bluuebunny/crossref_metadata_2025 huggingface dataset:
Iterative reconstructions of electrical conductivity from multiexperiment low‐frequency electromagnetic data
Authors: Abubakar Aria , van den Berg Peter M.
Published: February 2004

We discuss the extensions of the contrast source inversion (CSI) method to incorporate multiexperiment data. The simplest way to use the multiexperiment data is by using the frequency‐hopping method. In this method, we first solve the inverse problem for a particular experiment and then use the results as the initial estimates for the inversion of the next experiment. In this paper, we have first modified the CSI method so that the results of the inversion from one experiment can be used as input for the inversion of the next experiment, where we only have to invert the differences in material properties and fields. A more sophisticated way to incorporate the multiexperiment data is to invert all available data sets simultaneously. To that end, we introduce two cost functionals. The first is the traditional cost functional where the cost functional of each experiment is added and weighted. The second is the multiplicative cost functional, where instead of adding the cost functional of each experiment we take the product of the cost functional of each experiment. The main advantage of the last cost functional is that the weights of different experiments are determined by the iterative process itself. We consider both the inversion from multifrequency induction‐logging data and the joint inversion from induction‐ and electrode‐logging data in three‐dimensional cross‐well and single‐well configurations.
Impedance characteristics of probe‐fed L‐shaped plate antenna
Authors: Chen Zhi Ning
Published: February 2004

A probe‐fed L‐shaped plate antenna is presented for the enhancement of the impedance bandwidth. The radiating plate is suspended at a large height above a ground plane. The impedance characteristics of the antenna are investigated experimentally and numerically. The results demonstrate that the probe‐fed L‐shaped plate antenna operating in two adjacent modes has achieved a 24% impedance bandwidth for 2:1 VSWR without any additional matching network or any parasitic elements. The parametrical study on the spacing and the dimensions of the plate is performed using a commercial software package. Furthermore, a new parameter called “matching factor” is suggested to assess the matching performances of broadband antennas.
On the realization of the generalized soft‐and‐hard surface
Authors: Viitanen A. J. , Tretyakov S. A. , Lindell I. V.
Published: February 2004

Boundary conditions generalizing those of the soft‐and‐hard surface were recently introduced as an example of an ideal boundary on which the complex Poynting vector cannot have a normal component. It was shown that under such conditions a boundary surface can be used as a polarization transformer for a reflected plane wave. The question of how to realize the generalized soft‐and‐hard surface is considered here. Various possibilities are discussed based on a medium interface, a slab of an anisotropic medium, and a configuration of antennas.
Wavelet analysis of high‐resolution temperature data with applications to radio wave scattering
Authors: Chen C. Y. , Kelley M. C. , Walters D. L.
Published: February 2004

High‐resolution balloon‐borne temperature measurements have been made in the troposphere and stratosphere during late fall over the western plains of the United States. In one such experiment the data are of remarkable quality and quite suitable for investigating methods of separating organized and turbulent features from a geophysical data stream, exploring atmospheric dynamics, and estimating VHF radar backscatter. We are particularly interested in the mechanisms creating aspect sensitivity, i.e., a nonunity ratio of vertical to well off‐vertical radar backscatter. We find that very steep positive vertical temperature gradients, as high as 40.0 K/km, can be supported. On the other hand, negative gradients are limited to values near the marginal stability boundary. The structures are thus anisotropic and similar to ramp cliff features found in other fluids. We use wavelet analysis to isolate the organized components of the signal and, after subtraction, the residual signal is investigated to determine its character. The Fourier transform of the residual is Kolmogorov in nature, unlike that of the original data stream, and yields Cn2, the refractive index structure parameter, in good agreement with the higher‐order structure function approach; this supports the success of our partition. We calculate the Fresnel reflection coefficient using the wavelet coefficients; the turbulent scatter is found using Cn2. The ratio between the Fresnel scatter and the turbulent scatter, i.e., the aspect sensitivity, is ∼10 decibels in the troposphere and over 20 decibels in the stratosphere, in agreement with published observations and supporting our assumption that the organized features have horizontal extent exceeding the Fresnel scale.
Microwave flux density variations of compact radio sources monitored by real‐time very long baseline interferometry
Authors: Koyama Yasuhiro , Kondo Tetsuro , Kurihara Noriyuki
Published: February 2004

Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of correlated amplitudes obtained through the correlation processing of the observed data. From ∼5 years of the observed data, irregular variations in the flux densities were detected for several radio sources using the source 2134+004 as the calibrator. The results showed the monitoring of flux density variations by means of very long baseline interferometry is feasible. Since the correlation processing of the observed data are performed real‐time using a high‐speed digital communication technique, this method gives a capability to monitor flux density variations of quasars and BL‐Lac types of extragalactic radio sources.
Dynamics of a fast Maxwellian electron cloud in coronal plasma
Authors: Kontar E. P. , Mel'nik V. N. , Lapshin V. I.
Published: February 2004

