bluuebunny/crossref_metadata_2025 huggingface dataset:
Economics of water allocation to instream uses in a fully appropriated river basin: Evidence from a New Mexico wild river
Authors: Ward Frank A.
Published: February 2008

In fully appropriated multiple‐use river basins, a major potential competitor for a share of water may be publicly sponsored appropriations to supplement low streamflows for fish, wildlife, and recreation, which generates economic values not revealed in the marketplace. Based on a survey of instream recreationists on New Mexico's Rio Chama a travel cost model is developed to identify the potential recreation demand for instream flows. A discrete optimal control model is formulated that solves for the intraseasonal allocation of reservoir releases which maximizes the yearly value of instream recreation benefits, net of values of competing uses in the basin. Results indicate that in New Mexico, reservoir releases which augment low streamflows can return gross recreation benefits in the range of $900 to $1100 per acre‐foot (ac ft) of water consumed (1 ac ft = 1.233 × 103m3). This compares to a $40/ac ft cost of using the water. Consequently, results strongly support the hypothesis of potential economic payoff from public investments in and management of instream flow reservations.
An application of parametric mixed‐integer linear programming to hydropower development
Authors: Turgeon André
Published: February 2008

The problem consists in selecting the sites on the river where reservoirs and hydroelectric power plants are to be built and then determining the type and size of the projected installations. The solution obviously depends on the amount of money the utility is willing to invest, which itself is a function of what the new installations will produce. It is therefore necessary to solve the problem for all possible amounts of firm energy produced, since it is not known at the outset which production level the utility will select. This is done in the paper by a parametric mixed‐integer linear programming (MILP) method whose efficiency derives from the fact that the branch‐and‐bound algorithm for selecting the sites to be developed (and consuming most of the computer time) is solved a minimum number of times. Between the points where the MILP problem is solved, LP parametric analysis is applied.
Optimization of sampling locations for variogram calculations
Authors: Warrick A. W. , Myers D. E.
Published: February 2008

A method is presented and demonstrated for optimizing the selection of sample locations for variogram estimation. It is assumed that the distribution of distance classes is decided a priori and the problem therefore is to closely approximate the preselected distribution, although the dispersion within individual classes can also be considered. All of the locations may be selected or points added to an existing set of sites or to those chosen on regular patterns. In the examples, the sum of squares characterizing the deviation from the desired distribution of couples is reduced by as much as 2 orders of magnitude between random and optimized points. The calculations may be carried out on a microcomputer. Criteria for what constitutes best estimators for variogram are discussed, but a study of variogram estimators is not the object of this paper.
Periodic optimal control with application to pumped storage plants
Authors: Weiner Dan
Published: February 2008

This paper suggests how nonlinear periodic optimal control of a pumped storage plant can be realized. The control problem consists in maximization of the plant benefits over an operational period. An optimal control law is proposed, yielding a bang‐off‐bang mode of operation. An algorithm for numerical solution of the problem was developed, and its effectiveness was demonstrated by simulation experiments.
A study of cell models: 2. The effect of time delay on the limiting forms of cascade and manifold cell model response functions
Authors: Pegram G. G. S. , Diskin M. H.
Published: February 2008

A simplified cell model was used to investigate the behavior of the impulse response function (IRF) of cell models incorporating time delay elements, as the number of subdivisions increases. The simplified model used in this study was a single branch model consisting of a number of equal cells arranged in sequence. Two cell models were studied, a cascade model and a manifold model. In this study the model representing each cell is modified by the addition of a delay element which is taken to be linearly proportional to the distance of its corresponding cell from the outlet. The limiting forms of the IRFs are derived for the two models using Laplace transforms; the cascade cell model IRF is found to converge to a rectangle regardless of the values of its parameters, while the manifold cell model IRF converges to a family of curves whose shapes depend quite sensitively on the values of the parameters chosen. The expression derived contains the exponential integral function, which has not been used before in surface hydrology. For some combinations of the parameters, the IRF has the classical (or intuitively expected) shape consisting of a steep rise and mild recession. For other values, the IRF is reminiscent of the kinematic wave solution to the overland flow equations, and some comparisons are made. The manifold cell model is found to hold promise as a suitably behaved alternative to the cascade cell model in the context of distributed surface runoff systems.
Simulation of contaminant transport in three dimensions: 1. The alternating direction Galerkin Technique
Authors: Burnett R. D. , Frind E. O.
Published: February 2008

