bluuebunny/crossref_metadata_2025 huggingface dataset:
Low‐temperature thermochronology of the northern Cordillera Real, Ecuador: Tectonic insights from zircon and apatite fission track analysis
Authors: Spikings Richard A. , Seward Diane , Winkler Wilfried , Ruiz Geoffrey M.
Published: February 2004

The post Early Cretaceous tectonic development of the northern Andean segment (north of 5°S) is loosely constrained by a series of partially and fully reset K/Ar ages. New apatite and zircon fission track data from three traverses across the Cordillera Real of Ecuador reveal distinct periods of accelerated crustal cooling during the Cenozoic. Fission track ages range from 65 to 14 Ma (zircon) and 44 to 8 Ma (apatite), with apatite mean track lengths ranging from 14.6 to 12.1 μm. These results record rapid cooling rates during ∼ 43 – 30 Ma, 23–15 Ma, and 10–0 Ma occurring at temperatures below ∼ 275°C. The events reflect up to ∼ 9 km of crustal exhumation in various regions of the Cordillera in response to relative changes in the kinematics of the Farallon, Nazca, and South American plates. The exhumation history of the Cordillera Real was not continuous but rather evolved during separate phases. These phases persisted for ∼ 10 Myr and are partly coincident with tectonic phases of the central Andes, although extend for longer time periods, possibly as a result of isostatically driven crustal exhumation. Large spatial variations in the zircon and apatite fission track data within the Cordillera Real indicate that local fault‐bounded massifs have experienced different thermal histories since ∼ 43 Ma. Major variations are not restricted to the regional scale and exist within individual terranes. Faulted slivers within terranes demonstrate highly variable exhumation histories, suggesting that deformation was not dominated by terrane‐bounding faults. Additionally, N‐S variations in exhumation depths suggest that ∼ NE‐SW trending faults have been active since ∼ 10–9Ma.
On the relationship between surface uplift and gravitational extension
Authors: Stüwe Kurt , Barr Terence D.
Published: February 2004

This paper highlights the relationship between isostatically supported surface elevation and the lateral buoyancy forces of an orogen. The former is a linear function; the latter is a quadratic function of the thickness of the lithosphere. Thus surface elevation and lateral buoyancy forces may have differing evolutions in the development of an orogen. Indeed, during the evolution of collisional orogens the qualitative relationship between these two parameters may reverse, so that only in the early orogenic evolution, topographically high regions exert a net lateral buoyancy force on topographically low regions. Later in the evolution this may change to a counterintuitive scenario in which there is a net lateral buoyancy force from the topographically lower regions toward the topographically higher regions. To illustrate this fact, we use a simple one‐dimensional kinematic model for an orogen which undergoes simultaneous thickening due to convergence and erosion at the surface. It can be shown that for reasonable erosion and thickening rates, surface elevation increases through time until a steady state is reached, while the lateral buoyancy force changes from positive to negative, giving rise to the inward collapse of an orogen. In the field this may be recognized as the occurrence of two successive coaxial convergent deformation events. Present‐day examples may be the formation of back arc basins where extension in the low‐lying hinterland of an Andean‐type mountain belt is caused by a gravitational compressive force exerted by the hinterland onto a belt of significantly higher surface elevation but possibly of lower potential energy.
Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc
Authors: Rossetti Federico , Faccenna Claudio , Goffé Bruno , Monié Patrick , Argentieri Alessio , Funiciello Renato , Mattei Massimo
Published: February 2004

Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure‐temperature (P‐T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat‐lying ductile to brittle extensional shear zone. The upper complex consists of a nappe‐like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite‐bearing sequence showing typical high‐P/low‐T parageneses (Mg‐carpholite and Na‐amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene‐Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep‐seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post‐orogenic Neogene basin sedimentation (middle‐upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high‐P/low‐T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene‐Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.
Solute transport along preferential flow paths in unsaturated fractures
Authors: Su Grace W. , Geller Jil T. , Pruess Karsten , Hunt James R.
Published: February 2004

Laboratory experiments were conducted to study solute transport along preferential flow paths in unsaturated, inclined fractures. Qualitative aspects of solute transport were identified in a miscible dye tracer experiment conducted in a transparent replica of a natural granite fracture. Additional experiments were conducted to measure the breakthrough curves of a conservative tracer introduced into an established preferential flow path in two different fracture replicas and a rock‐replica combination. The influence of gravity was investigated by varying fracture inclination. The relationship between the travel times of the solute and the relative influence of gravity was substantially affected by two modes of intermittent flow that occurred: the snapping rivulet and the pulsating blob modes. The measured travel times of the solute were evaluated with three transfer function models: the axial dispersion, the reactors‐in‐series, and the lognormal models. The three models described the solute travel times nearly equally well. A mechanistic model was also formulated to describe transport when the pulsating blob mode occurred which assumed blobs of water containing solute mixed with residual pools of water along the flow path.
Geomorphic controls on regional base flow
Authors: Marani Marco , Eltahir Elfatih , Rinaldo Andrea
Published: February 2004

