bluuebunny/crossref_metadata_2025 huggingface dataset:
Groundwater Flow and Sand Body Interconnectedness in a Thick, Multiple‐Aquifer System
Authors: Fogg Graham E.
Published: February 2008

Many so‐called sandstone aquifers are actually multiple‐aquifer systems consisting of discontinuous sand bodies distributed complexly in a matrix of lower‐permeability silts and clays. The arrangement and Interconnectedness of these various lithofacies strongly influence spatial patterns of hydraulic conductivity (K) and, in turn, groundwater flow and mass transport. A promising technique of estimating such patterns of K involves careful analysis of both subsurface geologic and subsurface hydrologic data. In this study the three‐dimensional distribution of K was estimated for a numerical flow model of part of the Wilcox aquifer system in Texas, using K data from core samples and pumping tests and more than 100 geophysical logs. The aquifer system, which is up to 320 m thick, consists of multiple, elongate sand bodies and silts and clays deposited in a fluvial environment and is similar to many other systems found in the Gulf Coast and other sedimentary basins. The resulting deterministic‐conceptual flow model demonstrates the importance and methods of incorporating geologic information in groundwater models. Flow in the aquifer is shown to be controlled not so much by K of the sands as by their continuity and Interconnectedness. Much of the aquifer system consists of large zones in which the fluvial channel‐fill sands are sparse and apparently disconnected, resulting in groundwater flow rates lower by a factor of 101 to 103 than in adjacent, well‐interconnected belts of fluvial channel‐fill sand belts. Modeling results also raise serious doubts regarding our ability to predict regional scale flow and mass transport in complex aquifers such as the Wilcox, using current technology. Though sand body Interconnectedness is critically important, it is also very difficult to estimate. One or two well‐connected sands among a system of otherwise disconnected sands can completely alter a velocity field. This is particularly true if the sands are connected vertically and nonzero vertical hydraulic gradients exist. Because the model is three‐dimensional, sensitivity of hydraulic head to heterogeneity or Interconnectedness is much less than normally observed in two‐dimensional models, and therefore heads computed by the model give little to no indication of the location of well‐interconnected zones. Thus such zones can easily go undetected, even in carefully calibrated models which yield reasonably accurate hydraulic heads. This is a significant point for modeling of solute transport.
Can Unsaturated Flow During Gravity Drainage Be Represented by Boulton's Formulation?
Authors: El‐Kadi Aly I. , Brutsaert Wilfried
Published: February 2008

Despite recent advances in numerical methodology, the solution of the complete equations describing saturated‐unsaturated flow in an unconfined aquifer remains a difficult problem. Therefore in practical engineering calculations the formulation must be simplified. It is shown here that Boulton's equation (involving a time‐dependent drainable porosity) and also the linearized Boussinesq equation can provide results that are in agreement with certain aspects of the more realistic Richards solution, provided that an estimation of their parameters is possible. However, the performance of Boulton's formulation is only marginally superior to that based on the Boussinesq equation.
A Comparison of Chemical and Isotopic Hydrograph Separation
Authors: Hooper Richard P. , Shoemaker Christine A.
Published: February 2008

As part of the construction of a simulation model to test an acid precipitation neutralization mechanism, the stream hydrograph was separated into its base flow and event water components using stable environmental isotopes of water, naturally occurring conservative tracer. Three snowmelt events and one storm event during the winter and spring of 1984 were studied at an instrumented watershed in the Hubbard Brook Experimental Forest, New Hampshire. Conditions for use of the isotopic tracer were not always met, however. During the latter part of the snowmelt and the storm the isotopic content of the groundwater and event water were not distinguishable. Furthermore, the isotopic content of the meltwater varied considerably over time, thereby reducing the precision of the hydrograph separation. Frequent sampling of the meltwater is mandatory to assess this variability. Because the concentration of major cations and anions was measured as well, chemical tracers could be compared to the isotopic tracer, when the isotopic hydrograph separation was reliable, to test whether the chemical tracer was conservative. Dissolved silica was found to act as a conservative tracer for this watershed.
Hodograph Transformation for Flow Through a Dam With Sloping Walls and No Tail Water
Authors: Boast C. W.
Published: February 2008

A method is presented to transform the hodograph plane (velocity plane) of an earthen dam with sloping walls to a plane where the unconfined saturated‐flow boundary value problem associated with such a dam can be solved using the Poisson integral. The solution would give, for example, the rate of flow of water through the dam, the extent of the surface of seepage on the downstream side of the dam, and the shape and position of the water table within the dam. The method, which is presented for the no‐tailwater case, applies to a dam with perpendicular upstream and downstream walls. It is based on the fact that the hodograph of such a dam consists simply of a triangle, plus an internal circular arc cut that represents the water table. A key step in the method is possible because, in the plane generated by the first Schwarz‐Christoffel transformation, the curve corresponding to the water table approaches a parabola near the point at infinity.
Low‐Velocity Measurement in Water
Authors: Ellis Christopher , Stefan Heinz G.
Published: February 2008

