bluuebunny/crossref_metadata_2025 huggingface dataset:
On solute transport in a heterogeneous porous formation under saturated and unsaturated water flows
Authors: Russo David , Dagan Gedeon
Published: February 2004

The results of a recent study of solute transport in a hypothetical scale‐heterogeneous soil under transient unsaturated flow conditions (Russo, this issue) suggested that the time‐dependent components of the spatial covariance tensor, based on a single realization of the solute concentration in unsaturated flow, are in relatively good agreement with the components of the spatial covariance tensor, based on the ensemble‐average solute concentration derived by Dagan (1984), for solute transport under steady saturated flow. Furthermore, the asymptotic value of the longitudinal component of the dispersivity tensor λzz, estimated from the horizontally averaged solute concentration in the unsaturated flow domain, was found to be in good agreement with the asymptotic λzz derived by Dagan (1982, 1984) and Gelhar and Axness (1983) for saturated flow. These findings are explored in a general Lagrangian formulation, using small‐perturbation, first‐order approximations. The results of the analysis of the velocity field in the unsaturated flow domain of the hypothetical scale‐heterogeneous soil of Russo (this issue) suggest that there was good agreement between the result of the Lagrangian kinematic analysis and the result of the analysis of the spatial moments of the concentration distribution, in agreement with the stochastic theory of transport in saturated flow. This is explained by the finding that under unsaturated flow conditions there is an increase in the variability of the hydraulic conductivity K (and concurrently an increase in the variability of the velocity Vz), accompanied by a decrease in the correlation scale of K (and concurrently a decrease in the correlation scale of Vz) relative to the saturated conditions. The increase in the variability of the unsaturated conductivity stemmed from the variability in the water content θ and the strong nonlinear dependence of K on θ, while the accompanying decrease in its correlation scale stemmed from the negative correlation between F = log Ks and θ, which persisted for a relatively large separation distance in the longitudinal vertical direction. Consequently, the product between the conductivity variance and its correlation scale is of similar magnitude for both saturated and unsaturated flow regimes. The results suggest that in spite of the complexity of unsaturated flow, as compared with saturated flow, it might be amenable to a similar analysis.
Optimal data acquisition strategy for the development of a transport model for groundwater remediation
Authors: Tucciarelli Tullio , Pinder George
Published: February 2004

The reliability of groundwater quality management algorithms is limited in large part by the uncertainty present in the model parameters. Because the field parameter measurement costs and the remediation costs must be supplied by the same financial source, the classical optimization procedure does not minimize the real total remediation investment. This research presents an algorithm able to find the total minimum for the sum of both the measurement and the pumping costs. A chance‐constrained technique is used to cast the optimization problem in stochastic form, relating the concentration covariance matrix to the log‐transmissivity covariance matrix by means of the transport equations and a first‐order approximation for the uncertainty. The simulation model solves the steady state flow equations on a finite element triangular mesh and the transport equations using the backward method of characteristics. The resulting nonlinearly constrained optimization problem is solved using the quasi‐linearity algorithm; this algorithm is designed to find a good initial point for the local minimum search when the feasible domain is not convex.
Simulation of precipitation by weather type analysis
Authors: Hay Lauren E. , McCabe Gregory J. , Wolock David M. , Ayers Mark A.
Published: February 2004

A method of precipitation simulation that incorporates climatological information has been developed. A Markovian‐based model is used to generate temporal sequences of six daily weather types: high pressure; coastal return; maritime tropical return; frontal maritime tropical return; cold frontal overrunning; and warm frontal overrunning. Precipitation values are assigned to individual days by using observed statistical relations between weather types and precipitation characteristics. When this method was applied to an area in the Delaware River basin, the statistics describing average precipitation, extreme precipitation, and drought conditions for simulated precipitation closely matched those of the observed data. Potential applications of this weather type precipitation model include climatic change research and modeling of temperature and evapotranspiration.
On the estimation of parameters for frequency domain models
Authors: Ritzi Robert William , Sorooshian Soroosh , Gupta Vijai K.
Published: February 2004

The problem of estimating parameters for frequency domain models is considered. Past approaches have most commonly based estimation criteria upon modulus and phase transformations of the model and sample frequency response functions. As an alternative, a complex vector estimation criterion is proposed and is implemented in a nonlinear, Gauss‐Marquardt optimization algorithm. When compared to the previous methods of fitting modulus and phase transformations, the complex vector estimation methodology has less bias and variance and is more robust.
Goodness‐of‐fit tests for regional generalized extreme value flood distributions
Authors: Chowdhury Jahir Uddin , Stedinger Jery R. , Lu Li‐Hsiung
Published: February 2004

