bluuebunny/crossref_metadata_2025 huggingface dataset:
Alternative representation of dyadic Green's functions for circular cylindrical cavities with applications to the EMC characterization of space station modules
Authors: Daniele Vito , Gilli Marco , Graglia Roberto D.
Published: September 2002

This paper compares two different representations of the dyadic Green's functions for circular cylindrical cavities, also valid at the source point. The representations are obtained by considering the cavity either as a circular waveguide (longitudinal representation) or as a radial waveguide (radial representation), terminated by conducting surfaces. The radial representation of dyadic Green's functions for circular cylindrical cavities is not available in the previous literature and is applied to study the field penetrating a space station module through apertures, including lateral ones, or due either to elementary sources or to wire antennas active in the module. We show that for the typical dimensions of spatial modules the radial representations give rise to more convergent series and therefore are more suitable for the computation of the cavity electromagnetic fields. The problem of obtaining the self and mutual impedances of two wire antennas located in the cavity is also discussed in general, and various results are reported for particular cases. Simplified cavity models can be effectively applied to the electromagnetic compatibility (EMC) characterization of space station modules.
Remote sensing of hydrometeors by means of interferometric radiometry: Theory and experimental results
Authors: Bará J. , Corbella I. , Torres F. , Camps A. , Martin‐Neira M. , Nesti G. , Olhmer E.
Published: September 2002

Spaceborne and ground‐based radiometric measurements at microwave and millimeterwave frequencies have proved able to monitor hydrometeors, but at present the angular resolution of those systems is limited by the radiometer's antenna beam width. Efforts to improve the angular resolution of such systems by applying the interferometric techniques successfully used in radio astronomy [Thompson et al, 1986; Ruf et al., 1988] have encountered two main limitations. First, imaging interferometric radiometers are extremely complex, with a very large number of antennas and correlators. Second, because of the wide field of view, the array size is restricted by spatial decorrelation effects that cannot be simply compensated for by an adjustable delay, as in radio astronomy. Bandwidth segmentation overcomes this limitation at the expense of an even higher system's complexity. To overcome these limitations, this paper presents a technique that may achieve high angular resolution with a minimum number of antennas, taking advantage of the spatial filtering due to signal decorrelation. It describes both the basic theory behind this process and the experimental results at X band with a random static distribution of rubber bits and with simulated rain. The experimental results agree well with theoretical predictions and computer simulations, showing the inevitable trade‐off between angular resolution and radiometric sensitivity.
Static image principle for the sphere in isotropic or bi‐isotropic space
Authors: Lindell I. V. , Hänninen J. J.
Published: September 2002

The classical inversion transformation introduced by Kelvin is revisited. It is shown that it can be used to formulate the electrostatic image principle not only for the perfectly electrically conducting (PEC) sphere but also for the perfectly magnetically conducting (PMC) sphere and for the sphere with more general impedance boundary conditions. In the latter two cases the image of an electric point charge is seen to consist of electric point and line charges. More generally, it is shown that if the space is filled by the most general linear isotropic medium (bi‐isotropic medium), the same method can be extended to form the corresponding image principle for the PEC, the PMC, and the impedance sphere. In these cases an electric point charge is seen to have an image composed, in general, of both electric and magnetic point and line charges.
Influence of a magnetically permeable surface layer on electromagnetic induction using a horizontal current loop source
Authors: Mahmoud Samir F. , Wait James R.
Published: September 2002

Electromagnetic induction on a layered earth model is reexamined when the usually neglected magnetic permeability contrast is considered. It is shown that for a two‐layered earth model, where the upper layer is permeable, the electromagnetic induction response is modified over the nonpermeable case. Universal plots of normalized induced fields due to an electrically small current loop are produced. Special attention is paid to the case when the upper earth layer consists of a superparamagnetic material. This formulation should be useful for interpreting airborne electromagnetic systems and in cases where superparamagnetic mineral content is present.
Dominant lateral waves in the canopy layer of a four‐layered forest
Authors: Koh J. H. , Li L. W. , Kooi P. S. , Yeo T. S. , Leong M. S.
Published: September 2002

In this paper a forest model of four‐layered geometry is utilized for the theoretical analysis of radio wave propagation characteristics in the forest environment. Differing from those reported in previous publications, the current paper considers a receiving antenna that is located in the forest canopy layer instead of the trunk layer, and hence the results presented are not available elsewhere. Dyadic Green's functions in their eigenfunction expansion forms for the four‐layered geometry are used at first to obtain an exact integral representation of the radiated electric field in the canopy layer. The radiated electric field components due to an arbitrarily oriented, small dipole in the trunk layer are then evaluated using the saddle point technique and the branch cut integrations, leading to closed‐form expressions for the field. The total field is found to consist of the direct, the reflected, and the lateral waves. Although both the lateral waves propagating along the air‐canopy and the trunk‐ground interfaces play an important role in the propagation mechanism, only the lateral wave along the air‐canopy upper interface dominates the total field in the far zone. Path loss of the radio waves in the forest is computed numerically for both the vertically and the horizontally oriented dipoles.
Effectiveness of near‐grazing incidence reflection in creating the rotationally modulated lanes in the Jovian hectometric radio emission spectrum
Authors: Menietti J. D. , Gurnett D. A. , Kurth W. S. , Groene J. B.
Published: September 2002

