bluuebunny/crossref_metadata_2025 huggingface dataset:
Separating hydrological and geochemical influences on runoff acidification in spatially heterogeneous catchments
Authors: Kirchner James W. , Dillon Peter J. , Lazerte Bruce D.
Published: April 2004

Geochemical reactions and hydrologic regime both affect runoff chemistry in many catchments. Each factor can obscure the effects of the other. This complicates efforts to interpret and predict catchments' geochemical and hydrological behavior. Here we present simple empirical techniques for separating effects of discharge fluctuations and effects of changes in acid anion concentrations in catchment monitoring data. We illustrate these techniques with data from central Ontario. Statistically subtracting acid anion effects reveals that important water quality variables (e.g., sum of base cations, acid neutralizing capacity, H+, Ali) are simple functions of the logarithm of catchment discharge. Likewise, correcting for discharge effects reveals that these water quality variables are roughly linear functions of runoff acid anion concentrations. The linear functions linking acid anion concentrations to water quality can be reliably predicted from bulk catchment runoff chemistry. Therefore acid anions' effects on water quality can be reliably predicted, even if they are obscured by discharge fluctuations in catchment monitoring data.
Degradation kinetics of aromatic organic solutes introduced into a heterogeneous aquifer
Authors: MacIntyre William G. , Boggs Mark , Antworth Christopher P. , Stauffer Thomas B.
Published: April 2004

Degradation rates of benzene, p‐xylene, naphthalene, and o‐dichlorobenzene have been measured in a heterogeneous, unconfined aquifer during a pulse injection experiment at Columbus Air Force Base, Columbus, Mississippi. Dissolved oxygen in the pulse plume maintained aerobic conditions. Degradation kinetics calculated from the complete field data set were approximately first order with the following rate constants: benzene, 0.0070 d−1; p‐xylene, 0.0107 d−1; naphthalene, 0.0064 d−1; and o‐dichlorobenzene, 0.0046 d−1. Reaction rates were also calculated from a near‐field subset of the data using a model based on the hydrologic characteristics of the aquifer. Shapes of the degradation rate curves were consistent with microbial degradation processes. Maximum degradation rates obtained are presumed to be characteristic of the microbial population metabolism. Carbon 14‐labeled p‐xylene was included in the injection solution to permit detection of degradation products. This technique is suggested for future field experiments, because it distinguishes solute degradation from solute losses by sorption and evaporation and allows mass balance to be demonstrated throughout the course of the reaction in the aquifer.
Comparison of two approaches for implementing multireservoir operating policies derived using stochastic dynamic programming
Authors: Tejada‐Guibert J. Alberto , Johnson Sharon A. , Stedinger Jery R.
Published: April 2004

This paper compares two approaches for implementing reservoir operating policies derived using stochastic dynamic programming (SDP) models. In particular, operating policies for the Shasta‐Trinity system in Northern California are generated using SDP algorithms that employ either multilinear or multidimensional piecewise cubic functions to approximate the cost‐to‐go function. Release decisions in the simulations are then determined by either (1) interpolating in the policy tables or (2) reoptimizing the policy within the simulation, using the cost‐to‐go function generated by the SDP. The impact on simulated system performance of several discretization and interpolation schemes in the SDP is also evaluated. Reoptimizing the policy when a decision is made within the simulation resulted in better system performance, particularly when severe penalties were incurred for water and power shortages and coarse discretizations were employed in the SDP.
Area sinks in the analytic element method for transient groundwater flow
Authors: Zaadnoordijk Willem J. , Strack Otto D. L.
Published: April 2004

In the analytic element method, regional groundwater flow is modeled by superposition of particular solutions to the governing differential equation. The domain of the solutions is the x, y plane with the possible exception of isolated points. The solutions are referred to as analytic elements and represent a feature of flow in the aquifer, such as a well or the leakage through an aquitard. The analytic element method was originally developed for regional steady groundwater flow. In this paper the method is extended to transient flow. Several transient analytic elements and a method of determining a transient solution are presented. The governing differential equation that is used is the heat equation in two spatial dimensions with a sink term. Solutions for a transient well and a transient line sink are available in the literature. Both have a discharge that is equal to zero before the starting time and has a constant value after the starting time. A solution for a transient area sink is presented that also has a constant strength after the starting time. The area sink is a polygon with an extraction inside that is constant in space. A validation of the approach presented here is obtained by comparison with an exact solution for a case of one‐dimensional transient groundwater flow. The domain is semi‐infinite with a prescribed head at one side. Initially, the water is at rest, and then the head is suddenly raised at the boundary. The use of the transient analytic element method is illustrated by an example model with analytic elements both for steady and for transient flow. The former elements represent the initial steady state. The transient elements simulate variations of the groundwater flow due to seasonal variations in recharge and pumping.
Spatial moments of a kinetically sorbing solute plume in a heterogeneous aquifer
Authors: Dagan G. , Cvetkovic V.
Published: April 2004

