bluuebunny/crossref_metadata_2025 huggingface dataset:
Measuring and modeling the dissolution of nonideally shaped dense nonaqueous phase liquid pools in saturated porous media
Authors: Dela Barre Brian K. , Harmon Thomas C. , Chrysikopoulos Constantinos V.
Published: October 2002

A three‐dimensional physical aquifer model was used to study the dissolution of a dense nonaqueous phase liquid (DNAPL) pool. The model aquifer comprised a packing of homogeneous, medium‐sized sand and conveyed steady, unidirectional flow. Tetrachloroethene (PCE) pools were introduced within model aquifers atop glass‐ and clay‐lined aquifer bottoms. Transient breakthrough at an interstitial velocity of 7.2 cm/h, and three‐dimensional steady state concentration distributions at velocities ranging from 0.4 to 7.2 cm/h were monitored over periods of 59 and 71 days for the glass‐ and clay‐bottom experiments, respectively. Pool‐averaged mass transfer coefficients were obtained from the observations via a single‐parameter fit using an analytical model formulated with a second type boundary condition to describe pool dissolution [Chrysikopoulos, 1995]. Other model parameters (interstitial velocity, longitudinal and transverse dispersion coefficients, and pool geometry) were estimated independently. Simulated and observed dissolution behavior agreed well, except for locations relatively close to the pool or the glass‐bottom plate. Estimated mass transfer coefficients ranged from 0.15 to 0.22 cm/h, increasing weakly with velocity toward a limiting value. Pool mass depletions of 31 and 43% for the glass‐ and clay‐bottom experiments failed to produce observable changes in the plumes and suggested that changes in pool interfacial area over the period of the experiment were negligible. Dimensionless mass transfer behavior was quantified using a modified Sherwood number (Sh*). Observed Sh* values were found to be about 2–3 times greater than values predicted by an existing theoretical mass transfer correlation, and 3–4 times greater than those estimated previously for an ideally configured trichloroethene (TCE) pool (circular and smooth). It appeared that the analytical model's failure to account for pore‐scale pool‐water interfacial characteristics and larger scale pool shape irregularities biased the Sh* estimates toward greater values.
Groundwater flow in heterogeneous composite aquifers
Authors: Winter C. L. , Tartakovsky Daniel M.
Published: October 2002

We introduce a stochastic model of flow through highly heterogeneous, composite porous media that greatly improves estimates of pressure head statistics. Composite porous media consist of disjoint blocks of permeable materials, each block comprising a single material type. Within a composite medium, hydraulic conductivity can be represented through a pair of random processes: (1) a boundary process that determines block arrangement and extent and (2) a stationary process that defines conductivity within a given block. We obtain second‐order statistics for hydraulic conductivity in the composite model and then contrast them with statistics obtained from a standard univariate model that ignores the boundary process and treats a composite medium as if it were statistically homogeneous. Next, we develop perturbation expansions for the first two moments of head and contrast them with expansions based on the homogeneous approximation. In most cases the bivariate model leads to much sharper perturbation approximations than does the usual model of flow through an undifferentiated material when both are applied to highly heterogeneous media. We make this statement precise. We illustrate the composite model with examples of one‐dimensional flows which are interesting in their own right and which allow us to compare the accuracy of perturbation approximations of head statistics to exact analytical solutions. We also show the boundary process of our bivariate model is equivalent to the indicator functions often used to represent composite media in Monte Carlo simulations.
Measuring direct and indirect costs of land retirement in an irrigated river basin: A budgeting regional multiplier approach
Authors: Hamilton Joel , Whittlesey Norman K. , Robison M. Henry , Willis David
Published: October 2002

This analysis addresses three important conceptual problems in the measurement of direct and indirect costs and benefits: (1) the distribution of impacts between a regional economy and the encompassing state economy; (2) the distinction between indirect impacts and indirect costs (IC), focusing on the dynamic time path unemployed resources follow to find alternative employment; and (3) the distinction among the affected firms' microeconomic categories of fixed and variable costs as they are used to compute regional direct and indirect costs. It uses empirical procedures that reconcile the usual measures of economic impact provided by input/output models with the estimates of economic costs and benefits required for analysis of welfare changes. The paper illustrates the relationships and magnitudes involved in the context of water policy issues facing the Pecos River Basin of New Mexico.
Coupled rainfall model and discharge model for flood frequency estimation
Authors: Arnaud Patrick , Lavabre Jacques
Published: October 2002

A method was designed to study frequency distributions of hydrologic variables (rainfall and discharge) which combined a stochastic model for hourly rainfall with a general conceptual model for transforming rainfall into discharge. The model generates many different flood events over a given simulation period to evaluate hydrologic risks. The Simulated Hydrographs for flood Probability Estimation (SHYPRE) method was based on the use of observations to describe phenomena and statistically reproduce them. Frequency distributions of hydrological variables are built empirically from simulated rainfall and flood events. Extrapolation of these frequency distributions toward rare frequencies is performed by generating very different events over a long simulation period rather than by directly fitting a theoretical probability distribution on observed values. This method yields an original estimation of flood quantiles from common to rare frequencies and provides complete temporal data about these floods. In addition, the approach supplies more stable estimates of flood quantiles than statistical distributions fitted on observed values, even for frequent events. This is due to a better integration of rainfall data and the parametric design stability of the two models (rainfall model and rainfall‐discharge model).
Estimating trends in data from the Weibull and a generalized extreme value distribution
Authors: Clarke Robin T.
Published: October 2002

