bluuebunny/crossref_metadata_2025 huggingface dataset:
Benefit transfer protocol for long‐term health risk valuation: A case of surface water contamination
Authors: Kask Susan B. , Shogren Jason F.
Published: February 2004

In response to scarce financial resources, economists have promoted the concept of benefit transfer as a cost‐effective alternative to new nonmarket valuation studies. Recent discussion on benefit transfer for improved water quality has focused on recreational benefits. While useful, the discussion must now be expanded to include another key benefit from improved water quality: the reduction in risk to public health. This paper develops a protocol for benefit transfer of long‐term health risk reduction and presents a case study for surface water contamination. Challenges such as the multiple sources of risk, the mortality and morbidity effects indicated by a variety of symptoms, the long latency period between cause and effect, and an individual's ability to privately or collectively reduce the probability or severity of the risk are discussed.
Covariance properties of Great Lakes annual net basin supplies
Authors: Buchberger Steven G.
Published: February 2004

The cross‐correlation function (CCF) and autocorrelation function (ACF) for Great Lakes annual net basin supplies are derived under the assumptions that annual lake outflows and water levels are autoregressive lag 1 processes and that each lake behaves as a linear reservoir. Except for the pairing between Lakes Superior and Michigan‐Huron, there is reasonable agreement between the sample and predicted CCFs, especially among net basin supplies to the small lower basin lakes. The derived ACFs reduce to an expression identical in form to the ACF for an ARMA(1, 1) process at all the Great Lakes except Lake Michigan‐Huron. At the upper basin lakes, sample and predicted ACFs drop to zero rapidly. At the lower basin lakes the ACFs exhibit a much more gradual decay, suggesting the presence of long‐term persistence. Prominent tails in the ACFs of the annual net basin supplies have been attributed to historical shifts in the precipitation regime at the lower basin lakes. Results from this study show that the residual method currently used to estimate net basin supplies can also induce a similar artificial long tail in the ACF. This observation has important ramifications in efforts to simulate Great Lakes water levels, since simulation results are quite sensitive to the covariance structure of the annual net basin supplies.
Modeling reactive transport of organic compounds in groundwater using a partial redox disequilibrium approach
Authors: McNab W. W. , Narasimhan T. N.
Published: February 2004

The chemical transformation of organic contaminants in natural groundwater systems is clearly dependent upon local geochemistry which determines the thermodynamically favorable degradation reactions and the nature of local microbial populations. Conversely, groundwater geochemistry may be impacted significantly in terms of pH and redox couple speciation by the chemical transformation of sufficient quantities of organic compounds. Therefore an understanding of the coupling between degradation reactions, local geochemistry, and chemical transport is essential in predicting the chemical evolution of contaminated aquifers. Equilibrium‐based reactive chemical transport models are usually not utilized for problems involving the transport of degradable organic compounds due to slow reaction kinetics and the persistence of intermediate degradation products. In this study we propose a reactive geochemical transport model which considers these types of degradation reactions. An expert system approach is used to postulate a set of sequential, first‐order degradation reactions for the organic compounds based upon thermodynamic considerations and user‐defined rules. Redox disequilibrium provides the driving force for the abiotic or microbially mediated transformation of the organic compounds as well as the associated response of groundwater geochemistry. Coupling between local inorganic geochemistry and reacting organic compounds is achieved by assuring conservation of operational valence and mass balance. The composite geochemical model is in turn coupled with an integral finite difference transport algorithm using a two‐step sequential solution approach. The transport equation is solved separately for each inorganic aqueous species, complex, and dissolved organic species, allowing a high degree of flexibility in problem definition. We apply the model to an illustrative example problem concerning the introduction of aromatic hydrocarbons and chlorinated ethenes into an initially oxidizing aquifer. Modeling results agree well qualitatively with field and laboratory observations reported in the literature in terms of degradation patterns and the effects on groundwater geochemistry.
Eulerian‐Lagrangian approach for modeling of flow and transport in heterogeneous geological formations
Authors: Bellin Alberto , Rubin Yoram , Rinaldo Andrea
Published: February 2004

This paper presents a new Eulerian‐Lagrangian method for modeling flow and transport of passive solutes in heterogeneous porous formations. The physically plausible random velocity fields are generated by a geostatistical based model. The ability of the method to correctly reproduce the velocity field statistics and to satisfy mass balance is tested and demonstrated for the case of two‐dimensional flow. The method is found particularly useful in dealing with large fields where numerical, fixed grid methods such as finite difference or finite elements become very demanding from a computational point of view. The transport problem is solved in a Lagrangian framework by the particle‐tracking approach which uses a suitable grid refinement to correctly handle the velocity fluctuation at any a priori defined resolution. The new method is employed to compute the probability distribution functions of the concentration and travel times and to investigate the limitations of existing methods for predicting concentrations.
Apparatus for the rapid automated measurement of unsaturated soil transport properties
Authors: Salehzadeh A. , Demond A. H.
Published: February 2004

