borehole, seismic refraction, gravity, and aeromagnetic data to address long‐standing questions about the nature of the basement to the Great Valley, its origin, its tectonic history, and it's mechanism of incorporation into the North American continental margin. The geophysical models permit a 700‐km‐long, 70‐km‐wide, complete ophiolite sequence beneath the entire Great Valley. In the northern Great Valley, the ophiolite is overlain by ophiolitic breccia, the ophiolite crust is 7–8 km thick, and the mantle section is mostly unserpentinized. Beneath the southern Great Valley, there is no ophiolitic breccia, the crust may be up to 10 – 12 km thick, and the mantle section, if present at all, is serpentinized to such a degree that it cannot be distinguished from Sierran basement or mafic ophiolite members on the basis of velocity or density data. Geochemical, petrological, and paleomagnetic data support suprasubduction zone ophiolite formation at North American paleolatitudes, and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc. In the north, this was apparently followed by obduction of back arc crust onto older continental basement during the Late Jurassic Nevadan orogeny. In the south, the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the Nevadan orogeny but were not obducted onto it. Instead, the arc and back arc “docked” with the continental margin leaving the arc itself to become the basement to the Great Valley basin. Cretaceous Sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc. Irrespective of the detailed accretionary history, our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km³ km−1in less than 10 Myr.
Early Paleozoic orogenic collapse, tectonic stability, and late Paleozoic continental rifting revealed through thermochronology of K‐feldspars, southern Norway
Authors: Dunlap W. J. , Fossen H.
Published: February 2004

Southern Norway was subjected to contractional orogenesis and subsequent extensional collapse during the early to middle Paleozoic when Baltica collided with Laurentia, resulting in the Caledonian Orogen. The largely unknown post‐Caledonian Paleozoic thermal history of the crust in southern Norway has been assessed through application of multiple diffusion domain thermal modeling of K‐feldspar40Ar/39Ar data. Samples were collected along a 250 km E‐W traverse from the internides of the orogen along the western coast near Bergen through a series of middle to upper crustal nappes to the east. The K‐feldspar thermal models, which complement published mica and hornblende40Ar/39AR data, are consistent with rapid cooling at the end of the Caledonian orogenic cycle, when Early Devonian (∼400 Ma) extensional collapse resulted in exhumation through mainly midcrustal conditions. Following extensional collapse, a period of relative tectonic and thermal stability may have prevailed until ∼300 Ma. The K‐feldspar modeling suggests that southern Norway entered a new phase of relatively rapid cooling (a few °C/Ma) in the Permo‐Carboniferous and that the increase in cooling rate was diachronous across the traverse. The onset of this cooling phase occurs at about the time of the earliest faulting and magmatism in the Oslo Graben to the southeast. The increase in cooling rate may also indirectly date the onset of the first important rift phase and associated sedimentation in the North Sea graben system. The distribution of Permian and younger fission track ages in both southern continental Norway and its marginal basin sediments is consistent with the Permo‐Carboniferous rapid cooling recorded in the K‐feldspars. The simplest explanation for the late Paleozoic increase in cooling rate is that rifting around the margin of southern Norway has resulted in a decrease in the base level and an increase in the rate of erosional denudation, resulting in essentially uniform exhumation of ∼2–4 km along the traverse.
Flow in multiscale log conductivity fields with truncated power variograms
Authors: Di Federico Vittorio , Neuman Shlomo P.
Published: February 2004

