bluuebunny/crossref_metadata_2025 huggingface dataset:
Alpine plate kinematics revisited: The Adria Problem
Authors: Wortmann Ulrich G. , Weissert Helmut , Funk Hanspeter , Hauck Jürgen
Published: February 2004

Tectonic evolution of the Alpine Tethys is controlled by the plate movements of Africa, Europe, and the Adriatic microplate. It is, however, unclear to which extent and at what times the motion of Adria was related to Africa. Kinematic models which assume a rigid connection between Africa and Adria have difficulties in explaining the Alpine rock record. Reconstructions based on the Alpine record are, on the other hand, in conflict with the involved kinematics. To resolve these conflicts, they require complicated motions or the introduction of additional microplates. Here we present a solution which is based on a rigid connection between Africa/Adria during Jurassic/Cretaceous times. Our model requires only four plates, involving Africa, Europe, Iberia, and Adria. It describes a self‐consistent kinematic evolution of the western Tethyan microplate motions from Jurassic to Miocene times. The initial (Early Jurassic) plate configuration was found by iterative forward modeling. The resulting Jurassic plate configuration is unusual and provides new insights into Alpine geology. The obtained model is, however, in good agreement with the available geological data and suggests that the assumption of independent movements of Adria during Jurassic/Cretaceous times is not a necessity.
Style and history of Andean deformation, Puna plateau, northwestern Argentina
Authors: Coutand Isabelle , Cobbold Peter R. , de Urreiztieta Marc , Gautier Pierre , Chauvin Annick , Gapais Denis , Rossello Eduardo A. , López‐Gamundí Oscar
Published: February 2004

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high‐angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE‐SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line‐balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW‐ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE‐SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip‐slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.
A Bayesian Approach to the quantification of the effect of model error on the predictions of groundwater models
Authors: Gaganis Petros , Smith Leslie
Published: February 2004

Errors arising from the imperfect mathematical representation of the structure of a hydrologic system (model error) are not random but systematic. Their effect on model predictions varies in space and time and differs for the flow and solute transport components of a groundwater model. Such errors do not necessarily have any probabilistic properties that can be easily exploited in the construction of a model performance criterion. A Bayesian approach is presented for quantifying model error in the presence of parameter uncertainty. Insight gained in updating the prior information on the model parameters is used to assess the correctness of the model structure, which is defined relative to the accuracy required of the model predictions. Model error is evaluated for each measurement of the dependent variable through an examination of the correctness of the model structure for different accuracy levels. The effect of model error on each dependent variable, which is quantified as a function of location and time, represents a measure of the reliability of the model in terms of each model prediction. This method can be used in identifying possible causes of model error and in discriminating among models in terms of the correctness of the model structure. It also offers an improved description of the uncertainties associated with a modeling exercise that may be useful in risk assessments and decision analyses.
Macroinvertebrate community changes at coarse sediment recruitment points along two gravel bed rivers
Authors: Rice Stephen P. , Greenwood Malcolm T. , Joyce Christopher B.
Published: February 2004

Field data from two gravel bed rivers are used to investigate the response of mainstream macroinvertebrate communities to changes in bed sediment character at sites of significant coarse sediment recruitment. There is clear evidence that these points, where downstream fining trends are punctuated, are associated with significant changes in community composition. Community differences are greater between pairs of riffles that straddle recruitment points than between similarly spaced riffles that do not. Taxa that prefer coarse substrate and energetic flows tend to be more abundant below recruitment points where there is also a tendency for taxa diversity to increase. Classification procedures discriminate faunal assemblages at those sites which occur immediately downstream of recruitment points from assemblages at other sites. Although most of the lateral sediment sources examined are tributaries, a number of arguments suggest that, at least in this case, faunal changes are primarily due to abrupt changes in bed sediment character and are not due to concomitant changes in water quality or quantity. Our results suggest that at moderate spatial scales, patterns of coarse sediment recruitment are partly responsible for the spatial organization of macroinvertebrate communities in gravel bed rivers.
Measurement and approximate critical path analysis of the pore‐scale‐induced anisotropy factor of an unsaturated porous medium
Authors: Friedman Shmulik P. , Jones Scott B.
Published: February 2004