Steady water flow takes place in a heterogeneous aquifer of a three‐dimensional random stationary log conductivity of anisotropic, axisymmetric co variance. The latter has different vertical and horizontal integral scales of anisotropy ratio e. The mean flow is uniform in the horizontal plane. A solute plume of constant concentration is injected instantaneously in the formation. The plume is characterized by the expected values of its spatial moments: mass, centroid coordinates, and tensor of second spatial moment. Under ergodic conditions, assumed to prevail, the one‐realization spatial moments are approximately equal to their ensemble means. By adopting a first‐order approximation in the log conductivity variance and with neglect of pore‐scale dispersion, the spatial moments of a plume of a conservative solute were evaluated in the past as functions of the transport time or distance from the input zone (Dagan, 1988) by a Lagrangian approach. The moments were expressed in terms of a quadrature that had to be evaluated numerically. The present study generalizes these results to a reactive solute which undergoes kinetically controlled sorption, according to a linear relationship. The reaction depends on two coefficients: a reaction time tr and an equilibrium partition coefficient Kd, which are assumed to be constant. The solution of the transport problem is obtained by the Lagrangian approach, generalizing the one applied previously to conservative solutes. In the latter case, solute particles follow those of the fluid, whereas a wake of solute is left behind in the case of sorption. The zero‐order spatial moment, the plume mass, the first‐order approximation, and the centroid coordinates are determined in an analytical form. They depend only on the mean velocity and on the reaction coefficients. The second‐order longitudinal moment, characterizing dispersion with respect to the centroid, is made up from terms depending only on the reaction and on an interactive term which depends also on heterogeneity. The latter is effectively derived by a double quadrature as well as by an analytical approximate expression, presented here for the first time. The transverse spatial moments contain only interaction terms. The third spatial moment, characterizing the asymmetry of the plume, is also evaluated. Simple asymptotic results are presented for the cases of (1) large transport time reaction time ratio, resulting in retardation, (2) large transport time heterogeneity characteristic time ratio, for which the longitudinal transport of the conservative solute is “Fickian,” and (3) the joint limit, resulting also in a “Fickian” regime.
Sampling design for classifying contaminant level using annealing search algorithms
Authors: Christakos George , Killam Bart R.
Published: April 2004

A stochastic method for sampling spatially distributed contaminant level is presented. The purpose of sampling is to partition the contaminated region into zones of high and low pollutant concentration levels. In particular, given an initial set of observations of a contaminant within a site, it is desired to find a set of additional sampling locations in a way that takes into consideration the spatial variability characteristics of the site and optimizes certain objective functions emerging from the physical, regulatory and monetary considerations of the specific site cleanup process. Since the interest is in classifying the domain into zones above and below a pollutant threshold level, a natural criterion is the cost of misclassification. The resulting objective function is the expected value of a spatial loss function associated with sampling. Stochastic expectation involves the joint probability distribution of the pollutant level and its estimate, where the latter is calculated by means of spatial estimation techniques. Actual computation requires the discretization of the contaminated domain. As a consequence, any reasonably sized problem results in combinatorics precluding an exhaustive search. The use of an annealing algorithm, although suboptimal, can find a good set of future sampling locations quickly and efficiently. In order to obtain insight about the parameters and the computational requirements of the method, an example is discussed in detail. The implementation of spatial sampling design in practice will provide the model inputs necessary for waste site remediation, groundwater management, and environmental decision making.
Probing soil and aquifer material porosity with nuclear magnetic resonance
Authors: Hinedi Z. R. , Kabala Z. J. , Skaggs T. H. , Borchardt D. B. , Lee R. W. K. , Chang A. C.
Published: April 2004

Nuclear magnetic resonance relaxation measurements were used to identify different characteristic porosity domains in soil and aquifer materials. The porosity distribution can be inferred from these measurements by a regularization method applicable to any nuclear magnetic resonance (NMR) relaxation, or by an analytic method applicable only to multiexponential relaxations (D. Orazio et al., 1989). The porosity distribution obtained from NMR relaxation measurements strongly depends on the pore shape factor. For the Borden aquifer material, both the regularized and the analytic pore size distribution obtained from NMR relaxation measurements are consistent with those obtained by Ball et al. (1990) using Hg porosimetry and N2 adsorption. For the Eustis and the Webster soils, the measured porosity domains are qualitatively consistent with those expected based on their respective composition. Our findings suggest that due to the long time required to saturate fine pores, NMR measurements of porosity distribution that are collected at short saturation times are biased toward larger pore sizes.
Linear equilibrium adsorbing solute transport in physically and chemically heterogeneous porous formations: 1. Analytical solutions
Authors: Bellin Alberto , Rinaldo Andrea , Bosma Willem Jan P. , van der Zee Sjoerd E. A. T. M. , Rubin Yoram
Published: April 2004

