bluuebunny/crossref_metadata_2025 huggingface dataset:
The one‐dimensional quasi‐linear intercept on cumulative infiltration graphs
Authors: Weir G. J.
Published: February 2008

The asymptotic time behavior of one‐dimensional infiltration of water into a class of idealized soils is determined theoretically. First, we assume that all flows are one dimensional and investigate transient behavior between a steady initial state and another final steady state. Second, soil properties are idealized by making the quasi‐linear assumption of J. R. Philip (1969) that relative permeability is related exponentially to capillary pressure, which linearizes the flow terms. Third, and least importantly, we also assume later that changes in volumetric water content vary as a power of relative permeability. The transient flow equations then reduce to a nonlinear parabolic equation with relative permeability as the primary variable. One‐dimensional cumulative infiltration is then asymptotically a linear function of time, whose gradient depends on the saturated soil conductivity and whose intercept equals the change in field capacity (or near surface volumetric water content) divided by the quasi‐linear inverse length parameter. The equations for both the gradient and intercept for this asymptotic linear function are independent of time and independent of the relationship between volumetric water content and relative permeability. Finally, the three assumptions above imply that the time constant, representative of the duration needed to attain near steady conditions, essentially equals the sum of a term independent of the initial conditions, plus a term depending only on differences between the initial and final conditions.
Some space‐filling controls on the arrangement of tributaries in dendritic channel networks
Authors: Abrahams Athol D. , Updegraph John
Published: February 2008

The arrangement of tributaries in dendritic channel networks is controlled in part by the relationship between the spatial requirements of the tributaries and the availability of space. An investigation of 6105 tributaries in four dendritic networks reveals that the arrangement of tributaries of different sizes along subbasin main streams is influenced by two constraints on the availability of space on the acute side (inside) of the main streams. The first constraint arises from the tendency for semidivide angles (between the main stream and adjacent divides) at subbasin outlets to be larger on the obtuse (outside) than on the acute side of the main stream. This constraint not only causes a higher proportion of large tributaries than small ones to form on the obtuse side of subbasin main streams near their outlets but favors the development of large tributaries on the obtuse side farther upstream than small ones. The second constraint is imposed by the tendency for subbasin main streams to curve upstream. This constraint affects tributaries that are almost as large as the subbasin main stream they join and, like the first constraint, it favors the formation of large tributaries on the obtuse side farther upstream than small ones.
Use of contrasting D/H ratios of snows and groundwaters of eastern New York State in watershed evaluation
Authors: Lawrence James R.
Published: February 2008

The D/H ratios of snow on the ground and groundwaters in eastern New York State in early spring differ by 50‰ (δ notation). This is large in comparison to the analytical precision of ±1‰ and permits estimation of the input of snowmelt directly to runoff versus groundwater input to runoff resulting from increased hydraulic head as the snow melts and percolates into the ground. In 1978 the high overflow resulting from snowmelt contained only 25% meltwater; the rest was provided by an increased groundwater exfiltration.
The mobilization of aluminum in a natural soil system: Effects of hydrologic pathways
Authors: Cozzarelli Isabelle M. , Herman Janet S. , Parnell Roderic A.
Published: February 2008

A two‐component soil water flow model was used in conjunction with an equilibrium speciation model WATEQF to study aluminum mobility in soils of a forested watershed, White Oak Run, in the Shenandoah National Park, Virginia. Soil solution samples, taken from the O, E, B, C1, and C2 horizons, were collected from zero‐tension lysimeters designed to collect faster gravitational macropore flow and tension lysimeters designed to collect slower capillary micropore flow. Dissolved aluminum was fractionated into acid‐soluble, inorganic monomeric, and organic monomeric aluminum. Soil water aluminum concentrations decreased with depth indicating that the deep soil is a sink for aluminum. All waters contained significant concentrations of acid‐soluble aluminum and exhibited a negative correlation between pH and the inorganic monomeric aluminum concentrations. Water in the shallow soil showed distinctly different chemical compositions for the two flow types, while C horizon micropore and macropore waters were more similar. Because of its shorter residence time, water flowing in deep soil macropores underwent less extensive neutralization and immobilization of aqueous aluminum than micropore water. The O horizon macropore waters were undersaturated for all hydroxide, silicate, and sulfate mineral phases considered. The C horizon samples from both flow types were near equilibrium with respect to kaolinite and synthetic gibbsite, indicating that mineral solubility controls water chemistry in the deep soil, while organic substances are the key control in the shallow macropore waters.
Stochastic analysis of contaminant transport in porous media: Analysis of a two‐member radionuclide chain
Authors: Bonano Evaristo J. , Shipers Larry R. , Gutjahr Allan L.
Published: February 2008