This paper develops critical values and formulas for computing several goodness‐of‐fit tests for the generalized extreme value (GEV) distribution. These tests can check if data available for a site are consistent with a regional GEV distribution, except for scale, or if the data are consistent with a GEV distribution with a regional value of the shape parameter κ. Three tests employ unbiased probability‐weighted moment (PWM) estimators of theLmoment coefficient of variation (L‐CV), and coefficient of skewness (L‐CS) using formulas for their variances in small samples. In a Monte Carlo power study theL‐CV test was often more powerful than the Kolmogorov‐Smirnov test at detectingL‐CV inconsistencies. A test based uponL‐CS generally has equal or greater power than the probability plot correlation test at detectingL‐CS differences; both are poor at detecting thin‐tailed alternatives. Finally, a new chi‐square test based upon sample estimates of both theL‐CV andL‐CS, and their anticipated cross correlation, was much better than other tests at detecting departures from the assumedL‐CV,L‐CS, or both, particularly when the regional distribution was highly skewed.
Terrain‐based catchment partitioning and runoff prediction using vector elevation data
Authors: Moore Ian D. , Grayson Rodger B.
Published: February 2004

An automated method of partitioning catchments into interconnected elements using a “stream tube” approach and vector or contour‐based digital elevation models is briefly described. With this form of partitioning, hydrologic models can be structured based on the hydraulics of flow within a catchment and the effects of topography on runoff producing mechanisms and spatially distributed flow characteristics (such as flow depth and velocity) can be directly, and realistically, accounted for in the models. The method allows complex three‐dimensional flow problems to be reduced to a series of coupled one‐dimensional problems in areas with complex terrain. Two simple process‐oriented hydrologic models that demonstrate the utility of this form of partitioning are presented. The first models subsurface flow‐saturation overland flow and the second models Hortonian overland flow. Observed and predicted runoff hydrographs and the dynamic expansion and contraction of runoff source areas on a small laboratory microcatchment are presented. Also shown are predicted runoff hydrographs and surface flow velocities on a small rangeland catchment in the United States.
Flow through fissured porous media with deformable matrix: Implicit formulation
Authors: Khalili‐Naghadeh N. , Valliappan S.
Published: February 2004

An implicit coupled double‐porosity model has been presented for simulating the behavior of deformable fissured porous media. The finite element technique was used to develop the numerical approximation of the governing equations. Using the proposed model, the effect of matrix deformation on the behavior of fissured porous media was studied. It was found that (1) the response of a deformable fissured porous medium is identical to that of rigid formations with regard to three distinct periods of early, intermediate, and late time responses; (2) while matrix deformation can significantly contribute to the late time productivity of fissured porous media, it has very limited effect on the early time productivity; (3) during the early time response period, fluid pressure tends to rise in porous blocks before starting to fall; this is believed to be consistent with the coupling behavior of deformable fissured porous media; and (4) the fissure response of a deformable fissured porous medium may be practically simulated using the model proposed by Barenblatt et al. (1960) and Warren and Root (1963).
Kinematic routing using finite elements on a triangular irregular network
Authors: Goodrich David C. , Woolhiser David A. , Keefer Tim O.
Published: February 2004

Automated extraction of geometry for hydraulic routing from digital elevation models (DEM) is a procedure that must be easily accomplished for widespread application of distributed hydraulically based rainfall excess‐runoff models. One‐dimensional kinematic routing on a regular grid DEM is difficult due to flow division and convergence. Two‐dimensional kinematic routing on a triangular irregular network (TIN) surmounts many of these difficulties. Because TIN DEMs typically require far fewer points to represent topography than regular grid DEMs, substantial computational economy is also realized. One‐dimensional routing using vector contour data overcomes the grid‐based routing disadvantages but often requires several orders of magnitude more storage points than a TIN. The methodology presented in this paper represents a compromise between slightly increased computational complexity and the economy of TIN topographic representation. We take the unique approach of subdividing each topographic triangle (TIN facet) into a set of coplanar triangular finite elements, performing routing on a single facet and then routing the resulting excess hydrograph to downstream facets and channels via upstream boundary conditions. Results indicate that shock conditions are readily handled, computed depths match analytic results to within ±3% and volume balances are typically within 1%. This modeling system illustrates the viability of kinematic routing over a TIN DEM derived directly from digital mapping data.
Searching for chaotic dynamics in snowmelt runoff
Authors: Wilcox Bradford P. , Seyfried Mark S. , Matison Thor H.
Published: February 2004

Chaos analysis has altered the way we view natural systems. Complex or random‐appearing phenomena may be chaotic and thus deterministic, rather than random. In this study, we used the Grassberger‐Procaccia algorithm (GPA) to evaluate a runoff time series from a second‐order catchment in southwestern Idaho for chaotic dynamics. GPA can identify the presence of low‐dimensional chaotic dynamics for experimental time series. A daily runoff record, 8800 days in length, was examined. We found no evidence of chaotic dynamics in snowmelt runoff. Snowmelt runoff measured at a daily time step has a large number of degrees of freedom, which is characteristic of a random rather than chaotic process. These results suggest that the random‐appearing behavior of snowmelt runoff is generated from the complex interactions of many factors, rather than low‐dimensional chaotic dynamics.
Selection of methods for the detection and estimation of trends in water quality
Authors: Hirsch Robert M. , Alexander Richard B. , Smith Richard A.
Published: February 2004

