bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability analysis of the unsaturated water flow equation: 2. Experimental verification
Authors: Yao T. , Hendrickx J. M. H.
Published: February 2004

In this study, we verify the stability analysis of the unsaturated flow equation presented in paper 1. We compare finger sizes measured in laboratory experiments with predictions by the stability model. Using measured unsaturated hydraulic properties which are fitted and extrapolated over the entire water content range, the model enables us to predict finger sizes over a wide range of nonponding infiltration rates. The model predicts an increase of finger diameter at high and low infiltration rates for all soils. Such increases were also observed in our experiments. The model yields reliable predictions of finger diameters at high and intermediate infiltration rates but does not perform as well at infiltration rates <1 cm/h. This is caused by experimental limitations in accurately measuring hydraulic conductivities at low infiltration rates. A sensitivity analysis for initial water content and the shape parameter of the hydraulic conductivity curve showed that a small change of the parameters resulted in a relatively small change of finger size at high and intermediate infiltration rates but at low infiltration rates the finger size change was large, i.e., several orders of magnitude. We conclude that the stability model presented in paper 1 can be used for the assessment of wetting front instabilities in homogeneous soils over a wide range of nonponding infiltration rates.
Effect of sorbed oil on the dielectric properties of sand and clay
Authors: Li Camille , Tercier Paulette , Knight Rosemary
Published: February 2004

The dielectric response of geologic materials is related to the amount of water‐wetted solid surface present. This dependence raises the possibility that dielectric measurements may be used to detect changes in the state of the solid‐fluid interface such as those caused by the sorption of a chemical species. In this study, complex impedance data are collected for systems of clean and oil‐bearing sand and kaolinite vacuum‐ saturated with a 0.001 M NaCl brine in the frequency range 100 kHz to 10 MHz. The effective dielectric constant k and electrical conductivity σ of the samples are calculated from these measurements assuming quasi‐static conditions. We define the components in our system as brine, air, and a “wetted” solid phase. In this way, the contribution of the solid‐fluid interface to the bulk dielectric response is included in the last component. We use an inclusion‐based effective medium theory to extract the dielectric response of the wetted solid from the experimental data. The presence of sorbed oil is found to have little effect on the dielectric response of the wetted sand matrix, presumably because of the very low surface area (0.2 m2/g) of the solid‐fluid interface. In contrast, the kaolinite (5–12 m2/g) data indicate a decrease of the order of 50% in the dielectric constant of the wetted matrix phase as the amount of water‐wetted surface in the sample decreases. The results from this experimental study show that the presence of sorbed oil leads to a detectable change in the dielectric properties of high surface area geologic materials.
Total solar irradiance observations from DIARAD/VIRGO
Authors: Dewitte S. , Crommelynck D. , Joukoff A.
Published: February 2004

The Differential Absolute Radiometer is making measurements of the total solar irradiance as part of the Variability of Irradiance and Gravity Oscillations experiment on the Solar and Heliospheric Observatory. We present the measurements made during its 7.8 years of operation, from 1996 until the present (2003). The aging due to UV exposure of the continuously measuring left channel is determined by comparison with the backup right channel; the loss in sensitivity of the left channel is 0.5 W/m2 or 364 ppm over 7.8 years. A raise of the total solar irradiance from a level of 1365.5 W/m2 at the end of the solar minimum in 1996 towards a maximum level of 1368 W/m2 at the beginning of 2002 has been measured by DIARAD.
Seismicity and mean magnitude variations correlated to the strongest earthquakes of the 1997 Umbria‐Marche sequence (central Italy)
Authors: Murru Maura , Console Rodolfo , Lisi Arianna
Published: January 2004

Nearby faults can interact, affecting the timing of a future earthquake. A large earthquake can alter the static stress surrounding faults, possibly activating an aftershock sequence. Here we test the hypothesis of earthquake triggering by examining the earthquakes that affected the Umbria‐Marche region (central Italy) during September 1997 to April 1998. The analysis was performed combining information contained in parameters a and b of the Gutenberg‐Richter relationship, before and after the strongest main shocks, using a catalog of 6211 earthquakes (M ≥ 1.5). By analyzing the changes in space of the a parameter, we found that Colfiorito main shocks (Mw 5.7 and 6.0) on 26 September 1997 increased the seismicity rate within 40–50 km of the epicenter. We interpret this behavior as the effect of stress redistribution. After analyzing the b value changes in space and time we found the strongest increase in mean magnitude in the Sellano area, between the Colfiorito shocks and the two strong events (Mw 5.2 and 5.6, respectively) which hit this region on 12 and 14 October 1997. This area was also the location of the strongest increase in change of the local occurrence rate density. On the basis of these observations, we postulate that temporal variations in both a and b values indicate a probable causal connection between the largest earthquakes in the sequence.
Unstable layers in the mesopause region observed with Na lidar during the Turbulent Oxygen Mixing Experiment (TOMEX) campaign
Authors: Liu Alan Z. , Roble Raymond G. , Hecht James H. , Larsen Miguel F. , Gardner Chester S.
Published: January 2004