In this study we extend previous stochastic analyses of contaminant transport in geologic media for a single species to a chain of two species. Our particular application is the quantification of uncertainties due to lack of characterization of the spatial variability of hydrologic parameters on transport of radionuclides from a high‐level waste repository to the biosphere. Radionuclide chains can have a significant impact on demonstrating compliance (or violation) of standards regulating the release to the environment accessible to humans. Two approaches for determining the cross‐covariance terms in the mean concentration equations are presented. One uses a Taylor expansion to obtain the cross‐covariance between the velocity and concentration fluctuations, while the other is based on a Fourier‐Laplace double transform method. For the conditions of interest here, the differences between these two approaches are expected to be small. In addition, the variances are calculated in a unique way by solving another associated partial differential equation. A parametric study is carried out to examine the sensitivity of the mean concentration of the two species and their corresponding variances and cross‐covariance on the parameters associated with the structure of the stochastic velocity field. It is found that the dependent variables are most sensitive to the intensity and correlation length of the velocity fluctuations. The magnitude of the variances and cross‐covariance of the concentrations are proportional to the magnitude of the mean concentrations which depend on inlet concentration boundary conditions.
Stochastic modeling of the insecticide fenitrothion in water and sediment compartments of a stagnant pond
Authors: Leduc Roland , Unny T. E. , McBean Edward A.
Published: February 2008

A stochastic model is developed for the behavior of the insecticide fenitrothion's decomposition in a New Brunswick (Canada) stagnant pond after aerial spraying. The concentrations of fenitrothion are modeled in two pond compartments: water and bottom sediments. Assuming first‐order kinetics, a mass balance equation of fenitrothion is written for each of the compartments. The deterministic model is obtained by integrating the linear system of differential equations. The stochastic model arises by considering both the initial condition in the water compartment and the input term in the sediment compartment to be stochastic, which results in a stochastic differential equation (SDE). The moment equation, derived from the Itô stochastic differential rule, provides the moment solution to the SDE. Temporal expectations and heteroscedastic variance models are thus obtained. The model is calibrated using observations obtained from fenitrothion sampling at three different locations in each of the pond's compartments. The stochastic model performs satisfactorily.
Irrigation control in the presence of salinity: Extended linear quadratic approach
Authors: Bras Rafael L. , Seo Dong‐Jun
Published: February 2008

An intraseasonal irrigation scheduling problem is dealt with via extended linear quadratic (ELQ) control. The ELQ control is well‐suited for constrained multidimensional problems and provides openloop feedback control rules over the control horizon. A conceptual model is developed to describe the dynamics of water allocation and salt movement in the root zone of a crop. Moisture stress and osmotic stress are combined to obtain the integrated inhibitory effect of salinity on transpiration. For the intraseasonal model to be effective against perennial salt accumulation in the root zone, it should be able to yield control laws which will lead to favorable root zone conditions at the end of an irrigation season, thus avoiding any significant leaching prior to the next growing season. This long‐term aspect of salinity control is handled via probabilistic state constraints which impose desired salinity and moisture levels with desired confidence level. The ELQ control is employed in a case study of expected net benefit maximization over an irrigation season of corn in Fort Morgan, Colorado. The results, in general, correspond well with expected irrigation schedules under different conditions and provide valuable information on both short‐ and long‐term aspects of irrigation control under saline conditions. The ELQ control, being an analytic iterative solution scheme with theoretically guaranteed fast convergence, has a distinct computational advantage over state‐of‐the‐art procedures.
Estimation of flood frequency: An evaluation of two derived distribution procedures
Authors: Moughamian Matthew S. , McLaughlin Dennis B. , Bras Rafael L.
Published: February 2008

The derived distribution approach for predicting flood frequency distributions provides a potentially attractive way to analyze flood hazards in ungaged watersheds. This paper examines the performance of two typical derived distribution techniques (Hebson and Wood, 1982; Diaz‐Granados et al., 1984). Flood frequency distributions obtained with each technique are compared to sample distributions estimated from measured discharge records. The comparison is based on over 40 years of discharge data from each of three different watersheds. Input parameters required by the derived distribution methods are estimated from limited data sets to simulate conditions encountered in watersheds where rainfall and discharge records are available for only a few years. Both of the derived flood frequency methods performed poorly in every watershed when compared to sample distributions. The results of the comparison suggest that fundamental improvements are needed before the derived distribution approach can be applied with any confidence.
Estimates of acidification of lakes in the Mt. Zirkel Wilderness Area, Colorado
Authors: Turk J. T. , Campbell D. H.
Published: February 2008