In a previous paper we offered an interpretation for the observation that the log hydraulic conductivity of geologic media often appears to be statistically homogeneous but with variance and integral scale which grow with the size of the region (window) being sampled. We did so by demonstrating that such behavior is typical of any random field with a truncated power (semi)variogram and that this field can be viewed as a truncated hierarchy of mutually uncorrelated homogeneous fields with either exponential or Gaussian spatial autocovariance structures. The low‐ and high‐frequency cutoff scales λl and λu are related to the length scales of the sampling window and data support, respectively. We showed how this allows the use of truncated power variograms to bridge information about a multiscale random field across windows of different sizes, either at a given locale or between different locales. In this paper we investigate mean uniform steady state groundwater flow in unbounded domains where the log hydraulic conductivity forms a truncated multiscale hierarchy of Gaussian fields, each associated with an exponential autocovariance. We start by deriving an expression for effective hydraulic conductivity, as a function of the Hurst coefficient H and the cutoff scales in one‐, two‐, and three‐dimensional domains which is qualitatively consistent with experimental data. We then develop leading‐order analytical expressions for two‐ and three‐dimensional autocovariance and cross‐covariance functions of hydraulic head, velocity, and log hydraulic conductivity versus H, λl and λu; examine their behavior; and compare them with those corresponding to an exponential log hydraulic conductivity autocovariance. Our results suggest that it should be possible to bridge information about hydraulic heads and groundwater velocities across windows of disparate scales. In particular, when λl ≫ λu, the variance of head is infinite in two dimensions and grows in proportion to λl2+2H in three dimensions, while the variance and longitudinal integral scale of velocity grow in proportion to λl2H and λl, respectively, in both cases.
In situ determination of particle friction angles of fluvial gravels
Authors: Johnston Christopher E. , Andrews E. D. , Pitlick John
Published: February 2004

Particle friction angles Φ represent the physical resistance to initial movement of a sediment particle and are therefore useful for relating initiation of motion to particular flows. We determined over 8000 friction angle values at five natural rivers by applying a new method that uses a digital load cell to directly measure the force Fd required to pivot or slide a particle out of its natural resting place. Within each site, median Φ values were very similar to previously reported relations, yet different enough between sites that a location‐general predictive empirical relation would produce errors in Φ of ±10 degrees for Di/Ks > 1. Furthermore, within a Di/Ks class at a given site the range in Φ was as large as 80°, much greater than the range of median values between classes for natural sediment mixtures. Using estimates of τc* from extensive bed load measurements made by Andrews and Erman [1986] at Sagehen Creek and the in situ Φ measurements made in this study together with a theoretical model developed by Wiberg and Smith [1987], we show that Φ measurements made with this new method can be used to accurately predict τc* for natural, water‐worked sediments. Additionally, these results confirm that a Φ value ≪Φ50 is more appropriate for predicting τc* of a given size class.
Role of salinity‐derived variable‐density flow in the displacement of brine from a shallow, regionally extensive aquifer
Authors: Lahm Terry D. , Bair E. Scott , VanderKwaak Joel
Published: February 2004

Portions of many regional‐scale aquifers in midcontinent sedimentary basins exhibit large salinity gradients that significantly impact the velocity field and solute distribution through time. A two‐dimensional, numerical transport model was constructed to examine the role of salinity‐derived variable‐density flow on changes in the velocity field and solute distributions in a near‐surface, regionally extensive aquifer as brine is displaced by infiltrating meteoric water. The Silurian‐Devonian carbonate aquifer in the western portion of the Appalachian Basin was used as a framework to insure that realistic flow velocities and salinities were used in the assessment. The variable‐density effects on brine displacement are observed by examining the differences in the velocity fields and solute distributions produced by uniform‐density and variable‐density simulations. The effects include the change from an intraformational displacement pattern to a cross‐formational displacement pattern with the development of flow reversals and partitioning of regional flow cells into smaller flow cells. Variable‐density effects also are manifest in the solute distributions by slowing the displacement of brine and influencing the magnitude of the salinity gradient. A sensitivity analysis used to examine the influence of flow and transport parameters on the transient development and migration of salinity gradients shows that increasing cross‐formational leakage into the regional aquifer causes flow velocities to decrease, which magnifies the influence of the variable‐density behavior by slowing the displacement of brine. The sensitivity analysis also shows that increasing the value of dispersivity causes an increase in the variable‐density effects. However, the effects of variable‐density flow are relatively insensitive to changes in values of horizontal and vertical anisotropy assigned to the aquifer or to the presence of an overlying transmissive layer.
A general mathematical model for chemical‐enhanced flushing of soil contaminated by organic compounds
Authors: Ji Wei , Brusseau Mark L.
Published: February 2004