The Na wind/temperature lidar located at Starfire Optical Range near Albuquerque, New Mexico, provided real time measurements of wind, temperature, and Na density in the mesopause region during the TOMEX rocket campaign in October 2000. The state of the atmosphere in which the rocket was launched into was examined using the lidar measurements. Both convectively and dynamically unstable layers were observed at various times and altitudes during the night. The low convective stability region below 90 km was found to be associated with the diurnal tide. The unstable layers are the combined results of wave and tidal perturbations. Comparison with the thermosphere/ionosphere/mesopshere/electrodynamics general circulation model (TIME‐GCM) simulation showed that the model can produce the general feature of the observed atmospheric structure (but with a much smaller diurnal amplitude in temperature), which likely leads to underestimate of instability and gravity wave effects.
Upper crustal seismic velocity structure and microearthquake depths at the Endeavour Segment, Juan de Fuca Ridge
Authors: Barclay Andrew H. , Wilcock William S. D.
Published: January 2004

We present the results of a study to invert microearthquake and explosive shot data from the Endeavour segment of the intermediate‐spreading Juan de Fuca Ridge. The average isotropic P wave velocity structure, derived from the shot data, in the uppermost 1.5 km of the oceanic crust is characterized by an increase with age of ∼8% from the axis to at least 0.5 Ma, that is attributed to the sealing of layer 2A porosity by hydrothermal processes. Superimposed on this variation are axis‐parallel, 2‐km‐wide, alternating bands of high and low velocity with a peak‐to‐peak variation of 5–12%. High and low velocities away from the axis correspond to bathymetric trenches and ridges, respectively and are likely due to variations in layer 2A thickness. P wave azimuthal anisotropy is present in the data that is best fit with a model of 9% anisotropy at 750 m depth, decreasing to 1% at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along‐axis direction. Anisotropy and velocity heterogeneity are coupled; anisotropy alone may explain the form but not the magnitude of the axis‐parallel bands. There are strong trade‐offs between the hypocentral depths of microearthquakes and the P and S wave velocity structures. Changing the mean hypocentral depth by up to 0.5 km leads to only modest increases in the travel time RMS but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower.
Impact of GPS Zenith Tropospheric Delay data on precipitation forecasts in Mediterranean France and Spain
Authors: Vedel H. , Huang X.‐Y. , Haase J. , Ge M. , Calais E.
Published: January 2004

Forecasting precipitation in the western Mediterranean is difficult because of the interactions among dynamical forcing, orographic lifting and moisture advection from the warm Mediterranean Sea. Torrential rainfall events are not uncommon, especially during the autumn. This type of event motivated an effort to improve precipitation forecasting by incorporating additional information on the initial state of the humidity field from Global Positioning System measurements of refractive delay. In this study we process data from a network of sites in Western Europe and assimilate the data over a two week period into the HIRLAM numerical weather prediction model. The overall impact for the two week period is neutral, however, for a severe rain event taking place during that period, the forecasts show improved skill when including GPS data. The work implies that the GPS data have good potential for influencing numerical models in rapidly developing, high moisture flux situations.
Middle and upper thermospheric odd nitrogen: 2. Measurements of nitric oxide from Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) satellite observations of NO γ band emission
Authors: Minschwaner K. , Bishop J. , Budzien S. A. , Dymond K. F. , Siskind D. E. , Stevens M. H. , McCoy R. P.
Published: January 2004

Ultraviolet dayglow observed using the Ionospheric Spectroscopy and Atmospheric Chemistry (ISAAC) satellite instrument is used to determine the concentration of nitric oxide (NO) in the upper mesosphere and thermosphere. ISAAC was launched in February 1999 on board the Advanced Research and Global Observing Satellite (ARGOS). We focus on ISAAC dayside limb observations at tangent altitudes between 80 and 200 km and over the latitude range 40°N–70°S. The data analyzed here were obtained in November and December 1999 during a period of moderate to high solar activity. The NO density retrievals include constraints on instrument sensitivity, in‐flight assessments of stray light, and determinations of Rayleigh‐scattered background contributions. Residual spectra contain strong dayglow signatures of the NO γ bands which are used for inversion of measured radiances to absolute NO concentrations. Comparison between ISAAC measurements and photochemical calculations indicates improved agreement above 130 km using O2 and temperatures from the new NRLMSISE‐00, as compared to the MS model. In the lower thermosphere, observed daily variations in low‐latitude nitric oxide are strongly correlated with variations in the solar soft X‐ray flux.
Origin of the lithospheric stress field
Authors: Lithgow‐Bertelloni Carolina , Guynn Jerome H.
Published: January 2004