The subsurface hydrological response plays an important role in the hydrology of humid regions. In particular, the physical relationship between base flow dynamics and the fluctuations in spatially averaged water table depth, as described by the groundwater rating curve, determine to a significant extent the nature of statistical persistence of hydrological anomalies in the unconfined aquifers level and river flow. In this paper, we propose that the scale and shape of the groundwater rating curve reflect some of the geomorphological characteristics of the region such as relief, drainage density, and the hypsometric distribution of the elevation field. These connections between geomorphology and hydrology of river basins are investigated using a simple model of unconfined groundwater flow applied to synthetic basins as well as observed basins from Illinois.
Transport and intersection mixing in random fracture networks with power law length distributions
Authors: Park Young‐Jin , de Dreuzy Jean‐Raynald , Lee Kang‐Kun , Berkowitz Brian
Published: February 2004

The importance of fracture intersection mixing rules, complete mixing and streamline routing, on simulated solute migration patterns in random fracture networks is assessed. For this purpose, and based on geological evidence, two‐dimensional model networks having power law fracture length distributions and lognormal fracture permeability distributions are considered. Different fracture network structures are accounted for by the power law length distribution, ranging from networks composed of infinite length fractures to percolation networks with constant length fractures. Comparison of solute particle statistics shows that there is no significant difference between the bulk transport properties calculated with the two mixing rules. In fact, it is found that the choice of mixing assumptions has a significant influence in less than 5% of the total number of fracture intersections in most fracture networks. This result can be attributed to the small mean effective coordination number, defined as the number of branches connected to an intersection with nonzero flux. It is concluded that solute transport in fracture networks is strongly influenced by variability and uncertainty in parameters defining the geometrical structure of networks and that, by comparison, the choice of mixing rules at fracture intersections is of little importance.
Extended tailing of bacteria following breakthrough at the Narrow Channel Focus Area, Oyster, Virginia
Authors: Zhang Pengfei , Johnson William P. , Scheibe Timothy D. , Choi Keun‐Hyung , Dobbs Fred C. , Mailloux Brian J.
Published: February 2004

Extended tailing of low bacterial concentrations following breakthrough at the Narrow Channel focus area was observed for 4 months. Bacterial attachment and detachment kinetics associated with breakthrough and extended tailing were determined by fitting a one‐dimensional transport model to the field breakthrough‐tailing data. Spatial variations in attachment rate coefficient (kƒ) were observed under forced gradient conditions (i.e., kƒ decreased as travel distance increased), possibly because of decreased bacterial adhesion with increased transport distance. When pore water velocity decreased by an order of magnitude at 9 days following injection, apparent bacterial attachment rate coefficients did not decrease with velocity as expected from filtration theory, but, instead, increased greatly for most of the wells. The coincidence of the increase in apparent attachment rate coefficient with the occurrence of protist blooms suggested that the loss of bacteria from the aqueous phase during the protist blooms was not governed by filtration but rather was governed by predation. Simulations were performed to examine the transport distances achieved with and without detachment, using attachment and detachment rate coefficients similar to those obtained in this field study. Simulations that included detachment showed that transport distances of bacteria may significantly increase because of detachment under the conditions examined.
On the influence of local dispersion in solute transport through formations with evolving scales of heterogeneity
Authors: Fiori A.
Published: February 2004

The aim of the present study is to determine the longitudinal dispersion coefficient DL for transport in formations of long‐range permeability fields by considering both large‐scale advection and local‐scale dispersion; the nonergodicity of the plume will be considered throughout the work. The scope is twofold: (1) to analyze the mutual role played by both the “macroscale” and the local scales of heterogeneity in determining the overall transport properties and (2) to check the validity of the results obtained in the past, in particular concerning the occurrence of anomalous transport. The results are obtained through the Lagrangean formulation of transport, by the means of a few simplifying assumptions. Two models of permeability K variations are considered: (1) a stationary Y = ln K with unbounded integral scale and (2) a formation of Y of stationary increments. For both cases, the longitudinal macrodispersion coefficient DL always grows with time when local‐scale dispersion is present, indicating that transport is always anomalous for the random fields examined. The results are in variance with those obtained in the past by considering nonergodic transport but neglecting the local‐scale dispersion [e.g., Dagan, 1994; Bellin et al., 1996] and in qualitative agreement to those obtained by adopting the ergodic assumption [e.g., Neuman, 1990; Glimm and Sharp, 1991], which, however, predicted higher rates of growth of DL with time. We conclude that the interplay between large‐scale, advective displacement and local‐scale dispersion has a fundamental impact on the occurrence of anomalous transport in long‐range correlated permeability random fields.
Environmental controls of the seasonal variation in oxygen uptake in sulfidic tailings deposited in a permafrost‐affected area
Authors: Elberling Bo
Published: February 2004

