bluuebunny/crossref_metadata_2025 huggingface dataset:
A physically based model of heterogeneous hillslopes: 2. Effective hydraulic conductivities
Authors: Binley Andrew , Beven Keith , Elgy John
Published: February 2008

Using the results of a fully three‐dimensional model of variably saturated flow on a heterogeneous hillslope, the concept of equivalent homogeneous hillslopes is explored. By considering single realizations of random patterns of saturated hydraulic conductivity, attempts are made to determine single effective hydraulic conductivity values capable of reproducing both subsurface and surface flow hydrographs. For the case of high‐permeability soils, effective parameters were found to reasonably reproduce the hillslope hydrograph, although there was no consistent relationship between the effective values and the moments of the spatial distributions. For the case of low‐permeability soils, characterized by surface flow domination of the runoff hydrograph, single effective parameters were not found to be capable of reproducing both subsurface and surface flow responses. Furthermore, the event dependency of effective conductivity values under such conditions was demonstrated.
Some theoretical and field aspects of reinjection in geothermal reservoirs
Authors: Bodvarsson Gudmundur S. , Stefansson Valgardur
Published: February 2008

Theoretical and field aspects of reinjection into geothermal reservoirs is discussed using examples from various geothermal fields. Reinjection effects can be observed in terms of pressure maintenance, changes in chemical concentrations, and temperature decline. Pressure support due to injection is observed in many liquid‐dominated fields, and in two‐phase field pressure, effects are often felt through mobility changes. In most geothermal fields where injection is employed, changes in chemical concentrations of the produced fluids have been observed, due to the higher salinity of the injected fluids. In some cases the returns of the injected fluids observed through chemical changes have resulted in temperature declines at the producing wells. In those cases the fluid flow is probably concentrated through a few fractures or faults with insufficient surface area for heat exchange with the host rock.
Optimal control of groundwater by the feedback method of control
Authors: Makinde‐Odusola Babs A. , Mariño Miguel A.
Published: February 2008

This paper employs the feedback method of optimal control to solve groundwater hydraulic management problems such as maintaining a target piezometric surface within a confined aquifer. The feedback model for groundwater hydraulic management consists of a modified version of the groundwater model SUTRA coupled to a dynamic programming optimization model. A quadratic criterion representing a weighted sum of squared deviations from a target piezometric surface was chosen as the objective function. Under this criterion, a set of feedback rules for predicting optimal pumping strategies for the next time period based on current piezometric head values are a by‐product of the solution. The computed feedback rule coefficients are used to derive the optimal pumping strategy for the groundwater hydraulic management problem. The feedback model is shown to be an inverse groundwater simulation model in that piezometric heads are used to determine the optimal pumping rate.
Prediction of phosphate transport in small columns with an approximate sorption kinetics model
Authors: van der Zee Sjoerd , Leus Francisco , Louer Maarten
Published: February 2008

The reaction of phosphate (P) with soil is described with two processes: adsorption and diffusion precipitation. In case of small pH variations in the acid pH range (pH 4.5–6.5) P adsorption may be described with the Langmuir equilibrium or Langmuir kinetics equation. The diffusion‐precipitation process is described with an expression taking the effects of P concentration and reaction time into account. This description is based on the unreacted shrinking core model, where the metal oxides are converted into metal phosphates, and these two phases are separated by a sharp interface inside the particles. Assuming that the adsorption process is reversible and the diffusion‐precipitation process is irreversible, methods are discussed for parameter assessment of the sorption model. These methods are illustrated for a soil. The description of the experimental data is good. For an evaluation of the predictive capabilities of the sorption model, P transport in small columns is predicted, using independently assessed sorption and transport parameters. These predictions are compared to experimental data. The predicted P breakthrough curves are overall well in agreement with the experimental results obtained by percolation of a pulse of a P solution. The main difference between predicted and experimental breakthrough is observed in the sorption stage of the pulse with a feed concentration (c0) of 0.2 mol P m−3. After initial breakthrough the correspondence of the prediction and the data is good for these triplicated experiments at c0=0.2 mol/m3. At high feed concentrations (c0=3 mol/m3) only small differences between prediction and experiment are observed in the desorption stage of the concentration pulse. For experiments involving a first pulse at c0=3 mol/m3 followed by two pulses at c0=0.2 mol/m3 the same agreement of numerical and experimental breakthrough curves is found as for the experiments involving only one pulse. Because P transport involving both sorption and desorption stages was modelled reasonably for two different concentrations we conclude that the P sorption model developed describes the reactions of P with soil accurately.
Acidification of native brook trout streams in Virginia
Authors: Webb J. R. , Cosby B. J. , Galloway J. N. , Hornberger G. M.
Published: February 2008

