bluuebunny/crossref_metadata_2025 huggingface dataset:
Field study of hydraulic conductivity in a heterogeneous aquifer: Comparison of single‐borehole measurements using different instruments
Authors: Zlotnik Vitaly A. , Zurbuchen Brian R.
Published: May 2003

This field study compares three techniques for estimating the vertical distribution of horizontal hydraulic conductivity Kr in a heterogeneous aquifer and evaluates possible support volume effects. The dipole flow test (DFT), multilevel slug test (MLST), and borehole flowmeter test (BFT) are based on different kinematic flow structures and the shape and the size of the support volumes. The experiment design employed an identical characteristic linear scale for all tests. Vertical profiles of Kr ranging up to 260 m/day from tested wells in an alluvial aquifer exhibit a strong correlation in spite of the differences between test hydraulics. Results suggest that tested screen length is an important indicator of the averaging mechanism for hydraulic tests. Correlation between the DFT and MLST is especially strong. Correlation between data from the BFT and other tests is not as strong due to the absence of a distinct physical vertical scale, among other factors. The differences between the tests are discussed using the concept of a weighting function associated with the magnitude of instantaneous local velocity.
Analysis of nonuniform and dispersive time domain reflectometry measurement systems with application to the dielectric spectroscopy of soils
Authors: Lin C.‐P.
Published: May 2003

A practical time domain reflectometry (TDR) measurement system consists of a cable tester and a nonuniform transmission line. The transmission line includes a coaxial cable, a transitional device, and a measurement probe. Dielectric spectroscopy in a nonuniform transmission line requires a wave propagation model that accounts for multiple reflections in a nonuniform and dispersive TDR measurement system. Spectral analysis and the concept of input impedance were utilized to develop such a model. The system parameters were calibrated using the measured TDR waveform in an aqueous material with known dielectric permittivity. The excellent match between the simulated and measured waveforms validated the wave propagation model. A layer‐peeling algorithm was developed, based on the new formulation of the wave propagation model to remove the effect of the cable and transitional device on the measurement by TDR. The dielectric spectrum obtained by solving the scatter function was compared with that obtained by matching waveforms based on a dielectric relaxation model. The latter method was confirmed by the experimental results as being superior to the former at high frequencies if an appropriate dielectric relaxation model was used. Both the Debye relaxation model and the volumetric mixing model could model the dielectric dispersion of soils in the TDR frequency range. The latter method outperforms the former in modeling the dielectric dispersion of soils over the full range of frequencies.
Characterizing and quantifying the impact of immiscible‐liquid dissolution and nonlinear, rate‐limited sorption/desorption on low‐concentration elution tailing
Authors: Johnson G. R. , Zhang Z. , Brusseau M. L.
Published: May 2003

Long‐term elution tailing of organic contaminants, often observed when water or air is used to flush contaminated porous media, has been attributed to several factors. Characterization of this tailing behavior through the quantitative analysis of multiple coupled factors is necessary to enhance our understanding of contaminant transport. The objective of this study was to investigate the transport and elution behavior of trichloroethene in a naturally heterogeneous (poorly sorted) aquifer material, with a specific focus on characterizing and quantifying the relative contributions of rate‐limited immiscible‐liquid dissolution and nonlinear, rate‐limited sorption/desorption to low‐concentration elution tailing. A comparison of trichloroethene elution behavior for systems with and without immiscible‐liquid phase present suggests that the low‐concentration elution tailing observed in the former experiments is associated primarily with nonlinear, rate‐limited sorption/desorption. The transport and elution of trichloroethene was successfully simulated using a mathematical model that combines independent, coupled descriptions of rate‐limited dissolution and nonlinear, rate‐limited sorption/desorption. Specifically, immiscible‐liquid dissolution was described using a first‐order mass transfer approach with a temporally variable dissolution rate coefficient, and sorption/desorption was described using an approach incorporating a continuous distribution of rate‐limited domains. The results of this study indicate that multiple processes contributed to trichloroethene elution behavior when immiscible‐liquid phase was present and that a multiprocess model was required to accurately simulate the measured data.
Three‐dimensional steady state flow to a well in a randomly heterogeneous bounded aquifer
Authors: Guadagnini Alberto , Riva Monica , Neuman Shlomo P.
Published: May 2003