Water velocities in the centimeter per second range or less are measurable by only a few instruments. Experimental laboratory studies frequently require such measurements. A review of low water velocity measurement methods is presented. An inexpensive optical hydrogen bubble‐tracing technique is described for velocity measurements in the range 0.5 to 8 cm/s. Modification to a thymol blue (pH) tracer method extends its applicability to the range 0.1 to 1.0 cm/s. Design and operational characteristics of the hydrogen bubble/thymol blue current meter are described.
Location of an Interface Observation Well: A Bayesian Approach
Authors: Ben‐Zvi M. , Bachmat Y.
Published: February 2008

Coastal aquifers are affected by intrusion of seawater. The depth of intrusion is often measured by the distance from the coastline to the toe of the seawater‐freshwater interface. This paper considers the problem of estimating the location of the toe on the basis of prior information and of posterior information that can be obtained by adopting an interface model and installing an observation well. The solution to the problem involves decisions regarding the worthwhileness of drilling the well and its optimal location. A methodology is proposed for solving the problem by employing the Bayesian approach to decision making under uncertainty with the sum of expected risk and cost as the objective function. An alternative to installing a well may be the acquisition of additional prior information in order to reduce the prior range of uncertainty regarding the location of the toe. The problem then becomes one of making an optimal allocation of resources among competing alternatives. It is solved by comparing the expected net benefit from acquisition of prior information only against the net benefit from acquisition of prior information, adoption of an interface model, and installment of a well.
Group invariance and field‐scale solute transport
Authors: Sposito Garrison , Jury William A.
Published: February 2008

The conventional measurement and statistical characterization of solute dispersion coefficients and convection velocities in a field‐scale vadose zone are based on the assumption that the convection‐dispersion equation governs solute transport locally. The implications of this hypothesis are investigated mathematically through a study of the space and time coordinate transformations which leave the one‐dimensional convection‐dispersion equation (CDE) invariant in form. It is shown that there are just six nontrivial space‐time transformations under which this CDE is invariant. The pioneering field‐scale leaching experiment reported by J. W. Biggar and D. R. Nielsen (1976) is used to make a typical application of these results. When the boundary condition in the Biggar‐Nielsen experiment is imposed, the number of possible space‐time transformations of the CDE reduces to two, one of which is a scaling transformation related closely to scale transformations of the solute transport coefficients. These results and the assumption that sets of transport coefficients corresponding to widely separated regions in a vadose zone are statistically independent are sufficient to prove that the field‐wide probability distribution of the transport coefficients (taken as random variables) will be lognormal. The methodology developed in this study can be applied to any field‐scale solute transport phenomenon for which the partial differential equation governing solute movement locally is assumed known.
Estimating the effects of understory removal from a Douglas Fir forest using a two‐layer canopy evapotranspiration model
Authors: Kelliher F. M. , Black T. A. , Price D. T.
Published: February 2008

W. J. Shuttleworth's (1979) development of the Penman‐Monteith evaporation equation for multilayer, partially wet forest canopies was modified for application to the hypostomatous canopies of Douglas fir and salal. This theory was combined with standard hourly micrometeorological measurements, eddy diffusive, boundary layer and stomatal resistance functions, and canopy and root zone water balance equations to calculate evapotranspiration rates (E) from a Douglas fir forest with salal understory over extended periods during two growing seasons. Calculated values of E agreed to within 0.2 mm d−1 of values determined using Bowen ratio‐energy balance measurements. The courses of average root zone volumetric water content (θ) calculated for two extended periods agreed well with neutron probe measurements. Salal understory removal resulted in measured values of θ being only 0.01–0.03 m3 m−3 higher over the two periods, in close agreement with calculations. This corresponded to calculated tree transpiration rates being 0.4 mm d−1 higher on average, during the second half of both periods. These higher rates were confirmed by stomatal resistance measurements.
On the simulation of nonaqueous phase organic compounds in the subsurface
Authors: Pinder George F. , Abriola Linda M.
Published: February 2008

The movement of nonaqueous phase organic liquids (NAPL) in a groundwater system involves both miscible and immiscible flow phenomena. A broad overview of the task of modeling these flows is presented along with a discussion of various problematic issues related to such a modeling effort. Descriptive equations are obtained through combination of the species balance equations for multiphase fluid flow in a porous medium and appropriate constitutive relationships. Tabulation and discussion of these constitutive relations yields insight into model requirements. Under certain assumptions, the modeling problem can be reduced to the solution of two nonlinear partial differential equations in two unknowns. An example simulation of the movement of NAPL emanating from two point sources in a confined aquifer is presented to illustrate present modeling capabilities.
Urban water pricing and drought management
Authors: Moncur James E. T.
Published: February 2008