Oxygen consumption, sulfide oxidation, and acid mine drainage (AMD) of pyritic mine tailings were investigated at Nanisivik Mine, which is located in an area with continuous permafrost on Baffin Island in northern Canada. Tailings of varying age and water content have been deposited under alkaline conditions. One area consisting of tailings deposited on land in 1992 was selected for detailed measurements of in situ oxygen uptake rates at the tailing surface in the summers 1998 and 1999 and periodically during autumn and winter in 1998. Measurements included oxygen gas, water content, and temperature in profiles, as well as chemical analyses of pore solution and solids. Additional oxygen consumption rates were measured under controlled temperature conditions on columns filled with partly oxidized tailings. On the basis of temperature dependency of pyrite oxidation observed in the laboratory, an Arrhenius diffusion equation with soil temperature as input was used to simulate the observed temporal variation in oxygen uptake. Field data reveal that the ongoing sulfide oxidation of well‐drained tailings primarily takes place in the upper 30 cm and that oxidation has resulted in a depletion of pyrite, carbonates, and metals from this reaction zone. The model provides a reasonable fit to the observed trend in oxygen consumption and documents that oxidation of sulfide minerals in tailings is not reduced to neglectable levels at O°C. The AMD generation rate has been quantified based on the changes in concentration of oxidation products in the pore water and oxidation rates based on in situ measurements of oxygen consumption. The two rate descriptions provide comparable estimates of seasonal AMD generation and provide detailed information on weather‐related controls of AMD generation, i.e., ground temperature, freezing, water content, and snow cover. These environmental controls are crucial for the design of frozen cover schemes in permafrost regions, where the aim is to force the frost table to the top of the tailings, maintaining tailings at subzero temperatures year‐round and thereby controlling the AMD generation.
Deformation of stream surfaces in steady axisymmetric flow
Authors: Steward David R. , Janković Igor
Published: February 2004

Stream surfaces are used to examine the arrangement of neighboring streamlines in steady, divergence‐free axisymmetric flow. Expressions are obtained that relate the angle of intersection between stream surfaces with general orientations to the Stokes stream function and to the specific discharge vector. These expressions are used to quantify deformation to the geometry of streamlines and stream surfaces occurring in axisymmetric flow. The Stokes stream function is presented for an isolated inhomogeneity in hydraulic conductivity with spheroidal geometry. It is shown that axisymmetric flow generated by inhomogeneity rearranges the relative position of a streamline with respect to neighboring streamlines only locally. In contrast, a persistent rearrangement in the orientation and spacing of neighboring streamlines is observed between large distances upgradient and downgradient from groundwater withdrawal. This characterizes all possible rearrangement of neighboring streamlines in axisymmetric flow; withdrawal (or production) is required for a persistent rearrangement to occur.
A simple model of saturation excess runoff generation based on geomorphology, steady state soil moisture
Authors: Willgoose Garry , Perera Hemantha
Published: February 2004

A simple conceptual model of saturation excess runoff generation, the Hydrogeomorphic Steady State model (HGSS) is developed using widely observed geomorphic relationships: the saturation excess threshold, the area‐slope relationship, and the cumulative area diagram. HGSS provides theoretical insight into the link between runoff generation by saturation excess and the geomorphology that governs it. Current models for saturation excess runoff generation require detailed analysis of catchment digital elevation maps (DEMs), and their prediction accuracy depends on the DEM grid resolution and the analysis techniques used on the DEM data. HGSS can predict the saturated area at steady state without site‐specific DEM analysis. In its simplest form, HGSS requires only two parameters, each a combination of geomorphology parameters, together with soil and recharge properties. Effectively, HGSS predicts an antecedent recharge‐dependent runoff coefficient. A scaling solution indicates that the percentage of the catchment saturated decreases with the square root of catchment area over a broad range of catchment areas and that the proportion of the catchment saturated has a log‐log linear relationship with recharge. HGSS is tested by comparing its predictions with two DEM rainfall‐runoff models, the steady state TOPMODEL and a kinematic wave saturation excess model DISTFW‐EXT, with HGSS predictions matching for steady state conditions.
Analysis of streambed temperatures in ephemeral channels to determine streamflow frequency and duration
Authors: Constantz Jim , Stonestorm David , Stewart Amy E. , Niswonger Richard , Smith Tyson R.
Published: February 2004