Dynamics of a hot electron cloud with an initially Maxwellian electron distribution is considered both numerically and analytically. It is shown that only a small group of the electrons propagates into the plasma but the main group of the electrons is concentrated near the initial location of the cloud. The distribution of the electrons left presents a plateau with the maximum velocity growing linearly with distance.
A 10 MHz map of the galaxy
Authors: Cane H. V. , Erickson W. C.
Published: February 2004

A 10 MHz map of a large portion of the galaxy has been obtained by combining results from three separate surveys. The approximate resolution of the map is 5°. Although the map was produced over 20 years ago, it has not previously been published. Now that new instruments for space are being considered, this map is of use for estimating sky brightness levels.
Modal phenomena in the natural electromagnetic spectrum below 5 kHz
Authors: Porrat Dana , Bannister Peter R. , Fraser‐Smith Antony C.
Published: February 2004

This paper presents new measurements of the magnetic field component of naturally produced electromagnetic radiation in the ELF/VLF range. The measurements are compared to calculations based on modal propagation theory. The nocturnal spectrum below 5 kHz often contains a sharp increase in spectral level at the cutoff frequency of the first mode (∼1700 Hz) and a distinct variation of the spectral behavior at the cutoff frequency of the second mode (∼3400 Hz), where the indicated cutoff frequencies were calculated for perfectly conducting Earth and ionosphere, with the ionosphere 88 km above the Earth. These features are attributed to an enhancement in the first (quasi‐transverse electric (QTE1)) and second (QTE2) propagating modes at night, which makes their level comparable to the basic (transverse electromagnetic) mode. Another spectral feature, which also prevails during the night, is semiperiodic fluctuations of the spectrum between the two cutoff frequencies, with a short period at the low end of the band and a gradual increase of the period as frequency increases. A similar semiperiodic fluctuation is apparent above the cutoff frequency of the second mode up to the limit of the measured band at 5 kHz. These semiperiodic fluctuations of the spectrum are related to the modal nature of the electromagnetic wave propagation. We show that computations of the spectrum of the ELF/VLF radiation emitted by a vertical lightning in the Earth‐ionosphere waveguide gives spectra that compare well with the measurements.
Reliability analysis in aperture synthesis interferometric radiometers: Application to L band Microwave Imaging Radiometer with Aperture Synthesis instrument
Authors: Vall‐llossera M. , Duffo N. , Camps A. , Corbella I. , Torres F. , Bará J.
Published: February 2004

The Microwave Imaging Radiometer with Aperture Synthesis (MIRAS) instrument will be the first radiometer using aperture synthesis techniques for Earth observation. It will be boarded in the Soil Moisture and Ocean Salinity (SMOS) Earth Explorer Opportunity Mission of the European Space Agency and launched in 2005. The configuration under study in the MIRAS Demonstrator Pilot Project is a Y‐shaped array with 27 dual‐polarization L band antennas in each arm, spaced 0.89 wavelengths. In addition to these 81 antennas there are 3 additional ones between the arms for phase restoration and redundancy purposes and an extra one at the center of the Y array that is connected to a noise injection radiometer. The digitized in‐phase and quadrature outputs of each receiver are multiplexed in groups of four and optically transmitted to the hub where the complex cross correlations are computed. In this configuration there are 85 antennas‐receiving channels and 21 multiplexers. The objectives of this paper are twofold: (1) the study of the performance degradation of Y‐shaped aperture synthesis interferometric radiometers in case of single or multiple subsystem failures and (2) a reliability analysis at subsystem level.
Validation of WBMOD in the Southeast Asian region
Authors: Cervera M. A. , Thomas R. M. , Groves K. M. , Ramli A. G. , Effendy
Published: February 2004

The scintillation modeling code WBMOD, developed at North West Research, provides a global description of scintillation occurrence. However, the model has had limited calibration globally. Thus its performance in localized regions such as Australia‐Southeast Asia is required to be evaluated. The Defence Science and Technology Organisation, Australia, in conjunction with Indonesian National Institute of Aeronautics and Space (LAPAN), Defence Science and Technology Centre, Malaysia, Air Force Research laboratory, United States, and IPS Radio and Space Services of Australia, has commissioned a network of GPS receivers to measure scintillation from sites in the region. One of the objectives of this deployment is to carry out a validation of WBMOD in the region. This paper describes the network of GPS receivers used to record the scintillation data. The details of the procedure used to validate WBMOD are given and results of the validation are presented for data collected during 1998 and 1999 from two sites, one situated in the southern anomaly region and the other situated near the geomagnetic equator. We found good overall agreement between WBMOD and the observations for low sunspot numbers at both sites, although some differences were noted, the major one being that the scintillation activity predicted by WBMOD tended to cut off too early in the night. At higher levels of sunspot activity, while WBMOD agreed with the observations in the southern anomaly region, we found that it significantly underestimated the level of scintillation activity at the geomagnetic equator.
Reconstruction of the surface impedance of an inhomogeneous planar boundary beyond layered media
Authors: Yapar Ali , Akduman Ibrahim
Published: February 2004