In periods of drought, urban water systems commonly rely on nonmarket programs to induce temporary conservation, leaving the marginal price of water unchanged; an alternative is to raise the price. Using pooled cross‐sectional and time series observations on single‐family residential customers of the Honolulu Board of Water Supply (1982), demand for water is estimated as a function of price, income, household size, rainfall, and a dummy variable denoting a water restrictions program. Short‐run elasticities suggest that an increase in marginal price of less than 40% would achieve a 10% reduction in water use, even during a drought episode. An accompanying conservation program would mitigate the necessary price increase, but only slightly.
Small‐scale surface (tank) irrigation in Asia
Authors: Palanisami K. , Easter K. William
Published: February 2008

Tank irrigation is an ancient tradition in Asia which is now being reviewed as a potential model for future irrigation expansion. South India has thousands of tanks which are in need of rehabilitation after being in operation for over a century. This study evaluates tank irrigation in an area of south India which has the greatest concentration of tanks. Constraints and unique characteristics of tank irrigation are analyzed to provide a basis for devising strategies for improving tank irrigation. A combination of public and private investments along with institutional changes are recommended to help farmers organize to improve irrigation. Yet, only if public investment is carefully integrated with existing private efforts will farmers have incentives to maintain the irrigation systems.
Modeling the demand for outdoor recreation
Authors: Mendelsohn Robert
Published: February 2008

This paper critically reviews several of the new methodologies developed in the last 10 years to model the demand for recreation. There are three competing approaches to modeling heterogeneous recreation sites: partitioning, hedonic, and index models. Partitioning involves grouping sites into small homogeneous sets and treating each set as a unique good (multiple‐site travel cost models). Hedonic involves disaggregating goods into their component characteristics and modeling the prices and demands for the characteristics (hedonic property value and the hedonic travel cost method). The index models involve measuring choices among limited alternatives using an index of characteristics (generalized travel cost, gravity, and discrete choice models). Although much work has been done on each method, limitations are noted with each approach, and additional research needs are identified.
Stochastic modeling of vertically averaged concentration uncertainty in a perfectly stratified aquifer
Authors: Black T. C. , Freyberg D. L.
Published: February 2008

Solute concentration which results from transport in an aquifer with unknown variability in hydraulic conductivity can be subject to large predictive uncertainty. This uncertainty can be modeled using a stochastic approach in which conductivity is represented by a random field with known probabilistic structure. In this work we use Monte Carlo methods of stochastic modeling to study the uncertainty in vertically flux‐averaged concentration for transport in the simplified case of a perfectly stratified aquifer. We assume steady flow parallel to bedding and negligible transverse dispersion. We measure uncertainty with the ensemble coefficient of variation of concentration and find it to be large in most conditions examined. We consider the validity of an ergodic assumption (practical equivalence of ensemble mean and single‐realization concentrations) by investigating the dependence of uncertainty upon the vertical averaging length. Even for averaging over a depth of 100 times the vertical correlation scale of conductivity, uncertainty is large enough (around 20%) to be considered significant in many predictive situations. We conclude that prediction based solely on the ensemble mean concentration would be inappropriate for this simple system and emphasize the importance of the ensemble variance for quantification of predictive uncertainty in stochastic modeling of solute transport.
Stochastic identification of transmissivity and effective recharge in steady groundwater flow: 2. Case study
Authors: Rubin Yoram , Dagan Gedeon
Published: February 2008

A first‐order analytical solution of the inverse problem for aquifer steady flow, presented in paper 1 (Rubin and Dagan, this issue), is applied to the Avra Valley aquifer (Clifton and Neuman, 1982). First, the parameters characterizing the statistical structure of the log‐transmissivity Y and water head H fields are estimated by a maximum likelihood procedure. The results for Y are in good agreement with those of (Clifton and Neuman, 1982), in spite of the different methodologies. The incorporation of head measurements is shown to have definite advantages in reducing the estimation variances of Y parameters. Next, the best estimates of Y at various points are obtained by simultaneous conditioning on the measurements of Y and H. It is shown that a substantial reduction in the variance of the conditioned Y is achieved by accounting for H measurements, justifying a posteriori the solution of the inverse problem. Finally, the effective recharge, which is assumed to be uniform, but random, is estimated as part of the process. Although the latter is relatively small for Avra Valley, it might be a parameter of considerable interest in other cases. Further applications of the methodology are suggested.
Streambank erosion along two rivers in Iowa
Authors: Odgaard A. Jacob
Published: February 2008