Spatial and temporal patterns in streamflow are rarely monitored for ephemeral streams. Flashy, erosive streamflows common in ephemeral channels create a series of operational and maintenance problems, which makes it impractical to deploy a series of gaging stations along ephemeral channels. Streambed temperature is a robust and inexpensive parameter to monitor remotely, leading to the possibility of analyzing temperature patterns to estimate streamflow frequency and duration along ephemeral channels. A simulation model was utilized to examine various atmospheric and hydrological upper boundary conditions compared with a series of hypothetical temperature‐monitoring depths within the streambed. Simulation results indicate that streamflow events were distinguished from changing atmospheric conditions with greater certainty using temperatures at shallow depths (e.g., 10–20 cm) as opposed to the streambed surface. Three ephemeral streams in the American Southwest were instrumented to monitor streambed temperature for determining the accuracy of using this approach to ascertain the long‐term temporal and spatial extent of streamflow along each stream channel. Streambed temperature data were collected at the surface or at shallow depth along each stream channel, using thermistors encased in waterproof, single‐channel data loggers tethered to anchors in the channel. On the basis of comparisons with site information, such as direct field observations and upstream flow records, diurnal temperature variations successfully detected the presence and duration of streamflow for all sites.
Type curve interpretation of a cross‐hole pneumatic injection test in unsaturated fractured tuff
Authors: Illman Walter A. , Neuman Shlomo P.
Published: February 2004

Cross‐hole pneumatic injection tests have been conducted in 16 vertical and inclined boreholes in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. Their purpose was to characterize the bulk pneumatic properties and connectivity of fractures at the site on scales ranging from meters to several tens of meters. We describe the design, conduct, and type curve interpretation of one of these tests. Our cross‐hole type curves are modified after Hsieh and Neuman [1985] to consider single‐phase airflow and extended to consider the effects of storage and skin in monitoring intervals. Cross‐hole type curves of pressure derivatives and recovery are included for improved pneumatic characterization of the site. We analyze recorded pressures in each monitoring interval separately from those in other intervals while treating the fractured rock as a uniform, isotropic porous continuum. Each record yields an equivalent directional air permeability and air‐filled porosity for fractures that connect the corresponding monitoring and injection intervals. Both parameters are found to vary considerably from one monitoring interval to another, reflecting the nonuniform nature of pneumatic rock properties at the ALRS. The geometric mean of these equivalent permeabilities is found to be larger by a factor of 50 than that obtained from single‐hole pneumatic injection tests on a nominal scale of 1–3 m (single‐hole tests yield only limited information about porosities, which therefore cannot be meaningfully compared with cross‐hole results). Our results find support in numerical inverse modeling of cross‐hole tests at the site by Vesselinov [2000; see also Illman et al., 1998; Vesselinov et al, 2000]. Vesselinov [2000] has further demonstrated [see also Chen et al., 2000] that a similar scale effect is exhibited by fracture porosity at the ALRS and that both scale effects disappear when cross‐hole tests at the site are interpreted by means of a numerical inverse model, which resolves heterogeneities down to a scale of 1 m. When considered jointly with these and other studies of the site, our analysis implies that the pneumatic pressure behavior of unsaturated fractured tuffs at the ALRS can be described quite accurately by means of linearized single‐phase airflow equations; this behavior can be interpreted by treating the rock as a continuum on scales ranging from meters to tens of meters: the continuum is representative primarily of interconnected fractures; as these fractures are filled primarily with air, their pneumatic permeabilities and porosities are close to the bulk intrinsic properties of fractures at the site: these intrinsic properties vary randomly with location and direction across the ALRS, and they depend strongly on the scale at which they are determined.
A new tracer‐density criterion for heterogeneous porous media
Authors: Barth Gilbert R. , Illangasekare Tissa H. , Hill Mary C. , Rajaram Harihar
Published: February 2004

Tracer experiments provide information about aquifer material properties vital for accurate site characterization. Unfortunately, density‐induced sinking can distort tracer movement, leading to an inaccurate assessment of material properties. Yet existing criteria for selecting appropriate tracer concentrations are based on analysis of homogeneous media instead of media with heterogeneities typical of field sites. This work introduces a hydraulic‐gradient correction for heterogeneous media and applies it to a criterion previously used to indicate density‐induced instabilities in homogeneous media. The modified criterion was tested using a series of two‐dimensional heterogeneous intermediate‐scale tracer experiments and data from several detailed field tracer tests. The intermediate‐scale experimental facility (10.0×1.2×0.06 m) included both homogeneous and heterogeneous (σln k2 = 1.22) zones. The field tracer tests were less heterogeneous (0.24 < σln k2 < 0.37), but measurements were sufficient to detect density‐induced sinking. Evaluation of the modified criterion using the experiments and field tests demonstrates that the new criterion appears to account for the change in density‐induced sinking due to heterogeneity. The criterion demonstrates the importance of accounting for heterogeneity to predict density‐induced sinking and differences in the onset of density‐induced sinking in two‐ and three‐dimensional systems.
Visualization of surfactant‐enhanced nonaqueous phase liquid mobilization and solubilization in a two‐dimensional micromodel
Authors: Zhong Lirong , Mayer Alex , Glass Robert J.
Published: February 2004