Seventy lakes in the Mt. Zirkel Wilderness Area, Colorado, were sampled for alkalinity, and 20 of these lakes were sampled for major anions. Analysis of the data indicates that most of the lakes are extremely sensitive to acidification; about half of the lakes have alkalinities less than 100 μeq/L. A linear regression of alkalinity against lake altitude accounts for 55% of the variance in alkalinity among the lakes, whereas a regression of alkalinity against dominant geologic unit (exclusive of altitude) accounts for 63% of the alkalinity variance. A conceptual model of lake acidification allows estimation of the quantity of acidification that may have occurred. Acidity associated with strong acid anions probably neutralized no more than 9 μeq/L of alkalinity. This estimate is much smaller than a previous estimate for nearby Colorado lakes.
Pattern of distribution of radium 226 in drinking water of Texas
Authors: Cech Irina M. , Prichard Howard M. , Mayerson Adele , Lemma Mengistu
Published: February 2008

Concentrations of radium (Ra) as high as 65 pCi/L have been reported in some of the public water wells in Texas. Yet, the overall pattern of the geographic distribution of Ra or its decay products has not been determined or mapped. The present study is a step in this direction. The distribution of 226Ra in domestic water, public and individual, in selected regions of the state of Texas was investigated to identify areas where water may contain naturally occurring elevated concentrations of Ra and to provide much needed information for well development and water quality management. Two regions with anomalously high Ra concentrations were identified, one in the gulf coast, with some concentrations over 20 pCi/L downdip from uranium deposits, and the other in west‐central Texas, with some concentrations up to 65 pCi/L on the periphery of an igneous uplift. Otherwise, the concentrations of Ra were low, particularly in the west and northwest of the state.
Interacting precipitation/dissolution waves: The movement of inorganic contaminants in groundwater
Authors: Dria Myra A. , Bryant Steven L. , Schechter Robert S. , Lake Larry W.
Published: February 2008

Chemical reactions with minerals occur in groundwater flow in permeable media to produce zones of constant solid and aqueous phase compositions. These zones are separated by “waves” or step changes in composition. The occurrence of wave‐wave interactions resulting from nonuniform boundary conditions has important implications in inorganic pollutant transport through permeable formations. Three physical flow situations produce these conditions : injection of a finite volume of fluid followed in time by a compositionally different fluid, the flow of a reactive fluid through a compositionally layered media, and reversal in the direction of flow of a chemically reactive fluid. The theory we present when applied to contaminant transport indicates that a significant amount of contaminant initially precipitated can be redissolved during subsequent flow, and the patterns thus evolved can indicate the nature of the contaminating source.
On the main channel length‐area relationship for channel networks
Authors: Mesa Oscar J. , Gupta Vijay K.
Published: February 2008

Motivated by the empirical relationship between the main channel length and the basin area for river networks, a similar equation is derived analytically using the random model postulates and the additional assumption that link lengths have a common exponential probability distribution. Specifically, it is shown that in networks with large magnitude m, the main channel length l(m), and the magnitude are related by l(m) ∼ β(2π)½(2π)½, where β is mean link length. This result is utilized to explain another empirical relationship between the distance to the center of gravity lc(m) and the main channel length; lc(m) ∼ l(m)/2. The observed deviations of the random model prediction regarding the main channel length from empirical observations is used to discuss some important open problems.
Effects of streamflow variation on critical water quality for multiple discharges of decaying pollutants
Authors: Eheart J. W.
Published: February 2008

The assumption that the worst water quality occurs at the lowest streamflow may not always hold in instances involving multiple discharges and nonconservative pollutants. The additional dilution resulting from increased streamflow may be offset by adverse changes in the parameters that govern water quality and in decreased residence time, which allows the stream less time to recover from the effect of one discharge before receiving another. This paper addresses the question of whether, with multiple sources of decaying pollutants, water quality might worsen with increasing streamflow. For an isothermal uniform stream it is shown that the pattern of discharge that maximizes the derivative with respect to streamflow of critical dissolved oxygen deficit or the concentration of a substance exhibiting a first‐order decay is an infinite uniform distributed load. Whether the maximum value of the derivative is positive or negative depends on the values of the parameters that characterize the hydraulic geometry of the channel and the dependence of reaeration on flow. Theoretical results presented here indicate that for most natural streams the traditional assumption, that the lowest streamflow is the worst from a water quality perspective, will usually be valid for first‐order pollutants. Nevertheless, they also lead to the expectation that increases in impacts with increasing streamflow might occur for dissolved oxygen, especially in highly polluted and regulated streams (pollution, water quality models, rules and regulations, and management).
Dynamic model of in‐lake alkalinity generation
Authors: Baker Lawrence A. , Brezonik Patrick L.
Published: February 2008

