bluuebunny/crossref_metadata_2025 huggingface dataset:
Updating random hydraulic conductivity fields: A two‐step procedure
Authors: Massmann Joel W. , Freeze R. Allan
Published: February 2008

Predictions of groundwater travel times at field sites are generally uncertain because of uncertainty and variability in hydrogeological parameters, especially the hydraulic conductivity, K. Uncertainty is a subjective value that depends upon the person performing the analysis. Variability is an objective, though likely unknown, value that depends upon geology and depositional environment. Uncertainty and variability can be combined by treating hydraulic conductivity as a random field in a Bayesian framework [Hachich and Vanmarcke, 1983; Massmann and Freeze, 1987].
Uncertainties in streamflow measurement under winter ice conditions a case study: The Red River at Emerson, Manitoba, Canada
Authors: Pelletier Patrice M.
Published: February 2008

Computed streamflows on the Red River at Drayton in North Dakota, United States, have been generally higher under winter conditions than computed streamflows on the Red River at Emerson in Manitoba, Canada. The station at Drayton is located 85 km upstream from Emerson. Moreover, historically, the U.S. Geological Survey (USGS) has measured higher streamflows under ice conditions than the Water Survey of Canada (WSC) on the Red River at Emerson. The difference between their discharge measurements under ice conditions is generally between +10 and 20%. To explain these differences, field and laboratory experiments were conducted in 1986, which included an international field metering program at Drayton and Emerson, rating of current meters and suspension equipment at national calibration services in Canada and the United States, and testing of current meters in flowing water. Through the analysis of the field and calibration data it was determined that significant differences exist between a current meter rated on rod and on cable suspension, and that this suspension effect is responsible for the large differences in measurements between the USGS and WSC. Changes in discharge measurement techniques under ice conditions are suggested, including the sampling of vertical velocity curves and increasing the time of exposure for current meter observations. Several investigations are suggested to improve the overall quality of streamflow records for the winter ice period.
Source areas, drainage density, and channel initiation
Authors: Montgomery David R. , Dietrich William E.
Published: February 2008

An analysis of channel head locations provides insight into controls on drainage density, the response of landscapes to climatic change, and the delineation of source areas for channel network simulations. Channel heads and colluvial deposits were mapped in a roughly 2 km2 area near San Francisco, California, and, although channel heads are located within colluvial deposits in hollows, many such deposits do not support channel heads. Channel heads were classified as either gradual or abrupt. For either type of channel head, the channel reach immediately downslope may be contiguous with the channel network or may consist of a series of short discontinuous channel segments. The local valley slope at the channel head is inversely related to both source area and source‐basin length as well as to the contributing area per unit contour length at the channel head. In contrast, valley slope does not vary with drainage area upslope of channel heads. Field observations and a similarity between predicted and observed area‐slope relations suggest that the location of channel heads on steep slopes may be controlled by subsurface flow‐induced instability of the colluvial fill. Preliminary field observations also suggest that abrupt channel heads on gentle slopes are controlled by seepage erosion, whereas gradual channel head locations appear to be governed by saturation overland flow. Consideration of the geometric relationship between source areas and the first‐order drainages that contain them results in an inverse relation between mean source‐basin length and drainage density.
Simulation of three‐dimensional flow of immiscible fluids within and below the unsaturated zone
Authors: Faust Charles R. , Guswa John H. , Mercer James W.
Published: February 2008

This paper presents a two‐phase flow model based on a three‐dimensional, finite‐difference formulation. As three‐dimensional simulations can require substantial computer effort, a numerical technique that takes advantage of vector and parallel processing computer architecture is developed. The model is posed in terms of water saturation and nonwetting fluid pressure. It uses three‐phase capillary pressure and relative permeability relationships to permit simulation within or below the unsaturated zone. A modified formulation of slice successive overtaxation (an iterative matrix solution technique) is introduced. This technique is designed to use parallel processing capabilities of new computers. The model is applied to immiscible fluid flow at two chemical waste landfills near Niagara Falls, New York. At both sites, denser than water, nonaqueous liquids (NAPLs) are present in the groundwater regimes in relatively large quantities. The model applications address several technical concerns at the two sites, including the effectiveness of clay as a geologic barrier to NAPL migration owing to capillary pressure forces, the three‐dimensional aspects of dense NAPL flow, and the sensitivity of NAPL recovery in pumping wells due to various hydrogeologic and fluid properties. The results of the applications show that (1) even under a downward hydraulic gradient, natural differences in capillary pressure relationships for different lithologies can prevent downward migration of NAPL, (2) without any lithologic‐capillary barrier, an upward hydraulic gradient induced by a de watering system can prevent downward migration of NAPL, (3) NAPL recovery at wells is sensitive to relative permeability, a relationship that requires field calibration in many settings, and (4) the three‐dimensional aspects of two‐phase flow and hydrogeologic heterogeneity require explicit treatment in many settings.
Macrodispersion in sand‐shale sequences
Authors: Desbarats Alexandre J.
Published: February 2008

