bluuebunny/crossref_metadata_2025 huggingface dataset:
Valuing wildlife in benefit‐cost analyses: A case study involving endangered species
Authors: Boyle Kevin J. , Bishop Richard C.
Published: February 2008

The focus of the research presented in this paper is to ask what types of values are relevant in the valuation of wildlife species for benefit‐cost analyses of projects that may affect wildlife or its habitat. First, the components of value for wildlife resources are discussed, with emphasis on those particularly relevant to the valuation of endangered species. A simple model is then proposed and results from an application to valuing two of Wisconsin's endangered species of wildlife are presented. The empirical results indicate that significant values may be associated with endangered species of wildlife above and beyond those that arise from viewing these species in the wild. We conclude that to overlook values for wildlife species that go beyond common use values may result in a misleading project or policy decision.
Groundwater geochemistry of short‐term aquifer thermal energy storage test cycles
Authors: Holm Thomas R. , Eisenreich Steven J. , Rosenberg Heidi L. , Holm Nancy P.
Published: February 2008

The changes in the concentrations of Ca, dissolved silica, and alkalinity of the recovery waters of four short‐term aquifer thermal energy storage test cycles with respect to the injection waters and the correlation of these concentrations with recovery water temperatures indicate that quartz and calcite dissolved during hot water storage. This hypothesis was supported by chemical equilibrium modeling and mass balance calculations. Magnesium concentrations were lower in recovery waters than in injection waters. Chemical equilibrium modeling indicated that a Mg silicate (talc) could have precipitated. Potassium concentrations correlated well with temperatures, probably because of ion exchange involving potassium feldspars in the aquifer.
Calculations of the critical shear stress for motion of uniform and heterogeneous sediments
Authors: Wiberg Patricia L. , Smith J. Dungan
Published: February 2008

An expression for the critical shear stress noncohesive sediment is derived from the balance of forces on individual particles at the surface of a bed. The resulting equation, for a given grain size and density, depends on the near‐bed drag force, lift force to drag force ratio, and particle angle of repose. Calculated values of the critical shear stress for uniformly sized sediment correspond closely to those determined from Shields' diagram. The initial motion problem for mixed grain sizes additionally depends on the relative protrusion of the grains into the flow and the particle angle of repose. The latter decreases when the diameter of a moving grain, D, is larger than the length scale of the bed roughness, ks (D/ks > 1), and increases when D/ks < 1, producing a corresponding decrease or increase in critical shear stress. Using the Miller and Byrne experimental relationship between D/ks and particle angle of repose, which is consistent with Shields' definition of initial motion, we obtain results that are in good agreement with the available experimental critical shear stress data for heterogeneous beds.
Stochastic model for the long‐term transport of stored sediment in a river channel
Authors: Kelsey Harvey M. , Lamberson Roland , Madej Mary Ann
Published: February 2008

We develop a stochastic model for the transport of stored sediment down a river channel. The model is based on probabilities of transition of particles among four different sediment storage reservoirs, called active (often mobilized), semiactive, inactive, and stable (hardly ever mobilized). The probabilities are derived from computed sediment residence times. Two aspects of sediment storage are investigated: flushing times of sediment out of a storage reservoir and changes in the quantity of sediment stored in different reservoirs due to seasonal sediment transport into, and out of, a reach. We apply the model to Redwood Creek, a gravel bed river in northern California. Although the Redwood Creek data set is incomplete, the application serves as an example of the sorts of analyses that can be done with the method. The application also provides insights into the sediment storage process. Sediment flushing times are highly dependent on the degree of interaction of the stable reservoir with the more mobile sediment reservoirs. The most infrequent and highest intensity storm events, which mobilize the stable reservoir, are responsible for the long‐term shifts in sediment storage. Turnover times of channel sediment in all but the stable reservoir are on the order of 750 years, suggesting this is all the time needed for thorough interchange between these sediment compartments and cycling of most sediment particles from the initial reservoir to the ocean. Finally, the Markov model has adequately characterized sediment storage changes in Redwood Creek for 1947–1982, especially for the active reservoir. The model replicates field observation of the passage of a slug of sediment through the active reservoir of the middle reach of Redwood Creek in the 18 years following a major storm in 1964 that introduced large quantities of landslide debris to the channel.
Solute transfer and extraction from trickle irrigation source: The effective hemisphere model
Authors: Zemel A. , Ben‐Asher J. , Charach Ch. , Silberbush M.
Published: February 2008