A survey of 344 streams in the Appalachian Mountain region of Virginia that support reproducing populations of native brook trout (Salvelinus fontinalis) was conducted in the spring of 1987. The surveyed streams represent about 76% of the identified trout streams in the region. Relative to commonly applied sensitivity criteria, 93% of the streams are sensitive (alkalinity ≤ 200 μeq L−1), 49% of the streams are extremely sensitive (alkalinity ≤ 50 μeqL−1) and 10% of the streams are currently acidic (alkalinity ≤ 0.0 μeq L−1). Sulfate is the dominant anion in the streams, but all catchments associated with the streams are retaining significant amounts of atmospherically deposited sulfur (median retention=68%). Estimates of past and potential future acidification were obtained using a simple linear model relating changes in concentrations of base cations to changes in concentrations of sulfate. Sulfate concentration changes were determined as the difference between currently observed concentrations and estimated past and future steady state concentrations. Changes in concentrations of base cations were calculated, assuming base cation increase factors equal to 0.4 and 0.8 times the sulfate increase. The median historical alkalinity loss for the sampled population is estimated as 29 and 9 μeq L−1 for the two assumed factors, with 3 and 0% of the streams estimated to have had historical alkalinities < 0.0 μeq L−1. The median future alkalinity loss is estimated as 90 and 30 μeq L−1 for the two assumed factors, with 88 and 32% of the streams estimated to have future alkalinities < 0.0 μeq L−1.
Diffusion approximation for equilibrium distribution of reservoir storage
Authors: Buchberger Steven G. , Maidment David R.
Published: February 2008

The Fokker‐Planck equation is used to describe the evolution through time of the probability density function of storage levels between the boundaries of a finite reservoir. Analytical solutions for the equilibrium density function are obtained under the assumption that potential storage displacements are independent stationary Gaussian random variables. The potential storage displacement represents the change in storage that would occur during a unit time interval in a reservoir of infinite capacity. Results show that the storage density function is a mixed truncated exponential distribution with mass concentrations on the lower and upper boundaries corresponding to the steady state probability that the reservoir is empty or full, respectively. Parameters of the equilibrium storage density function are computed from the reservoir capacity and the mean and variance per unit time of the potential storage displacement. A dimensionless parameter, analogous to a Peclet number, identifies conditions for which the finite reservoir effectively behaves as one with semi‐infinite capacity. Analytical solutions from the diffusion approximation show excellent agreement with independent estimates of the storage distribution function obtained using probability matrix methods, Monte Carlo simulation, and numerical methods.
Characterization and isotopic composition of organic and inorganic carbon in the Milk River Aquifer
Authors: Murphy Ellyn M. , Davis Stanley N. , Long Austin , Donahue Douglas , Jull A. J. Timothy
Published: February 2008

Carbon isotope analyses were performed by accelerator mass spectrometry on fractions of dissolved organic carbon (DOC) in groundwater from the Milk River aquifer, Alberta, Canada. The DOC was separated into high molecular weight (HMW) and low molecular weight (LMW) fractions for analysis. Characterization information and 14C activities suggest that the HMW fractions (primarily humic substances) originate in the interstitial waters of the soil zone in the area of recharge and are transported through the porous media of the aquifer. Transformations of HMW fractions down the flow path, including an increase in carbon, decrease in oxygen, and a pronounced increase in the aliphatic nature of this fraction, are consistent with decreasing 14C activity. The LMW fractions show a dominant kerogen origin. This fraction of the DOC has low 14C activities (generally <1 percent modern carbon), and gas chromatograph/mass spectrometry analyses identified a homologous series of substituted alkanes, further evidence that these compounds may have originated from organic‐rich shales in the aquifer and/or confining beds. Geochemical and stable isotope measurements of the dissolved inorganic carbon (DIC) indicate that microbial activity is occurring in the Milk River aquifer. The alteration of these DIC δ13C ratios by microbial activity render corrections for groundwater age in this aquifer difficult, if not impossible. Although 14C analyses of DOC fractions will not replace other groundwater dating techniques, these analyses do provide additional information on groundwater age and perhaps a more complete interpretation of groundwater age than DIC alone.
Air permeability and trapped‐air content in two soils
Authors: Stonestrom David A. , Rubin Jacob
Published: February 2008