We consider flow in a confined aquifer of uniform thickness due to a well of zero radius that fully penetrates the aquifer and discharges at a constant rate. If the lateral extent of the aquifer is infinite, a steady state flow regime never develops. It is, however, well known that if the aquifer is additionally uniform, a quasi‐steady state region extends from the well out to a cylindrical surface whose radius expands as the square root of time. On the expanding surface, head is uniform and time invariant. Inside this surface, head at any time is described by a steady state solution. A rigorous analysis of the analogous situation in a randomly heterogeneous aquifer would require the solution of a three‐dimensional transient stochastic flow problem in an aquifer of infinite lateral extent. Here we take a different approach by developing a three‐dimensional steady solution for mean flow to a well in a randomly heterogeneous aquifer with a cylindrical prescribed head boundary. In analogy to the uniform case we expect our solution to approximate a quasi‐steady state region whose radius is initially small in comparison to the horizontal correlation scale of log conductivity but grows with time to become eventually much larger. We treat log conductivity as a statistically homogeneous random field characterized by a Gaussian spatial covariance function that may have different horizontal and vertical correlation scales. Our solution consists of analytical expressions for the ensemble mean and variance of head in the aquifer to second order in the standard deviation of log conductivity. It is based on recursive approximations of exact nonlocal moment equations that are free of distributional assumptions and so apply to both Gaussian and non‐Gaussian log conductivity fields. The analytical solution is supported by numerical Monte Carlo simulations. It clarifies the manner in which relationships between the horizontal and vertical scales of the quasi‐steady state region and those of statistical anisotropy impact the statistical moments of drawdown and the equivalent and apparent hydraulic conductivities of the aquifer. Both conductivities are shown to exhibit a scale effect by growing with distance from the well within a radius of one to two horizontal integral scales from it.
Numerical experiments on bar growth
Authors: Defina Andrea
Published: May 2003

A two‐dimensional finite element model for the morphodynamic evolution of a cohesionless bed is presented and tested against experimental observations of the formation and evolution of free bars. The model is then employed to study the strongly nonlinear interactions governing bar development. The attention is focused on the influence of the initial perturbation introduced in the flow field for bar inception. It is found that the type of the disturbance plays a role of utmost importance as it can strongly affect both the developing process and the equilibrium conditions.
A sediment transport model for incision of gullies on steep topography
Authors: Istanbulluoglu Erkan , Tarboton David G. , Pack Robert T. , Luce Charles
Published: May 2003

We have conducted surveys of gullies that developed in a small, steep watershed in the Idaho Batholith after a severe wildfire followed by intense precipitation. We measured gully length and cross sections to estimate the volumes of sediment loss due to gully formation. These volume estimates are assumed to provide an estimate of sediment transport capacity at each survey cross section from the single gully‐forming thunderstorm. Sediment transport models commonly relate transport capacity to overland flow shear stress, which is related to runoff rate, slope, and drainage area. We have estimated the runoff rate and duration associated with the gully‐forming event and used the sediment volume measurements to calibrate a general physically based sediment transport equation in this steep, high shear stress environment. We find that a shear stress exponent of 3, corresponding to drainage area and slope exponents of M = 2.1 and N = 2.25, match our data. This shear stress exponent of 3 is approximately 2 times higher than those for bed load transport in alluvial rivers but is in the range of shear stress exponents derived from flume experiments on steep slopes and with total load equations. The concavity index of the gully profiles obtained theoretically from the area and slope exponents of the sediment transport equation, θc = (M − 1)/N, agrees well with the observed profile concavity of the gullies. Our results, although preliminary because of the uncertainty associated with the sediment volume estimates, suggest that for steep hillslopes such as those in our study area, a greater nonlinearity in the sediment transport function exists than that assumed in some existing hillslope erosion models which calculate sediment transport capacity using the bed load equations developed for rivers.
A method for enforcing parameter nonnegativity in Bayesian inverse problems with an application to contaminant source identification
Authors: Michalak Anna M. , Kitanidis Peter K.
Published: May 2003

When an inverse problem is solved to estimate an unknown function such as the hydraulic conductivity in an aquifer or the contamination history at a site, one constraint is that the unknown function is known to be everywhere nonnegative. In this work, we develop a statistically rigorous method for enforcing function nonnegativity in Bayesian inverse problems. The proposed method behaves similarly to a Gaussian process with a linear variogram (i.e., unrestricted Brownian motion) for parameter values significantly greater than zero. The method uses the method of images to define a prior probability density function based on reflected Brownian motion that implicitly enforces nonnegativity. This work focuses on problems where the unknown is a function of a single variable (e.g., time). A Markov chain Monte Carlo (MCMC) method, specifically, a highly efficient Gibbs sampler, is implemented to generate conditional realizations of the unknown function. The new method is applied to the estimation of the trichloroethylene (TCE) and perchloroethylene (PCE) contamination history in an aquifer at Dover Air Force Base, Delaware, based on concentration profiles obtained from an underlying aquitard.
Downstream hydraulic geometry and hydraulic similitude
Authors: Griffiths George A.
Published: May 2003

A theoretical basis is provided for the empirical equations of downstream hydraulic geometry, written as scaling relations, using the equations for momentum, flow resistance, and continuity of gradually varied open channel flow. Width, depth, velocity, and bed slope are expressed as power functions of water discharge and bed sediment size. These theoretical relations apply in alluvial, ice, and bedrock channels and hold for any value of water discharge as opposed to just mean or bank‐full values used in empirical equations. A scaling relation for bed load transport rate in alluvial channels is also derived using dimensional analysis. Threshold and regime theories of stable channel design are shown to be consistent with the requirements of hydraulic similitude as defined by the scaling relations. If field measurements confirm that the scaling relations apply in braided alluvial channels, then predictions of a threshold bed slope between states of meandering and braiding will not be valid.
Influence of transitional volcanic strata on lateral diversion at Yucca Mountain, Nevada
Authors: Flint Lorraine E. , Flint Alan L. , Selker John S.
Published: May 2003