One result of increased scientific and public interest in water quality over the past few decades has been the gradual accumulation of reliable long‐term water quality data records and an interest in examining these data for long‐term trends. This paper summarizes and examines some of the major issues and choices involved in detecting and estimating the magnitude of temporal trends in measures of stream water quality. The first issue is the type of trend hypothesis to examine: step trends versus monotonic trend. The second relates to the general category of statistical methods to employ: parametric versus nonparametric. The third issue relates to the kind of data to analyze: concentration data versus flux data. The fourth relates to issues of data manipulation to achieve the best results from the trend analysis. These issues include the use of mathematical transformations of the data and the removal of natural sources of variability in water quality due to seasonal and stream discharge variations. The final issue relates to the choice of a trend technique for the analysis of data records with censored or “less than” values. The authors' experiences during the past decade with the development of several trend detection techniques and application of these techniques to a large number of water quality records provide insight into the issues related to a choice of a statistical test for trend in water quality.
Two‐phase flow in heterogeneous porous media: 1. Model development
Authors: Kueper Bernard H. , Frind Emil O.
Published: February 2004

A two‐dimensional finite difference model to study the simultaneous movement of a dense, nonaqueous phase liquid and water in heterogeneous porous media is developed. A distinctive feature of the solution is that the primary variables solved for, wetting phase pressure and wetting phase saturation, are both existent throughout the solution domain regardless of whether the nonwetting phase is present. This eliminates the need to specify small, fictitious saturations of nonwetting fluid ahead of the advancing front where only wetting fluid is present, as is often required in conventional simulators. The model is therefore well suited for the simulation of ground water contamination problems involving the advance of immiscible liquids into previously uncontaminated groundwater systems. The finite difference equations are solved fully implicitly using Newton‐Raphson iteration. In order to minimize computer storage and execution time a Dupont‐Kendall‐Rachford iterative solver utilizing Orthomin acceleration has been incorporated. The numerical model is verified against an exact analytical solution which incorporates fully the effects of both relative permeability and capillary pressure. The model is validated through comparison to a parallel‐plate laboratory experiment involving the infiltration of tetrachloroethylene into a heterogeneous sand pack.
Centrifugal techniques for measuring saturated hydraulic conductivity
Authors: Nimmo John R. , Mello Karen A.
Published: February 2004

Centrifugal force is an alternative to large pressure gradients for the measurement of low values of saturated hydraulic conductivity (Ksat). With a head of water above a porous medium in a centrifuge bucket, both constant‐head and falling‐head measurements are practical at forces up to at least 1800 times normal gravity. Darcy's law applied to the known centrifugal potential leads to simple formulas for Ksat that are analogous to those used in the standard gravity‐driven constant‐ and falling‐head methods. Both centrifugal methods were tested on several fine‐textured samples of soil and ceramic with Ksat between about 10−10 and 10−9 m/s. The results were compared to falling‐head gravity measurements. The comparison shows most measurements agreeing to within 20% for a given sample, much of the variation probably resulting from run‐to‐run changes in sample structure. The falling‐head centrifuge method proved to be especially simple in design and operation and was more accurate than the constant‐head method. With modified apparatus, Ksat measurements less than 10−10 m/s should be attainable.
Further developments of the neyman‐scott clustered point process for modeling rainfall
Authors: Cowpertwait Paul S. P.
Published: February 2004

This paper provides some useful results for modeling rainfall. It extends work on the Neyman‐Scott cluster model for simulating rainfall time series. Several important properties have previously been found for the model, for example, the expectation and variance of the amount of rain captured in an arbitrary time interval (Rodriguez‐Iturbe et al., 1987a), In this paper additional properties are derived, such as the probability of an arbitrary interval of any chosen length being dry. In applications this is a desirable property to have, and is often used for fitting stochastic rainfall models to field data. The model is currently being used in rainfall time series research directed toward improving sewage systems in the United Kingdom. To illustrate the model's performance an example is given, where the model is fitted to 10 years of hourly data taken from Blackpool, England.
A stochastic model of infiltration which simulates “macropore” soil water flow
Authors: Grant S. A. , Jabro J. D. , Fritton D. D. , Baker D. E.
Published: February 2004

