bluuebunny/crossref_metadata_2025 huggingface dataset:
Upscaling experiments conducted on a block of volcanic tuff: Results for a bimodal permeability distribution
Authors: Tidwell Vincent C. , Wilson John L.
Published: September 2002

Permeability upscaling is physically investigated by making over 31,000 permeability measurements on a meter‐scale block of volcanic tuff. The experiments are made possible by a specially adapted minipermeameter test system. Here we present and analyze 5185 permeability values, corresponding to five different sample supports (i.e., sample volumes) collected from one of the six block faces. The results show that the measured spatial permeability patterns, bimodal permeability distribution, and semivariogram structure/length scales are closely related to the strong textural contrast characterizing the tuff sample (i.e., highly porous pumice fragments embedded in a tight rock matrix). Each of the summary statistics shows distinct and consistent trends with increasing sample support (i.e., upscaling). As the sample support increases, the mean and variance decrease according to a power law relation, and the semivariogram range increases linearly, while the general structure of the semivariogram (isotropic, spherical model) remains unchanged. Interpretation of these results is pursued from two very different points of view; one addresses upscaling of the ensemble statistics, while the second examines upscaling from a local or pointwise perspective. We find the general upscaling trends exhibited by the ensemble statistics (given above) to be consistent with the basic concepts of volume averaging, albeit nonlinear volume averaging. The bimodal characteristics of the tuff sample and the nonuniform flow conditions imparted by the minipermeameter contribute to the nonlinearity. The local analysis reveals strong variability in permeability upscaling from point to point throughout the sampling domain. Specifically, the permeability upscaling exhibited by zones rich in pumice is very different from zones dominated by matrix, unless the averaging volume is significantly larger than the spatial correlation scale.
A Stochastic Tokunaga Model for stream networks
Authors: Cui Gurong , Williams Brian , Kuczera George
Published: September 2002

The Tokunaga cyclic model describes average network topology. A stochastic generalization is proposed. The stochastic model assumes that actual tributary numbers are random realizations from a negative binomial distribution whose mean is defined by the Tokunaga parameters ϵ1 and K. These parameters can be interpreted as representing the effects of regional controls. Upon these regional controls is superimposed an inherent spatial variability in network topology. A third parameter α characterizes this spatial variability. When α becomes large, the negative binomial model approaches a Poisson model. A goodness‐of‐fit test based on a χ2 test statistic is developed, and an inference framework for estimation of parameters and stream‐related statistics is described. This methodology is illustrated on tributary data from three catchments, one of the order of 5 and two of the order of 8. It is shown that the stochastic Tokunaga model using the negative binomial distribution is not inconsistent with the tributary data, whereas the Poisson model is unambiguously rejected by the data. Monte Carlo Bayesian methods are used to evaluate the uncertainty in the Tokunaga parameters and in stream number related statistics such as the bifurcation ratio. It is shown that tributary data from the order‐5 network provide little power for discriminating between model hypotheses. The tributary data for the two order‐8 basins are significantly different from the asymptotic stream number statistics predicted by Shreve's random network model. Finally, the problem of space filling or preservation of nontopological properties is considered in the context of the stochastic Tokunaga model.
Longitudinal and lateral dispersion in an unsaturated field soil
Authors: Forrer Irène , Kasteel Roy , Flury Markus , Flühler Hannes
Published: September 2002

Lateral and longitudinal dispersion was quantified in a field soil under water‐unsaturated conditions. The relatively mobile dye tracer Brilliant Blue FCF was applied as a line source and leached into the soil at two different rates of infiltration, 4 and 24 mm d−1, respectively. The resulting tracer plume was photographically recorded at vertical soil profiles excavated perpendicularly to the line source after ∼50, 100, and 200 mm of cumulative infiltration. An image analysis technique was used to determine two‐dimensional concentration distributions from the photographs. Average horizontal and vertical concentration distributions were analyzed using the two‐dimensional advection‐dispersipn equation. Model parameters were fitted to optimize the agreement between measured and modeled averaged concentration profiles in both horizontal as well as vertical directions. Dispersivities showed a dependency on flow rates and amount of cumulative infiltration, but this dependency appeared to be related to the degree of irregularities of observed flow patterns. Large dispersivities were associated with higher degree of irregularities in the flow patterns and vice versa. Layer boundaries played a significant role for redirecting flow when flow rates were high and cumulative infiltration was large. This study demonstrates (1) that more than just the vertical concentration profiles are needed to define the transport regime under unsaturated conditions, and (2) that even subtle layer boundaries affect the lateral mixing regime and exert a marked influence on the transport in the main flow direction.
Adjoint method for obtaining backward‐in‐time location and travel time probabilities of a conservative groundwater contaminant
Authors: Neupauer Roseanna M. , Wilson John L.
Published: September 2002