The alternating direction Galerkin technique for the simulation of advective‐dispersive transport in three dimensions is developed. The technique is designed for high efficiency in handling the large and detailed grids that are often required in simulations of natural groundwater systems. It is comparable to an alternating direction finite difference scheme, except for two advantages: first, the capability to handle certain curvilinear grids that may conform to either a flow net geometry or a natural stratigraphy and, second, the option to enhance solution accuracy through a choice between a finite difference or a finite element representation of the time derivative. Criteria for controlling numerical dispersion and accuracy can be applied easily. Three options for the time‐stepping algorithm are developed and analyzed for stability, and their accuracy is investigated for simple flow systems. The technique is shown to be far more efficient than a conventional three‐dimensional finite element model. In a companion paper, an application to a field‐scale contaminant transport problem is described.
Synthesis of a simplified water use simulation model for predicting wheat yields
Authors: Arora V. K. , Prihar S. S. , Gajri P. R.
Published: February 2008

A simplified water use simulation model has been developed that predicts components of field water balance for wheat using information on atmospheric evaporative demand, extractable water and draining characteristics of soil profile, leaf area development, root growth characteristics, and soil wetting events. The model accounts for soil water flow with redistributing water below field capacity and estimates layerwise water use. Predicted evaportranspiration and soil water content in different soil layers matched well with the measured values for a wide range of water management situations and the root‐meansquare error varied between 10 and 15%. Transpiration‐yield relations, estimated through a multiplicative approach, showed that booting‐heading phase of wheat development was most sensitive to water stress. Validation of this model against independent data was reasonably well, thereby suggesting that the model dependably predicts grain yield. Small data required for the operation of the model increase its capability as management tool for efficient on‐farm water use.
Effect of soil water hysteresis on solute movement during intermittent leaching
Authors: Jones M. J. , Watson K. K.
Published: February 2008

The use of a computer‐based numerical solution of the hydrodynamic dispersion equation, coupled with a program for the movement of soil water incorporating an interpolative hysteresis model, is described. Results are presented for nonreactive solute movement during an intermittent leaching regime under a constant concentration boundary condition at the surface, both including and excluding the effects of soil water hysteresis. When hysteresis data is included in the analysis, the solute mass profiles indicate that the solute moves through the soil profile in a series of well‐defined peaks. This is in marked contrast to the situation in which hysteresis is neglected. For this case the well‐defined peaks disappear after the first infiltration event, and the solute becomes fairly uniformly distributed much deeper in the profile. The possible application of intermittent leaching is discussed briefly.
On the nonconvexity of the reservoir design problem
Authors: Turgeon André , Brosseau Gilles
Published: February 2008

The reservoir design problem consists of choosing the sites on a river where reservoirs are to be built and determining the size of the installations. The problem has nonconvexities, and ignoring them may lead to a completely incorrect solution. A branch‐and‐bound method using concave overestimation is developed for dealing with these nonconvexities. It guarantees a globally optimal solution to be found in a finite number of steps.
Determination of aquifer transmissivity from Earth tide analysis
Authors: Hsieh Paul A. , Bredehoeft John D. , Farr John M.
Published: February 2008

The water level in an open well tapping an artesian aquifer responds to pressure head disturbance caused by Earth tide dilatation of the aquifer. Because a finite amount of time is needed for water to flow into and out of the well, there exists a phase shift (or time lag) between the tidal dilatation of the aquifer and the water level response in the well. We derive an analytical solution that expresses the phase shift as a function of the aquifer transmissivity, storage coefficient, well radius, and the period of the harmonic disturbance. This solution is rather insensitive to the storage coefficient. Thus if the phase shift is known for a harmonic disturbance, the transmissivity can be calculated given a rough estimate of the storage coefficient. Theoretical analysis shows that a significant phase shift may be present even if the disturbance is slowly varying, as in the case of Earth tides. This opens the possibility of estimating aquifer transmissivity from water level records that show Earth tide fluctuations. A case study, using data from a site near Parkfield, California, is presented to illustrate application of the theory. Phase shifts of the O1 (25.82‐hour period) and M2 (12.42‐hour period) tidal constituents are chosen for analysis because they are free of systematic contamination by fluctuations in barometric pressure. A brief error analysis suggests that the computed O1 phase shift is subject to large uncertainty, while the computed M2 phase shift is substantially more accurate. Based on an assumed storage coefficient range of 10−4 to 10−6, the estimated transmissivity range is 8 × 10−6 to 2 × 10 −5 m2/s. While hydraulic tests have not been performed to validate these estimates, the range is consistent with the transmissivity value determined by other investigators from analysis of the water level response to an earthquake.
Monte Carlo studies of sampling strategies for estimating tributary loads
Authors: Richards R. Peter , Holloway Jim
Published: February 2008