A first‐order analytical solution for the transport of reactive solutes in physically and chemically heterogeneous porous media is derived and discussed. The solution relies on the assumption of chemical activity described by the local linear equilibrium assumption postulating the existence of a (spatially variable) retardation factor. Retardation factors and log permeabilities modeling heterogeneities are described statistically by random space functions with assigned correlation structure. Correlated as well as uncorrelated physical and chemical heterogeneities are studied. The analytical expressions derived reduce to Dagan's classic solution for the case of nonreactive solute transport and obey asymptotic limits already known from the literature.
Linear equilibrium adsorbing solute transport in physically and chemically heterogeneous porous formations: 2. Numerical results
Authors: Bosma Willem Jan P. , Bellin Alberto , van der Zee Sjoerd E. A. T. M. , Rinaldo Andrea
Published: April 2004

Numerical Monte Carlo simulations were conducted to assess dispersion of reactive solutes in two‐dimensional physically and chemically heterogeneous porous media, using random fields with assigned correlation structure for hydraulic conductivity and linear adsorption coefficient. Conditions under which linearization of adsorption is valid are discussed. Lognormal distributions of hydraulic conductivity and adsorption coefficient were assumed. Calculations have been performed for positive and negative correlation between hydraulic conductivity and adsorption coefficient, and for uncorrelated cases. Effects of varying different properties including mean and average sorption coefficient, physical and chemical integral scale, and variance of hydraulic conductivity on dispersive behavior are shown. A larger mean sorption coefficient enhances plume spreading in uncorrelated and in negatively correlated cases. In positively correlated cases, counteracting effects of physical and chemical heterogeneity play an important role. The outcome of these counteracting effects depends on the mean, variance, and integral scales of the spatially variable properties. The analytical solutions, derived in paper 1 (Bellin et al., this issue), reveal a good agreement with the numerical results in a significant range of heterogeneities. The generally surprisingly good agreement of the analytical solutions with the numerically obtained results can possibly be attributed to opposing effects of nonlinearities neglected in the derivation of the analytical solutions. In the case of strong physical heterogeneity the analytical solutions perform slightly better than in the case of strong chemical heterogeneity.
Stochastic modeling of unsaturated flow in heterogeneous media with water uptake by plant roots: Tests of the parallel columns model under two‐dimensional flow conditions
Authors: Or Dani , Rubin Yoram
Published: April 2004

In the first paper of this series (Rubin and Or, 1993) we developed a stochastic model predicting spatial moments of soil water matric potential ψ and saturation S under conditions of unsaturated steady state flow as function of the spatial moments of log saturated hydraulic conductivity Y, capillarity index α, and plant rooting depth δ. In this study, we investigate the ability of the proposed parallel columns model to reconstruct two‐dimensional flow solutions, and apply the model to spatial estimation using the geostatistical formalism. A finite difference numerical scheme was developed to solve the two‐dimensional Richards equation in heterogeneous flow domain and obtain detailed images of the spatial distribution of ψ and S and their moments. Two‐dimensionally‐based images representing “true” distributions of ψ and S in a vertical cross section (x‐z plane) of laterally heterogeneous soil were used in a synthetic spatial estimation problem for comparison with the one‐dimensionally‐based solution and with kriging estimates. The results show that (1) the means of ψ and S were practically identical for the one‐ and two‐dimensional solutions; (2) ψ variance obtained from the two‐dimensional solution was smaller than the one‐dimensional, whereas S variance was larger in the two‐dimensional case; (3) a smoothing of ψ under two‐dimensional flow regime resulted in spatial covariance function of a Bessel type, whereas two‐dimensional spatial covariance of S exhibited a steep decay near its origin denoting spatial discontinuity; (4) the least smoothing of ψ was observed near the surface where nearly one‐dimensional conditions exist; (5) during transient flow, spatial covariances assume their steady state form quite rapidly; (6) spatial covariances predicted by the parallel columns model provided good estimates at unmeasured locations with conservative confidence regions for ψ estimation and less conservative for S estimation; and (7) analytically predicted spatial covariances provide a means for network design.
A tracer migration experiment in a small fracture zone in granite
Authors: Birgersson Lars , Moreno Luis , Neretnieks Ivars , Widén Hans , Ågren Thomas
Published: April 2004