Backward location and travel time probabilities can be used to determine the prior location of contamination in an aquifer. For a contaminant particle that was detected in an aquifer, the backward location probability is the probability of where the particle was located at some prior time. Backward travel time probability is the probability of when the particle was located at some position upgradient of the detection. These probabilities can be used to improve characterization of known sources of groundwater contamination, to identify previously unknown contamination sources, and to delineate capture zones. For simple model domains, backward probabilities can be obtained heuristically from a forward model of contaminant transport. For multidimensional problems and complex domain geometries, the heuristic approach is difficult to implement and verify. The adjoint method provides a formal approach for obtaining backward probabilities for all model domains and geometries. We formally show that the backward model probabilities are adjoint states of resident concentration. We provide a methodology for obtaining the governing equations and boundary and final conditions for these probabilities. The approach is illustrated using a one‐dimensional, semi‐infinite domain that mimics flow to a production well, and these results are compared to equivalent probabilities derived heuristically.
Impact of intermittent rainwater and wastewater flow on coated and uncoated fractures in chalk
Authors: Weisbrod Noam , Nativ Ronit , Adar Eilon M. , Ronen Daniel
Published: September 2002

Two coated and two uncoated slices from the fracture surface of an unsaturated chalk were exposed to short flow events (24, 8, and 9 hours) of industrial wastewater and/or synthetic rainwater, followed by long drying periods (weeks). The topography of the fracture surface was shown to be unstable due to the detachment of colloidal and large‐sized particles during the first 3–7 hours of flow. Following rainwater flow, erosion was more pronounced on the coated than on the uncoated surface (mean erosion of 0.313 and 0.134 mm, respectively). Interaction with industrial wastewater generated a skin of organic matter and gypsum that collapsed following contact with rainwater, leading to a deeper erosion of the uncoated surface (1.238 mm) than of the coated one (0.549 mm). Erosion was measured using a laser‐scanning system and was calculated from high‐resolution topographical maps (elevationz≤ ± 0.01 mm) generated by Geographic Information System (GIS, ARCInfo) prior to and following the flow experiments. The mean thickness of the erosion was found to be strongly correlated with the thickness of a layer calculated from the total accumulated mass of particles and soluble salts released from the fracture surface. This relationship can be used to evaluate fracture surface erosion in large field and laboratory experiments.
Transport of dissolving colloidal particles in porous media
Authors: Corapcioglu M. Yavuz , Jiang Shiyan , Kim Seung‐hyun
Published: September 2002

A mathematical model was developed to simulate the transport and dissolution of colloidal particles with a water‐soluble component in a porous medium. The model is based on mass balance equations describing the mass transport, dissolution, sorption, capture, and release of colloids in a three‐phase medium consisting of a solid matrix, an aqueous phase, and colloids. Colloid dissolution, contaminant sorption on solid matrix, and colloid capture mechanisms are represented by first‐order kinetics. A numerical comparison of the model of a water‐soluble component associated with fly ash–mobilized transport and transport due only to dissolved component migration indicates that the presence of colloids significantly increases the aqueous phase contaminant concentration.
A moment method for analyzing breakthrough curves of step inputs
Authors: Yu C. , Warrick A. W. , Conklin M. H.
Published: September 2002

This paper presents a new and simple moment method for analyzing breakthrough curves for step inputs. An advantage of moment analyses, in general, is that the underlying physical model is not needed, unlike other techniques such as fitting convective (or advective) dispersion equations. Therefore moment calculations allow multispecies (such as partitioning tracers) to be considered in one, two, or three dimensions. Previous analyses using moments have emphasized the response for a pulse input. Generally, the travel (residence) time is of importance, and previous solution techniques do not perform well for long pulse or step inputs. In this paper the presented method is used to analyze breakthrough curves for step inputs, and the results are compared to fitted analytical solutions for the one‐dimensional convection dispersion equation. Only negligible differences between the new moment method and the fitted analytical solutions are found for both simulated and experimental data. Superior performance over previous methods used for evaluating moments for step inputs is demonstrated.
Entrainment response of bed sediment to time‐varying flows
Authors: Admiraal David M. , García Marcelo H. , Rodríguez José F.
Published: September 2002

Unsteady flows are ubiquitous in nature. In order to understand the behavior of sediment when subjected to unsteady flows, a set of experiments was performed in a rectangular duct with a mobile bed. A computer‐operated valve governed the velocity of the water in the duct, and the flow velocity, wall shear stress, and vertical distribution of suspended sediment were simultaneously measured. Beds composed of 120 μm and 580 μm diameter sand were investigated. Both quasi‐steady flows and pulse flows were simulated in the duct. For the pulse flows the water was accelerated at a constant rate to a peak velocity and then decelerated at a constant rate to zero velocity. Phase lags were observed between the bed shear stress and the upward flux (entrainment) of sand from the bed. The phase lags were larger for tests with fine sand than for tests with coarse sand. Differences were attributed to differences in bed roughness and flow Reynolds numbers. Relations based on flow acceleration and sediment size were developed for predicting the entrainment phase lag. Large phase lags can have a considerable impact on the amount of sediment transported by boat wakes, waves, and other unsteady flows.
Effects of sediment supply on surface textures of gravel‐bed rivers
Authors: Buffington John M. , Montgomery David R.
Published: September 2002