Determination of the boundary conditions of a surface from the remote field measurements is an important and interesting topic in the inverse scattering theory. As an example of such problems a method is developed for the reconstruction of the surface impedance of a planar boundary beyond a layered structure. The surface impedance is recovered from the boundary condition itself, which requires that the field distribution and its derivative on the surface be known. The measured data are analytically continued to the boundary to obtain the required field expressions. The computed results obtained for some illustrative examples through the theory established here have very good agreement with the exact values, which shows that the method is accurate and effective.
An evaluation of range accuracy in the Super Dual Auroral Radar Network over‐the‐horizon HF radar systems
Authors: Yeoman T. K. , Wright D. M. , Stocker A. J. , Jones T. B.
Published: February 2004

The Super Dual Auroral Radar Network (SuperDARN) of over‐the‐horizon HF radars forms a powerful diagnostic of large‐scale ionospheric and magnetospheric dynamics in the Northern and Southern Hemispheres. Currently, the ground location of the HF radar returns is routinely determined by a simple range‐finding algorithm, which takes no account of the prevailing HF propagation conditions. This is in spite of the fact that both direct E and F region backscatter and 1½‐hop E and F region backscatter are commonly used in geophysical interpretation of the data. Here HF radar backscatter which has been artificially induced by the high‐power RF facility (ionospheric heater) operated by the European Incoherent Scatter Scientific Association at Tromsø is used to provide a range calibration for the SuperDARN radars. The known ground range, the measured radar slant range, and the group path calculated by a ray‐tracing simulation are compared. The standard algorithm for backscatter ground range location is found to be accurate to within 16 km and 60 km for direct and 1½‐hop backscatter, respectively.
Numerical computation of Hankel functions of integer order for complex‐valued arguments
Authors: Heckmann David L. , Dvorak Steven L.
Published: February 2004

The accurate computation of a series of integer‐order Bessel functions is often required in applications in engineering and physics. Numerous authors have shown how a recurrence relationship can be used in the backward and forward directions to accurately compute integer‐order Bessel functions of the first and second kinds, respectively, for real‐ valued arguments. However, significant round‐off errors can result when these standard recurrence algorithms are employed and the argument for the Bessel function is complex valued. C. F. du Toit recently developed an algorithm that overcomes these numerical instabilities for Bessel functions of the first and second kinds. However, if one needs to compute integer‐order Hankel functions with complex arguments, then we have found that numerical round‐off errors can lead to inaccurate results if Bessel functions of the first and second kinds are superimposed to obtain the desired Hankel functions. To address this problem, a technique is presented in this paper that uses a combination of forward and backward recurrence for the computation of integer‐order Hankel functions with complex‐valued arguments.
Application of the GPS network to estimate the effect of the terrestrial ionosphere on the radio occultation measurements of planetary ionospheres
Authors: Noguchi K. , Imamura T. , Oyama K.‐I. , Saito A.
Published: February 2004

A feasibility study to observe the low‐density Martian nightside ionosphere and the cislunar electron layer by radio occultation technique is presented. Since the total electron contents (TECs) along the ray paths of radio occultation in the ionospheres of Mars and the Moon are comparable to the fluctuation of the TEC of the terrestrial ionosphere, the distortions of radio occultation data by the terrestrial ionosphere should be taken into account. Local time and seasonal dependences of the terrestrial TEC fluctuations are investigated using the GPS receiver network, showing that the terrestrial ionosphere is calm at night in winter and that this period is suitable for the radio occultation measurements of the planetary ionospheres. A method is also developed to estimate the terrestrial TEC fluctuation along the ray path of radio occultation from GPS TEC data.
Realization of a genetic‐algorithm‐optimized wire antenna with 5:1 bandwidth
Authors: Rogers Shawn D. , Butler Chalmers M. , Martin Anthony Q.
Published: February 2004

Techniques for developing and realizing a broadband, loaded wire antenna with a transmission line transformer matching network are presented. The component values and position of a parallel inductor‐resistor load circuit are optimized via a genetic algorithm to improve the bandwidth of a straight‐wire monopole antenna. Candidate antennas are analyzed by solving the electric field integral equation with a lumped‐load model included. This model has been enhanced with fast algorithms from the literature in order to improve computational efficiency during optimization. Subsequent to optimization, the curved‐wire integral equation solution is used to more closely model a real‐world monopole loaded with a helical coil and parallel resistor. The measured input voltage standing wave ratio of the loaded antenna with matching network is below 3.5 over a 5:1 bandwidth (200–1000 MHz). The computed system gain is above −4.0 dBi over the band 220–1000 MHz.
Preconditions for market solution to urban water scarcity: Empirical results from Hyderabad City, India
Authors: Saleth R. Maria , Dinar Ariel
Published: February 2004