To describe the flow of water in unsaturated soils, the constitutive relationships of capillary pressure and relative permeability are necessary. To measure these relationships using traditional methods may require different experimental apparatuses and can be time‐consuming, particularly for multiple drainage and imbibition relationships for systems containing surfactants at various concentrations. Thus a critical need exists for an apparatus that can make rapid measurements. This paper outlines the design of an automated apparatus that can produce both capillary pressure and relative permeability relationships simultaneously and rapidly. The rapidity of the measurements is based on the use of a thin soil sample, a highly conductive capillary barrier, and stripper tensiometers. The capillary pressure is changed externally, with the change within the sample monitored through the use of tensiometers. Saturation is determined by monitoring the cumulative effluent, with the accuracy of the mass balance enhanced through the active removal of air bubbles from beneath the capillary barrier. A comparison of capillary pressure relationships for a fine‐grained sandy porous medium produced with this apparatus with those produced with a Tempe cell shows that this apparatus yields comparable measurements in about 2% of the time. Thus the burden of making multiple transport property measurements can be reduced considerably through the use of this apparatus.
Evaluation of cluster‐based rectangular pulses point process models for rainfall
Authors: Velghe T. , Troch P. A. , De Troch F. P. , Van de Velde J.
Published: February 2004

This technical note presents a comparison of cluster‐based point rainfall models using the historical hourly rainfall data observed between 1949 and 1976 at Denver, Colorado. The Denver data are used to analyze the performance of three classes of models, namely, the Bartlett‐Lewis model, the geometric Neyman‐Scott model and the Poisson Neyman‐Scott model. The original formulation of the structure of each model, as well as the modified description developed in order to improve the zero depth probability, is considered in this study. Rodriguez‐Iturbe et al.(1987a) concluded that it is unlikely that empirical analysis of rainfall data can be used to choose between the Bartlett‐Lewis model and the Neyman‐Scott model. In a subsequent paper, Rodriguez‐Iturbe et al. (1987b) argued that the choice of the distribution of the number of cells per storm for the Neyman‐Scott model, either geometric or Poisson, has no general bias effect on the stochastic structure. Some investigators (e.g., Burlando and Rosso, 1991), however, reported results contradictory to those of the previous authors. In light of these observations this note investigates the performance of the cluster‐based models. For the Denver data the geometric Neyman‐Scott model yields better results compared to the Poisson Neyman‐Scott model. Moreover, the Bartlett‐Lewis model is shown to be very sensitive to the sets of moment equations used in the parameter estimation. This sensitivity is not observed in the Neyman‐Scott scheme and is believed to be a drawback for applying the Bartlett‐Lewis model in hydrologic simulation studies.
Application of the mixed hybrid finite element approximation in a groundwater flow model: Luxury or necessity?
Authors: Mosé R. , Siegel P. , Ackerer P. , Chavent G.
Published: February 2004

Selected groundwater flow scenarios are used in a two‐way comparison between the mixed hybrid finite element method and the standard finite element method (also called the conforming finite element method). The simulations presented are performed in the bidimensional case with a triangular space discretization because of its practical interest for hydrogeologists. The basic idea of the mixed procedure is to approximate both the hydraulic potential and the velocity simultaneously and to satisfy an exact water balance for each element. By contrast, the conforming finite element method calculates the potential field everywhere and then calculates the velocity by differentiation of the potential. The conventional approach results in an elementwise constant velocity which can be subject to significant problems because of the normal component discontinuity of the velocity. The mixed hybrid finite element method provides velocities everywhere in the field, as well as potentials at the center of each element and each edge. Moreover, the normal component of the velocity field is continuous between adjacent elements. The results of the simulations are presented in the form of streamlines. To avoid the problem of velocity discontinuity, the method of Cordes and Kinzelbach (1992) is used; it allows the construction of a continuous velocity field from potentials obtained by the conforming finite element method. The comparison studies show that the mixed hybrid finite element is superior to the conforming method in terms of accuracy. It is also superior to the conforming method in terms of computational effort. The potential fields obtained by the mixed hybrid and the conforming finite element methods are the same.
Investigation of flow and transport in certain cases of nonstationary conductivity fields
Authors: Rubin Yoram , Seong Kwanjae
Published: February 2004

This paper describes an investigation of flow and transport phenomena in heterogeneous, nonstationary formations, where the log conductivity is the superposition of a linear trend and a stationary fluctuation. The problem of flow and transport in a steady, two‐dimensional flow without recharge is investigated analytically for an infinite flow domain. General results are obtained for any space dimensionality, but applications were limited to the case of two‐dimensional flow in the horizontal plane. The log conductivity field is assumed to have a linear trend. We limited the scope of this study to the cases where the trend is either parallel or normal to the head gradient and to small variance in the log conductivity. This paper presents results for the moments of the velocity and the hydraulic head, the effective conductivity, macrodispersion coefficients, solute mass breakthrough curves, and travel time distributions, all in terms of the parameters of the log conductivity field and the boundary conditions. The effects of the trend in the conductivity on these results are analyzed and compared with those obtained when the apparent trend is neglected. The results may be used for predicting transport phenomena extending over distances of many integral scales, as well as for interpretation of field data where the presence of a trend is suspected.
Characterization of a High‐Transmissivity Zone by Well Test Analysis: Steady State Case
Authors: Tiedeman Claire R. , Hsieh Paul A. , Christian Sarah B.
Published: February 2004