The Galileo plasma wave instrument has identified a narrow (in frequency) attenuation band in the hectometric emission that varies in frequency with system III longitude. It is possible to model this emission band assuming a high‐latitude cyclotron source region with emission that is efficiently attenuated when the ray path is nearly tangent to an L shell that is close to the Io flux tube [Gurnett et al, 1998]. The data suggest that the mechanism for attenuating the emission is very efficient, with the ratio of attenuated to unattenuated emission I/I0 < 0.02, and not a strong function of frequency. In this paper we demonstrate that incoherent scattering alone cannot explain the attenuation lane, which does not preclude coherent scattering by uncertain processes. We find rather that the source of attenuation is consistent with near‐grazing incidence reflection of emission from an L shell that is near the Io flux tube (a caustic surface).
Chaotic scattering of microwaves
Authors: Kottos Tsampikos , Smilansky Uzy , Fortuny Joaquim , Nesti Giuseppe
Published: September 2002

In this paper we analyze a recent experiment conducted in an anechoic chamber, where the scattering of microwaves from an array of metallic cylinders was measured. This is a system which displays chaotic scattering in the short wave limit. The analysis of the experimental data is aimed at elucidating the effects of the underlying chaos. We describe a robust numerical method which provides the scattering matrix for any number of nonoverlapping reflecting cylinders. We use this method to calculate the scattering from one, two, and three cylinders and to compare the results in the numerical simulations with those obtained in the experiment. Both simulations and measurements validate the presented theory.
Derivation of higher‐order impedance boundary conditions for stratified coatings composed of inhomogeneous‐dielectric and homogeneous‐bianisotropic layers
Authors: Galdi Vincenzo , Pinto Innocenzo M.
Published: September 2002

A spectral domain framework is presented for deriving exact electromagnetic impedance boundary conditions for isotropic, longitudinally inhomogeneous, dielectric coatings on a general (polarization‐rotating) impedance plane. The derived expressions are shown to be well approximated over a reasonably wide range of parameters by means of rational functions of the spectral variables, from which higher‐order approximate impedance boundary conditions are readily obtained by simple Fourier transformation. The proposed method is readily extended to multilayer coatings consisting of any combination of inhomogeneous dielectric layers and homogeneous, arbitrarily complex (e.g., bianisotropic, nonreciprocal) materials. Application to curved boundaries is also possible. A number of examples are included to validate the proposed approach and show its versatility and reliability.
Leaky‐wave phenomena viewed from the time domain
Authors: Shigesawa Hiroshi , Tsuji Mikio
Published: September 2002

It is known until now that when most printed‐circuit transmission lines are excited by a practical source of a monochromatic high frequency, the near‐field amplitude decays exponentially along the guide axis due to the excitation of a leaky mode. This is true if the excitation frequency is in the leaky portion of the guide and the leaky mode is the only physical mode at this frequency. Then, the decay rate of the field amplitude is given by the leakage constant of the physical leaky mode. However, we have recently found that although the excitation is set at the frequency at which the leaky mode is the only physical solution, there is a situation in which the amplitude decay of the near field becomes much smaller than that expected from the leaky‐mode eigenvalue. Then, contrary to our earlier belief that nonphysical solutions exert little influence on the physical field, we have recently discovered that such a slow decay in the physical near‐field amplitude is strongly influenced by an improper‐real solution, which is in the leakage portion as a nonphysical solution. We report here for the first time that the slow decay in the near field depends on how close the nonphysical improper‐real solution is to the surface‐wave curve relevant to power leakage. In this paper we present the evidence for this discovery by a discussion based on the generalized pencil of function analysis applied to the finite difference time domain data. To reinforce these discussions, we also present discussions viewed from the mutual relation between the excited‐field distributions on the guide and the evolution of the improper‐real solution on the steepest‐descent plane.
Solving the volume integral equation in axisymmetric inhomogeneous media using the conjugate gradient fast Hankel transform method
Authors: Chen S. Y. , Chew W. C. , Kennedy W. D.
Published: September 2002

In this paper we present a new method to solve the volume integral equation governing the electromagnetic wave propagation in axisymmetric inhomogeneous media. The integral equation is first formulated using a one‐dimensional Green's function and then solved using the conjugate gradient fast Hankel transform method. The use of the one‐dimensional Green's function reduces the number of unknowns significantly, thereby accelerating the iterative procedure. Several numerical results are used to show the efficiency and accuracy of this new approach as well as its applications in electromagnetic subsurface sensing.
Electromagnetic wave propagation through simulated atmospheric refractivity fields
Authors: Gilbert Kenneth E. , Di Xiao , Khanna Samir , Otte Martin J. , Wyngaard John C.
Published: September 2002