Solute distribution around a trickle source for three‐dimensional steady state flow is studied in terms of an approximate analytic model. Ion and water uptake by the root system are taken into account. Moisture flow and ion concentration profiles are assumed to have hemispherical symmetry. The distributions are nearly constant near the source, with a sudden increase or decrease closer to the wetting front, depending on the ion balance. A simple criterion for the highest allowed concentration is developed. The calculated profiles are compared with the results of field measurements on potassium and nitrate ions.
The infiltration joining problem
Authors: Philip J. R.
Published: February 2008

Since the 1950s there has been continuing interest in the problem of joining the intermediate‐time series solution for one‐dimensional infiltration and the large time traveling wave solution. In practical field terms, however, observational errors, and errors produced by minor heterogeneities and nonuniformities of initial moisture content, will be far larger than the errors of even the crudest joining technique. The practical problem thus warrants only the minimum number of parameters and maximum simplicity. For soils which are initially relatively dry, two parameters suffice, and these are ideally taken as the physically meaningful and measurable sorptivity S and hydraulic conductivity K1. Polynomial fractions are unsuitable; piecewise representation is simple and adequate for most purposes, but a technique using the Knight infiltration solution of Burgers’ equation gives greater accuracy. It is well suited when a continuous representation and/or accuracy is desired.
A siting model for regional wastewater treatment systems: The chain configuration case
Authors: Zhu Zhongping , ReVelle Charles
Published: February 2008

The techniques of locations systems analysis are adapted to develop a siting model for wastewater treatment facilities when the wastewater sources and treatment facilities are in a chain or linear configuration. The regional facility problem is considered as a special variant of the fixed charge plant location model rather than the general transshipment type model. The definition of the additional cost of a waste source joining a regional facility and the development of sequential priority constraints in the siting model allow the siting model to be conveniently solved by widely available linear programming packages. The model deals with the situation in which no current treatment is provided, no bypassed plants are allowed, a fixed level of treatment is required, and water quality constraints are either not included or are not binding. The concave cost of treatment at a plant is approximated by a fixed charge and one straight‐line segment. Numerical examples are included to demonstrate the computational efficiency of the siting model.
Numerical analysis of the nonsteady transport of interacting solutes through unsaturated soil: 1. Homogeneous systems
Authors: Russo David
Published: February 2008

Transient one‐dimensional vertical transport of mixed Na/Ca solutions in a saturated‐unsaturated homogeneous soil during infiltration was studied using a modified version of the transport model of Bresler (1973a), taking into account physicochemical interactions between the soil solution and the soil matrix expressed in terms of changes in the hydraulic conductivity and retentivity functions, anion exclusion, and cation exchange. These latter, in turn, were derived from theoretical considerations based on the mixed ion diffuse double‐layer theory, the structure of the clay particles, the pore size distribution of the soil, and hydrodynamic principles. The solute transport was analyzed for three soils of different textures and for different sets of boundary and initial conditions. Results of the analyses suggested that the effect of soil‐matrix‐soil‐solution interactions on the transport of water and solutes may be significant and generally increases as the wetting zone soil water content and soil solution sodium adsorption ratio increase, as its solute concentration decreases, as the clay fraction of the soil increases; and as the soil texture becomes finer. For a given soil the magnitude of these interactions and their effect on the transport process are affected by the surface boundary conditions and the initial conditions. For a given set of boundary and initial conditions the retardation of the water and solute movement due to the soil‐solution‐soil‐matrix interactions, relative to a reference inert state, depends on the soil texture. The implication of this finding with respect to solute transport in spatially variable fields is discussed briefly.
Combined forced‐convection and buoyancy flow at a two‐fluid interface in a porous medium: Analytical studies
Authors: Hellström Göran , Tsang Chin‐Fu , Claesson Johan
Published: February 2008

The motion of a two‐fluid interface in an aquifer is a combination of buoyancy flow and forced convection. The buoyancy flow, which is due to density variations of the fluid, and the forced convection, which acts on the viscosity variations, will cause the two‐fluid interface to tilt. The basic case of a plane interface between two immiscible fluids of different density and viscosity in an infinite anisotropic aquifer bounded by two horizontal planes is analyzed in detail. The groundwater flow in the aquifer is treated as a superposition of buoyancy flow and forced convection. The buoyancy flow and the forced‐convection components are shown by analytical methods to be related to each other. The tilting angle of the two‐fluid interface is given by a simple formula containing only two parameters. There exists a certain stationary tilting angle, the stability of which depends on the two viscosities.
Internal inconsistencies in dispersion‐dominated models that incorporate chemical and microbial kinetics
Authors: Molz Fred J. , Widdowson Mark A.
Published: February 2008