Using previously published data from flume studies, we test a new approach for quantifying the effects of sediment supply (i.e., bed material supply) on surface grain size of equilibrium gravel channels. Textural response to sediment supply is evaluated relative to a theoretical prediction of competent median grain size (D50′). We find that surface median grain size (D50) varies inversely with sediment supply rate and systematically approaches the competent value (D50′) at low equilibrium transport rates. Furthermore, equilibrium transport rate is a power function of the difference between applied and critical shear stresses and is therefore a power function of the difference between competent and observed median grain sizes (D50′ and D50). Consequently, we propose that the difference between predicted and observed median grain sizes can be used to determine sediment supply rate in equilibrium channels. Our analysis framework collapses data from different studies toward a single relationship between sediment supply rate and surface grain size. While the approach appears promising, we caution that it has been tested only on a limited set of laboratory data and a narrow range of channel conditions.
A gas‐phase partitioning tracer method for the in situ measurement of soil‐water content
Authors: Nelson N. T. , Brusseau M. L. , Carlson T. D. , Costanza M. S. , Young M. H. , Johnson G. R. , Wierenga P. J.
Published: September 2002

The purpose of this paper is to describe a gas‐phase partitioning tracer method for the in situ measurement of soil‐water content and to illustrate the application and performance of the tracer method in well‐defined systems. The method is based on the use of a tracer test with nonpartitioning and partitioning tracers introduced into the system in the gas phase. Partitioning tracers dissolve into the water, which retards their gas‐phase transport relative to that of the nonpartitioning tracers. Retardation of the partitioning tracers is a function of the amount of water present. The method provides an integrated field‐scale value that complements smaller‐scale methods, such as neutron moderation, and regional‐scale methods based on remote sensing. Experiments were conducted in the laboratory and in a large weighing lysimeter to test the performance of the gas‐phase partitioning tracer method. Soil‐water contents estimated from the tracer tests were reasonably close to values obtained using gravimetric and time domain reflectometry measurements, indicating the tracer method has the potential to provide accurate measurements of soil‐water content at the field scale.
A Full‐Bayesian Approach to parameter inference from tracer travel time moments and investigation of scale effects at the Cape Cod Experimental Site
Authors: Woodbury Allan D. , Rubin Yoram
Published: September 2002

A method for inverting the travel time moments of solutes in heterogeneous aquifers is presented and is based on peak concentration arrival times as measured at various samplers in an aquifer. The approach combines a Lagrangian [Rubin and Dagan, 1992] solute transport framework with full‐Bayesian hydrogeological parameter inference. In the full‐Bayesian approach the noise values in the observed data are treated as hyperparameters, and their effects are removed by marginalization. The prior probability density functions (pdfs) for the model parameters (horizontal integral scale, velocity, and log K variance) and noise values are represented by prior pdfs developed from minimum relative entropy considerations. Analysis of the Cape Cod (Massachusetts) field experiment is presented. Inverse results for the hydraulic parameters indicate an expected value for the velocity, variance of log hydraulic conductivity, and horizontal integral scale of 0.42 m/d, 0.26, and 3.0 m, respectively. While these results are consistent with various direct‐field determinations, the importance of the findings is in the reduction of confidence range about the various expected values. On selected control planes we compare observed travel time frequency histograms with the theoretical pdf, conditioned on the observed travel time moments. We observe a positive skew in the travel time pdf which tends to decrease as the travel time distance grows. We also test the hypothesis that there is no scale dependence of the integral scale λ with the scale of the experiment at Cape Cod. We adopt two strategies. The first strategy is to use subsets of the full data set and then to see if the resulting parameter fits are different as we use different data from control planes at expanding distances from the source. The second approach is from the viewpoint of entropy concentration. No increase in integral scale with distance is inferred from either approach over the range of the Cape Cod tracer experiment.
Colorado River sediment transport: 2. Systematic Bed‐elevation and grain‐size effects of sand supply limitation
Authors: Topping David J. , Rubin David M. , Nelson Jonathan M. , Kinzel Paul J. , Corson Ingrid C.
Published: September 2002