Rates and processes of bank erosion were evaluated for reaches of two alluvial, equiwidth to wide bend, point bar streams in Iowa: East Nishnabotna River and Des Moines River. The evaluation was made using historical records (airphotos, maps, and streamflow records), field measurements, and soil analyses. The erosion rates were correlated with channel characteristics such as width, depth, curvature, arc angle of channel centerline, channel slope, friction factor, and degree of vegetation on the banks. The correlation was studied using a meander flow model developed by the author. The meander flow model simulated the velocity distribution in the bends and the location of the erosion occurrences. Estimates were made of total sediment influx to the rivers from cutbank erosion.
Coupled thermomechanical response of an axisymmetric cold ice sheet
Authors: Hutter K. , Yakowitz S. , Szidarovszky F.
Published: February 2008

This study proposes a detailed model for ice sheets having a radially symmetric geometry. Governing relations are derived from first principles of physics, and the interactive effects of mechanical and thermal energy are taken into account. The resulting model is a nonlinear partial differential equation relating the temperature, velocity, and stress fields. Boundary conditions of accumulation rate, geothermal flux, sliding laws, and surface temperatures having been specified, and the model determines a steady state response in terms of ice sheet profile and internal temperatures, velocities, and stresses. We describe the computational methodology for numerically determining solutions from which one can assess the credibility of the model itself, and to the extent that it is credible, examine effects such as perturbing of external temperatures, accumulation rates, and so forth. Piloting examples illustrate this.
Two adjacent pipe diameters at the optimal solution in the water distribution network models
Authors: Fujiwara Okitsugu , Dey Debashis
Published: February 2008

In the study of optimization models for the design of water distribution networks, most of the literature has either indicated or explicitly claimed that at the optimal solution each link will consist of at most two pipe segments with adjacent diameters. This paper presents the necessary and sufficient conditions that a given set of pipe diameters is in an optimal solution and as a special case shows that the adjacency property holds if and only if pipe costs are a strictly convex function of a power of pipe diameters.
Cross‐channel mixing and its effect on sedimentation in the Orinoco River
Authors: Stallard R. F.
Published: February 2008

At high discharge, cross‐channel mixing of water in the Orinoco River main stem requires several days and up to 500 km of downriver transit for completion. During mixing, substantial amounts of < 0.063 mm sediment are lost from suspension. The water closest to the left bank appears to lose the most, probably by deposition onto the floodplain. This occurs in two ways. First, sediment‐laden left‐bank tributaries draining the Andes, such as the Apure, deposit sediment in distributary systems following impoundment by the Orinoco backwater. Second, when sediment‐laden waters from the Andes actually join the Orinoco, the slow nature of cross‐channel mixing causes the water and sediment to be confined more to the left side of the channel. Significant sediment loss or storage probably occurs only near flood stage. Since sedimentation occurs preferentially on the left (Andean foreland basin) bank, the Orinoco remains stably situated on the border of the foreland basin with the Guayana shield.
Evaluation of uncertainty in flood magnitude estimator on annual expected damage costs of hydraulic structures
Authors: Bao Yixing , Tung Yeou‐Koung , Hasfurther Victor R.
Published: February 2008

In the risk‐based design for hydraulic structures, the major task is the evaluation of the annual expected damage costs caused by floods. Due to the use of a limited amount of data in flood frequency analysis, the computed flood magnitude of a specified return period is subject to uncertainty. A methodology to integrate such uncertainty in the evaluation of annual expected flood damage is developed and illustrated through an example in culvert design. The effect of uncertainty in estimating flood magnitude using different hydrologic probability models with different sample sizes on the annual expected damage cost is examined. Results of the study show that the effect of the uncertainty in a flood magnitude estimate on annual expected damage is quite significant and is sensitive to the sample sizes and the probability distribution models used.
Probability distributions and their partitioning
Authors: Karlsson Per‐Ola , Haimes Yacov Y.
Published: February 2008

The partitioned multi‐objective risk method (PMRM) was developed for solving risk‐based multi‐objective decision making problems. Based on the premise that the expected value concept is not sufficient for proper decision making, the PMRM generates a number of conditional expected value functions (or risk functions) by partitioning the probability axis into probability ranges. The goal of partitioning the probability axis is to have better information on extreme events for decision making purposes. These conditional expectations are dependent on the chosen partitioning points. This paper analyzes how conditional expectations are sensitive to variations in partitioning. One of the risk functions is a measure of extreme and catastrophic events. By using the relationship between this particular risk function and the statistics of extremes, the sensitivity analysis is simplified. In many practical applications, it is difficult to determine which type of distribution function best represents the random process. Conditional expectations also depend on the choice of distribution, and the impact of this selection is discussed.
Scale dependence and the temporal persistence of spatial patterns of soil water storage
Authors: Kachanoski R. G. , de Jong E.
Published: February 2008