An understanding of the tectonic stress field is geologically important because it is the agent that preserves in the crust a memory of dynamical processes. In an effort to elucidate the origin of the present state of stress of the lithosphere we use a finite element model of the Earth's lithosphere to calculate stresses induced by mantle flow, crustal heterogeneity, and topography and compare these to observations of intraplate stresses as given by the World Stress Map. We explore two models of lithospheric heterogeneity, one based directly on seismic and other observational constraints (Crust 2.0), and another that assumes isostatic compensation. Mantle tractions are computed from two models of mantle density heterogeneity: a model based on the history of subduction of the last 180 Myr, which has proved successful at accurately reproducing the present‐day geoid and Cenozoic plate velocities, and a model inferred from seismic tomography. We explore the effects of varying assumptions for the viscosity structure of the mantle, and the effects of lateral variations in viscosity in the form of weak plate boundaries. We find that a combined model that includes both mantle and lithospheric sources of stress yields the best match to the observed stress field (∼60% variance reduction), although there are many regions where agreement between observed and predicted stresses is poor. The stress field produced by mantle tractions alone shows a greater degree of long‐wavelength structure than is apparent in the stress observations but agrees very well with observations in some areas where radial mantle tractions are particularly strong such as in southeast Asia and the western Pacific. The stress field produced by lithospheric heterogeneity alone depends strongly on the assumed crustal model: Whereas the isostatically compensated model yields very poor agreement with observations, the model based on Crust 2.0 matches the observations about as well as mantle tractions alone and matches very well in certain areas where the influence of high topography is very important (e.g., Andes, East Africa). A possible interpretation of our results is that the stress field is significantly influenced by lateral variations in the viscosity of the mantle, which leads to variable amounts of decoupling between lithosphere and mantle, allowing the mantle signature to dominate in some areas and the crustal signature to dominate in others. The poor fit between the isostatically compensated crustal model and observations and the large differences between the two crustal models point toward the importance of dynamic topography and remaining uncertainties in crustal structure and rheology. We also consider the possibility that observations of stress from the shallow crust may not reflect the state of stress of the entire plate; stresses in the upper plate may be at least partially decoupled from broader‐scale plate driving forces by lateral and vertical variations in lithospheric rheology.
A high‐resolution oxygen A‐band and water vapor band spectrometer
Authors: Min Q.‐L. , Harrison L. C. , Kiedron P. , Berndt J. , Joseph E.
Published: January 2004

A prototype high‐resolution oxygen A‐band and water vapor band spectrometer (HAWS) and new theoretical framework have been developed and demonstrated to study the applicability of photon path length statistics in the remote sensing of clouds, aerosols, and water vapor, particularly in addressing current challenges such as the detection of thin layers of clouds and aerosols especially over surfaces with high albedo, reducing retrieval errors due to the presence of non‐spherical particles, and enhancing the vertical resolution of retrieved atmospheric constituents. This work also provides a basis for the application of path length distribution in the development and validation of radiative transfer parameterizations that account for the effects of cloud inhomogeneity. The HAWS successfully achieves an out‐of‐band rejection of better than 10−5, a resolution of better than 0.5 cm−1, and high signal‐to‐noise ratio, which are crucial to retrieval of atmospheric information through high‐resolution spectrometry in the A‐band and water vapor band. A field campaign was conducted to demonstrate the capabilities of HAWS and the new retrieval algorithm at the cloud and radiation testbed at ARM SGP site in Oklahoma, where a comprehensive set of radiometric, passive, and active sensors provide continuous and concurrent measurements of clouds, aerosols, water vapor, and other atmospheric properties. Results show that in the A‐band thick and multiple layer clouds significantly enhance the mean and variance of the photon path length distribution, thin cirrus condition produce relatively small mean distribution and variance, and mean path lengths comparable to or smaller than the solar airmass were associated with clear sky cases at large solar zenith angles. The mean path length and variance in the water vapor band differs from that in the A‐band due to the spatial inhomogeneity of water vapor amounts, particularly in association with cloud layers.
Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana
Authors: Whitmeyer Steven J. , Simpson Carol
Published: January 2004

Metamorphic and ductile deformation fabrics within the Sierra de San Luis, central Argentina, provide evidence for the Early to Middle Paleozoic development of the paleo‐Pacific margin of Gondwana. Presumed Vendian‐aged metasedimentary rocks within both the Sierra de San Luis and the Sierras de Córdoba preserve early pressure solution cleavage development. Cambrian peak metamorphism in the Sierras de Córdoba and Ordovician peak metamorphism in the Sierra de San Luis indicate that juxtaposition of these two terranes did not occur prior to the Late Ordovician. Peak metamorphism and regional folding of the Sierra de San Luis metasedimentary rocks is equated with early stages of the development and shortening of the Famatinian magmatic arc along the western margin of Gondwana. Extensive, NNE trending zones of upper greenschist‐ to amphibolite‐facies ductile deformation within the Sierra de San Luis and along the western margin of the Sierra de Comechingones record the dextral oblique juxtaposition of the Sierra de San Luis terrane against the southwestern margin of the Sierras de Córdoba, coincident with, or closely followed by, the suture of the exotic Precordillera terrane to the western margin of Gondwana by the Early Devonian. Middle to Late Devonian restricted, lower greenschist‐grade reactivation of ductile faults may record the accretion of the Chilenia terrane, the final stage of Paleozoic convergent tectonism along this segment of western Gondwana.
Distance of movement of coarse particles in gravel bed streams
Authors: Hassan Marwan A. , Church Michael , Schick Asher P.
Published: February 2004