Where changes in hydrologic regime occur, whether as a result of change in land use or climate, statistical procedures are needed to test for the existence of trend in hydrological data, particularly those expected to follow extreme value distributions such as annual peak discharges, annual minimum flows, and annual maximum rainfall intensities. Furthermore, where trend is detected, its magnitude must also be estimated. A later paper [Clarke, 2002] will consider the estimation of trends in Gumbel data; the present paper gives results on tests for the significance of trends in annual and minimum discharges, where these can be assumed to follow a Weibull distribution. The statistical procedures, already fully established in the statistical analysis of survival data, convert the problem into one in which a generalized linear model is fitted to a power‐transformed variable having Poisson distribution and calculates the trend coefficients (as well as the parameter in the power transform) by maximum likelihood. The methods are used to test for trend in annual minimum flows over a 19‐year period in the River Paraguay at Cáceres, Brazil, and in monthly flows at the same site. Extension of the procedure to testing for trend in data following a generalized extreme value distribution is also discussed. Although a test for time trend in Weibull‐distributed hydrologic data is the motivation for this paper, the same approach can be applied in the analysis of data sequences that can be regarded as stationary in time, for which the objective is to explore relationships between a Weibull variate and other variables (covariates) that explain its behavior.
Modeling the impact of ethanol on the persistence of benzene in gasoline‐contaminated groundwater
Authors: Molson J. W. , Barker J. F. , Frind E. O. , Schirmer M.
Published: October 2002

The effect of ethanol on the persistence of benzene in gasoline‐contaminated aquifers is simulated using a multicomponent reactive transport model. The conceptual model includes a residual gasoline source which is dissolving at the water table into an aquifer containing a limited amount of dissolved oxygen. The coupled processes include nonaqueous phase liquid (NAPL) source dissolution, transport of the dissolved components, and competitive aerobic biodegradation. Comparisons are made between dissolved benzene plumes from a gasoline spill and those from an otherwise equivalent spill containing 10% ethanol (gasohol). Simulations have shown that under some conditions a 10% ethanol component in gasoline can extend the travel distance of a benzene plume by up to 150% relative to that from an equivalent ethanol‐free gasoline spill. The increase occurs because ethanol preferentially consumes oxygen, which reduces the biodegradation rate of benzene. The impact is limited, however, because sufficient oxygen disperses behind the ethanol plume into the slightly retarded benzene plume. A sensitivity analysis for two common spill scenarios showed that background oxygen concentrations and benzene retardation have the most significant influence on ethanol‐induced benzene persistence. The results are highly relevant in light of the increasing use of ethanol‐enhanced fuels throughout the world and the forthcoming ban of methyl tertiary‐butyl‐ether (MTBE) in California and its probable replacement by ethanol by the end of 2002.
Sources of iodine and iodine 129 in rivers
Authors: Moran Jean E. , Oktay Sarah D. , Santschi Peter H.
Published: October 2002

A spatial survey of iodine and its long‐lived isotope, 129I, in 40 rivers of the USA, Canada, and western Europe, reveals that the ratio of 129I/I is a sensitive indicator for the influence of nuclear fuel reprocessing facilities. Rivers with point sources for 129I in their watersheds are drastically affected, while all rivers sampled show evidence for atmospherically transported 129I from the world's major nuclear fuel reprocessing facilities. Varying mixtures of oceanic cyclic salt and soil‐derived iodine account for the observed spatial variation in iodine concentrations. A comparison of 129I concentrations in river and rainwater reveals concentration effects due to evapotranspiration.
Possible problems of scale dependency in applications of the three‐dimensional fractional advection‐dispersion equation to natural porous media
Authors: Lu Silong , Molz Fred J. , Fix George J.
Published: October 2002

A three‐dimensional (3‐D) analysis of transport and macrodispersion at the Macrodispersion Experiment (MADE) site [Boggs et al., 1993] using the Fractional Advection‐Dispersion Equation (FADE) developed by Meerschaert et al. [1999, 2001] shows that the Levy dispersion process is scale dependent. Levy dispersion may be superior to Gaussian dispersion on a sufficiently small scale; on larger scales, both theories are likely to suffer from the fact that because of depositional structures most flow fields display an evolving, nonstationary structure. Motion in such fields is advection‐dominated, displays a lot of memory and therefore is not modeled well by Markov random processes which underlie the derivation of both the Gaussian and Levy advection‐dispersion equations [Berkowitz et al., 2002]. To improve plume simulation of an advection‐dominated transport process, one would have to bring in more advective irregularity while simultaneously decreasing the Levy dispersion coefficient. Therefore, on a 3‐D basis, first‐order Levy dispersion has limitations similar to Gaussian dispersion. However, this and related theories, such as the continuous time random walk (CTRW) formalism, are in the early stages of development and thus may be fruitful areas for further research.
Representation of subsurface storm flow and a more responsive water table in a TOPMODEL‐based hydrology model
Authors: Shaman Jeffrey , Stieglitz Marc , Engel Victor , Koster Randal , Stark Colin
Published: October 2002