Current understanding of transport processes in aquifers is limited by lack of precise point chemical concentration measurements. Recently, however, some careful measurements of vertical chemical concentration profiles have been made at several locations around the world that appear to support a consistent picture concerning the persistence of large vertical concentration gradients in aquifers and, by implication, the existence of very small vertical transverse dispersivities. These data were obtained in aquifers supporting microbial activity. Data analysis using a mathematical model which considers microbial degradation coupled to nutrient and oxygen transport indicates that a vertical transverse dispersivity on the order of 0.1 cm or less is consistent with the concentration gradients that were measured. The existence of such large gradients and low dispersivities is not consistent with the use of two‐dimensional vertically averaged (areal) models as currently applied, especially if one is interested in the development of transport models with predictive capability beyond that associated with standard calibration and extrapolation. Even three‐dimensional models with large vertical transverse dispersivities compared to those measured will produce results inconsistent with measurements. Microbial‐chemical activity is very sensitive to concentration distributions. Smearing of the oxygen profile can result in the prediction of aerobic activity where, in fact, none exists.
Effect of storm scale on surface runoff volume
Authors: Milly P. C. D. , Eagleson P. S.
Published: February 2008

Dynamic hydrologic models of areas that are potentially larger than characteristic storm sizes must give explicit consideration to the effect of storm size on the rainfall‐runoff process. The local response of an element of the modeled area can be parametrized in terms of proportion of saturated and impermeable area and infiltration parameters of the unsaturated permeable areas. When a very simple spatial description of the storm depth and duration is provided, it is possible to integrate the local response over the entire area to obtain the average, or lumped, input‐output behavior of the large area on a volume basis. Where infiltration‐excess runoff is significant, the rainfall‐runoff response of a large area is extremely sensitive to the storm size, with a fixed volume of precipitation producing more runoff when it is concentrated over smaller areas. Where saturated or impermeable source areas provide most of the surface runoff, this scale effect on runoff volume is absent. Whenever the effect is significant, it becomes important for rainfall‐runoff models to include not only the rainfall volume but also some description of its areal variability, as forcing variables.
Soil liming as a measure to mitigate acid runoff
Authors: Warfvinge Per , Sverdrup Harald
Published: February 2008

Watershed liming is one method to decrease the acidity of surface waters. To gain an understanding of the mechanisms involved in terrestrial liming and to develop a tool for planning and evaluation, a mathematical model has been developed. The model includes key chemical processes such as limestone dissolution, cation‐exchange reactions, and leaching and accumulation of dissolved species. The ability of the model to describe the short and long‐term improvement in stream water quality following wetland liming is demonstrated by comparing the model calculations with data from two Swedish full‐scale liming experiments. The influence of temporal hydrological and chemical variations on model output is assessed. The sensitivity of the system response to liming is analyzed with respect to three design parameters: the amount and the fineness of the liming material, and the fraction of the watershed that is treated. The simulations illustrate the importance of careful soil and hydrological characterization of treated watersheds to ensure that the limestome dose and the treated area are sufficient to ensure a satisfactory increase in stream pH and resistance to reacidification.
A new boundary element formulation for time‐dependent confined and unconfined aquifer problems
Authors: Aral Mustafa M. , Tang Yi
Published: February 2008

The boundary element method (BEM) is used extensively for the solution of unsteady groundwater flow problems in confined or unconfined aquifers. In recent literature most of these studies concentrate on the efficient solution of resultant matrix equations in time, since the advantages and efficiency of BEM over other domain methods is well‐established for spatial approximation aspects of the technique. Here a new numerical time integration procedure is presented for boundary element solution of time‐dependent aquifer problems. The proposed algorithm reduces the resulting finite difference time integration process to a boundary only computation, if desired, and extends the efficiency of the BEM to time‐dependent problems which is an important step forward for boundary formulation approach. The proposed method is tested with considerable success on two classical aquifer problems. Results obtained are presented comparatively with analytical, finite element, dual reciprocity, and standard boundary element time integration procedures, clearly demonstrating the efficiency and accuracy of the proposed method.
Groundwater flow systems in mountainous terrain: 1. Numerical modeling technique
Authors: Forster Craig , Smith Leslie
Published: February 2008

A coupled model of fluid flow and heat transfer is developed to characterize steady groundwater flow within a mountain massif. A coupled model is necessary because high‐relief terrain can enhance groundwater flow to depths where elevated temperatures are encountered. A wide range in water table form and elevation expected in high‐relief terrain is accommodated using a free‐surface method. This approach allows us to examine the influence of thermal conditions on the patterns and rates of groundwater flow and the position of the water table. Vertical fluid flow is assumed to occur within the unsaturated zone to provide a simple basis for modeling advective heat transfer above the water table. This approach ensures that temperatures at the water table, and throughout the domain, are consistent with temperature conditions specified at the bedrock surface. Conventional free‐surface methods provide poor estimates of the water table configuration in high‐relief terrain. A modified free‐surface approach is introduced to accommodate recharge at upper elevations on the seepage face, in addition to recharge at the free surface.
Experimental study of incipient motion in mixed‐size sediment
Authors: Wilcock Peter R. , Southard John B.
Published: February 2008