Macrodispersion in sand‐shale sequences is investigated by a series of numerical tracer tests. Hydraulic conductivity is modeled as a binary, spatially correlated random function. Realizations of the random conductivity field are simulated on a nodal grid discretizing the heterogeneous formation. Corresponding realizations of the random velocity field are obtained by solving the equation for saturated steady state flow. Particle tracking, with flux‐weighted tracer injection and detection, is used to generate experimental residence time distributions (RTDs). Moments of the RTD are used to characterize longitudinal tracer spreading. Results show that macrodispersive transport in sand‐shale sequences cannot be represented by a Fickian model. RTDs display a bimodal structure caused by the fast arrival of particles traveling along preferential sandstone channels and by the much slower arrival of particles following tortuous routes through sandstone and shale. The relative importance of channeling and tortuous flow transport mechanisms is determined by sand‐shale conductivity contrast, shale volume fraction, and conductivity spatial correlation structure. Channeling is promoted by high conductivity contrasts, low shale fractions, and flow parallel to bedding in anisotropic media. Low contrasts, high shale fractions, and flow perpendicular to bedding act to break up channels and to enhance tracer spreading.
Efficient numerical methods for infiltration using Richards' equation
Authors: Ross P. J.
Published: February 2008

Two efficient finite difference methods for solving Richards' equation in one dimension are presented, and their use in a range of soils and conditions is investigated. Large time steps are possible when the mass‐conserving mixed form of Richards' equation is combined with an implicit iterative scheme, while a hyperbolic sine transform for the matric potential allows large spatial increments even in dry, inhomogeneous soil. Infiltration in a range of soils can be simulated in a few seconds on a personal computer with errors of only a few percent in the amount and distribution of soil water. One of the methods adds points to the space grid as an infiltration or redistribution front advances, thus gaining considerably in efficiency over the other fixed grid method for infiltration problems. In 17‐s computing, this advancing front method simulated infiltration, redistribution, and drainage for 50 days in an inhomogeneous soil with nonuniform initial conditions. Only 16 space and 21 time steps were needed for the simulation, which included early ponding with the development and dissipation of a perched water table.
Modeling fracture flow with a stochastic discrete fracture network: Calibration and validation: 2. The transport model
Authors: Cacas M. C. , Ledoux E. , de Marsily G. , Barbreau A. , Calmels P. , Gaillard B. , Margritta R.
Published: February 2008

As part of the development of a methodology for investigating flow and transport in fractured rocks, a large‐scale experiment was recently performed at Fanay‐Augères, France. In a companion paper (Cacas et al., this issue) (paper 1) the results of the flow measurements were analyzed. In this paper, the results of the tracer experiments are interpreted. A particle following is coupled to the flow model, described in paper 1. Microscopic dispersion in the fractures and retardation effects due to unevenness of the flow paths are taken into account. The transport model is calibrated on in situ tracer tests, whereas the parameters of the hydraulic model were initially fitted on structural and hydraulic measurements (paper 1). The dispersive properties of the model are reasonably comparable to those of the real site. It tends to confirm the validity of the preliminary hydraulic calibration of the model and thus to validate further the approach used to simulate hydraulic and transport phenomena.
Optimal multicrop allocation of seasonal and intraseasonal irrigation water
Authors: Rao N. H. , Sarma P. B. S. , Chander Subhash
Published: February 2008

The problem of allocation of a limited water supply for irrigation of several crops grown in the same season is addressed. Both seasonal and intraseasonal competition for water between crops are considered. Since intraseasonal irrigation decisions are not independent, the allocation problem is solved in a dynamic framework by decomposition to two levels, seasonal and intraseasonal competition for water. A single crop irrigation scheduling model provides the input to the models at both levels. The optimization models at the two levels and the single crop model are solved by dynamic programming. Economic coefficients, crop areas, and crop growth stage stress effects are included in the mathematical formulation at both levels. The final output is an implementable set of weekly irrigation programmes of individual crops. Results of model application to a two crop situation are presented.
Lumped versus distributed mass matrices in the finite element solution of subsurface flow
Authors: Wood W. L. , Calver A.
Published: February 2008

There is confusing advice in the literature on whether Richards' equation for saturated‐unsaturated subsurface flow when space discretized by finite elements should use a lumped or distributed mass matrix. This paper discusses the difference made in the solution for the flow in a hillslope with rainfall input and typical physical parameters and various finite element grids. There is very little difference in run times and rate of convergence of the iteration. The difference in the hydraulic potential solution may exceed observational error under rapidly varying conditions. The significant difference is in the discharge from the slope. This difference may be considerably in excess of field measurement error. Use of a distributed mass matrix is strongly recommended.
On the fractal interpretation of the mainstream length‐drainage area relationship
Authors: Robert André , Roy André G.
Published: February 2008