Nonspherical particles aligned parallel to preferential bedding planes form sedimentary rocks and soils, which are anisotropic in their resistance to flow of fluids, solutes, heat, and electrical current at the microscopic scale. Understanding the extent of the anisotropy of an unsaturated medium, containing both water and air, and its quantification is of major importance for improving our prediction capability of pollutant transport occurring in those media. The anisotropy factor is defined as the ratio of the conductivities parallel to the bedding plane and perpendicular to it. Measurements of the anisotropy factor of the apparent electrical conductivity in initially saturated packings of platy mica particles resulted in an initial moderate increase of the anisotropy factor with desaturation, followed by its decrease when approaching the drier region of residual moisture contents. The anisotropy factor for the hydraulic conductivity is expected to follow a pattern similar to that found for the apparent electrical conductivity. This behavior is opposite to that predicted by previous theoretical models, which suggest a possible initial decrease of the anisotropy factor of the hydraulic conductivity with desaturation, followed by its divergence to infinity toward the dry end. A critical path analysis of a simplified three‐dimensional pore network model, based on different characterization of the pores parallel and normal to the bedding plane, served to evaluate the saturation degree‐dependent anisotropy factors. The critical path analysis reconstructed the experimentally determined pattern qualitatively and helped to explain the observed dependence of the apparent electrical conductivity's anisotropy factor on saturation degree. The critical path analysis also provided an indication of the anisotropy factor for hydraulic conductivity, which is expected to be of higher magnitude than that of the apparent electrical conductivity.
The ecohydrological role of soil texture in a water‐limited ecosystem
Authors: Fernandez‐Illescas Coral Pilar , Porporato Amilcare , Laio Franscesco , Rodriguez‐Iturbe Ignacio
Published: February 2004

Soil texture is a key variable in the coupled relationship between climate, soil, and vegetation. This coupling and its dependence on soil texture are studied in this paper using analytical descriptions for soil moisture dynamics and the corresponding vegetation water stress. Results confirm the importance of soil texture in partitioning rainfall into the mean values of the water balance loss components, namely, evapotranspiration, leakage, and runoff, and in determining vegetation water stress for the vegetation and climatic regimes of the La Copita Research Area in Texas. Two separate mechanisms by which this sensitivity to soil texture can impact the ecological structure at La Copita are illustrated. First, dynamics similar to the inverse texture effect, whereby the optimal soil texture for a given vegetation type changes with rainfall amount, are demonstrated for the two most common species at this site. Second, it is shown that soil texture plays a major role in the modulation of the impact that interannual rainfall fluctuations have on the fitness and coexistence of trees and grasses.
Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 2. Equivalent parameters, high‐resolution stochastic imaging and scale effects
Authors: Vesselinov Velimir V. , Neuman Shlomo P. , Illman Walter A.
Published: February 2004

In paper 1 of this two‐part series we described a three‐dimensional numerical inverse model for the interpretation of cross‐hole pneumatic tests in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. Our model is designed to analyze these data in two ways: (1) by considering pressure records from individual borehole monitoring intervals one at a time, while treating the rock as being spatially uniform, and (2) by considering pressure records from multiple tests and borehole monitoring intervals simultaneously, while treating the rock as being randomly heterogeneous. The first approach yields a series of equivalent air permeabilities and air‐ filled porosities for rock volumes having length scales ranging from meters to tens of meters, represented nominally by radius vectors extending from injection to monitoring intervals. The second approach yields a high‐resolution geostatistical estimate of how air permeability and air‐filled porosity, defined on grid blocks having a length scale of 1 m, vary spatially throughout the tested rock volume. It amounts to three‐dimensional pneumatic “tomography” or stochastic imaging of the rock. Paper 1 described the field data, the model, and the effect of boreholes on pressure propagation through the rock. This second paper implements our inverse model on pressure data from five cross‐hole tests at ALRS. We compare our cross‐hole test interpretations by means of the two approaches with earlier interpretations by means of type curves and with geostatistical interpretations of single‐hole test data. The comparisons show internal consistency between all pneumatic test interpretations and reveal a very pronounced scale effect in permeability and porosity at ALRS.
Robust simulation of root zone soil moisture with assimilation of surface soil moisture data
Authors: Montaldo Nicola , Albertson John D. , Mancini Marco , Kiely Ger
Published: February 2004

An operational framework is presented for assimilating surface soil moisture remote sensing measurements into a soil‐vegetation‐atmosphere‐transfer (SVAT) model for the robust prediction of root zone moisture time series. The proposed approach is based on analytical treatment of the dynamical equations coupling surface and deeper soil reservoirs. The resulting framework uses biases between observed and modeled time rates of change of surface soil moisture to quantify biases between modeled and actual root zone average soil moisture contents. The approach is based on the popular interactions between soil‐biosphere‐atmosphere (ISBA) force‐restore SVAT model. An experimental data set, collected near Cork, Ireland, is analyzed both for a long data series of 183 days and four short periods that were selected to focus on different hydrometeorological conditions. The results demonstrated that the proposed framework performs uniformly robust over 3 orders of magnitude of misspecification of saturated hydraulic conductivity. In the presence of uncertain initial conditions, the results demonstrated a marked increase in model skill (over the original ISBA model) for periods when average precipitation was less than average potential evaporation.
Assessment of dam trapping efficiency from water residence time: Application to fluvial sediment transport in the Adour, Dordogne, and Garonne River Basins (France)
Authors: Maneux Eric , Probst Jean Luc , Veyssy Eric , Etcheber Henri
Published: February 2004