Utilizing both primary and secondary information pertaining to the water sector of Hyderabad City, India, this paper (1) evaluates the economics of various technically feasible supply augmentations options; (2) estimates the group‐specific water demand and consumption response functions under alternative pricing behaviors; (3) calculates the net willingness to pay (NWTP, considered to be the value of raw water at source) of different user groups as derived from their respective price elasticities; (4) shows how inadequate the NWTP is to justify most supply augmentation options including intersectoral water transfers under the existing water rate structure; (5) argues that the economic and institutional conditions internal to urban water sector cannot justify an externally imposed water transfers, whether market‐based or otherwise, as long as the water rate structure is inefficient and regressive; and (6) concludes by underlining the central role that the pricing option, both the level and structure, plays not only in activating a number of nonprice options but also in generating incentives for the emergence of new and the consolidation of existing institutional conditions needed to support economically rooted water transfers and conservation initiatives.
Effect of particle size distribution on the effective dielectric permittivity of saturated granular media
Authors: Robinson D. A. , Friedman S. P.
Published: February 2004

The effective permittivity of an unconsolidated porous material is shown to be not only a function of the permittivity of the individual components but also of the distribution of particle sizes present. Increasing width of particle size distribution for glass spheres and sieved quartz sand is demonstrated to reduce the effective permittivity of a mixture compared to the effective permittivity of the corresponding monosize material. The results are shown to lie between predictions made using the Maxwell‐Garnett [1904] formula for monosize spheres and a self‐similar model [Sen et al., 1981] and can be predicted by a multiple Maxwell‐Garnett‐based mixing model. The results demonstrate that the degree of sorting of a material will directly influence the effective permittivity of the mixture. This is beyond the effect of the change in permittivity caused by the significant change in porosity resulting from the mixing of different particle sizes.
Surface‐zone flow along unsaturated rock fractures
Authors: Tokunaga Tetsu K. , Wan Jiamin
Published: February 2004

Although fractures in rock are well recognized as pathways for fast percolation of water, processes which permit fast flow along unsaturated fracture pathways remain to be identified and understood. Earlier aperture‐based models of flow in partially saturated fractures permit fast flow only through a continuous network of locally saturated segments. Film flow was recently identified as a mechanism capable of sustaining fast flow along truly unsaturated fractures when the matric potential is very close to zero. Another mechanism for fast flow along unsaturated fractures is introduced in this study, “surface‐zone flow,” which can be important when the permeability of the rock along fractures (fracture skin) is significantly greater than that of the bulk rock matrix. In such systems the fracture surface zone provides low resistance pathways through which fast flow (relative to the bulk matrix rock) can occur, even when the fractures are at very low water saturation. Initial experimental tests of surface‐zone fast flow were performed. Surface‐zone fast imbibition of water was measured on a welded tuff and a rhyolite. However, because (1) imbibition rates are also strongly influenced by rock wettability, (2) these initially air‐dry rocks exhibited finite contact angles upon exposure to water, and (3) we lack methods to reliably measure permeabilities of thin regions on rock surfaces, quantification of permeability contrasts was not possible in these initial tests.
A new formulation of evaporation‐temperature dynamics of saline solutions
Authors: Oroud I. M.
Published: February 2004

This paper presents a new formulation which connects the variations in evaporation among adjacent shallow saline solutions to the differences in their surface temperatures. The derivations are based on the theory of water vapor transfer and energy balance considerations. Evaporation differences among adjacent shallow saline solutions are evaluated without the need to measure their activity coefficients or the prevailing meteorological forcings. The only parameters needed to evaluate the differences in evaporation among saline solutions exposed to the same meteorological forcings are their surface temperatures and a general heat transfer coefficient term for sensible heat. When combined with conventional or remote‐sensing data, the procedure can be used for operational purposes and as a diagnostic tool to analyze evaporation and heat balance dynamics of saline water bodies under different environmental conditions. The present formulation will prove useful in evaluating the transfer coefficient of sensible heat using only evaporation from and temperature of saline solutions having different activity coefficients. In addition to the above applications this formulation can be extended to examine the efficiency of monolayers or other chemicals in suppressing evaporation from freshwater lakes, particularly in arid and semiarid regions. This can be achieved by observing the increase in temperature of these water bodies compared to untreated or controlled dams or lakes.
Modeling flow and transport in irrigation catchments: 1. Development and testing of subcatchment model
Authors: Connell L. D. , Jayatilaka C. , Gilfedder M. , Mein R. G. , Vandervaere J. P.
Published: February 2004