A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer composed of two homogeneous zones with contrasting transmissivities. Zone 1 is laterally unbounded and encloses zone 2, which is elliptical in shape and is several orders of magnitude more transmissive than zone 1. The solution for head is obtained by the boundary integral equation method. Nonlinear least squares regression is used to estimate the model parameters, which include the transmissivity of zone 1, and the location, size, and orientation of zone 2. The method is applied to a hypothetical aquifer where zone 2 is a long and narrow zone of vertical fractures. Synthetic data are generated from three different well patterns, representing different areal coverage and proximity to the fracture zone. When zone 1 of the hypothetical aquifer is homogeneous, the method correctly estimates all model parameters. When zone 1 is a randomly heterogeneous transmissivity field, some parameter estimates, especially the length of zone 2, become highly uncertain. To reduce uncertainty, the pumped well should be close to the fracture zone, and surrounding observation wells should cover an area similar in dimension to the length of the fracture zone. Some prior knowledge of the fracture zone, such as that gained from a surface geophysical survey, would greatly aid in designing the well test.
Hydrograph Separation Using Continuous Open System Isotope Mixing
Authors: Harris David M. , McDonnell Jeffrey J. , Rodhe Allan
Published: February 2004

We propose a new hydrograph separation method for runoff source modeling based on continuous open system isotope mixing using a variable source area and three isotopic reservoirs. The three reservoirs are (1) direct precipitation on saturated areas, (2) a near‐stream saturated zone, and (3) subsurface water in upslope areas. The reservoir sizes and isotopic compositions evolve due to continuous, open system mixing. Streamflow is composed of rainfall on saturated areas and water exfiltrated from the near‐stream saturated zone. The contributing source fractions are determined using the observed isotopic compositions of the stream and the modeled isotopic compositions of the contributing reservoirs, which are functions of time. The near‐stream saturated zone receives inflow from upslope areas which are not surface saturated. The surface‐saturated area changes dynamically based on the difference between lateral inflow and contributions to streamflow from the near‐stream saturated zone. The upslope zone (reservoir 3) evolves by mixing with rainfall. This hydrograph separation method requires (1) a function which relates the amount of water stored in the near‐stream saturated zone to saturated area, (2) a function which relates saturated area fraction to total streamflow, and (3) knowledge of the water storage capacity in reservoirs 2 and 3. The hydrograph separation method was applied to oxygen isotope data for rainfall and runoff during a 36‐hour storm at the Gårdsjön F1 catchment in southwestern Sweden. The hydrograph separation method was used to estimate time‐dependent streamflow contributions due to overland flow of storm rainfall and subsurface flow of mainly preevent water. We also used the separation method to estimate parameters for relations between saturated area fraction and streamflow, and saturated area and subsurface water storage. We show that streamflow contributions from overland flow may be described by a simple analytic function of antecedent conditions and catchment parameters.
A Control Volume Model of Solute Transport in a Single Fracture
Authors: Kennedy Christopher A. , Lennox William C.
Published: February 2004

A control volume model of solute transport through a single fracture in a porous matrix is developed. Application to problems of contaminant transport through fractured clay demonstrates several strong features of the method. The control volume approach inherently conserves mass and treats dispersivity at interfaces in a physically correct manner. By employing an upstream weighting scheme, based on the exact solution to the one‐dimensional steady state advection‐dispersion equation, the model proves to be more efficient than previous single‐fracture models. The significance of matrix diffusion in the direction parallel to the fracture axis is investigated. For the transport of a nonreactive tracer through a 20‐micrometer‐wide fracture in clay material, analytical solutions based on one‐dimensional matrix diffusion are erroneous for flow velocities of less than 1 m/day. The influence of boundary conditions on two‐dimensional matrix diffusion is considered, and the clean‐up of a contaminated fracture is simulated.
Diffusion of tritium in arid disposal sites
Authors: Smiles D. E. , Gardner W. R. , Schulz R. K.
Published: February 2004

Tritiated water, released at a point in a uniform, relatively dry soil, diffuses in both the liquid and vapor phases. A model which accords well with published data indicates that the flux density of tritium in the liquid exceeds that in the vapor phase provided the water content is greater than approximately 20% of the total soil porosity. Thus tritium redistribution should be modeled recognizing transfer “in parallel” in both phases. The diffusion equation cast in spherical coordinates, and taking into account radioactive decay, then provides a basis for design of field experiments and predicts the long‐term fate of tritiated water released in these experiments. We calculate the evolution of profiles of tritium concentration, within and external to the sphere of released solution, assuming the initial concentration to be uniform. We also predict the speed and the envelope of the tritium maximum as it advances and attenuates in the soil. We briefly discuss effects of variation in the volume fractions of soil water and air on the effective diffusion coefficient of tritium in soil and comment on possible effects of convective transfer by moving water and air.
Using Genetic Algorithms to Solve a Multiobjective Groundwater Monitoring Problem
Authors: Cieniawski Scott E. , Eheart J. Wayland , Ranjithan S.
Published: February 2004