Transport rates of five sediments were measured in a laboratory flume. Three of these sediments had the same mean size, the same size distribution shape, and different values of grain size distribution standard deviation. The critical shear stress for incipient motion of the individual size fractions within these sediments was estimated as that shear stress that produced a small dimensionless transport rate. The sorting of the sediment mixture had little effect on the critical shear stress of individual fractions, once the median size (D50) of the mixture and a fraction's relative size (Di/D50) are accounted for. Our data, combined with previously published data, show a remarkably consistent relation between the critical shear stress of individual fractions and the fraction's relative grain size, despite a broad variation in the available data of mixture sorting, grain size distribution shape, mean grain size, and grain shape. All fractions in a size mixture begin moving at close to the same value of bed shear stress during steady state transport conditions. This result is apparently true for transport systems where the transport rates of individual fractions are determined solely by the flow and bed sediment (recirculating systems), as well as for systems where the fractional transport rates are imposed on the system (feed systems). This equivalence in initial‐motion results is important because natural transporting systems often show attributes of both types of behavior in an unknown combination.
Assessment of flood frequency models using empirical distribution function statistics
Authors: Ahmad M. I. , Sinclair C. D. , Spurr B. D.
Published: February 2008

Tests of goodness of fit of the generalized extreme value (GEV) and the generalized logistic (GL) distribution based on the empirical distribution function are described. Significance points for the Anderson‐Darling (Anderson and Darling, 1952, 1954) and a recently introduced modified Anderson‐ Darling (Sinclair et al., 1987) test statistic are obtained by simulation. For each of these distributions the unknown parameters are estimated from the sample data using probability weighted moments. Approximations for the probability of significance of these test statistics are provided. This enables assessment of GEV and GL flood frequency models for a hydrological region based on all the annual maximum series in the region.
Ex post analysis of flood control: Benefit‐cost analysis and the value of information
Authors: Ramirez Jorge , Adamowicz Wiktor L. , Easter K. William , Graham‐Tomasi Theodore
Published: February 2008

Two analytical tools which aid decision making flood control design, ex post benefit‐cost analysis and the ex post value of information, are presented for the case study of Rushford, Minnesota. The ex post analysis is conducted using a coincident frequency analysis which provides a more accurate description of discharge rates and flood levels. The ex post benefit‐cost analysis shows that the Corps of Engineers substantially underestimated the value of potential property damage. The ex post value of information model provides an analysis of the project sensitivity to stream discharge. In addition, the optimum project size is determined under risk of project destruction. The results illustrate that net benefits are sensitive to changes in discharge information but less sensitive to project size and the risk of project destruction. This leads us to recommend more sensitivity analyses on information structures, project size, and the probability of project destruction in ex ante benefit‐cost analysis.
Temporal variability in spatial correlation of daily rainfall
Authors: Berndtsson Ronny
Published: February 2008

Composite correlation fields based on a 5‐year record of daily rainfall at 67 stations throughout Tunisia were used to investigate spatial and temporal properties of rainfall fields. The composite correlation fields had in general banded or elliptical patterns and with the elongations generally along the coastline or the topographical ridges. The banded patterns occurred usually during the rainy season, and the elliptical patterns occurred toward the summer or early autumn. The ratio of width to length of the elongated correlation fields varied in general between 0.1 and 0.4. The composite correlation fields defined as the area within the 0.7 correlation isoline were usually less than about 8.000 km2. The mean monthly rainfall was moderately correlated with the size of the composite correlation fields. The major orientation of the coastline seems to be a more important factor for the rainfall generation than are local irregularities in the coastal orientation and local topographical differences.
The influence of basin morphology on groundwater outflow
Authors: Zecharias Yemane B. , Brutsaert Wilfried
Published: February 2008

Twenty geomorphologic parameters which constitute a well‐balanced representation of the area, length, elevation, form, and shape aspects of watersheds as well as the texture of their drainage networks were considered. On the basis of groundwater theory and the results of previous related studies, eight of these parameters were identified as being relevant to the process of groundwater discharge. Values of the selected parameters were obtained for representative watersheds located in a section of the Appalachian Plateaus. The data, which were generated from U.S. Geological Survey topographic maps, were then subjected to a factor analysis to assess the relative importance of the parameters with respect to their influence on groundwater outflow behavior. The analytical results and their interpretation showed that among the parameters that are related to groundwater outflow, total length of perennial streams, average basin slope, and drainage density are the ones that are most closely related to the process. The influences of these parameters on groundwater outflow behavior are independent of each other. Thus in relating groundwater outflow characteristics to basin morphology, only these parameters need be considered and the inclusion of additional parameters does not necessarily yield a better relationship and may result in redundancy. The findings of the study are consistent with existing concepts regarding base flow (or drought flow) and have been confirmed by an independent study that investigated the process based on a more mechanistic approach.
Multicomponent mass transport with homogeneous and heterogeneous chemical reactions: Effect of the chemistry on the choice of numerical algorithm: 1. Theory
Authors: Kirkner David J. , Reeves Howard
Published: February 2008