The exponent of the mainstream Length‐Drainage area relationship is interpreted as a fractal quantity, superseding the previous allometric interpretation. The fractal interpretation is based on the assumption that cartographic generalization is applied evenly to all map scales. Twenty‐three drainage basins of the Eaton River (Quebec, Canada) were delineated from topographic maps at three different scales (1:20,000, 1:50,000, and 1:125,000). The exponent of the length‐area relation is much lower (0.546) at the largest scale than at the smallest scales (0.65), and its value corresponds to that obtained from a Richardson analysis of 10 interior stream segments. At the 1:20,000 map scale, the exponent is entirely fractal. The larger exponent values obtained at the smallest scales exceed the fractal value and incorporate an allometric component. This component is not functional, however, and it merely reflects the generalization process of cartographic abstraction of stream heads as scale is reduced. The fractal dimension of streams should not be inferred from the exponent of the length‐area relation because its value is likely to be scale‐dependent.
A quasi‐linear theory of non‐Fickian and Fickian subsurface dispersion: 2. Application to anisotropic media and the Borden site
Authors: Zhang You‐Kuan , Neuman Shlomo P.
Published: February 2008

When the quasi‐linear theory developed in paper 1 is applied to anisotropic media it shows, in contrast to the isotropic case, that longitudinal and transverse dispersivities may become asymptotically proportional to σY when the log hydraulic conductivity variance σY2 is much smaller than 1. It further implies, among other phenomena, that when the mean seepage velocity vector μ is at an angle to the principal axes of statistical anisotropy, the long axis of a plume is generally offset toward the direction of the largest log hydraulic conductivity correlation scale; when μ is at 45° to the bedding in strongly stratified media, the longitudinal axis is nearly parallel to the bedding under non‐Fickian conditions. As Fickian conditions are approached, the plume rotates toward μ and stabilizes asymptotically at a relatively small angle of deflection depending on σY2. Application of the quasi‐linear theory to depth‐averaged concentration data from a tracer experiment at Borden, Ontario, yields a consistent and improved fit to a two‐dimensional model without any need for parameter adjustment. Three‐dimensional models are shown to be in fundamental conflict with observed behavior at Borden and in other stratified formations; we show that, in principle, this conflict is easy to resolve by accounting for local hydraulic anisotropy.
Evaluating dam safety retrofits with uncertain benefits: The Case of Mohawk Dam (Walhonding River, Ohio)
Authors: Resendiz‐Carrillo Daniel , Lave Lester B.
Published: February 2008

Mohawk Dam, part of the Muskingum basin flood control system, was built in 1938 and is operated by the U.S. Army Corps of Engineers (Corps). Since this high‐hazard dam could not survive a probable maximum flood (PMF), the Corps conducted a study to determine the least expensive means of enabling the dam to survive a PMF. Applying a previously proposed framework to select the social cost minimizing capacity of a dam, we show that Mohawk Dam had sufficient capacity that any retrofit has a social cost larger than expected benefits. Sensitivity analyses were performed adjusting the peak flow distribution, the costs of modification, and downstream flood damage, as well as the possibility of loss of life. For any reasonable value of these variables the conclusion does not change that the structure already met so high a safety goal regarding extreme floods that no retrofit is needed. Using risk‐based methods to perform reservoir safety evaluations, as recommended by a National Research Council committee in 1985, is indeed feasible. Furthermore, their use provides valuable insight and guidance into the selection of strategies to enhance the safety of dams.
Enhancing instream flow benefits in an era of water marketing
Authors: Colby Bonnie G.
Published: February 2008

Growing populations in the western United States demand water not only for residential use and to support urban development but also for recreation, water quality enhancement, improvement of fish and wildlife habitat and to preserve the aesthetics of riparian areas. Instream flows contribute substantial economic benefits, and emerging pressure to reserve water instream comes at a time when markets are evolving to reallocate water among offstream uses such as agriculture, industry and municipal expansion. This article examines current instream flow policies in the western states and outlines the economic values generated by stream flows. The author argues that instream values are high enough to compete in the market for water rights with offstream uses when important recreation sites and wildlife species are involved. The paper suggests how western state policies might be altered to accommodate instream flow protection within the context of water marketing, with the objective of improving the efficiency of water allocation among instream and consumptive uses.
Solute transport in aggregated media: Aggregate size distribution and mean radii
Authors: Fong Fred K. , Mulkey Lee A.
Published: February 2008