Monte Carlo techniques were used to evaluate the accuracy and precision of tributary load estimates, as these are affected by sampling frequency and pattern, calculation method, watershed size, and parameter behavior during storm runoff events. Simulated years consisting of 1460 observations were chosen at random with replacement from data sets of more than 4000 samples. Patterned subsampling of these simulated years produced data appropriate to each sampling frequency and pattern, from which load estimates were calculated. Thus results for all sampling strategies were based on the same series of simulated years. Sampling frequencies ranged from 12 to roughly 600 samples per year. Unstratified and flow‐stratified sampling were examined, and loads were calculated with and without the use of the Beale Ratio Estimator. All loads were evaluated by comparison with loads calculated from all 1460 samples in the simulated year. Studies consisting of 1000 iterations were repeated twice for each of five parameters in each of three watersheds. The results show that bias and precision of loading estimates are affected not only by the frequency and pattern of sampling and the calculation approach used, but also by the watershed size and the behavior of the chemical species being monitored. Furthermore, considerable interaction exists between these factors. In every case, loads based on flow‐stratified sampling and calculated using the Beale ratio estimator provided the best results among the strategies examined. Differences in bias and precision among watersheds and among transported materials are related to the variability of instantaneous fluxes in the systems being monitored. These differences are qualitatively predictable from knowledge of the time behavior of the material and hydrological systems involved. Attempts to derive quantitative relationships to predict the sampling effort required to achieve a specified level of precision have not been successful.
A model for hysteretic constitutive relations governing multiphase flow: 2. Permeability‐saturation relations
Authors: Lenhard R. J. , Parker J. C.
Published: February 2008

A theoretical model is described for the prediction of relative permeability‐saturation (k‐S) relations in two‐phase (air‐water) and three‐phase (air‐oil‐water) porous media systems subject to arbitrary saturation paths. Integral expressions for air, water, and oil realtive permeabilities are presented which extend the nonhysteretic relative permeability model of Parker et al. (1987) to accomodate effects of pore blockage by air trapped in water and oil phases and oil trapped in the water phase. The parametric model for saturation‐pressure (S‐P) relations and fluid entrapment of paper 1 (Parker and Lenhard, this issue) is employed in the integral equations to enable derivation of closed‐form expressions for air, water, and oil relative permeabilities as functions of current fluid saturations and saturation history. Three‐phase k‐S relations are calculated for main drainage and imbibition paths for a hypothetical soil to illustrate usage of the model and to evaluate the magnitude of fluid entrapment effects on relative permeabilities. Water permeability‐saturation relations are predicted to exhibit mild hysteretic effects except at high saturations, while hysteresis in air permeability‐saturation relations is much more pronounced. Predicted hysteresis in oil permeability is low at low water saturations but becomes quite marked as water saturation increases. Predictions of k‐S‐P relations for a hypothetical NAPL contamination scenario are presented using model parameters determined for a sandy soil by two methods in paper 1 (Parker and Lenhard, this issue). The results indicate that hysteresis and nonwetting fluid entrapment effects on k‐S‐P relations may be quite substantial. Sensitivity to calibration method is found to be rather small.
Risk‐based analysis of extreme events
Authors: Karlsson Per‐Ola , Haimes Yacov Y.
Published: February 2008

Mathematical expectation has traditionally been used in solving risk‐based decision making problems. However, this concept is not appropriate for decision making that affects public policy because it conceals extremes by commensurating events of different magnitudes and probabilities of occurrence. If a decision maker instead uses an expected value function in addition to a conditional expectation that captures extreme and catastrophic events, he will gain a more meaningful and more encompassing representation of the trade‐offs at hand. A theory relating conditional expectation to the statistics of extremes is developed in this paper. This expectation can now be viewed as the conditional expected risk given the occurrence of an event with a return period that equals or exceeds n years. The theory highlights the importance of using both the conditional and the unconditional expected risk in decision making. This fact has previously been recognized in the partitioned multi‐objective risk method (PMRM), a risk analysis methodology based on the concept of conditional expectation. The theory proposed in this paper provides a formulation for analyzing the sensitivity of subjectively chosen parameters in the PMRM. This theory will also simplify the practical implementation of the method.
Bed load transport fluctuations in a gravel bed laboratory channel
Authors: Kuhnle Roger A. , Southard John B.
Published: February 2008