This paper presents an analysis of several methods for approximately solving the equations governing multicomponent mass transport with homogeneous and heterogeneous chemical reactions. A derivation of the governing equations allowing a fairly general description of the chemistry is given. It is shown that a number of different formulations are possible depending on the choice of primary variables. Discretization of the governing equations reduces the problem to solving a set of nonlinear algebraic equations each time step. It is shown that the choice of method to solve the algebraic equations should be based on the nature of the chemical reactions. An analysis of the equations yields recommendations in some cases and direction for numerical experimentation in others.
Modeling the transport of volatile organics in variably saturated media
Authors: Sleep B. E. , Sykes J. F.
Published: February 2008

The understanding of the processes of dissolution, volatilization, and gas‐liquid partitioning in porous media is very limited. The few models which attempt to characterize the transport of volatile organics in variably saturated media all assume that mass transfer processes are at equilibrium. In addition, gas phase advection is neglected by assuming that gas phase pressures are uniformly atmospheric and that density gradients are negligible. In this study a model was developed to solve for water phase flow and transport and density dependent gas phase flow and transport. Simple expressions for dissolution, volatilization, and gas‐liquid partitioning, employing the concept of an overall mass transfer coefficient, were incorporated into the model. The transport of trichloroethylene in a variably saturated vertical cross section, under a variety of conditions, was simulated. Results of the simulations appeared qualitatively correct. The importance of gas phase processes in increasing subsurface contamination from volatile organics, and in dissipating residual amounts of these substances, was demonstrated. The lack of similar analytical and/or numerical models, or suitable experimental studies, excluded the possibility of validating, or verifying, the model.
Efficient control of cropland sediment: Storm event versus annual average loads
Authors: Wu Pei‐Ing , Braden John B. , Johnson Gary V.
Published: February 2008

The sediment economics model is used to determine whether optimal responses of farmers to constraints on cumulative sediment loads from cropland are similar to or different from optimal responses to constraints on episodic loads. Stream impacts are not directly addressed. Control costs for episodic loads are found to be consistently higher than control costs for the same proportionate reductions in annual average loads. A shift of policy emphasis from reducing cumulative sediment loads to reducing impacts from storm events may entail different analytical procedures, different management responses, and greater costs.
Fluid pressures in deforming porous rocks
Authors: Palciauskas V. V. , Domenico P. A.
Published: February 2008

The constitutive relations describing the fluid pressure response of a porous medium to changes in stress and temperature must reflect the microscopic processes that are operative over the time scale allowed for the deformation. Short‐duration deformations are readily described by undrained moduli, and intermediate duration deformations by drained moduli, both of which are formulated through linear elastic theory. Long‐term deformations that operate over geologic time are normally dominated by irreversible processes and result in considerably larger deformations, for the same applied stress conditions, than would be expected from their elastic counterparts. Model constitutive equations are developed for both elastic and irreversible processes and the magnitude and interpretation of the relevant material properties examined. Although the theory is presented in general terms, a sample calculation shows that for sandstone the inelastic deformation is one and one half orders of magnitude greater than the elastic deformation at the same applied stress. This difference in magnitude has a significant effect on the effective hydraulic diffusivity, various pore pressure coefficients, and the prospective fluid pressure development of the sediment.
Regional flood frequency analysis using extreme order statistics of the annual peak record
Authors: Smith James A.
Published: February 2008

A regional flood frequency model is developed for estimating recurrence intervals of extreme floods. The regionalization procedure developed in this paper differs notably from the U.S. Geological Survey index flood method (Dalrymple, 1960) in that a large quantile, rather than the mean annual flood, is used as the “index flood.” Based on a result from extreme value theory, exceedances of the specified quantile are modeled by a generalized Pareto distribution. The generalized Pareto distribution has two parameters: a scale parameter and a shape parameter. It is assumed that the shape parameter does not vary from basin to basin, implying that annual peak distributions for all basins have the same upper‐tail thickness. The scale parameter, on the other hand, not only varies from basin to basin, but may also depend on covariate information, such as drainage area or indicator functions for basin geology. Likelihood‐based inference procedures are developed for the regional flood frequency model. The model is applied to extreme floods of the Central Appalachian region of the United States.
Hydrodynamic description of the erosional development of drainage patterns
Authors: Roth Giorgio , Siccardi Franco , Rosso Renzo
Published: February 2008