An aggregate model was employed to study the effects of aggregate size distribution (ASD) on solute transport in a column. Instantaneous local sorption equilibria and nondegradation were assumed. The aggregate model was solved using an effective new iterative numerical scheme. Three ASDs from the literature were used in the calculations. The effects of aggregate size and size distribution can not be separated and are influenced by the flow regimes of the system. A mean variance radius was derived. This mean variance radius depends on the internal diffusion and mass transfer coefficient of the aggregates. It provided the best approximation among other types of mean aggregate radii used in the literature. Multiple mean variance radii should be used to represent the ASDs.
A dynamic optimization model for irrigation investment and management under limited drainage conditions
Authors: Knapp Keith C. , Stevens Brandt K. , Letey John , Oster J. D.
Published: February 2008

Irrigation management in a field underlain by a water table aquifer is considered. The water table varies with deep percolation which is in turn a function of the irrigation system and applied water quantities. A drainage system is installed once the water table reaches a maximum height. Optimal irrigation systems and applied water quantities are calculated using dynamic programming. For low confining layer conductivities, deep percolation is progressively reduced through time under optimal management by a combination of improved irrigation systems and reduced water applications. This delays the water table build‐up and postpones drainage costs. Optimal management for a moderately conductive confining layer results in a cyclical policy in which periods of improved irrigation systems and reduced water applications are alternated with periods of traditional irrigation systems and water applications to maintain the water table below the depth requiring drainage system installation.
Safety goals for high‐hazard dams: Are dams too safe?
Authors: Lave Lester B. , Resendiz‐Carrillo Daniel , Mcmichael Francis C.
Published: February 2008

The 1977 National Dam Inspection Program determined that many high‐hazard dams in the United States were incapable of passing a probable maximum flood (PMF). Retrofitting these dams was estimated to cost at least $10 billion. Since the PMF is revised upward periodically, retrofit is a continual issue. But surviving a PMF is a more stringent safety criterion than preventing other sources of dam failure; in addition, it is more stringent than safety criteria for other structures with respect to wind, earthquakes, or storm surges. This higher safety goal has large social costs. We propose an alternative safety goal, separating property damage from possible loss of lives. For a proposed dam whose failure could cause large loss of life or property damage, a careful evaluation should be done as to whether the dam should be built. For dams that impose smaller hazards, property damage should be handled by an analysis based on expected values of annualized benefits and costs. An adjustment for scale could be used if the property damage were extremely large. Danger to lives should be handled by establishing programs to warn and evacuate people. Our proposal should (1) lead to less injury and death, (2) use society's limited resources more efficiently, and (3) put the determination of safety goals on a more scientific and sensible basis.
Modeling the movement of volatile organic chemicals in columns of unsaturated soil
Authors: Gierke John S. , Hutzler Neil J. , Crittenden John C.
Published: February 2008

Mechanisms affecting the fate of nondegradable volatile organic chemicals in soils include (1) advection in air and water, (2) dispersion in air and water, (3) air‐water mass transfer and equilibrium, (4) diffusion in immobile water, (5) mass transfer between mobile and immobile water, and (6) sorption. A deterministic model was developed to account for these processes in laboratory columns of unsaturated soil. The general form of the model was solved numerically. The numerical solution was verified with analytic solutions for simplified conditions. Column experiments were conducted to validate the model and to determine the relative importance of each mechanism in two soil types. The movement of trichloroethene was measured in a column packed with a uniform sand and one packed with uniformly sized aggregates that were made from clay. Parameter values for the model predictions were independently determined from direct measurements and literature correlations. Bromide tracer studies were performed to determine parameter values that could not be measured directly or were not estimated accurately by literature correlations. For the sand column the amount of immobile water, the rate of liquid diffusion, and the liquid dispersion coefficient were measured in a tracer study. A batch rate study was used to measure the rate of intraaggregate diffusion in the clay aggregates. The liquid dispersion coefficient for the column containing aggregates was measured in a tracer study. These parameter values were used in the model to predict the breakthrough and elution of trichloroethene in the two columns. To describe the column data, however, Henry's constant was increased from a literature value of 0.4 to 0.7, and the predicted gas dispersion coefficient was reduced by a factor of 10.
Multidimensional simulation applied to water resources management
Authors: Camara A. S. , Ferreira F. C. , Loucks D. P. , Seixas M. J.
Published: February 2008

A framework for an integrated decision aiding simulation (IDEAS) methodology using numerical, linguistic, and pictorial entities and operations is introduced. IDEAS relies upon traditional numerical formulations, logical rules to handle linguistic entities with linguistic values, and a set of pictorial operations. Pictorial entities are defined by their shape, size, color, and position. Pictorial operators include reproduction (copy of a pictorial entity), mutation (expansion, rotation, translation, change in color), fertile encounters (intersection, reunion), and sterile encounters (absorption). Interaction between numerical, linguistic, and pictorial entities is handled through logical rules or a simplified vector calculus operation. This approach is shown to be applicable to various environmental and water resources management analyses using a model to assess the impacts of an oil spill. Future developments, including IDEAS implementation on parallel processing machines, are also discussed.
A stepwise time series regression procedure for water demand model identification
Authors: Miaou Shaw‐Pin
Published: February 2008