The Colorado River in Marble and Grand Canyons displays evidence of annual supply limitation with respect to sand both prior to [Topping et al, this issue] and after the closure of Glen Canyon Dam in 1963. Systematic changes in bed elevation and systematic coupled changes in suspended‐sand concentration and grain size result from this supply limitation. During floods, sand supply limitation either causes or modifies a lag between the time of maximum discharge and the time of either maximum or minimum (depending on reach geometry) bed elevation. If, at a cross section where the bed aggrades with increasing flow, the maximum bed elevation is observed to lead the peak or the receding limb of a flood, then this observed response of the bed is due to sand supply limitation. Sand supply limitation also leads to the systematic evolution of sand grain size (both on the bed and in suspension) in the Colorado River. Sand input during a tributary flood travels down the Colorado River as an elongating sediment wave, with the finest sizes (because of their lower settling velocities) traveling the fastest. As the fine front of a sediment wave arrives at a given location, the bed fines and suspended‐sand concentrations increase in response to the enhanced upstream supply of finer sand. Then, as the front of the sediment wave passes that location, the bed is winnowed and suspended‐sand concentrations decrease in response to the depletion of the upstream supply of finer sand. The grain‐size effects of depletion of the upstream sand supply are most obvious during periods of higher dam releases (e.g., the 1996 flood experiment and the 1997 test flow). Because of substantial changes in the grain‐size distribution of the bed, stable relationships between the discharge of water and sand‐transport rates (i.e., stable sand rating curves) are precluded. Sand budgets in a supply‐limited river like the Colorado River can only be constructed through inclusion of the physical processes that couple changes in bed‐sediment grain size to changes in sand‐transport rates.
Determination of hydrologic pathways during snowmelt for alpine/subalpine basins, Rocky Mountain National Park, Colorado
Authors: Suecker Julie K. , Ryan Joseph N. , Kendall Carol , Jarrett Robert D.
Published: September 2002

Alpine/subalpine ecosystems in Rocky Mountain National Park may be sensitive to atmospherically derived acidic deposition. Two‐ and three‐component hydrograph separation analyses and correlation analyses were performed for six basins to provide insight into streamflow generation during snowmelt and to assess basin sensitivity to acidic deposition. Three‐component hydrograph separation results for five basins showed that streamflow contained from 42 to 57% direct snowmelt runoff, 37 to 54% subsurface water, and 4 to 13% direct rain runoff for the May through October 1994 study period. Subsurface contributions were 89% of total flow for the sixth basin. The reliability of hydrograph separation model assumptions was explored. Subsurface flow was positively correlated with the amount of surficial material in a basin and was negatively correlated with basin slope. Basins with extensive surficial material and shallow slopes are less susceptible to ecosystem changes due to acidic deposition than basins with less surficial material and steeper slopes. This study was initiated to expand the intensive hydrologic research that has been conducted in Loch Vale basin to a more regional scale.
On the second‐order correction to velocity covariance for two‐dimensional statistically isotropic porous media
Authors: Hsu Kuo‐Chin , Lamb G. L.
Published: September 2002

Second‐order perturbation theory has recently been applied to flow and transport in statistically isotropic porous media. The second‐order head correction has been found to be important in the velocity covariance and thus also in transport. This result was obtained by using numerical integration on the second‐order correction for velocity covariance with the higher‐order head fluctuation. The present paper is a continuation of previous studies and aims at deriving an analytical, closed‐form expression for the second‐order correction term involving higher‐order head fluctuation in lognormal statistically isotropic media with exponential log conductivity correlation. The higher‐order correction for velocity covariance is found to be most significant in the transverse direction, a result that is consistent with previous analysis.
Flume study of the effect of relative depth on the incipient motion of coarse uniform sediments
Authors: Shvidchenko Andrey B. , Pender Gareth
Published: September 2002

An experimental study of the incipient motion of coarse uniform sediments was undertaken in an 8 m long by 0.3 m wide tilting flume. The concept of critical shear stress for the initial motion of streambeds has been linked to the probability of sediment entrainment through the intensity of sediment transport. The experiments have revealed that critical flow conditions for uniform sediment motion are dependent not only on the grain size but also on the ratio of flow depth to grain diameter. A revised Shields diagram relating critical stress, grain Reynolds number, and depth to grain size ratio has been derived. This has implications for the calculation of sediment transport rates, design of stable alluvial channels, and physical hydraulic modeling.
A model for forecasting drought from teleconnections
Authors: Cordery Ian , McCall Mark
Published: September 2002

Strong relationships have been developed between both global and local phenomena in one season and precipitation in the next four seasons for years when the precipitation is likely to be low (drought years). The relations explain more than 65% of the variance in the precipitation for regions of up to 500,000 km2 and more than 35% of the variance for up to 1.3 × 106 km2 in eastern Australia. The strong relationships occurred when precipitation was regressed against one of the variables in years that were selected on the basis of the magnitude of a third, so‐called partitioning, variable. The relations are shown to be able to provide forecasts of low precipitation in all seasons of the year. Statistical tests show that the strong forecasting relations could not occur by chance.
Upscaling of anisotropy in unsaturated Miller‐similar porous media
Authors: Ursino Nadia , Roth Kurt , Gimmi Thomas , Flühler Hannes
Published: September 2002

Geological and pedological processes rarely form isotropic media as is usually assumed in transport studies. Anisotropy at the Darcy or field scale may be detected directly by measuring flow parameters or may become indirectly evident from movement and shape of solute plumes. Anisotropic behavior of a soil at one scale may, in many cases, be related to the presence of lower‐scale directional structures. Miller similitude with different pore‐scale geometries of the basic element is used to model macroscopic flow and transport behavior. Analytical expressions for the anisotropic conductivity tensor are derived based on the dynamic law that governs the flow problem at the pore scale. The effects of anisotropy on transport parameters are estimated by numerical modeling.
Random self‐similar river networks and derivations of generalized Horton Laws in terms of statistical simple scaling
Authors: Veitzer Seth A. , Gupta Vijay K.
Published: September 2002