Estimates of “macropore” soil water phenomena are calculated from a simple stochastic model of infiltration. Probability density functions (pdf) for cumulative infiltration (I), infiltration rate (I′), and infiltrating‐water flux (ν) are proposed. Cumulative infiltration is defined here as the maximum quantity of water the soil can accept from unponded water supplied at a given location on the soil surface. Similarly, infiltration rate is defined as the maximum rate at which unponded water can enter the soil. The pdfs for cumulative infiltration (fI) and infiltration rate (fI′) are derived by assuming that the saturated hydraulic conductivity (Ks) is a lognormal‐distributed random variable. The actual infiltrating‐water flux will depend on I′, rainfall rate (R0), slope, and surface roughness. For a flat and smooth soil surface the pdf for infiltrating‐water flux fν is derived from fI′, by assuming that water failing to infiltrate into portions of the soil with lower infiltration rates will flow over the soil surface, eventually being accepted by areas with higher infiltration rates. The pdf fν predicts that macropore soil water flow phenomena will become more pronounced as the rainfall rate approaches the mean infiltration rate, when macropore flux predicted by fν, is many times the rainfall rate. These results indicate that stochastic description of soil hydraulic properties shows promise in describing macropore soil water phenomena. Models of the field‐scale effects of macropore soil water flow should include infiltration rate variability and runoff contributions to infiltrating‐water flux.
Theoretical analysis of flow rate into perforated drain tubes
Authors: Hazenberg G. , Panu U. S.
Published: February 2004

Perforated drain tubes have, in many places, supplanted the use of drain tiles in drainage from agricultural lands, airports and the like. However, the few existent formulae for predicting drainage rates are complex and unwieldy. In this paper, a formula is derived by using a series solution of Laplace's equation and techniques of solution of dual trigonometric series, for the rate of water flow into a perforated drain tube, showing the effects of such parameters as hole size, number and spacing of holes, and drain size and depth. The formula applies to pipes perforated by small circular holes or by longitudinal slits. Recommendations for the design of perforated pipes are made, based on the delivery ratio.
Stable Isotopes as an Indicator of Evaporation in the River Murray, Australia
Authors: Simpson H. J. , Herczeg A. L.
Published: February 2004

The stable isotope composition of water (δD and δ18O) and chloride concentrations were measured monthly at 26 stations along the River Murray, Australia and several major tributaries from December 1988 to June 1989. The first four months had low natural runoff plus large irrigation diversions from storage reservoirs. The following three months had much higher (up to a factor of 5) river discharge. Deuterium and oxygen 18 compositions had a maximum range of 50‰ and 9‰, respectively, from headwaters to mouth. Chloride concentrations increased downstream with a maximum range from 1 to 180 × 10−3 kg m−3. These trends can be attributed to (1) evaporation from storage facilities and river channels, (2) inflows of irrigation drainage water enriched in heavy isotopes, and (3) inflows of saline groundwater. Water losses by evaporation from the river surface and irrigation areas amounted to about 40 ± 15% of releases from reservoir storage, based on the integrated enrichment of deuterium, which is comparable to the total transpiration flux within the basin. About half of the chloride acquired by the Murray as it traversed the middle third of the basin during summer months appears to have resulted from return flows of irrigation drainage water.
A Solute Transport Model for Channelized Flow in a Fracture
Authors: Johns Robert A. , Roberts Paul V.
Published: February 2004

Recent experimental and theoretical investigations of transport in fractures indicate that water flows preferentially through channels in the fracture plane. This channeling of flow may imply that solutes transported in large‐aperture channels have limited exposure to the bulk of the rock matrix surface which contacts the fracture. We present a solute transport model for large‐aperture channels that includes both mass transfer to small‐aperture regions of the fracture and the adjacent rock matrix to investigate the interaction of solute in large‐aperture channels with the surrounding rock mass. Modeling channels similar to those observed in the crystalline rock at the Stripa mine show that solute can be retarded considerably due to mass exchange between the channels, the small‐aperture regions of the fracture, and the rock matrix. The extent of solute retardation is insensitive to the solute capacity in the small‐aperture regions and the areal spacing between channels in the fracture plane but is very sensitive to the mass transfer rates between the channel and the small‐aperture region.
Spatial Variability of Strontium Distribution Coefficients and Their Correlation With Hydraulic Conductivity in the Canadian Forces Base Borden Aquifer
Authors: Robin M. J. L. , Sudicky E. A. , Gillham R. W. , Kachanoski R. G.
Published: February 2004