Dam‐trapping efficiency can be estimated by using the hydraulic residence time. On the regional scale, the global impact of several dams can be assessed by taking into account the spatial organization of dams along the river network. Therefore, in this study, a method is proposed to estimate the global‐trapping efficiency, TEw, for any watershed containing more than one dam. This method is applied to the Adour, Dordogne, and Garonne River watersheds (southwestern France). The spatial organization of dams and its impact on TEw and on sediment yields measured at 37 TSM sampling stations over 2 years are discussed. Positive correlation between drainage areas and river sediment loads corrected from dam regulation using TEw, as well as comparisons between TEw‐corrected sediment yields and sediment yields measured upstream from dams, point out the interest of the method in order to reconstitute the natural sediment yields.
Patterns of nitrogen transport in streams of the Lake Tahoe Basin, California‐Nevada
Authors: Coats Robert N. , Goldman Charles R.
Published: February 2004

In an effort to characterize the spatial and temporal patterns of nitrogen concentration and load in streams of the Lake Tahoe basin, we analyzed 10 years of data from 10 streams, developing discharge‐concentration relationships and total load estimates for nitrate‐N, ammonium‐N, and organic N. The results indicate that (1) most of the nitrate transport occurs early in the snowmelt season or during large winter rainstorms; (2) dissolved organic nitrogen concentrations peak early in the runoff season, decline during snowmelt, and in some streams peak again during the summer low‐flow period; (3) organic nitrogen accounts for over 90% of the total nitrogen load in basin streams; (4) averaged over 10 years and the area of 10 watersheds, the nitrogen flux rates in kg ha−1 yr−1 are 0.081 for nitrate‐N, 0.017 for ammonium‐N, 0.58 for dissolved organic N, and 0.47 for particulate organic N; (5) the variation in annual runoff explains most of the interannual and interwatershed variability in total nitrogen load; and (6) the dominance of organic nitrogen relative to nitrate‐nitrogen in Tahoe basin streams contrasts with sites in eastern North America.
Curing the misbehavior of reservoir capacity statistics by controlling shortfall during failures using the modified Sequent Peak Algorithm
Authors: Adeloye A. J. , Montaseri M. , Garmann C.
Published: February 2004

In a recent article, Pretto et al. [1997] found that certain statistics of storage capacity estimates, for example, mean and higher quantiles, obtained using a behavior analysis do misbehave for medium record lengths in that they exhibit a hump in their functional relationships with the data record length. Further, this upward bias was observed to disappear at longer record lengths. Pretto et al. proffered several reasons for this apparently anomalous behavior. In this paper, we investigate another reason that we suspect might also be causing behavior analysis storage estimates to misbehave. In particular, we use a modified sequent peak methodology, which, unlike a behavior analysis, enables the analyst to restrict the amount of water shortfall during any failure period to fixed amounts. The results show that when the shortfall is made the same for all failure periods, the hump in the storage capacity statistics versus length of record functions largely disappears.
Use of April 1 SWE measurements as estimates of peak seasonal snowpack and total cold‐season precipitation
Authors: Bohr Gregory S. , Aguado Edward
Published: February 2004

Many previous studies of snow accumulation and ablation have been based on snow course data obtained at monthly intervals, using April 1 snow‐water equivalents (SWE) as surrogates for the total seasonal accumulation and for the maximum seasonal snowpack on the ground. This article uses daily snow pillow data to determine how well the April 1 SWEs represent the total accumulation and the maximum snowpack within the entire Rocky Mountain region and in individual subregions. April 1 SWE tends to underestimate the peak SWE by ∼6 cm (12%). April 1 SWE provides a more accurate estimate of the total seasonal precipitation, with mean errors of approximately −1.7 cm (4%) for most of the study area and approximately +2 cm (6%) in the eastern portion. The error incurred by the use of April 1 SWE in the estimation of peak SWE does not vary systematically with elevation, but errors in the estimation of total seasonal precipitation do.
Longitudinal profiles in simple alluvial systems
Authors: Rice Stephen P. , Church Michael
Published: February 2004