Distributions of distance of bedload particle movement were examined in two gravel bed streams using several hundred magnetically tagged cobbles and pebbles. The compound Poisson model of Einstein‐Hubbell‐Sayre and a simple gamma function model were compared with observed distributions of moved particles, and of all particles. Both models fit the data reasonably well for small mean displacements, but notable misfits occurred in an event with large mean displacement. When mean particle travel distance approaches the scale of bar spacing, trapping in the bars interrupts particle progress and the dispersion process. The data remain very noisy, so definitive discrimination of suitable models will require trials with more than 103 particles.
A three‐dimensional field study of solute transport through unsaturated, layered, porous media: 2. Characterization of vertical dispersion
Authors: Ellsworth T. R. , Jury W. A.
Published: February 2004

Solute plumes were created in an unsaturated field soil with either flux application or by leaching an initial resident distribution (see Ellsworth et al., this issue). The spatial variance of the plumes initially increased with time between the soil surface and a depth of 2.5 m, within which the soil was a nearly structureless loamy sand. Below this depth, the plumes were observed to compress in the vertical direction as they moved into, and through, a region of subangular blocky structure and loam texture (between 2.5 and 4.0 m depth). As the solute moved below the layer of fine texture, the plume variance again increased with time. Using a transformed advection‐dispersion equation description, two constant, field‐averaged transport coefficients, V* and Dzz*, were determined in a scaled coordinate system from the moment equations. These two constant parameters were then used to predict the observed local, or plot scale, transport. Results indicate that the two constant parameters describe transport reasonably well at each plot site and over all sampling depths.
Three‐dimensional spatial moments analysis of the Borden Tracer Test
Authors: Rajaram Harihar , Gelhar Lynn W.
Published: February 2004

The large‐scale tracer test conducted at the Borden site included detailed three‐dimensional plume monitoring, but most previous studies have emphasized the interpretation of the data in a two‐dimensional sense. Here, an independent analysis of the fully three‐dimensional nature of the plume is developed using a linear spatial interpolation scheme which differs significantly from the gridded higher‐order interpolation scheme used in earlier analyses. A detailed examination of the data base reveals significant truncation of the plume during several sampling sessions. This feature explains the low mass estimates obtained during the later sampling sessions. Consequently, there are significant uncertainties associated with the corresponding second‐moment estimates. This uncertainty, which calls into question any refined comparisons of second‐moment evolution with stochastic theories, has not been recognized in previous interpretations of the second‐moment estimates. The estimated vertical macrodispersivity is found to be about twice that attributable to molecular diffusion. The estimated horizontal transverse macrodispersivity is much larger than the vertical macrodispersivity. Temporal variations in the flow appear to provide a plausible explanation of the large horizontal transverse macrodispersivity. Anamolous behavior of the second‐moment evolution suggests the role of a large‐scale heterogeneity during the later portion of the test, indicating possible mutual interactions between the second‐moment components.
The one‐dimensional approximation for infiltration in heterogeneous soils
Authors: Protopapas Angelos L. , Bras Rafael L.
Published: February 2004

Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009. Document W91–004; $2.50. Payment must accompany order.

By using analytical solutions to the problem of one‐ and two‐dimensional linearized unsteady infiltration from a strip source in a soil with saturated hydraulic conductivity varying exponentially in space, explicit criteria for the validity of the one‐dimensional approximation are derived, in terms of parameters describing the soil type, the uniformity of the infiltrating source at the surface, and the soil heterogeneity. Numerical solutions for the linearized infiltration problem are developed for any variation of hydraulic conductivity in space. Extensive simulations suggest that for a sizable strip source and moderate variation of hydraulic conductivity, a set of independent neighboring soil columns represents well the multidimensional infiltration problem. The possibility of defining sufficient parameters controlling the accuracy of the one‐dimensional approximation is discussed.
Comparison between natural and forced gradient tests to determine the vertical distribution of horizontal transport properties of aquifers
Authors: Ronen Daniel , Magaritz Mordeckai , Molz Fred J.
Published: February 2004