To improve understanding of hysteretic air permeability relations, a need exists for data on the water content dependence of air permeability, matric pressure, and air trapping (especially for wetting‐drying cycles). To obtain these data, a special instrument was designed. The instrument is a combination of a gas permeameter (for air permeability determination), a suction plate apparatus (for retentivity curve determination), and an air pycnometer (for trapped‐air‐volume determination). This design allowed values of air permeability, matric pressure, and air trapping to be codetermined, i.e., determined at the same values of water content using the same sample and the same inflow‐outflow boundaries. Such data were obtained for two nonswelling soils. The validity of the air permeability determinations was repeatedly confirmed by rigorous tests of Darcy's law. During initial drying from complete water saturation, supplementary measurements were made to assess the magnitude of gas slip. The extended Darcy equation accurately described the measured flux gradient relations for each condition of absolute gas pressure tested. Air permeability functions exhibited zero‐permeability regions at high water contents as well as an abruptly appearing hysteresis at low water contents. Measurements in the zero‐permeability regions revealed that the total amount of air in general exceeded the amount of trapped air. This indicates that the medium' s air space is partitioned into three measurable domains: through‐flowing air, locally accessible air (i.e., air accessible from only one flow boundary), and trapped air. During repeated wetting and drying, the disappearance and reappearance of air permeability coincided closely with the reappearance and disappearance, respectively, of trapped air. The observed relation between critical features of the air permeability functions and those of the air‐trapping functions suggest that water‐based blockages play a significant role in the disruption of gas‐phase connectivity and in preventing air flow, and must be considered in any effectual model of air permeability relations.
Flow channeling in a single fracture as a two‐dimensional strongly heterogeneous permeable medium
Authors: Tsang Y. W. , Tsang C. F.
Published: February 2008

The void space of a rock fracture is conceptualized as a two‐dimensional heterogeneous system with variable apertures as a function of position in the fracture plane. The apertures are generated using geostatistical methods. Fluid flow is simulated with constant head boundary conditions on two opposite sides of the two‐dimensional flow region, with closed boundaries on the remaining two sides. The results show that the majority of flow tends to coalesce into certain preferred flow paths (channels) which offer the least resistance. Tracer transport is then simulated using a particle tracking method. The apertures along the paths taken by the tracer particles are found to obey a distribution different from that of all the apertures in the fracture. They obey a distribution with a larger mean and a smaller standard deviation. The shift in the distribution parameters increases with increasing values of variance for the apertures in the two‐dimensional fracture. Provided that the correlation length is no greater than one fifth of the scale of measurement, the aperture density distributions of tracer particle paths remain similar for flow in two orthogonal directions, even with anisotropy ratio of spatial correlation up to 5. These results may be applicable in general to flow and transport through a two‐dimensional strongly heterogeneous porous medium with a broad permeability distribution, where the dispersion of the system may be related to the parameters of the permeability distribution along preferred flow channels.
Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry
Authors: Zegelin S. J. , White I. , Jenkins D. R.
Published: February 2008

Volumetric water content θ and soil electrical conductivity σ may be measured in situ using time domain reflectometry (TDR). The parallel‐wire or two‐wire transmission line TDR probes currently in field use suffer from unwanted noise and information loss due to impedance mismatch between the probe and the coaxial connecting cable. Here we describe symmetric, multiwire probes designed to minimize these problems and eliminate the need for a balancing transformer between probe and TDR device. Analysis of the electric field distributions around these new probes shows that they emulate a coaxial transmission line, and their measured characteristic probe impedances approach that of coaxial probes. Signals from the new probes permit more reliable and accurate θ and σ measurement and are superior to those of two‐wire probes with balancing transformer. The enhanced signal clarity of the new probes extends to sample diameters of at least 0.2 m. We show that electrical conductivity determined with the new probes is identical to that found with a coaxial cell and substantially different from that measured by a two‐wire probe. Our results indicate that values of σ, determined using the Giese‐Tiemann thin sample approach and measured characteristic probe impedances of coaxial or multiwire probes, agree with values of σ measured using an ac bridge for both electrolyte solutions and soil samples to within ±10%, provided σ exceeds 10 mS m−1. Finally, we give an example of the use of multiplexed three‐wire probes in following rainfall infiltration and redistribution during and after a simulated rainfall event in the field. Infiltrated quantities of water estimated from the TDR water content profiles agreed within ±10% with the amount applied.
Evaluation of volatilization by organic chemicals residing below the soil surface
Authors: Jury William A. , Russo David , Streile Gary , El Abd Hesham
Published: February 2008

Although volatile organic compounds located in buried waste repositories or distributed through the unsaturated soil zone have the potential to migrate to the atmosphere by vapor diffusion, little attention has been paid in the past to estimating the importance of volatilization losses. In this paper a screening model is introduced which evaluates the relative volatilization losses of a number of organic compounds under standard soil conditions. The model is an analytic solution to the problem wherein the organic chemical is located at time zero at uniform concentration in a finite layer of soil covered by a layer of soil devoid of chemical. The compound is assumed to move by vapor or liquid diffusion and by mass flow under the influence of steady upward or zero water flow while undergoing first‐order degradation and linear equilibrium adsorption. Loss to the atmosphere is governed by vapor diffusion through a stagnant air boundary layer. Calculations are performed on 35 organic compounds in two model soils with properties characteristic of sandy and clayey soil. The model identifies those compounds with high potential for loss during 1 year after incorporation under 100 cm of soil cover and also is used to calculate the minimum soil cover thickness required to reduce volatilization losses to insignificant levels during the lifetime of the compound in the soil. From the latter calculation it was determined that certain compounds may volatilize from deep subsurface locations or even groundwater unless the soil surface is sealed to prevent gas migration.
An assessment of irrigation technology performance in the southern San Joaquin Valley of California
Authors: Vaux H. J. , Handley Dale F. , Giboney Paul M.
Published: February 2008