Theoretical considerations suggest that exponential or quadratic functions are apt for describing the longitudinal profiles of aggrading, alluvial systems that are unaffected by significant lateral inputs of water or sediment. A new set of field data confirms this for individual sedimentary links along a wandering gravel‐bed river in British Columbia, Canada. Each link is viewed as a simple alluvial system, without major water or sediment inputs, within which grain size typically fines downstream in a relatively systematic manner. Such homogeneous reaches are suitable for the investigation of simple profile form. It is found that quadratic approximations are the most flexible descriptor for link longitudinal profiles. Overall, the river forms an irregularly cuspate long profile structured by these fundamental length‐scale units. Both link gradients and link fining rate reflect size‐selective aggradation associated with the bounding lateral inputs.
Global optimization of water distribution networks through a reduced space branch‐and‐bound search
Authors: Costa André L. H. , de Medeiros José Luiz , Pessoa Fernando L. P.
Published: February 2004

A branch‐and‐bound approach to the problem of optimal design of water distribution networks is presented. Global optimum is reached through the generation of convergent sequences of upper and lower bounds. The relaxations responsible for the lower bounds correspond to linear programming problems formulated through the enlargement of the original feasible region by outer approximations of the constraints. Although formulated within an arc‐based framework, the proposed scheme does not apply the branching process to all flow variables; by utilizing the mass conservation principle, only a reduced set of variables is assigned for branching. The algorithm was applied to three variants of a classical problem from the literature. Comparisons with previous results indicates a faster convergence to the optimum (fewer linear programming problems solved) in several situations.
Modeling flow and transport in irrigation catchments: 2. Spatial application of subcatchment model
Authors: Connell L. D. , Jayatilaka C. J. , Nathan R.
Published: February 2004

An alternative to spatial distribution is presented for physically based process modeling of catchment flow and solute transport. The approach is based on an integral relation for catchment streamflow which describes the accumulation through the stream network of the hillslope contributions. This relation is redefined from integration with respect to position along the stream to an integral that is with respect to the physical properties determining the hillslope flow and transport with probability distributions to represent the range of parameter values and areas associated with these. Since the integrand is now a relatively smooth function of the variables of integration, standard numerical integration can be used to evaluate the catchment totals. The assumptions allowing this redefinition are that hillslope hydrology is primarily determined by driving forces that are local to the hillslope and that catchment flow can be lumped in time so that streamflow routing is not required. The interpolation of the integrand involved in the numerical integration results in significant savings over physically based spatially distributed catchment flow and transport models. The methodology is tested through application to an irrigated catchment where model simulations of flow and transport were found to be in close agreement with those observed.
Regional variations in small‐basin floods in the United States
Authors: Michaud Jené D. , Hirschboeck Katherine K. , Winchell Michael
Published: February 2004

This paper presents a reconnaissance study of regional flood patterns in the United States, focusing on peak discharges at several recurrence intervals and characteristics of flood‐causing rainfall. Because of an interest in flash floods, attention was restricted to watersheds between 10 and 200 km2 in area. Data were obtained from 130 stream gaging stations with a consistent 30‐year period of record and from reports documenting 90 exceptional floods occurring mostly on ungaged watersheds. Peak discharges vary considerably within local regions. Roughly 60% of the local variability can be explained by watershed characteristics, but watershed area is not a reliable predictor of peak discharge within the narrow range of watershed sizes examined. On a continental scale the spatial patterns of the median and 25‐year floods are similar. In both cases a concentration of large floods is found in the southeastern Great Plains and parts of the southeast. In the west, north, and northeast, floods tend to be small, but large floods still occur in scattered locations. The pattern and seasonality of the exceptional floods, which are presumed to have relatively long recurrence intervals, are different from the pattern of median and 25‐year floods. The largest of the exceptional floods are concentrated in the central and southern Great Plains during May and June. They occur farther west (and several months later) than the largest median floods. Exceptional floods occurring in the semiarid west were caused by as little as 5–10 cm of rain in 30–60 min, whereas in humid areas most of the exceptional floods resulted from 13–32 cm of rain in 1–12 hours.
Release and transport of colloidal particles in natural porous media: 1. Modeling
Authors: Grolimund Daniel , Borkovec Michal
Published: February 2004