Flume experiments were conducted to investigate the mechanisms of transport of a gravel‐sand mixture by shallow unidirectional flows. Two water recirculating sediment feed flumes were used: one with a 6 m long and 0.15 m wide channel and other with an 11 m long channel with widths of 0.74 m and 0.53 m. The sediment, poorly sorted gravel with a mean size of 3 mm, was fed at the upstream end of the channel at steady rates from 0.03 kg s−1 m−1 to 1.0 kg s−1 m−1. Sediment transport rate out of the channel varied in all runs, at approximate periods of 3 min in the runs with high transport rates to 14 min in the runs with low transport rates. The runs with low transport rates also showed fluctuations in total transport rate at periods of about 25 min. Transport rates of each size fraction varied with time in a distinctive pattern in all runs. The time variations were caused by the migration of very long and low bed load sheets in the runs with low to moderate transport rates and dunelike bed forms in the run with the highest transport rate. The bed surface in all runs was coarser in size than the original sediment mix except that with the highest transport rate (run H5) in which the size distribution was nearly the same as the original.
Dependence of anisotropy on saturation in a stratified sand
Authors: Stephens Daniel B. , Heermann Stephen
Published: February 2008

A laboratory experiment was performed to investigate the movement of water infiltrating from a point source into a porous medium comprised of alternating layers of fine and coarse sand packed in a clear plastic tank. The purpose of the experiment is to demonstrate whether the effective anisotropy of stratified materials is dependent upon saturation. The noncolinearity of the specific discharge and hydraulic head gradient directions is used as a measure of anisotropy. Dyes were used to map the specific discharge direction and tensiometers were used to characterize the hydraulic gradient field under unsaturated, steady state conditions. The results indicate a divergence between the direction of the specific discharge and gradient which increases as the pressure head decreases. Anisotropy determined from the experiment exceeds that calculated from the hydraulic conductivities of the individual materials.
Analysis of the inverse problem for transient unsaturated flow
Authors: Kool J. B. , Parker J. C.
Published: February 2008

The inverse problem of determining unsaturated soil hydraulic properties from one‐dimensional, transient infiltration and redistribution events is analyzed. Hydraulic properties are assumed to be described by an extension of van Genuchten's (1980) model which allows for hysteresis in the retention function and air entrapment. Unknown parameters in the model are estimated from observed water contents and heads during transient flow by numerical inversion of the unsaturated flow equation. The inverse problem is formulated as a weighted least squares problem and solved using an efficient Levenberg‐Marquardt algorithm. The flow event consists of ponded infiltration followed by gravity drainage with evaporation at the soil surface. Sensitivity analyses indicate that observations during ponded infiltration should be made near the position of the wetting front. The location of observation points during the drying stage is less critical than during the infiltration stage, but for the relatively high imposed evaporative flux sensitivity of pressure head is highest near the soil surface. Large differences in sensitivity are observed among the various model parameters. Unknown evaporative fluxes are approximated in the inverse solution as an equivalent first‐type boundary condition requiring only periodic measurements of surface water content during the drying stage. Little error is incurred provided accurate measurements are possible. Corruption of input data with random error is shown to have a larger effect on the predicted conductivity function than on the retention function and more effect on the wetting branch of the hysteretic retention function than on the drying. When measurements are subject to error and the assumed parametric model for water retention and conductivity relations is not exact, it may no longer be possible to detect hysteresis in the retention function.
The performance of the two‐component extreme value distribution in regional flood frequency analysis
Authors: Arnell Nigel W. , Gabriele Salvatore
Published: February 2008

The performance of the regional two‐component extreme value (TCEV) estimation procedure in estimating both dimensionless regional frequency curves showing the T‐year flood as a multiple of the mean annual flood and site curves incorporating local and regional information was assessed using computer simulation experiments. It was found that the TCEV procedure produces estimates of both dimensionless regional curves and scaled site quantiles that are relatively unbiased but very variable when data are drawn from TCEV or Wakeby parents, although estimates are very biased when data follow a GEV distribution. The TCEV procedure is robust in the sense that its worst efforts are less bad than those of the alternatives considered, but performance declines significantly as sample sizes reduce.
Cost efficiency of transferable discharge permit markets for control of multiple pollutants
Authors: Lence Barbara J. , Eheart J. Wayland , Brill E. Downey
Published: February 2008

Transferable discharge permit (TDP) programs for managing multiple pollutants may be complicated by interdependent treatment costs. Under such systems pollutants may be managed as several individual commodities or grouped together and managed as a single commodity. This paper presents an approach for evaluating the cost efficiency of TDP programs in which permits for several pollutants are traded as individual commodities. Two market scenarios, representing simultaneous and sequential TDP markets, are developed to provide high and low benchmarks for cost efficiency, respectively. The scenarios demonstrate the control of biochemical oxygen demand (BOD), phosphorus, and nitrogen discharges in a typical river basin. A method for estimating feasible transfer payments under the market scenarios is also described and demonstrated for markets for BOD and phosphorus permits. The costs of these scenarios are compared with each other and with that of a uniform treatment approach. These comparisons suggest that a TDP program that manages several pollutants on an individual basis is a cost effective management strategy for conventional municipal pollutants and that any interdependencies that exist among these pollutants in terms of waste treatment costs do not limit the effectiveness of this simple program.
Predicting regional episodic acidification of surface waters using empirical models
Authors: Eshleman Keith N.
Published: February 2008