This study presents two new modeling strategies. First, a methodology for representing the physical process of subsurface storm flow within a TOPMODEL framework is developed. In using this approach, discharge at quick flow timescales is simulated, and a fuller depiction of hydrologic activity is brought about. Discharge of water from the vadose zone is permitted in a physically realistic manner without a priori assumption of the level within the soil column at which subsurface storm flow saturation can take place. Determination of the subsurface storm flow contribution to discharge is made using the equation for groundwater flow. No new parameters are needed. Instead, regions in excess of field capacity that develop during storm events, producing vertical recharge, are also allowed to contribute to soil zone discharge. These subsurface storm flow contributions to river runoff, as for groundwater flow contributions, are a function of catchment topography and hydraulic conductivity at the depth at which such regions in excess of field capacity occur. The second approach improves groundwater flow response through a reduction of porosity and field capacity with depth in the soil column. Large storm events are better captured and a more dynamic water table develops with application of this modified soil column profile (MSCP). The MSCP predominantly reflects soil depth differences in upland and lowland regions of a watershed. Combined, these two approaches, subsurface storm flow and the MSCP, provide a more accurate representation of the timescales at which discharge responds and a more complete depiction of hydrologic activity. Storm events large and small are better simulated, and some of the biases previously evident in TOPMODEL simulations are reduced.
Theoretical interpretation of a pronounced permeability scale effect in unsaturated fractured tuff
Authors: Hyun Yunjung , Neuman Shlomo P. , Vesselinov Velimir V. , Illman Walter A. , Tartakovsky Daniel M. , Di Federico Vittorio
Published: October 2002

Numerous single‐hole and cross‐hole pneumatic injection tests have been conducted in unsaturated fractured tuff at the Apache Leap Research Site (ALRS) near Superior, Arizona. Single‐hole tests have yielded values of air permeability at various locations throughout the tested rock volume on a nominal scale of ∼1 m. Cross‐hole tests have yielded equivalent air permeabilities (and air‐filled porosities) for a rock volume characterized by a length scale of several tens of meters. Cross‐hole tests have also provided high‐resolution tomographic estimates of how air permeability (and air‐filled porosity), defined over grid blocks having a length scale of 1 m, vary throughout a similar rock volume. The results have revealed a highly pronounced scale effect in permeability (and porosity) at the ALRS. We examine the extent to which the permeability scale effect is amenable to interpretation by a recent stochastic scaling theory, which treats the rock as a truncated random fractal.
Regional estimation of flood quantiles: Parametric versus nonparametric regression models
Authors: Latraverse Marco , Rasmussen Peter F. , Bobée Bernard
Published: October 2002

A recent trend in regional frequency analysis is to consider floating regions where only basins that are sufficiently similar to the design site are considered for information transfer. Similarity is measured in some suitable metric of catchment characteristics. This paper discusses the analogy between this idea and nonparametric regression. Some of the techniques developed recently in the area of nonparametric regression are employed to develop improved regional flood estimators. The additive model used here to a large extent overcomes the curse of dimensionality often associated with nonparametric regression on multivariate predictor space. The application of the proposed methodology to selected areas of the United States suggests that there can be substantial gains over the traditional log linear models currently employed.
Braided channels: Self‐similar or self‐affine?
Authors: Walsh Jeremy , Hicks D. Murray
Published: October 2002

The dimensions of islands in seven braided river reaches are analyzed as an indicator of the scaling behaviour of braided channels at small scales. We examined the relationship, W∼LH, where L and W are island length and width which are measured along the major and minor axes respectively and H is Hurst's exponent, which is a measure of scaling anisotropy. Islands are shown to exhibit near isotropic scaling (H = 0.88–1.02), or self‐similarity, at length scales up to 1.5 times the braid plain width, with reasonable consistency occurring between all reaches studied despite large differences in scaling and hydrologic/geomorphologic characteristics. We suggest that previous conclusions that braided channels show more strongly self‐affine scaling behavior have been influenced by the inappropriate application of simple linear regression and by basing morphological feature dimensions on their projection parallel and orthogonal to the braid plain axis rather than in terms of their absolute (major axis) dimensions.
Estimating the economic value of water quality protection in the Catawba River basin
Authors: Kramer Randall A. , Eisen‐Hecht Jonathan I.
Published: October 2002