Distribution coefficients (Kd), defined as the ratio of the concentration of solute associated with the solids to the concentration in solution, are widely used in the prediction of reactive solute transport. With the advent of stochastic approaches to describe solute transport, there is a need to examine the spatial distribution of Kd, and its correlation with the hydraulic conductivity (K). Distribution coefficients were measured in triplicates for strontium on 1279 subsamples of cores from Canadian Forces Base Borden for which K measurements were available. The Kd values ranged from 4.4 to 29.8 mL/g, with a mean of 9.9 and standard deviation of 2.89 mL/g. The standard error on the triplicate means was 0.95 mL/g or approximately 10% of the mean. The spatial behavior of Kd and K (expressed as In (Kd) and ln (K)) was examined in three directions: horizontally along two orthogonal transects and vertically. The two variables each behaved nearly identically in the two horizontal directions, suggesting horizontal isotropy. Horizontally, ln (Kd) appeared as “white noise” suggesting that the horizontal spacing between cores (1 m) was too large to detect any self‐correlation. The distribution coefficient displayed increasing power spectral density with increasing scale in the vertical direction, while In (K) showed these trends in all directions. Depending on the model used, the, correlation lengths obtained by least squares fits of the power spectra varied from 1 to 7.5 m horizontally and from 10 to 30 cm vertically for ln (K); and from 30 cm to 2 m horizontally and from 30 to 70 cm vertically for ln (Kd). The ln (Kd) values showed a significant but very weak negative overall correlation with ln (K) at the 99.95% confidence level. The cross‐spectral and coherency analysis showed that the sign and degree of correlation between ln (Kd) and ln (K) depended on the scale and direction considered. The correlations in all directions and at all scales were weak, and could not always be declared significantly different than zero at the 95% confidence level.
A “Conductance” Mesh Approach to the Permeability of Natural and Simulated Fracture Patterns
Authors: Odling Noelle E. , Webman Itzhak
Published: February 2004

The “conductance” mesh method is used to determine the permeability properties of natural and simulated, two‐dimensional, fracture patterns. In the method, the fractures and their matrix are discretized on a grid of 200 × 200 elements. The permeability of the whole pattern in the direction of the pressure gradient is determined from the total flow through the network. By applying the model in varying orientations, a picture of the directional permeability is constructed. Using the model, the effects of varying matrix permeability and fracture aperture on the global permeability of a natural fracture pattern and 10 simulations are investigated. The simulated fracture patterns were found to increasingly underestimate the permeability of the natural pattern as the permeability contrast between rock matrix and fractures increased. The lower permeability of the simulations was found to relate to their poorer connectivity, as reflected in the sizes of their “backbones” (comprising all paths across the region along the fracture network) and in the number and type of fracture intersections. The results show that the effect of the natural pattern spatial distribution is to promote fracture intersection, thereby increasing connectivity and permeability.
Early Water Yield Effects of Conversion of Slopes of a Eucalypt Forest Catchment to Radiata Pine Plantation
Authors: Bren L. J. , Papworth M.
Published: February 2004

Between 1975 and 1987 the water yield from three small, contiguous, forested catchments carrying similar vegetation in southeastern Australia was measured. These were located in humid, steep foothill areas subject to a major plantation program. At the project start all catchments carried mature, natural eucalypt forest. In their natural state the catchments exhibited similar annual hydrologic variation and water yields, with a pronounced low‐flow period in summer and autumn, and high flows in winter and spring. One catchment was converted from native eucalypt forest to radiata pine by clearing, burning, and planting in December 1979. A 30‐m buffer was retained along the stream. The treatment increased the water yield of the catchment by up to 3.5 ML ha−1, a 47% increase on average. The actual yield increase varied from year to year, and appeared to decline slowly with time from the conversion. Most of the increase was as increased storm flow in the early part of winter. The relation between the storm flow, causal rainfall, and antecedent flow did not appear to be changed by the treatment, suggesting that most of the storm flow response is attributable to increased catchment wetness at the end of the dry summer period.
Sub‐Grid‐Scale Characterization of Channel Lengths for Use in Catchment Modeling
Authors: Hoover K. A. , Foley M. G. , Heasler P. G. , Boyer E. W.
Published: February 2004

We explored methods for extrapolating mainstream channel lengths of first‐order drainage basins from synoptic data to characterize them for numerical watershed modeling. We analyzed four catchments in a climatologically semiarid arid geologically homogeneous region east of the Cascade Mountains in Washington state. Within each of these catchments, we identified stream channel networks manually from 1:24,000‐scale topographic maps, and from 50‐m resolution digital elevation models using commercially available drainage network extraction methods. A least squares fit of logarithms of mainstream length plotted against basin area established a regression relation to use for predicting mainstream lengths in the smallest subbasins. To test our relation, we compared predicted mainstream lengths with first‐, second‐ and third‐order channel lengths measured from low‐altitude aerial photographs of a representative fourth‐order basin. Our results indicate that relations of mainstream length to basin area derived from coarsely gridded data (e.g., 30 m) cannot be used to characterize stream and basin morphometry in the smallest basins due to the presence of hydrologic and geometric controls (i.e., thresholds) that limit the mainstream channel length and total basin length in first‐, second‐, and third‐order basins. The presence of these thresholds potentially constrains the range over which theoretically self‐similar, or fractal, relationships can be applied to stream‐channel networks.
An Empirical Model of Intrinsic Permeability in Reactive Clay‐Bearing Sands
Authors: Raffensperger Jeff P. , Ferrell Ray E.
Published: February 2004