Seasonal applied water measurements were obtained for 1710 irrigated fields in the southern San Joaquin Valley of California. Most of the fields were planted to one of five major crops: citrus, almonds, grapes, cotton, and small grains. These crops were irrigated with a wide array of irrigation technologies, including drip, sprinkler, furrows with tailwater reuse facilities, conventional furrows, and border irrigation systems. The data were analyzed within an accounting framework to standardize for a variety of climatic and cultural variations. Analyses of the mean depths of applied water by crop and irrigation technology and of the standardized results reveal that drip irrigation systems were associated with the lowest levels of applied water on permanent crops and that the levels of water applied with sprinklers did not differ significantly from those applied with surface systems on either permanent or annual crops.
A simple iterative method for the simultaneous determination of soil hydraulic properties from one‐step outflow data
Authors: Valiantzas John D. , Kerkides Petros G.
Published: February 2008

Assuming a Brooks and Corey family of moisture retention curves and a Θ power hydraulic conductivity function, the parameter identification problem is solved with reasonable accuracy and simplicity, through the analysis of one‐step outflow data. Additional information regarding the moisture retention curve may improve the results. Our method does not require a numerical solution of the flow initial boundary value problem and when tested against numerical and experimental outflow data gives very satisfactory results.
A class of time series urban water demand models with nonlinear climatic effects
Authors: Miaou Shaw‐Pin
Published: February 2008

A new class of monthly time series urban water demand model is proposed. The model postulates that water use is made up of base use and seasonal use; and the latter consists of three components: a potential use that is dependent on temperature in the absence of rainfall, a water use reduction due to rainfall occurrences, and a random component. The proposed model utilizes three observations that were established in recent daily water use studies: (1) “hysteresis” temperature effect: under the same temperature, water use has different levels and response rates (to a unit change of temperature) in different seasons, (2) “dynamic” rainfall effect: a rainfall causes a temporary reduction in seasonal use that diminishes over time, and (3) “state‐dependent” rainfall effect: the higher the seasonal use level prior to the occurrence of a rainfall, the more significant the effect is expected. Monthly rainfall effects are derived through the time aggregating and averaging of a daily response model. The model so obtained is nonlinear in structure. The performance of the model is compared with conventional linear models using monthly data in Austin, Texas, from 1975 to 1984. The proposed nonlinear models outperform the linear models in describing seasonal water use variations in terms of adjusted R2, Akaike information criterion value, and ability to estimate the high summer use in dry and wet years.
Variable‐rate pumping tests for radially symmetric nonuniform aquifers
Authors: Butler James J. , McElwee Carl D.
Published: February 2008

Conventional pumping test methodology is of limited effectiveness for defining the spatial distribution of aquifer properties because of the nonuniqueness of the parameter estimates. Sensitivity analysis can be used to develop a pumping test procedure that significantly decreases the uncertainty associated with the estimated parameters. This approach employs systematic variations in pumpage rates to achieve reductions in parameter uncertainty. These reductions are obtained by increasing the sensitivity of drawdown to flow properties while simultaneously constraining the growth in the correlation between the effects of different flow properties on observation well drawdown. Numerical experiments demonstrate the importance of the magnitude and frequency of the rate variations, the spatial and temporal pattern of data collection, as well as the dependence of the technique on the total duration of the pumping test. Significant decreases in parameter uncertainty can be expected in any flow system in which the primary component of flow is in the radial direction. This study demonstrates that sensitivity analysis can be an important tool in the development of methodology for the characterization of subsurface properties.
Evaporation from heated water bodies: Predicting combined forced plus free convection
Authors: Adams E. Eric , Cosler Douglas J. , Helfrich Karl R.
Published: February 2008

Recent measurements at the National Geothermal Test Facility at East Mesa, California, and the Savannah River Plant, South Carolina, have spurred a renewed look at evaporation from heated water bodies. Theoretical analyses are provided for each of four evaporation regimes that could be experienced at heated ponds, ranging from free convective layers to forced convective layers. Differences among regimes are highlighted by calculations appropriate for the Savannah River site where water temperatures reach 70°C and underscore the difficulty in finding a single evaporation equation to adequately cover the entire range. Consequently, a new equation is developed which combines free and forced convection in a different manner from previous equations: evaporation is computed as the square root of the sum of the squares of the respective components. The new equation and several existing equations are compared with evaporation measured at water bodies at the Savannah River and the East Mesa sites.
Modeling village water demand behavior: A discrete choice approach
Authors: Mu Xinming , Whittington Dale , Briscoe John
Published: February 2008