This study used stated preference methods to estimate the economic value of protecting water quality in the Catawba River basin of North and South Carolina at its current level. Telephone interviews were completed with 1085 randomly selected households, who were also mailed a short information booklet about these issues. Respondents expressed a mean willingness to pay $139 for a management plan designed to protect water quality at its current level over time. Aggregation of this mean willingness to pay value amounted to an annual economic benefit of over $75 million for all taxpayers in Catawba basin counties. By using a split‐sample survey design, this study also compared the effectiveness of different combined mail and telephone survey formats. Results indicated that while a phone‐mail‐phone approach is preferred for some reasons over a mail‐phone approach, the survey format did not significantly affect the economic valuation results.
Critical factors for achieving multiple goals with water tariff systems: Combining limited data sources and expert testimony
Authors: Dalhuisen Jasper , Nijkamp Peter
Published: October 2002

Price instruments are well‐known policy handles to influence effectively residential water demand. Prices used to be set by water authorities in such a way that the principle of cost coverage was respected; they acted as prominent instruments in residential water policies in the past decades. More recently, however, price instruments are increasingly used to meet simultaneously financial, environmental, and social goals. This paper addresses four conditions for an appropriate tariff system for residential water use which are often found in the recent literature on the economics of water use. The paper analyzes the importance of background factors (e.g., low water availability) of these four principles as well as the extent to which actual tariff systems are employed in five mutually contrasting cities (Amsterdam, Athens, London, Seville, and Tel Aviv). Meta‐analytic techniques, in particular, rough set analysis stemming from artificial intelligence, are applied to identify the common underlying relations between background factors and success of achieving multiple goals in these five urban case studies. The paper concludes with policy recommendations.
Applicability of prewhitening to eliminate the influence of serial correlation on the Mann‐Kendall test
Authors: Yue Sheng , Wang Chun Yuan
Published: October 2002

Prewhitening has been used to eliminate the influence of serial correlation on the Mann‐Kendall (MK) test in trend‐detection studies of hydrological time series. However, its ability to accomplish such a task has not been well documented. This study investigates this issue by Monte Carlo simulation. Simulated time series consist of a linear trend and a lag 1 autoregressive (AR(1)) process with a noise. Simulation results demonstrate that when trend exists in a time series, the effect of positive/negative serial correlation on the MK test is dependent upon sample size, magnitude of serial correlation, and magnitude of trend. When sample size and magnitude of trend are large enough, serial correlation no longer significantly affects the MK test statistics. Removal of positive AR(1) from time series by prewhitening will remove a portion of trend and hence reduces the possibility of rejecting the null hypothesis while it might be false. Contrarily, removal of negative AR(1) by prewhitening will inflate trend and leads to an increase in the possibility of rejecting the null hypothesis while it might be true. Therefore, prewhitening is not suitable for eliminating the effect of serial correlation on the MK test when trend exists within a time series.
A travel time approach to exclusion on transport in porous media
Authors: Ginn Timothy R.
Published: October 2002

Transport of suspended colloidal particles in porous media often appears faster relative to molecular‐scale tracers under the same flow field in both laboratory and field studies. A similar phenomenon appears in the transport of ionic molecules in like‐charged fine‐grained porous media and is known as anion exclusion. Here a simplified empirical model of the travel time density function for excluded tracer arrivals is developed in terms of the travel time density of an unexcluded tracer in the same flow field by equating the excluded particle trajectories with unexcluded particle trajectories modified with “detours.” The detours are defined generally without invoking assumptions about the detoured trajectory velocities or proximity to pore walls, although such may be introduced. This conceptual basis provides a representation of the effects of exclusion on the transport alone, exclusive of reactions terms, that is consistent with respect to mass balance as well as particle kinematics. The analysis involves a constitutive “speedup” function that tells how travel times of unexcluded particles map to those of excluded particles. A simple inverse operator that identifies the speedup function given experimentally observed cumulative arrival distributions (e.g., breakthrough curves) of excluded and unexcluded particles in the same flow field is derived. The inverse analysis requires breakthrough curve data for which the reactions and transport are separable, as in the case of irreversible kinetic reactions. The relations developed are exercised in calculating the breakthrough curve of excluded particles given the breakthrough curve of a neutral and inert molecular tracer. The method is then applied to recently published data to determine an empirical speedup function, which is found to be in this case invariant of solute flow rate.
Gas phase advection and dispersion in unsaturated porous media
Authors: Costanza‐Robinson Molly S. , Brusseau Mark L.
Published: October 2002

Gas phase miscible displacement experiments were conducted to quantitatively investigate the advective and dispersive contributions to gas phase transport in unsaturated porous media over a range of soil water contents. Furthermore, the independence of measured dispersivity values was evaluated through comparison of nonreactive and reactive tracer transport. Methane was used as a nonreactive tracer, while difluoromethane (DFM) and trichloroethene (TCE) were used as reactive tracers. At soil water contents below 17%, measured dispersivity values are observed to be independent of the tracer compound and of the soil water content. Conversely, the dispersivities are tracer dependent at the highest soil water contents, wherein the values for DFM and TCE are 3 and 6 times larger than that of methane's, respectively. The significantly larger dispersivity values obtained for DFM and TCE are most likely due to rate‐limited mass transfer of these compounds between the gas phase and soil water, which is not observed for methane because of its minimal water partitioning. The dispersivity values obtained here range between 0.3 and 3 cm and are similar to those reported in the literature. The results indicate that within a given “ideal transport” range, dispersivities measured at one soil water content with a given tracer may be representative of the same porous media system at other soil water contents and for other compounds.
Estimating time trends in Gumbel‐distributed data by means of generalized linear models
Authors: Clarke Robin T.
Published: October 2002