Studies of individual lakes and streams have documented the occurrence of transient, short‐term acidification of surface waters during hydrologic events, but a regional assessment of episodic chemical effects has not been made. Application of a two‐box mixing model, together with regional chemistry and deposition data, indicates that acidic episodes (acid neutralizing capacity <0) are likely an important regional phenomenon. Population estimates of the total proportion of acidic stream reaches increased by 40–640% in six subrogions of the eastern United States when episodes were taken into account. Data from a small sample of lakes in the Adirondacks (which appear to be representative of the lake population) show that fall “index” acid neutralizing capacity is an excellent predictor of the minimum episodic ANC measured at the outlets of these lakes during spring snowmelt. While 11% of the Adirondack lakes were acidic at fall overturn, a linear regression model predicts that more than 35% were acidic at their outlets during the spring of 1986.
Nonaqueous phase liquid transport and cleanup: 1. Analysis of mechanisms
Authors: Hunt James R. , Sitar Nicholas , Udell Kent S.
Published: February 2008

Groundwater contamination by nonaqueous liquids such as organic solvents and petroleum hydrocarbons frequently occurs as a result of surface spills, tank leaks, and improper disposal practices. This first of two papers examines the physics governing the emplacement and movement of a separate phase in porous media, the role of sorption, and the conditions necessary to mobilize a separate phase. The movement of the separate phase is controlled by capillary forces, and ganglia displacement by groundwater is not possible under reasonable hydraulic gradients. In addition, because of mass transfer limitations in liquid phase dissolution, groundwater extraction at contaminated sites is shown to be ineffective in removing the nonaqueous contaminant within a reasonable time frame. Therefore other means of mobilizing the trapped second phase are needed, steam displacement is proposed and steam front propagation through contaminated porous media is evaluated. The results of laboratory experiments supporting some of these analytical results are presented in the second paper (Hunt et al., this issue).
A multisignal automatic calibration methodology for hydrochemical models: A case study of the Birkenes Model
Authors: de Grosbois Ed , Hooper Richard P. , Christophersen Nils
Published: February 2008

Most hydrochemical models employ immeasurable or “artificial” parameters that need to be estimated from calibration data. A methodology is presented here for the automatic calibration of artificial parameters, which improves parameter identifiability through increasing the information available for determining the parameter values. Instead of calibrating the model using only one signal (e.g., the hydrograph), multiple signals (e.g., chemical signals as well as the hydrograph) are considered simultaneously. Either a simple least squares or a weighted least squares objective function may be used. The methodology is applied to the hydrologic module of the Birkenes hydrochemical model using artificial data. From a wide range of starting points, a gradient search optimization technique is able to consistently locate the correct parameter values uniformly better when using two signals (the hydrograph and a conservative tracer) than when only one signal (the hydrograph) is used.
Time‐dependent macrodispersion for solute transport in anisotropic heterogeneous aquifers
Authors: Dagan Gedeon
Published: February 2008

The expected values of the spatial second‐order moments of a solute body transported by groundwater are derived for flow through heterogeneous formations of a stationary random anisotropic structure. They are based on a general formulation, which reduces to most existing results in the literature as particular cases. Detailed results are given for the spatial variance as a function of time in the case of axisymmetric anisotropy, average flow parallel to the plane of isotropy, first‐order approximation in the log conductivity variance σY2, and high Peclet numbers. These results fill the gap existing between the studies of Dagan (1982, 1984), on one hand, and those of Gelhar and Axness (1983) and Neuman et al. (1987), on the other. A preliminary investigation of the higher‐order effects in σY2 suggests that the use of the first‐order approximations is warranted, at present, only for σY2 ≪ 1. The impact of the errors of estimation of the parameters on which transport depends is briefly analyzed.
Mass exchange between mobile freshwater and immobile saline water in the unsaturated zone
Authors: Gvirtzman Haim , Paldor Nathan , Magaritz Mordeckai , Bachmat Yehuda
Published: February 2008