The development of a physically based model for flow and transport in hillslope transects is presented. In the second paper of this set, Connell et al. [this issue] use this model as the fundamental unit in a catchment application to describe flow within flood‐irrigated catchments. The hillslope transect model is based on the assumption that surface and groundwater flow is one‐dimensional, flowing down the topographic gradient. This approximation is particularly accurate for flood‐irrigated catchments where the land surface is divided into irrigation bays. To maximize the model's utility for catchment modeling, a number of simplifications to the process descriptions are introduced to reduce model complexity and computational overhead. The description of the unsaturated zone makes a distinction between infiltration and capillary rise, allowing infiltration to be a mixture of macropore and matrix flow and capillary rise to be a porous medium process. The capillary rise description exploits the transition in flow behavior as soil dries from being atmosphere‐limited to profile‐controlled. An efficient Laplace space quasi‐analytical solution to Richards' equation is used for this second stage. This quasi‐analytical procedure is also used to solve the advection‐dispersion equation for unsaturated solute transport and for Darcian groundwater flow. The model is tested through application to a field site where detailed observations of surface and subsurface flow and transport are available. The good agreement with observations in both calibration and long‐duration simulations of a wide range of physical processes promotes confidence in the model representation.
Arsenic in groundwater: Testing pollution mechanisms for sedimentary aquifers in Bangladesh
Authors: McArthur J. M. , Ravenscroft P. , Safiulla S. , Thirlwall M. F.
Published: February 2004

In the deltaic plain of the Ganges‐Meghna‐Brahmaputra Rivers, arsenic concentrations in groundwater commonly exceed regulatory limits (>50 µg L−1) because FeOOH is microbially reduced and releases its sorbed load of arsenic to groundwater. Neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution. The most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat. Concentrations of ammonium up to 23 mg L−1come from microbial fermentation of buried peat and organic waste in latrines. Concentrations of phosphorus of up to 5 mg L−1come from the release of sorbed phosphorus when FeOOH is reductively dissolved and from degradation of peat and organic waste from latrines. Calcium and barium in groundwater come from dissolution of detrital (and possibly pedogenic) carbonate, while magnesium is supplied by both carbonate dissolution and weathering of mica. The87Sr/86Sr values of dissolved strontium define a two‐component mixing trend between monsoonal rainfall (0.711±0.001) and detrital carbonate (<0.735).
Soil seal formation and its effect on infiltration: Uniform versus nonuniform seal approximation
Authors: Assouline S. , Mualem Y.
Published: February 2004

Two different approaches to the solution of the problem of flow through the dynamic stage of seal formation as well as through soil with a fully developed seal are studied. The first approach considers the disturbed seal layer and the undisturbed soil underneath as a continuous nonuniform soil profile. The second replaces the nonuniform seal by a uniform equivalent layer, thereby generating a homogeneous two‐layer flow system. The depth‐dependent properties of the nonuniform seal are expressed in terms of the exponential model of Mualem and Assouline [1989]. The dynamics of seal formation are modeled according to Assouline and Mualem [1997]. During the first rainfall on an undisturbed soil profile, when the seal layer is formed, the application of the first or the second approach has only a minor effect on the calculated infiltration curves. However, there is a significant difference between the two solutions regarding the dynamic changes of the water content in the soil surface and, consequently, within the seal layer. During subsequent rainfalls on a sealed soil profile, when the seal layer is completely developed, the differences between the two ways of accounting for the seal layer become evident, and their effects on the infiltration curve are much more significant. Representing the seal as an equivalent uniform layer increases the ponding time and the infiltration rates at the early stage of the process. The amplitude of these effects is increased when the rainfall rate is higher and the seal layer is thicker. An important result is that the relationship between infiltration rate and cumulative infiltration is unique in the case of a completely developed seal and when the seal is considered as a nonuniform layer. However, this relationship is not unique during seal formation, independent of the approach applied to represent the seal layer.
Methyl tert‐butyl ether degradation in the unsaturated zone and the relation between MTBE in the atmosphere and shallow groundwater
Authors: Baehr Arthur L. , Charles Emmanuel G. , Baker Ronald J.
Published: February 2004

Atmospheric methyl tert‐butyl ether (MTBE) concentrations in southern New Jersey generally exceeded concentrations in samples taken from the unsaturated zone. A simple unsaturated zone transport model indicates that MTBE degradation can explain the attenuation with half‐lives from a few months to a couple of years. Tert‐butyl alcohol (TBA), a possible degradation product of MTBE, was detected in unsaturated‐zone samples at concentrations exceeding atmospheric levels at some sites, suggesting the possible conversion of MTBE to TBA. At sites where MTBE was detected in shallow groundwater, the concentration was typically higher than the overlying unsaturated‐zone concentration. This observation is consistent with outgassing from the aquifer and combined with the unsaturated‐zone attenuation suggests some of the MTBE detections in shallow groundwater are nonatmospheric in origin, coming from leaking tanks, road runoff, or other sources. The identification of sources of MTBE in groundwater and attenuation mechanisms through the hydrologic cycle is critical in developing an understanding of the long‐term effect of MTBE releases.
Tide‐induced groundwater fluctuation in a coastal leaky confined aquifer system extending under the sea
Authors: Li Hailong , Jiao Jiu J.
Published: February 2004