This paper shows how Gumbel‐distributed data can be related to explanatory variables by using generalized linear models (GLMs) fitted by using a modified form of the iteratively weighted least squares algorithm (IWLS). Typical applications include (1) testing for trend in annual flood data, as a possible consequence of changing land cover or other factors; (2) testing for trend in annual maximum rainfall intensities of different durations, as a possible consequence of climate change; and (3) testing how annual maximum rainfall intensity is related to weather conditions at the times that annual maximum intensities were recorded. Given a first estimate of the Gumbel scale parameter α, the coefficients β of explanatory variablesxare estimated by casting the model in GLM form, and the scale parameter α is updated by solution of relevant maximum likelihood equation for this parameter. The parameters α, β can be readily estimated using currently available statistical software for fitting GLMs, which can also be used to test the significance of trends in annual flood data for which the Gumbel distribution is a plausible hypothesis. A plotting procedure to indicate departures from the Gumbel hypothesis is also given. The proposed procedure avoids the illogicality in which, when a trend in flood data is suspected, it is tested either by linear regression methods that assume Normally distributed residuals, or by nonparametric methods, both of which discard the Gumbel hypothesis. Simulated samples from Gumbel distributions were used to compare estimates of linear trend obtained by (1) the GLM procedure and (2) straightforward use of a Newton‐Raphson procedure to locate the maximum of the likelihood surface; the GLM procedure converged more rapidly and was far less subject to numerical instabilities. Simulated samples from Gumbel distributions were also used to compare estimates of a linear trend coefficient β given by the GLM procedure, with estimates of β obtained by simple linear regression (LR). The variance of the distribution of GLM estimates of β was less than the variance of the distribution of LR estimates, while comparison of the powers of the two tests showed that GLM was more powerful than LR at detecting the existence of small trends, although for large linear trends there was little to choose between the two methods.
Hydrograph separations in a mesoscale mountainous basin at event and seasonal timescales
Authors: Uhlenbrook Stefan , Frey Markus , Leibundgut Christian , Maloszewski Piotr
Published: October 2002

The spatial and temporal (event and seasonal timescale) variability of major runoff components in the mountainous Brugga basin (Black Forest, Germany) were examined. The mesoscale (40 km2) study basin represented an extraordinary challenge as comparable studies have been undertaken mainly in smaller headwater basins. Discharge data, tracer concentrations of 18O, 3H, CFCs, and dissolved silica, and major anions and cations were analyzed during single events and over a period of 3 years. Three main runoff components were defined: event water with a residence time of several hours to a few days contributed up to 50% during flood peaks, quantified by a classical hydrograph separation technique using 18O. However, this component is of minor importance for longer periods, comprising ∼11.1% of total runoff as estimated for the period August 1995 to April 1998. The other two flow components originated from shallow and deep groundwater. Source areas for these are the upper drift and debris cover for the shallow groundwater and the deeper drift, weathering zone and hard rock aquifer for the deep groundwater. Mean residence times ranged from 28 to 36 months on the basis of 18O data for the shallow groundwater and from 6 to 9 years on the basis of 3H and CFC data for the deep groundwater. The importance of the upper drift and debris cover of the slopes for runoff generation at the test site was clearly demonstrated at the seasonal timescale, showing a contribution of 69.4% based on a mixing model with a monthly time step. The deep groundwater contribution was 19.5%. With this information a conceptual model of runoff generation for the study site was constructed.
Multispecies reactive tracer test in an aquifer with spatially variable chemical conditions, Cape Cod, Massachusetts: Dispersive transport of bromide and nickel
Authors: Hess Kathryn M. , Davis James A. , Kent Douglas B. , Coston Jennifer A.
Published: October 2002