Annual time series water demand has traditionally been studied through multiple linear regression analysis. Four associated model specification problems have long been recognized: (1) the length of the available time series data is relatively short, (2) a large set of candidate explanatory or “input” variables needs to be considered, (3) input variables can be highly correlated with each other (multicollinearity problem), and (4) model error series are often highly autocorrelated or even nonstationary. A step wise time series regression identification procedure is proposed to alleviate these problems. The proposed procedure adopts the sequential input variable selection concept of stepwise regression and the “three‐step” time series model building strategy of Box and Jenkins. Autocorrelated model error is assumed to follow an autoregressive integrated moving average (ARIMA) process. The stepwise selection procedure begins with a univariate time series demand model with no input variables. Subsequently, input variables are selected and inserted into the equation one at a time until the last entered variable is found to be statistically insignificant. The order of insertion is determined by a statistical measure called between‐variable partial correlation. This correlation measure is free from the contamination of serial autocorrelation. Three data sets from previous studies are employed to illustrate the proposed procedure. The results are then compared with those from their original studies.
On two‐phase relative permeability and capillary pressure of rough‐walled rock fractures
Authors: Pruess K. , Tsang Y. W.
Published: February 2008

This paper presents a conceptual and numerical model of multiphase flow in fractures. The void space of real rough‐walled rock fractures is conceptualized as a two‐dimensional heterogeneous porous medium, characterized by aperture as a function of position in the fracture plane. Portions of a fracture are occupied by wetting and nonwetting phase, respectively, according to local capillary pressure and global accessibility criteria. Phase occupancy and permeability are derived by assuming a parallel‐plate approximation for suitably small subregions in the fracture plane. For lognormal aperture distributions, a simple approximation to fracture capillary pressure is obtained in closed form; it is found to resemble the typical shape of Leverett's j function. Approximations to wetting and nonwetting phase relative permeabilities are calculated by numerically simulating single phase flows separately in the wetted and nonwetted pore spaces. Illustrative examples indicate that relative permeabilities depend sensitively on the nature and range of spatial correlation between apertures. It is also observed that interference between fluid phases flowing in a fracture tends to be strong, with the sum of wetting and nonwetting phase relative permeabilities being considerably less than 1 at intermediate saturations.
Variational sensitivity analysis, data requirements, and parameter identification in a leaky aquifer system
Authors: Yeh William W.‐G. , Sun Ne‐Zheng
Published: February 2008

The adjoint state sensitivity method is developed to aid the analysis of a pumping test design for parameter identification in a leaky aquifer system. The system considered consists of an unconfined aquifer overlaying an aquitard which overspreads a confined aquifer. The adjoint equations are derived in the continuous form without discretizing the governing equations and are expressed in terms of “convection‐diffusion” equations with time dependent coefficients. The multiple cell balance (MCB) method is used to solve the nonlinear governing equations as well as their adjoint equations. Algorithms are developed for calculating sensitivity coefficients of heads in each layer of an aquifer with respect to various parameters. A concept called “contribution of an observation” is introduced which can be used to measure the usefulness of an observation in connection with data requirements for parameter identification. It links experimental design with observation errors and the requirement of model application. The sufficiency of an observation system for parameter identification can be evaluated at the design stage before extensive resources are committed to data collection. A numerical example is given to verify the proposed method and to explain its applicability to the design of a pumping test.
Mean square error of regression‐based constituent transport estimates
Authors: Gilroy E. J. , Hirsch R. M. , Cohn T. A.
Published: February 2008

Estimates of long‐term transport of constituents commonly are obtained by summing retransformed estimates from regressions of logarithmically transformed response variables. Typical explanatory variables for these regressions include functions of flow, change in flow, time, and time of year. The mean and mean square error of four estimators of long‐term transport at periodically measured stations are presented as a function of the observed values of the explanatory variables from the long‐term record and summary statistics of the regression data. Estimates of the mean square errors can be used in designing sampling strategies to attempt to minimize the uncertainty in the estimation of long‐term transport subject to a constraint on the number of samples to be taken. This uncertainty is expressed in terms of the explanatory variables in the long‐term record, the regression coefficients and standard error of the regression and the mean and covariance structure of the explanatory variables used in the regression.
Some anomalous features of flow and solute transport arising from fracture aperture variability
Authors: Moreno Luis , Tsang Chin‐Fu , Tsang Yvonne , Neretnieks Ivars
Published: February 2008