This study presents a discrete choice model of households' water source choice decisions in developing countries. This model is estimated with data collected by in‐depth personal interviews with 69 households in Ukunda, Kenya, a small town south of Mombasa. The results suggest that households' source choice decisions are influenced by the time it takes to collect water from different sources, the price of water, and the number of women in a household. Household income, however, did not have a statistically significant effect. Essentially the same data were used to estimate a traditional water demand model which attempts to explain the quantity of water demanded by a household as a function of collection time, income, and other socioeconomic variables. The results of the discrete choice and traditional water demand models are compared in this paper.
An optimal control method for real‐time irrigation scheduling
Authors: Protopapas Angelos L. , Georgakakos Aris P.
Published: February 2008

In this paper a systematic methodology for making real‐time irrigation decisions is presented. A physically based representation of the dynamics of the soil‐crop‐atmosphere system is used. The variables characterizing the crop and soil status are concurrently simulated with an integrated state space model. Soil moisture and salinity conditions, which synergistically control the plant water uptake, are obtained by using lumped parameter mass balance models for the root zone. Crop yield is predicted by explicitly modeling the plant growth processes, such as assimilation, respiration, and transpiration, which are driven by the climatic inputs. The control model is an analytical optimization method for multistage multidimensional sequential decision‐making problems. It is suitable for systems with nonlinear dynamics and objective functions. The method is based on local iterative approximations of the nonlinear problem with a linear quadratic problem. This approach is evaluated in a series of case studies, where optimal irrigation schedules are obtained on an hourly basis over the growing season.
A stochastic approach to the problem of upscaling of conductivity in disordered media: Theory and unconditional numerical simulations
Authors: Rubin Yoram , Gómez‐Hernández J. Jaime
Published: February 2008

A general problem in groundwater modeling is how to assign conductivity values to blocks of a numerical model. In this study, a method is proposed to upscale the conductivity measurements observed at a given scale to block conductivity values for arbitrary block size. Assuming a multilognormal distribution for the point conductivities, the block conductivity appears as a random function whose distribution, conditional to point measurements, can be derived. This conditional distribution allows filling the flow simulator blocks with equiprobable realizations of block conductivities locally conditioned to actual data. These realizations could also be conditioned to conductivity values measured at any scale if the size of the measurement support is known. The analytical results are compared to results obtained through a Monte Carlo analysis, and a good agreement was found even for variances of ln (T) larger than 1.
The influence of temperature variations on interception loss and water storage in vegetation canopies
Authors: Lockwood J. G.
Published: February 2008

Results of a numerical simulation using a multilayer model pine canopy are presented showing that if incoming radiation is kept at the realistically low levels associated with temperate latitude rainfall events, then interception loss tends to decrease with increasing temperature. If other meteorological conditions are kept constant, increasing temperature is associated with an increasing tendency for intercepted rainwater to be stored in the lower parts of the canopy for long time periods, while the upper canopy dries relatively quickly.
R‐5 revisited: 1. Spatial variability of infiltration on a small rangeland catchment
Authors: Loague Keith , Gander Gene A.
Published: February 2008

In this paper, results from a large set of steady state infiltration experiments made for a small rangeland catchment are presented. We report a geostatistical characterization of infiltration variability for the new data. The scale of spatial correlation between infiltration measurements is determined to be less than 20 m. Steady state infiltration rates are linked to vegetation type and found to be variant with time. The infiltration data described in this paper are used in a companion paper (Loague, this issue) to reexcite and reevaluate the quasi‐physically based rainfall‐runoff model employed by Loague and Freeze (1985).
One‐dimensional solute transport in porous media with partial well‐to‐well recirculation: Application to field experiments
Authors: Chrysikopoulos Constantinos V. , Roberts Paul V. , Kitanidis Peter K.
Published: February 2008