Natural hydraulic barriers exist at Yucca Mountain, Nevada, a potential high‐level nuclear waste repository, that have been identified as possible lateral diversions for reducing deep percolation through the waste storage area. Historical development of the conceptual model of lateral diversion has been limited by available field data, but numerical investigations presented the possibility of significant lateral diversion due to the presence of a thin, porous rock layer, the Paintbrush nonwelded tuffs. Analytical analyses of the influence of transitional changes in properties suggest that minimal lateral diversion is likely at Yucca Mountain. Numerical models, to this point, have not accounted for the gradual transition of properties or the existence of multiple layers that could inadvertently influence the simulation of lateral diversion as an artifact of numerical model discretization. Analyses were made of subsurface matric potential measurements, and comparisons were made of surface infiltration estimates with deeper percolation flux calculations using chloride‐mass‐balance calculations and simulations of measured temperature profiles. These analyses suggest that insignificant lateral diversion has occurred above the repository horizon and that water generally moves vertically through the Paintbrush nonwelded tuffs.
Modeling of contaminants mobility in underground domains with multiple free/porous interfaces
Authors: Das D. B. , Nassehi V.
Published: May 2003

Existing simulation techniques for subsurface water quality management remain inadequate despite the phenomenal progress made in the computing capabilities in the past 2 decades. One major lacunas found in mathematical models for groundwater hydrodynamics in combined free flow zones (zones that are devoid of porous materials) and porous domains (e.g., soil water regions) in the subsurface is that these models impose continuity of all flow/transport parameters to describe the flow behavior at the intermediate surfaces between the two zones. Prescription of such interfacial conditions cannot always be physically justified and, in general, result in ill‐posed mathematical formulations. The present paper aims to remove the above limitations of the mathematical models by imposing well‐posed mathematical formulations of mass and momentum transfer across the boundaries between free and porous zones underground for simulating subsurface flow. The possibility of the existence of a multiple number of pervious boundaries in the domain across which water may flow is considered in this work, as it represents the most realistic scenario for the subsurface transport processes. The hydrodynamics of coupled fluid flow and behavior of contaminants transport are described. Combined effects of Darcy and Reynolds numbers on the subsurface transport of contaminants are discussed. Sensitivity of the flow variables to a coefficient of the permeability and porosity of interfacial boundary, which is determined by its structural properties, is also considered.
Well hydraulics theory and data analysis of the constant head test in an unconfined aquifer with the skin effect
Authors: Chen Chia‐Shyun , Chang Chien‐Chieh
Published: May 2003

In a constant‐head test (CHT), groundwater is withdrawn from a well that has a fixed drawdown. Accordingly, the CHT is particularly useful in aquifers of low transmissivity because the constant‐head well would not be dewatered owing to overpumping. The problem of interest pertains to a CHT in a homogeneous and anisotropic unconfined aquifer, where water released by the water table decline is assumed to be instantaneous and the skin effect surrounding the constant‐head well is represented by a skin factor. A two‐dimensional transient well hydraulics model is developed, and its Laplace domain solutions of aquifer drawdown and the constant‐head well discharge are determined. It is found that the three distinct segments typically noted in the time drawdown history of a constant‐rate test (CRT) are also characteristic of the temporal drawdown variation of the CHT. However, the early time and late time asymptotes conform to the radial flow solution of a CHT in confined aquifers as given by Hantush [1964]. Under the late time radial flow conditions the asymptotic solution is in terms of an apparent storage coefficient equal to the sum of the storage coefficient and the specific yield, and a specific mass balance relation exists among the constant‐head well discharge, the discharge due to aquifer storage, and the discharge resulted from the water table decline. While aquifer drawdown and the constant‐head well discharge are under the influence of well skin, their ratios are free from it. A curve matching method using this ratio is developed to determine the horizontal and the vertical hydraulic conductivities, the storage coefficient, and the specific yield. When the prescribed constant drawdown is not negligibly small in comparison to the initial saturated thickness, aquifer drawdown in late times may be large, and thus a drawdown correction is needed for data analysis. It is found that the Jacob correction for large drawdown of a CRT can also be used for the late drawdown correction in a CHT.
Distributed assessment of contributing area and riparian buffering along stream networks
Authors: McGlynn Brian L. , Seibert Jan
Published: May 2003