Dispersive transport of groundwater solutes was investigated as part of a multispecies reactive tracer test conducted under spatially variable chemical conditions in an unconfined, sewage‐contaminated sand and gravel aquifer on Cape Cod, Massachusetts. Transport of the nonreactive tracer bromide (Br) reflected physical and hydrologic processes. Transport of the reactive tracer nickel (Ni) complexed with an organic ligand (NiEDTA) varied in response to pH and other chemical conditions within the aquifer. A loss of about 14% of the Ni mass was calculated from the distribution of tracers through time. This loss is consistent with reversible adsorption of NiEDTA onto the iron and aluminum oxyhydroxide coatings on the aquifer sediments. The Ni consistently lagged behind Br with a calculated retardation coefficient of 1.2. Longitudinal dispersivities reached constant values of 2.2 and 1.1 m for Br and Ni, respectively, by at least 69 m of travel. The smaller dispersivity for Ni possibly was due to nonlinear or spatially variant adsorption of NiEDTA. In the upper, uncontaminated zone of the aquifer, longitudinal dispersion of Ni was greater than that of Br early in the test as a result of reversible adsorption of NiEDTA. In general, transverse dispersivities were much smaller (horizontal: 1.4–1.5 × 10−2 m; vertical: 0.5–3.8 × 10−3 m) than the longitudinal dispersivities. The Br results are similar to those from a test conducted eight years earlier, suggesting that transport parameters are spatially stationary within the aquifer at the scale of 300 m covered by the spatially overlapping tests. A significant difference between the two tests was the travel distance (69 and 26 m) needed to reach a constant longitudinal dispersivity.
A note on unsaturated flow in two‐dimensional fracture networks
Authors: Liu H. H. , Bodvarsson G. S. , Finsterle S.
Published: October 2002

Although considerable progress has been made in understanding unsaturated flow processes in a single fracture, our knowledge of unsaturated flow in fracture networks remains incomplete. In this study, we present a numerical investigation of steady flow behavior in two‐dimensional fracture networks containing thousands of fractures within a 10 m × 10 m domain. Simulation results indicate that flow paths are generally vertical (as a result of gravity‐dominated flow behavior), with subhorizontal fractures providing pathways for communications between vertical flow paths, inducing horizontal spreading of these paths. Although many fractures with small trace lengths do not contribute to the global flow through a fracture network, some of them are still connected to the major flow paths and thus contribute to the overall connectivity of the network. They may also considerably affect the interaction between fractures and the matrix. On the basis of our simulation results we hypothesize that average spacing between flow paths in a layered system tends to increase with depth as long as flow is gravity driven. We also discuss the concept of an influence zone of a capillary barrier to describe seepage from fracture networks to underground openings (drifts). Our simulation results imply that three‐dimensional fracture network models are needed for providing a more realistic evaluation of capillary barrier effects.
Two‐dimensional unsaturated flow through a circular inclusion
Authors: Warrick A. W. , Knight J. H.
Published: October 2002

Two‐dimensional unsaturated flow is considered through a circular inclusion. The hydraulic conductivity is of the form Kiexp(αh) where the saturated conductivity Ki is different in the main flow regime and the inclusion, α is a constant in the entire flow domain, and h is the pressure head. The problem reduces to the Helmholtz equation as previously used by J. R. Philip and colleagues for solving problems with impermeable regions or cavities in an unsaturated regime. The pressure heads are continuous on the interface of the inclusion. The normal flow velocities at the interface are matched approximately using the analytical element method recently exploited for saturated domains in several studies. Flow enhancement and exclusion through the circular inclusions are dependent on the value of α and the radius of the cylinder but otherwise are similar to that for the saturated case. For example, for ratios of the inclusion to background saturated conductivity of 0.5 the flow is 0.74 of what it would be without the inclusion compared to 0.67 for the saturated case. This was calculated for a dimensionless radius (0.5 α multiplied by the physical radius) of 1. When the ratio of the inclusion saturated conductivity to the background is 5, the comparable value for the unsaturated case is 1.45 and for the saturated case is 1.67.
Transport and retention of coarse woody debris in mountain streams: An in situ field experiment of log transport and a field survey of coarse woody debris distribution
Authors: Haga Hirokazu , Kumagai Tomo'omi , Otsuki Kyoichi , Ogawa Shigeru
Published: October 2002

Although coarse woody debris (CWD) is an important component of stream ecosystems in forested areas, the processes of CWD distribution, transport, and retention have not been clarified. In this study the distribution process of CWD pieces shorter than the bankfull width (S‐CWD) is discussed using an in situ field experiment of log transport and a field survey of CWD distribution in mountain streams. The transport experiment showed that transport distance has a close relation to flow depth and also implied that the magnitude and sequence of a series of flows were important factors for S‐CWD transport and retention in streams. The survey of CWD distribution indicated that in‐stream obstructions played an important role in the S‐CWD retention in deeper channels where S‐CWD pieces were potentially transported distances more than spacing between trapping sites of CWD. Overall, the in situ field experiment and the segment‐ to reach‐scaled analysis using h* (=depth/diameter) helped us understand the actual movement and distribution of CWD.
Effects of heterogeneity on influent and effluent concentrations from horizontal permeable reactive barriers
Authors: Elder Carl R. , Benson Craig H. , Eykholt Gerald R.
Published: October 2002