This paper presents the analytical solution of groundwater response to tidal fluctuation in a coastal multilayer aquifer system consisting of a leaky confined aquifer, a semipermeable layer, and an unconfined aquifer. The semipermeable layer, or the roof of the leaky confined aquifer, extends under the sea. Comparisons are made with solutions derived previously, which considered only a single aquifer, zero extension of the roof, or infinite roof length, and demonstrate that the previous solutions are special cases of the solution presented here. A hypothetical example is used to discuss the impact of the dimensionless roof length, dimensionless leakage, and tidal efficiency on the groundwater level fluctuations in the inland part of the confined aquifer. The fluctuation decreases significantly with the roof length when the roof length is small but is insensitive to the change of roof length when the roof length is greater than a threshold. The impacts of leakage from the offshore and inland portions of the confining unit are different. Leakage from the offshore portion tends to increase the fluctuation of the groundwater level, while leakage from the inland portion tends to decrease the fluctuation. The fluctuation increases as the tidal efficiency increases. This fact is significant only when the roof length is great and leakage is small.
Tropical storms and the flood hydrology of the central Appalachians
Authors: Sturdevant‐Rees Paula , Smith James A. , Morrison Julia , Baeck Mary Lynn
Published: February 2004

Flooding from Hurricane Fran is examined as a prototype for central Appalachian flood events that dominate the upper tail of flood peak distributions at basin scales between 100 and 10,000 km2. Hurricane Fran, which resulted in 34 deaths and more than $3.2 billion in damages, made land fall on the North Carolina coast at 0000 UTC, September 6, 1996. By 1200 UTC on September 6, Fran had weakened to a tropical storm, and the center of circulation was located at the North Carolina–Virginia border. Rain bands surrounding the tropical depression produced extreme rainfall and flooding in Virginia and West Virginia, with the most intense rainfall concentrated near ridge tops in the Blue Ridge and Valley and Ridge physiographic provinces. The most severe flooding occurred in the Shenandoah River watershed of Virginia, where peak discharges exceeded the 100‐year magnitude at 11 of 19 U.S. Geological Survey stream‐gaging stations. The availability of high‐resolution discharge and rainfall data sets provides the opportunity to study the hydrologic and hydrometeorological mechanisms associated with extreme floods produced by tropical storms. Analyses indicate that orographie enhancement of tropical storm precipitation plays a central role in the hydrology of extreme floods in the central Appalachian region. The relationships between drainage network structure and storm motion also play a major role in Appalachian flood hydrology. Runoff processes for Hurricane Fran reflected a mixture of saturation excess and infiltration excess mechanisms. Antecedent soil moisture played a significant role in the hydrology of extreme flooding from Hurricane Fran. Land use, in particular, the presence of forest cover, was of secondary importance to the terrain‐based distribution of precipitation in determining extreme flood response.
Effect of interaggregate capillary cracks on the hydraulic conductivity of swelling clay soils
Authors: Chertkov V. Y. , Ravina I.
Published: February 2004

Clay soils, as a result of shrinking‐swelling, usually have a capillary crack network. The objective of this presentation is to develop a physically based model of the hydraulic conductivity of a swelling clay matrix with interaggregate capillary cracks only. It is based on a number of assumptions relating in the main to the geometrical characteristics of the capillary crack network and the use of the cubic law for water flow in a water‐filled slit. The key assumption speaks about the contraction of the interaggregate capillary crack volume in a drying but saturated clay matrix. The expression for vertical hydraulic conductivity of the network of cracks of width <R is determined by network tortuosity and specific length of crack traces at a horizontal cross section. To check the model, we compared the available data on the hydraulic conductivity of an Israeli clay soil with model predictions of the hydraulic conductivity of an interaggregate capillary crack network in combination with the independent predictions of hydraulic conductivity of the soil matrix available in literature. This comparison demonstrated satisfactory agreement.
The complementary relationship in estimation of regional evapotranspiration: The complementary relationship areal evapotranspiration and advection‐aridity models
Authors: Hobbins Michael T. , Ramírez Jorge A. , Brown Thomas C. , Claessens Lodevicus H. J. M.
Published: February 2004

Two implementations of the complementary relationship hypothesis for regional evapotranspiration, the Complementary Relationship Areal Evapotranspiration (CRAE) model and the Advection‐Aridity (AA) model, are evaluated against independent estimates of regional evapotranspiration derived from long‐term, large‐scale water balances (1962–1988) for 120 minimally impacted basins in the conterminous United States. The CRAE model overestimates annual evapotranspiration by 2.5% of mean annual precipitation, and the AA model underestimates annual evapotranspiration by 10.6% of precipitation. Generally, increasing humidity leads to decreasing absolute errors for both models, and increasing aridity leads to increasing overestimation by the CRAE model and underestimation by the AA model, with the exception of high, arid basins, where the AA model overestimates evapotranspiration. Overall, the results indicate that the advective portion of the AA model must be recalibrated before it may be used successfully on a regional basis and that the CRAE model accurately predicts monthly regional evapotranspiration.
A Markov Chain Monte Carlo Scheme for parameter estimation and inference in conceptual rainfall‐runoff modeling
Authors: Bates Bryson C. , Campbell Edward P.
Published: February 2004