We present a simple approach for quantifying the local contributions of hillslope area and riparian area along a stream network based on gridded digital elevation data. The method enables one to compute catchment characteristics such as the distribution of riparian and hillslope inputs to the stream network, the variation of riparian‐area percentage along the stream network, and subcatchment area distributions. We applied the technique to the 280‐ha Maimai research area in New Zealand. We found that 85% of the catchment area contributed to streams with a local catchment area of <20 ha, whereas only 28% of the riparian area was found along these small streams. The potential of riparian zones to buffer hillslope runoff depends partially on the size of the riparian zone relative to the adjacent hillslope or upland area. Our approach enables calculation of a spatially distributed measure of riparian to hillslope area ratios. At the 280 ha Maimai research area we found that the ratio between riparian and hillslope area was 0.14. When we calculated this “buffer capacity” for each 20 m stream reach along the stream network, the values were below 0.14 for 75% of the stream length and the median was 0.06. Using the catchment‐wide ratio would thus significantly overestimate the “effective” riparian‐to‐hillslope‐area ratio.
Role of discrete landscape units in controlling catchment dissolved organic carbon dynamics
Authors: McGlynn Brian L. , McDonnell Jeffrey J.
Published: May 2003

The spatial sources and delivery mechanisms of dissolved organic carbon (DOC) to streams are poorly understood. We examined the relationship between storm DOC dynamics, catchment landscape units, and catchment scale to elucidate controls on DOC export dynamics at the Maimai watersheds, a group of highly responsive, steep, wet catchments located on the west coast of the South Island of New Zealand. Specifically, we address the controls on the characteristic hysteresis in DOC export dynamics (i.e., DOC concentrations higher on the rising than falling limb of the discharge hydrograph) previously ascribed to a flushing mechanism. We found that during the storm event, the proportion of riparian runoff was larger on the rising than falling limb of the hydrograph, while the proportion of hillslope runoff was smaller on the rising than falling limb of the hydrograph. The delayed response of hillslope runoff resulted in a disconnection between hillslope and riparian areas early in the event and higher DOC concentrations on the rising limb than the falling limb of the event hydrograph. Later in the event, hillslope and riparian areas became connected once the hillslope soil moisture deficits were satisfied. We suggest that the relative timing of riparian and hillslope source contributions and the connections and disconnections of dominant runoff contributing areas are the first‐order catchment controls on stream DOC concentrations and mass export.
Mechanistic modeling of fingering, nonmonotonicity, fragmentation, and pulsation within gravity/buoyant destabilized two‐phase/unsaturated flow
Authors: Glass Robert J. , Yarrington Lane
Published: May 2003

Fingering, nonmonotonicity, fragmentation, and pulsation within gravity/buoyant destabilized two‐phase/unsaturated flow systems has been widely observed with examples in homogeneous to heterogeneous porous media, in single fractures to fracture networks, and for both wetting and nonwetting invasion. To model this phenomena, we consider a mechanistic approach based on forms of modified invasion percolation (MIP) that include gravity, the influence of the local interfacial curvature along the phase‐phase interface, and the simultaneous invasion and reinvasion of both wetting and nonwetting fluids. We present example simulations and compare them to experimental data for three very different situations: (1) downward gravity‐driven fingering of water into a dry, homogeneous, water‐wettable, porous medium; (2) upward buoyancy‐driven migration of gas within a water saturated, heterogeneous, water‐wettable, porous medium; and (3) downward gravity‐driven fingering of water into a dry, water‐wettable, rough‐walled fracture.
Microbiotic crust control of runoff generation on sand dunes in the Negev Desert
Authors: Kidron Giora J. , Yair Aaron , Vonshak Ahuva , Abeliovich Aharon
Published: May 2003

The general belief that dune sands in arid regions are not likely to generate runoff and that runoff necessitates wet conditions is examined. Runoff generation was measured within an arid dune field in the western Negev Desert, Israel, during 1990–1994 on 16 plots (1.2–6.6 m2), constructed along a continuum of crust‐covered crust. The relationships between chlorophyll a and carbohydrates of the microbiotic crust and runoff yield were studied. Whereas chlorophyll a (15–45 mg m−2) and carbohydrates (4.9–17.6 g m−2) of the cyanobacterial crusts showed positive linear relationships with runoff coefficients, a significant reduction in runoff yield was obtained once the moss‐dominated crust (with 53–54.5 mg m−2 chlorophyll a and 28.5–28.9 g m−2 of carbohydrates), which inhabited the wettest habitat within the dune field, was also included in the analysis. Contrary to previous assumptions that an increase in runoff generation occurs on arid and semiarid sand dunes in wet climates, wet climates may lead to the establishment of moss‐dominated crusts that in turn decrease runoff yield.
Transport of colloids in saturated porous media: A pore‐scale observation of the size exclusion effect and colloid acceleration
Authors: Sirivithayapakorn Sanya , Keller Arturo
Published: May 2003