Colloidal particles in natural porous media can be mobilized in situ through traveling chromatographic fronts. This process is often modeled by a single population of particles whose release and deposition is described by a first‐order kinetic process with fixed rate constants. We present a new, more realistic approach considering the physicochemical coupling between the corresponding rates and the (changing) chemical composition of the pore water. Neglecting particle deposition, analytical solutions are presented for various relevant situations. In the absence of dispersion the effluent curves are given by simple exponential functions; while in the presence of dispersion, more complicated relations are obtained. Particle deposition influences the release and transport of particles, and analytical and numerical solutions are discussed for this situation as well. In most practical situations, particle release is a strongly nonexponential process, which can be modeled by assuming multiple particle populations, whose properties are described with the corresponding probability distribution. Such a model can be used to rationalize the experimentally observed nonexponential decay.
Pricing ecological services: Willingness to pay for drought mitigation from watershed protection in eastern Indonesia
Authors: Pattanayak Subhrendu K. , Kramer Randall A.
Published: February 2004

In this study we estimate local economic values of ecological services provided by protected forest watersheds in Ruteng Park in eastern Indonesia. Our use of contingent valuation (CV) methodology for pricing drought mitigation benefits to local farmers extends previous work by deriving measures of willingness to pay in terms of incremental agricultural profits. On the basis of the theoretical and content validity of estimated models we find that CV can be used to value complex ecological services in a rural developing country setting. The estimated parameters provide policy and management information regarding the economic magnitude and spatial distribution of the value of drought mitigation.
Generation of stable microbubbles and their transport through porous media
Authors: Wan Jiamin , Veerapaneni Srinivas , Gadelle Fred , Tokunaga Tetsu K.
Published: February 2004

Laboratory experiments were conducted to generate stable microbubbles and to measure their transport properties in porous media in order to evaluate their possible use in subsurface remediation. A mathematic model was developed as a special case of filtration theory for predicting microbubble transport in porous media. Several physical‐chemical parameters were tested, including surfactant type, surfactant concentration, generation method, and pressure. A combination of an anionic surfactant and a hydrophobic nonionic surfactant was found to yield the highest concentrations of stable microbubbles (3 × 109 bubbles mL−1) in the size range of 0.7 to 20 μm. The specific surface areas of the microbubble suspensions were in the range of 50 cm2 mL−1, and specific air volumes were >0.07 mL mL−1. For 1 pore volume of injected bubble suspension, effluent recoveries of 100, 80, and 30% were achieved from columns of coarse (415–500 μm), medium (150–212 μm), and fine (53–106 μm) sands, respectively. Effluent recovery in the fine sand column increased to 63% following a 3 pore volume injection. Microbubble generation and injection under pressure were shown to minimize microbubble loss due to gas dissolution. Results from the modeling suggest that retention of microbubbles could be adequately described by filtration theory.
Tracer tests in a fractured dolomite: 3. Double‐porosity, multiple‐rate mass transfer processes in convergent flow tracer tests
Authors: McKenna Sean A. , Meigs Lucy C. , Haggerty Roy
Published: February 2004

Convergent flow tracer tests conducted in the Culebra dolomite (Rustler Formation, New Mexico) are analyzed with both single‐ and multiple‐rate, double‐porosity models. Parameter estimation is used to determine the mean and standard deviation of a lognormal distribution of diffusion rate coefficients as well as the advective porosity and longitudinal dispersivity. At two different test sites both multirate and single‐rate models are capable of accurately modeling the observed data. The single‐well injection‐sswithdrawal test provides more precise estimates of the mass transfer parameters than the convergent flow tracer tests. Estimation of the multirate distribution parameters is consistent across locations for the two types of tests. Limits of resolution are calculated for the multirate distribution, and these limits explain the precision with which the standard deviation of the multirate distribution can be estimated. These limits also explain the necessary increase in the advective porosity for the single‐rate model at one location and not the other. Implications of the multirate mass transfer model at time and length scales greater than those of the tracer tests include the instantaneous equilibrium of a significant fraction of the matrix and the possibility of a fraction of the diffusive porosity not reaching an equilibrium solute concentration at long times.
Tracer tests in a fractured dolomite: 2. Analysis of mass transfer in single‐well injection‐withdrawal tests
Authors: Haggerty Roy , Fleming Sean W. , Meigs Lucy C. , McKenna Sean A.
Published: February 2004