A fully Bayesian approach to parameter estimation and inference in conceptual rainfall‐runoff models (CRRMs) is presented. Computations are performed using a Markov chain Monte Carlo (MCMC) method based on the Metropolis‐Hastings algorithm. Single‐site and block updating schemes are used for model parameters subject to nonnegativity restrictions as well as interval, equality, and order constraints. Diagnostics for the convergence of the Markov chain and CRRM assessment are also considered. The MCMC approach produces samples from the joint posterior distribution of the model parameters. This provides more information than single‐point estimates and avoids the need to use a normal approximation to the posterior as the basis for inference. The methodology is demonstrated using an eight‐parameter conceptual rainfall‐runoff model and two case studies from southeastern Australia. The first case study considers a watershed with high runoff yield over a 12‐year period. The second case study considers a watershed with low yield over a 17‐year period. The results indicate that (1) Bayesian methods provide an objective framework for model criticism and choice, (2) the proposed strategies for handling constraints on model parameters are effective, (3) the model parameters are sensitive to likelihood function selection, (4) the conventional approach of using a power transformation and an autoregressive process to stabilize error variance and model dependence in the residuals may have limited success, and (5) some care is required in the implementation of the MCMC approach and reliable results will be difficult to obtain when CRRM complexity exceeds the limitations of the rainfall‐runoff data at hand. A key finding is that the MCMC scheme presented herein provides a powerful means of identifying specific inadequacies in the structure of CRRMs.
Benefit‐cost estimation for alternative drinking water maximum contaminant levels
Authors: Gurian Patrick L. , Small Mitchell J. , Lockwood John R. , Schervish Mark J.
Published: February 2004

A simulation model for estimating compliance behavior and resulting costs at U.S. Community Water Suppliers is developed and applied to the evaluation of a more stringent maximum contaminant level (MCL) for arsenic. Probability distributions of source water arsenic concentrations are simulated using a statistical model conditioned on system location (state) and source water type (surface water or groundwater). This model is fit to two recent national surveys of source waters, then applied with the model explanatory variables for the population of U.S. Community Water Suppliers. Existing treatment types and arsenic removal efficiencies are also simulated. Utilities with finished water arsenic concentrations above the proposed MCL are assumed to select the least cost option compatible with their existing treatment from among 21 available compliance strategies and processes for meeting the standard. Estimated costs and arsenic exposure reductions at individual suppliers are aggregated to estimate the national compliance cost, arsenic exposure reduction, and resulting bladder cancer risk reduction. Uncertainties in the estimates are characterized based on uncertainties in the occurrence model parameters, existing treatment types, treatment removal efficiencies, costs, and the bladder cancer dose‐response function for arsenic.
Frictional retardation of laminar flow by plant litter and surface stones on dryland surfaces: A laboratory study
Authors: Dunkerley David , Domelow Peter , Tooth David
Published: February 2004

The relative contributions of plant litter and surface stones to the frictional retardation of laminar flows were investigated in a series of laboratory experiments on a glued sand board. A computer‐controlled measuring gantry was used for precise recording of flow depths, and retardation was quantified using the Darcy‐Weisbach friction factor ƒ. Surface stones had relatively little effect on flow retardation or flow speeds but increased flow depths in comparison with the same discharge across a stone‐free surface. In contrast, plant litter produced larger reductions in flow speed and increases in flow depth, associated with large increases in flow retardation. These effects were most marked at the highest loading tested, 80 g m−2. Results show that the retardation caused by a given surface cover fraction of litter significantly exceeds that caused by the same cover of surface stones, emphasising the need for measures other than surface cover to be employed in work on overland flow. Neither nonsubmerged obstacles such as surface stones nor plant litter contribute increased roughness with rising discharge in the laminar regime. Instead, values of ƒ decline as Reynolds number rises. These findings contrast notably with relationships established for turbulent flow and suggest that for correct modeling of overland flow in drylands, the temporal and spatial ranges of laminar flows must be distinguished from those where turbulent flows occur.
A multipopulation genetic algorithm to solve the inverse problem in hydrogeology
Authors: Karpouzos D. K. , Delay F. , Katsifarakis K. L. , de Marsily G.
Published: February 2004

The inverse problem of groundwater flow is treated with an automatic method that can produce several alternative solutions at once. During their joint optimization, these solutions can exchange information in order to maintain some diversity and thus avoid a systematic premature convergence toward a single local minimum. Although genetic algorithms are capable of doing this, they have not often been used in groundwater inverse optimization. First, a specific multipopulation genetic algorithm is developed. It is then tested on two synthetic cases of steady state flow with transmissivity values extending over 4 orders of magnitude. The first test is nonparametric and optimizes as many parameters as those used to define the reference case. The second test uses a sort of “pilot point” parametrization. The optimization is carried out on a limited number of perturbations that are interpolated and superimposed on an initial transmissivity field. In view of the good quality of the results, these initial attempts provide incentives to further develop genetic algorithms in groundwater inverse problems.
Visualization of microscale phase displacement processes in retention and outflow experiments: Nonuniqueness of unsaturated flow properties
Authors: Mortensen Annette P. , Glass Robert J. , Hollenbeck Karl , Jensen Karsten H.
Published: February 2004