A profile of tritium concentrations measured in the unsaturated zone in loessial sediments in a semiarid area is interpreted in terms of mobile and immobile water domains, according to a nonequilibrium transport model. The mobile domain is represented by percolating freshwater from both rain and irrigation, and the immobile one is represented by isolated fossil saline water pockets. The two domains are connected by partially saturated narrow passages within dispersed clay minerals. The transport of the mobile water is described by convective‐dispersive flow and by mass exchange between the two water domains. The relevant equations with the given initial and boundary conditions are solved numerically, and the simulated profile is adjusted to fit the measured one. In this study we concentrate on examination of the mass exchange law between the two domains. It was assumed that matrix characteristics vary in time due to the dispersion of clays at the interface between fresh and saline waters. Accordingly, a time‐dependent mass exchange was adopted, which made it possible to obtain an adequate reconstruction of the measured tritium profile. By using a least squares optimization procedure it was found that the best fit between the simulated and measured profiles is attained when the fraction of mobile water is 30%, and the rate of mass exchange decreases from 0.60 to 0.01 year−1 in 26 years. The proposed model implies is that it is the immobile water domain which contains the memory of the “high tritium period” (thermonuclear tests period) of the 1960s.
Stochastic analysis of solute arrival time in heterogeneous porous media
Authors: Shapiro Allen M. , Cvetkovic Vladimir D.
Published: February 2008

Longitudinal advective solute movement in heterogeneous porous media is investigated by considering the solute arrival time at a plane perpendicular to the mean fluid velocity. The moments of the solute arrival time are defined in terms of the stochastic properties of a statistically anisotropic hydraulic conductivity field. The flux‐averaged concentration is specified by introducing the moments of the arrival time into a probability density function for the arrival time. The quadratic dependence of the arrival time variance on position in the vicinity of an injection point is indicative of a nondiffusive process. For the assumed spatial correlation of the hydraulic conductivity, the variance of the arrival time asymptotically approaches a linear dependence on the position from the injection point similar to a diffusion process. The impact of assuming solute movement to be a diffusive process from the onset of the solute injection causes erroneous estimates of flux‐averaged concentrations at distances from the injection point that are of the order of the correlation length of the hydraulic conductivity. The arrival time analysis and the particle position analysis given in Dagan (1982, 1984) are complementary interpretations of advective solute movement that yield different definitions of the solute concentration; the position analysis intrinsically defines the resident or volume‐averaged concentration, while the flux‐averaged concentration is defined from the arrival time analysis. The temporal variation of the resident and flux‐averaged concentration are similar at a given position for small values of the variance of the hydraulic conductivity, or at distances from the solute injection point that are large relative to correlation length of the hydraulic conductivity.
A note on the recent natural gradient tracer test at the Borden Site
Authors: Naff R. L. , Yeh T.‐C. Jim , Kemblowski M. W.
Published: February 2008

The variance in particle position, a measure of dispersion, is reviewed in the context of certain models of flow in random porous media. Asymptotic results for a highly stratified medium and an isotropic medium are particularly highlighted. Results of the natural gradient tracer test at the Borden site are reviewed in light of these models. This review suggests that the moments obtained for the conservative tracers at the Borden site could as well be explained by a model that more explicitly represents the three‐dimensional nature of the flow field.
Interaction of fine sediment with alluvial streambeds
Authors: Jobson Harvey E. , Carey William P.
Published: February 2008

More knowledge is needed about the physical processes that control the transport of fine sediment moving over an alluvial bed. The knowledge is needed to design rational sampling and monitoring programs that assess the transport and fate of toxic substances in surface waters because the toxics are often associated with silt‐ and clay‐sized particles. This technical note reviews some of the past research in areas that may contribute to an increased understanding of the processes involved. An alluvial streambed can have a large capacity to store fine sediments that are extracted from the flow when instream concentrations are high and it can gradually release fine sediment to the flow when the instream concentrations are low. Several types of storage mechanisms are available depending on the relative size distribution of the suspended load and bed material, as well as the flow hydraulics. Alluvial flow tends to segregate the deposited material according to size and density. Some of the storage locations are temporary, but some can store the fine sediment for very long periods of time.
Probabilistic representation of the temporal rainfall process by a modified Neyman‐Scott Rectangular Pulses Model: Parameter estimation and validation
Authors: Entekhabi Dara , Rodriguez‐Iturbe Ignacio , Eagleson Peter S.
Published: February 2008

The capability of the Neyman‐Scott clustered stochastic point process model of rainfall to preserve various observed statistics is considered. Randomization of the cell duration parameter from storm to storm is shown to considerably improve the wet‐dry period, joint distribution, and extreme value statistics. A simple procedure for parameter estimation is introduced and applied.
Numerical modeling of tide and current in Central Puget Sound: Comparison of a three‐dimensional and a depth‐averaged model
Authors: Chu Wen‐Sen , Liou Jiing‐Yih , Flenniken Kathleen Dillon
Published: February 2008

