bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Equivalence of Two Conceptual Models for Describing Ion Exchange During Transport Through an Aggregated Oxisol
Authors: Nkedi‐Kizza P. , Biggar J. W. , Selim H. M. , Van Genuchten M. Th. , Wierenga P. J. , Davidson J. M. , Nielsen D. R.
Published: February 2008

Breakthrough curves (BTCs) of the cation 45Ca2+, an anion 36Cl−, and 3H2O were measured during miscible displacement through water‐saturated soil columns packed with aggregates of an Oxisol. Two conceptual models were used to simulate the observed asymmetry and tailing in the BTCs caused by an apparent nonequilibrium situation in the porous medium. In both models the exchange process on one type of site was assumed to be instantaneous while the rate of isotopic exchange on another type of site was assumed to be either a diffusion‐controlled process (model 1) or a first‐order reversible kinetic process (model 2). Isotopic exchange in both models was described with a linear isotherm. It is shown that the two models are mathematically equivalent with respect to the derived BTCs.
Boundary Integral Solution to Seawater Intrusion Into Coastal Aquifers
Authors: Taigbenu Akpofure E. , Liggett James A. , Cheng Alexander H‐D.
Published: February 2008

The boundary integral equation method is used to solve problems of seawater intrusion into a freshwater aquifer that is being pumped. The transient problem of intrusion (lens type) into a deep aquifer is solved, as is the steady state problem of intrusion (wedge type) into a shallow aquifer. Both cases are compared with analytical solutions that are available for simple geometries. The numerical models uses the Dupuit‐Forchheimer approximation to reduce the three‐dimensional problem to two dimensions. It then uses the Ghyben‐Herzberg approximation, which simplifies the treatment of flow in the saltwater region. A “special potential“ eliminates the need to track explicitly the freshwater‐saltwater interface. The numerical program adds to the capability of the general groundwater basin model, which was based on the same techniques.
Alteration of Streamflow Characteristics Following Road Construction in North Central Idaho
Authors: King John G. , Tennyson Larry C.
Published: February 2008

Effects of logging access roads on seven streamflow variables were monitored on six forested headwater watersheds in north central Idaho. The streamflow variables were annual maximum streamflow, date of maximum streamflow, annual minimum streamflow, annual water yield, and streamflow equaled or exceeded 5% of the year, 25% of the year, and 75% of the year. The watersheds, ranging in area from 28.3 to 147.7 ha, had less than 5% of their area in roads. Two statistically significant (α = 0.05) changes occurred following road construction: an increase in the 25% exceedance flows in one watershed and a decrease in the 5% exceedance flows in another watershed. No significant changes were detected in other flow parameters on any of the watersheds. The results indicate that the hydrologic behavior of small forested watersheds may be altered when only a small area is disturbed by roads.
Solution of surface water flow equations using Clebsch variables
Authors: Zijl Wouter
Published: February 2008

Conventionally, the flow equations for surface waters are obtained by integrating vertically the basic continuum conservation equations governing three‐dimensional motion, thus reducing the problem to two dimensions. However, when vertical transport is important, this model fails, and the basic equations for three‐dimensional motion must be solved directly. With the advent of super computers and attached array processors this has become feasible. In this paper a formulation based on so‐called Clebsch variables is presented. This formulation is basically equivalent to a formulation based on the vorticity. However, the formulation based on Clebsch variables does not exhibit the disadvantages of the formulation based on the vorticity. Since in the Clebsch representation an explicit expression for the pressure is available (generalized Bernoulli equation), this formulation may be considered as an extension of the well‐known potential theory for irrotational flow. A groundwater reservoir simulator solving for the pressure, temperature, and concentration can, in principle, be applied to determine the Clebsch variables describing flow in surface water, thus unifying the mathematical description of surface and subsurface hydrology. The conclusion is that a computer program based on a formulation with Clebsch variables is quite competitive, especially for three‐dimensional calculations where it should lead to a more economical use of the computer.
Impacts, costs, and techniques for mitigation of contaminated groundwater: A review
Authors: Sharefkin Mark , Shechter Mordechai , Kneese Allen
Published: February 2008

A framework is developed for evaluating the impacts, costs, benefits, and techniques for mitigating groundwater contamination. The framework is a generalized cost‐benefit analysis. Contamination of the Cohansey aquifer in New Jersey is the subject of an illustrative case study. The plume of contamination is modeled and potential health impacts are projected based on dose‐response information available from the Environmental Protection Agency. The economic value of such damages is estimated using alternative mortality risk values that span those found in the recent literature. Costs for controlling the contamination from the site are estimated for alternative levels of control and techniques of control adapting information from the Environmental Protection Agency and other sources. While both the benefits and cost estimates should be mainly viewed as the result of illustrating a method and not as accurate estimates, they do suggest that in a case like the Cohansey, both potential damages (or damages averted, i.e., benefits) and the cost of containment once contamination has occurred can be quite high. Prevention in such cases appears to be the best cure. The paper closes with a discussion of the potential role of economic incentives in the generation of hazardous wastes and their “safe” disposal.
Infiltration in Swelling Soils
Authors: Giraldez Juan V. , Sposito Garrison
Published: February 2008