The water flow rate and tracer distribution has been studied in a fracture zone and averagely fractured rock in the Stripa underground research laboratory. The experimental site is located in granitic rock at a depth of 385 m below the ground surface. A 50‐m‐long drift with a diameter of 3 m was excavated. The drift was intersected by a 6‐m‐wide fracture zone. The upper part of the drift was covered by 150 plastic sheets in which water was collected. The water in the lower part of the drift was collected in sump holes. Different tracers were injected in seven locations at distances between 9.5 and 25 m from the drift. More than 50% of the water was found in one sampling area in the drift, and more than 90% in only eight sampling areas. The recovery of the tracers was also concentrated in a few sheets. Most of the flow preferentially takes place in a few paths. Two sets of tracers were used in tandem; one set of dyes which had previously been used in Stripa and one set of lanthanide‐DTPA complexes. Although shown to be nonsorbing in long‐time high‐sensitivity laboratory tests, the dyes were found to have a somewhat smaller recovery than the metal tracers. This observation was used in an attempt to estimate the flow‐wetted surface into which the dyes can diffuse and sorb. The flow porosity was estimated from the tracer residence times for all injection points. The flow in the intersected fracture zone is not like that in a porous medium; it is highly channeled.
Numerical investigation of steady liquid water flow in a variably saturated fracture network
Authors: Kwicklis Edward M. , Healy Richard W.
Published: April 2004

Numerical simulation was used to study steady liquid water movement in a 5‐m by 5‐m vertical section containing a hypothetical fracture network under conditions of variable saturation. The fracture network was assumed to be embedded within an impermeable rock matrix. Three variations of a network were considered. The “mixed” network consisted of two fracture sets, a subvertical set containing five 125 μm average aperture fractures and a subhorizontal set containing four 25 μm average aperture fractures. The other two networks had identical fracture orientation and contained either all 125 μm or all 25 μm average aperture fractures. The TOUGH simulator was used to calculate the total steady liquid water flux through the network, the flux through individual fracture segments, and the pressure head at each fracture segment. A unit hydraulic gradient was imposed on the network by applying fixed pressure head boundaries (ranging from −0.25 to 0.0 m of water) of equal value to the top and bottom. Saturation and permeability versus pressure head relations for the two sets of fractures were determined with the VSFRAC model, which assumed that aperture was variable within an individual fracture. Results showed that the spatial distributions of pressure head and flux within the network, as well as the location of the dominant pathways, depended strongly on the prescribed boundary pressure head. For the mixed network, both pressure head and flux tended to become more spatially uniform when the boundary pressure head approached the pressure head at which the permeability thickness products of the large‐ and small‐aperture fractures are equal (the crossover pressure head). These results imply that for systems similar to the one considered here, interpretation of actual measurements of pressure head and flux may be quite complex, and that representation of variably saturated fracture networks as an equivalent continuum may be more valid for some ranges in pressure head than for others. Equivalent permeability as a function of pressure head was calculated for the fracture network, illustrating how information collected on individual fractures may be used to estimate the flow properties of rock at larger scales.
Alternative analyses of hydraulic data from disc tension infiltrometers
Authors: Hussen A. A. , Warrick A. W.
Published: April 2004

Hydraulic conductivity values were compared based on alternative analyses of data from disc tension infiltrometers. The first method was based on a single disc and tension and depends on the estimate of sorptivity and steady state flow. A second method used steady state flow measurements for two different disc radii, 52 and 118 mm. A third method used a single disc with multiple tensions from which steady state flow was obtained at three or more tensions which could be at the same location. A fourth method used a single disc with two tensions from which steady state flow was obtained at two tensions. Finally, values based on soil cores were compared. The results show reasonable agreement between methods for the hydraulic conductivity with the largest differences for data collected for zero tension. For the most part, there were no significant differences in hydraulic conductivity due to the disc infiltrometer radius. A single‐disc method with multiple tensions (more than 3 points) and large disc radius gave results which were the most stable, accurate, and repeatable.
Channel network source representation using digital elevation models
Authors: Montgomery David R. , Foufoula‐Georgiou Efi
Published: April 2004

Methods for identifying the size, or scale, of hillslopes and the extent of channel networks from digital elevation models (DEMs) are examined critically. We show that a constant critical support area, the method most commonly used at present for channel network extraction from DEMs, is more appropriate for depicting the hillslope/valley transition than for identifying channel heads. Analysis of high‐resolution DEMs confirms that a constant contributing area per unit contour length defines the extent of divergent topography, or the hillslope scale, although there is considerable variance about the average value. In even moderately steep topography, however, a DEM resolution finer than the typical 30 m by 30 m grid size is required to accurately resolve the hillslope/valley transition. For many soil‐mantled landscapes, a slope‐dependent critical support area is both theoretically and empirically more appropriate for defining the extent of channel networks. Implementing this method for overland flow erosion requires knowledge of an appropriate proportionality constant for the drainage area‐slope threshold controlling channel initiation. Several methods for estimating this constant from DEM data are examined, but acquisition of even limited field data is recommended. Finally, the hypothesis is proposed that an inflection in the drainage area‐slope relation for mountain drainage basins reflects a transition from steep debris flow‐dominated channels to lower‐gradient alluvial channels.
A stochastic model of river water quality: Application to salinity in the Colorado River
Authors: Lee Donna J. , Howitt Richard E. , Mariño Miguel A.
Published: April 2004