We investigated multiple‐rate diffusion as a possible explanation for observed behavior in a suite of single‐well injection‐withdrawal (SWIW) tests conducted in a fractured dolomite. We first investigated the ability of a conventional double‐porosity model and a multirate diffusion model to explain the data. This revealed that the multirate diffusion hypothesis/model is consistent with available data and is capable of matching all of the recovery curves. Second, we studied the sensitivity of the SWIW recovery curves to the distribution of diffusion rate coefficients and other parameters. We concluded that the SWIW test is very sensitive to the distribution of rate coefficients but is relatively insensitive to other flow and transport parameters such as advective porosity and dispersivity. Third, we examined the significance of the constant double‐log late time slopes (−2.1 to −2.8), which are present in several data sets. The observed late time slopes are significantly different than would be predicted by either conventional double‐porosity or single‐porosity models and are believed to be a distinctive feature of multirate diffusion. Fourth, we found that the estimated distributions of diffusion rate coefficients are very broad, with the distributions spanning a range of up to 3.6 orders of magnitude. Fifth, when both heterogeneity and solute drift are present, late time behavior similar to multirate mass transfer can occur. Although it is clear that multirate diffusion occurs in the Culebra, the number of orders of magnitude of variability may be overestimated because of the combined effects of drift and heterogeneity.
Tracer tests in a fractured dolomite: 1. Experimental design and observed tracer recoveries
Authors: Meigs Lucy C. , Beauheim Richard L.
Published: February 2004

A series of tracer tests has been conducted in a 7‐m‐thick fractured dolomite at two sites in southeastern New Mexico. The tests were designed to evaluate transport processes, especially matrix diffusion, in fractured, permeable media. Both single‐well injection‐withdrawal (SWIW) and multiwell convergent flow (MWCF) tests were conducted. Seventeen different organic tracers (he fluorobenzoic and chlorobenzoic acids) and iodide were used as conservative tracers for the tests. The MWCF tests included repeated tracer injections while pumping the central well at different rates, injection of tracers with different aqueous diffusion coefficients, and injection of tracers into both the full and partial formation thickness. This paper describes the tracer test sites and aquifer characteristics, the experimental methods, and the tracer data produced. The tracer test results provide a high‐quality data set for a critical evaluation of the conceptual model for transport. Both the SWIW and MWCF tracer test data showed gradual mass recovery and breakthrough (or recovery) curve tailing consistent with matrix diffusion. However, the SWIW recovery curves did not display the −1.5 log‐log slope expected from a conventional double‐porosity medium with a single rate of diffusion. The breakthrough curves from MWCF tests conducted at two different pumping rates showed similar peak heights, which is also not what was expected with a conventional double‐porosity model. However, the peak heights were different for two tracers with different aqueous diffusion coefficients that were injected simultaneously in one test, consistent with the effects of matrix diffusion. The complexity of the tracer test results suggests that a simple double‐ porosity conceptual model for transport in the Culebra with a single rate of diffusion is overly simplistic.
Laboratory measurements of electromagnetic wave velocity in layered sands
Authors: Chan Christina Y. , Knight Rosemary J.
Published: February 2004

The velocity of an electromagnetic (EM) wave in a system of layered geologic materials is determined by the velocities and volume fractions of the individual layers and by the ratio between the wavelength of the EM wave (λ) and the average layer thickness (t). For the case λ < t the EM wave velocity is given by ray theory; for the case λ > t the velocity is given by effective medium theory (EMT). Previous numerical studies have shown the transition from ray theory to effective medium theory to occur at λ/t approximately equal to 4. Using laboratory time domain reflectometry (TDR), we measured the EM velocities of six layered samples composed of 50% dry coarse sand and 50% saturated fine sand. The λ/t for these measurements range from ∼0.5 to 25. For λ/t < 4 we measured the faster EM velocity predicted by ray theory; and when λ/t > 4, we measured the slower EM velocity predicted by EMT. Reasonable agreement is found with numerical modeling of the experiments. In these experiments a 20% change in velocity is caused by a change in the thickness of the layers, with no change in the volume fraction of the coarse and fine sands. This illustrates the importance of accounting for heterogeneity in the interpretation of EM velocity.
Processes regulating the solute concentrations of snowmelt runoff in two subalpine catchments of the Sierra Nevada, California
Authors: Williams Michael R. , Leydecker Al , Brown Aaron D. , Melack John M.
Published: February 2004

Geochemical processes regulating solute concentrations in snowmelt runoff were investigated for the snowmelt periods of 1992 and 1993 in two subalpine catchments in Sequoia National Park, California. The catchments, 0.5 and 0.2 ha, have 10 and 25% soil coverage, respectively; the remaining area is exposed granodiorite bedrock. Mineral weathering was the major source of solutes in runoff. Calcium export in excess of stoichiometric plagioclase weathering was attributed to dry deposition and the weathering of mafic minerals and disseminated calcite. Cation exchange was important in the regulation of K and increased base cation export in response to rain‐on‐snow events and LiBr tracer. Sulfate fluxes were attributed to snowpack elution and SO4 desorption after the beginning of melt. The selective retention of Li during tracer experiments suggests that acid‐neutralizing capacity is linked to soil stocks along convoluted flow paths and is not necessarily compromised in areas of sparse soil cover.
Evaluation of an integrated biogeochemical model (PnET‐BGC) at a northern hardwood forest ecosystem
Authors: Gbondo‐Tugbawa Solomon S. , Driscoll Charles T. , Aber John D. , Likens Gene E.
Published: February 2004