Infiltration phenomena in swelling soils were investigated theoretically. The approach taken consisted of applying both the approximate analytical techniques developed by J.‐Y. Parlange and co‐workers and conventional finite difference numerical methods to study the generalized Richards equation for one‐dimensional infiltration in a swelling soil. Equations were derived for the ponding time and the post‐ponding infiltration rate that are generalizations of the Parlange‐Smith model expressions for rigid soils. Ponding times for swelling soils were shown to be shorter than those for nonswelling analogs, and post‐ponding infiltration rates in swelling soils were shown to approach zero instead of becoming equal to the hydraulic conductivity, as in rigid soils. These results were confirmed, both qualitatively and quantitatively, with the numerical model, which also provided instantaneous moisture profiles and surface swelling predictions in agreement with field observations. A three‐parameter infiltration equation proposed recently by J.‐Y. Parlange et al. (1982) was generalized to describe swelling soils and shown to be in good agreement with published laboratory and field data. It appears that the generalized analytical model equations developed can be employed conveniently in hydrologic applications which do not require high accuracy in predictions.
Modeling the Effects of Acid Deposition: Assessment of a Lumped Parameter Model of Soil Water and Streamwater Chemistry
Authors: Cosby B. J. , Hornberger G. M. , Galloway J. N. , Wright R. F.
Published: February 2008

Quantitative predictions of the effects of acid deposition onterrestrial and aquatic systems require physically based, process‐oriented models of catchment soil water and streamwater chemistry. A desirable characteristic of such models is that they include terms to describe the important phenomena controlling a system's chemical response to acidic deposition, yet be restricted in complexity so that they can be implemented on diverse systems with a minimum of a priori data. We present an assessment of a conceptual model of soil water and streamwater chemistry based on soil cation exchange, dissolution of aluminum hydroxide, and solution of carbon dioxide, all processes that occur in catchment soils and that have rapid equilibration times. The model is constructed using an “average” or lumped representation of these spatially distributed catchment processes. The adequacy of the model is assessed by applying it to 3 years of soil water and streamwater chemistry data from White Oak Run, Virginia, a second‐order stream in the Shenandoah National Park. Soil properties predicted by the model are in good agreement with presently available measurements of those soil properties. The success of the model suggests that lumped representations of complex and spatially distributed chemical reactions in soils can efficiently describe the gross chemical behavior of whole catchments (e.g., pH, alkalinity, and major ionic concentrations in surface waters). Further assessment of the adequacy of this conceptual approach will require more detailed empirical knowledge of the soil processes being modeled, particularly soil cation exchange and the variability of soil CO2 partial pressures.
An Empirical Study of Flood Measurement Error
Authors: Potter Kenneth W. , Walker John F.
Published: February 2008

At a gaging station, very large peak discharges are usually estimated by extending the rating curve or by making an indirect measurement; hence these discharges are subject to much larger errors than are directly measured discharges. A crude measure of the uncertainty of the largest observed peak discharge at a gaging station is the ratio of the largest directly measured discharge to the largest observed peak discharge. For 46 gaging stations in the State of Wisconsin this ratio ranges from 0.16 to 0.99, with a median of 0.72. For the annual flood series from the half of the gaging stations with the lowest ratios, the coefficients of skewness have considerably higher mean and standard deviation than do those of the remaining flood series. These results are consistent with the impact of measurement error, though they can also be explained by the occurrence of single extremely large floods in several of the annual flood series.
Comparison of the Thermal Regimes for Freezing and Thawing of Moist Soils
Authors: Civan Faruk , Sliepcevich C. M.
Published: February 2008

The spatial and temporal temperature distributions for freezing or thawing of moist soils are compared. For this purpose an enthalpy formulation in terms of dimensionless variables combined with similarity transformation and coordinate scaling is used to obtain accurate numerical solutions for heat conduction with phase change, irrespective of whether the transition occurs at a single temperature or over a temperature interval. Differences between the results obtained herein and those published earlier by Y. Nakano and J. Brown (1971) are noted.
Effects of incorrectly removed periodicity in parameters on stochastic dependence
Authors: Yejevich Vujica , Obeysekera Jayantha T. B.
Published: February 2008