This research combines mass transport theory with stochastic methods to derive a stochastic, variable flow, surface water quality model for water quality policy analysis. Steady‐state flow assumptions and a series of differential equations reduce the information and number of equations required to completely specify the model. The newly developed hydrology model is applied to river salinity in the Colorado Basin to evaluate the effectiveness of alternative salt load reduction strategies in water quality management. Model results are used to illustrate the influence of salt load reduction, stochastic river flows, and water conservation on river water quality.
An efficient method for simulating steady unsaturated flow in random porous media: Using an analytical perturbation solution as initial guess to a numerical model
Authors: Harter Thomas , Yeh T.‐C. Jim
Published: April 2004

Numerical simulation of flow through multidimensional heterogeneous soils under unsaturated conditions is a computationally intensive task. The governing unsaturated flow equation is nonlinear. The degree of nonlinearity depends on the unsaturated hydraulic properties of the soil and the degree of heterogeneity. In this paper, we present a highly efficient method to simulate unsaturated steady state flow through random porous media. Our method utilizes the analytical (approximate) solution derived by a perturbation‐spectral method as an initial guess solution for a numerical model to simulate two‐dimensional vertical infiltration problems. It is found that this approach, which we call “ASIGNing,” reduces the required CPU time by one to two orders of magnitude. ASIGNing is demonstrated to operate successfully under a wide variety of boundary conditions which may substantially deviate from those imposed on the initial guess solution, A large range of mean and variances in the independent variables ln Ks and α or alternatively In α, has been tested and it is shown that the method works well for variances of the unsaturated hydraulic conductivity σln2K′ ≤ 5 and average 〈α〉 ≤ 0.1 [cm−1].
Adaptive estimation of the log fluctuating conductivity from tracer data at the Cape Cod Site
Authors: Deng F. W. , Cushman J. H. , Delleur J. W.
Published: April 2004

An adaptive estimation scheme is used to obtain the integral scale and variance of the log‐fluctuating conductivity at the Cape Cod site based on the fast Fourier transform/stochastic model of Deng et al. (1993) and a Kalmanlike filter. The filter incorporates prior estimates of the unknown parameters with tracer moment data to adaptively obtain improved estimates as the tracer evolves. The results show that significant improvement in the prior estimates of the conductivity can lead to substantial improvement in the ability to predict plume movement. The structure of the covariance function of the log‐fluctuating conductivity can be identified from the robustness of the estimation. Both the longitudinal and transverse spatial moment data are important to the estimation.
The use of velocity and conductivity data for the quantification of heterogeneity: A comparison
Authors: Pfleiderer S. , Moltyaner G. L.
Published: April 2004

Hydraulic conductivities of a portion of the Twin Lake aquifer are derived from grain size analyses performed at 3‐cm vertical sampling intervals in four boreholes, spaced l m apart. Correlation of conductivity logs between four boreholes leads to a geometrical model which corresponds closely to the spatial structure derived from the velocity field of previous tracer tests. Histogram and correlogram analyses of conductivities agree closely with those of velocities, and the two data sets appear equally useful for characterizing the aquifer and its spatial variability. Using Darcy's law, flow velocities and conductivities are combined to calculate a theoretical, hydraulic gradient. Comparison to gradients observed at the site reveals agreement on a large scale. However, differences exist in detail because observed piezometric heads fail to reflect small‐scale heterogeneity. Since these differences are significant for contaminant transport prediction, it is recommended for future studies to directly measure flow velocities from a tracer experiment, rather than to reply on head measurements and on Darcy's law for the calculation of transport parameters.
Using genetic algorithms to solve a multiple objective groundwater pollution containment problem
Authors: Ritzel Brian J. , Eheart J. Wayland , Ranjithan S.
Published: April 2004

The genetic algorithm (GA), a new search technique, is applied to a multiple objective groundwater pollution containment problem. This problem involves finding the set of optimal solutions on the trade‐off curve between the reliability and cost of a hydraulic containment system. The decision variables are how many wells to install, where to install them, and how much to pump from each. The GA is an optimization technique patterned after the biological processes of natural selection and evolution. A GA operates on a population of decision variable sets. Through the application of three specialized genetic operators: selection, crossover, and mutation, a GA population “evolves” toward an optimal solution. In the paper, simple GAs and GAs that can solve multiple objective problems are described. Two variations of a multiple objective GA are formulated: a vector‐evaluated GA (VEGA) and a Pareto GA. For the zero‐fixed cost situation, the Pareto GA is shown to be superior to the VEGA and is shown to produce a trade‐off curve similar to that obtained via another optimization technique, mixed integer chance constrained programming (MICCP). The effect on the VEGA and Pareto GA of parameter variation is shown. The Pareto GA is shown to be capable of incorporating the fixed costs associated with installing a system of wells. Results for several levels of fixed cost are presented. A comparison of computer resources required by the GAs and the MICCP method is given. Future research plans are discussed, including the incorporation of the objective of pump‐out time into the model and the development of parallelized GAs.
Spectrum estimation of moderately overspread radar targets using aperiodic transmitter coding
Authors: Uppala Sathyadev V. , Sahr John D.
Published: April 2004