Recent data analyses of a dozen or so large networks support the presence of simple scaling in the probability distributions of many topologic, geometric, and hydraulic‐geometric variables. We have labeled them “generalized Horton laws” in contrast to traditional Horton laws which are stated only in terms of means. These data sets also show that Horton's bifurcation ratios are significantly larger than 4, which can not be predicted by the well‐known random model. Motivated by the need to understand these data analyses of large river networks within a common theoretical framework, a new class of models with statistically self‐similar network topology is introduced. The network construction process can be viewed as a statistical generalization of the type of a recursive process which generates many fractal geometrical objects. Unlike the well‐known random model, our random channel networks do not belong to the class of binary Galton‐Watson stochastic branching processes. The generalized Horton laws are predicted in the limit of large Horton‐Strahler order as a scale parameter for a subclass of random self‐similar network models. A derivation of Horton's law of stream numbers as a consequence of the simple scaling of distributions of basin magnitude gives it a new theoretical significance. A further subclass also exhibits Tokunga self‐similarity, which is an average property of the side tributary structure in river networks. Comparisons with empirical observations for three large basins analyzed here show that the generalized Horton laws are well predicted by this new class of random self‐similar network models, and analytically predicted relationships between topologic and geometric Horton ratios are also seen in data.
Eddy covariance measurements of evaporation from Great Slave Lake, Northwest Territories, Canada
Authors: Blanken Peter D. , Rouse Wayne R. , Culf Alistair D. , Spence Chris , Boudreau L. Dale , Jasper Jesse N. , Kochtubajda Bob , Schertzer William M. , Marsh Philip , Verseghy Diana
Published: September 2002

The first direct measurements of evaporation from a large high‐latitude lake, Great Slave Lake, Northwest Territories, Canada, were made using eddy covariance between July 24 and September 10, 1997, and June 22 and September 26, 1998. The main body of the lake was ice‐free between June 20 and December 13, 1997, and June 1, 1998, and January 8, 1999, with the extended ice‐free season in 1997–1998 coinciding with 4°C above normal air temperatures and an abnormally strong El Niño. Measurements extending roughly 5.0 to 8.5 km across the lake were made from a small rock outcrop located near the main body of the lake. The lake was thermally stratified between mid‐July and September, with the thermocline extending down to approximately 15 m. High winds were effective in mixing warm surface waters downward and, when accompanied by cold fronts, resulted in large, episodic evaporation events typically lasting 45 hours. The daily total evaporation was best described as a function of the product of the horizontal wind speed and vapor pressure difference between the water surface and atmosphere. Seasonally, the latent heat flux was initially negative (directed toward the surface) followed by a steady increase to positive values (directed away from the surface) shortly after ice breakup. The latent heat flux then remained positive for the remainder of the ice‐free period, decreasing midsummer and then steadily increasing until freeze‐up. The sensible heat flux was small and often negative most of the spring and summer yet switched to positive and began to increase in the early fall. Extrapolation of evaporation measurements for the entire ice‐free periods gave totals of 386 and 485 mm in 1997 and 1998–1999, respectively.
Infiltration with controlled air escape
Authors: Culligan Patricia J. , Barry D. A. , Parlange J.‐Yves , Steenhuis Tammo S. , Haverkamp Randel
Published: September 2002

Infiltration into the soil is restricted if the movement of displaced air is hampered. In this study, separate infiltration experiments were performed where the air could escape only through capillary glass tubes (three different diameters were used). Control experiments, where the air movement was not restricted, were performed also. During the experiments both air pressure and the cumulative infiltration were measured. Air pressure measurements showed a rapid rise to a maximum after the water was ponded, followed by a gradual decrease. A first aim of this study was to show that even for a small increase in pressure relative to the case where the air was free to escape, e.g., <1 cm of water, there was a small but measurable reduction in infiltration. The air movement was obtained as a function of the air pressure via Poiseuille's law. By equating air and water movement (both fluids assumed incompressible) it was shown that the air pressure measurements could be used in predicting the water flux into the column and hence the cumulative infiltration. Then, by using the precise air pressure measurements for the various capillary tubes we were able to assess the sensitivity of hydraulic conductivity and sorptivity to minor increases in air pressure.
Analysis and mapping of field‐scale soil moisture variability using high‐resolution, ground‐based data during the Southern Great Plains 1997 (SGP97) Hydrology Experiment
Authors: Mohanty B. P. , Skaggs T. H. , Famiglietti J. S.
Published: September 2002