Model fractures with variable apertures are generated by a statistical method, and water flow and tracer transport in these fractures are calculated. Tracer injection and collection are simulated for parallel and convergent flow fields. The impact of the use of different injection flow rates and the different locations of injection and collection points are studied. The results show that the transport time, dispersion, and the so‐called mass balance fracture aperture are very sensitive to the location of the injection point and the injection flow rate. These anomalous effects appear to be consistent with observations in several recent field experiments. The implications of the calculated results on the analysis of tracer tests are discussed. It is concluded that because of the stochastic nature of variable fracture apertures (and their permeabilities) a point tracer test in a fracture is not sufficient to characterize the properties of the fracture. Dispersivities and apertures calculated from different tracer tests in the same fracture may vary by orders of magnitude depending on the flow rate and the location of the injection. However, a line injection of tracer, averaged over a series of adjacent points, may avoid some of these problems.
Incorporating hillslope effects into the geomorphologic instantaneous unit hydrograph
Authors: van der Tak Laurens D. , Bras Rafael L.
Published: February 2008

Use of gamma distributions of stream holding times, rather than the traditional exponential distributions, results in geomorphologic instantaneous unit hydrographs (GIUHs) that better fit data‐based IUHs. In this paper, hillslope effects are incorporated into the gamma GIUH (GGIUH) model by assuming that the hillslope travel distance in an area of a given order is approximated by the inverse of twice the local drainage density and introducing a hillslope velocity term. A method of moments fitting procedure is developed to estimate the channel and hillslope velocity terms in the GGIUH model from the moments of rainfall input and basin discharge output. It was found that hillslope velocities are 2 orders of magnitude smaller than channel velocities. The values found for the latter are reasonable given the range of values given in the literature for channel velocities. Similarly, the hillslope velocity term found by the method of moments procedure matches macropore velocities reported in the literature.
Solution of a nonlinear absorption model of mixed saturated‐unsaturated flow
Authors: Broadbridge P.
Published: February 2008

A nonlinear diffusion model of Fujita is adapted to solve a mixed saturated‐unsaturated flow problem representing absorption of water by a soil with a constant pond depth at the surface. From the closed‐form solution, there emerges an equation relating dimensionless sorptivity, dimensionless pond depth, and a single soil hydraulic nonlinearity parameter. For the whole range of soil types, the ratio of ponded sorptivity to unponded sorptivity increases significantly with pond depth but the ponding effect is stronger in weakly nonlinear soils than in highly nonlinear soils. In weakly nonlinear soils the ponding effect is greatly enhanced by increasing the initial water content, but in extremely nonlinear soils the initial water content has little influence.
Modeling infiltration into a sealing soil
Authors: Baumhardt R. L. , Römkens M. J. M. , Whisler F. D. , Parlange J.‐Y.
Published: February 2008

Rain infiltration is often controlled by a less permeable layer known as a seal. Infiltration through this layer was modeled numerically by continuously updating the seal properties as a function of rainfall characteristics. The seal remains unchanged until the surface has reached incipient ponding. Subsequently, a rapid change in the hydraulic properties of the seal is imposed to reflect the effect of raindrop impact. Finally, an equilibrium state develops between seal formation due to raindrop impact and seal erosion due to the rainstorm intensity effects. Model‐predicted infiltration was similar to that observed during simulated rainstorms having various combinations of intensities and kinetic energy rates per millimeter of rain. A sensitivity analysis was performed to determine the effects of changes in seal and bulk soil properties, as well as simulation parameters, on infiltration. Seal formation was dependent on cumulative rainfall energy and the rainstorm intensity.
Analytically Derived Runoff Models Based on Rainfall Point Processes
Authors: Bierkens Marc F. P. , Puente Carlos E.
Published: February 2008

This work introduces four stochastic and lumped in space rainfall‐runoff models. They arise combining two stochastic rainfall models with two conceptualizations of a basin's response. Rainfall is given by rectangular pulses whose arrivals are described by Poisson and Neyman‐Scott models. Runoff is obtained routing such pulses via two alternative conceptualizations, both based on simple linear reservoirs. General expressions for the first‐ and second‐order moments are given for both the instantaneous and locally averaged processes. The mathematical structure of the alternative parameterizations is illustrated via computer simulations.
A Spatial Analysis of Variance Applied to Soil‐Water Infiltration
Authors: Gotway Carol A. , Cressie Noel A. C.
Published: February 2008

A spatial analysis of variance uses the spatial dependence among the observations to modify the usual inference procedures associated with a statistical linear model. When spatial correlation is present, the usual tests for presence of treatment effects may no longer be valid, and erroneous conclusions may result from assuming that the usual F ratios are F distributed. This is demonstrated using a spatial analysis of soil‐water infiltration data. Emphasis is placed on modeling the spatial dependence structure with geostatistical techniques, and this spatial dependence structure is then used to test hypotheses about fixed effects using a nested linear model.
The Cell Analytical‐Numerical Method for Solution of the Advection‐Dispersion Equation: Two‐Dimensional Problems
Authors: Elnawawy Osman A. , Valocchi Albert J. , Ougouag Abderrafi M.
Published: February 2008