Large‐eddy simulation (LES) provides three‐dimensional, time‐dependent fields of turbulent refractivity in the atmospheric boundary layer on spatial scales down to a few tens of meters. These fields are directly applicable to the computation of electromagnetic (EM) wave propagation in the megahertz range but not in the gigahertz range. We present an approximate technique for extending LES refractivity fields to the smaller scales needed for calculating EM propagation at gigahertz frequencies. We demonstrate the technique by computing refractivity fields through 1283LES, extending them to smaller scales in two dimensions, and using them in a parabolic equation EM propagation model. At 0.263 GHz the very small scale structure in the extended fields has a negligible effect on the predicted EM levels. At 2 GHz, however, it increases the predicted levels by 15–25 dB. We relate these results to the refractivity structure sampled by EM waves at 0.263 and 2 GHz. We also show that at long range an EM field calculated through an LES‐based refractivity field is generally less coherent and significantly weaker than one computed from a “plywood” (i.e., stratified, range‐independent) model of the small‐scale refractivity field. We give a physical explanation for the differences in the EM fields computed in these two ways. Finally, although the plywood model gives results that fit the EM levels observed in the recent Variability of Coastal Atmospheric Refractivity (VOCAR) experiment, it is not physically realistic. The instantaneous top of the atmospheric boundary layer is known to be sharp and horizontally varying, and we show that using such a top also yields a fit to the VOCAR data.
An iterative numerical method for inverse scattering problems
Authors: Rekanos Ioannis T. , Tsiboukis Theodoros D.
Published: September 2002

In this paper an inverse scattering method for reconstructing the constitutive parameters of two‐dimensional scatterers is proposed. The inversion is based on measurements of the scattered magnetic field component, while the scatterer domain is illuminated by transverse electric waves. The spatial distribution of the inverse of the relative complex permittivity is estimated, iteratively, by minimizing an error function. This minimization procedure is based on a nonlinear conjugate gradient optimization technique. The error function is related to the difference between the measured and estimated scattered magnetic field data. Moreover, an additional term, which is associated with the Tikhonov regularization theory, is introduced to the error function in order to cope with the illposedness of the inverse problem. For an estimate of the scatterer profile the direct scattering problem is solved by means of the finite element method. On the other hand, the gradient of the error function is computed by a finite element based sensitivity analysis scheme. The latter is enhanced by introducing the adjoint state vector methodology. This approach reduces dramatically the computational burden. The capabilities of the proposed method are investigated by applying it to synthetic field measurements, which are affected by additive noise. Different levels of the regularization are also examined.
A novel formulation for the on‐surface measured equation of invariance method and its applications to scattering and radiation problems
Authors: Lan K. , Liu Y. W. , Mei K. K.
Published: September 2002

A novel formulation, surface current measured equation of invariance (SCMEI), for the on‐surface measured equation of invariance method (OSMEI) is deduced from the reaction integral equation. This formulation directly couples the current densities and scattered tangential electrical fields on the perfect conducting surface in the MEI equations. Compared with the original OSMEI, the advantage of the SCMEI is that when finding the MEI coefficients, only measuring functions of electric fields need to be integrated from the selected metrons, and the surface current can be directly obtained from the solution of the MEI equations. The computing burden for the measuring functions is thus reduced. The SCMEI can be used to analyze open boundaries, such as strips and wire antennas, etc., while the OSMEI was applicable only to closed boundaries because of the requirement of simultaneous integrals of electric and magnetic fields. The limitation for the present achievement, which needs to be further improved, is that the number of the nodes coupled in the SCMEI equations should be larger than that of the general OSMEI equations. The validity of this formulation is verified by modeling scattering of perfect conducting cylinders and radiation of thin‐wire antennas.
First synthesis of wind‐profiler signals on the basis of large‐eddy simulation data
Authors: Muschinski A. , Sullivan P. P. , Wuertz D. B. , Hill R. J. , Cohn S. A. , Lenschow D. H. , Doviak R. J.
Published: September 2002