We present a new method of spectrum estimation of moderately overspread targets using a pulse‐to‐pulse method, with nonuniform pulse spacing. This method is especially applicable to VHF observations of the auroral electrojet irregularities, for which the ratio of the target range extent and the autocorrelation time exceeds the speed of light by a modest amount. For a suitable choice of pulse spacings, the range and frequency aliasing can be uniquely unraveled, and the estimation algorithm can be expressed in terms of the fast Fourier transform, for excellent computational efficiency. This technique should find application to other moderately overspread targets, such as Doppler weather radar and equatorial spread F.
Use of the very large array for measurement of time variable Faraday rotation
Authors: Sakurai Takayuki , Spangler Steven R.
Published: April 2004

In this paper we present a method for applying a second‐order correction to the polarization data taken at the very large array (VLA). We found that the standard polarization calibration method is inadequate for obtaining a polarization angle time series with position angle constancy better than a few degrees over several hours. Even after application of a more sophisticated ionospheric Faraday rotation model than is commonly employed in VLA polarization measurements, the rms scatter in position angle exceeded 2° at 20 cm. In order to reduce this scatter, a polarization “self‐calibration” method was developed to correct the data for residual instrumental polarization, here noted by the array‐averaged instrumental polarization error A to be defined and discussed in section 5, and ionospheric Faraday rotation errors δθFR. Using four extragalactic radio sources with fractional polarization ranging from 1 to 4.8%, the residual errors A and δθFR were estimated. A typical value of A was 10−3 and the magnitude of δθFR was less than 2°. These residuals, however, are enough to limit the VLA polarization angle measurement to roughly 2° at 20 cm. The instrumental polarization error was found to be time variable with a timescale of a few hours. These errors were removed from the data, and the corrected data showed an improvement in polarization time series with a smaller scatter in position angle, except for the source 0010 + 005. It was close to the Sun on the day of the observation, and we believe the excess fluctuations are due to coronal plasma turbulence. We believe the technique described in this paper will be of use to observations in which the Faraday rotation varies during the course of an observing session, such as measurements of coronal Faraday rotation, or in which it is important to eliminate or minimize the Faraday rotation noise. An example of the latter type of observation would be the study of pulsar rotation measure variations over long time intervals. Of course, a result of this procedure is a precision determination of ionospheric rotation measure, in principle in various directions. Such observations might aid radio remote sensing studies of the ionosphere.
Estimation of the transmitter and receiver differential biases and the ionospheric total electron content from Global Positioning System observations
Authors: Sardón E. , Rius A. , Zarraoa N.
Published: April 2004

In the estimation of the ionospheric total electron content from the Global Positioning System (GPS) observables, various instrumental systematic effects such as the biases in the GPS satellites and receivers must be modeled. This paper describes a procedure, based on a Kalman filtering approach, for estimating these instrumental biases as well as the total electron content at each GPS station, using dual GPS data. The method is applied to six data sets, of 48 hours each, spanning one year, from the Deep Space Network with GPS stations in Australia, Spain, and the United States. The formal errors for the estimated satellite biases and for the total electron content at each station are about 0.07 ns and 0.2×1016 el/m2, respectively. The variation in time of the satellite biases (relative to the mean of all of them) estimated in different epochs during 1‐year period, is below 1 ns.
Mid‐latitude spread F occurrence related to geomagnetic activity at preferred local times
Authors: Bowman G. G.
Published: April 2004

Superposed‐epoch methods have been used to investigate the level of spread F occurrence relative to high geomagnetic activity. The controls used were determined from high values of the AE index during two 6‐hour periods representing early morning intervals at two high midlatitude stations separated by 10 hours of local time. Delayed spread F occurrences at these stations (Lannion in France and Canberra in Australia) were considered using these controls. The results show that spread F occurrence is enhanced (delayed, however, sometimes by as much as 3 days) by using controls for the early morning periods of the station being investigated, but not when controls for the early morning periods of the other station are used. It is suggested that in examining relationships between other geophysical phenomena and geomagnetic activity, it may be profitable to consider geomagnetic activity at specific local times rather than employing the use of a daily index such as ΣKp.
Numerical simulation of conical diffraction of tapered electromagnetic waves from random rough surfaces and applications to passive remote sensing
Authors: Li L. , Chan C. H. , Tsang L.
Published: April 2004