In this paper we present the development and evaluation of a new numerical scheme for efficient solution of groundwater solute transport problems. The scheme, which we have named the cell analytical‐numerical (CAN) method, is an extension of the so‐called nodal methods which have been developed in the nuclear engineering area for solving neutron diffusion problems. The CAN method is based upon decomposition of the solution domain into a number of rectangular cells (volume subdomains); each cell is homogeneous so that a local analytic solution to the solute transport equation can be obtained. A first‐order accurate finite difference approximation of the time derivative followed by a transverse averaging procedure is used to transform the governing partial differential equation into a set of coupled one‐dimensional ordinary differential equations for the transverse moments of the concentration. The local analytical solution is therefore found in terms of these moments. Solute mass flux continuity across cell surfaces, along with the local analytical solution, is used to construct an algebraic relationship between concentration moment values at adjacent cell surfaces. Assembling all the cells together results in a set of coupled tridiagonal matrix equations, one set for each spatial direction, which can be solved very efficiently. The CAN method is applied to several simple test problems involving uniform flow in homogeneous aquifers. While the method lacks geometrical flexibility, it offers a spatial approximation that is demonstrated to have high accuracy and minimal grid orientation error, even when applied to coarse meshes.
Comparison of Photon Correlation Spectroscopy With Photosedimentation Analysis for the Determination of Aqueous Colloid Size Distributions
Authors: Rees Terry F.
Published: February 2008

Colloidal materials, dispersed phases with dimensions between 0.001 and 1 μm, are potential transport media for a variety of contaminants in surface and ground water. Characterization of these colloids, and identification of the parameters that control their movement, are necessary before transport simulations can be attempted. Two techniques that can be used to determine the particle‐size distribution of colloidal materials suspended in natural waters are compared. Photon correlation Spectroscopy (PCS) utilizes the Doppler frequency shift of photons scattered off particles undergoing Brownian motion to determine the size of colloids suspended in water. Photosedimentation analysis (PSA) measures the time‐dependent change in optical density of a suspension of colloidal particles undergoing centrifugation. A description of both techniques, important underlying assumptions, and limitations are given. Results for a series of river water samples show that the colloid‐size distribution means are statistically identical as determined by both techniques. This also is true of the mass median diameter (MMD), even though MMD values determined by PSA are consistently smaller than those determined by PCS. Because of this small negative bias, the skew parameters for the distributions are generally smaller for the PCS‐determined distributions than for the PSA‐determined distributions. Smaller polydispersity indices for the distributions are also determined by PCS.
Diffusion in the Matrix of Granitic Rock: Field Test in the Stripa Mine
Authors: Birgersson Lars , Neretnieks Ivars
Published: February 2008

Three field experiments in granitic rock were performed at 360 m depth in a mine to investigate diffusion in the undisturbed rock matrix. The results from all three experiments showed that tracers had migrated through the disturbed zone close to the injection hole and a distance into the undisturbed rock matrix. These results show that it is possible for dissolved compounds to migrate into the rock matrix in its natural stress environment. Diffusivities and hydraulic conductivities obtained in this in situ experiment compare well with those obtained in laboratory experiments.
Mechanisms Affecting Streamflow and Stream Water Quality: An Approach via Stable Isotope, Hydrogeochemical,and Time Series Analysis
Authors: Turner Jeffrey V. , Macpherson Duncan K.
Published: February 2008