In‐lake alkalinity generation (IAG) is important in regulation of alkalinity in lakes with long residence times, particularly seepage lakes. An IAG model based on input/output modeling concepts is presented that describes budgets for each ion involved in alkalinity regulation by a single differential equation that includes inputs, outputs, and a first‐order sink term. These equations are linked to an alkalinity balance equation that includes inputs, outputs, IAG (by sulfate and nitrate reduction), and internal alkalinity consumption (by ammonium assimilation). Calibration using published lake budgets shows that rate constants are generally similar among soft water lakes (kSO4 ≈ 0.5 m/yr; kNO3 ≈ 1.3 yr−1; kNH4 ≈ 1.5 yr−1). Sensitivity analysis shows that predicted alkalinity is sensitive to water/residence time, but less sensitive to modest changes in rate constants. The model reflects the homeostatic nature of internal alkalinity generation, in which internal alkalinity production increases with increasing acid input and decreases with decreasing acid inputs of HNO3 or H2SO4.
Constant rate rainfall infiltration: A versatile nonlinear model: 2. Applications of solutions
Authors: White I. , Broadbridge P.
Published: February 2008

In paper 1 (Broadbridge and White, this issue) an analytical nonlinear model for constant rate rainfall infiltration was proposed which promised considerable versatility. In it a wide range of soil hydraulic properties are generated through the variation of a single free parameter C. Here three techniques are advanced for determining this parameter. The first, a one‐dimensional technique, involves simultaneous determination of sorptivity and wetting front position. The second uses measured values of surface water content at long infiltration times for rainfall rates less than the saturated conductivity. In the third, three‐ and one‐dimensional flow rates are measured on the same soil sample. All are suitable for field applications. The practical range of the C parameter, for a variety of repacked and in situ soils, is found to be restricted to between 1 and 2. The hydraulic conductivities and diffusivities of the model are in general agreement with independent measurements. Its moisture characteristics ψ(θ), which are not matched in any way to measured characteristics, follow closely those observed. Also, the model permits predictions of the dependence of sorptivity on antecedent water content given a single measurement of sorptivity. The analytic solutions for constant flux infiltration given in paper 1 (Broadbridge and White, this issue) describe satisfactorily the evolution of water content profiles and surface water pressure potential in the laboratory and field without a posteriori adjustments. The mathematically simple, traveling wave approximation agrees well with observations at comparatively short infiltration times. Finally, field and laboratory measured times to ponding are predicted satisfactorily by the model's analytic expression.
Unidimensional solute transport incorporating equilibrium and rate‐limited isotherms with first‐order loss: 2. An approximated simulation after a pulsed input
Authors: Lassey Keith R.
Published: February 2008

I develop and illustrate a simple approximation to the simulated concentration history at a position in a unidimensional flow path downstream of an idealized (Dirac delta function) pulse input. The simulation model incorporates combined Freundlich and rate‐limited isotherms as previously formulated. The approximation circumvents the numerical integration which the simulation would otherwise require; it is particularly reliable either for large Péclet number or for minor kinetic influence upon exchange or for both, and it is superior to invoking the “local equilibrium assumption” as an approximant.
Sequential estimation of aquifer parameters
Authors: Lu Allen H. , Schmittroth F. , Yeh William W‐G.
Published: February 2008

A two‐dimensional “inverse” or “parameter estimation” code was developed for use in the Basalt Waste Isolation Project site characterization program. The method uses a sequential least squares algorithm that incorporates a finite element groundwater flow code. Adjoint equations and flow equations are solved to provide an efficient calculation of sensitivity coefficients. The least squares algorithms are formulated to sequentially include the output information for updating a priori parameter estimates. Several cases were studied to demonstrate the methodology developed; included were an analysis of pumping from a single well and a sequential analysis in which results obtained by pumping from one well were combined with an analysis of pumping from a second well. The approach emphasizes the sequential nature of the problem whereby the characterization of site hydrology can be systematically improved.
Measurement and simulation of one‐dimensional transient three‐phase flow for monotonic liquid drainage
Authors: Lenhard R. J. , Dane J. H. , Parker J. C. , Kaluarachchi J. J.
Published: February 2008

Simultaneous movement of oil, water, and air in a sandy porous medium was investigated experimentally under transient flow conditions and results were compared to numerical simulations employing a finite element multiphase flow code. The liquid hydrocarbon was Soltrol 170, a low‐density branched alkane mixture. Liquid saturations were measured using a collinear dual‐energy gamma radiation apparatus and liquid pressures were measured using hydrophilic (untreated) and hydrophobic (treated) ceramic tensiometers connected to pressure transducers. The experimental regime was selected to impose monotonically draining water and total liquid saturation paths to avoid hysteretic effects. Measured saturations and pressures are compared to values obtained from numerical simulations of the experiment using a finite element solution of the governing multiphase flow equations assuming negligible gas pressure gradients. Functional relationships between permeabilities k, saturations S, and capillary pressures P employed in the numerical model were estimated by two calibration methods which require different degrees of experimental effort. Measured transient water saturation versus oil‐water capillary head data agreed well with predictions from static air‐water S‐P relations and interfacial tension data. Transient total liquid saturation versus air‐oil capillary head data deviated more severely from the scaled air‐water S‐P data, possibly reflecting noncompliance with the assumption of negligible gas pressure gradients. Reasonably good agreement was observed between measured and numerically simulated water and oil saturations and pressures in space and time. Sensitivity of the numerical results to calibration method was not great.
Estimation of the time scale of the geomorphologic instantaneous unit hydrograph from effective streamflow velocity
Authors: Agnese C. , D'Asaro F. , Giordano G.
Published: February 2008