Surfactant‐enhanced aquifer remediation is an emerging technology for aquifers contaminated with nonaqueous phase liquids (NAPLs). A two‐dimensional micromodel and image capture system were applied to observe microscale NAPL mobilization and solubilization phenomena. In each experiment a common residual NAPL field was established, followed by a series of mobilization and solubilization experiments. Mobilization floods included pure water floods with variable flow rates and surfactant floods with variations in surfactant formulations. At relatively low capillary numbers (Nca < 10−3) the surfactant mobilization floods resulted in higher NAPL saturations than for similar Nca pure water floods. These differences in macroscopic saturations are explained by differences in microscale mobilization processes. Sqlubilization of the residual NAPL remaining after the mobilization stage was dominated by the formation of microscale dissolution fingers, which produced nonequilibrium macroscale NAPL solubilization. A macroemulsion phase also was observed to form spontaneously and persist during the solubilization stage of the experiments.
Effective matrix diffusion in kilometer‐scale transport in fractured crystalline rock
Authors: Shapiro Allen M.
Published: February 2004

Concentrations of tritium (3H) and dichlorodifluoromethane (CFC‐12) in water samples taken from glacial drift and fractured crystalline rock over 4 km2 in central New Hampshire are interpreted to identify a conceptual model of matrix diffusion and the magnitude of the diffusion coefficient. Dispersion and mass transfer to and from fractures has affected the 3H concentration to the extent that the peak 3H concentration of the 1960s is no longer distinguishable. Because of heterogeneity in the bedrock the sparsely distributed chemical data do not warrant a three‐dimensional transport model. Instead, a one‐dimensional model of CFC‐12 and 3H migration along flow lines in the glacial drift and bedrock is used to place bounds on the processes affecting kilometer‐scale transport, arid model parameters are varied to reproduce the measured relation between 3H and CFC‐12, rather than their spatial distributions. A model of mass exchange to and from fractures that is dependent on the time‐varying concentration gradient at fracture surfaces qualitatively reproduces the measured relation between 3H and CFC‐12 with an upper bound for the fracture dispersivity approximately equal to 250 m and a lower bound for the effective matrix diffusion coefficient equal to 1 m2 yr−1. The diffusion coefficient at the kilometer scale is at least 3 orders of magnitude greater than laboratory estimates of diffusion in crystalline rock. The large diffusion coefficient indicates that diffusion into an immobile fluid phase (rock matrix) is masked at the kilometer scale by advective mass exchange between fractures with large contrasts in trarismissivity. The measured transmissivity of fractures in the study area varies over more than 6 orders of magnitude. Advective mass exchange from high‐permeability fractures to low‐permeability fractures results in short migration distances of a chemical constituent in low‐permeability fractures over an extended period of time before reentering high‐permeability fractures; viewed at the kilometer scale, this process is analogous to the chemical constituent diffusing into and out of an immobile fluid phase.
Macrodispersion in a radially diverging flow field with finite Peclet Numbers: 1. Perturbation theory approach
Authors: Neuweiler I. , Attinger S. , Kinzelbach W.
Published: February 2004

In this paper large‐scale dispersion coefficients for tracer transport in a radially diverging flow field with cylindrical geometry in an unbounded domain are investigated. The effect of small‐scale dispersion as well as small‐scale diffusion on the large‐scale dispersion is analyzed. Macrodispersion coefficients are derived analytically from ensemble‐averaged second radial cumulants of the tracer concentration distribution. The cumulants are calculated to second order in the fluctuations of permeability of the heterogeneous porous medium. A macrodispersion coefficient is found, which is proportional to the mean velocity field. It is shown that the macrodispersivity is modified because of the impact of the small‐scale diffusion and small‐scale dispersion. The vertical small‐scale mixing leads to a decrease of the macrodispersivity. Small‐scale diffusion makes this decrease time‐dependent. Horizontal diffusion, however, leads to an increase of the macrodispersivity.
New insights about the climatic and geologic control on the probability distribution of floods
Authors: Fiorentino Mauro , Iacobellis Vito
Published: February 2004

An application of the theoretically based distribution of floods recently derived by Iacobellis and Fiorentino [2000] was carried out with the aim of analyzing the climatic and geologic control on the distribution itself. In particular, 20 basins in a wide area of southern Italy were considered. These basins were classified by the use of a climatic index I depending on the average annual rainfall and on the potential evapotranspiration. This index takes positive values in humid climates and negative values in dry zones. With regard to the geology, attention was paid to the watershed permeability during intense storms; in each basin the pervious area was estimated after identification of three permeability classes. The analyses demonstrated that the sign of the climatic index strongly discriminates the behavior of both the parameters ƒA and E[a], the former representing, for each basin, the characteristic total abstraction rate and the latter being the expected value of the basin area contributing to the flood peak. In dry zones, ƒA was found to decrease with increasing A, and the ratio r = E[a]/A tended to decline linearly, as the percentage of pervious areas increased. On the contrary, in humid zones the basin area did not seem to have any effect on ƒA, which was instead particularly sensitive to the climatic index. Moreover, in these zones the parameter r was steadily low without showing any significant dependence on climate, geology, and morphology of the basin. In this paper, an interpretation of these results is indeed provided.
Response of mean annual evapotranspiration to vegetation changes at catchment scale
Authors: Zhang L. , Dawes W. R. , Walker G. R.
Published: February 2004