A solute transport model incorporating well‐to‐well recirculation was developed to facilitate the interpretation of pilot‐scale field experiments conducted for the evaluation of a test zone chosen for in situ restoration studies of contaminated aquifers, where flow was induced by recirculation of the extracted fluid. A semianalytical and an approximate analytical solution were derived to the one‐dimensional advection‐dispersion equation for a semi‐infinite medium under local equilibrium conditions, with a flux‐type inlet boundary condition accounting for solute recirculation between the extraction‐injection well pair. Solutions were obtained by taking Laplace transforms to the equations with respect to time and space. The semianalytical solution is presented in Laplace domain and requires numerical inversion, while the approximate analytical solution is given in terms of a series of simple nested convolution integrals which are easily determined by numerical integration techniques. The applicability of the well‐to‐well recirculation model is limited to field situations where the actual flow field is one dimensional or where an induced flow field is obtained such that the streamlines in the neighborhood of the monitoring wells are nearly parallel. However, the model is fully applicable to studies of solute transport through packed columns with recirculation under controlled laboratory conditions. The model successfully simulated tracer breakthrough responses at a field solute transport study, where an induced flow field superimposed on the natural gradient within the confined aquifer was created by a well pair with extraction to injection rates of 10:1.4.
Saltation of snow
Authors: Pomeroy J. W. , Gray D. M.
Published: February 2008

Saltation of snow, the transport of snow in periodic contact with and directly above the snow surface, is governed by the atmospheric shear forces applied to the erodible snow surface, the nonerodible surface, and the moving snow particles. Empirical data measured over a snow‐covered plain suggest functions for parameters important to the apportionment of atmospheric shear forces; the aerodynamic roughness height during saltation, the mean horizontal velocity of saltating particles, and the efficiency of the saltation process. The resulting mass transport expression shows an approximately linear increase in snow saltation transport rate with friction velocity, in agreement with the measurements presented. The expression is sensitive to the cohesion of the snow surface, as indexed by the threshold wind speed, that wind speed at which transport ceases; for wind speeds well above the threshold condition, higher threshold wind speeds correspond to higher transport rates. An adaptation of the expression allows calculation of the mass concentration of saltating snow from measured data and the transport rate of saltating snow from the mean wind speed at 10 m height. Application of the transport rate expression using measured wind speeds, directions, and weather observations demonstrates that the directional component of annual saltating snow transport does not always correspond with wind direction frequency.
Universal scaling of hydraulic conductivities and dispersivities in geologic media
Authors: Neuman Shlomo P.
Published: February 2008

An interpretation is offered for the observation that dispersivities increase with scale. Apparent longitudinal dispersivity data from a variety of hydrogeologic settings are assumed to represent a continuous hierarchy of log hydraulic conductivity fields with mutually uncorrelated increments, each field having its own exponential autocovariance, associated integral scale, and variance that increases as a power of scale. Such a hierarchy is shown theoretically to form a self‐similar random field with homogeneous increments. Regardless of whether or not the underlying assumption is valid, one can justify interpreting the apparent dispersivities in a manner consistent with a recent quasi‐linear theory of non‐Fickian and Fickian dispersion in homogeneous media which supports the notion of a self‐similar hierarchy a posteriori. The hierarchy is revealed to possess a semivariogram γ(s;) ≊ cs½, where c is a constant, and a fractal dimension D ≊ E + 0.75, where E is the topological dimension of interest. This can be viewed as a universal scaling rule about which large deviations occur due to local influences including the existence of discrete natural scales at which log hydraulic conductivity is statistically homogeneous. As such homogeneity is at best a local phenomenon occurring intermittently over narrow bands of the scale spectrum, one must question the utility of associating medium properties with representative elementary volumes and relying on Fickian models of dispersion over more than relatively narrow scale intervals. Porous and fractured media appear to follow the same idealized scaling rule for both flow and transport, raising a question about the validity of many distinctions commonly drawn between such media. Finally, the data suggest that conditioning transport models through calibration against hydraulic measurements has the effect of filtering out large‐scale modes from the hierarchy.
Flow to a heated borehole in porous, thermoelastic rock: Analysis
Authors: McTigue D. F.
Published: February 2008

Exact solutions are obtained for fluid flow induced by the heating of a borehole. The rock is modeled as a fluid‐saturated, porous, thermoelastic medium. The temperature and pore pressure fields are governed by a pair of diffusion equations, which are coupled through a source term in the pressure equation proportional to the temperature rate. The pressure profile exhibits a maximum that grows in magnitude and propagates away from the borehole. For a constant heat flux applied as an instantaneous step, the fluid flux to the borehole takes a finite initial value, and decays monotonically. When the heat flux exhibits a finite rise time, the fluid flux is initially zero, rises to a maximum, and then decays. At late time, the inverse of the fluid flux is linear in ln t; this observation can be exploited to estimate the permeability and fluid diffusivity of low‐permeability rock. Sample calculations are shown for Westerly granite.
Geochemical Modeling of the Madison Aquifer in Parts of Montana, Wyoming, and South Dakota
Authors: Plummer L. Niel , Busby John F. , Lee Roger W. , Hanshaw Bruce B.
Published: February 2008