A depth‐averaged and a three‐dimensional hydrodynamics model were applied in a study to evaluate quantitative methods for characterizing tide and current in one of the major basins of Puget Sound. The objective of the investigation was to assess the models' performance and operational requirements for future management applications. The study found that at the chosen horizontal spatial resolution scale of 762 m, both models were capable of reproducing major observed tide and tidal current characteristics in the study area and the differences between the results of the two models are small. Typical runs of the models with the current resolution require about 6–20 System Resources Units on a CRAY X/MP‐48 supercomputer. For future simulation of general tidal circulation and transport features in the Sound, the use of depth‐averaged models with spatial resolution of 800 m or less is recommended. For certain management problems around the Sound (for example, outfall siting, dredge material disposal) in which more detailed knowledge of the tidal current might be useful, the use of three‐dimensional models with finer spatial resolution (300 m or less horizontally and 15–50 m vertically) is suggested. The study also demonstrated that with increasing availability of computing power, wider use of multidimensional hydrodynamics models for estuarine environmental decision making as well as basic scientific research is very promising.
Simulation of fractionation of 13C during nonequilibrium reactive solute transport in geologic systems: Formulation and example calculation
Authors: Noorishad J. , Carnahan C. L. , Benson L. V.
Published: February 2008

Equations of transport for the fractionating stable carbon isotope 13C have been formulated and implemented in a nonequilibrium reactive chemical transport code. Application is demonstrated by solution of a generic problem. The new modeling capability adds a new dimension to investigation of groundwater evolution.
Verifiable evaporation modeling on the Laurentian Great Lakes
Authors: Croley Thomas E.
Published: February 2008

Water or energy balance estimates of Great Lakes evaporation require storage change data, not available in simulations or forecasts, and errors in the components of the balances are summed in the residual, giving large estimation errors. Neither these balance estimates nor evaporation models, which use the aerodynamic equation with mass transfer coefficients developed originally in the Lake Hefner studies, can be verified, since independent estimates of evaporation are not available with sufficient accuracy. However, water surface temperatures can be used to verify energy budgets. The mass transfer coefficient research is combined here with lumped concepts of classical energy conservation and a new superposition heat storage model to provide continuous simulation capability of both water surface temperatures and lake evaporation for use in outlooks and forecasts of lake levels. Calibration matches remotely sensed water surface temperatures for those Great Lakes with observations over the past 20 years. Model sensitivities are analyzed and heat and water budgets are compared.
A probabilistic storm transposition approach for estimating exceedance probabilities of extreme precipitation depths
Authors: Foufoula‐Georgiou E.
Published: February 2008

A storm transposition approach is investigated as a possible tool of assessing the frequency of extreme precipitation depths, that is, depths of return period much greater than 100 years. This paper focuses on estimation of the annual exceedance probability of extreme average precipitation depths over a catchment. The probabilistic storm transposition methodology is presented, and the several conceptual and methodological difficulties arising in this approach are identified. The method is implemented and is partially evaluated by means of a semihypothetical example involving extreme midwestern storms and two hypothetical catchments (of 100 and 1000 mi2(˜260 and 2600 km2)) located in central Iowa. The results point out the need for further research to fully explore the potential of this approach as a tool for assessing the probabilities of rare storms, and eventually floods, a necessary element of risk‐based analysis and design of large hydraulic structures.
Fluxes of N2O at the sediment‐water and water‐atmosphere boundaries of a nitrogen‐rich river
Authors: Hemond Harold F. , Duran Alexander P.
Published: February 2008

The biogeochemistry of N2O was studied in a reach of a small river in Massachusetts which receives wastewater treatment plant effluent. Fluxes of N2O across the sediment‐water and water‐air interfaces were measured using a mass‐balance approach in which gas exchange coefficients were measured in situ. The observed atmospheric flux averaged 0.25 mg N2O m−2 h−1, and was among the highest reported fluxes for aquatic systems on a per‐area basis, although the significance of such rivers as global N2O sources may not be large. The largest fraction of the N2O released to the atmosphere was produced in the treatment plant per se. During the cold season, N2O was produced in the river sediment; during the warm season, the sediment was a sink for N2O. Throughout the year, water column production was minimal. During the season when N2O was consumed by the sediment, the benthic N2O flux could be predicted by a simple oxygen penetration depth model.
Estimating constituent loads
Authors: Cohn Timothy A. , Delong Lewis L. , Gilroy Edward J. , Hirsch Robert M. , Wells Deborah K.
Published: February 2008

