bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic analysis of solute transport in heterogeneous, variably saturated soils
Authors: Harter Thomas , Yeh T.‐C. Jim
Published: February 2004

Statistical moments of solute plumes from small sources in variably saturated, heterogeneous porous media are analyzed by using a newly developed, efficient high‐resolution Monte Carlo technique. In agreement with previous theoretical work, it is illustrated that the prediction of such solute plumes is associated with large uncertainties for dimensionless travel times, t′, exceeding 40, particularly predictions of plumes in highly heterogeneous soils (σy2>2). Uncertainty about the travel path of the plume center contributes significantly to overall concentration uncertainty as flux fields become more variable. It is shown that the concentration coefficient of variation at the center of the plume initially increases but stagnates or decreases at later times. For highly heterogeneous soil flux conditions or for the common case of soils with strongly anisotropic conditions, analytical models underestimate transverse spreading of the mean concentration plume at any given time, while overestimating longitudinal spreading. At identical mean plume displacement distances, analytical models underestimate both transverse and longitudinal spreading and overestimate the variance of solute flux (breakthrough curve) by up to a factor 4. As an alternative to the statistical analysis of solute flux, we propose to analyze statistical properties of time associated with peak solute flux and with first exceedance of a given solute flux level.
Soil‐water flux in the Southern Great Basin, United States: Temporal and spatial variations over the last 120,000 years
Authors: Tyler S. W. , Chapman J. B. , Conrad S. H. , Hammermeister D. P. , Blout D. O. , Miller J. J. , Sully M. J. , Ginanni J. M.
Published: February 2004

The disposal of hazardous and radioactive waste in arid regions requires a thorough understanding of the occurrence of soil‐water flux and recharge. Soil‐water chemistry and isotopic data are presented from three deep vadose zone boreholes (>230 m) at the Nevada Test Site, located in the Great Basin geographic province of the southwestern United States, to quantify soil‐water flux and its relation to climate. The low water contents found in the soils significantly reduce the mixing of tracers in the subsurface and provide a unique opportunity to examine the role of climate variation on recharge in arid climates. Tracing techniques and core data are examined in this work to reconstruct the paleohydrologic conditions existing in the vadose zone well beyond the timescales typically investigated. Stable chloride and chlorine 36 profiles indicate that the soil waters deep in the vadose zone range in age from approximately 20,000 to 120,000 years. Secondary chloride bulges that are present in two of the three profiles support the concept of recharge occurring at or near the last two glacial maxima, when the climate of the area was considerably wetter and cooler. The stable isotopic composition of the soil water in the profiles is significantly more depleted in heavy isotopes than is modern precipitation, suggesting that recharge under the current climate is not occurring at this arid site. Past and present recharge appears to have been strongly controlled by surface topography, with increased incidence of recharge where runoff from the surrounding mountains may have been concentrated. The data obtained from this detailed drilling and sampling program shed new light on the behavior of water in thick vadose zones and, in particular, show the sensitivity of arid regions to the extreme variations in climate experienced by the region over the last two glacial maxima.
Dissolved gas tracers in groundwater: Simplified injection, sampling, and analysis
Authors: Sanford William E. , Shropshire Robin G. , Solomon D. Kip
Published: February 2004

Simplified injection, sampling, and analytical procedures using dissolved gases as groundwater tracers are presented for use in saturated conditions at both the laboratory and field scales. The injection of gases into the groundwater is accomplished by allowing the gas to diffuse through semipermeable tubing, minimizing the formation of bubbles that could modify the hydraulic properties around the well. We have simplified the collection of dissolved gases by developing a passive in situ headspace sampler the employs a semipermeable membrane and copper tubing equipped with a schrader valve. The headspace within the sampler equilibrates with the dissolved gases in the groundwater in around 24 hours, and no groundwater is collected, which is of great advantage for use in contaminated sites. The design parameters and the time to equilibrium of the headspace sampler can be adjusted for investigation requirements using the analytical equation presented. The analysis of the gases for tracer content is performed by using a common gas Chromatograph fitted with a thermal conductivity detector. Examples of the use of these methods at both the laboratory and field scales are presented.
Dissolution Fingering During the Solubilization of Nonaqueous Phase Liquids in Saturated Porous Media: 2. Experimental Observations
Authors: Imhoff Paul T. , Thyrum Geoffrey P. , Miller Case T.
Published: February 2004