The erosional development of drainage networks is analyzed by modeling two‐dimensional flow of water and sediment over a bare slope. Deterministic equations of mass and momentum conservation are coupled with random fluctuations of sediment pickup and transportation processes acting at the hydrodynamic scale. These fluctuations are then shown to be able to provide an active source of channelization at the hillslope scale. Controlled experiments are also carried out by simulation runs aimed at investigating model performance (1) to describe formation of natural drainage patterns, (2) to account for different factors (i.e., soil characteristics, hillslope geometry, and rainfall intensity) which control network formation and development, and (3) to reproduce differentiation of drainage patterns due to above factors.
The determination of flood control volumes in a multireservoir system
Authors: Kelman J. , Damazio J. M. , Mariën J. L. , Da Costa J. P.
Published: February 2008

A methodology is presented for the optimal design of flood control volumes in a multireservoir system with one downstream critical section. The design is optimal in the sense that the objective function attempts to minimize the penalties associated with providing the flood protection. Moreover, the method explicitly considers a set of probability constraints on the occurrence of floods. The proposed calculation scheme is easily applied to almost any type of multireservoir system. The methodology is applied to the problem of determining the flood control volumes to be provided in a hydropower system of eight reservoirs on the Parana river in Brazil. In that case the objective function consists of minimizing the total firm energy loss.
A non‐Gaussian multicomponent model for river flow
Authors: Vandewiele G. L. , Dom A.
Published: February 2008

In order to model satisfactorily sharp rises and slow decreases appearing in any pure runoff time series with a small enough time base, exponentially decaying recession and base flow terms are first subtracted from total runoff. The remaining part is deseasonalized by a truncated Fourier series multiplicator and then modeled as a second‐order Markov stationary series. The use of non‐Gaussian distributions allows the modeling of zero contributions, which correspond to dry spells. An example has been optimized and simulated and the observed and simulated series have been compared, even on hydrologically interesting properties such as return periods, which were not modeled explicitly. The results were satisfactory.
Application of the discrete fracture network concept with field data: Possibilities of model calibration and validation
Authors: Dverstorp Björn , Andersson Johan
Published: February 2008

The uneven flow distribution observed in a migration experiment in the Stripa research mine is analyzed with a discrete fracture network model. Data on the measured inflow distribution and trace geometry in the experimental drift are the basic sources for estimation of the model parameters. Direct estimates cannot provide a unique definition of input data to the model; thus different combinations of input data need to be analyzed in the network model. Detailed measurements of the inflow distribution in the experimental drift provided an opportunity to determine the model parameters through calibration in the network model. Calibration was performed on the mean inflow, distribution of flow, and trace geometry observed in the roof of the experimental drift. The simulation results show that the model can be calibrated to produce an areal flow distribution that is consistent with the measured one. The uncertainties in the input parameters are thus reduced, but different combinations of input data are still possible. The calibration simulations show that the length of conductive fractures per area might be used as a calibration parameter. Simulations based on different combinations of fracture size and density but with the same length of conductive fracture traces produced similar flow distributions. The validity of the calibrated model is explored by predicting the flow into the boreholes at the experimental site. The resulting inflow distributions accord well with those measured in two of the three boreholes. The properties of the third borehole, which differ substantially from the other two, could not be explained by the simulation results.
Modeling one‐dimensional infiltration into very dry soils: 2. Estimation of the soil water parameters and model predictions
Authors: Hills R. G. , Hudson D. B. , Porro I. , Wierenga P. J.
Published: February 2008

Using a water content based one‐dimensional finite difference algorithm, we model infiltration into a 6‐m‐lysimeter column containing alternating layers of air dried clay loam and sand. We use van Genuchten's equation to model the h−θ relationship and both Campbell's and Mualem's equations to model the K−θ relationship. Several sets of model parameters are estimated using data generated from various combinations of laboratory drainage experiments, laboratory measurements of saturated hydraulic conductivity, lysimeter observations of initial conditions, and lysimeter observations of water redistribution. We find that predictions of infiltration based on field redistribution data give the best agreement with the observed infiltration into the lysimeter. We also find that the use of Campbell's K−θ relationship results in closer agreement between the infiltration model predictions and the lysimeter observations than does the use of Mualem's K−θ equation. The results of this study show that infiltration model predictions for a carefully controlled field scale lysimeter are very sensitive to the field and laboratory techniques used to estimate the soil water parameters.
Solute advection in stratified formations
Authors: Cvetkovic Valdimir D. , Shapiro Allen M.
Published: February 2008