The use of chemical agents to enhance the in situ removal of hydrophobic organic compounds (HOCs) from porous media is an emerging remediation technology. Whereas surfactants and cosolvents are the primary agents examined to date, others, such as natural organic matter and complexing agents, have also been examined for their ability to enhance the solubilization of HOCs. While the mode of action of each type of enhanced‐solubilization agent may be different, they all induce similar responses. In this paper, a general mathematical model is developed to simulate the enhanced‐solubilization process for various chemical agents, including cosolvents, surfactants, natural organic matter, and complexing agents. This model is developed using a master‐equation approach that incorporates the solubilization mechanisms associated with each type of agent. A limited evaluation of the model is conducted by comparing simulations to the results of two laboratory experiments. A sensitivity analysis is performed to illustrate the influence of various factors on contaminant removal.
The effect of temperature on capillary pressure‐saturation relationships for air‐water and perchloroethylene‐water systems
Authors: She Hugh Y. , Sleep Brent E.
Published: February 2004

The temperature dependence of capillary pressure‐saturation relationships was measured for air‐water and perchloroethylene‐water systems in silica sand. Changes in capillary pressures, irreducible water phase saturations, and residual nonwetting saturations with temperature were determined. Relationships for temperature dependence of contact angle and interfacial tension were incorporated into the van Genuchten [1980] model and fitted to the data. Capillary pressures at constant degrees of saturation decreased as temperature increased. Hysteresis decreased, irreducible water saturations increased, and residual nonwetting saturations decreased as temperature increased. The magnitude of the change in capillary pressures could not be explained by the temperature dependence of wetting‐nonwetting interfacial tensions alone. Derived parameters for the temperature dependence of the contact angle predicted an increase of contact angle of roughly 45°–50° for air‐water and perchloroethylene systems with a temperature increase from 20° to 80°C, while literature studies suggest that contact angles should decrease with increasing temperature. It was concluded that the parametric relationship for temperature effects incorporated into the van Genuchten [1980] model fit the data well, but other effects in addition to changes in interfacial tension and contact angle played a role in the temperature dependence of capillary pressure‐saturation relationships.
Viscous drag, driving forces, and their reduction to Darcy's Law
Authors: Li Jiang , Helm Donald
Published: February 2004

On the basis of the conservation of momentum for pore water flow, a new relation between drag and driving forces is developed for flow within a saturated aquifer. Such a relation inherently includes inertial terms of both the solid phase and the fluid phase that play an important role under dynamic conditions (e.g., earthquakes) and under some transient conditions (e.g., aquifer pumping). For laminar flow of a fluid relative to the skeletal frame a new relation is reached by simplification from the more general case. The Darcy‐Gersevanov law and Darcy's law are found to be special cases that result from further simplifications.
Merging and error analysis of regional hydrometeorologic anomaly forecasts conditioned on climate precursors
Authors: Liu Zhongjian , Valdés Juan B. , Entekhabi Dara
Published: February 2004

Forecasts of hydroclimatic variables and incorporation of their error bounds are invaluable in water resources planning and operations under uncertainty. In this study, regional long‐term operational hydrologic forecast models conditioned on climatic precursor are presented. The forecasts also include uncertainty intervals and confidence limits. The forecasts are based on the temporal and spatial variability of hydrometeorologic anomalies and their relationships with climatic interannual and intraseasonal El Niño‐Southern Oscillation (ENSO). The forecast skills of the proposed model, which incorporates ENSO forecasts on tropical rainfall and streamflow, are compared with those that are unconditional and do not incorporate ENSO. Significantly improved skills are achieved by incorporating forecasted ENSO indices and their errors. The seasonal variability of the forecast model skills are also evaluated. These ENSO‐based forecasts of regional and seasonal‐to‐interannual hydrometeorologic variables consistently merged with systematic error analysis can provide outputs for direct use in water resources planning and operation under uncertainty.
Multiple‐input transfer function modeling of daily streamflow series using nonlinear inputs
Authors: Astatkie T. , Watt W. E.
Published: February 2004