Modeling of seasonal hydrologic time series often requires the identification of periodic functions of its basic parameters such as the mean, standard deviation, and autocorrelation coefficients. The consequences of incorrect estimation of harmonics on dependence models of the stochastic component are investigated. The underremoval of harmonics in the periodic mean introduces four additional terms in the inferred autoregressive moving average model of stochastic component for each underestimated harmonic. Overremoval has a small effect on the dependence structure. In both cases, however, the total number of parameters to be estimated increases. The underremoval of harmonics in the periodic standard deviation decreases the values of autocorrelation coefficients of the stochastic component, whereas the overremoval may increase them. In the case of the periodic standard deviations the effect on model identification of stochastic component appears to be minimal. The neglect of periodicity in the auto‐ correlation coefficients can lead to erroneous stationary models of the stochastic component.
One‐dimensional analytical solutions for the migration of a three‐member radionuclide decay chain in a multilayered geologic medium
Authors: Gureghian A. B. , Jansen G.
Published: February 2008

Analytical solutions are presented for the one‐dimensional convective‐dispersive and nondispersive equilibrium transport equations of a three‐member radionuclide decay chain in a multilayered porous media. The solution for the nondispersive case is exact. The solution for the general case, although analytic in the first layer, takes a semianalytical form in the subsequent layers by virtue of its numerical integration requirements. The solutions are based on the Laplace transformation technique. Two types of boundary conditions are considered at the source, i.e., a continuous and a band release mode. The accuracy of the solutions was satisfactorily tested on a selected number of problems for which experimental and analytical solutions were available. The practical use of the solutions in a two‐dimensional domain is illustrated by a scenario of radionuclide migration from a high‐level waste repository located in a saturated multilayered aquifer.
Kinematic Wave Approximation to Infiltration Into Soils With Sorbing Macropores
Authors: Germann Peter F. , Beven Keith
Published: February 2008

Infiltration into soils with macropores is treated as two‐domain flow. The first domain is the soil matrix, in which water is subjected to capillarity, and infiltration is approached by Philip's sorptivity concept. The second domain is the soil macropore system in which water moves only under gravity and flow is approached by kinematic wave theory. A sink function with respect to flow in the macropore system accounts for water sorption by the soil matrix. The two‐domain flow model is then applied to infiltration into a block of undisturbed soil containing macropores.
A Model for Steady Fluid Flow in Random Three‐Dimensional Networks of Disc‐Shaped Fractures
Authors: Long Jane C. S. , Gilmour Peggy , Witherspoon Paul A.
Published: February 2008

A model for steady fluid flow in three‐dimensional, random networks of fractures has been developed. In this model the fractures are disc shaped discontinuities in an impermeable matrix. The fracture discs can be arbitrarily located within the rock volume and can have any desired distribution of aperture, radius orientation, and density. Thus where the disc model is appropriate it is possible to calculate flow through fracture networks which are statistically similar to those that occur in nature. After the boundary conditions and the desired fractures are specified, the intersections (nodes) between these discs (elements) are identified. Then steady flow through the network is calculated using a mixed analytical‐numerical technique. In each fracture, analytic equations for flow into or out of each node as a function of the average head at each node are developed. The equations are based on image theory and the assumption that each node is a source (or sink) of uniform strength. A set of mass balance equations is constructed which equate flow into a node from one of its associated fractures to flow out of the node into the other associated fracture. These equations are solved for the average head at each node, and flux between fractures can then be calculated by substituting the average head values back into the analytical equations. The model has been successfully checked against analytical results for several cases of two and three intersecting fractures. We plan to use these techniques to measure the permeability of fracture networks.
Modeling Three‐Dimensional Flow in Confined Aquifers by Superposition of Both Two‐ and Three‐Dimensional Analytic Functions
Authors: Haitjema Henk M.
Published: February 2008

A technique is presented to incorporate three‐dimensional flow in a Dupuit‐Forchheimer model. The method is based on superposition of approximate analytic solutions to both two‐ and three‐dimensional flow features in a confined aquifer of infinite extent. Three‐dimensional solutions are used in the domain of interest, while farfield conditions are represented by two‐dimensional solutions. Approximate three‐ dimensional solutions have been derived for a partially penetrating well and a shallow creek. Each of these solutions satisfies the condition that no flow occurs across the confining layers of the aquifer. Because of this condition, the flow at some distance of a three‐dimensional feature becomes nearly horizontal. Consequently, remotely from a three‐dimensional feature, its three‐dimensional solution is replaced by a corresponding two‐dimensional one. The latter solution is trivial as compared to its three‐dimensional counterpart, and its use greatly enhances the computational efficiency of the model. As an example, the flow is modeled between a partially penetrating well and a shallow creek that occur in a regional aquifer system.
Nonparametric Kernel Estimation of Flood Frequencies
Authors: Adamowski Kaz
Published: February 2008