The greatest difficulty in applying the geomorphologic instantaneous unit hydrograph is the estimation of the time scale of the hydrologic response of a basin. We derive the time scale from the effective streamflow velocity of the highest‐order stream and the spatial distribution of velocity throughout the stream network. The latter is described by a dimensionless geomorphologic function with a single‐shape parameter. The proposed method is applied to some Italian basins.
Drainage basin control of acid loadings to two Adirondack lakes
Authors: Booty W. G. , DePinto J. V. , Scheffe R. D.
Published: February 2008

Two adjacent Adirondack Park (New York) calibrated watersheds (Woods Lake and Cranberry Pond), which receive identical atmospheric inputs, generate significantly different unit area of watershed loading rates of acidity to their respective lakes. A watershed acidification model is used to evaluate the watershed parameters which are responsible for the observed differences in acid loadings to the lakes. The greater overall mean depth of overburden on Woods Lake watershed, which supplies a greater buffer capacity as well as a longer retention time of groundwater, appears to be the major factor responsible for the differences.
Evaluation of a homogeneous point process description of Arizona thunderstorm rainfall
Authors: Jacobs Bruce L. , Rodriguez‐Iturbe Ignacio , Eagleson Peter S.
Published: February 2008

Rain gage observations of storm events at Walnut Gulch, Arizona, are used to evaluate the Rodriguez‐Iturbe and Eagleson (1987) stochastic, point process model of storm events. Five hundred and fifty eight summer storm events are identified over the years 1970–1977 and the parameters of the model estimated by the method of moments for each event as well as for the entire ensemble of events. The model is evaluated by comparing the observed correlation function of the temporally averaged rainfall process to its theoretical analogue.
Contaminant transport in fractured porous media: Steady state solutions by a boundary integral method
Authors: Fogden A. , Landman K. A. , White L. R.
Published: February 2008

The problem of steady state transport of a decaying contaminant in a fractured porous rock matrix by two‐dimensional diffusion and vertical advection is solved using a powerful (and readily generalizable) numerical method. Worst case predictions of contaminant penetration are obtained by assuming that the uniform porous medium contains either a single vertical fracture or a periodic array of vertical fractures. The method requires construction of appropriate Green's functions, which are here obtained analytically in both the fracture and matrix regions. Application of Green's theorem in the plane then yields representations for the contaminant concentration in both regions in terms of integrals along their boundaries of known quantities and the unknown fracture‐matrix interfacial flux. This flux is given by a Fredholm integral equation which can be solved numerically to great accuracy using a simple discretization technique. At sufficiently large distances vertically from the source the concentration is observed to be exponentially decaying. Although the concentration can, in general, only be obtained numerically, an analytical method is developed which produces a uniformly valid and extremely accurate approximation to its vertical decay length. Also, an assessment of the accuracy of analytical approximations made by various authors to solution of the full problem is provided. The parameter space of validity of these simpler psuedo‐two‐dimensional models is determined.
Improved methodology for a collinear dual‐energy gamma radiation system
Authors: Stillwater Roger , Klute Arnold
Published: February 2008

Improved procedures are presented for the measurement of water content and soil bulk density using a collinear 241Am and 137Cs gamma radiation system. More accurate procedures than those previously available are offered to correct the americium count rate for dead time and Compton‐scattered cesium radiation. Procedures are also presented for the direct measurement of system dead times, based on single‐channel analyzer control of dead time. The concept of a maximum system processing rate is discussed, particularly emphasizing its implications for the theoretically achievable measurement precision using available technology. A calibration procedure is offered to improve this precision, by reducing or eliminating the highest (empty cell) count rate from the calibration procedure. To also improve the theoretical system precision, the optimal range of the ratio of corrected americium to cesium count rates is found. A method is presented for determining the actual precision and accuracy of measurements.
A theoretical analysis of seepage flow of the confined groundwater into the lake bottom with a gentle slope
Authors: Fukuo Yoshiaki , Kaihotsu Ichirow
Published: February 2008

Recently, in Japan, a new interest has been shown to the specific discharge of groundwater from the lake bottom in order to elucidate its influence on the quality and quantity of lake water. Using conformal mapping techniques, the authors describe analytically the steady two‐dimensional groundwater flow in an idealized confined aquifer, which extends infinitely with uniform thickness and constant inclination and is exposed to the lake bottom surface with a gentle slope. The techniques enable analytical calculation of the head and stream functions of groundwater in the aquifer and also the distribution of specific discharge at lake bottom surface. From the results of calculation, it is found that most of the aquifer water flows into the lake through the upper part of the surface of contact between lake and aquifer. This analysis may be useful in the evaluation of the role of groundwater seepage in the water balance of large lakes.
On the use of change constraints in reservoir design and operation modeling
Authors: Loaiciga Hugo
Published: February 2008