Soil moisture is an important state variable in the hydrologic cycle, and its spatiotemporal distribution depends on many geophysical processes operating at different spatial and temporal scales. To achieve a better accounting of the water and energy budgets at the land‐atmosphere boundary, it is necessary to better understand the spatiotemporal variability of soil moisture under different hydrologic and climatic conditions and at different hierarchical space scales and timescales. During the Southern Great Plains 1997 (SGP97) Hydrology Experiment the 0–6 cm soil water content was measured on consecutive afternoons at 400 locations in a small, gently sloping range field (Little Washita field site 07). The soil moisture measurements were made using portable impedance probes. Spatiotemporal data analyses of the two sampling events showed a significant change in the field variance but a constant field mean, suggesting moisture was redistributed by (differential) base flow, evapotranspiration, and condensation. Among the different relative landscape positions (hilltop, slope, valley) the slope was the largest contributor to the temporal variability of the soil moisture content. Using a sequential aggregation scheme, it was observed that the relative position influencing the field mean and variance changed between the two sampling events, indicating time instability in the spatial soil moisture data. Furthermore, high‐resolution (impedance probe) sampling and limited (gravimetric) sampling gave different field means and variances.
Electromagnetic alpha/proton instabilities in the solar wind
Authors: Gary S. Peter , Yin Lin , Winske Dan , Reisenfeld Daniel B.
Published: September 2002

The average flow speed of alpha particles relative to protons in the solar wind is observed to be of the order of or less than the Alfvén speed. This flow, if sufficiently large, can lead to two distinct types of electromagnetic alpha/proton instabilities: magnetosonic and Alfvén. Spatially homogeneous two‐dimensional hybrid simulations of collisionless plasmas are used to study these growing modes and the consequent wave‐particle scattering of each ion component. This scattering reduces the alpha/proton average flow speed, heats the alphas more strongly than the protons, and increases the alpha temperature in the directions perpendicular to the background magnetic field, all of which are consistent with solar wind observations.
Outflow of ions from Ganymede: A reinterpretation
Authors: Vasyliūnas Vytenis M. , Eviatar Aharon
Published: September 2002

The cold, dense plasma in the polar magnetosphere of Ganymede has been observed to contain one species of positive ions. The usual assumption that they are protons leads to severe problems, e.g. finding an adequate ionization mechanism for hydrogen, or explaining why the expected oxygen ions are not found and what happens to the oxygen. We propose that the observed ions are in fact singly charged oxygen ions rather than protons, which immediately removes all these problems. We show that the assumption of O+ instead of H+ is fully consistent with the plasma ion measurements and, by multiplying the inferred number density by 4 and speed by 1/4, brings them into better agreement with independent estimates of electron density and ion bulk flow.
On galactic cosmic ray flux decrease near solar minima and Imf intensity
Authors: Ahluwalia H. S.
Published: September 2002

We have analyzed the annual mean hourly rate data obtained with a variety of detectors located at the global sites as well as at the balloon altitudes, for the sunspot number (SSN) cycles 20, 21, 22. A systematic decrease is observed in all of them near the solar minimum epochs, for the period 1965 to 1987. The median rigidity of response (Rm) for these detectors, to the galactic cosmic ray (GCR) spectrum, lies in the range: 3GV < Rm < 67 GV. The observed decrease in GCR flux corresponds to an observed increase in the baselevel of the interplanetary magnetic field (IMF) intensity (B) during the same time period; no increase is observed in SSN minimum levels during this interval. The available data for GCR (1937 to 1998) and B (1963 to 1998) are reviewed. We argue that the cause of the observed GCR flux decrease (increase in the residual modulation) must be located in the heliosphere and not in the local interstellar medium.
On the occurrence and evolution of extremely high temperatures at the polar winter stratopause — A GCM study
Authors: Braesicke Peter , Langematz Ulrike
Published: September 2002

Sudden warmings in the stratopause region are a well known phenomenon in middle atmosphere dynamics. In this study, these stratopause warmings are analyzed using a ten year integration of the Berlin Climate Middle Atmosphere Model. The good spatial and temporal resolution of the model allows to investigate the main properties on a statistical basis. It turns out that strong stratopause warmings occur in almost each northern hemisphere winter. The associated temperature maximums and the descent of the stratopause agree very well with observed events. Two selected warmings are compared in terms of EP flux diagnostics and planetary wave amplitudes to study the links between stratopause events and (major) warmings in the lower/middle stratosphere. Only with supply of upward propagating wave energy from the troposphere and lowest stratosphere, stratopause warmings develop to lower stratospheric warmings.
Seismic and micromechanical studies of rock fracture
Authors: Young R. Paul , Hazzard James F. , Pettitt Will S.
Published: September 2002