Radar wind profilers detect scatter from clear‐air refractive‐index irregularities. The Doppler shift calculated from the time series of the backscattered signal provides an estimate of the radial velocity of the air within the radar's resolution volume. It is known that there are quite a number of effects that can give rise to intrinsic biases in these radial‐velocity estimates. It is extremely difficult to unambiguously identify these biases on the basis of observations since it is impossible to independently probe the three‐dimensional, time‐dependent fine structure of the relevant atmospheric variables (wind and refractive index) within the radar's resolution volume down to the relevant spatial and temporal scales associated with the scattering process itself. In this paper the large‐eddy simulation (LES) technique is used to generate, with high spatial and temporal resolution (vertical resolution, 8 m; horizontal resolution, 16 m; time step, 0.8 s), data of atmospheric variables within a convective boundary layer. The LES data that are used in this study do not have the spatiotemporal solution that would be needed for a deterministic, full solution of the scattering integral. However, a parameterized model of the scattering integral can be constructed on the basis of Kolmogorov‐scaling arguments and a random phase model. This model can be numerically solved on the basis of LES data. For the first time, LES‐generated fields of virtual temperature, humidity, and three‐dimensional wind are used to construct time series of the real and imaginary parts of wind‐profiler signals. The simulated wind‐profiler signals are processed to derive Doppler velocities, and the simulated Doppler velocities are compared with the (simulated) meteorological "in situ truth" provided directly by the LES wind‐velocity data set. Examples of simulation results and the potential of the LES‐based, wind‐profiler simulation technique as well as some limitations are discussed.
Predicting the effects of anomalous refraction on the telescope point spread function: Tip‐tilt compensation at millimeter wavelengths
Authors: Olmi Luca
Published: September 2002

Radio‐seeing effects on centimeter‐ and millimeter‐wavelength interferometers are now reasonably well known. On the other hand, radio seeing also shows up on filled‐aperture telescopes as an anomalous refraction, i.e., an apparent displacement of a radio source from its true position. The magnitude of this effect, as a fraction of the beam width, is bigger on larger telescopes, and thus its impact on the pointing is likely to become critically important in the next generation of electrically large filled‐aperture radio telescopes (D/λ ≳ 3 ‐ 5×104, where D is the diameter). Also, because of the increasing number of focal plane arrays being used for both spectroscopic and continuum astronomical observations, it is of great interest to study the atmospherically distorted shape of the telescopic point spread function and the resulting cross talk among adjacent pixels. Here we present a model study of anomalous refraction effects by producing simulations of a two‐dimensional phase screen located in the aperture plane of the antenna, using the Large Millimeter Telescope as a benchmark. Then we discuss the basic concept and simulate the operation of a tip‐tilt compensation method at millimeter wavelengths that would use an aperture plane array of 183 GHz radiometers.
Azimuthal variations of the microwave radiation from a slightly non‐Gaussian sea surface
Authors: Irisov V. G.
Published: September 2002

Microwave radiation from the ocean rough surface is considered using a small slope expansion. The emphasis is on azimuthal variations of the brightness temperature, related to the anisotropy of wind‐generated sea waves. The first and second harmonics, describing the upwind‐downwind and upwind‐crosswind differences, respectively, are considered using existing models of the sea spectrum. For modeling of the first harmonic the small deviation from Gaussian statistics of the sea surface is introduced to estimate the effect of longwave asymmetry and the distribution of ripples over the long waves. Numerical analysis shows that the longwave asymmetry cannot explain the observed values of the first harmonic in azimuthal variations of the brightness temperature. The ripple modulation by long waves is a possible mechanism explaining the first harmonic in moderate winds, although foam and wave breaks have to be included in electromagnetic modeling to achieve a quantitative agreement with experiments.
Estimating dynamic ionospheric changes without a priori models
Authors: Granat Robert A. , Na Helen
Published: September 2002

We present a novel artificial‐neural‐network‐based computerized ionospheric tomography technique, capable of imaging through time in three‐dimensional space. Total electron content (TEC) data collected from a satellite passing over the region of interest are used to train a neural network. The trained network creates estimates of the difference in ionospheric electron density between time steps based on the TEC data. Application of the difference estimate at each time step to the previous electron density image results in a time‐varying ionospheric electron density estimate. Experimental results on synthetic data are presented that demonstrate that the algorithm is capable of detecting short‐term localized disturbances in the ionosphere.
Response of intracontinental deformation in the central Andes to late Cenozoic reorganization of South American Plate motions
Authors: Marrett Randall , Strecker Manfred R.
Published: September 2002

New stratigraphic, chronologic, and fault kinematic data from the Quebrada del Toro in northwestern Argentina provide a detailed view of late Cenozoic kinematic reorganization in the southern central Andes. The Quebrada del Toro is a thrust‐bounded basin filled with 2 km of synorogenic clastic deposits, ranging in age from early Miocene to Holocene. As observed elsewhere in northwestern Argentina, faults in the Quebrada del Toro record an early phase of horizontal NW‐SE contraction (beginning between early and late Miocene and ending after 0.98 Ma) and a later phase of horizontal NE‐SW contraction (beginning between late Miocene and 4.17 Ma and still active). These results require both kinematic regimes to have been active between 4.17 and 0.98 Ma, compatible with timing and kinematic data from the adjacent central Andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes. A hypothesis that intraplate deformation in the central Andes is driven by the absolute motion of the South American Plate with respect to the hotspot reference frame is tested by comparing Quebrada del Toro kinematics to plate reconstructions and space‐based geodetic results. Nazca‐South America relative motion and absolute Nazca Plate motion show consistent azimuths throughout the past 20.5 Myr. In contrast, absolute South American Plate motion azimuths mimic the changing contraction directions determined from intraplate Andean faults. Although the timing of the South American Plate motion change is poorly resolved by current data, it is compatible with the Quebrada del Toro data. Absolute upper plate motion might control intraplate kinematics along many of the Earth's convergent margins.
Central Costa Rica deformed belt: Kinematics of diffuse faulting across the western Panama block
Authors: Marshall Jeffrey S. , Fisher Donald M. , Gardner Thomas W.
Published: September 2002