This work examines (1) the set of probabilistic and deterministic constraints binding the objective function of a reservoir design and operation problem, (2) the conditions under which the solution of a reservoir design and operation problem becomes infeasible, and (3) the discrepancies between the exact chance constrained formulation of the reservoir design and operation problem and alternative models. The crux of the paper is the difference between chance‐constrained feasibility sets that restrict reservoir levels to be within zero and maximum capacity with probability one and those sets that impose a less stringent chance constraint on reservoir levels. We derive the probability that a reservoir solution computed with the less stringent feasibility set is physically feasible (i.e., reservoir levels are bounded by zero and reservoir capacity). Such probability is found to be a function of prespecified reliability levels, the length of the planning horizon, the joint distribution of streamflows and initial reservoir storage, reservoir capacity, and two chance‐constrained parameters. It is concluded that physically feasible solutions yielded by alternative models are valuable if they serve as guidelines that are updated (as they are implemented) in response to unforeseen, uncontrollable events.
The extent of lateral water movement in the sediments of a New England Salt Marsh
Authors: Nuttle William K.
Published: February 2008

Models of depth‐averaged hydraulic head are used with data from a water balance study in a New England salt marsh to describe horizontal pore water water fluxes near a creek bank. Three hydrologically distinct regions are identified in the sediment. In the marsh studied, semidiurnal tides may drive an oscillating horizontal flux in the narrow region within about 2.5 m of the creek bank. Farther than 15 m from the creek there is essentially no horizontal water movement. In the region between 2.5 and 15 m, drainage to the creek is driven by alternating periods of surface flooding and nonflooding due to the spring neap variation in tidal amplitudes. There is little or no input of fresh groundwater to the marsh sediment at this site. The spatial extent of the drained region depends on the duration of the nonflooded period, the morphology of the sediment surface, and the ratio of hydraulic conductivity to specific storage of the sediment.
Solutions approximating solute transport in a leaky aquifer receiving wastewater injection
Authors: Chen Chia‐Shyun
Published: February 2008

A mathematical model amenable to analytical solution techniques is developed for the investigation of contaminant transport from an injection well into a leaky aquifer system, which comprises a pumped and an unpumped aquifer connected to each other by an aquitard. A steady state groundwater flow field is assumed, where the injected fluids move horizontally in the pumped aquifer and vertically in the aquitard. The unpumped aquifer is assumed to remain in hydrostatic condition due to its large transmissivity. Descriptions of groundwater velocities are based on appropriate leaky aquifer well hydraulics. The model assumes that contaminants are transmitted in the pumped aquifer by radial advection, and in the aquitard by vertical one‐dimensional advection and longitudinal dispersion. Two coupled linear differential equations are formulated to deal with this transport problem; they involve variable coefficients dependent on Bessel functions. The unpumped aquifer is incorporated into the model as a boundary condition. Semianalytical transient solutions for concentration distributions in the pumped aquifer and the aquitard can be obtained by numerically inverting the Laplace domain solutions of the model with the Crump [1976] method. However, the steady state solutions of the model are determined in closed forms. In general, the modeling approach neglecting longitudinal dispersion in the pumped aquifer and lateral dispersion in the aquitard does not introduce significant error to the modelling results, as shown by the good agreement between numerical solutions including these two transport mechanisms and the solutions of the proposed model. Within the context of the assumed hydrogeological conditions, the proposed model appears to be a valid approximation to the problem of interest. Type curves for concentration evaluated at the well bore and the top of the aquitard are given, which can be employed to estimate whether the unpumped aquifer would be polluted as the consequence of the injection. Neglecting the leakage effects in a leaky aquifer can cause significant error in studying the transport problem.
Recursive parameter estimation of hydrologic models
Authors: Rajaram Harihar , Georgakakos Konstantine P.
Published: February 2008

Proposed is a nonlinear filtering approach to recursive parameter estimation of conceptual watershed response models in state‐space form. The conceptual model state is augmented by the vector of free parameters which are to be estimated from input‐output data, and the extended Kaiman filter is used to recursively estimate and predict the augmented state. The augmented model noise covariance is parameterized as the sum of two components: one due to errors in the augmented model input and another due to errors in the specification of augmented model constants that were estimated from other than input‐output data (e.g., topographic and rating curve constants). These components depend on the sensitivity of the augmented model to input and uncertain constants. Such a novel parameterization allows for nonstationary model noise statistics that are consistent with the dynamics of watershed response as they are described by the conceptual watershed response model. Prior information regarding uncertainty in input and uncertain constants in the form of degree‐of‐belief estimates of hydrologists can be used directly within the proposed formulation. Even though model structure errors are not explicitly parameterized in the present formulation, such errors can be identified through the examination of the one‐step ahead predicted normalized residuals and the parameter traces during convergence. The formulation is exemplified by the estimation of the parameters of a conceptual hydrologic model with data from the 2.1‐km2 watershed of Woods Lake located in the Adirondack Mountains of New York.
The winter cover of a high‐mountain Mediterranean lake (Estany Redó, Pyrenees)
Authors: Catalan Jordi
Published: February 2008