An integrated biogeochemical model (PnET‐BGC) was formulated to simulate chemical transformations of vegetation, soil, and drainage water in northern forest ecosystems. The model operates on a monthly time step and depicts the major biogeochemical processes, such as forest canopy element transformations, hydrology, soil organic matter dynamics, nitrogen cycling, geochemical weathering, and chemical equilibrium reactions involving solid and solution phases. The model was evaluated against soil and stream data at the Hubbard Brook Experimental Forest, New Hampshire. Model predictions of concentrations and fluxes of major elements generally agreed reasonably well with measured values, as estimated by normalized mean error and normalized mean absolute error. Model output of soil base saturation and stream acid neutralizing capacity were sensitive to parameter values of soil partial pressure of carbon dioxide, soil mass, soil cation exchange capacity, and soil selectivity coefficients of calcium and aluminum. PnET‐BGC can be used as a tool to evaluate the response of soil and water chemistry of forest ecosystems to disturbances such as clear‐cutting, climatic events, and atmospheric deposition.
Information content of time domain reflectometry waveforms
Authors: Weerts A. H. , Huisman J. A. , Bouten W.
Published: February 2004

The possibility to link model parameters with soil properties is obstructed if identification problems in inverse modeling of time domain reflectometry (TDR) waveforms occur. Therefore multiple objective functions, each associated with one model parameter, are sought for identification of probe (resistance and length) and Debye (static permittivity, high‐frequency permittivity, relaxation frequency, and bulk electrical conductivity) parameters in TDR waveforms. An objective function could be defined for all six model parameters. Some of the defined objective functions correspond with theoretical and experimental observations of earlier work. Other new insights have been obtained. For instance, it is possible to obtain estimates of high‐frequency permittivity even when the reflection of the end of the probe has vanished because of high salinity. Application of the found objective functions on measured TDR waveforms yielded parameter values that compared well with values obtained from several references. However, identification of a parameter is only possible if its true value lies within the frequency bandwidth of TDR.
Adjoint‐derived location and travel time probabilities for a multidimensional groundwater system
Authors: Neupauer Roseanna M. , Wilson John L.
Published: February 2004

Backward location and travel time probabilities can be used to determine the former location of contamination in an aquifer. For a contaminant parcel that was detected in an aquifer the backward location probability describes its position at some time prior to sampling, and the backward travel time probability describes the amount of time required for it to travel to the sampling location from some upgradient position. These probabilities, which can provide information about the source of contamination, are related to adjoint states of resident concentration. The governing equations of the backward probabilities are adjoints of the forward governing equation, e.g., the advection‐dispersion equation. We derive these backward governing equations and their boundary and final conditions for both location and travel time probabilities in a multidimensional system. Each governing equation contains the adjoint of the advection‐dispersion operator and a load term that defines the particular adjoint state (probability). The load term depends on both the type of probability (location or travel time) and the sampling device (pumping well or monitoring well) with which the contamination was detected. The adjoint equation can also be used to efficiently determine forward location and travel time probabilities describing the future location of groundwater contamination, a feature most useful for delineating pumping well captures zones. We illustrate the use of the backward model for obtaining location and travel time probabilities in a hypothetical two‐ dimensional domain.
Impact of variable reservoir releases on management of downstream water temperatures
Authors: Carron John C. , Rajaram Harihar
Published: February 2004

A coupled unsteady flow and heat transport model is used to determine the impacts of fluctuating reservoir releases on downstream water temperatures. Maintenance of stream temperatures is one of the most common reasons cited for imposition of minimum flow requirements in regulated (reservoir controlled) rivers. Minimum flow constraints for temperature control are typically developed using worst‐case scenarios (i.e., maximum air temperature, clear sky, etc.) of atmospheric conditions. We show that short‐ term modifications to reservoir releases based on local meteorological conditions can reduce the volume of water released, while still meeting temperature objectives. A case study of the Green River below Flaming Gorge Dam shows that for certain sets of temperature objectives and atmospheric conditions, a diurnally varying release may be the only way to meet multiple temperature objectives at different downstream locations. In the examples discussed, savings of nearly 20% in total release volume could be realized by using variable releases.
Downstream variations in the width of bedrock channels
Authors: Montgomery David R. , Gran Karen B.
Published: February 2004