Permeameter experiments were conducted to determine the permeability response of a clay‐bearing sand to changes in fluid composition. These experiments examined the effects of varying cation type and concentration using NaCl and CaCl2 solutions, as well as the effects of freshwater flushing of sediments equilibrated with seawater and large versus small stepwise reductions in NaCl concentration. The major goal of the study was to correlate permeability with measured parameters of the pore microstructure. Sediment samples from the tests were analyzed using Brunauer, Emmett, Teller (BET) and ethylene glycol monoethyl ether (EGME) adsorption methods to determine specific surface area. Other samples were examined using scanning electron microscopy and digital image analysis methods. For image analysis, backscattered electron images were acquired and specific measurements of pore structure parameters (porosity, pore‐size distribution) were made. Stepwise multiple regression was used to construct an empirical model of permeability, which included terms of porosity, mean pore diameter, and specific surface area. The Kozeny‐Carman model was applied to the data and led to predicted permeabilities 2 to 3 orders of magnitude lower than measured values. This was attributed to the presence of dead‐end pores and the importance of flow through large conducting channels.
Evaluation of Unsaturated Zone Air Permeability Through Pneumatic Tests
Authors: Baehr Arthur L. , Hult Marc F.
Published: February 2004

Predicting the steady state distribution of air pressure in the unsaturated zone resulting from a pneumatic test provides a method for determining air‐phase permeability. This technique is analogous to the inverse problem of well hydraulics; however, air flow is more complicated than ground water flow because of air compressibility, the Klinkenberg effect, variations in air density and viscosity that result from temperature fluctuations in the unsaturated zone and the possibility of inducing water movement during the pneumatic test. An analysis of these complicating factors reveals that, when induced water movement can be neglected, a linear version of the airflow equation can provide an appropriate approximation for the purpose of determining air‐phase permeability. Two analytical solutions for steady state, two‐dimensional, axisymmetric airflow to a single well partially screened in the unsaturated zone are developed. One solution applies where there is a stratum of relatively low air permeability, separating the stratum in which the well is completed, from the atmosphere. The other solution applies where there is no separating stratum between the domain and atmosphere. In both situations the water table forms the lower horizontal boundary. Applications of both solutions to determine air permeability from data collected during pneumatic tests are presented.
Crossing Theory for Non‐Gaussian Stochastic Processes With an Application to Hydrology
Authors: Desmond A. F. , Guy B. T.
Published: February 2004

The theoretical properties of level crossings for stationary Gaussian processes have been applied in the past to model empirical crossing behavior of streamflow data with some success. However, when the marginal distributions exhibit departures from normality, such as high coefficients of skewness, the theoretical Gaussian results perform poorly in modeling empirical crossing behavior. In this paper we present theoretical results for crossings of levels by two non‐Gaussian processes, namely, the chi‐square process and the lognormal process. The chi‐square process does not appear to have been applied previously in the hydrologic literature. We apply both models to a skewed annual flow series. Comparisons with the Gaussian model are made and noticeable improvements are found in both cases.
Measuring Contingent Values for Wetlands: Effects of Information About Related Environmental Goods
Authors: Whitehead John C. , Blomquist Glenn C.
Published: February 2004

A model of contingent market behavior is developed which emphasizes the role of household information about wetlands and related environmental goods. Information is acquired through previous experience with wetlands and through the contingent market. Households which are unaware of substitute or complement environmental goods when participating in contingent markets may overstate or understate willingness to pay values. This paper estimates willingness to pay for preservation of the Clear Creek wetland in western Kentucky when faced with surface coal mining. We test for the effects of explicit information about related environmental goods on contingent values by measuring the difference in stated willingness to pay. Willingness to pay for preservation of the Clear Creek wetland decreases with information about surface coal mine lake reclamation and, in the initial, independent format increases with information about a nearby publicly owned, wetland area. These findings suggest that the lack of explicit information about related environmental goods in contingent markets can contribute to a misstatement of willingness to pay.
Application of Energy Concepts to Groundwater Flow: Time Step Control and Integrated Sensitivity Analysis
Authors: Karney Bryan W. , Seneviratne Asitha
Published: February 2004