A modified natural gradient point dilution test using a multilayer sampler was conducted in a 20‐m‐thick confined aquifer in a research field near Mobile, Alabama. The horizontal component of the specific discharge along the profile varies by an order of magnitude, from 2.0 to 21.1 m/yr. The modified point dilution test was capable of detecting very large q changes over small vertical intervals. The results obtained by the point dilution test agree with the results obtained in the same aquifer by a forced gradient tracer test over a distance of 6 m (Molz et al., 1988).
Approximating catastrophic risk through statistics of extremes
Authors: Mitsiopoulos James , Haimes Yacov Y. , Li Duan
Published: February 2004

This paper studies the approximation of the partitioned multiobjective risk method's (PMRM) extreme‐event risk function ƒ4. The analytic expression of the approximation for ƒ4 is derived through the use of the statistics of extremes for cases where the underlying distribution is of an extreme‐value type I, II, or III, and it thus provides an effective theoretical tool for understanding the behavior of conditional expected values for a large class of distribution functions used in science and engineering. The results are confirmed for example problems of normal, Gumbel, Weibull, Pareto, lognormal, and uniform distributions.
Short‐term flood risk prediction: A comparison of the Cox Regression Model and a Conditional Distribution Model
Authors: Futter M. R. , Mawdsley J. A. , Metcalfe A. V.
Published: February 2004

Two models for estimating the risk of a flood exceeding some critical threshold within a few days (Smith and Karr, 1986; Ettrick et al., 1987), which take account of the season and prevailing catchment conditions, have recently been published. The models are fitted to a 1000‐year synthetic data set, to compare the results with empirically determined risk estimates. After some modifications to the Smith and Karr method both models demonstrated reasonable accuracy. A second comparison is then made using data from a United Kingdom catchment. The Smith and Karr model was further modified to allow risk estimates for up to 30 days ahead to be made. Both models were applied to both data sets for 30 day ahead estimates, and the results demonstrate the sensitivity of the risk to prevailing conditions at the beginning of the period. The assumptions, data requirements, and accuracy of the models are compared and discussed.
Stochastic Control of Groundwater Systems
Authors: Georgakakos Aris P. , Vlatsa Dimitra A.
Published: February 2004

In groundwater management, uncertainty mainly stems from imprecise parameters and boundary conditions. This paper first formulates a stochastic groundwater management problem and subsequently proposes an appropriate solution approach. The equations of flow are converted to a dynamical state‐space system using finite element and difference techniques. Parameter and boundary condition uncertainty is incorporated using the small perturbation method. Management objectives are expressed as a composite performance index which may be used to minimize pumping costs, maintain hydraulic heads and pumping rates in the vicinity of target sequences, or optimally compromise among various system goals. This problem is solved via a numerical optimal control method which exhibits good computational properties. The approach is applied to the management of a two‐layer aquifer system with various boundary conditions and uncertainty levels and sources. The results provide useful insights of the system response under uncertainty and quantify the trade‐offs between accomplishing average system goals and minimizing uncertainty.
Small‐perturbation solution for steady nonuniform infiltration into soil surface of a general shape
Authors: Wallach Rony , Israeli Moshe , Zaslavsky Dan
Published: February 2004

A steady state solution is developed for three‐dimensional infiltration where both the water flux at the soil surface and the soil surface topography are nonuniform. By identifying a small parameter, which is defined as a ratio between the vertical and the horizontal characteristic lengths, the solution of the linearized equation can then be found by using perturbation methods. The spatial variation of both the soil surface topography and the water flux entering the profile appears only in parametric form in the equations and boundary conditions to all orders of approximation, thus allowing easy evaluation of the solution. The water accumulation due to soil surface topography is related to the inner produce between the gradients of the surface elevation and the rate of infiltration at the soil surface. Therefore the nonuniformity of the soil surface should be accompanied by a spatial nonuniformity of the applied flux in order to produce lateral variations in the solution. This method can be used to evaluate the water accumulation in the soil due to general rainfall or irrigation distributions. It can also be used for design purposes in cases where one or both sources of nonuniformities in the horizontal directions can be controlled, as for example, in an irrigation system in a field with a given topography.
New perspective on the Vedernikov Number
Authors: Ponce V. M.
Published: February 2004

There are three characteristic speeds in open channel hydraulics: (1) the mean flow velocity u, (2) the relative celerity of kinematic waves υ, and (3) the relative celerity of dynamic waves w. These three speeds give rise to two independent dimensionless ratios: (1) the dimensionless relative celerity of kinematic waves ck = υ/u, and (2) the dimensionless relative celerity of dynamic waves cd = w/u, The reciprocal of cd is the Froude number Fr = u/w. The Vedernikov number is the third ratio V = υ/w. Recent developments in the linear theory of surface runoff show that the Vedernikov number has a significant role in extending the concept of hydraulic diffusivity to the realm of dynamic waves. This new perspective makes possible the modeling of catchment dynamics using diffusion waves and including inertial effects, under a wide range of boundary friction and cross‐sectional shape specifications.
Three Case Studies of Lake Temperature and Stratification Response to Warmer Climate
Authors: Hondzo Midhat , Stefan Heinz G.
Published: February 2004