Fault kinematics, seismicity, and geodetic data across central Costa Rica reveal a diffuse fault zone, here named the Central Costa Rica Deformed Belt (CCRDB). The CCRDB defines the western margin of the Panama block and links the North Panama Deformed Belt (NPDB) along the Caribbean coast with the Middle America Trench (MAT) along the Pacific coast. The junction of the CCRDB and the MAT coincides with an abrupt transition from smooth to rough crust on the subducting Cocos plate (rough‐smooth boundary). Shallow subduction of rough, thickened oceanic crust associated with the Cocos Ridge shifts active shortening into the volcanic arc along faults of the CCRDB. Variable fault kinematics along this zone may reflect three combined deformation mechanisms: horizontal shortening and shear from oceanic ridge indentation, basal traction from shallow subduction, and localized block uplift from subducting seamount roughness. Within the forearc (domain 1), mesoscale faults express transtension where steep NE striking regional‐scale faults intersect the Pacific coast. Across the volcanic arc (domain 2), mesoscale faults exhibit mostly sinistral and dextral slip on NE and NW striking conjugate faults, respectively. Approaching the NPDB in the back arc (domain 3), transcurrent faulting is modified by transpression and crustal thickening. Fault kinematics are consistent with earthquake focal mechanisms and Global Positioning System (GPS) measurements. Radiometric age constraints confirm that faulting postdates the late Neogene onset of shallow subduction. The ensuing deformation front has propagated northward into the volcanic arc to its present position along the seismically active CCRDB. Within the forearc, the effect of shallow subduction is overprinted by local uplift related to underthrusting seamounts.
Thermochronologic constraints on the breakup of the Pacific Gondwana margin: The Paparoa metamorphic core complex, South Island, New Zealand
Authors: Spell Terry L. , McDougall Ian , Tulloch Andy J.
Published: September 2002

Continental extension preceding the breakup of Gondwana in the Cretaceous produced a metamorphic core complex preserved in the Paparoa Range on South Island, New Zealand. Most features of classic Cordilleran core complexes are present including high metamorphic grade lower plate rocks separated from low‐grade upper plate rocks by detachment faults, syntectonic granitic intrusions and volcanism, and thick sequences of subaerial breccias and conglomerates. Dating of lower plate rocks by the40Ar/39Ar method indicates rapid cooling rates up to 110 °C Myr−1from temperatures of ∼500°–170 °C during the Cretaceous interval from ∼110 to 90 Ma, followed by lower cooling rates (∼5 °C Myr−1) beginning at ∼90 Ma. In contrast, granites intruding the upper plate underwent slow cooling (<5 °C Myr−1) during extended cooling histories (>200 Ma) beginning in the Devonian and ending in the Cretaceous. Combined with published U/Pb and fission track dates, the K/Ar‐ and40Ar/39Ar data define complete thermal histories (∼700°–100 °C), indicating rapid unroofing of lower plate rocks during a brief interval around 100 Ma. Rapid cooling rates recorded in lower plate rocks contrast with the extended slow cooling histories of upper plate rocks. Cooling ages for core rocks relative to distance from the southern (Pike) detachment fault indicate extension rates of ∼4 mm yr−1and suggest that the Pike detachment was responsible for most of the unroofing. The presence of syntectonic granitic plutons supports models in which magmatism is intimately associated with core complex formation. Slower cooling rates beginning at ∼90 Ma may record cessation of continental extension and the inception of seafloor spreading in the Tasman Sea (oldest basaltic crust, ∼84 Ma). These data establish a temporal and spatial link between continental extensional tectonics of Gondwana at ∼110–90 Ma and inception of seafloor spreading in the Tasman Sea (∼90–80 Ma) leading to separation of New Zealand from Australia and Antarctica.
Continental collision, gravity spreading, and kinematics of Aegea and Anatolia
Authors: Martinod J. , Hatzfeld D. , Brun J. P. , Davy P. , Gautier P.
Published: September 2002

We have carried out experiments using a layered medium of sand and silicone to investigate the lateral extrusion of a material which spreads over its own weight while being compressed by the advance of a rigid indenter. Boundary conditions in the box mimic those prevailing in the Anatolian‐Aegean system. Both shortening in front of the rigid piston, which models the northward motion of Arabia, and extension resulting from the gravity spreading of the sand‐silicone layer are necessary to initiate the lateral extrusion. Strike‐slip faults accommodate the lateral escape and link the normal faults accompanying gravity spreading with the thrust faults in front of the rigid indenter. Strike‐slip faults begin to accommodate extrusion at a late stage in the experiments after the normal and thrust faults have developed. Experiments also show that the initial geometry of the boundary of the spreading layer may result in the formation of two arcs behind which material extends, in a manner analogous to the Hellenic and Cypriot arcs, without invoking a rheological change at the junction of the two arcs. The experiments also suggest that southward motion of the eastern, part of the spreading region is compensated by the northward advance of the piston, which is a possible explanation for the slower movement of the Cypriot arc compared to the Aegean arc.
Water supply planning under interdependence of actions: Theory and application
Authors: Rajabi S. , Hipel K. W. , Kilgour D. M.
Published: September 2002