The concept of time stability as defined by Vachaud et al. (1985) is expanded to include general linear transformations in time and to account for the occurrence of spatial scale dependency. Time stability is described as the temporal persistence of a spatial pattern and is evaluated using correlation analysis of successive measurement dates. Spatial coherency analysis is suggested as a method for examining the temporal persistence of a spatial pattern as a function of spatial scale. Spatial coherency analysis was used to examine the temporal persistence of soil water storage (0–1.7 m) measured every 10 m in a 720‐m‐long transect for drying and recharge events. Soil water recharge altered the spatial pattern of water storage at small scales which was significantly related to surface (topographic) curvature. Drying did not alter the spatial pattern of soil water storage. The study supports the concept that soil water storage at a point is the product of hydrologic processes operating at different spatial scales. The analysis can be used to relate the spatial scale of processes to independent factors.
Tree ring‐based reconstruction of annual precipitation in the south‐central United States from 1750 to 1980
Authors: Blasing T. J. , Stahle D. W. , Duvick D. N.
Published: February 2008

A 231‐year reconstruction of annual precipitation, from 1750 through 1980 A.D., was developed from 10 tree ring chronologies (9 post oak, Quercus stellata, and 1 white oak, Q. alba, series) in the south‐central United States. Straight line regression was used to calibrate regionally averaged precipitation with ring width data, and the derived reconstruction was verified with independent climatic data and historical evidence. A variance trend in the tree ring data, which may have resulted from nonclimatic factors, was removed. The reconstructed precipitation series indicates that (1) a drought which appears to have been more severe than any in the instrumental record occurred about 1860 and (2) severe and prolonged droughts comparable to twentieth century events have occurred at roughly 15‐ to 25‐year intervals throughout the past 231 years. It follows that serious droughts in the south‐central United States could be expected to recur even in the absence of projected CO2‐induced warming.
Information, the decision forum, and third‐party effects in water transfers
Authors: Nunn Susan Christopher , Ingram Helen M.
Published: February 2008

Transfers of water from irrigation to municipal and industrial uses are seen as a low‐cost approach to the water supply problems of western cities. Rural areas of origin protest that market transfers ignore indirect economic, political, social, fiscal, and environmental effects of changes in water use. The capacity of five different water transfer institutions: the market, courts, legislature, special purpose districts, and administrative agencies, to develop and weigh information about indirect and nonuser impacts is analyzed and compared. All five forums are found to have biases regarding the type of information used. Markets process information on direct economic costs and benefits well but ignore third‐party costs; legislative bodies are sensitive to information about indirect and nonuser impacts but distort information on direct benefits and costs; neither the judiciary nor the water agency is likely to consider community and social impacts of water transfers. Special districts could consider both direct and indirect values but are often controlled by a leadership elite, pursuing narrow goals with minimum membership participation. The appropriate forum for decision making depends upon our priorities among values and on the values that are at stake in particular issues.
Simulated response of an acidic Adirondack Lake Watershed to various liming mitigation strategies
Authors: Davis James E. , Goldstein Robert A.
Published: February 2008

The Integrated Lake Watershed Acidification Study model was modified to accept pulse inputs of calcium carbonate dry deposition on user‐specified dates and over selective areas of the Woods Lake watershed. Responses at the lake outlet and in several soil horizons in the catchment were simulated for various liming scenarios comparing separate applications to both lake and land surfaces, during both spring and winter seasons, and at two different dose levels over the land area. The direct lake application was characterized by large, immediate, short‐duration increases in lake outlet pH and alkalinity, attributable to a sudden influx of alkaline material that quickly flushed through the aquatic system. The terrestrial application was characterized by lower magnitude, more gradual and longer duration increases in both lake outlet, and soil pH and alkalinity, attributable to enhancement of the base saturation at soil cation exchange sites. Inorganic aluminum concentrations, a concern with respect to fishery health, were only temporarily reduced following the lake application, while dramatic reductions were sustained for several decades following the terrestrial application; this suggests terrestrial liming slows the rate of chemical weathering and leaching of aluminum. The seasonal timing of the lime application significantly affected the magnitude and duration of the response in the lake application, but not in the land treatment, suggesting that the effectiveness of land treatment may not depend on seasonal flow dynamics. In addition, this indicates that mean hydraulic residence time may have certain limitations as an indicator of the retention time of chemical species introduced to an entire watershed, depending on the extent to which the land catchment influences adjacent water chemistry. These results suggest that for lakes and streams having short retention times, liming part or all of the land catchment might be advantageous to liming only the lake as a means of mitigating lake acidity. This conclusion and the potential for concurrent mitigation of terrestrial effects employing land catchment liming requires further experimental study.
Numerical modeling of separation eddies in shallow water
Authors: Yeh Harry H. , Chu Wen‐Sen , Dahlberg Olle
Published: February 2008