We present experimental evidence of the effect of colloid exclusion from areas of small aperture sizes, using direct observations at the pore‐scale using a realistic micromodel of porous media. Four sizes of hydrophobic latex spheres in aqueous suspension, from 0.05 to 3 μm, were introduced into the micromodel at three different pressure gradients. We observed the frequency of occurrence of the size exclusion effect and the influence of relative size of pore throats and colloids (T/C ratio) and flow velocity. From our observations the smallest T/C ratio entered by these different colloids was 1.5. We also observed certain preferential pathways through the pore space for different colloid sizes, such that size exclusion eventually results in distinct pathways. These preferential paths become more important for larger colloids and for greater pressure gradients. Measured colloid velocities were 4–5.5 times greater than estimated pore water velocities. Acceleration factors (ratio of colloid to water velocity) increased for all colloid sizes with increasing pore‐scale Pe. Smaller particles appeared to travel along faster streamlines in pore throats, while larger particles travel along with a number of streamlines, thus at a slightly lower velocity than the small colloids. At larger scales the acceleration factor is decreased owing to Brownian motion, adsorption, colloid and straining filtration, and other factors, but these pore‐scale results shed light on the size exclusion effect and its role in determining early colloid breakthrough.
Impact of water quality on municipal water price and residential water demand and implications for water supply benefits
Authors: Piper Steven
Published: May 2003

This analysis evaluates the extent to which water quality influences the cost of supplying domestic water and the extent to which finished water quality affects household water demand from price impacts. The water demand and cost models are based on nationwide data for individual systems collected by the American Water Works Association [1996]. The water cost model estimated in this analysis indicates that factors such as the size of the water supply system, the population density of the service area, the source of water supplies, and water quality all significantly affect the cost per unit of water delivered. The water use model indicates water demand is influenced significantly by household income, household size, temperature, precipitation, the price of water, and finished water quality. Combining the results of the models allows an analyst to evaluate the full impacts of water quality benefits on municipal water supplies.
Countervailing effects of atrazine on water recreation: How do recreators evaluate them?
Authors: Earnhart Dietrich , Smith Val
Published: May 2003

This paper examines the countervailing effects of atrazine on water recreational choices. The presence of atrazine in waterbodies potentially reduces the symptoms of eutrophication, which is a condition of low water quality due to nutrient enrichment. Eutrophication frequently undermines recreational enjoyment and diminishes recreational use of affected waterbodies. Thus increased atrazine concentrations could induce greater recreation. However, atrazine also potentially decreases the mass and size of fish populations; this loss potentially reduces recreational use. To examine empirically these countervailing effects on recreational use, this study gathers and generates data on actual recreation under initial water quality conditions and stated recreation under hypothetical quality conditions, which vary eutrophication‐related and fish‐related quality independently and jointly. This economic study uses a travel cost framework and the associated contingent behavior framework to analyze these data.
On stochastic study of well capture zones in bounded, randomly heterogeneous media
Authors: Lu Zhiming , Zhang Dongxiao
Published: May 2003

Accurately determining well capture zones is of great importance in aquifer cleanup and for the protection of drinking water. Earlier models in delineating capture zones assumed the medium to be homogeneous. Recently, stochastic approaches have been applied to studying capture zones in heterogeneous media. The Monte Carlo method is commonly used to infer the probability distribution of the resulting capture zones from multiple realizations of the aquifer of interest. In this study, we present a moment‐equation‐based approach to derive the time‐dependent mean capture zones and their associated uncertainties. The flow statistics are obtained by solving the first two moments of flow, and the mean capture zones are determined by reversely tracking the nonreactive particles released at a small circle around each pumping well. The uncertainty associated with the mean capture zones is calculated based on the particle displacement covariance Xij for nonstationary flow fields. For comparison purposes, we also conducted Monte Carlo simulations. It has been found that our model results are in excellent agreement with Monte Carlo results.
Influence of a solubility‐enhancing agent (cyclodextrin) on NAPL‐water partition coefficients, with implications for partitioning tracer tests
Authors: Dugan Pamela J. , McCray John E. , Thyne Geoffrey D.
Published: May 2003

The partitioning tracer test (PTT) is used to quantify subsurface nonaqueous phase liquid (NAPL) saturations (SN). The laboratory‐measured NAPL‐water partitioning coefficient (KNW) is required to calculate the SN. The complexing agent hydroxypropyl−β−cyclodextrin (HPCD) has been used to successfully remediate NAPL‐contaminated sites. Residual concentrations of HPCD may remain in the groundwater during a postremediation PTT. This investigation examines the influence of HPCD on the trichlorethene NAPL‐water partitioning coefficients of several alcohol tracers. Laboratory results indicate that aqueous HPCD concentrations of 2000 mg/L result in the reduction of KNW values by up to 33%. This effect can result in up to 33% error in the PTT‐estimated SN. The effect decreases as the original KNW value of a tracer decreases. The relationship between KNW values and HPCD concentrations is nonlinear and yields curves characteristic of each tracer. Empirical relationships are offered to reproduce the characteristic curves based on tracer KNW values to allow estimates of corrected KNW values for an arbitrary tracer.
A semianalytical solution of the Saint‐Venant equations for channel flood routing
Authors: Wang G.‐T. , Chen S. , Boll J.
Published: May 2003