Single‐ and multiple‐input transfer function (TF) models and their applications in modeling streamflow systems are reviewed. To account for nonlinearities in streamflows, a procedure for generating the two most important inputs, snowmelt and effective rain, using nonlinear filters is proposed. A multiple‐input TF model identification procedure that uses a vector autoregressive model and the Corner method is proposed. These procedures are illustrated using 3 years of daily data for the Saugeen River near Port Elgin in Ontario, Canada. The final TF model was found to be very parsimonious and more accurate (in terms of mean‐squared error) than the more complex, nonlinear Nested Threshold AutoRegressive model.
Eulerian and Lagrangian correlation structures of convective rainstorms
Authors: May Don R. , Julien Pierre Y.
Published: February 2004

The Eulerian and Lagrangian correlation structures of 13 convective rainstorms were investigated using rainfall data from the 1987 Convective Initiation Downburst Experiment in Denver, Colorado. One minute rainfall data were available for two superposed raingage networks: 46 raingages on the portable automated mesonet (PAM) network at a mean spacing of 10.6 km and 31 additional raingages on the Federal Aviation Administration Lincoln Laboratory Operational Weather Studies (FLOWS) network at a mean spacing of 2.4 km. Eulerian correlation coefficients are low and often negative for high‐resolution 1 min rainfall data. Correlation increases with time‐averaging, beyond 10–15 min for single rain cells on the FLOWS network and small‐mesoscale rainstorms on the PAM network. The average speed of rain cells and small‐mesoscale rainstorms was found to be 76.5 and 56.4 km hr−1, respectively. Storm kinematic is identified as the cause of data scatter and low Eulerian correlations. Convective rainstorms are essentially uncorrelated in the Eulerian reference frame at a typical raingage spacing of 2–3 km for rain cells and 10–15 km for small mesoscale rainstorms. A spatial cross‐correlation analysis in a Lagrangian reference frame moving with the center of mass of the storm separates the kinematic component from the structural component of a rainfall field. The spatial cross correlations in the Lagrangian rainfall field show considerable improvement over the Eulerian spatial correlation plots, and data scatter is greatly reduced. The increase in correlation coefficients from Eulerian to Lagrangian reference frames typically ranged from 0.5 to 1.1.
Experimental study of a bimolecular reaction in Poiseuille Flow
Authors: Kapoor Vivek , Jafvert Chad T. , Lyn Dennis A.
Published: February 2004

A bimolecular chemical reaction (reactantl + reactant2 → products) in laminar Poiseuille flow is experimentally observed using a spectrophotometer. The reaction rate (rm) follows the second‐order rate law; that is, rm = кC1C2, where cm(m=1, 2) are the reactant concentrations. The reaction rate constant к is independently estimated by monitoring the reaction kinetics in a completely mixed batch reactor using the stopped‐flow technique. In the reactive transport experiments, the reactants are introduced in a tube and are initially separated by a sharp interface. The variation of the fluid velocity over the cross section of the tube causes the concentrations of the reactants to vary around their cross‐sectional average values (c¯m). These spatial variations in the concentrations (c′m) influence the overall reaction rate. The cross‐sectional average reaction rate is given by
, where
is the segregation intensity. The experimentally observed breakthrough concentration of the product is in agreement with a numerical model that accounts for the effects of the segregation intensity. On ignoring the influence of the segregation intensity, the predicted product concentration substantially exceeds the experimental observations. This shows that for initially non‐overlapping reactants the segregation intensity is negative (s < 0) and that the overall chemical transformation rate in flowing systems can be significantly different from that implied by substituting the mean concentrations in the expression for the reaction rate.
Incorporation of constraints on hydraulic head gradients near no‐flow boundary conditions in the determination of spatial drift and their use in an inverse groundwater flow model
Authors: Lamorey Gregg , Jacobson Elizabeth
Published: February 2004