The modeling of free‐surface flows is usually made based on the time‐averaged form of the Navier‐Stokes equations (i.e., the Reynolds equations) to filter out the turbulent fluctuations. For coastal waters and estuaries, the time‐averaged hydrodynamic equations are often further simplified by averaging over depth. Assuming the hydrostatic pressure field, the depth‐averaged equation of conservation of mass is

urn:x-wiley:00431397:media:wrcr4526:wrcr4526-math-0001
where H is the total water depth, u = (u, v) is the depth‐averaged horizontal velocity components, and div denotes the divergence in the horizontal directions. Neglecting the Earth's rotation effects and shear stress at the free surface, and assuming ∇H/H ≪ 1, the depth‐averaged equation of conservation of momentum can be expressed as

urn:x-wiley:00431397:media:wrcr4526:wrcr4526-math-0002
where ξ is the departure of the water surface for a horizontal datum, v; is the kinematic viscosity of the fluid, ρ is the fluiddensity, u′ is the velocity deviation from u due to vertical velocity profile, u″ ; u″ is the time‐averaged value of a dyadic product of velocity fluctuation due to turbulence, τb is shearstress at the bottom boundary.
A single‐well tracing method for estimating regional advective velocity in a confined aquifer: Theory and preliminary laboratory verification
Authors: Leap Darrell I. , Kaplan Paul G.
Published: February 2008

An equation is derived by which advective groundwater velocity in a confined aquifer may be estimated by a single‐well tracer test in which a single tracer pulse is allowed to drift from the well and then pumped back to the well and sampled to obtain a breakthrough curve. Although similar in methodology to preexisting methods, this method differs in that it takes into account ambient groundwater movement during the pumpback phase. Using sodium chloride as a tracer solution, a series of small‐scale tests were run in a laboratory sand tank model to test the theory. Results of linear flow tracer tests through the model, simulating unperturbed regional advective flow at known velocities, were compared to results of single‐well drift‐and‐pumpback tests conducted during linear flow through the model. Advective velocities computed by both types of tests were identical, thus proving the validity of the equation.
Stochastic analysis of paths of high hydraulic conductivity in porous media
Authors: Silliman S. E. , Wright A. L.
Published: February 2008

Recent investigations of heterogeneity within subsurface media have shown the importance of understanding the structure of hydraulic conductivity fields when considering either flow or transport within porous media. In the present study, a Monte Carlo approach is utilized to investigate the existance and structure of continuous paths of high hydraulic conductivity within low conductivity media. The rock mass is modeled as a discretized grid of elements, each element characterized by a single value for the hydraulic conductivity. By transforming the hydraulic conductivity utilizing the cumulative distribution function of the conductivity, a new random variable is obtained which is uniformly distributed in the range (0, 1). Utilizing this parameter, we can locate the path (termed the extreme path) which (1) connects two specified faces of the simulation grid and (2) contains the maximum value for the minimum conductivity along all such paths. The transform of this maximized minimum is termed the extreme path value. Grids are analyzed which are characterized either by (1) independence between grid elements, (2) correlation between grid elements, or (3) conditioning on field values obtained from a site in southern Arizona. It is shown that for all grids analyzed, there will exist, on average, at least one path crossing the grid along which the hydraulic conductivity is everywhere greater than the effective conductivity of the medium. Further, when the minimum conductivity along such extreme paths is analyzed, it is observed that the expected value of this minimum conductivity increases with correlation. Finally, conditioning simulations on field values resulted in a decrease in the estimation variance of the minimum conductivity.
Physical and numerical studies of a fracture system model
Authors: Piggott Andrew R. , Elsworth Derek
Published: February 2008

Physical and numerical studies of transient flow in a model of discretely fractured rock are presented. The physical model is a thermal analogue to fractured media flow consisting of idealized disc‐shaped fractures. The numerical model is used to predict the behavior of the physical model. The use of different insulating materials to encase the physical model allows the effects of differing leakage magnitudes to be examined. A procedure for determining appropriate leakage parameters is documented. These parameters are used in forward analysis to predict the thermal response of the physical model. Knowledge of the leakage parameters and of the temporal variation of boundary conditions are shown to be essential to an accurate prediction. Favorable agreement is illustrated between numerical and physical results. The physical model provides a data source for the benchmarking of alternative numerical algorithms.
Efficient total enumeration of water resources alternatives
Authors: Ng Elliot K. , Heaney James P.
Published: February 2008

Successful regional water resources planning involves efficiency analysis to find the system that maximizes benefits minus costs or minimizes the cost of providing a specified level of output. The required input to the financial analysis is the optimal solution for the entire project and the optimal solutions for each participant acting independently and with any subset of the total group. For an nmember problem this necessitates the calculation of 2n″ ‐ 1 optimal solutions. This paper presents a reliable total enumeration procedure to solve this problem for small‐scale regional water supply systems. This procedure allows the use of realistic cost functions or detailed cost estimates to find the optimal solution, as well as to examine nonoptimal but good solutions.
Hydrogeologic factors affecting acid neutralization in Cadwell Creek Watershed, west central Massachusetts
Authors: Yuretich Richard , Leonard Wendy , Pohanka Susan
Published: February 2008