Numerical models were used to evaluate how aquifer and barrier heterogeneity affect influent and effluent concentrations for permeable reactive barriers (PRBs). Spatial variability in the reaction rate constant kr and hydraulic conductivity KP of the PRB and hydraulic conductivity of the aquifer (in terms of variations in the mean μlnK, standard deviation σlnK, and correlation scale λ of the logarithm of hydraulic conductivity (ln K)) were considered. Spatial variability of kr and KP was found to change influent and effluent concentrations by less than an order of magnitude. Spatial continuity in hydraulic conductivity parallel to flow, described by the correlation length λx, has a modest effect, with greater continuity yielding higher effluent concentrations. Decreasing the hydraulic conductivity of the aquifer (i.e., decreasing μlnK) does not affect influent concentrations but decreases the median effluent concentrations and broadens the distribution of effluent concentration due to increased residence time in the PRB. Increasing the variability of the aquifer hydraulic conductivity (i.e., increasing σlnK) decreases the median influent concentration and broadens the distribution of influent concentrations due to additional dispersion caused by greater heterogeneity. Greater variability in aquifer hydraulic conductivity also results in a higher median effluent concentrations and a broader distribution of effluent concentration. Comparison of effluent concentrations predicted using a one‐dimensional deterministic model and the three‐dimensional numerical model shows that longer residence times and lower effluent concentrations are usually predicted by the one‐dimensional model. The results indicate that designers should carefully consider factors of safety used for design, perhaps opting for a more conservative approach until more guidance on designing amidst aquifer heterogeneity is available.
Correlated geophysical, geochemical, and volcanological manifestations of plume‐ridge interaction along the Galápagos Spreading Center
Authors: Detrick R. S. , Sinton J. M. , Ito G. , Canales J. P. , Behn M. , Blacic T. , Cushman B. , Dixon J. E. , Graham D. W. , Mahoney J. J.
Published: October 2002

As the Galápagos hot spot is approached from the west along the Galápagos Spreading Center there are systematic increases in crustal thickness and in the K/Ti, Nb/Zr, 3He/4He, H2O, and Na2O content of lavas recovered from the spreading axis. These increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth, residual mantle Bouguer gravity anomaly, magma chamber depth, average lava Mg #, Ca/Al ratio, and the frequency of point‐fed versus fissure‐fed volcanism. Magma chamber depth and axial morphology display a “threshold” effect in which small changes in magma supply result in large changes in these variables. These correlated variations in geophysical, geochemical, and volcanological manifestations of plume‐ridge interaction along the western Galápagos Spreading Center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes, and the consequences of these variations on magma supply, axial thermal structure, basalt chemistry, and styles of volcanism.
A direction‐dependent instability of sporadic‐E layers in the nighttime midlatitude ionosphere
Authors: Cosgrove Russell B. , Tsunoda Roland T.
Published: October 2002

The configuration of a midlatitude sporadic‐E (Es) layer at a zonal wind‐shear node has been assumed to be a stable equilibrium, as long as the wind shear is hydrodynamically stable. In this paper, we show that this equilibrium is in fact unstable, at night, to electrodynamical forces that arise when plane wave perturbations in altitude or density are imposed on the Es layer. We show that the growth rate of the instability depends on the azimuthal alignment of the phase fronts of the plane‐wave distortion, a feature that is reminiscient of the Perkins instability [Perkins, 1973]. The dependence on azimuth is consistent with observations of structure in Es layers that show the same preferred alignment.
Regional and local ionospheric models based on Millstone Hill incoherent scatter radar data
Authors: Holt John M. , Zhang Shun‐Rong , Buonsanto Michael J.
Published: October 2002

Local and regional statistical models to describe Millstone Hill incoherent scatter radar observations of electron density, electron temperature and ion temperature since 1976 are developed using a bin‐fit technique. The local models generate ionospheric variations with local time, day number, and altitude from 150–1000 km. The prior day's F107 and the Ap index from the previous 3 hour period are keyed inputs to specify solar and geomagnetic activity. The regional models have a latitude coverage of 32–55° geodetic and an altitude coverage of 200–600 km. These climatology models are capable of reproducing primary ionospheric variation features seen in previous studies as well as several newly revealed features, such as the semiannual variation of electron density. They are accessible through the World Wide Web at the URL http://www.openmadrigal.org.
Evidence of surface cooling from absorbing aerosols
Authors: Krishnan R. , Ramanathan V.
Published: October 2002

Anthropogenic emissions over the Asian region have grown rapidly with increase in population and industrialization. Air‐pollutants from this region lead to a brownish haze over most of the North Indian Ocean and South Asia during winter and spring. The haze, with as much as 10–15% of black carbon (by mass), is known to reduce the surface solar insolation by about 10% (−15 Wm−2) and nearly double the lower atmospheric solar heating. Here we present an analysis of observed surface‐temperature variations over the Indian subcontinent, which filters out effects of greenhouse gases and natural variability. The analysis reveals that the absorbing aerosols have led to a statistically significant cooling of about 0.3°C since the 1970s. The seasonally asymmetric cooling, which is consistent with the seasonality of the South Asian aerosol forcing, raises the new possibility that the surface cooling underneath the polluted regions, is balanced by warming elsewhere.
The influence of water on the development of lattice preferred orientation in olivine aggregates
Authors: Kaminski É.
Published: October 2002