A currently used approach to flood frequency analysis is based on the concept of parametric statistical inference. In this analysis the assumption is made that the distribution function describing flood data is known, for example, a log‐Pearson type III distribution. However, such an assumption is not always justified and often leads to other difficulties; it could also result in considerable variability in the estimation of design floods. A new method is developed in this article based on the nonparametric procedure for estimating probability distribution function. The results indicate that design floods computed from the different assumed distribution and from the nonparametric method provide comparable results. However, the nonparametric method is a viable alternative with the advantage of not requiring a distributional assumption, and has the ability of estimating multimodal distributions.
Flow to a Well in a Multiaquifer System
Authors: Hunt Bruce
Published: February 2008

Solutions are calculated for both steady and unsteady flow to a well in a multiaquifer system with N‐horizontal aquifers. Aquifer and aquitard properties may vary from one layer to the next but are assumed constant in the horizontal direction. The solutions, which are expressed in closed form, require the solution of generalized eigenvalue problems for their numerical evaluation. It is shown that the matrices appearing in these eigenvalue problems are symmetric and positive definite, which means that the eigenvalues are real and positive and that the solution always exists. Finally, a solution is calculated for a numerical example.
Risk Management of Groundwater Contamination in a Multiobjective Framework
Authors: Kaunas John R. , Haimes Yacov Y.
Published: February 2008

This paper addresses the issue of uncertainty in groundwater contamination by applying risk analysis concepts to the problem of industrial chemical spills. A hypothetical aquifer system is considered that includes a factory and two water supply wells. Accidental spills of solvent at the factory enter the aquifer, causing well solute concentrations to exceed a mandated limit. Regulation forces the company owning the factory to reduce the frequency and magnitude of the spills. Its managers need to determine the optimal levels of investment in spill control technologies that will achieve three objectives: minimize the cost of contamination prevention, minimize the proportion (ratio) of time in which a maximum contaminant limit (MCL) is exceeded, and minimize the sensitivity of the MCL exceedance ratio to uncertainties in aquifer dispersivity. Simulation with a stochastic time series of spills gives sample values of the MCL exceedance ratio for values of the investment decision variables and dispersivity; the investment decisions determine the statistics of the time series. Use of regression enables calculation of a continuous function relating the contamination time ratio objective to investments and dispersivity. The third objective is an approximation to the standard deviation of the MCL exceedance ratio and is computed through the risk dispersion index method (RDIM). The RDIM incorporates the surrogate worth trade‐off method for optimizing the resulting multiple objectives. The simulations assume that the aquifer is in a steady state and behaves linearly. The concentration impulse response at the wells for a single spill is computed via a mass transport model. The well solute concentration over time, which is determined from the convolution of a series of spills, provides the basis for calculating the exceedance ratio. This ratio is defined as the portion of time that the pollution concentration limit is exceeded in some chosen time span. To obtain credible values of the exceedance ratio, several simulations, each encompassing a 20‐year planning horizon, are run for each scenario (policy option).
Input‐Output Models, Economic Surplus, and the Evaluation of State or Regional Water Plans
Authors: Young Robert A. , Gray S. Lee
Published: February 2008

The adaptation of input‐output models for normative (benefit‐cost) appraisal of state or regional water plans is discussed. The economic surplus approach is taken to be appropriate for regional, as well as for national program appraisals. A conceptual framework, based on a willingness to pay concept, is developed to provide a formula for estimating direct economic benefits of state water development plans from input‐output models. Key elements of the formula are the costs (including opportunity costs) of primary resources utilized by water users particularly capital, labor, management, and land. A number of influential empirical studies are shown to incorrectly account for opportunity costs facing water users in producing sectors, thereby reporting erroneously high measures of economic benefits.
On Measurement, Scale, and Scaling
Authors: Cushman John H.
Published: February 2008

The relationships between measurement, scale, and scaling are discussed and analyzed in detail. It is pointed out that only certain fundamental properties can be measured and that these properties do not include constitutive variables. The interrelationships between scale and measurement are shown to manifest themselves in two types of heterogeneity which we label as explicit and implicit. An important issue that is addressed for the first time is the necessity of scaling instruments when scaling laboratory experiments up to field scale or vice versa. Explicit constraints that scaled instruments must satisfy are derived for the frequency response of the instrument. It is shown that linear interpolators can be viewed as filters (instruments), but that these interpolators don't preserve the scale constraints of the instrument used in making the field measurements. The relationship between signal conditioning and the instrument's window is isolated and shown to be a multiple convolution over the spatial coordinates (corresponding to the window) followed by a temporal convolution for the signal conditioning.
Treatment Optimization of Municipal Wastewater and Reuse for Regional Irrigation
Authors: Dinar Ariel , Yaron Dan
Published: February 2008