Transmissivities determined by an inverse groundwater model are dependent on the prior hydraulic heads used to calibrate the model. It has been observed in a previous case study of the Avra Valley aquifer in southern Arizona that when gradients in the prior head distribution obtained by kriging do not match assumed no‐flow boundary conditions, the inverse model produced low transmissivity values along these boundaries in an attempt by the model to reconcile inconsistencies in the head gradient and the boundary conditions. To alleviate this problem, an estimation method that includes constraints on the hydraulic head gradient across no‐flow boundaries is presented and used to obtain prior heads estimated at node points for the Avra Valley aquifer. The method involves estimating the drift in the hydraulic head data and then kriging the hydraulic head residuals. The method is multiobjective in that the goal in the drift estimation is to fit simultaneously both the measured heads at well locations and the head gradient along the no‐flow boundary conditions. The appropriate weighting of the no‐flow boundary constraints is investigated by examining the tradeoff in fitting head measurements and fitting the no‐flow boundary conditions. Changes in the semivariogram of the head residuals and the estimation error of the heads are also considered. It is found that boundary constraints can improve the fit of the estimated heads to the no‐flow boundaries with little deterioration of the fit to the head measurements. Inverse modeling of the Avra Valley aquifer based on these estimated prior heads did not produce the low transmissivities along the no‐flow boundaries. The constraints on the head gradient resulted in lower estimation errors on the prior heads and the subsequent lower estimation error of the transmissivities in regions of the aquifer in the vicinity of a no‐flow boundary where few head measurements exist. Including boundary conditions in estimating prior heads for use in inverse modeling yielded more realistic transmissivities with lower estimation errors.
Groundwater drainage flow in a soil layer resting on an inclined leaky bed
Authors: Koussis Antonis D. , Smith Martin E. , Akylas Evangelos , Tombrou Maria
Published: February 2004

We study subsurface storm flow from a planar hill slope, a problem that is similar hydraulically to lateral flow toward drains in landfills. Our analysis is based on the linearized one‐dimensional Boussinesq equation (Dupuit‐Forchheimer approximation), which is extended to allow for leakage through the underlying barrier. This linear advection‐diffusion equation has a greater range of validity than the kinematic wave equation. Stating it in terms of the discharge, the variable of primary hydrologic interest, we integrate it numerically, using an adaptation of the Muskingum‐Cunge routing scheme. A single‐step computation of the outflow hydrograph, which combines the convenience of an analytical solution formula with acceptable accuracy, is proposed as a design tool and as a means of parameterization of drainage from hill slopes. Depth profiles are determined afterwards by a simple integration of Darcy's law. Examples of the buildup and recession phases, with and without leakage, demonstrate the application of the computational method.
Exploring functional similarity in the export of Nitrate‐N from forested catchments: A mechanistic modeling approach
Authors: Creed I. F. , Band L. E.
Published: February 2004

Functional similarity of catchments implies that we are able to identify the combination of processes that creates a similar response of a specific characteristic of a catchment. We applied the concept of functional similarity to the export of NO3−‐N from catchments situated within the Turkey Lakes Watershed, a temperate forest in central Ontario, Canada. Despite the homogeneous nature of the forest, these catchments exhibit substantial variability in the concentrations of NO3−‐N in discharge waters, over both time and space. We hypothesized that functional similarity in the export of NO3−‐N can be expressed as a function of topographic complexity as topography regulates both the formation and flushing of NO3−‐N within the catchment. We tested this hypothesis by exploring whether topographically based similarity indices of the formation and flushing of NO3−‐N capture the observed export of NO3−‐N over a set of topographically diverse catchments. For catchments with no elevated base concentrations of NO3−‐N the similarity indices explained up to 58% of the variance in the export of NO3−‐N. For catchments with elevated base concentrations of NO3−‐N, prediction of the export of NO3−‐N may have been complicated by the fact that hydrology was governed by a two‐component till, with an ablation till overlying a basal till. While the similarity indices captured peak NO3−‐N concentrations exported from shallow flow paths emanating from the ablation till, they did not capture base NO3−‐N concentrations exported from deep flow paths emanating from the basal till, emphasizing the importance of including shallow and deep flow paths in future similarity indices. The strength of the similarity indices is their potential ability to enable us to discriminate catchments that have visually similar surface characteristics but show distinct NO3−‐N export responses and, conversely, to group catchments that have visually dissimilar surface characteristics but are functionally similar. Furthermore, the similarity indices provide a potentially powerful method to scale and generalize NO3−‐N export responses to other regions.