A semianalytical solution for channel hydraulic routing has been derived from the Saint‐Venant equations. The Saint‐Venant equations were converted into a nonlinear diffusion equation by introducing the Froude number. Subsequently, the mixing cell method is used to discretize a nonlinear diffusion equation in space, transforming it into a first‐order nonlinear ordinary differential equation where the optimal space interval is obtained to be the same as the characteristic reach length. The nonlinear ordinary differential equation was solved by integration with respect to time to achieve a nonlinear implicit algebraic equation. The resulting implicit equation is practical and easy to program on a computer. This method is based on the results of a numerical simulation using the Lambda scheme which indicates that the Froude number is only dependent on the channel roughness and bottom slope. The method was tested with numerical examples and compared with the Lambda scheme and observed data. The hydrographs produced by this method were of comparable accuracy.
Hillslope and channel contributions to the hydrologic response
Authors: D'Odorico Paolo , Rigon Riccardo
Published: May 2003

This paper develops a new framework to analyze the relevance of hillslope with respect to channel processes in the generation of the hydrologic response at different scales. The dependence between basin response and geomorphic features of channels and hillslopes is analyzed through the mean and variance of travel times under simple but realistic dynamical hypotheses and within the framework of the rescaled width function. The distribution of travel times is also studied in association with variations of the fraction of saturated areas: with small values of the saturated fraction, the channel network controls most of the mean and variance of travel time distribution. Vice versa, when the basin is saturated, the properties of this distribution may be significantly affected by hillslope processes. These properties depend also on the basin size, with the role of hillslope processes becoming more important in the smaller basins. Furthermore, the average residence time is related to contributing area and drainage density through a simple expression which is shown to be consistent with previous empirical laws.
A Dupuit formulation for modeling seawater intrusion in regional aquifer systems
Authors: Bakker Mark
Published: May 2003

A new formulation is presented for the modeling of seawater intrusion in coastal multiaquifer systems. The formulation is intended for the simulation of three‐dimensional regional seawater intrusion starting from a specified salinity distribution. The Dupuit approximation is adopted, and diffusion and dispersion are not taken into account. The formulation is based on a vertical discretization of the groundwater into zones of either constant density (stratified flow) or continuously varying density (piecewise linear in the vertical direction). The zones are separated from each other by curved surfaces, representing interfaces for stratified flow and isosurfaces of the density for variable density flow. During a simulation both the change of the elevations of the surfaces and the change in head are computed through consistent application of continuity of flow; a simple tip and toe tracking algorithm is applied to simulate the horizontal movement of the surfaces. The resulting differential equations are equivalent in form to the differential equation for single‐density flow. The density effects are incorporated through the addition of pseudosource terms. This makes it easy to implement the formulation in existing numerical groundwater codes. The main advantage of the formulation is the tremendous reduction of the number of cells needed for a simulation because every aquifer may be represented by a single layer of cells. The accuracy of the formulation is demonstrated through comparison with existing exact solutions and a numerical solution.
Stochastic groundwater quality management: Role of spatial variability and conditioning
Authors: Bakr Mahmoud I. , te Stroet Chris B. M. , Meijerink Allard
Published: May 2003

In this paper a methodology for the stochastic management of groundwater quality problems is presented. A stochastic algorithm to solve the coupled flow and mass transport inverse problem is combined with a stochastic management approach to design a reliable pump‐and‐treat scheme. Three main differences exist between this research and previously published research in the field of groundwater management. The first is the method of parameterization, where we have explicitly considered spatial variability of flow and mass transport parameters in our method. Second, we have used adjoint and/or sensitivity equations to calculate first and second derivatives of state variables with respect to hydraulic parameters. Finally, the third point is the nonlinear multiobjective formulation we adopt which enables us to explore the whole trade‐off curve between reliability and cost of remediation. A number of key results are listed here. The stochastic inverse algorithm used here, the representers method, makes it possible to estimate heterogeneity as reflected by the measurements of parameters and state variables. Meanwhile, using the adjoint operator to accurately estimate sensitivity of state variables with respect to the parameters has facilitated the use of the head and concentration measurements in the inverse algorithm. Moreover, stochastic management, which is conditioned on head and concentration measurements simultaneously are shown to provide more realistic and reliable remediation schemes. Finally, with the aid of the multiobjective formulation of the management problem and analysis of its results, it is possible to identify a significant remediation scheme. Such a scheme has the advantage of producing maximum possible reliability with the least possible increase in the total cost of remediation.
Inverse problem in avalanche dynamics models
Authors: Ancey C. , Meunier M. , Richard D.
Published: May 2003