Deep lakes freeze late in the Pyrenees, and time for black ice growth is short because snow falls abundantly. More than 70% of the winter cover water equivalent comes from the lake, mainly in the form of flooding water. The quantity of flooding water is proportional to the accumulated precipitation. Silicate, bicarbonate, calcium, and inorganic nitrogen are the compounds more effectively excluded when ice forms. Fresh snow shows a high variability in its chemistry, especially in nitrogen compounds and chloride. The destructive metamorphism of the snow is accentuated during high radiation periods, promoting an enrichment of the slush layers with some ions by preferential elution. Nevertheless, the flooding process is the principal factor controlling the chemical composition of the cover and causes the cover composition to approach the lake water composition. The important role of the flooding water has implications in the evaluation of water resources and chemical budgets of the lake.
Water and surface energy balance model with a multilayer canopy representation for remote sensing purposes
Authors: van de Griend A. A. , van Boxel J. H.
Published: February 2008

A surface energy balance model with a multilayer canopy representation has been developed to simulate the vertical temperature distribution of the foliage elements and the soil underneath. This model allows one to simulate the radiative temperature of the complex vegetation‐covered surface as a function of the angle of observation. The model was implemented for a two‐layer canopy and has been tested by comparing simulated radiative surface temperatures (observed under a look angle of 45°), latent heat, sensible heat, momentum, and ground heat fluxes, and soil temperatures at different depths, with observed data, gathered in a maize field. The fluxes above the canopy were measured by eddy correlation methods. This paper describes the model structure and simulation results as compared with the field measurements. Except for the ground heat fluxes, all the simulated and measured parameters match reasonably well. Further, the model was used to study the sensitivity of the radiative temperature of the complex surface to variations in the roughness length z0, the leaf area index LAI, and the drag coefficient Cd. The radiative surface temperature turned out to be very insensitive to variations in Cd and LAI, and it was found that resulting variations in the radiative surface temperature fall well within the detection error of the remotely sensed surface temperature.
Optimum experimental design for parameter identification in groundwater hydrology
Authors: Hsu Nien‐Sheng , Yeh William W‐G.
Published: February 2008

The experimental design for parameter identification of a confined groundwater system is formulated as a nonlinear mixed integer programming problem. The decision variables considered are the number and locations of the pumping and observation wells, and the pumping rates. The constraints include the allowable drawdown, the restricted locations of pumping wells and observation wells, the permissible pumping rates, and the required information. The amount of the required information is determined by the prescribed reliability of the response for the future operation of the groundwater system. The Galerkin finite element‐Crank Nicolson approach is used to develop the simulation models for solving the groundwater governing equation for the system responses. The sensitivity coefficients are computed by the influence coefficient method. The principle of superposition is used to combine the simulation models with the optimization model. The characteristics of the optimization problem have been investigated and are clarified, and its difficulties are pointed out. An efficient heuristic solution approach is proposed for solving this problem. A test problem is constructed to demonstrate the usefulness of the proposed methodology. Observations generated from the optimal design are used to identify the parameters. A covariance analysis is then carried out to verify the validity of the assumptions made in the formulation of the optimization problem. The results obtained indicate that the proposed methodology is feasible and can be used to solve complicated experimental design problems associated with a confined groundwater system.
Hydrodynamic dispersion at the local scale of continuum representation
Authors: Moltyaner G. L.
Published: February 2008

Two types of measurement devices, a fully penetrating dry observation well with a gamma ray detector and a multilevel piezometer with a 7.5‐cm‐long screen, were used for measurement of radiotracer concentrations in three natural gradient dispersion tests performed at the Twin Lake aquifer, Chalk River, Ontario over 20, 40, and 260 m distances. The devices are classified according to the method of measurement of tracer concentration as through‐the‐wall and mixing‐cup measurement devices. The weighting function associated with the through‐the‐wall measurement device is defined by considering the process of interaction of gamma radiation with aquifer materials in a laboratory experiment and is found to follow an exponential attenuation law. As a result, a continuum description of the porous medium is introduced in which the field variable is defined as a convolution product of associated microscopic values with the exponential weighting function. An experimental method for measuring the field variable according to the convolution procedure is proposed. It is shown that in the field tests the weighting function associated with the through‐the‐wall and mixing‐cup measurement devices can be approximated by a “table” function with the scale of support of the order of centimeters. The scale of support is referred to as the local scale of continuum representation. It is postulated that the transport of a tracer at the Twin Lake aquifer can be quantified by means of the local scale advection‐dispersion equation. Dispersivity values obtained from three field‐tracer dispersion experiments using the local scale advection‐dispersion equation are found to be similar to dispersivity values obtained from 23 laboratory columns. The comparison between results of field and laboratory experiments showed that patterns of dispersion observed in field tests at the local scale (using different measurement devices with the same scale of support) are similar to the patterns observed in small‐scale laboratory experiments. The comparison between results of three field tests performed under different experimental and hydrogeological conditions and over different scales showed the universal nature of the local scale characteristics of the dispersion process.
A symmetric‐matrix time integration scheme for the efficient solution of advection‐dispersion problems
Authors: Leismann Heinz Michael , Frind Emil O.
Published: February 2008