This paper builds on the work of Meyer and Brill (1988) and subsequent work by Meyer et al. (1990, 1992) on the optimal location of a network of groundwater monitoring wells under conditions of uncertainty. We investigate a method of optimization using genetic algorithms (GAs) which allows us to consider the two objectives of Meyer et al. (1992), maximizing reliability and minimizing contaminated area at the time of first detection, separately yet simultaneously. The GA‐based solution method has the advantage of being able to generate both convex and nonconvex points of the trade‐off curve, accommodate nonlinearities in the two objective functions, and not be restricted by the peculiarities of a weighted objective function. Furthermore, GAs have the ability to generate large portions of the trade‐off curve in a single iteration and may be more efficient than methods that generate only a single point at a time. Four different codings of genetic algorithms are investigated, and their performance in generating the multiobjective trade‐off curve is evaluated for the groundwater monitoring problem using an example data set. The GA formulations are compared with each other and also with simulated annealing on both performance and computational intensity. Simulated annealing relies on a weighted objective function which can find only a single point along the trade‐off curve for each iteration, while all of the multiple‐objective GA formulations are able to find a larger number of convex and nonconvex points of trade‐off curve in a single iteration. Each iteration of simulated annealing is approximately five times faster than an iteration of the genetic algorithm, but several simulated annealing iterations are required to generate a trade‐off curve. GAs are able to find a larger number of nondominated points on the trade‐off curve, while simulated annealing finds fewer points but with a wider range of objective function values. None of the GA formulations demonstrated the ability to generate the entire trade‐off curve in a single iteration. Through manipulation of GA parameters certain sections of the trade‐off curve can be targeted for better performance, but as performance improves at one section it suffers at another. Run times for all GA formulations were similar in magnitude.
A comparison of Picard and Newton iteration in the numerical solution of multidimensional variably saturated flow problems
Authors: Paniconi Claudio , Putti Mario
Published: February 2004

Picard iteration is a widely used procedure for solving the nonlinear equation governing flow in variably saturated porous media. The method is simple to code and computationally cheap, but has been known to fail or converge slowly. The Newton method is more complex and expensive (on a per‐iteration basis) than Picard, and as such has not received very much attention. Its robustness and higher rate of convergence, however, make it an attractive alternative to the Picard method, particularly for strongly nonlinear problems. In this paper the Picard and Newton schemes are implemented and compared in one‐, two‐, and three‐dimensional finite element simulations involving both steady state and transient flow. The eight test cases presented highlight different aspects of the performance of the two iterative methods and the different factors that can affect their convergence and efficiency, including problem size, spatial and temporal discretization, initial solution estimates, convergence error norm, mass lumping, time weighting, conductivity and moisture content characteristics, boundary conditions, seepage faces, and the extent of fully saturated zones in the soil. Previous strategies for enhancing the performance of the Picard and Newton schemes are revisited, and new ones are suggested. The strategies include chord slope approximations for the derivatives of the characteristic equations, relaxing convergence requirements along seepage faces, dynamic time step control, nonlinear relaxation, and a mixed Picard‐Newton approach. The tests show that the Picard or relaxed Picard schemes are often adequate for solving Richards' equation, but that in cases where these fail to converge or converge slowly, the Newton method should be used. The mixed Picard‐Newton approach can effectively overcome the Newton scheme's sensitivity to initial solution estimates, while comparatively poor performance is reported for the various chord slope approximations. Finally, given the reliability and efficiency of current conjugate gradient‐like methods for solving linear nonsymmetric systems, the only real drawback of using Newton rather than Picard iteration is the algebraic complexity and computational cost of assembling the derivative terms of the Jacobian matrix, and it is suggested that both methods can be effectively implemented and used in numerical models of Richards' equation.
Influence of evaporation on waste package environment and radionuclide release from a tuff repository
Authors: Walton John C.
Published: February 2004

Conceptual and mathematical models are presented for predicting waste package environment and radionuclide release in an unsaturated repository. Presence and ionic strength of water are viewed as a balance between water influx, evaporation, and condensation as modified by salinity‐caused vapor pressure lowering. The model is used to examine the influence of thermal loading on waste package environment. Increased thermal gradients near the waste packages are shown to be as important as overall temperature in drying the waste package. The waste package environment is predicted to be highly variable, ranging from a concentrated brine to more dilute solutions. Under certain sets of conditions, water vapor condensation is predicted to occur on the waste package in the absence of liquid water influx.
Power law distribution of discharge in ideal networks
Authors: de Vries H. , Becker T. , Eckhardt B.
Published: February 2004