Avalanche dynamics models are increasingly used to estimate the features of extremely rare events for avalanche zoning. They employ a frictional coefficient, which reflects something close to snow viscosity. As this coefficient is more conceptual than physical, it cannot be measured and must be fitted by matching avalanche dynamics model results and field data. However, most of the time, the historical record is not long enough to fit this coefficient for extremely rare events. Here we propose a deterministic inversion method to obtain the probability density function of this coefficient. The method has been applied to two avalanche paths in the French Alps, each with a sustained avalanche activity over the last century. For applications the Voellmy avalanche dynamics model has been used with no loss of generality. It is shown that the friction coefficient is a random variable whose marginal probability distribution varies rapidly and exhibits two or more peaks.
Travel cost demand model based river recreation benefit estimates with on‐site and household surveys: Comparative results and a correction procedure
Authors: Loomis John
Published: May 2003

Past recreation studies have noted that on‐site or visitor intercept surveys are subject to over‐sampling of avid users (i.e., endogenous stratification) and have offered econometric solutions to correct for this. However, past papers do not estimate the empirical magnitude of the bias in benefit estimates with a real data set, nor do they compare the corrected estimates to benefit estimates derived from a population sample. This paper empirically examines the magnitude of the recreation benefits per trip bias by comparing estimates from an on‐site river visitor intercept survey to a household survey. The difference in average benefits is quite large, with the on‐site visitor survey yielding $24 per day trip, while the household survey yields $9.67 per day trip. A simple econometric correction for endogenous stratification in our count data model lowers the benefit estimate to $9.60 per day trip, a mean value nearly identical and not statistically different from the household survey estimate.
Optimal replacement of water pipes
Authors: Mailhot Alain , Poulin Annie , Villeneuve Jean‐Pierre
Published: May 2003

Pipe breaks are used as indicators of the structural state of pipe network. The approach used considers times to failure between pipe breaks as random variables. Pipe lifespan is divided into two periods, the first one characterized by time‐dependent hazard functions (nonexponential period) and the second one characterized by constant hazard functions (exponential period). Closed‐form expressions have been derived for probability density functions of occurrence of breaks for all break orders as well as expressions for the time evolution of the average number of pipe breaks per unit time. An optimal replacement criterion is defined on a pipe‐to‐pipe basis based on a cost function using conditional probabilities to estimate the expected future costs. Minimization of this cost function leads to a replacement criterion involving hazard functions. When applied to models with constant hazard functions, this criterion identifies a critical pipe break order at which replacement should be made.
Strong carbon cycle feedbacks in a climate model with interactive CO2 and sulphate aerosols
Authors: Jones Chris D. , Cox Peter M. , Essery Richard L. H. , Roberts David L. , Woodage Margaret J.
Published: May 2003

Carbon cycle feedbacks are a significant source of uncertainty in climate change projections, with the potential for strong positive feedbacks to accelerate the rate of anthropogenic global warming during the 21st century. A climate change experiment is presented which uses a General Circulation Model (GCM) in which both interactive carbon and sulphur cycles have been included for the first time, along with the natural climate forcings due to solar changes and volcanic aerosol. These extra climate forcing factors have a significant impact on both 20th century climate change and the contemporary land and ocean carbon sinks. The additional forcings act to delay by more than a decade the conversion of the land carbon sink to a source, but ultimately result in a more abrupt rate of CO2 increase with the land carbon source (which reaches 7 GtC yr−1 by 2100) exceeding the ocean carbon sink (which saturates at 5 GtC yr−1 by 2100) beyond about 2080.
Tropical Pacific Ocean heat content variations and ENSO persistence barriers
Authors: McPhaden Michael J.
Published: May 2003

Data from the tropical Pacific Ocean for the period 1980–2002 are used to examine the persistence of sea surface temperature (SST) and upper ocean heat content variations in relation to El Niño and the Southern Oscillation (ENSO). The present study demonstrates that, unlike for SST, there is no spring persistence barrier when considering upper ocean heat content. Conversely, there is a persistence barrier for heat content in boreal winter related to a seasonal reduction in variance. These results are consistent with ENSO forecast model studies indicating that accurate initialization of upper ocean heat content often reduces the prominence of the spring prediction barrier for SST. They also suggest that initialization of upper ocean heat content variations may lead to seasonally varying enhancements of forecast skill, with the most pronounced enhancements for forecasts starting early and late in the development of ENSO events.
Simultaneous thermospheric observations during the geomagnetic storm of April 2002 from South Pole and Arrival Heights, Antarctica
Authors: Hernandez G. , Roble R. G.
Published: May 2003

Simultaneous ground‐based optical measurements of the response of the thermosphere to the geomagnetic storm of April 2002 were made at South Pole (90°S) and Arrival Heights (78°S). Large meridional and zonal winds were observed, with values reaching 600 m/s. The kinetic temperature during the storm showed an increase nearly 3‐fold from the normal values during the time of peak wind, while the emission of the 630 nm increased over 20‐fold. Simulation of the storm with the TIME‐GCM model, including low energy electron precipitation observed by the DMSP satellites, shows good agreement with the measurements. Without low energy electron precipitation, the model simulation underestimates the kinetic temperature and emission.
The magnetic field in the pile‐up region at Mars, and its variation with the solar wind
Authors: Vennerstrom S. , Olsen N. , Purucker M. , Acuña M. H. , Cain J. C.
Published: May 2003