Cadwell Creek in west central Massachusetts, like many New England watersheds, is underlain by crystalline metamorphic bedrock with glacial till serving as the main unconfined aquifer. Groundwater hydrology and chemistry were monitored in a network of 10 wells. Low air‐equilibrated pH (lpar;4.7–5.4), low alkalinity (near 0 mg L−1 CaCO3), and low conductivity (23–34 μmhos) occur in the groundwater in the downstream parts of the watershed where hydraulic gradients are steep and groundwater residence times are short (about 3 months). On the other hand, the headwater areas are typified by low hydraulic gradients and slower groundwater velocities, with correspondingly higher air‐equilibrated groundwater pH (7.0–7.4), alkalinity up to 26 mg L−1, and higher conductivity (32–71 μmhos). Therefore even in this supposedly acid‐sensitive terrain, groundwater is neutralized if mineral‐water contact time is sufficiently long (here ∼1 year). Because some 70% of Cadwell Creek base flow originates as shallow groundwater in the lower reaches of the watershed, the stream is very acidic most of the year. Only during late summer, when deeper base flow from headwater sources prevails, does the pH and alkalinity of the streamwater rise appreciably.
A random spatial network model based on elementary postulates
Authors: Karlinger Michael R. , Troutman Brent M.
Published: February 2008

A model for generating random spatial networks that is based on elementary postulates comparable to those of the random topology model is proposed. In contrast to the random topology model, this model ascribes a unique spatial specification to generated drainage networks, a distinguishing property of some network growth models. The simplicity of the postulates creates an opportunity for potential analytic investigations of the probabilistic structure of the drainage networks, while the spatial specification enables analyses of spatially dependent network properties. In the random topology model all drainage networks, conditioned on magnitude (number of first‐order streams), are equally likely, whereas in this model all spanning trees of a grid, conditioned on area and drainage density, are equally likely. As a result, link lengths in the generated networks are not independent, as usually assumed in the random topology model. For a preliminary model evaluation, scale‐dependent network characteristics, such as geometric diameter and link length properties, and topologic characteristics, such as bifurcation ratio, are computed for sets of drainage networks generated on square and rectangular grids. Statistics of the bifurcation and length ratios fall within the range of values reported for natural drainage networks, but geometric diameters tend to be relatively longer than those for natural networks.
Redox‐controlled multiple‐species reactive chemical transport: 2. Verification and application
Authors: Liu Chen Wuing , Narasimhan T. N.
Published: February 2008

A redox‐controlled multiple‐species, multidimensional reactive chemical transport model, DYNAMIX, has been presented in part 1 of this paper. In part 2 the model is checked against two other independently developed reactive chemical transport codes. Simulated results from DYNAMIX agree reasonably well with those obtained from the other two models. Two approaches for simulating redox reactions, an external approach (based on hypothetical electron activity) and an effective internal approach (based on conservation of electrons), have been examined. The results show that in the external approach the simulated redox front is smeared out by diffusion and dispersion, whereas in the effective internal approach the redox front is sharp and is controlled by the redox reactions. The evolution of solid phases from the two approaches give markedly different mineral assemblages. It is suggested that the external approach is of relevance in industrial processes such as electrometallurgy, in which mineral dissolution is driven by externally supplied electric power. The effective internal approach is applicable to hydrogeochemical systems (groundwater contamination, diagenesis, ore formation, rock weathering, and soil genesis) in which redox potential is dictated by the states of the redox species. The results also suggest that in the presence of precipitation (and consequent retardation of the concentration front), spreading due to hydrodynamic dispersion may be significantly inhibited. To demonstrate the applicability of DYNAMIX for realistic field problems, two field cases are simulated by this model. In the first case, the supergene enrichment of copper at Butte, Montana, the simulated mineral assemblages and their distributions agree reasonably with the mineral assemblages and the ore grade observed in the field. In the second case, involving two‐dimensional contaminant transport in a hypothetic aquifer, the simulated results suggest that DYNAMIX is capable of handling a realistic multi‐dimensional field problem with several hundred grid blocks and time scales of practical interest.
Evaluation of groundwater replenishment coefficients from the record of a borehole penetrating the unsaturated zone
Authors: Bachmat Yehuda , Gvirtzman Haim , Magaritz Mordeckai
Published: February 2008

The coefficients of groundwater replenishment from surficial sources are the most critical parameters in assessing the exploitable yield of an aquifer. So far, the methods used for estimating these coefficients failed to yield plausible results, either because of crude models of the system or because of a simultaneous estimation of many parameters requiring long and diverse data records. This paper proposes a method for obtaining the in situ chronological sequence of the coefficients of replenishment at a given site, independently from the estimates of other parameters. It is based on the profiles of water content and concentrations of tritium and chloride in a single borehole penetrating the unsaturated zone. A combination of these data with the historical record of their inputs at the surface of the ground enables dating of the moisture content in each depth interval. This yields a series of seasonal replenishment coefficients from both natural and anthropogenic sources. The application of the method is illustrated at two sites in the central and southern parts of Israel.
Contaminant transport and biodegradation: 2. Conceptual model and test simulations
Authors: Kindred J. Scott , Celia Michael A.
Published: February 2008