Data for several river networks show algebraically decaying distribution functions for the mean annual discharge and the dissipation rate (Rodríguez‐Iturbe et al., 1992). We derive a relation between the exponent a in the integrated mean annual discharge distribution P(Q ≥ q) ∼ q−α and the topological dimension Dt of the network, α = 1 − 1/Dt. Using the experimentally determined value Dt ≈ 1.8 (Tarboton et al., 1988) we find α ≈ 0.45, in good agreement with the data of (Rodríguez‐Iturbe et al., 1992). For the random model (Shreve, 1967, 1969; Smart and Werner, 1974) we find an exponent of 1/2.
Predicting the transport of atrazine in soils: Second‐order and multireaction approaches
Authors: Ma Liwang , Selim H. M.
Published: February 2004

As a follow‐up to a previous paper (Ma and Selim, 1994a), this study was designed to further validate a modified second‐order, two‐site (SOTS) model for describing atrazine transport in column miscible experiments. Moreover, the capability of the SOTS model was compared with that of the multireaction transport model (MRTM) of Selim (1989). For both models the necessary parameters were derived from batch experiments and tritium tracer breakthrough results. The models were used solely to predict atrazine breakthrough curves (BTCs) for different experimental conditions. We also proposed an alternative way of analyzing tritium BTCs, where the hydrodynamic dispersion coefficient D and an effective solute transport length Le were used as fitting parameters. Tritium fitted D and Le were used in both models to arrive at atrazine BTC predictions. The SOTS model provided superior predictions over MRTM for all (14) atrazine BTCs regardless of (1) input concentration C0, (2) soil column length L, (3) pore water velocity ν, (4) multiple pulse applications, and (5) flow interruption (incubation). We conclude that the SOTS model was capable of describing chemical heterogeneity of atrazine retention and transport. BTC predictions lend credence to the transport parameter Le in predicting solute transport in soils.
Eulerian‐Lagrangian Analysis of Transport Conditioned on Hydraulic Data: 3. Spatial Moments, Travel Time Distribution, Mass Flow Rate, and Cumulative Release Across a Compliance Surface
Authors: Zhang Dongxiao , Neuman Shlomo P.
Published: February 2004

In paper 1 of this series we described an analytical‐numerical approach to predict deterministically solute transport under uncertainty. The approach allows conditioning such predictions on hydraulic measurements and assessing the corresponding reduction in uncertainty. In paper 2 we examined the effects of log transmissivity and hydraulic head data on conditional predictions of concentration due to instantaneous point and nonpoint sources. In this paper we show how the same approach can be used directly to estimate mass flow rate across a “compliance surface,” cumulative mass release, and the probability distribution of travel times across this surface and the associated uncertainty. Contrary to some other methods in the literature, our approach requires neither a special theory for travel times nor a prior assumption about their probability distribution. We also show how one can compute explicitly the second spatial moment of the conditional mean plume about its center of mass, the conditional mean second spatial moment of the actual plume about its center of mass, and the conditional covariance of the plume center of mass. We illustrate these quantities and the effect of conditioning on some of them by considering instantaneous point, line, and area sources in a two‐dimensional, statistically homogeneous, isotropic, mildly varying log transmissivity field under uniform prior (unconditional) mean flow.
Pilot Point Methodology for Automated Calibration of an Ensemble of conditionally Simulated Transmissivity Fields: 1. Theory and Computational Experiments
Authors: RamaRao Banda S. , LaVenue A. Marsh , De Marsily Ghislain , Marietta Melvin G.
Published: February 2004

A new methodology for solution of the inverse problem in groundwater hydrology is proposed and applied to a site in southeastern New Mexico with extensive hydrogeologic data. The methodology addresses the issue of nonuniqueness of the inverse solutions by generating an ensemble of transmissivity fields considered to be equally likely, each of which is in agreement with the measured transmissivity and pressure data. It consists of generating a selected number of conditionally simulated transmissivity fields and then calibrating each of the fields to match the measured steady state or transient pressures, in a least squares sense. The calibration phase involves an iterative implementation of an automated pilot point approach coupled with conditional simulations. Pilot points are the parameters of calibration. They are synthetic transmissivity data which are added to the transmissivity database to produce a revised conditional simulation during calibration. Coupled kriging and adjoint sensitivity analysis is employed for the optimal location of pilot points, and gradient search methods are used to derive their optimal transmissivities. The pilot point methodology is well suited for characterizing the spatial variability of the transmissivity field in contrast to methods using zonation. Pilot points are located where their potential for minimizing the objective function is the highest. This minimizes the perturbations in the transmissivities which are optimally assigned to the pilot point and results in minimal changes to the covariance structure of the transmissivity field. The calibrated fields honor the transmissivity measurements at their locations, preserve the variogram, and match the measured pressures in a least squares sense. Since there are numerous options in the execution of this methodology, computational experiments have been conducted to identify the most efficient among them. The method has been applied to the Waste Isolation Pilot Plant (WIPP) site, in southeastern New Mexico, where the U.S. Department of Energy is conducting probabilistic system assessment for the permanent disposal of transuranic nuclear waste. The resulting calibrated transmissivity fields are input to a Monte Carlo analysis of the total system performance. The present paper, paper 1 of a two‐paper presentation, describes the methodology. Paper 2, a companion paper, presents the methodology's application to the WIPP site.
Hydroclimatology of Continental Watersheds: 1. Temporal Analyses
Authors: Georgakakos Konstantine P. , Bae Deg‐Hyo , Cayan Daniel R.
Published: February 2004