A new time integration scheme for the time‐marching numerical solution of the advection‐dispersion equation is formulated. The scheme leads to a symmetric positive‐definite coefficient matrix that can be solved with any symmetric‐matrix solver. Matrix symmetry is achieved by placing the advective term at the old time level. The resulting numerical errors are minimized by introduction of an artificial diffusion term and by optimal time weighting of all terms on the basis of a Taylor expansion of the governing differential equation. The scheme is unconditionally stable and effectively second‐order accurate, giving results equivalent to those obtained with the Crank‐Nicolson scheme. The use of highly efficient symmetric versions of iterative solution techniques, such as the conjugate gradient method, in the solution of advection‐dispersion problems thus becomes possible. Such techniques can provide substantial advantages in the case of grids with large numbers of nodal points, particularly in three dimensions.
The seepage exclusion problem for sloping cylindrical cavities
Authors: Philip J. R.
Published: February 2008

Solutions of the seepage exclusion problem for sloping cylinders of arbitrary cross section are simply deduced from those for horizontal cylinders. The three‐dimensional flow field around sloping cylinders may be specified by two scalar functions of two space coordinates. Liability to water entry from downward unsaturated seepage decreases as the slope of the cylinder axis increases.
Asymptotic solutions of the seepage exclusion problem for elliptic‐cylindrical, spheroidal, and strip‐ and disc‐shaped cavities
Authors: Philip J. R.
Published: February 2008

Asymptotic roof boundary layer solutions are obtained for the quasi‐linear seepage exclusion problem for elliptic‐cylindrical and spheroidal cavities of arbitrary aspect ratio h. The remarkably simple result for ϑmax, the maximum dimensionless potential at the cavity wall, is ϑmax(σ) ∼ 2σ + f0(h) + [f1(h)/σ] + ]f2(h)/σ2 + … where σ=½ αk−1 (α is sorptive number; k is apical total curvature) and the fn are polynomials in h−2 of degree (n + 1). The results agree, as they should, with previous results for the circular cylinder and sphere (h=1) and the parabolic cylinder and paraboloid (h=∞). Connections noted in earlier studies between apical sharpness and the variation of potential along the cavity wall are generalized and made quantitative. The singular (h =0) strip and disc are of especial interest: they indicate behavior near the critical upstream stagnation point on cavities (and impermeable obstacles) with flat strip‐ and disc‐shaped roofs but otherwise (within some limits) arbitrary. For the strip, ϑmax(S1) ∼ 4S12 + 9 ‐ (36/S12) + ⋯, and for the disc, ϑmax(S1) ∼ 2S12 + 6 ‐ (27/S12) + ⋯, where s1=½αl1 (l1 is semiwidth or radius). The central importance of ϑmax is that seepage water enters, or is excluded from, the cavity, depending on whether K0/K1 (K0 is downward steady seepage velocity far from the cavity; K1 is saturated hydraulic conductivity) exceeds or does not exceed ϑmax−1.
Two‐dimensional transport and fate of chemicals emitted by arbitrarily placed sources in confined aquifers
Authors: Lindstrom F. T. , Boersma L.
Published: February 2008

A two‐dimensional mathematical model (TWODIMPL) is presented that describes the advective‐dispersive transport and the transformations of chemicals leaking into confined aquifers. Linear equilibrium rules are assumed to apply to the sorbing soil components made up of weakly sorbing and strongly sorbing fractions and an organic matter fraction. First‐ or zero‐order loss rates of compound due to microbial or other irreversible loss processes are included. Aquifers of rectangular shape and infinite in the transverse dimensions with constant strength sources of chemical, emitting uniformly across the finite vertical aquifer thickness are assumed. The linear parabolic transport equation is solved via the classical Green's function approach. The spatial distribution of chemical concentration at any time derived from any and all distributed sources is given as a time convolution integral. The integral is approximated numerically by a specialized Simpson's quadrature procedure. Several specific scenarios are evaluated and discussed.