An ongoing water supply planning problem in the Regional Municipality of Waterloo, Ontario, Canada, is studied to select the best water supply combination, within a multiple‐objective framework, when actions are interdependent. The interdependencies in the problem are described and shown to be essential features. The problem is formulated as a multiple‐criteria integer program with interdependent actions. Because of the large number of potential actions and the nonconvexity of the decision space, it is quite difficult to find nondominated subsets of actions. Instead, a modified goal programming technique is suggested to identify promising subsets. The appropriateness of this technique is explained, and the lessons learned in applying it to the Waterloo water supply planning problem are described.
Denitrification in marine shales in northeastern Colorado
Authors: McMahon P. B. , Böhlke J. K. , Bruce B. W.
Published: September 2002

Parts of the South Platte River alluvial aquifer in northeastern Colorado are underlain by the Pierre Shale, a marine deposit of Late Cretaceous age that is <1000 m thick. Ground water in the aquifer is contaminated with NO3‐, and the shale contains abundant potential electron donors for denitrification in the forms of organic carbon and sulfide minerals. Nested piezometers were sampled, pore water was squeezed from cores of shale, and an injection test was conducted to determine if denitrification in the shale was a sink for alluvial NO3− and to measure denitrification rates in the shale. Measured values of NO3−, N2, NH4+, δ15N[NO3−], δ15N[N2], and δ15N[NH4+] in the alluvial and shale pore water indicated that denitrification in the shale was a sink for alluvial NO3−. Chemical gradients, reaction rate constants, and hydraulic head data indicated that denitrification in the shale was limited by the slow rate of NO3− transport (possibly by diffusion) into the shale. The apparent in situ first‐order rate constant for denitrification in the shale based on diffusion calculations was of the order of 0.04–0.4 yr−1, whereas the potential rate constant in the shale based on injection tests was of the order of 60 yr−1. Chemical data and mass balance calculations indicate that organic carbon was the primary electron donor for denitrification in the shale during the injection test, and ferrous iron was a minor electron donor in the process. Flux calculations for the conditions encountered at the site indicate that denitrification in the shale could remove only a small fraction of the annual agricultural NO3‐input to the alluvial aquifer. However, the relatively large potential first‐order rate constant for denitrification in the shale indicated that the percentage of NO3− uptake by the shale could be considerably larger in areas where NO3− advection.
Injection mode implications for solute transport in porous media: Analysis in a stochastic Lagrangian Framework
Authors: Demmy George , Berglund Sten , Graham Wendy
Published: September 2002

The effect of solute injection mode is examined in the stochastic‐advective framework. For three‐dimensional heterogeneous aquifers, uniform resident injection of solute results in nonlinear propagation of mass arrival time mean and variance with distance. Injection in flux results in a linear propagation of mean mass arrival time and mass arrival time variance that is both lower and appears to reach a linear regime more rapidly than uniform resident injection. Implementation of instantaneous injections for both modes as boundary conditions for mathematical and numerical models is discussed.
Spatial distribution of deposited bacteria following Miscible Displacement Experiments in intact cores
Authors: Bolster C. H. , Mills A. L. , Hornberger G. M. , Herman J. S.
Published: September 2002

Miscible displacement experiments were performed on intact sand columns ranging from 15 to 60 cm in length to determine whether bacterial deposition varies at the centimeter scale within aquifer sediments. A 1‐pore‐volume pulse of radiolabeled cell suspension was introduced into the columns followed by a 2‐pore‐volume flush of artificial groundwater. The columns were then drained and dissected along the axis of flow. At ∼1‐cm intervals, nine samples were removed for the enumeration of sediment‐associated bacteria. Concentrations of sediment‐associated (deposited) bacteria varied by up to 2 orders of magnitude in the direction perpendicular to flow demonstrating that bacterial deposition cannot be described mechanistically by a single rate coefficient. Incorporation of a distribution of sediment size and porosity values into Monte Carlo simulations indicates that physical heterogeneities are only partially responsible for the observed variability in deposited bacteria. A simple first‐order model (classic filtration theory) adequately described the average spatial distribution of bacteria with depth within the 15‐cm column. For the longer columns, however, the average concentration of deposited bacteria did not decrease exponentially with depth. A second‐order model, modified to include an influent suspension of bacteria consisting of two subpopulations with separate sticking efficiencies (dual‐alpha population), was required to describe the observed decreases of deposited bacteria with depth. A sensitivity analysis was performed with a first‐order dual‐alpha model to understand the effects of an influent suspension with two subpopulations of bacteria on the decrease of deposited bacteria with flow path length. Numerical simulations show that even for small fractions (0.01) of nonsticky bacteria, the decrease in deposited bacteria may deviate substantially from the exponential decrease expected from colloid‐filtration theory. Results from experimental as well as numerical studies demonstrate the importance of column dissections for understanding bacterial deposition in saturated porous media.
Extension of “Transient flow in bounded randomly heterogeneous domains: 1, Exact conditional moment equations and recursive approximations”
Authors: Tartakovsky Daniel M. , Neuman Shlomo P.
Published: September 2002