A new tapered wave integral equation method was derived to simulate the conical diffraction of electromagnetic waves from rough surfaces. Both the full matrix inversion and the banded matrix iterative approaches are developed. By using the principle of reciprocity and energy conservation, all four Stokes parameters are calculated for polarimetric passive remote sensing of rough surfaces. We show in this paper that for a moderately rough surface, the third Stokes parameter can be as high as ±20°K. The tapered wave integral equation approach can deal with a rough surface with a large slope. The new method presented in this paper is compared with the previously published plane wave integral equation method and the extended boundary condition method. Very good agreement is obtained. Unlike the plane wave integral equation method and the extended boundary condition method, the tapered wave integral equation method does not have the kinks imposed by Floquet models and it requires a shorter surface length in most applications.
Automated Mapping of Montane Snow Cover at Subpixel Resolution from the Landsat Thematic Mapper
Authors: Rosenthal Walter , Dozier Jeff
Published: April 2004

A fully automated method uses Landsat Thematic Mapper data to map snow cover in the Sierra Nevada and make quantitative estimates of the fractional snow‐covered area within each pixel. We model winter and spring reference scenes as linear mixtures of image end member spectra to produce the response variables for tree‐based regression and classification models. Decision trees identify cloud cover and fractional snow‐covered area. We test the algorithm on a different Thematic Mapper scene and verify with high‐resolution, large‐format, color aerial photography. The accuracy of the automated classification of Thematic Mapper data equals that obtainable from the aerial photographs but is faster, cheaper, and covers a vastly larger area. The mapping method is insensitive to the choice of lithologic or vegetation end members, the water equivalent of the snow pack, snow grain size, or local illumination angle.
Summer monsoon response of the Northern Somali Current, 1995
Authors: Schott Friedrich , Fischer Jürgen , Garternicht Ulf , Quadfasel Detlef
Published: April 2004

Preliminary results on the development of the northern Somali Current regime and Great Whirl during the summer monsoon of 1995 are reported. They are based on the water mass and current profiling observations from three shipboard surveys of R/V Meteor and on the time series from a moored current‐meter and ADCP array. The monsoon response of the GW was deep‐reaching, to more than 1000m. involving large deep transports. The northern Somali Current was found to be disconnected from the interior Arabian Sea in latitude range 4°N–12°N in both, water mass properties and current fields. Instead, communication dominantly occurs through the passages between Socotra and the African continent. From moored stations in the main passage a northward throughflow from the Somali Current to the Gulf of Aden of about 5 Sv was determined for the summer monsoon of 1995.
Cloud optical thickness variations during 1983–1991: Solar cycle or ENSO?
Authors: Kuang Zhiming , Jiang Yibo , Yung Yuk L.
Published: April 2004

Based on a detailed analysis of the cloud data obtained by the International Satellite Cloud Climatology Project (ISCCP) in the years 1983–1991, we show that besides the reported 3% variation in global cloudiness (Svensmark and Friis‐Christensen, 1997), the global mean cloud optical thickness (MCOT) also has significant variation which is out of phase with that of the global cloudiness. The combined effect of the two opposing variations may be a null effect on the cloud reflectivity. These results are consistent with the Total Ozone Mapping Spectrometer (TOMS) reflectivity measurements. The MCOT variation is further shown to be correlated with both the solar cycle and the ENSO cycle. Our present analysis cannot distinguish which of the above two provides better correlation, although independent data from the High resolution Infrared Radiation Sounder (HIRS) from 1990 to 1996 favor the solar cycle. Future data are needed to identify the true cause of these changes.
Overlapping ion populations in the cusp: polar/TIMAS results
Authors: Trattner K. J. , Coates A. J. , Fazakerley A. N. , Johnstone A. D. , Balsiger H. , Burch J. L. , Fuselier S. A. , Peterson W. K. , Rosenbauer H. , Shelley E. G.
Published: April 2004

The 90 degree inclined orbit of the Polar satellite crosses the cusp at intermediate altitudes (4RE–6RE). The Toroidal Imaging Mass‐Angle Spectrograph (TIMAS) on the Polar spacecraft can obtain full energy/pitch angle distributions for four masses simultaneously. Many examples of magnetosheath particle injections with velocity dispersion, sometimes overlapping, have been observed.

We report on two crossings of the cusp where TIMAS observed several overlapping cusp ion injections. These injections showed simultaneous onset of cusp ion precipitation at two different energies as well as the brief occurrence of ion precipitation above and below the energy of a preexisting population. Existing models of cusp precipitation do not provide an adequate explanation of these observations.
Hydraulic power capacity of water distribution networks in uncertain conditions of deterioration
Authors: Park Joshua I. , Lambert James H. , Haimes Yacov Y.
Published: April 2004