Municipal wastewater warrants increased attention as a potential environmental pollution and a possible irrigation water source. A long‐run mathematical programming model aimed at regional optimization with regard to plant capacity, treatment level, allocation of the effluent to the participating farms, and cropping patterns at each farm is presented. The model is applied to an agricultural region in Israel which includes a town and several farms. The results indicate the necessity of a subsidy for regional cooperation and provide a regional setup of reusing municipal wastewater for irrigation. The optimal solution enables each farm to reallocate the freshwater quota among the different land sections, to cultivate new land areas, and to expand the irrigated crops by using effluent. The acceptability of the regional optimal solution by the town and the farms is discussed in another paper.
Investigation of Water Movement in the Unsaturated Zone Under an Irrigated Area Using Environmental Tritium
Authors: Gvirtzman Haim , Magaritz Mordeckai
Published: February 2008

A 14‐ayear record of convective‐dispersive flow in the unsaturated zone was reconstructed using the difference between the environmental tritium content of rain and of irrigation water. Samples were taken from a loess profile in the northern Negev, Israel, in an area where precipitation is 200 mm/winter and irrigation is 650 mm/summer. The difference between the measured tritium profile and the input tritium profile was interpreted in terms of mobile and immobile water domains. The percentage of the total water which is immobile was estimated to be about 40% at the surface and 55% at 8.5 m depth. This leads to the estimate that 8 ± 1% of the rain and irrigation water percolated downward, with a velocity of 0.66 ± 0.03 m yr−1 and a maximal effective dispersion coefficient of 5 × 10−11 m2s−1. Using these values, we determined the limits of dispersivity (0.5–1.5 mm) and of tortuosity (0.05–0.11) in the medium studied.
Migration Patterns of an Asymmetric Meandering River: The Rouge River, Quebec
Authors: Lapointe M. F. , Carson M. A.
Published: February 2008

The migration patterns responsible for the asymmetry of highly sinuous Rouge River meanders are described and related to the hydraulic and sediment transport patterns controlling bank erosion in these bends. Two elements of the overall migration patterns play important roles in maintaining asymmetry, and raise fresh questions concerning our understanding of erosion processes: significant outer bank erosion starts remarkably early at bend entrances, well before crossover of the high velocity filament. In addition, such erosion extends far along the cross‐valley reaches, despite the very gentle channel curvatures between meander apices. Data on high‐flow suspended sediment transport and helix structure suggest how, beyond the control exerted by near‐bank shear stresses, the spatial patterns of near‐bed primary and secondary flows can also exert an influence on the distribution of bank erosion, by controlling the evacuation of the influxes of eroded bank materials.
State Estimation of a Scalar Dynamic Precipitation Model From Time‐Aggregate Observations
Authors: Georgakakos Konstantine P.
Published: February 2008

Developed is an exact, recursive, state estimator for linear, scalar systems whose observables are time integrals, over an interval Δt, of linear functions of the state. In general, implementation of the estimator results in excessive computations. The computationally inexpensive, ordinary Kalman filter with inflated observation error variance is used to approximate the developed estimator in terms of prediction accuracy. Comparison of the exact estimator and of the Kalman filter, using precipitation data of various aggregation intervals Δt and a scalar dynamic precipitation model, shows that the Kalman filter develops prediction bias as Δt increases.
A Statistical Methodology for Estimating Transport Parameters: Theory and Applications to One‐Dimensional Advectivec‐Dispersive Systems
Authors: Wagner Brian J. , Gorelick Steven M.
Published: February 2008

A simulation nonlinear multiple‐regression methodology for estimating parameters that characterize the transport of contaminants is developed and demonstrated. Finite difference contaminant transport simulation is combined with a nonlinear weighted least squares multiple‐regression procedure. The technique provides optimal parameter estimates and gives statistics for assessing the reliability of these estimates under certain general assumptions about the distributions of the random measurement errors. Monte Carlo analysis is used to estimate parameter reliability for a hypothetical homogeneous soil column for which concentration data contain large random measurement errors. The value of data collected spatially versus data collected temporally was investigated for estimation of velocity, dispersion coefficient, effective porosity, first‐order decay rate, and zero‐order production. The use of spatial data gave estimates that were 2–3 times more reliable than estimates based on temporal data for all parameters except velocity. Comparison of estimated linear and nonlinear confidence intervals based upon Monte Carlo analysis showed that the linear approximation is poor for dispersion coefficient and zero‐order production coefficient when data are collected over time. In addition, examples demonstrate transport parameter estimation for two real one‐dimensional systems. First, the longitudinal dispersivity and effective porosity of an unsaturated soil are estimated using laboratory column data. We compare the reliability of estimates based upon data from individual laboratory experiments versus estimates based upon pooled data from several experiments. Second, the simulation nonlinear regression procedure is extended to include an additional governing equation that describes delayed storage during contaminant transport. The model is applied to analyze the trends, variability, and interrelationship of parameters in a mourtain stream in northern California.
Historic Trends in the Sediment Flow Regime of the Mississippi River
Authors: Keown Malcolm P. , Dardeau Elba A. , Causey Etta M.
Published: February 2008