It is now well established that forested catchments have higher evapotranspiration than grassed catchments. Thus land use management and rehabilitation strategies will have an impact on catchment water balance and hence water yield and groundwater recharge. The key controls on evapotranspiration are rainfall interception, net radiation, advection, turbulent transport, leaf area, and plant‐available water capacity. The relative importance of these factors depends on climate, soil, and vegetation conditions. Results from over 250 catchments worldwide show that for a given forest cover, there is a good relationship between long‐term average evapotranspiration and rainfall. From these observations and on the basis of previous theoretical work a simple two‐parameter model was developed that relates mean annual evapotranspiration to rainfall, potential evapotranspiration, and plant‐available water capacity. The mean absolute error between modeled and measured evapotranspiration was 42 mm or 6.0%; the least squares line through the origin had as lope of 1.00 and a correlation coefficient of 0.96. The model showed potential for a variety of applications including water yield modeling and recharge estimation. The model is a practical tool that can be readily used for assessing the long‐term average effect of vegetation changes on catchment evapotranspiration and is scientifically justifiable.
First‐order reliability method for estimating reliability, vulnerability, and resilience
Authors: Maier Holger R. , Lence Barbara J. , Tolson Bryan A. , Foschi Ricardo O.
Published: February 2004

Reliability, vulnerability, and resilience provide measures of the frequency, magnitude, and duration of the failure of water resources systems, respectively. Traditionally, these measures have been estimated using simulation. However, this can be computationally intensive, particularly when complex system‐response models are used, when many estimates of the performance measures are required, and when persistence among the data needs to be taken into account. In this paper, an efficient method for estimating reliability, vulnerability, and resilience, which is based on the First‐Order Reliability Method (FORM), is developed and demonstrated for the case study of managing water quality in the Willamette River, Oregon. Reliability, vulnerability, and resilience are determined for different dissolved oxygen (DO) standards. DO is simulated using a QUAL2EU water quality response model that has recently been developed for the Oregon Department of Environmental Quality (ODEQ) as part of the Willamette River Basin Water Quality Study (WRBWQS). The results obtained indicate that FORM can be used to efficiently estimate reliability, vulnerability, and resilience.
Transient storage and hyporheic flow along the Willamette River, Oregon: Field measurements and model estimates
Authors: Fernald Alexander G. , Wigington P. J. , Landers Dixon H.
Published: February 2004

Transient storage is a measure of the exchange of main channel flow with subsurface hyporheic flow and surface water dead zones. Hyporheic flow, in which river water enters the channel bed and banks to reemerge downstream, promotes biochemical processes that are important for water quality and aquatic habitat. Previous studies have quantified transient storage and hyporheic flow on small streams but were not specifically developed to identify both of these processes over long reaches of large rivers. We studied transient storage on the eighth‐order upper Willamette River, which flows through high‐ porosity gravel deposits conducive to hyporheic flow. We used main channel dye tracer studies and solute transport modeling to estimate transient storage on nine study reaches in a 26‐km‐long study area. We also took dye measurements within the transient storage zone to identify transient storage flow paths. We obtained estimates of transient storage exchange coefficient, αs (mean equals 1.6×10−4 s−1), and transient storage to main channel cross‐sectional area, As/A (mean equal to 0.28), that show that significant amounts of water follow flow paths with 0.2–30 hour transient storage zone residence times. Our dye measurements from the transient storage zone itself showed the occurrence of both subsurface and surface flow paths, confirming that hyporheic flow is an important component of estimated transient storage. We found that the two highest As/Aestimates were for reaches that spanned the only length of active main channel in our study area that is unconstrained and where the river can rework large gravel deposits. Much of the natural channel complexity that historically promoted hyporheic flow no longer exists on the upper Willamette River. River management targeting the ecological functions provided by hyporheic flow might best focus on restoring historic hydrogeomorphic processes for creating sites conducive to hyporheic flow.
Sediment classification and estimation of suspended sediment fluxes in the Changjiang Estuary, China
Authors: Wu Jia‐Xue , Shen Huan‐Ting , Xiao Chen‐You
Published: February 2004