Water distribution systems throughout the United States are in various and uncertain stages of deterioration. Large capital expenditures are needed for their maintenance, modernization, and revitalization. Failures of the systems to supply adequate flow and pressure are adverse to the long‐term economic growth and prosperity of the nation. Modeling the uncertain capacity of the distribution network is essential to allocating limited resources to components requiring the most rehabilitation. Network capacity and reliability are related to the uncertain conditions of pipes in the system. Some approaches to measuring a hydraulic capacity have been based on specifying flow volume alone without accounting for pressure. This paper introduces the combining of requirements for volume and pressure in a single dimension, the hydraulic power. Next, the hydraulic power capacity is defined as the probability that there exists a feasible flow of hydraulic power in the water distribution network. The probabilistic assessment of hydraulic power capacity is demonstrated in a small example under the assumptions of (1) fixed, known maximum inflows (supply) and minimum outflows (demand), (2) uncertain capacities for hydraulic power in individual pipes, reflecting uncertain conditions of pipe decay and roughness, and (3) an optional operating rule corresponding to a fixed minimum total loss of power in the network.
Inverse hydrologic modeling using stochastic growth algorithms
Authors: Hestir Kevin , Martel Stephen J. , Vail Stacy , Long Jane , D'Onfro Pete , Rizer William D.
Published: April 2004

We present a method for inverse modeling in hydrology that incorporates a physical understanding of the geological processes that form a hydrologic system. The method is based on constructing a stochastic model that is approximately representative of these geologic processes. This model provides a prior probability distribution for possible solutions to the inverse problem. The uncertainty in the inverse solution is characterized by a conditional (posterior) probability distribution. A new stochastic simulation method, called conditional coding, approximately samples from this posterior distribution and allows study of solution uncertainty through Monte Carlo techniques. We examine a fracture‐dominated flow system, but the method can potentially be applied to any system where formation processes are modeled with a stochastic simulation algorithm.
Geophysical characterization, redox zonation, and contaminant distribution at a groundwater/surface water interface
Authors: Lendvay J. M. , Sauck W. A. , McCormick M. L. , Barcelona M. J. , Kampbell D. H. , Wilson J. T. , Adriaens P.
Published: April 2004

Three transects along a groundwater/surface water interface were characterized for spatial distributions of chlorinated aliphatic hydrocarbons and geochemical conditions to evaluate the natural bioremediation potential of this environmental system. Partly on the basis of ground penetrating radar measurements, a conductive sediment layer was detected from the shore out to at least 300 m offshore which exhibited gradients in redox pairs and contaminant profiles. The cis‐Dichloroethene and 1‐chloroethene were predominant in the presence of elevated methane and ferrous iron concentrations and depressed sulfate and aquifer solids‐bound iron concentrations. The shallow monitoring points were generally hypoxic to aerobic and exhibited values of specific conductance reflective of near‐shore lake water, indicating reoxygenation of the contaminant plume due to wave infiltration. The barge transect yielded trace contaminant concentrations and showed evidence of sulfate reduction. These analyses contributed to the understanding of processes affecting contaminant fate and transport at near‐shore mixing zones.
Human DNA polymerase‐η, an A‐T mutator in somatic hypermutation of rearranged immunoglobulin genes, is a reverse transcriptase
Authors: Franklin Andrew , Milburn Peter J , Blanden Robert V , Steele Edward J
Published: April 2004

We have proposed previously that error‐prone reverse transcription using pre‐mRNA of rearranged immunoglobulin variable (IgV) regions as templates is involved in the antibody diversifying mechanism of somatic hypermutation (SHM). As patients deficient in DNA polymerase‐η exhibit an abnormal spectrum of SHM, we postulated that this recently discovered Y‐family polymerase is a reverse transcriptase (RT). This possibility was tested using a product‐enhanced RT (PERT) assay that uses a real time PCR step with a fluorescent probe to detect cDNA products of at least 27−37 nucleotides. Human pol‐η and two other Y‐family enzymes that are dispensable for SHM, human pols‐ι and ‐κ, copied a heteropolymeric DNA‐primed RNA template in vitro under conditions with substantial excesses of template. Repeated experiments gave highly reproducible results. The RT activity detected using one aliquot of human pol‐η was confirmed using a second sample from an independent source. Human DNA pols‐β and ‐µ, and T4 DNA polymerase repeatedly demonstrated no RT activity. Pol‐η was the most efficient RT of the Y‐family enzymes assayed but was much less efficient than an HIV‐RT standard in vitro. It is thus feasible that pol‐η acts as both a RNA‐ and a DNA‐dependent DNA polymerase in SHM in vivo, and that Y‐family RT activity participates in other mechanisms of physiological importance.
From immune system to health systems − Challenges for health research
Authors: Pang Tikki
Published: April 2004

There are many similarities between the immune system and health systems. Both are knowledge‐based learning systems with an ultimate common objective of maintaining the health of individuals and populations, respectively. The immune system learns from the environment, and knowledge and discovery are key drivers of health systems improvement and improving health status. But discovery goes well beyond immunology, virology and genomics, for example, in the quest to develop new drugs, vaccines and diagnostic tests. New ways must also be discovered to make sure that these products reach the people who need it most. We must discover ways for knowledge to be turned into effective policies, to inform the practice of healthcare and to inform the public and society at large. Only if discovery is seen in its broadest perspective will we be able to reach the goal of health for all.