Several recent articles have called attention to the problem of retransformation bias, which can arise when log linear regression models are used to estimate sediment or other constituent loads. In some cases the bias can lead to underestimation of constituent loads by as much as 50%, and several procedures have been suggested for reducing or eliminating it. However, some of the procedures recommended for reducing the bias can actually increase it. This paper compares the bias and variance of three procedures that can be used with log linear regression models: the traditional rating curve estimator, a modified rating curve method, and a minimum variance unbiased estimator (MVUE). Analytical derivations of the bias and efficiency of all three estimators are presented. It is shown that for many conditions the traditional and the modified estimator can provide satisfactory estimates. However, other conditions exist where they have substantial bias and a large mean square error. These conditions commonly occur when sample sizes are small, or when loads are estimated during high‐flow conditions. The MVUE, however, is unbiased and always performs nearly as well or better than the rating curve estimator or the modified estimator provided that the hypothesis of the log linear model is correct. Since an efficient unbiased estimator is available, there seems to be no reason to employ biased estimators.
On mass response functions
Authors: Rinaldo Andrea , Marani Alessandro , Bellin Alberto
Published: February 2008

Field transport of reactive solute species is investigated through a class of stochastic models, here termed mass response functions (MRFs), which incorporate simplified concepts of chemical/physical nonequilibrium kinetics in the formulation of transport by travel time distributions. MRFs are probability density functions (pdfs) associated with solute particles' travel time within transport volumes. The theory hinges on recent advances in modeling transport of solutes in groundwater and in basin scale transport volumes and links the approaches of surface hydrologists with recent subsurface transport models. The relationship between MRFs and the theory of solute transport by continuous motions is investigated. It is found that MRFs extend the basic formulation of transport of inert solutes to a particular case of sorption process. The relationship between MRFs and the basic differential convection‐dispersion equation incorporating linear sorption is also investigated. It is found here that not only are transfer functions of solutes consistent with any mechanistic three‐dimensional (3‐D) model of convection dispersion, but also that they are, under limit conditions, the product of the travel time distribution of the carrier flow with a bounded continuous function. The latter is the solution to an initial value problem which results from solving the general 3‐D differential equations of convection dispersion with sorption under some simplifying assumptions, and formally coincides with the resident concentration included (as an assumption) in the original MRF formulation. Travel time distributions and MRFs underlain by statistical constraints rather than by dynamical models are proposed. Non‐Gaussian distributions are studied by statistical‐mechanical tools and are found to represent the norm, rather than the exception, in this formulation of transport of reactive solutes. The concepts are applied to a field study and are shown to yield reliable models of solute migration in nonpoint pollution problems.
Bed load transport of mixed size sediment: Fractional transport rates, bed forms, and the development of a coarse bed surface layer
Authors: Wilcock Peter R. , Southard John B.
Published: February 2008

Fractional transport rates, bed surface texture, and bed configuration were measured after a mixed size sediment had reached an equilibrium transport state for seven different flow strengths in a recirculating laboratory flume. Fractional transport rates were also measured at the beginning of each run when the bed was well mixed and planar. The start‐up observations allow us to describe the variation of fractional transport rates with bed shear stress for a constant bed surface texture and bed configuration. The start‐up and equilibrium observations together allow, for the first time, an unambiguous description of the mutual adjustment among the transport, the bed configuration, and the bed surface, as the transport system moves toward equilibrium. We find that a substantial interaction exists among the transport, bed surface, and bed configuration. Bed forms and a coarse surface layer coexist over a range of bed shear stress. Under some flow conditions the size and shape of the bed forms are controlled by the presence of the coarse surface layer. At higher flows the coarse surface layer is eliminated by scour in the lee of the bed forms. If the bed surface is defined as that over which the bed forms move, a coherent relation between the bed surface texture and the transport grain size distribution may be defined. At equilibrium the transport rates of all fractions were not equally mobile, defined as identical transport and bed grain size distributions, although equal mobility was approached for runs in which the bed shear stress was more than twice that for initial motion of the mixture. Under some flow conditions the transport was observed to adjust away from equal mobility as the bed adjusted from a well‐mixed start‐up condition to an equilibrium state. Development of a partial static armor, wherein some individual grains become essentially immobile even though other grains in the same fraction remain in transport, is suggested to explain these adjustments between the transport and bed surface grain size distributions. Constraints on equilibrium mixed size sediment transport are defined. The special conditions for which equal mobility must hold and the relevance to natural conditions of flume results and the equal mobility concept are discussed.
A potential theory‐based finite element algorithm for freshwater recharge of saline aquifers: A sharp interface model
Authors: Singh Satya P. , Stammers William N.
Published: February 2008

The size and shape of the freshwater lens developed during the freshwater recharge of thick, unconfined saline aquifers was investigated through a potential theory‐based finite element method (implicit difference‐in‐time scheme) in which the air‐freshwater free surface and the fresh‐saltwater interface boundary conditions have been integrated in the Galerkin's finite element formulation of the flow equation through Green's theorem in an iterative scheme using a sharp interface assumption. The model can also be applied to locate the positions of the fresh‐saltwater interface and the air‐freshwater free surface for recharge of strip oceanic islands. The model results are verified through experimental studies conducted on a Hele‐Shaw analogue.