The calculation of the lattice preferred orientation of crystals in mantle flow has become a major step in the interpretation of the upper mantle seismic anisotropy. Available models for such calculations are based on experimental data obtained on dry crystal aggregates. Recent simple shear experiments showed that the presence of water may radically change the fabrics produced, which casts doubt on the validity of the models in water rich environments like subduction zones. Water may change both the mechanisms of dynamic recrystallization and the mechanical properties of the crystals. This paper illustrates how a change of the critical resolved shear stress of the slip systems is sufficient to reproduce the new experimental results. In contrast, no changes are necessary in the theory of dynamic recrystallization. For subduction zones, first order calculations predict that trench parallel flow rather than water is more likely to be responsible for the anisotropic signature.
Arctic Oscillation variability generated through inter‐ocean interactions
Authors: Dima Mihai , Rimbu Norel , Dima Ioana
Published: October 2002

We investigate the structure of the Northern Hemisphere atmospheric circulation anomaly patterns associated with the dominant mode of interannual variability in the Tropical Pacific and the dominant mode of quasi‐decadal variability in the Atlantic basin. There is evidence that the superposition of the atmospheric patterns associated with these modes possesses an Arctic Oscillation‐like structure. We suggest that part of the Arctic Oscillation variability is due to interactions between coupled ocean‐atmosphere modes originating in the Tropical Pacific and Atlantic basins.
Tearing‐mode instability in anisotropic plasmas: Effects of energy closure
Authors: Chiou S.‐W. , Hau L.‐N.
Published: October 2002

The resistive tearing‐mode instability of tangential‐discontinuity current sheets is studied within the framework of linear double‐polytropic MHD theory. Four sets of polytropic exponents, γ∥ and γ⟂, are chosen to describe various thermodynamic states. It is shown that the dependence of tearing growth rate on the energy equations, plasma beta as well as the magnetic By component is much more pronounced than that in isotropic plasmas; in particular, the growth rate is larger for smaller γ∥ and γ⟂ as well as for larger p⟂/p∥ and β⟂. For certain parameter regime, the growth rate may even increase with increasing magnetic Reynolds number, a result in contrast to the pure tearing instability. Cases of large growth rate on the order of Alfvén time scale are associated with oscillatory slow‐mode structures that may sometimes exhibit positive density‐magnetic field correlation.
Dual‐wavelength Raman lidar observations of the extinction‐to‐backscatter ratio of Saharan dust
Authors: Mattis Ina , Ansmann Albert , Müller Detlef , Wandinger Ulla , Althausen Dietrich
Published: October 2002

For the first time, height profiles of the extinction‐to‐backscatter ratio (lidar ratio) of desert dust particles were simultaneously measured at 355 and 532 nm. The observations were performed with an advanced Raman lidar during two long‐range Saharan dust outbreaks at Leipzig, Germany (51.3°N, 12.4°E), in August and October 2001. Measured desert‐dust lidar ratios are needed for a proper profiling of the climate‐relevant volume extinction coefficient of the dust particles with widely used standard backscatter lidars. Unexpectedly large lidar ratios, mainly between 50 and 80 sr, were found in the Saharan dust plumes. The lidar ratios at 355 nm were, on average, higher by 10%–30% than the ones at 532 nm, probably due to enhanced light absorption in the UV. The large lidar ratios can be explained by model calculations, available in the literature for 532 nm, which focus on the deviations between the scattering characteristics of spheres and spheroids.
Reply to the ‘Comment on “Return stroke transmission line model for stroke speed near and equal that of light” by R. Thottappillil, J. Schoene, and M.A. Uman’ by B. Kordi, R. Moini, and V.A. Rakov
Authors: Thottappillil R. , Uman M. A.
Published: October 2002

The ideal and exact solution for a vertical wire antenna of infinite length above a ground plane excited at the bottom, all conductors being perfect, is spherical TEM. Currents along such an antenna do not suffer from attenuation and dispersion. The energy lost due to radiation from such an antenna appears to originate from the source at the bottom. Practical systems are different from this ideal case because there are no ideal sources and no ideal conductors. However, ideal analytical solutions should be used, when possible, as a validation of numerical solutions in the appropriate limit.
5‐cm‐Precision aircraft ocean altimetry using GPS reflections
Authors: Lowe Stephen T. , Zuffada Cinzia , Chao Yi , Kroger Peter , Young Larry E. , LaBrecque John L.
Published: October 2002

We present the first two aircraft Global Positioning System (GPS)‐reflection altimetry measurements, the most precise GPS ocean‐altimetry measurement, and demonstrate the altimetric precision and spatial resolution necessary to map mesoscale eddies. Our first experiment demonstrated a 14‐cm precision single‐satellite ocean altimetry measurement while our more recent experiment demonstrates 5 cm altimetric precision with 5‐km spatial resolution. The new results show significant improvement over our previous effort, due to improved modeling, greater aircraft altitudes and velocities, improved receiver positioning, and better experimental control. Plans to further reduce speckle and refine models to obtain 5‐cm altimetric accuracy are presented.