Stable isotope data for dissolved carbonate, sulfate, and sulfide are combined with water composition data to construct geochemical reaction models along eight flow paths in the Madison aquifer in parts of Wyoming, Montana, and South Dakota. The sulfur isotope data are treated as an isotope dilution problem, whereas the carbon isotope data are treated as Rayleigh distillations. All reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the Madison Limestone. The geochemical reaction models indicate that the dominant groundwater reaction in the Madison aquifer is dedolomitization (calcite precipitation and dolomite dissolution driven by anhydrite dissolution). Sulfate reduction, [Ca2+ + Mg2+]/Na+ cation exchange, and halite dissolution are locally important, particularly in central Montana. The groundwater system is treated as closed to CO2 gas from external sources such as the soil zone or cross‐formational leakage but open to CO2 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer. The computed mineral mass transfers and modeled sulfur isotopic composition of Madison anhydrites are mapped throughout the study area. Carbon 14 groundwater ages, adjusted for the modeled carbon mass transfer, range from modern to about 23,000 years B.P. and indicate flow velocities of 7–87 ft/yr (2.1–26.5 m/yr). Most horizontal hydraulic conductivities calculated from Darcy's Law using the average 14C flow velocities are within a factor of 5 of those based on digital simulation. The calculated mineral mass transfer and adjusted 14C groundwater ages permit determination of apparent rates of reaction in the aquifer. The apparent rate of organic matter oxidation is typically 0.12 μmol/L/yr. Sulfate and, to a lesser extent, ferric iron are the predominant electron acceptors. The (kinetic) biochemical fractionation of 34S between sulfate and hydrogen sulfide is approximately −44‰ at 25°C, with a temperature variation of −0.4‰ per °C. The rates of precipitation of calcite and dissolution of dolomite and anhydrite typically are 0.59, 0.24, and 0.95 μmol/L/yr, respectively.
Stream functions and equivalent freshwater heads for modeling regional flow of variable‐density groundwater: 1. Review of theory and verification
Authors: Senger Rainer K. , Fogg Graham E.
Published: February 2008

Steady state flow of variable‐density groundwater is simulated using equivalent freshwater heads and stream functions. On the basis of the work by De Josselin de Jong (1960,1969), fluids with different fluid densities are replaced by one hypothetical fluid, and singularities are introduced along interfaces where the actual fluids change densities. The basic flow equation describing variable‐density flow, written in terms of equivalent freshwater head, is used to derive the corresponding stream function equation and associated boundary conditions. Neumann boundary conditions for the stream function equation can be determined from gradients of equivalent freshwater heads along the boundary. Stream functions provide a direct representation of the groundwater flow pattern and flow rates where fluid densities vary in space. In comparison, equivalent freshwater heads and fluid densities describe the two driving forces, hydraulic gradient and buoyancy force, but head gradients do not necessarily describe the flow direction of variable‐density groundwater in isotropic media. Comparison of the finite element formulations indicates that potential errors in the centroid‐consistent velocity calculation based on the stream function solution can be expected to be smaller than those in the velocity calculation based on the head solution for cross‐sectional flow models, because discretization in the vertical direction is typically finer than in the horizontal direction.
Practical aspects of low‐flow frequency analysis
Authors: Nathan R. J. , McMahon T. A.
Published: February 2008

This paper considers some practical aspects concerning the application of the Weibull distribution to low‐flow frequency analysis. Two‐ and three‐parameter forms of the distribution are fitted to a total of 987 distributions derived from the daily flow data of 134 catchments located in southeastern Australia. The relative performance of three estimation methods (moments, maximum likelihood, and probability weighted moments) is investigated, and it is found that the different estimation methods provide distinct sets of quantile estimates. The method of probability weighted moments is more likely to give unsatisfactory estimates of the smallest drought and in general tends to yield less severe estimates of drought volumes relative to the other two methods. The method of maximum likelihood, however, occasionally provides estimates of drought volumes that are many times greater than that yielded by the methods moments or probability weighted moments. In addition, the differences between low‐flow frequency estimates based on calendar and hydrologic years is investigated, and recommendations are given for the selection of an appropriate subset of the data to be fitted.
Biofilm growth and the related changes in the physical properties of a porous medium: 3. Dispersivity and model verification
Authors: Taylor Stewart W. , Jaffé Peter R.
Published: February 2008

In the first part of this paper the change in dispersivity resulting from the growth of a biofilm in a porous medium is derived from an existing model of dispersivity and a cut‐and‐random‐rejoin‐type model of the pore geometry. The change in dispersivity due to a biofilm is also estimated from experimental data. Tracer experiments were conducted in biofilm column reactors and dispersion coefficients estimated by solving the inverse solute transport problem by nonlinear, least squares regression. Due to the presence of the biofilm in the porous media, solute flux into the biofilm is an important transport process and is given special attention. Both the dispersivity model and experimentally estimated dispersivities show order of magnitude increases in dispersivity as a result of significant biofilm growth. In the second part of the paper the models for the biofilm‐affected permeability, porosity, and specific surface derived in a companion paper (Taylor et al., this issue) are verified using data from biofilm column reactors. These models are used to parameterize an equation describing the transport of substrate in the experimental columns. Numerical simulations were performed and compared to observed substrate data. Results show that the models for permeability and porosity can be used to make estimates of these parameters, while the model for specific surface appears to be inadequate.
Climatic influences on streamflow variability: A comparison between southeastern Australia and southeastern United States of America
Authors: Kuhnel I. , McMahon T. A. , Finlayson B. L. , Haines A. , Whetton P. H. , Gibson T. T.
Published: February 2008