Advection‐dominated solute movement in stratified formations is investigated using a Lagrangian interpretation of particle motion. A probability density function (pdf) for particle position quantifies the expected depth‐integrated resident concentration. A pdf for particle arrival time quantifies the expected depth‐integrated rate of mass arrival, from which the flux‐averaged concentration can be defined. The difference between the flux‐averaged and resident concentrations is shown to be significant for the variability in the hydraulic conductivity that is commonly encountered in field applications. The influence of porosity variations on the advection‐dominated solute movement in stratified porous media is shown to be notable only for large variability in the effective porosity.
Circular convection during subsurface injection of liquid waste, St. Petersburg, Florida
Authors: Hickey John J.
Published: February 2008

Injection of liquid waste into a highly transmissive, saltwater‐bearing, fractured dolomite underlying the city of St. Petersburg, Florida, provided an opportunity to study density‐dependent flow associated with two miscible and density‐different liquids. The injection zone was 98 m thick with a radial hydraulic conductivity of 762 m/d and a vertical hydraulic conductivity of 152 m/d. Mean chloride concentrations of the injectant during two tests of 91 and 366 days duration were 180 and 170 mg/L, respectively, whereas chloride concentration of native salt water ranged from 19,000 to 20,000 mg/L. During the 366‐day test, chloride concentration in water from a well open to the upper part of the injection zone 223 m from the injection well approximately stabilized at about 4000 mg/L. Relatively constant chloride concentrations in water from this observation well at a level significantly greater than the injectant concentration suggested the hypothesis that circular convection with saltwater flow added chloride ions to the injection zone flow sampled at the observation well. In order to assess the acceptability of the circular convection hypothesis, information was required about the velocity field during injection. Mass transport model simulations were used to provide this information, after determining that the fractured injection zone could be treated as an equivalent porous medium with a single porosity. The mass transport model was calibrated using the 91‐day test data from two observation wells 223 m from the injection well. The model was then run without parameter changes to simulate the 366‐day test. Mass fractions of injectant computed for four observation wells during the 366‐day test compared favorably with observed mass fractions. Observed mass fractions were calculated as a function of chloride concentration and density. Comparisons between model‐computed mass fraction and velocity fields in a radial section showed circular convection, with salt water flowing toward the injection well in the lower part of the injection zone. The salt water then mixed with the injectant, and the mixture flowed away from the injection well in the upper part of the injection zone. On the basis of the model results and the assumed reasonableness of treating the injection zone as an equivalent porous medium with a single porosity, the hypothesis of circular convection with saltwater flow during subsurface injection of liquid waste into a highly transmissive saltwater‐bearing fractured dolomite was judged acceptable.
Semianalytic boundary element solution of groundwater seepage problems
Authors: Aral Mustafa M.
Published: February 2008

A numerical solution is developed for the analysis of groundwater flow problems using semianalytic time integration procedures and the boundary element method. The secondary reduction method, developed earlier, is an integral part of this computation process. Compared to finite difference procedures, which are commonly used for time integration in boundary element applications, the semianalytic computation yields more efficient and accurate results. The proposed method is tested here for two‐dimensional confined flow applications. Numerical results obtained are verified using analytical procedures that are available in the literature and are compared with other boundary element solutions.
Chaos in rainfall
Authors: Rodriguez‐Iturbe Ignacio , Febres De Power Beatriz , Sharifi Mohammad B. , Georgakakos Konstantine P.
Published: February 2008

The question is posed regarding possible chaotic dynamics in the temporal rainfall of storm events. Chaotic dynamics implies a nonlinear deterministic system very sensitive to initial conditions which yields outputs indistinguishable from a stochastic process by standard techniques. The trajectories of these systems in the phase space are characterized by being contained in a strange attractor of fractal dimension. A time series of 1990 points of 15‐s rainfall for the storm of October 25, 1980, in Boston is analyzed in detail toward this objective. It is found that both the characteristics of the correlation integral and the Lyapunov exponents of the historical data give preliminary support to the presence of chaotic dynamics with a strange attractor. The implications of these findings for the modeling of storm rainfall and for the limits of predictability of the process are discussed jointly with the research challenges lying ahead in this field.
Air‐water gas transfer in open channels
Authors: Gulliver John S. , Halverson Martin J.
Published: February 2008