Prior to cultural development in the Mississippi River Basin, the main stem was a heavy sediment carrier due to the character of the climate and soils in the basin. The placement of flood control structures and other channel improvement features and the implementation of improved land management practices have significantly changed the suspended‐sediment flow regime of the main stem. The estimated annual sediment yield of the basin is 900 million (817 million metric) tons. That portion of the yield passing into the Gulf of Mexico, as monitored at suspended‐sediment sample collection stations near the mouths of the Mississippi and Atchafalaya rivers, totaled 434 million (394 million metric) tons prior to 1963; however, this value has now declined to 225 million (204 million metric) tons. Long‐term trends in the bed material gradation of the Lower Mississippi downstream from the mouth of the Arkansas River have remained relatively constant; however, from Donaldsonville, Louisiana, downstream to Head of Passes, there has been a pronounced shift from the sand to silt fraction.
Contaminant transport through a fractured porous rock: Impact of the inlet boundary condition on the concentration profile in the rock matrix
Authors: Moreno Luis , Rasmuson Anders
Published: February 2008

In tracer tests in single fractures in fissured rock it is of interest to determine the concentration profile in the adjacent rock matrix. In the present note, concentration profiles in the rock matrix are calculated for the case of contaminant transport in a single fracture. Two different inlet boundary conditions are used: constant concentration and constant flux at the inlet of the fracture. An analytical solution is derived for the concentration in the fissure and the matrix for constant flux condition at the inlet. Concentration profiles are shown for different values of the sorption coefficient within the rock matrix and at different locations along the fracture. For low Petlet numbers, it is shown that the concentration in the matrix is strongly influenced by the inlet boundary condition.
A generalized stochastic hydrometeorological model for flood and flash‐flood forecasting: 1. Formulation
Authors: Georgakakos Konstantine P.
Published: February 2008

An attempt to couple meteorological and hydrological models and procedures within the real‐time flood forecasting framework is made. A local quantitative precipitation model is coupled to a soil model and a channel routing model through mass conservation differential equations and an automatic updating procedure. Automatic updating is performed through the use of the Extended Kalman Filter that provides the capability for real‐time probabilistic forecasts of flood occurrence and flood magnitude. To complement the coupled system, a methodology was developed for consistent spatial interpolation of sparse observations of the pertinent meteorological input variables. The interpolation methodology takes into account topographic relief and atmospheric lapse rates. The result of the modeling effort is a stochastic‐dynamic hydrometeorological system suitable for use in real‐time flood and flash‐flood forecasting.
Classification of seasonal snow cover crystals
Authors: Colbeck S. C.
Published: February 2008

Snow cover crystals must be classified in a physically meaningful way. Previous classification systems are not sufficiently detailed or not based on sufficient knowledge of the physical processes. A new system is proposed based on our current knowledge of the physical processes of metamorphism. As more information about snow metamorphism is developed, the labels attached to snow grains should evolve too. Two levels of classification are proposed here. For practical purposes only a few terms like rounded and faceted are necessary, but for a more complete description a more detailed system is also given. The most basic description given in the table could be useful to many practitioners, while the more complete description given in the appendix will be necessary for many purposes.
Water flow and solute transport processes in the unsaturated zone
Authors: Nielsen D. R. , Th. Van Genuchten M. , Biggar J. W.
Published: February 2008

This paper gives a review of our current conceptual understanding of the basic processes of water flow and chemical transport in the untsaturated (vadose) zone and of various deterministic mathematical models that are being used to describe these processes. During the past few decades, tremendous effort has been directed toward unravelling the complexities of various interactive physical, chemical, and microbiological mechanisms affecting unsaturated flow and transport, with contributions being made by soil scientists, geochemists, hydrologists, soil microbiologists, and others. Unfortunately, segmented, disciplinary research has contributed to a lack of experimental and theoretical understanding of the vadose zone, which, in turn, has precluded the accurate prediction and management of flow and contaminant transport through it. Thus a more unified and interdisciplinary approach is needed that considers the most pertinent physical, chemical, and biological processes operative in the unsaturated zone. Challenges for both fundamental and applied researchers to reveal the intricacies of the zone and to integrate these with currently known concepts are numerous, as is the urgency for progress inasmuch as our soil and ground water resources are increasingly subjected to the dangers of long‐term pollution. Specific research areas in need of future investigation are outlined.
Flood estimates: How good are they?
Authors: Linsley Ray K.
Published: February 2008

The few tests available suggest that procedures and assumptions in common use for estimating flood peak frequency for ungaged streams are subject to large errors and are biased toward overestimates. Economic analysis of costs and benefits is ignored by many agencies in favor of designing to a arbitrarily selected probability level, commonly 1%. In view of the large expenditures for storm drainage and flood damage mitigation, one may question the wisdom of this approach. This paper discusses the causes of the current situation and suggests some steps which might help to correct it.
Estimating freshwater inflow needs for Texas estuaries by mathematical programming
Authors: Martin Quentin W.
Published: February 2008