The linkage between meteorology/climate and hydrology of temperate latitude catchments on daily to decade time scales is studied. Detailed hydrology is provided by a hydrologic catchment model, adapted from the operational streamflow forecast model of the National Weather Service River Forecast System. The model is tuned to respond to observed daily precipitation and potential evaporation input. Results from the Bird Creek basin with outlet near Sperry, Oklahoma, and from the Boone River basin with outlet at Webster City, Iowa, indicate that the model quite accurately simulates the observed daily discharge over 40 years at each of the two 2000‐km2 basins. Daily cross‐correlations between observed and simulated basin outflows were better than 0.8 for both basins over a 40‐year historical period. Soil moisture variability over a period of four decades is studied, and an assessment of temporal and spatial (as related to the separation distance of the two basins) scales present in the estimated soil moisture record is made. Negative soil water anomalies have larger magnitudes than positive anomalies, and comparison of the simulated soil water records of the two basins indicates spatial scales of variability that in several cases are as long as the interbasin distance. The temporal scales of soil water content are considerably longer than those of the forcing atmospheric variables for all seasons and both basins. Timescales of upper and total soil water content anomalies are typically 1 and 3 months, respectively. Linkage between the hydrologic components and both local and regional‐to‐hemispheric atmospheric variability is studied, both for atmosphere forcing hydrology and ydrology forcing atmosphere. For both basins, cross‐ correlation analysis shows that local precipitation strongly forces soil water in the upper soil layers with a 10‐day lag. There is no evidence of soil water feedback to local precipitation. However, significant cross‐correlation values are obtained for upper soil water leading daily maximum temperature with 5–10 day lags, especially during periods of extremely high or low soil water content. Complementary results of a spatial hydroclimatic analysis are presented in a companion paper (Cayan and Georgakakos, this issue).
Effect of Temperature‐Controlled Motility on Transport of Bacteria and Microspheres Through Saturated Sediment
Authors: McCaulou Douglas R. , Bales Roger C. , Arnold Robert G.
Published: February 2004

Continuous flow column experiments were used at different temperatures to study the importance of motility on advective transport of bacteria through repacked, but otherwise unaltered, natural aquifer sediment. The bacterium used was A0500, a flagellated, spore‐forming rod isolated from the deep subsurface (180 m). At 4°C, A0500 was nonmotile because here was no flagellar metabolism. Bacteria removal was greater at 4°C than at 18°C. Similar experiments with microspheres showed an opposite effect, ith greater removal at 18° than 4°C, which was consistent with colloid filtration theory. The sticking efficiency (α) for nonmotile A0500 (4°C), estimated using a steady state colloid filtration model, was over 3 times that of the motile A0500 (18°C), 0.073 versus 0.022. Analysis of complete breakthrough curves using a nonsteady, kinetically limited, transport model suggested that motile A0500 bacteria traveled twice as far as nonmotile A0500 bacteria before becoming attached to the sediment grains. Once attached, nonmotile bacteria detached on a timescale of 9–17 days versus 4–5 days for the motile bacteria. Bacterial motility facilitates advective transport through sediments by changing the attachment‐detachment kinetics to effectively reduce retardation and increase the fraction of time bacteria spend in a detached versus an attached state. Consequently, travel times to deep aquifers from recharge waters could be significantly affected by bacterial motility.
Issues in Single‐Fracture Transport Modeling: Scales, Algorithms, and Grid Types
Authors: Ewing Robert P. , Jaynes Dan B.
Published: February 2004

Transport in single fractures has recently been intensely examined. Potential applications of these studies include nuclear waste storage and infiltration of rainwater into soil desiccation cracks. We modeled hydrodynamic dispersion in single fractures, using a variable‐aperture model and particle‐tracking techniques. We examined issues of scale of heterogeneity, particle‐tracking method, and grid topology. Hydrodynamic dispersion tends to zero as the scale of the transport path increases in relation to the scale of heterogeneity. Since this is not observed in nature, it implies either that fractures have fractal structure or that hydrodynamic dispersion alone does not account for all the dispersion that occurs in fractures. Dispersion and retardation as simulated using a node‐to‐node or mixing type algorithm are greater than when they are simulated using an interpolation algorithm, and the difference cannot be attributed to molecular diffusion. Differences in conductivity and dispersion between different grid types (serial, parallel, square, and random field) are related to the coordination number (degree of connectedness) of the grid, with lower coordination number grids having higher dispersion.
Modeling for Optimal Management of Agricultural and Domestic Wastewater Loading to Streams
Authors: Ejaz Muhammad Shafqat , Peralta Richard C.
Published: February 2004