Estuarine suspended sediment is transported in a mixed nonuniform way under unsteady flows. Sediment of different grain sizes has different characteristics and transport behavior and has a different effect on the ecological system. Therefore classification and fractionization of the mixed sediment are required before the flux is estimated. A fuzzy clustering approach is applied to the classification of suspended fine‐grained sediment in the Changjiang Estuary. Two populations are objectively found by considering the standard grain‐size distribution statistics of each cluster. The critical grain size of ∼10 μm in diameter is the size limit for cohesive sediments. A grid with equal cell areas is used to estimate fractional sediment fluxes through an estuarine cross section since this type of grid introduces less statistical error in the flux calculation. The sediment transport mechanism is analyzed.
Analytical solutions for transport in porous media with Gaussian source terms
Authors: Walker Douglas D. , White Janis E.
Published: February 2004

Analytical solutions for the advective‐dispersion equation for solute transport in porous media commonly assume a uniform distribution of mass within the source term. This paper derives an analytical solution for transport in porous media for a source term whose mass is distributed as a bivariate Gaussian spatial function. The solution is an extension of existing analytical solutions using a Green's function approach to separate out one‐dimensional terms in a manner similar to previous authors. This approach illustrates the relationship of the bivariate Gaussian source term solution to other Green's function solutions and thus leads to a set of solutions for advective‐dispersive transport with various source term and domain geometries. Comparison of point, bivariate Gaussian, and uniform source term solutions finds the greatest differences near the source, with discrepancies decreasing with travel distance.
A particle‐tracking method for advective transport in fractures with diffusion into finite matrix blocks
Authors: Tsang Y. W. , Tsang C. F.
Published: February 2004

A particle‐tracking method has been developed to calculate tracer transport in fractures with diffusion into finite rock matrix blocks. The method is an extension of the work of Yamashita and Kimura [1990], which is only applicable to diffusion into an infinite matrix. The new method has been verified against a number of analytic or semianalytic solutions for transport in a homogeneous fracture medium with matrix diffusion. The method is applied to the calculation of tracer breakthrough curves for a hypothetical tracer injection‐withdrawal experiment in a heterogeneous fracture zone, with variable hydraulic properties and finite matrix blocks.
Estimating the hydraulic conductivity at the south oyster site from geophysical tomographic data using Bayesian Techniques based on the normal linear regression model
Authors: Chen Jinsong , Hubbard Susan , Rubin Yoram
Published: February 2004

This study explores the use of ground penetrating radar (GPR) tomographic velocity, GPR tomographic attenuation, and seismic tomographic velocity for hydraulic conductivity estimation at the South Oyster Site, using a Bayesian framework. Since site‐specific relations between hydraulic conductivity and geophysical properties are often nonlinear and subject to a large degree of uncertainty such as at this site, we developed a normal linear regression model that allows exploring these relationships systematically. Although the log‐conductivity displays a small variation (σ2 = 0.30) and the geophysical data vary over only a small range, results indicate that the geophysical data improve the estimates of the hydraulic conductivity. The improvement is the most significant where prior information is limited. Among the geophysical data, GPR and seismic velocity are more useful than GPR attenuation.
Seasonal subsidence and rebound in Las Vegas Valley, Nevada, observed by Synthetic Aperture Radar Interferometry
Authors: Hoffmann Jörn , Zebker Howard A. , Galloway Devin L. , Amelung Falk
Published: February 2004

Analyses of areal variations in the subsidence and rebound occurring over stressed aquifer systems, in conjunction with measurements of the hydraulic head fluctuations causing these displacements, can yield valuable information about the compressibility and storage properties of the aquifer system. Historically, stress‐strain relationships have been derived from paired extensometer/piezometer installations, which provide only point source data. Because of the general unavailability of spatially detailed deformation data, areal stress‐strain relations and their variability are not commonly considered in constraining conceptual and numerical models of aquifer systems. Interferometric synthetic aperture radar (InSAR) techniques can map ground displacements at a spatial scale of tens of meters over 100 km wide swaths. InSAR has been used previously to characterize larger magnitude, generally permanent aquifer system compaction and land subsidence at yearly and longer timescales, caused by sustained drawdown of groundwater levels that produces intergranular stresses consistently greater than the maximum historical stress. We present InSAR measurements of the typically small‐magnitude, generally recoverable deformations of the Las Vegas Valley aquifer system occurring at seasonal timescales. From these we derive estimates of the elastic storage coefficient for the aquifer system at several locations in Las Vegas Valley. These high‐resolution measurements offer great potential for future investigations into the mechanics of aquifer systems and the spatial heterogeneity of aquifer system structure and material properties as well as for monitoring ongoing aquifer system compaction and land subsidence.
New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190
Authors: Moore Gregory F. , Taira Asahiko , Klaus Adam , Becker Luann , Boeckel Babette , Cragg Barry A. , Dean Allison , Fergusson Christopher L. , Henry Pierre , Hirano Satoshi , Hisamitsu Toshio , Hunze Sabine , Kastner Miriam , Maltman Alex J. , Morgan Julia K. , Murakami Yuki , Saffer Demian M. , Sánchez‐Gómez Mario , Screaton Elizabeth J. , Smith David C. , Spivack Arthur J. , Steurer Joan , Tobin Harold J. , Ujiie Kohtaro , Underwood Michael B. , Wilson Moyra
Published: February 2004