In a recent paper we developed exact nonlocal conditional moment equations for transient flow in bounded domains driven by random forcing terms (sources, initial head, and boundary conditions). Whereas our conditional mean equations took into account the randomness of forcing terms, our conditional second moment equations did not. We do so in this brief addendum.
Field studies of in situ colloid mobilization in a southeastern coastal plain aquifer
Authors: Swartz Christopher H. , Gschwend Philip M.
Published: September 2002

The release of colloids to groundwater was investigated in situ in an iron‐oxyhydroxide‐rich, sandy aquifer. Groundwater amended with various solutes was injected into and immediately withdrawn from the shallow aquifer. Turbidity and colloid composition were monitored in the retrieved injectate. The response of the aquifer material to the amendments generally mimicked that observed in an earlier study using packed columns containing the sediment, demonstrating the viability of the single‐well method for testing colloid mobilization in situ. The decline of turbidity in the retrieved injectates with increasing withdrawal volume was analyzed to determine a “reaction order” n, describing the redeposition of mobilized colloids to the immobile matrix. Differences in the reaction order for the amendments tested presumably indicated the effectiveness of these amendments to generate repulsive colloid‐immobile matrix interactions.
Data for snowmelt model development, calibration, and verification at an Alpine Site, Colorado Front Range
Authors: Williams Mark W. , Cline Don , Hartman Mike , Bardsley Tim
Published: September 2002

Logistical constraints have caused data collection in seasonally snow covered areas to generally be on a campaign basis with limited instrumentation. The problems of winter access, cold air temperatures, and blowing snow cause both equipment malfunctions and problems with consistent and timely maintenance. At the Long‐Term Ecological Research program network site in an alpine area of Colorado we have been operating a meteorological station and subnivean (below snow) laboratory at 3517 m since the spring of 1994 to collect information that will allow us to better understand snow‐surface energy exchanges and the mass flux of water during snowmelt. This unique and high‐quality data set was designed to measure the meteorologic and hydrologic parameters necessary to compute the surface energy and snowpack mass balances at a point. All meteorological parameters are directly measured. Turbulent fluxes are calculated using the aerodynamic profile method. The timing and magnitude of snowmelt is measured with 18 snow lysimeters. Meteorologic parameters and energy fluxes are available at 10‐min, hourly, and daily time steps. Complementary information includes a high‐resolution digital elevation model, snowpack parameters, and stream discharge.
A constructive dual dynamic programing for a reservoir model with correlation
Authors: Yang Miao , Read E. Grant
Published: September 2002

The reservoir management problem for a hydrothermal power system is well suited to modeling via dynamic programing. In this paper we describe a dual approach which we term “constructive dynamic programming” (CDP) which has been successfully applied to optimize releases in a stochastic two‐reservoir model of the New Zealand power system. That model ignores serial correlations of inflows, though, and hence assumes that current inflow observations do not have any impact on future release decisions. Tests show, however, that better decision rules can be produced by accounting for inflow correlation. Hence we have developed an extension to the standard CDP to explicitly deal with serial correlation of reservoir inflows, and we report on those extensions also.
Phenanthrene adsorption‐desorption hysteresis in soil described using discrete‐interval equilibrium models
Authors: Burgos William D. , Munson Christopher M. , Duffy Christopher J.
Published: September 2002

Both adsorption and desorption data were used to estimate sorption parameters for phenanthrene with four soils of varying organic carbon content. Adsorption parameters were determined using linear and nonlinear equilibrium models for four different initial concentrations that had equilibrated with each soil for 70 days. Desorption parameters were determined from successive short‐term decant‐and‐refill batch water extractions following the 70‐day adsorption period. Separate nonsuccessive desorption kinetics experiments revealed that equilibria were attained rapidly (approximately minutes). Linear and nonlinear equilibrium forms of a discrete‐interval one‐stage model were then used to describe phenanthrene batch desorption from the soils. Adsorption‐based linear distribution coefficients normalized to organic carbon (OC) content of the soils (log KOC) ranged from 3.22 to 3.73 L kg−1 OC. Adsorption isotherms were fairly linear, n ranged from 0.80 to 1.0. Desorption‐based parameters predicted with the discrete‐interval linear one‐stage equilibrium model (log KOC) ranged from 3.69 to 4.04 L kg−1 OC. Desorption‐based parameters predicted with the discrete‐interval nonlinear model (KF) ranged from 67.4 to 493 (L kg−1)−n, and n ranged from 0.46 to 0.63. The nonlinear desorption model was significantly more accurate then the linear model. The adsorption‐desorption hysteresis of phenanthrene was closed loop and not an experimental artifact, indicating that adsorption and desorption were distinctly different processes in these soils.
Bias and variance of planning level estimates of pollutant loads
Authors: Schwartz Stuart S. , Naiman Daniel Q.
Published: September 2002