Laboratory flume experiments on waterside‐controlled gas transfer, such as reaeration, are described. The experiments are well described by Danckwerts' surface renewal (1951) model, as originally proposed by O'Connor and Dobbins (1958), for an open channel flow. The primary cause of surface renewal in flumes is proposed to be the upwelling of large streamwise vortices and is documented by relating prior velocity measurements of these vortices and observations of surface upwellings to gas transfer measurements. A dimensionless liquid film coefficient (Stanton number) is related to a shear Peclet number and a shear Reynolds number. The shear Reynolds number represents the effectiveness of the large streamwise vortices in penetrating the water surface. The proposed hypothesis is essentially a large eddy model, except that coherent structures in the flow have been identified as the predominant mechanism of surface renewal, rather than random turbulence. The coherent structures of importance to surface renewal are the large streamwise vortices with a size similar to the depth that occurs in an open channel flow. Thus a streamwise vortex model of gas transfer in open channels is developed. The implications of this model on the prediction of gas transfer in open channels is discussed.
Boundary conditions along permeable fracture walls: Influence on flow and conductivity
Authors: Berkowitz Brian
Published: February 2008

It is usually assumed in the literature that an impermeable parallel plate approximation is suitable for modeling flow in permeable wall fractures. Experimental evidence which casts doubt on the validity of this assumption is examined, and an alternative formulation for flow in permeable wall fractures and the surrounding porous medium is developed. Analysis of the model indicates that assuming that the tangential fracture fluid velocity vanishes along the interface between fracture and porous medium can in certain cases lead to significant underestimation of the net flow rate and of the coefficient of equivalent fracture conductivity.
Colloid mobilization in two Atlantic coastal plain aquifers: Field studies
Authors: Ryan Joseph N. , Gschwend Philip M.
Published: February 2008

The geochemical mechanisms leading to the mobilization of colloids in groundwater were investigated in the Pine Barrens of New Jersey and in rural central Delaware by sampling pairs of wells screened in oxic and anoxic groundwaters in the same geologic formations. Samples were carefully taken at very low flow rates (∼100 mL min−1) to avoid suspending immobilized particles. The colloidal matter was characterized by light‐scattering photometry, scanning electron microscopy, energy‐dispersive X ray analysis, microelectrophoresis, and Fe, Al, Si, and organic carbon analyses. The colloids, composed primarily of clays, were observed at high concentrations (up to 60 mg colloids/L) in the anoxic groundwaters, while the oxic groundwaters exhibited ≤1 mg colloids/L. Colloidal organic carbon was present in all groundwaters; but under anoxic conditions, one‐third to one‐half of the total organic carbon was associated with the inorganic colloids. The field evidence indicates that anoxic conditions cause the mobilization of soil colloids by dissolving the ferric oxyhydroxide coatings cementing the clay particles to the aquifer solids. The depletion of oxidized iron on the surfaces of immobile particles and the addition of organic carbon coatings on the soil particles and colloids apparently stabilizes the colloidal suspension in the anoxic groundwaters.
Deuterium variations in storm rainfall: Implications for stream hydrograph separation
Authors: McDonnell J. J. , Bonell M. , Stewart M. K. , Pearce A. J.
Published: February 2008

Isotopic variation in storm rainfall is an important consideration in hydrograph separation using the mass balance approach but is rarely considered when determining the accuracy of old water estimates. Study of a small watershed on the South Island of New Zealand in which new water is a major component of the storm hydrograph shows that, in addition to the within‐storm isotopic variations themselves, rainfall weighting techniques may substantially influence estimates of old/new water as a function of both total runoff and total quick flow production. Two incremental approaches to rainfall weighting are presented. Results show that within‐storm incremental weighting is better than the standard weighting technique, which imposes a total storm rainfall value exogenously on the mass balance equation.
Modeling fracture flow with a stochastic discrete fracture network: calibration and validation: 1. The flow model
Authors: Cacas M. C. , Ledoux E. , de Marsily G. , Tillie B. , Barbreau A. , Durand E. , Feuga B. , Peaudecerf P.
Published: February 2008

A large‐scale investigation of fracture flow was recently conducted in a granite uranium mine at Fanay‐Augères, France. Its aim was to develop a methodology for the investigation of possible nuclear waste repository sites in crystalline environments, and thus to determine what measurements to make and what models to use in order to predict the flow and transport properties of the medium, i.e., their average behaviors and spatial variabilities at different scales. Four types of data were collected: (1) geometry of the fracture network; (2) local hydraulic properties measured by injection tests in boreholes; (3) global hydraulic behavior from flow rate and piezometric head distribution at a 106 m3 scale; and (4) tracer tests performed at a scale of up to 40 m. A stochastic fracture network model assuming negligible matrix permeability was developed and calibrated essentially on data 1 and 2 above; this was then used to predict data 3 and 4 in an attempt to validate both the parameters and the structure of the model. In this first part, only the flow problem (data 1) is discussed.