When fluid passes into or out of an aquifer, work is done at the boundaries which is used partly to change the internal energy of the system and partly to overcome resistance to flow. For a saturated medium, the change in internal energy is further partitioned into two terms, the strain energy stored in the elastic soil matrix and the strain energy stored in the pore water due to compression. A technique is developed in this paper which interprets the dynamic behavior of an aquifer in terms of its energy transformations. The central feature of this approach is the quantification of the physical processes into individual energy and work parameters which together characterize the response of an entire aquifer to a given set of excitations. By this means, the rate of change of strain energy is shown to be a natural index of the unsteadiness of a system, an insight which leads to an adaptive algorithm for adjusting the time step of a transient groundwater flow model. Further, the energy approach is used to assess differential compaction in a heterogeneous aquifer, thereby providing a basis for efficient computation and for rational acquisition of compressibility data.
An approach to calculating wind‐driven currents and transport of substances in Unstratified water bodies using curvilinear coordinates
Authors: Plis Yuri M.
Published: February 2004

A model of wind‐driven currents and transport of substances in unstratified water bodies is presented. The model employs a coordinate system, consisting of curvilinear functions that are orthogonal in the horizontal plane and a coordinate of “normalized depth.” The method of merging asymptotic expansions was used to obtain analytical expressions for components of current velocity as functions of wind stress and pressure anomaly gradient. The pressure anomaly was calculated using an equation that describes the influence of bottom topography, Coriolis force, inertial forces, wind friction, bottom friction and lateral friction.
Surface runoff contamination by soil chemicals: Simulations for equilibrium and first‐order kinetics
Authors: Wallach Rony , Shabtai Rina
Published: February 2004

A model was developed to predict the potential contamination of overland flow by chemicals removed from soil water by rainfall on sloping soil. The model accounts for transient water infiltration and convective‐dispersive solute transport in the soil and also considers rate‐limited mass transfer through a laminar boundary layer at the soil surface/runoff water interface. Sorption‐desorption interactions between soil and chemicals are assumed to be subject to linear and nonlinear isotherms or to first‐order kinetics. The dissolved‐chemical concentrations at the soil surface and in the surface runoff were determined for different antecedent soil moistures and rainfall intensities. These concentrations are lower when the antecedent moisture is low because the time of ponding for drier soil is longer and because during that period soil solutes are displaced by greater volumes of infiltrating water. For a specified initial soil water content, higher rainfall rates cause higher dissolved‐chemical concentrations at the soil surface. The degree of nonlinearity of the equilibrium isotherm greatly affects the transient dissolved‐chemical concentrations and the linear isotherm cannot always be used as an alternative. These concentrations are also greatly affected by the value of the kinetics rate coefficient. In the first‐order kinetics model there is a recovery of the dissolved‐chemical concentration at the soil surface during the period between rainstorms. As a result, the initial concentration at the soil surface for the subsequent rainstorm is higher than that expected when equilibrium is assumed.
Transport of Organic Chemicals by Gas Advection in Structured or Heterogeneous Porous Media: Development of a Model and Application to Column Experiments
Authors: Brusseau Mark L.
Published: February 2004

The use of soil venting or vacuum extraction for remediation of contaminated soil has lead to an increased interest in modeling gas‐phase processes. The majority of existing transport models have been developed assuming homogeneous porous media properties. It is well known, however, that the assumption of homogeneity will almost always be invalid for field systems. In addition, sorption has been described using the local equilibrium assumption. This assumption has also been shown to be invalid under certain conditions. A one‐dimensional model is presented that accounts for a structured or heterogeneous porous medium and for rate‐limited sorption. The model is designed for cases where transport occurs by advection and dispersion in the gas phase and where the liquid phase is immobile. A sensitivity analysis is presented, and parameter determination is discussed. The performance of the model was evaluated by comparing predicted simulations to data obtained from the literature. The model performed well, especially considering that the values for all input parameters were obtained independent of curve fitting.
Application of stochastic unsaturated flow theory, numerical simulations, and comparisons to field observations
Authors: Jensen Karsten Høgh , Mantoglou Aristotelis
Published: February 2004

A stochastic unsaturated flow theory and a numerical simulation model have been coupled in order to estimate the large‐scale mean behavior of an unsaturated flow system in a spatially variable soil. On the basis of the theoretical developments of Mantoglou and Gelhar (1987a, b, c), the theory assumes that the local hydraulic soil properties are realizations of three‐dimensional random fields, and it derives the large‐scale flow structure by averaging the local governing flow equation over the ensemble. The resulting mean model representation is in the form of a partial differential equation, similar to the local flow equation, in which effective model parameters occur (e.g., effective hydraulic conductivity). Further, the theory predicts the variance (prediction error) of the capillary tension head due to the spatial variability of the local hydraulic soil properties. The governing unsaturated flow equation representing the mean system behavior is solved using a finite difference numerical solution technique. The effective parameters are evaluated from the stochastic theory formulas before entering them into the numerical solution for each iteration. The stochastic model is applied to a field site in Denmark, where information is available on the spatial variability of soil parameters and variables. Numerical simulations have been carried out, and predictions of the mean behavior and the variance of the capillary tension head and the soil moisture content have been compared to field observations. The stochastic model predicts the average system behavior reasonably well. Reasonable results are also obtained for the range of variation of the capillary tension head, despite the limited number of measurements and corresponding statistical inference issues. The stochastic model presented here seems to offer a rational framework for modeling large‐scale unsaturated flow and estimating areal averages of soil‐hydrological processes in spatially variable soils.
Multivariate analysis of stream water chemical data: The use of principal components analysis for the end‐member mixing problem
Authors: Christophersen Nils , Hooper Richard P.
Published: February 2004