Field surveys of channel width w and drainage area A in bedrock channel reaches reveal relationships where w = cAb, similar to the classic hydraulic geometry of alluvial channels. Data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b = 0.3–0.5 holds for bedrock channel systems. Although there is substantial local variability in channel width in bedrock channel systems, there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal Oregon and Washington. In contrast, bedrock channels were narrower, and therefore had deeper flow, than alluvial channels with equal drainage areas in the granite and limestone terrain of the Yuba River, California. In addition, data from the Mokelumne River show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion‐resistant granite, but that channel slope does not change appreciably across contacts between these two lithologies. Data from coastal Oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts. We conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history.
An analytic, approximate method for modeling steady, three‐dimensional flow to partially penetrating wells
Authors: Bakker Mark
Published: February 2004

An analytic, approximate solution is derived for the modeling of three‐dimensional flow to partially penetrating wells. The solution is written in terms of a correction on the solution for a fully penetrating well and is obtained by dividing the aquifer up, locally, in a number of aquifer layers. The resulting system of differential equations is solved by application of the theory for multiaquifer flow. The presented approach has three major benefits. First, the solution may be applied to any groundwater model that can simulate flow to a fully penetrating well; the solution may be superimposed onto the solution for the fully penetrating well to simulate the local three‐dimensional drawdown and flow field. Second, the approach is applicable to isotropic, anisotropic, and stratified aquifers and to both confined and unconfined flow. Third, the solution extends over a small area around the well only; outside this area the three‐dimensional effect of the partially penetrating well is negligible, and no correction to the fully penetrating well is needed. A number of comparisons are made to existing three‐dimensional, analytic solutions, including radial confined and unconfined flow and a well in a uniform flow field. It is shown that a subdivision in three layers is accurate for many practical cases; very accurate solutions are obtained with more layers.
Bayesian recursive parameter estimation for hydrologic models
Authors: Thiemann M. , Trosset M. , Gupta H. , Sorooshian S.
Published: February 2004

The uncertainty in a given hydrologic prediction is the compound effect of the parameter, data, and structural uncertainties associated with the underlying model. In general, therefore, the confidence in a hydrologic prediction can be improved by reducing the uncertainty associated with the parameter estimates. However, the classical approach to doing this via model calibration typically requires that considerable amounts of data be collected and assimilated before the model can be used. This limitation becomes immediately apparent when hydrologic predictions must be generated for a previously ungauged watershed that has only recently been instrumented. This paper presents the framework for a Bayesian recursive estimation approach to hydrologic prediction that can be used for simultaneous parameter estimation and prediction in an operational setting. The prediction is described in terms of the probabilities associated with different output values. The uncertainty associated with the parameter estimates is updated (reduced) recursively, resulting in smaller prediction uncertainties as measurement data are successively assimilated. The effectiveness and efficiency of the method are illustrated in the context of two models: a simple unit hydrograph model and the more complex Sacramento soil moisture accounting model, using data from the Leaf River basin in Mississippi.
Analytical solutions for linearized Richards Equation with arbitrary time‐dependent surface fluxes
Authors: Chen Jiann‐Mou , Tan Yih‐Chi , Chen Chu‐Hui , Parlange J.‐Y.
Published: February 2004

Watrick's [1975] solution was for general time dependence although the analytical form was for a constant flux or step functions. A new method has been extended to derive analytical solutions of a linearized Richards' equation for arbitrary input fluxes before surface saturation. The new solution also determines the time to surface saturation.
Stochastic frontier approach to measuring irrigation performance: An application to rice production under the two systems in the Tarai of Nepal
Authors: Sharma Khem R. , Pradhan Naresh C. , Leung PingSun
Published: February 2004

This study employs the stochastic frontier production function approach to measuring the productive performance of canal irrigation in terms of farm‐specific technical efficiency. The 1993 production data from samples of rice farmers from Khageri (government managed) and Pithuwa (farmer managed) irrigation systems in the Tarai of Nepal were analyzed using a stochastic production frontier, including a model for the technical inefficiency effects. Relative both to a common or pooled production frontier for the two systems and a separate or own frontier for each system, on average, the farmers from the farmer‐managed system were found to be more efficient than those from the government system. Furthermore, the government system showed higher spatial variability in technical efficiency in rice production than the farmer‐managed one, which suggests that irrigation management has a significant influence on production and hence on the distribution of productivity gains among the farmers. However, better performance of the farmer‐managed irrigation system relative to the government system should be weighed against the higher transaction costs associated with irrigation management by farmers. The results indicate that substantial potential exists for increasing rice production by increasing the application of chemical fertilizer, the planting of seed, and the use of irrigation and by improving technical efficiency at the farm level.