Differences in runoff and rainfall variability between southeastern Australia and southeastern United States have been examined, using various standard techniques such as principal component and spectral analyses, and examination of the quick and base flow components of runoff. It is shown that the higher runoff variability in Australia can be partly explained by the large‐scale circulation and rainfall patterns associated with the Southern Oscillation which are unlike those in the southeastern United States. Whereas the Southern Oscillation signal is easily detectible in the southeast Australian rainfall and runoff data, it is, with the exception of several small areas, absent in the southeastern United States data. In the case of this particular comparison, the differences in runoff variability between the two selected regions were found to be largest in winter and spring months and smallest in summer.
Coupled inverse problems in groundwater modeling: 1. Sensitivity analysis and parameter identification
Authors: Sun Ne‐Zheng , Yeh William W.‐G.
Published: February 2008

Some basic concepts and methods for solving the coupled inverse problem in groundwater modeling are presented in a series of two papers. Because there are crossover effects between state variables and parmeters, coupled inverse problems are different from single inverse problems in several aspects, especially in the selection of objective functions, identifiability, and experimental design. This paper discusses the general definition of coupled inverse problems posed in the framework of multiobjective optimization. In order to obtain gradient vectors of objective functions as well as all sensitivity coefficients, a general procedure for deriving the adjoint state equations and their associated conditions is described. Using the rules of adjoint operation presented in this paper, the derivation of adjoint problems becomes very easy, even for nonlinear and transient coupled problems. As examples, the adjoint problem of flow‐mass transport (or flow‐heat transport), the adjoint problem of saltwater intrusion, and the adjoint problem of two‐phase flow are derived. Numerical experiments on a two‐dimensional flow‐mass transport problem are given to explain the procedure of sensitivity analysis and parameter identification for coupled problems. The hydraulic conductivity can be identified by using observations of head and/or solute concentration.
Numerical simulation of solute transport in three‐dimensional, randomly heterogeneous porous media
Authors: Tompson Andrew F. B. , Gelhar Lynn W.
Published: February 2008

A three‐dimensional solute transport model has been developed to study detailed contaminant movements through large heterogeneous flow systems in porous media. The model is based upon a random walk particle method (RWPM) which can readily treat multidimensional advection and dispersion processes in saturated or unsaturated media in a computationally efficient manner. The transport simulations are used to examine the large time and spatial effects of the variable flow field on developing solute plumes, and, in particular, to investigate the nature of the large‐scale dispersive behavior. Numerical transport experiments were conducted using single realizations of random hydraulic conductivity fields with three different degrees of heterogeneity. Experiments with different source locations were used to investigate preasymptotic and nonergodic effects that would appear as differences in plume evolution among the experiments. Analyses of the simulations indicate the spatial moments of the particle distributions in statistically isotropic saturated media compare favorably with stochastic theory predictions in terms of longitudinal advection and mixing, but differ markedly from predictions of transverse mixing. The simulations also demonstrate that significant nonergodic effects occur, as reflected in strong differences in the second moment evolution curves among the individual experiments and as predicted from the ensemble stochastic theory.
Isotopic Evolution of River Water in the Northern Chile Region
Authors: Aravena R. , Suzuki O.
Published: February 2008

Two main northern Chilean rivers, the Loa and Tarapaca, were investigated regarding their isotopic characteristics. Groundwater associated with various recharge zones, and the input of tributaries along their courses, mainly control their (18O, 2H) isotopic composition of the rivers. In the Loa river, carbon isotopic exchange between dissolved inorganic carbon (DIC) and atmospheric CO2 plays a minor role in the inorganic carbon cycle of the Loa river. The carbon isotopic composition (14C, 13C) of this river is probably controlled by carbon source associated with volcanic and geothermal activities and by the deposition of travertines. For the Tarapaca river the carbon isotope content of the DIC reflects the input of recirculated water from irrigated areas along the river course and carbon isotopic exchange. These findings imply that the input of volcanic and/or geothermal CO2 into the DIC pool has to be evaluated in order to use carbon isotopes as a dating tool for groundwater in the Loa basin and that modern conditions are not analogous to the paleohydrology of the Tarapaca river.