The impact of climatic warming on lakes will most likely have serious implications for water resources and water quality. Rather than using model predictions of greenhouse warming, this paper looks at the changes in heat balance and temperature profiles in a particularly warm year (1988) compared to a more normal one (1971). The comparisons are made for three different morphometrically different lakes located 45°N latitude and 93°W longitude (north central United States) and for the summer period (April 1 to October 31). Water temperatures are daily values simulated with a model driven by daily weather parameters and verified against several sets of measurements. The results show that in the warmer year epilimnetic water temperatures were higher, evaporative water loss increased, and summer stratification occurred earlier in the season.
Drawdown, Velocity, Storage, and Capture Response Functions for Multiaquifer Systems
Authors: Maddock Thomas , Lacher Laurel J.
Published: February 2004

Response functions for drawdown, velocity, storage losses, and capture describe the spatial and temporal reaction of an aquifer to a unit pumping stress. This paper extends the use of response functions to the notion of capture for multilayered aquifer systems. Capture describes the pumping‐induced quantity of water gained by the aquifer from internal or boundary sources. Internal sources include rivers and hydraulically connected aquifers. Boundary sources include constant head and head‐dependent boundaries. Although response functions are defined only for linear systems, a methodology is demonstrated for implementing response functions in cases of nonlinear capture. The value of response functions lies in their utility as hydrologic constraints in management optimization models. An example demonstrates the use of separable programming to determine the nonlinear effects of drawdown below the bottom of a streambed.
Distributed Snowmelt Simulations in an Alpine Catchment: 2. Parameter Study and Model Predictions
Authors: Blöschl G. , Gutknecht D. , Kirnbauer R.
Published: February 2004

A distributed grid‐based model is used (1) to analyze the importance of selected model parameters, (2) to simulate spatial distributions of snow cover properties in a small basin and (3) for a comparison with less sophisticated models as typically used in operational applications. Results indicate that variations of water equivalent with slope and local relief are of utmost importance for realistic distributed simulations but more moderately influence mean basin melt. Snow cover variables of which spatial distributions are simulated include the thermal and hydraulic state of the pack and hourly melt water release. All variables exhibit substantial variations in space and time. They are primarily controlled by topography and the delay of melt water in deep packs. The grid model is compared with a snow band model and a parametric model. The latter estimates the snowpack's areal extent from water equivalent. Simulated snow‐covered areas suggest the grid model to be the most realistic. Differences in terms of mean basin melt derive from different assumptions associated with model structure.
Uncorrelated measurement error in flood frequency inference
Authors: Kuczera George
Published: February 2004

When measurement error (ME) is present, the true value of the annual maximum discharge and of the censoring threshold discharge is unknown. The impact of using estimated rather than true discharges on the inference of flood quantiles is considered. The likelihood function is formulated for data which consist of annual maximum discharges (systematic data) and occurrences of floods above some threshold discharge (binomial‐censored data). The formulation makes explicit allowance for the corruption of discharges by statistically independent ME. This formulation can be used to get maximum likelihood estimates or perform a Bayesian analysis. A limited sampling experiment was conducted to assess the performance of two‐parameter lognormal maximum likelihood quantile estimators in the presence of simple uncorreiated discontinuous ME. It was found that ME reduces the information content of systematic and binomial‐censored data with the reduction being greatest for distributions with low coefficients of variation. When only using systematic data it was found that the performance of estimators making allowance for ME was, at best, only modestly better than that of estimators ignoring ME. The principal finding, however, was that the performance of maximum likelihood estimators using binomial‐censored data but ignoring ME can be substantially worse than that of estimators making explicit allowance for ME. In fact, the loss in performance can be so great that in some cases, it may be preferable to ignore the binomial‐censored data. Finally, it is shown that a Bayesian analysis of flood data affected by ME provides insights not offered by maximum likelihood estimation. When the threshold discharge is in error, the posterior distribution of the lognormal parameters can become bimodal as the length of the binomial‐censored record increases, indicating an inconsistency between the information in the systematic and binomial‐censored data.
Groundwater chemical evolution in a sandy silicate aquifer in northern Wisconsin: 2. Reaction modeling
Authors: Kenoyer Galen J. , Bowser Carl J.
Published: February 2004