“Earthquakes” occur as the result of stress redistribution on major fractures in the earth's crust and are also observed as scaled phenomena along grain boundaries and microcracks. Earthquake seismology has significantly contributed to our knowledge of fault processes, but our fundamental understanding of how micro‐fractures progressively weaken rocks and how this contributes to macro‐deformation processes is far from understood. Recent advances in particulate mechanics now mean fracture processes can be modelled dynamically to study the micromechanics of fracturing in rock. In addition, advances in recording and analysing very high frequency acoustic emissions (AE) allow for detailed examination of micro‐cracking. The paper describes how particle models and AE monitoring techniques can be used in conjunction to test specific hypotheses about natural and induced rock fracture processes at the grain scale. Intermediate scale processes (between laboratory and field studies) are also studied by examining rock fracture in an underground research laboratory.
Morphology and origin of the Osbourn Trough
Authors: Billen Magali I. , Stock Joann
Published: September 2002

The Osbourn Trough is a 900 km long, east‐west trending gravity low, visible in satellite altimetry data, which intersects the Tonga Trench at 25°30′S. A recent survey collected gravity, magnetic, echo sounder, and swath bathymetry data on three adjacent, north‐south trending ship tracks centered on the trough. The linear gravity low is 20–30 mGal less than the regional value and is accompanied by a flat‐lying, 200–500 m deep sediment‐filled valley. Swath bathymetry images reveal several parallel, east‐west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers, including a prominent inside corner high and possible pseudo‐fault trace. The presence of magnetic anomalies (possibly anomalies 33 and 32) suggests that the seafloor here was formed after the end of the Cretaceous Normal Superchron (anomaly 34). These data support the conclusion that this trough is a spreading center, which stopped spreading in Late Cretaceous time. The existence of this feature has important implications for tectonic reconstructions in this region. The Osbourn Trough could be part of the fossil spreading center between the Pacific Plate and a fragment of the Phoenix Plate, the Bellingshausen Plate. This would require the seafloor between the Osbourn Trough and the Chatham Rise to the south to be a remnant fragment of the Bellingshausen Plate. This remnant may have joined to the Pacific Plate when the Hikurangi Plateau entered the Gondwana subduction zone at the Chatham Rise possibly causing the cessation of spreading on the Osbourn Trough.
Fold style inversion: Placing probabilistic constraints on the predicted shape of blind thrust faults
Authors: Brooks Benjamin A. , Sandvol Eric , Ross Andrew
Published: September 2002

We develop a new methodology which compares quantitatively styles of folding from seismic reflection data. The goal of the “fold style inversion” (FSI) method is to provide an objective choice of the most appropriate model used when solving for the shape of an unimaged blind fault from folded layer geometry. FSI is a discretization of the dip isogon fold classification scheme reformulated as simple vector transformations. A data set's goodness of fit to parallel (class 1c) or similar (class 2) fold geometry is assessed by calculating misfit between the predicted and observed bed geometries through a grid search of the parameter space specific to each transformation; the two fold types correspond to the constant bed length and arbitrarily inclined simple shear (AISS) fault solution routines, respectively. For seismic reflection data, confidence estimates may be placed on the preference of fold style and its corresponding fault solution by Monte Carlo simulations of depth correlative, spatially limited depth conversion errors. For synthetic geometric examples FSI determines fold style preference and parameters exactly. At low fold limb dips (<∼15°) the actual geometric difference between parallel and similar folds is very small, and the difference between fold styles cannot be resolved, highlighting a general difficulty in the analyses of young blind thrust structures. For a synthetic seismic line of an AISS fault‐related fold the method chooses the correct folding style and leads to the correct fault geometry at depth. In examples of real data from the Barrancas/Lunlunta‐Carrizal anticlinal complex in Mendoza, Argentina, FSI analysis determines 71% and 54% probability of similar preference for two seismic lines on separate structures. The corresponding fault solutions for the first example are well constrained, whereas for the second example the solutions are widely variant. This analysis helps to quantify the relationship between the predicted sub surface fault trajectories and hypocenter and aftershock data of the 1985 Mw 5.9 Mendoza earthquake, showing that the earthquake and the fault causing the Barrancas/Lunlunta‐Carrizal anticlinorium are most likely unrelated.
Surface layer balance of the southern Antarctic Circumpolar Current (prime meridian) used to derive carbon and silicate consumptions and annual air‐sea exchange for CO2 and oxygen
Authors: Hoppema Mario , Fahrbach Eberhard , de Baar Hein J. W.
Published: September 2002