As mandated by the Texas State Legislature, the Texas Department of Water Resources conducted studies of the effect of freshwater inflows upon the bays and estuaries of Texas. Developed as part of these studies, a mathematical programming model is described for computing estimates of the monthly and seasonal freshwater inflows necessary to meet specified environmental conditions in each of the major estuaries of the Texas Gulf Coast. The optimization model relates freshwater inflow to the key estuarine indicators of salinity, marsh inundation, and commercial fisheries harvests. Three management proposals are formulated for each estuary, corresponding to ecosystem subsistence, maintenance of fisheries harvests, and fisheries harvest enhancement. Linear and nonlinear mathematical programming techniques are used to determine the optimal inflows for each of these three management alternatives in all but one of the seven major estuaries, where only one of the management proposals could be solved.
Reforestation and the reduction of water yield on the Southern Piedmont since circa 1940
Authors: Trimble Stanley W. , Weirich Frank H. , Hoag Barbara L.
Published: February 2008

The southern Piedmont has undergone extensive cropland reversion during the twentieth century with row crops being replaced by forest and pasture. Ten continguous river basins with a total area of 54,020 km 2 had 10 to 28% of their respective areas reforested during the period 1919–1967. During the same period, water yield decreased 3 to 10 cm according to both regression and double‐mass analysis. These reductions in water yield constituted a 4 to 21% decrease in annual stream discharge and were statistically significant for a majority of the basins. The reduction of water yields by forests tends to be greater for dry years than for wet years. There was little or no relation between the degree of reforestation and reductions of water yield at the scale of this study, but when our data are included with the universe of data, the variance of our data from the overall model is much less than in the universal set. The inclusion of our results extends the range and predictive power of the universal model, giving it greater utility for water yield planning.
A multigrid model for steady flow in partially saturated porous media
Authors: McKeon Thomas J. , Chu Wen‐Sen
Published: February 2008

A numerical model for steady flow in a partially saturated porous medium is presented. The model uses the multigrid solution algorithm for solving the system of equations resulting from the discrete approximation to the partial differential equation. The model solutions checked favorably with an existing analytical solution. Used for a hypothetical problem with realistic soil characteristics the model has been shown to be very efficient for problems with the number of unknowns on the order of 10,000 and more. Specifically, the multigrid scheme was shown to be 22 times faster in terms of CPU time than the line successive overtaxation method for the particular problem chosen. In addition, the required CPU time is shown to be a linear function of the number of unknowns within the solution domain. This indicates that the convergence rate of the multigrid solution algorithm is independent of the grid resolution. Because of the significant improvement in computational efficiency, the multigrid model can be used to simulate field scale partially saturated flow problems with very fine grid resolution as is often required by highly nonlinear soil characteristics. Further studies and application of the multigrid solution technique are suggested.
A parametric model for constitutive properties governing multiphase flow in porous media
Authors: Parker J. C. , Lenhard R. J. , Kuppusamy T.
Published: February 2008

A parametric model is developed to describe relative permeability‐saturation‐fluid pressure functional relationships in two‐ or three‐fluid phase porous media systems subject to monotonic saturation paths. All functions are obtained as simple closed‐form expressions convenient for implementation in numerical multiphase flow models. Model calibration requires only relatively simple determinations of saturation‐pressure relations in two‐phase systems. A scaling procedure is employed to simplify the description of two‐phase saturation‐capillary head relations for arbitrary fluid pairs and experimental results for two porous media are presented to demonstrate its applicability. Extension of two‐phase relations to three‐ phase systems is obtained under the assumption that fluid wettability follows the sequence water > nonaqueous phase liquid > air. Expressions for fluid relative permeabilities are derived from the scaled saturation‐capillary head function using a flow channel distribution model to estimate effective mean fluid‐conducting pore dimensions. Constraints on model application are discussed.
More on Stochastic Models
Authors: Cushman John H.
Published: February 2008

In a recent review article, G. Sposito et al. (1986) examined the various stochastic theories which are concerned with transport of solutes in porous media. In this short note we expand on their discussion to include several topics which had been omitted. We begin by looking at two definitions of probability theory and their relation to the concept of an ensemble. An REV ensemble of soils is defined and examined. The concept of ergodicity is reviewed, and it is pointed out that most stochastic models are theoretically unverifiable. The relationship between scale of measurement and stochasticity is briefly reviewed, and an equation that combines the two concepts is presented.
A stochastic model for sediment yield using the Principle of Maximum Entropy
Authors: Singh V. P. , Krstanovic P. F.
Published: February 2008