Traditional multivariate data analysis techniques, such as principal components analysis (PCA), have often been used in an attempt to identify source solutions from potential mixtures, such as stream water. Artificial data, generated from conservative mixing of known source solutions in random proportions, are employed to demonstrate that PCA should be used only to determine the rank of the mixture and not to determine the composition of the source solutions. The rank of the mixture is related to the number of source solutions. Unambiguous identification of the source solution compositions from the mixture alone is impossible; thus it is necessary that potential source solutions be derived from independent measurements. In the case of stream water, possible source solutions are groundwater and soil water from different horizons. A multivariate screening procedure is presented for the evaluation of these potential source solutions.
Numerical modeling of steam injection for the removal of nonaqueous phase liquids from the subsurface: 2. Code validation and application
Authors: Falta Ronald W. , Pruess Karsten , Javandel Iraj , Witherspoon Paul A.
Published: February 2004

The multiphase steam injection simulator developed in part 1 (Falta et al., this issue) is used to simulate two laboratory column steam displacement experiments. In the first simulation, steam is injected into a clean, water‐saturated column, while in the second simulation, steam is injected into a column containing both water and separate phase trichloroethylene. In both cases, the numerical results are in good quantitative agreement with the experimental data. Based on the assumption of local chemical equilibrium between the phases, a simple criterion is derived for determining the major mechanism of nonaqueous phase liquid (NAPL)/chemical transport during the steam displacement process. Several one‐dimensional simulations of the steam displacement of high‐boiling‐point NAPLs are discussed. These results are consistent with theoretical predictions and indicate that steam may efficiently displace organic liquids having boiling points substantially greater than that of water.
Optimal design of run‐of‐river power plants
Authors: Najmaii M. , Movaghar A.
Published: February 2004

In optimization of any hydro plant system, the analysis should make use of all information available, including a thorough study of streamflow and civil engineering and electromechanical cost effectiveness. For this purpose a general mathematical model is used for an overall design optimization of run‐of‐river power plants. The design criteria for such power plants are fundamentally based on some important and critical cost‐effective parameters such as discharge design, penstock or tunnel diameter, turbine capacity, number of units and type of turbines. The optimization process consists of three main steps: (1) find the maximum yearly energy; (2) maximize the yearly benefits of all the maximized sets of power plants; and (3) select the most beneficial of the feasible sets of run‐of‐river power plants on the basis of a combination of the cited criteria. The computational analysis is based upon two major objective functions and a computer program which is developed for fast maximization and optimization of the generated equations.
A model for the entrainment and transport of sediment grains of mixed sizes, shapes, and densities
Authors: Bridge John S. , Bennett Sean J.
Published: February 2004

A model for the entrainment and bed load transport of sediment grains of different sizes, shapes and densities by a unidirectional turbulent flow is developed in terms of (1) sediment types available for transport; (2) the mean and turbulent fluctuating values of fluid forces acting upon the sediment grains; and (3) the nature of the interaction between turbulent fluid forces and available sediment, resulting in entrainment and transport of grains as bed load or in suspension. The behavior of the model is explored extensively, and compared with natural data from flumes and rivers. The predicted threshold of entrainment of individual size fractions within a mixed‐size bed agrees well with observations as long as the pivoting angle is specified appropriately as a function of grain size. The rate and size distribution of bed load transport generally agrees well with natural data as long as effective bed shear stress in the presence of bed forms can be defined, bed load transport measurements are reliable, and the size distribution of available sediment is accurately specified.
Water conservation in irrigated agriculture: A stochastic production frontier model
Authors: McGuckin J. Thomas , Gollehon Noel , Ghosh Soumen
Published: February 2004

A stochastic production frontier model of irrigation is used to analyze sources of economic inefficiency in irrigation practices of Nebraska com producers and the extent that field information from soil moisture monitoring, commercial scheduling and/or weather reports increases economic efficiency. The results indicate that farm irrigation practices have an average technical efficiency of 81% (defined as the ratio of actualized production to maximum potential production for a level of inputs). Field information from moisture sensors can improve technical efficiency by 3.9%. The value of information provided by moisture sensors depends on the technical efficiency of the farmer and ranges from $58.23 per hectare for an efficient farmer to $40.29 for an inefficient producer. The elasticity of derived demand for water is estimated to be −1.095.