The Nankai Trough accretionary prism is considered an “end‐member” prism accreting a coarse terrigenous sediment section in a setting with structural simplicity, unparalleled resolution by seismic and other geophysical techniques, and large historic earthquakes. It therefore has been the focus of Ocean Drilling Program (ODP) drilling to address several unresolved questions concerning accretionary processes and prism evolution. At six sites cored along two transects across the Nankai Trough accretionary prism during ODP Leg 190, lithostratigraphy and sediment diagenesis vary markedly. For the first time, reference sites at the seaward ends of the two transects defined the stratigraphic framework of the accreting/subducting Shikoku Basin sedimentary section. A thick section of Miocene turbidites and smectite‐rich mudstone is present within the subducting section at the Ashizuri site. The turbidites and mudstones are absent in the correlative section at the Muroto site; variations in lithology, mineralogy, and hydrologic properties of the incoming sediments probably contribute to the difference in prism wedge taper between the two transects, while possibly controlling the seismic character of the active plate boundary. The décollement in both transects is localized within a common stratigraphic unit (∼5.9–7 Ma) within the lower Shikoku Basin facies. The décollement is also a major boundary for both physical and mechanical properties. A broad low‐chloride pore water anomaly in the lower Shikoku Basin unit, first identified at Site 808, progressively decreases in magnitude from prism to basin along the Muroto Transect. Physical properties relationships, evidence for mineralogic changes in the sediments, and pore fluid chemistry suggest that the chloride anomaly results primarily from in situ diagenetic reactions in the sediments, possibly augmented by flow of freshened fluid from depth. New constraints on stratigraphy and age of units along more landward parts of the Muroto Transect have dramatically changed our ideas about the tectonic evolution of the prism in this area. Growth of the seaward‐most part of the prism took place very rapidly, with 40 km of accretion within the past 2 Myr. This rate is at least 3 times greater than growth rates in a comparable prism.
Aftershock locations and rupture characteristics of the 1995 Mariana Deep Earthquake
Authors: Tibi Rigobert , Wiens Douglas A. , Hildebrand John A.
Published: February 2004

The August 23, 1995 (Mw=7.0, depth 586 km) Mariana event produced one of the strongest deep earthquake aftershock sequences known. We combined teleseismic arrival times with arrival times picked from a local network operating directly above the event in the Mariana Islands, and relocated the main shock and aftershocks. We derived the source characteristics from teleseismic body waves by inverting waveform data of the Global Seismograph Network. The main shock ruptured downward across the entire width of the active slab along a plane dipping southwestward. Most of the 17 well‐located aftershocks occurred along or very near the rupture plane. The different focal mechanisms of the main shock and aftershocks indicate that the stress field varies across the width of the Mariana slab at the area of the August 1995 sequence.
Trace gas transport and scavenging in PEM‐Tropics B South Pacific Convergence Zone convection
Authors: Pickering Kenneth E. , Thompson Anne M. , Kim Hyuncheol , DeCaria Alex J. , Pfister Leonhard , Kucsera Tom L. , Witte Jacquelyn C. , Avery Melody A. , Blake Donald R. , Crawford James H. , Heikes Brian G. , Sachse Glen W. , Sandholm Scott T. , Talbot Robert W.
Published: February 2004

Analysis of chemical transport on Flight 10 of the 1999 Pacific Exploratory Mission (PEM) Tropics B mission clarifies the role of the South Pacific Convergence Zone (SPCZ) in establishing ozone and other trace gas distributions in the southwestern tropical Pacific. The SPCZ is found to be a barrier to mixing in the lower troposphere but a mechanism for convective mixing of tropical boundary layer air from northeast of the SPCZ with upper tropospheric air arriving from the west. A two‐dimensional cloud‐resolving model is used to quantify three critical processes in global and regional transport: convective mixing, lightning NOx production, and wet scavenging of soluble species. Very low NO and O3 tropical boundary layer air from the northeastern side of the SPCZ entered the convective updrafts and was transported to the upper troposphere where it mixed with subtropical upper tropospheric air containing much larger NO and O3 mixing ratios that had arrived from Australia. Aircraft observations show that very little NO appears to have been produced by electrical discharges within the SPCZ convection. We estimate that at least 90% of the HNO3 and H2O2 that would have been in upper tropospheric cloud outflow had been removed during transport through the cloud. Lesser percentages are estimated for less soluble species (e.g., <50% for CH3OOH). Net ozone production rates were decreased in the upper troposphere by ∼60% due to the upward transport and outflow of low‐NO boundary layer air. However, this outflow mixed with much higher NO air parcels on the southwest edge of the cloud, and the mixture ultimately possessed a net ozone production potential intermediate between those of the air masses on either side of the SPCZ.