A conceptual model of subsurface contaminant transport with biodegradation is presented. This model incorporates both aerobic and anaerobic reactions, with several models of differing complexity presented for the anaerobic case. Nutrient uptake is described using Michaelis‐Menten expressions, which are incorporated into a multiple nutrient uptake model. Several uptake inhibition factors are also included in the model. The mathematical model that results takes the form of a series of reactive transport equations. These equations are coupled nonlinearly through the reaction terms, which are themselves coupled to growth equations for subsurface bacterial populations. Example simulations are presented for a variety of cases, ranging from one‐substrate, one‐population aerobic degradation to multisubstrate, multipopulation anaerobic consortium degradation.
Dynamics of dissolved organic carbon in forested and disturbed catchments, Westland, New Zealand: 1. Maimai
Authors: Moore T. R.
Published: February 2008

Dissolved organic carbon (DOC) dynamics were investigated in 2 undisturbed catchments (beech‐podocarp‐hardwood forest) and 6 catchments which had been clear‐cut 8–10 years previously and planted with pine. Over a 4‐month sampling period, DOC concentrations in an undisturbed catchment averaged 1.4, 16.0, 356.0, and 55.7 mg/L in precipitation, throughfall, stemflow, and surface organic horizon, respectively. DOC concentrations decreased to averages of 11.8 and 4.5 mg/L in the subsoil and stream, respectively. The annual flux of DOC was estimated as an input through precipitation of 3.4 g/m2, 83.6 g/m2 through the organic horizon, and an export of 6.8 g/m2 in the stream. The pronounced decrease in DOC concentration and flux in the subsoil horizon is related to the high concentrations of amorphous, oxalate‐extractable Fe, and Al with high DOC‐adsorption capacities. In streams draining the 8 catchments, discharge‐weighted DOC concentrations ranged from 4.5 to 10.2 mg/L. DOC and log discharge showed a significant positive relationship and the rising limb of the storm hydrograph contained higher DOC concentrations than the falling limb, at similar discharges. Export of DOC in 1986 ranged from 8 to 21 g/m2 and was related to the type of disturbance: the lowest values were recorded in undisturbed catchments and the highest in catchments that had been clear‐cut and the slash had not been burnt. Organic debris in the stream channel appear to be important sources of stream DOC and probably account for the higher DOC concentrations in the disturbed catchments, 8–10 years after disturbance.
Stochastic‐deterministic nature of an elemental rainfall runoff process
Authors: Muzik I. , Beersing A. K.
Published: February 2008

The transformation process of probability density functions of rainfall intensity into the densities of peak discharge is investigated for the case of runoff from a uniformly sloping impervious plane. Normal, two‐parameter gamma, and exponential distributions of rainfall intensity are used in Monte Carlo simulations to obtain densities of peak discharge. The kinematic wave and experimentally derived relations are utilized to compute the peak discharge. The probability density function of peak discharge is shown to be the outcome of deterministic relationships operating on stochastic inputs. Conclusions are made about the effect of runoff plane physical characteristics on the probability density of peak discharge.
Changes in the chemical composition of stream water in two catchments in the Shenandoah National Park, Virginia, in response to atmospheric deposition of sulfur
Authors: Ryan P. F. , Hornberger G. M. , Cosby B. J. , Galloway J. N. , Webb J. R. , Rastetter E. B.
Published: February 2008

Rates of atmospheric deposition of sulfur are as high in areas of the southeastern United States as they are in the northeastern United States and southeastern Canada. Sulfate concentrations and acidity of surface waters in the southeastern United States, however, remain low because of sulfate adsorption by soils. Because the capacity of soils to adsorb sulfate is finite, the questions of when and how fast the surface waters in the southeast might respond to the high rates of acidic deposition are of some importance. Modeling studies have suggested that time constants for sulfate breakthrough may be of the order of decades. Given that acidic deposition levels have been high in the southeastern United States since the 1940s, it might be expected that trends of increasing sulfate concentration and acidity would be apparent in data from monitored catchments. An analysis of 8 years of discharge chemistry data for two catchments in Shenandoah National Park, Virginia, indicates that sulfate concentrations and acidity are increasing in these catchments. The discharge‐weighted mean annual sulfate concentrations increased at a rate of 2 μeq L−1 yr−1 for both streams over the 8 years of the study. Both catchments are currently retaining approximately 65% of deposited sulfate. The ratios of the rate of decrease of alkalinity to the rate of increase of sulfate concentration are 34% and 25% for the two streams. If deposition rates remain constant, the observed rates of increase indicate that steady state conditions will be approached in 100 years, at which time the stream alkalinities may be as low as −65 μeq L−1.