The magnetic measurements from the Mars Global Surveyor satellite are used to study the magnetic field on the Martian dayside, and its variation with the solar wind. Because of the lack of solar wind measurements near Mars, solar wind measurements near Earth during a period centered on a Mars‐Earth conjunction are used. Concurrent variations at Mars and Earth related to the interplanetary sector‐structure and dynamic pressure variations are demonstrated. The study is confined to the northern hemisphere of Mars in regions where the crustal anomalies are weak. Here we find a close association between the solar wind dynamic pressure and the magnetic pressure in the pile‐up region, and also a strong asymmetry with the Interplanetary magnetic field (IMF) By‐component, probably related to solar wind pick‐up of planetary ions.
Identification of sources of potential fields with the continuous wavelet transform: Application to VLF data
Authors: Boukerbout Hassina , Gibert Dominique , Sailhac Pascal
Published: May 2003

A method to localize and characterize the sources of VLF tilt anomalies is proposed. It relies on the continuous wavelet transform computed with particular analyzing wavelets possessing remarkable properties with respect to potential fields. An example with a synthetic dyke model shows how the method allows to locate the top of conductive structures both horizontally and vertically. An application to real data acquired over a conductive dyke illustrates the robustness of the method with respect to noise.
Radon anomaly in the soil of Taal volcano, the Philippines: A likely precursor of the M 7.1 Mindoro earthquake (1994)
Authors: Richon P. , Sabroux J.‐C. , Halbwachs M. , Vandemeulebrouck J. , Poussielgue N. , Tabbagh J. , Punongbayan R.
Published: May 2003

The soil‐gas 222Rn concentration had been monitored almost continuously from June 1993 till November 1996 on Taal volcano, Luzon Island, the Philippines. During this measurement period, a singular Mb 7.1 earthquake occurred on November 15, 1994, between Luzon and Mindoro, 48 km south of the volcano. Twenty‐two days before the earthquake, an anomalous increase in soil‐gas radon (peak to background ratio = 6) was recorded, unique in the whole time series. The possible generation of this anomaly by typhoon Teresa, which struck Luzon Island a few days before, was ruled out one year later when super typhoon Angela, the most powerful storm to hit the Philippines in ten years, crossed Luzon Island along almost the same track without triggering a similar disturbance in the radon signal. Consequently, there is strong evidence that the Taal radon anomaly originated in stress accumulation preceding the Mindoro earthquake.
Near micron‐sized cirrus cloud particles in high‐resolution infrared spectra: An orographic case study
Authors: Kahn Brian H. , Eldering Annmarie , Clough Shepard A. , Fetzer Eric J. , Fishbein Evan , Gunson Michael R. , Lee Sung‐Yung , Lester Peter F. , Realmuto Vincent J.
Published: May 2003

The high‐resolution spectra of the Atmospheric Infrared Sounder (AIRS) provide a global view of small‐ particle‐dominated cirrus clouds, and they exist over much larger spatial extents than seen in previous aircraft campaigns. As shown by simulations using a plane‐parallel scattering radiative transfer (RT) model and realistic ice particle shapes, the shape of the radiance spectra in the atmospheric windows is uniquely influenced by small ice crystals. Minima in the brightness temperature (BT) spectra between 800 to 850 cm−1 are seen for ice particles smaller than 3 μm in the RT simulations and AIRS spectra. A case study of an orographic cirrus cloud observed on October 2, 2002, over the central Andes of South America is presented with spectral BT differences up to 63K between 998 and 811 cm−1.
Minimum variance free wave identification: Application to Cluster electric field data in the magnetosheath
Authors: Balikhin M. A. , Pokhotelov O. A. , Walker S. N. , Amata E. , Andre M. , Dunlop M. , Alleyne H. S. C. K.
Published: May 2003

The electric field measurements provided by the EFW instrument on board Cluster spacecraft are used to identify the mode of plasma waves observed in the magnetosheath. Previous identifications of magnetosheath wave modes have involved either the use of dimensionless transport ratios (DR) calculated using magnetic field and plasma parameters or the use of magnetic field measurements together with a Minimum Variance (MV) analysis. However, by utilising simultaneous four point measurements it is possible to identify the observed wave dispersion without having to resort to the use of either the calculations of DR or MV. It is, therefore, possible to identify the mode of the observed waves using measurements of any parameter (electric field, density etc.). In the present paper low frequency waves observed in the electric field by Cluster spacecraft travelling through the magnetosheath are studied. The wave dispersion relation obtained from the data indicates that observed waves are mirror waves.