Stable isotope (deuterium) and hydrogeochemical (chloride, nitrate) data in rainfall, streamflow and deep and shallow groundwaters have been applied to develop an understanding of the mechanisms of streamflow generation and nutrient discharge from a 27 km2 rural catchment near Perth, Western Australia. Intensive sampling of the stable isotopic composition of rainfall, groundwater and streamflow during the winter flow periods of 1987 and 1988 identified several significant rainfall events that were up to 90‰ depleted in deuterium relative to deep and shallow groundwaters in the catchment. Isotopic responses in streamflow to the depleted rainfall could be discerned on three time scales: (1) the rainfall events themselves (i.e., 1 to 2 days); (2) isotopic relaxation periods lasting up to 6 weeks as streamflow δ values slowly returned to their preevent values; and (3) a longer seasonal scale response of 2 to 3 months caused by a seasonal isotopic trend in rainfall composition and an increase in the proportion of groundwater in streamflow toward the end of the flow period. The sharp isotopic responses in streamflow observed in this study were in marked contrast to the attenuated responses observed in a previous study on a 1 km2 catchment under native forest in the same region. This contrast reflects the different roles of the shallow groundwater system in streamflow generation between the two catchments under different land use. Kalman filtering techniques were applied to investigate the numerical functions linking rainfall amount and streamflow volume and their respective stable isotopic compositions. The purpose of the numerical analysis was to estimate the average lag between rainfall and streamflow, thereby quantifying the time scales of streamflow generation. Analysis of streamflow water quality has been shown that the bulk of nutrients and solutes are discharged from the catchment at the onset of flow, in response to the first major rainfall events of the winter period. Integration of the isotopic, hydrogeochemical and numerical analysis presents a consistent view that surface and subsurface storm flow are important streamflow generating mechanisms over the time scale of a rainfall event. Postevent periods of isotopic relaxation in streamflow are due to discharge of groundwater to streamflow from an ephemeral perched aquifer. There is evidence for an increasing component of deep groundwater in streamflow as streamflow declines toward the end of the flow period in October and November.
Transport properties across the La2∕3Ca1∕3MnO3∕SrTiO3 heterointerface
Authors: Balcells Ll. , Abad Ll. , Rojas H. , Perez del Pino A. , Estrade S. , Arbiol J. , Peiro F. , Martinez B.
Published: February 2008

The transport properties across La2∕3Ca1∕3MnO3∕SrTiO3 (LCMO/STO) heterostructures with different thicknesses of the STO insulating barrier have been studied by using atomic force microscopy measurements in the current sensing (CS) mode. To avoid intrinsic problems of the CS method we have developed a nanostructured contact geometry of Au dots. The conduction process across the LCMO/STO interface exhibits the typical features of a tunneling process. The analysis of I(V) curves by using the Simmons model allows us to determine the barrier height (φ0≈0.6eV) of STO barriers.
Attenuation of microwaves propagating through parallel-plate helium glow discharge at atmospheric pressure
Authors: Srivastava A. K. , Prasad G. , Atrey P. K. , Kumar Vinay
Published: February 2008

The experimental study of microwave-plasma interaction has been performed to demonstrate the transmission and attenuation of microwaves in atmospheric pressure glow discharge plasma. The cold-collisional plasma produced at atmospheric pressure can absorb the microwave energy because of its complex dielectric constant. The microwave of 10 GHz frequency was launched into the plasma and attenuation was measured as a function of electron plasma density, plasma thickness, electron-neutral collision frequency, etc. It was observed that the attenuation significantly depends on electron plasma density and thickness. The microwave attenuation measurement was also used as a diagnostic to estimate electron plasma density. It was validated by optical emission spectroscopic measurements with helium line intensity ratio method. Both the methods show good agreement.
Optimal negative electrodes for poly(3-hexylthiophene): [6,6]-phenyl C61-butyric acid methyl ester bulk heterojunction photovoltaic devices
Authors: Reese Matthew O. , White Matthew S. , Rumbles Garry , Ginley David S. , Shaheen Sean E.
Published: February 2008

The role of the work function and interfacial chemistry on organic device performance was investigated by using a series of contact materials. The active layer was a standard blend of poly(3-hexylthiophene) and [6-6]-phenyl C61-butyric acid methyl ester. Over 100 devices were fabricated and measured to obtain good statistics. Ba∕Al and Ca∕Al electrodes performed best, with similar open-circuit voltages and power conversion efficiencies. Device stability studies showed devices with these two electrodes remained similar after six weeks with degradation of 11%–16% in net conversion efficiency observed. The incorporation of silver into the electrodes led to considerably more degradation than other electrode types.
Torsional resonance atomic force microscopy in water
Authors: Mullin Nic , Hobbs Jamie
Published: February 2008

An atomic force microscope for use in liquids including water, in which a torsionally resonating microcantilever controls the tip-sample interaction, is presented. Magnetostrictive actuators made from a particulate composite of Terfenol-D are used to excite the torsional oscillations of the cantilever. The high quality factor (Q) of torsional oscillation gives high sensitivity to tip-sample forces, and the ability to work using the true resonance peak of the cantilever allows “phase” images corresponding to real mechanical contrast to be collected under liquid. High quality topographic and phase images of a delicate crystallizing polymer sample submerged in water are presented.
Moisture induced electron traps and hysteresis in pentacene-based organic thin-film transistors
Authors: Gu Gong , Kane Michael G.
Published: February 2008

Previous work revealed that electron traps in pentacene formed in air cause hysteresis in thin-film transistor characteristics. Here, we experimentally showed that water vapor, rather than oxygen, is responsible for these hysteresis-causing trap states. Photogenerated and injected electrons are trapped at the pentacene-dielectric interface under positive gate bias, and induce extra holes, resulting in the observed extra drain current. The electron detrapping causes the decay of the extra hole population with time under negative gate bias and, therefore, that of the drain current.