Major chemical trends along flow paths in a sandy silicate aquifer (Kenoyer and Bowser, this issue) are interpreted using a mass balance approach and reaction simulation modeling. Because of the diverse mineral assemblage present and uncertainties in the thermodynamic constants, it is not possible to derive a unique solution. However, considering relative reaction kinetics and mineral abundance, a model involving the dissolution of plagioclase and small amounts of biotite and pyroxene to kaolinite in an open‐CO2 system is a simple, thermodynamically valid and reasonable explanation for the major chemical changes. The favored reaction model would result in apparent rates of dissolution of 2 × 10−18 mol cm−2 s−1 plagioclase (An41), 3 × 10−20 mol cm−2 s−1 biotite, and 3 × 10−17 mol cm−2 s−1 diopside. These rates are lower than published rates from laboratory studies, that range from 10−11 to 10−17 mol cm−2 s−1 for plagioclase, but are in the same range as rates derived from field studies. In predictive simulations where current silicate dissolution rates are assumed to hold constant but the pH of the recharge water is lowered from 6.4 to 3.99 (simulating increasing acidity of precipitation), the pH and alkalinity of the groundwater are reduced significantly.
Institutional response to climate change: Water provider organizations in the Denver Metropolitan Region
Authors: Rhodes Steven L. , Miller Kathleen A. , MacDonnell Lawrence J.
Published: February 2004

This study addresses regional adjustments to possible reductions in water availability which may occur with a future climate change. The study assesses potential responses of urban water suppliers to reduced water availability by examining an analogous case of reduced supply in the Denver metropolitan region. In this case the U.S. Environmental Protection Agency's veto of a large surface storage project (the Two Forks dam) prompted water providers in metropolitan Denver to take other actions to ensure adequate future water supply. In some cases, actions taken since the veto process began represent a departure from previous water provider policies. It is argued that similar institutional adjustments may occur due to climate change‐induced reductions in regional water supplies and that useful lessons may be drawn from an analysis of institutional responses to the Two Forks veto. This paper introduces what is envisioned as a long‐term assessment of regional adjustments to reduced future water supplies.
Network design for predicting groundwater contamination
Authors: McKinney Daene C. , Loucks Daniel P.
Published: February 2004

A new network design algorithm for improving the reliability of groundwater simulation model predictions is developed. The objective of the algorithm is to minimize the simulation model prediction variance by choice of new aquifer property measurement locations. This method, which uses parameter measurements to minimize prediction error, is different in concept from conventional parameter estimation or inverse methods which use state variable measurements to minimize parameter estimation error. The variance of predicted state variables, hydraulic head and contaminant concentration, is used as a measure of model prediction reliability. Prediction variance depends on the extent and quality of aquifer property measurements used to estimate simulation model parameters. Kriging is used in the first‐order uncertainty analysis to estimate the model parameters. First‐order uncertainty analysis is used in the algorithm to compute the prediction variance and select new measurement locations. A network design example is presented, the results of which show that a significant increase in simulation model prediction reliability is achieved by measuring aquifer properties at locations selected by the algorithm. The predicted mean was found to be insensitive to new measurements, while the predicted state variable variance was found to be quite sensitive to new measurements. The selection of measurement locations was found to be greatly affected by the type and extent of boundary conditions existing in the aquifer. Fixed value boundary conditions tend to reduce prediction variance in the neighborhood of new measurements and throughout the entire aquifer. Specified flux and mixed boundary conditions tend to reduce prediction variance in the neighborhood of the measurement locations, with little reduction occurring in other areas of the aquifer. The results also indicate that both the mean head and concentration are relatively insensitive to the number of data points in the network as long as there is a good estimate of the statistics of the ln (K) distribution and covariance function. This is not the case with the standard deviation of both the head and concentration which are significantly affected by the addition of data points to the network.
Effects of atmospheric pressures on gas transport in the vadose zone
Authors: Massmann Joel , Farrier Daniel F.
Published: February 2004

Temporal variations in barometric pressure due to weather patterns may induce air intrusion into the subsurface. This air intrusion can affect monitoring activities aimed at characterizing the composition and movement of gases in the vadose zone. Expressions are presented to estimate gas fluxes due to the combined effects of Knudsen diffusion, multicomponent molecular diffusion, and viscous flow. These expressions are used to evaluate the validity of the single‐component advection‐dispersion equation for simulating gas transport in the presence of atmospheric pressure variations. The single‐component equation provides reasonable results when used to simulate transport in media with relatively high gas permeability. Computer simulations of vertical transport at sites with homogeneous soils indicate that “fresh” air can migrate several meters into the subsurface during a typical barometric pressure cycle. Horizontal pressure gradients can develop at sites with near‐surface heterogeneities. These gradients may cause fresh air to intrude meters or tens of meters into the vadose zone during a storm event.
Time domain reflectometry for measuring volumetric water content in processed oil shale waste
Authors: Reeves T. L. , Elgezawi S. M.
Published: February 2004