A simulation(optimization (S/O) model to aid managing multiobjective wastewater loading to streams while maintaining adequate downstream water quality is presented. The conflicting objectives are to maximize the human and dairy cattle populations from which treated wastewater can be discharged to the river system. Nonindustrial municipal (domestic) wastewater undergoes primary and secondary treatment by a sewage treatment plant (STP) before entering as a steady point source. Dairy wastewater is treated by overland flow (OLF) land treatment before entering the stream as a controlled steady diffuse source. Maximum dual‐source loading strategies which do not degrade downstream water quality beyond specified limits are developed. For each computed loading strategy, an optimal OLF system design is also determined. The E constraint method is used to obtain sets of noninferior solutions. Sets of noninferior solutions are represented graphically to show the trade‐off between human and bovine populations that can be maintained. Each set is computed for a different upstream flow rate to illustrate sensitivity to nondeterministic upstream flow rates. The nonlinear constraints utilized restrict downstream concentrations of 5‐day biochemical oxygen demand, dissolved oxygen, nitrogen (organic, ammonia, nitrite, and nitrate), organic and dissolved phosphorus, and chlorophyll a. Concentrations are described via regression equations. The new regression expressions, surrogates for the complex advective‐dispersive equation, permit rapid and feasible solutions by this unique S/O model.
Solving the Estimation‐Identification Problem in Two‐Phase Flow Modeling
Authors: Finsterle Stefan , Pruess Karsten
Published: February 2004

In this paper a procedure is presented to solve the estimation‐identification problem in two‐phase flow modeling. Given discrete observations made on the system response, an optimum parameter set is derived for an appropriate conceptual model by solving the inverse problem using standard optimization techniques. Subsequently, a detailed error analysis is performed, and nonlinearity effects are considered. We discuss the iterative process of model identification and parameter estimation for a ventilation test performed at the Grimsel Rock Laboratory, Switzerland. A numerical model of the ventilation drift and the surrounding crystalline rock matrix is developed. Evaporation of moisture at the drift surface and the propagation of the unsaturated zone into the formation are simulated. A sensitivity analysis is performed to identify the parameters to be estimated. Absolute permeability and two parameters of van Genuchten's characteristic curves are subsequently determined based on measurements of negative water potentials, evaporation rates, and gas pressure data. The performance of the minimization algorithm and the system behavior for the optimum parameter set are discussed. The study shows that a field experiment conducted under two‐phase flow conditions can be successfully reproduced by taking into account a variety of physical processes and that it is possible to reliably determine the two‐phase hydraulic properties that are related to the given conceptual model.
Brine Chemistry of Groundwater Discharge Zones in the Murray Basin, Australia
Authors: Chambers L. A. , White I. , Ferguson James , Radke B. M. , Evans W. R.
Published: February 2004

Control of rising groundwater levels in Australia's Murray Basin requires the pumping of groundwater, and natural salt lakes seem ideal areas in which to dispose of the pumped water. A thorough understanding of the interaction of chemical and physical processes in particular geologic settings should be the basis for selection and management of appropriate disposal sites. In this work we use chemical modeling to predict the equilibrium evolutionary path of evaporating brines in two operating disposal basins and two natural salt lake complexes in the Murray Basin. Evaporative brines from an engineered and a natural, salt lake, groundwater disposal basin showed close correspondence with the predicted equilibrium evaporation pathways for the relevant inlet waters. There was no evidence of re‐solution of salts during evaporation in the salt lake disposal basin. The predicted evaporite suites for these two disposal basins differ because the inlet waters are derived from different sedimentary regions of the basin. In the two natural salt lake complexes studied, the comparison of field samples with predicted evaporation of regional groundwater indicates that brine evolution is a complex process of evaporation and re‐solution of salts, and this is strongly influenced by the lithostratigraphic setting. Re‐solution is favored where thick clayey sediments overlie the regional, unconfined aquifer, and such a site is favorable for disposal of pumped groundwater.
Aquifer Characteristics Derived From the Interaction Between Water Levels of a Terminal Lake (Dead Sea) and an Adjacent Aquifer
Authors: Yechieli Y. , Ronen D. , Berkowitz B. , Dershowitz W. S. , Hadad A.
Published: February 2004

The water level of the Dead Sea, a terminal hypersaline lake (total dissolved solids, approximately 340 g/L) has decreased at an average rate of 0.5 m/yr since 1960 and by 0.8 m/yr between 1981 and 1989. The dramatic long‐term water level variation of the Dead Sea and the seasonal short‐term fluctuations are accompanied by parallel variations of groundwater levels in an adjacent aquifer. A general methodology based on a simplified yet reliable one‐dimensional flow model, together with continuous measurements of groundwater levels in observation wells, enables analysis of aquifer structural and hydraulic properties. Furthermore, this analysis enables prediction of future groundwater levels in unconfined and confined aquifers due to future changes in lake levels.
New Results for Self‐Similar Trees with Applications to River Networks
Authors: Peckham Scott D.
Published: February 2004