A simple model, using concentrations of nitrate and phosphate in austral winter 1992, reveals that the Antarctic Surface Water (AASW) of the southernmost Antarctic Circumpolar Current (ACC) between the Southern ACC Front and the Weddell Front is made up of ∼90% Upper Circumpolar Deep Water (UCDW) and 10% northward flowing AASW from the Weddell Gyre. With a typical timescale of ∼1 year the upwelling velocity was calculated to be as high as 60–100 m yr−1. Knowing the composition of the surface water with respect to its sources, changes due to several processes in the surface layer were deduced for carbon dioxide, oxygen, and silicate. As the timescale of changes in the surface layer of the southern ACC is ∼1 year, this allows us to calculate changes on an annual basis without interference of short‐term variations. Balancing the contributions by upwelling, biological activity, and air‐sea exchange to the concentrations in the surface layer, the area was found to be a large sink for atmospheric oxygen of 6.0 mol m−2 yr−1 (53 μmol kg−1) and a small sink for atmospheric carbon dioxide of 1.0 mol m−2 yr−1 (9 μmol kg−1). The most important cause for the oxygen sink is the upwelling of oxygen‐poor UCDW, which surpasses the oxygen‐elevating effect of primary productivity. This large oxygen sink, in between areas to the north and south which are only a small sink or even a source, conforms with the latitudinal distribution of atmospheric oxygen. The small CO2 sink is largely brought about by biological activity. The annual carbon utilization amounts to 76±22 g C m−2 yr−1, which is relatively high for an open ocean region in the Antarctic. However, it supports recent estimates of primary production of the Antarctic Ocean that are higher than early published values. The annual silicate consumption was calculated to be 126±19 g Si m−2 yr−1. This is considerably higher than the Southern Ocean mean in current estimates. Although the southernmost ACC may be atypical for the Southern Ocean, the current estimate for Southern Ocean silica production may well be an underestimation. The silicate to carbon utilization ratio derived here is 0.53 which aligns with investigations on Antarctic phytoplankton and thus underscores the consistency of our results.
An oxygen isotope data set for marine waters
Authors: Bigg Grant R. , Rohling Eelco J.
Published: September 2002

The proportion of 18O in a sample of seawater is an excellent tracer of its past history as, away from the surface, it is conservative and also nondynamical. The range of values in source waters is also large, and the accuracy achievable in modern measurement high. Here we bring together for the first time a global data set of over 6000 individual measurements from the past 40 years. The properties of this data set are described. Noteworthy features include the hitherto unnoticed, but distinctive, contribution of North Pacific Upper Water to the δ18O:salinity relationship, and different origins of the deeper waters of the Atlantic and Pacific Oceans. We also make a plea here to the international community to contribute unpublished data to the archive for the use of all.
Assimilation of TOPEX/Poseidon altimetric data in a primitive equation model of the tropical Pacific Ocean during the 1992–1996 El Niño‐Southern Oscillation period
Authors: Gourdeau L. , Verron J. , Delcroix T. , Busalacchi A. J. , Murtugudde R.
Published: September 2002

Four years of TOPEX/Poseidon data (October 1992 to October 1996) are assimilated into a high‐resolution primitive equation model [Gent and Cane, 1989] of the tropical Pacific Ocean. The assimilation method used is a degraded version of the Singular Evolutive Extended Kaiman filter approach [Pham et al., 1998] in which the reduced order basis is fixed. The along‐track TOPEX/Poseidon sea surface height residuals are introduced in the model every 3 days. Initialization of the model runs appears to be crucial. This is particularly true for the 1992–1996 period, which is characterized by several El Niño and La Niña events and exhibits quite anomalous climatological behavior. An in situ database dedicated to this period was built and used to initialize the model properly. The simulation results with and without assimilation were compared with expendable bathythermograph and Tropical Atmosphere‐Ocean in situ observations. The impact of sea surface height assimilation is observed both on the surface and subsurface temperature and velocity fields. The deficiencies of the free model in terms of means and variability structures are significantly reduced. More importantly, the evolution of the model fields over time is considerably improved by the assimilation. These supposedly more realistic assimilated four‐dimensional fields are used to investigate the mechanism responsible for the 1992–1996 warm pool replenishment and depletion. This approach provides insights into the combined role of zonal and meridional transports in balancing volume changes in the warm pool.
Response of the Adriatic sea level to the air pressure and wind forcing at low frequencies (0.01–0.1 cpd)
Authors: Pasarić Miroslava , Pasarić Zoran , Orlić Mirko
Published: September 2002

Low‐frequency (0.01–0.1 cpd) variability of air pressure, wind, and sea level is examined through 6‐ to 8‐year records of data collected at three locations along the east coast of the Adriatic and one on the west coast. Seasonal energy spectra show that processes at these timescales are more energetic in winter than in summer. There is substantial wind energy at timescales corresponding to planetary atmospheric waves. In order to explain the stronger‐than‐isostatic adjustment of sea level at low frequencies to the air pressure forcing, recorded in different parts of the Mediterranean, the present empirical analysis is based on a physically more tractable model, relating sea level slope to the air pressure gradient and wind stress integral. The multiple input regression and the cross‐spectral analysis yield a spatially variable response: over the deeper sea region sea level slope is fully explained by isostatic adjustment to the air pressure gradient alone; over the shelf a much stronger‐than‐isostatic response (−1.7 cm/mbar) is greatly reduced (−1.3 cm/mbar), but not fully accounted for, by the action of wind. Next the multiple linear regression method is carefully reexamined; a simple statistical model is developed to show that in multiple‐input linear models with mutually correlated inputs, small errors in one of the inputs produce biased estimates of all the response parameters. The apparent discrepancy between the theoretically predicted and the estimated response is attributed to the bias.