The principle of maximum entropy was applied to derive a stochastic model for sediment yield from upland watersheds. By maximizing the conditional entropy subject to certain constraints, a probability distribution of sediment yield conditioned on the probability distribution of direct runoff volume was obtained. This distribution resulted in minimally prejudiced assignment of probabilities on the basis of given information. The parameters of this distribution were determined from such prior information about the direct runoff volume and sediment yield as their means and covariance. The stochastic model was verified by using three sets of field data and was compared with a bivariate normal distribution. The model yielded sediment yield reasonably accurately.
Macroscopic and microscopic capillary length and time scales from field infiltration
Authors: White I. , Sully M. J.
Published: February 2008

Estimates of characteristic times to approach steady state flow in multidimensional infiltration in the landscape depend on the magnitude and character of the capillary length scale λc and the associated capillary time scale tc. Here we derive relationships between λc and tc and readily measured field properties sorptivity S and hydraulic conductivity K or S at two supply heads. We explore the relations between λc and tc and other macroscopic and microscopic length, potential, and time scales. In addition, we show that the microscopic characteristic length λm associated with λc gives physically plausible estimates of flow‐weighted mean pore dimensions. We contrast values of λc, tc, and λm for undisturbed field soils with those of repacked materials for water supply potentials close to zero. Large λm for the undisturbed surface soils are attributed to preferential flow. Data from here and elsewhere reveal no apparent trend of λc with soil texture, with most λc of the order of 100 mm. We suggest that the characteristic size of devices used to determine hydraulic properties of field soils should be greater than or equal to λc for representative measurements. The geometric mean time of approach to steady state flow when water is supplied at potentials near or greater than zero is found to be 1.7 hours. This value together with published results suggest that the time of approach to steady state flow from multidimensional cavities is of the order of 1 hour for many field situations.
Critical chemical reaction rates for multicomponent groundwater contamination models
Authors: Jennings Aaron A.
Published: February 2008

A method for determining the validity of the Local Equilibrium Assumption (LEA) for multicomponent contamination models is presented. The LEA may be justified only when interaction rate coefficients are large enough to force solutions of kinetic models to be indistinguishable from their pointwise equilibrium counterparts. When rate constants are not sufficiently large, LEA models can yield large prediction errors. A pulse error method is presented that offers more easily interpreted and less functionally complex measures of LEA error than existing procedures. Pulse error measures are also well conditioned for the questions most often asked of groundwater contamination models (arrival times and arrival concentrations at strategic downstream points). Nondimensional results for 1:1 ion exchange are presented to illustrate how pulse error analysis may be used to establish critical Damkohler number constraints for classes of interaction mechanisms. For the example problem discussed the Damkohler number constraint (Dar > 100) appears to be more stringent than previously reported.
An experimental study of solute transport in a stony field soil
Authors: Schulin Rainer , van Genuchten Martinus Th. , Flühler Hannes , Ferlin Peter
Published: February 2008

The transport of two conservative tracers, bromide and chloride, was studied in a stony field soil under natural soil, vegetative, and climatic conditions. Small tracer pulses were applied evenly over the 94‐m2 plot area in April 1982 (bromide) and October 1982 (chloride). A 15‐m‐long, 3‐m‐deep transect through the plot area was extensively sampled in May 1983, approximately 400 days after the bromide and 200 days after the chloride was applied. Tracer concentrations were obtained from 842 samples taken at 61 locations along the transect. Two‐dimensional contour plots of the data showed a relatively uniform displacement in the vertical direction, as well as a significant horizontal redistribution during the study period. The data were analyzed with the classical convection‐dispersion equation (CDE) and with a regional stochastic model that assumes logarithmic distributions of the pore water velocity and the dispersion coefficient across the field. Both models successfully described momentary field‐averaged concentration distributions but failed to predict observed concentration data in October 1984 when another, less intensive sampling was carried out. Pore water velocities could be estimated reasonably well with as few as five vertical sampling lines, whereas dispersion coefficients and solute loads required numerous additional samplings. A field scale dispersivity of about 8 cm was obtained for the CDE model. Between 75 and 130% of the applied tracers were recovered in 1982. The effects of sampling size (0.3 versus 3 kg dry soil weight) were found to be relatively small.
Simulation of solute transport using a continuous time Markov process: 1. Theory and steady state application
Authors: Knighton R. E. , Wagenet R. J.
Published: February 2008

A continuous time Markov process is developed and used to describe chemical movement through the soil under steady state water flow regimes. Theoretical considerations are made to include chemical and biological degradation, crop uptake, and adsorption as processes which affect the probability of movement of a molecule of solute between adjacent soil layers. Probability distributions of solute are given for each soil layer given the flux of soil water and the intensity of transition for the source and sink terms. Predicted solute concentrations of nitrate and bromide compared favorably with observed concentrations collected from a laboratory soil column study under steady state water flow. The model, however, shows a dependency on the number of layers used to simulate the soil profile. The approach offers substantial promise for description of spatially variable solute movement in field soils, which is examined in a companion paper.