Time domain reflectometry (TDR) was evaluated and developed to monitor volumetric water content (θυ) in oil shale solid waste retorted and combusted by the Lurgi‐Ruhrgas process. A TDR probe was designed and tested that could be buried and compacted in waste embankments and provide in situ measurements for θυ in the high‐saline and high‐alkaline conditions exhibited by this waste. TDR was found to be accurate for measurement of θυ across a broad range of water contents in the processed oil shale waste. A computer algorithm to automate the analysis of TDR traces to determine θυ, was developed and tested. A sensitivity test was performed to analyze differences between three smoothing algorithms on the measurement. No significant differences were found between smoothing algorithms or between the number of points applied for smoothing.
The possibility of community‐wide flood control benefits: Evidence from voting behavior in a bond referendum
Authors: Shabman Leonard , Stephenson Kurt
Published: February 2004

Voting behavior in a flood control referendum in Roanoke, Virginia, provides evidence that people living and working outside the flood prone area are willing to pay for flood control project construction. This voting behavior supports the argument that flood control benefits exist at the community level. In providing the cost sharing required under recent federal legislation, local government financing which distributes project costs over the whole population of a local jurisdiction, and not just those persons living or working in protected areas, may increase both economic efficiency and expand communities' financial capacity to pay for such projects.
Does Titan have an ocean? A review of current understanding of Titan's surface
Authors: Lunine Jonathan I.
Published: February 2004

The nature of Titan's surface has been a fascinating puzzle since the first definitive detection of an atmosphere in 1944. Pre‐Voyager models of the surface based largely on cosmochemistry and the detection of atmospheric methane permitted a range of possibilities from clathrate hydrate to pure methane snow. The Voyager 1 flyby ruled out this last possibility but pointed to a mixed ocean of methane, higher hydrocarbons, and nitrogen about a kilometer in thickness. Subsequent ground‐based radar observations reveal a reflective surface, inconsistent with pure light hydrocarbons. The variation in radar reflectivity from night to night is surprisingly high. Passive radiometry at centimeter wavelengths indicates a surface emissivity distinct from that of the icier Galilean satellites, suggesting that water ice does not dominate the surface either. Observations in the near‐infrared region are consistent with dirty water ice as well as a suite of other materials, including areas of hydrocarbon seas. Reconciling the Voyager, radar and near‐infrared data continue to be a challenge. Possibilities include the following: a hydrocarbon ocean, which is frothy or contains a significant amount of aerosol contaminants, maintained near the surface by stirring; an ocean stored in pore spaces of the ice regolith or in an aquiferlike system; an ocean that is somewhat smaller than estimated from the methane photolysis rate because of impact‐driven chemistry. Models involving significant amounts of dry land exposed above a shallow ocean potentially run afoul of tidal constraints imposed by Titan's nonzero free orbital eccentricity. The Cassini/Huygens mission will investigate the nature of the surface through a variety of techniques and hopefully solve this long‐standing planetary puzzle.
Macrodispersion of sorbing solutes in heterogeneous porous formations with spatially periodic retardation factor and velocity field
Authors: Chrysikopoulos Constantinos V. , Kitanidis Peter K. , Roberts Paul V.
Published: February 2004

Expressions for the macroscopic velocity vector and dispersion tensor for sorbing solute transport in heterogeneous porous formations whose hydrogeologic properties are repeated at intervals were derived via Taylor‐Aris‐Brenner moment analysis. An idealized three‐dimensional porous formation of infinite domain with spatially periodic retardation factor, velocity field, and microdispersion coefficients in all three directions was considered. Sorption was assumed to be governed by a linear equilibrium isotherm under local chemical equilibrium conditions. The analytical expressions presented are based on a perturbation method where all of the spatially periodic parameters employed were assumed to have “small” fluctuations. It was shown that the effective velocity vector is given by the volume‐averaged interstitial velocity vector divided by the volume‐averaged retardation factor, and the effective dispersion dyadic (second‐order tensor) is given by the volume‐averaged microdispersion dyadic divided by the volume‐averaged dimensionless retardation factor plus a dyadic expressing the increase in solute spreading caused by the spatial variability of the parameters.
Solute transport with multiple equilibrium‐controlled or kinetically controlled chemical reactions
Authors: Friedly John C. , Rubin Jacob
Published: February 2004

A new approach is applied to the problem of modeling solute transport accompanied by many chemical reactions. The approach, based on concepts of the concentration space and its stoichiometric subspaces, uses elements of the subspaces as primary dependent variables. It is shown that the resulting model equations are compact in form, isolate the chemical reaction expressions from flow expressions, and can be used for either equilibrium or kinetically controlled reactions. The implications of the results on numerical algorithms for solving the equations are discussed. The application of the theory is illustrated throughout with examples involving a simple but broadly representative set of reactions previously considered in the literature. Numerical results are presented for four interconnected reactions: a homogeneous complexation reaction, two sorption reactions, and a dissolution/precipitation reaction. Three cases are considered: (1) four kinetically controlled reactions, (2) four equilibrium‐controlled reactions, and (3) a system with two kinetically controlled reactions and two equilibrium‐controlled reactions.