In a little‐known series of papers beginning in 1966, Tokunaga introduced an infinite class of tree graphs based on the Strahler ordering scheme. As recognized by Tokunaga (1984), these trees are characterized by a self‐similarity property, so we will refer to them as self‐similar trees, or SSTs. SSTs are defined in terms of a generator matrix which acts as a “blueprint” for constructing different trees. Many familiar tree constructions are absorbed as special cases. However, in Tokunaga's work an additional assumption is imposed which restricts from SSTs to a much smaller class. We will refer to this subclass as Tokunaga's trees. This paper presents several new and unifying results for SSTs. In particular, the conditions under which SSTs have well‐defined Horton‐Strahler stream ratios are given, as well as a general method for computing these ratios. It is also shown that the diameters of SSTs grow like mβ, where m is the number of leaves. In contrast to many other tree constructions, here β need not equal 1/2; thus SSTs offer an explanation for Hack's law. Finally, it is demonstrated that large discrepancies exist between the predictions of Shreve's well‐known model and detailed measurements for large river networks, while other SSTs fit the data quite well. Other potential applications of the SST framework include diffusion‐limited aggregation (DLA), lightning, bronchial passages, neural networks, and botanical trees.
Global ocean phosphate and oxygen simulations
Authors: Anderson Laurence A. , Sarmiento Jorge L.
Published: February 2004

We examine the role of dissolved organic matter (DOM) and the stoichiometric ratios of organic matter remineralization in determining the magnitude and distribution of remineralization of organic matter in the oceanic water column, and the impact of this remineralization on tracer distributions. Our aim is to improve the parameterization of relevant processes in ocean general circulation models by bringing the models into closer agreement with new observational constraints that suggest substantial differences from previous work (lower DOM levels and higher −O2/P ratios). We used phosphate and apparent oxygen utilization (AOU) to analyze the effect of the remineralization profile on water column tracer distributions. The primary impact of DOM cycling is to modify the distribution of remineralization over that obtained from a model with particle cycling only. Changing the oxygen‐to‐phosphorus stoichiometric ratio modifies the magnitude of oxygen utilization, but preserves its basic distribution. We find that even a small amount of DOM is sufficient to prevent the problem of nutrient trapping. Improved phosphate and AOU simulations are obtained when the amount of DOM is reduced and the deep ocean −O2/P ratio is increased in accord with recent observations of these properties.
Belowground cycling of carbon in forests and pastures of eastern Amazonia
Authors: Trumbore Susan E. , Davidson Eric A. , Barbosa de Camargo Plínio , Nepstad Daniel C. , Martinelli Luiz Antonio
Published: February 2004

Forests in seasonally dry areas of eastern Amazonia near Paragominas, Pará, Brazil, maintain an evergreen forest canopy through an extended dry season by taking up soil water through deep (>1 m) roots. Belowground allocation of C in these deep‐rooting forests is very large (1900 g C m−2 yr−1) relative to litterfall (460 g C m−2 yr−1). The presence of live roots drives an active carbon cycle deeper than l m in the soil. Although bulk C concentrations and 14C contents of soil organic matter at >l‐m depths are low, estimates of turnover from fine‐root inputs, CO2 production, and the 14C content of CO2 produced at depth show that up to 15% of the carbon inventory in the deep soil has turnover times of decades or less. Thus the amount of fast‐cycling soil carbon between 1 and 8‐m depths (2–3 kg C m−2, out of 17–18 kg C m−2) is significant compared to the amount present in the upper meter of soil (3–4 kg C m−2 out of 10–11 kg C m−2). A model of belowground carbon cycling derived from measurements of carbon stocks and fluxes, and constrained using carbon isotopes, is used to predict C fluxes associated with conversion of deep‐rooting forests to pasture and subsequent pasture management. The relative proportions and turnover times of active (including detrital plant material; 1–3 year turnover), slow (decadal and shorter turnover), and passive (centennial to millennial turnover) soil organic matter pools are determined by depth for the forest soil, using constraints from measurements of C stocks, fluxes, and isotopic content. Reduced carbon inputs to the soil in degraded pastures, which are less productive than the forests they replace, lead to a reduction in soil carbon inventory and Δ14C, in accord with observations. Managed pastures, which have been fertilized with phosphorous and planted with more productive grasses, show increases in C and 14C over forest values. Carbon inventory increases in the upper meter of managed pasture soils are partially offset by predicted carbon losses due to death and decomposition of fine forest roots at depths >1 m in the soil. The major adjustments in soil carbon inventory in response to land management changes occur within the first decade after conversion. Carbon isotopes are shown to be more sensitive indicators of recent accumulation or loss of soil organic matter than direct measurement of soil C inventories.