Planning level techniques typically use the product of runoff volume and a characteristic concentration to estimate mean annual contaminant loads when monitoring data are inadequate or unavailable. In contrast to the extensive literature on sampling properties, bias, and precision of loads estimated from monitoring data, the unconstrained and often inconsistent alternatives for choosing “representative” runoff volumes and concentrations for use in planning level estimates limit the opportunities of generalizing analytical results on the properties of these estimators. The ease with which these simple load estimates can be calculated belies their inherent uncertainty, motivating this examination of their bias and variability. The mean and variance of planning level load estimators are derived both under mild parametric assumptions and using a distribution free approximation. Common use of the mean, median, or geometric mean of event concentrations is shown to result, in general, in biased estimates of the mean annual load. Sensitivity analysis of the mean and variance demonstrates the need to incorporate the relative variance as well as the correlation of cumulative discharge and characteristic concentration in planning level load estimates. While analogous to load estimation from monitoring data, the results presented here are distinct and unrelated to retransformation or sampling biases that have been well documented in the river load literature. Substantive implications for regional assessments, planning, and watershed management are illustrated with a simple example drawn from Chesapeake Bay.
Calibration of a conceptual rainfall‐runoff model for flood frequency estimation by continuous simulation
Authors: Lamb Robert
Published: September 2002

An approach is described to the calibration of a conceptual rainfall‐runoff model, the Probability Distributed Model (PDM), for estimating flood frequencies at gauged sites by continuous flow simulation. A first step was the estimation of routing store parameters by recession curve analysis. Uniform random sampling was then used to search for parameter sets that produced simulations achieving the best fit to observed, hourly flow data over a 2‐year period. Goodness of fit was expressed in terms of four objective functions designed to give different degrees of weight to peaks in flow. Flood frequency results were improved, if necessary, by manual adjustment of parameters, with reference to peaks extracted from the entire hourly flow record. Although the primary aim was to reproduce observed peaks, consideration was also given to finding parameter sets capable of generating a realistic overall characterization of the flow regime. Examples are shown where the calibrated model generated simulations that reproduced well the magnitude and frequency distribution of peak flows. Factors affecting the acceptability of these simulations are discussed. For an example catchment, a sensitivity analysis shows that there may be more than one set of parameter values well suited to the simulation of peak flows.
Nonlocal and localized analyses of conditional mean steady state flow in bounded, randomly nonuniform domains: 2. Computational examples
Authors: Guadagnini Alberto , Neuman Shlomo P.
Published: September 2002

In paper 1 [Guadagnini and Neuman, this issue] of this two‐part series we presented exact nonlocal equations for first and second conditional ensemble moments of hydraulic head and flux under steady state groundwater flow in bounded, randomly nonuniform domains in the presence of uncertain source and boundary terms. We derived recursive approximations for these equations based on expansion in powers of a small parameter σY, which represents the standard estimation error of log hydraulic conductivity; developed a two‐dimensional finite element scheme for their solution, under the action of deterministic forcing terms, to first order in σY2 showed how to localize the conditional mean flow equations so as to yield familiar‐looking, Darcian differential equations in which, however, all quantities are conditional on data and therefore nonunique; and proposed that traditional deterministic flow models be reinterpreted in light of these localized equations. We stressed that all quantities which enter into our nonlocal and localized conditional moment equations are defined on a consistent support scale thereby obviating the need for upscaling (from support to grid scale) or the a priori definition of a computational grid; that such a grid can therefore be defined a posteriori based on the degree of smoothness one expects the conditional moments to exhibit; and that conditioning points are often spaced widely enough to render these moments relatively smooth and hence resolvable on a coarser grid than is necessary for Monte Carlo simulations. This notwithstanding, we use in this paper a high‐resolution grid to explore superimposed mean‐uniform and convergent flows in unconditional and conditional log hydraulic conductivity domains and to compare the relative accuracies of corresponding nonlocal, localized, and Monte Carlo finite element solutions. Whereas our nonlocal solution is nominally restricted to mildly nonuniform media with σY2 ≪ 1, we find that it actually yields remarkably accurate results for strongly nonuniform media with σY2 at least as large as 4. Localized solutions deteriorate relative to nonlocal results as one approaches some points of conditioning and singularity or as σY2 increases. Though they require much less computer time than do nonlocal solutions, localized solutions have the added disadvantage that they yield no information about predictive uncertainty.