Methods to determine unsaturated hydraulic properties can exhibit random and nonunique behavior. We assess the causes for these behaviors by visualizing microscale phase displacement processes that occur during equilibrium retention and transient outflow experiments. For both types of experiments we observe the drainage process to be composed of a mixture of fast air fingering and slower air back‐filling. The influence of each of these microscale processes is controlled by a combination of the size and the speed of the applied boundary step, the initial saturation and its structure, and small‐scale heterogeneities. Because the mixture of these microscale processes yields macroscale effective behavior, measured unsaturated flow properties are also a function of these controls. Such results suggest limitations on the current definitions and uniqueness of unsaturated hydraulic properties.
Sources and fluxes of inorganic carbon in a deep, oligotrophic lake (Loch Ness, Scotland)
Authors: Jones R. I. , Grey J. , Quarmby Christopher , Sleep Darren
Published: February 2004

The main river inflows to Loch Ness and several depths in the water column within the loch were sampled over an annual cycle. The carbon isotope composition of total dissolved inorganic carbon (DIC) from the samples was determined as well as that of phytoplankton from the loch. Values of δ13C for DIC in the rivers indicated that this DIC was derived from soil respiration in the catchment and achieved only partial equilibrium with the atmosphere during river transport. Riverine loading accounted for most of the DIC in Loch Ness, and the great depth of the loch relative to its surface area allows only limited exchange with the atmosphere. Despite the low productivity in Loch Ness, DIC concentrations in the low alkalinity water are appreciably influenced by plankton metabolism, and seasonal fluctuations in δ13C of DIC and phytoplankton revealed the particular impact of photosynthetic carbon fixation on DIC. However, the photosynthetic depletion of DIC during summer does not offset the riverine loading sufficiently to prevent the loch waters being supersaturated with CO2 throughout the year. Annual efflux of CO2 from Loch Ness is estimated to be 253 × 106 mol yr−1, of which around one quarter may be due to net heterotrophic mineralization within the loch of organic carbon of terrestrial origin. The remainder is attributable to inorganic carbon input to the lake via river inflow and derived from prior mineralization of soil organic matter within the drainage area. This annual efflux of CO2 can represent around 6% of net ecosystem production in the catchment.
Synthesis and thermal decomposition of tetragonal RbClO4 and volume of fluid O2 from 2 to 9 GPa
Authors: Walker David , Hughes Gwyneth , Cranswick Lachlan M. D. , Clark Simon M. , Buhre Stephan
Published: February 2004

Orthorhombic (O) RbClO4 transforms to a new tetragonal (T) form at pressures above 20 kbar. Thermal decomposition proceeds through the reversible equilibrium reaction T RbClO4 = B2 RbCl + 2O2 at modestly increasing temperatures centered ∼550°C over the pressure range investigated from 20 to 90 kbar. Cell volumes for T RbClO4 and B2 RbCl were determined by high‐pressure, high‐temperature, in situ X‐ray diffraction on station 16.4 of the Daresbury Synchrotron Radiation Source, from which the partial molar volume of O2 along the decomposition curve was determined. This volume (∼10 cm3/mol) is in close agreement with volumes determined from studies of the thermal decomposition of KClO3 and KClO4 but is difficult to reconcile with fluid O2 volumes determined using shock wave techniques. If these new O2 partial molar volumes are relevant to molten iron solutions, then storage of substantial amounts of O2 in the core becomes feasible.
Mean winds, tides, and gravity waves during the westward phase of the mesopause semiannual oscillation (MSAO)
Authors: Gurubaran S. , Rajaram R.
Published: February 2004

MF radar observations of mean winds, tides, and gravity waves (in the period range 30–120 min) over Tirunelveli (8.7°N, 77.8°E) during April 1993, when the mesopause semiannual oscillation (MSAO) was in the westward phase, are presented in this work. The focus herein is on the observed relationship between these dynamical parameters, which is explained in the context of the current understanding of the behavior of MSAO winds, tidal dissipation, and gravity wave‐tide interactions. The observed diurnal tide amplitude and phase variations with height indicate (1,1) wave damping and subsequent westward momentum deposition. The gravity wave variances are shown to be modulated by the diurnal tide. Though the analysis does not lead to an estimation of gravity wave contributions to the time‐mean zonal flow and its shears, the observations do suggest that the short‐term tidal variability contributes, either directly, or indirectly through the gravity wave‐tidal interactions, to the variation of the strength of the time‐mean westward flow associated with the MSAO.