Nonaqueous phase liquids (NAPLs) are a common source of contamination at polluted groundwater sites, where they frequently remain trapped within the pore space at residual saturation and reduce the permeability of the medium to aqueous phase flow. The model presented in a companion paper [Imhoff and Miller, this issue] suggested that when fluid flow is imposed on such a system, the aqueous phase may interact with dissolution‐induced permeability changes, and lead to fingered patterns. In this investigation, a two‐dimensional flow cell was used to study the effects of porous medium structure, Darcy flux, initial residual NAPL saturation, median particle diameter, gravity, and NAPL composition on dissolution fingering. Fingering occurred when two conditions were met: (1) 11 to 80 e‐fold times had elapsed, where e‐fold time is the time required for the instability to grow by a factor e and was predicted from the linear stability analysis in the companion paper; and (2) the length of the dissolution front before finger development was smaller than the zone of NAPL residual. Where fingers formed, finger structure was similar and showed no systematic variation within the parameters investigated. Observed finger wavelengths compared well with model predictions. A single experiment in a three‐dimensional cell, 1 m long, demonstrated that fingers can grow to at least 30 cm in length. When experimental observations in this cell were compared with predictions of NAPL dissolution based on models that did not include fingering, the measurements of changing NAPL saturation differed significantly from model predictions.
Regional scale framework for modeling water resources and health risk problems
Authors: Pelmulder Susan D. , Yeh William W.‐G. , Kastenberg William E.
Published: February 2004

A framework of simulation models for including human exposure to contaminants in regional scale aquifer management problems is presented. The framework includes horizontal flow and transport of contaminant plumes in the aquifer and multiple‐pathway human exposure. Well water from the aquifer simulation model is used as the source of contaminant in the human exposure model. The exposure pathways considered for regional analysis are ingestion of foods grown using well water as part of the irrigation supply; ingestion and dermal absorption of contaminants in tap water; and inhalation of vaporized contaminants while bathing. An environmental compartment model is used to track the contaminant in irrigation water into the soil layers in contact with food products. The simulation framework is demonstrated in a study of the sensitivity of exposure to various aquifer and water supply parameters. The region used is hypothetical; however, the parameters are typical of California.
Soil vapor extraction system design by combinatorial optimization
Authors: Sun Yung‐Hsin , Davert Marshall W. , Yeh William W‐G.
Published: February 2004

Soil vapor extraction is a cost‐effective technique for the removal of volatile organic compounds (VOCs) immobilized in the unsaturated zone. The design of soil vapor extraction system (SVES) requires determination of the number and locations of extraction wells and corresponding extraction rates. At present, few guidelines for design are available. Thus most designs are based on empirical assessments, simplified analytical approximations, or limited trial‐and‐error simulations. In this paper, three optimization‐based design methodologies are introduced: (1) implicit enumeration, (2) systematic reduction, and (3) local search. These methodologies address the combinatorial nature of the design problem and properly evaluate the well interference effects in a multiwell system. Through a benchmark problem, a comparative evaluation of the three methodologies is made with regard to their solution accuracy, computational efficiency, optimality heuristic, and application limitations.
Application of a model accounting for kinetic sorption and degradation to in situ microcosm observations on the fate of aromatic hydrocarbons in an aerobic aquifer
Authors: Bjerg Poul L. , Brun Adam , Nielsen Per H. , Christensen Thomas H.
Published: February 2004

The fate of seven aromatic hydrocarbons was investigated under aerobic conditions by in situ microcosm (, one‐dimensional in situ column) in a sandy aquifer low in organic carbon. The results were simulated by an one‐dimensional reactive solute transport model accounting for kinetically controlled sorption and degradation in order to obtain degradation parameters. The kinetic sorption process affected the fate of the aromatic hydrocarbons and must be accounted for in simulation of the degradation process. The kinetic sorption constants were determined either by laboratory batch sorption experiments or from the fate curves of a biological deactivated ISM. The fate curves of biological active ISMs were adequately described both by the first‐order degradation model, including a lag phase, and by the Monod model. The first‐order degradation model was preferred to the Monod model, because the Monod model approaches the first‐order degradation model at the low concentrations studied (initial concentrations of the organic compounds were 150 μg L−) and because the basic assumptions of the Monod model may be violated in a system where several aromatic hydrocarbons are present at the same time. The results obtained by the first‐order degradation model showed lag phase ranging from 2–20 days. First‐order degradation rate constants for aromatic hydrocarbons determined in situ ranged between 0.05 and 0.8 day−1. Sensitivity analysis revealed the importance of independent determination of sorption constants in order to obtain reliable first‐order degradation rate constants.
On instability during immiscible displacement in fractures and porous media
Authors: Neuman Shlomo P. , Chen Guoliang
Published: February 2004

Existing linear theories for the onset of instability during immiscible fluid displacement in fractures and porous media are generalized to account for arbitrary fracture, mean flow, and fluid interface orientations. The generalized analysis shows that contrary to common belief, the interface is not perpendicular (and hence incipient fingers do not extend in parallel) to the mean direction of displacement except in special cases.
Stochastic analysis of sorption macrokinetics in heterogeneous aquifers
Authors: Miralles‐Wilhelm F. , Gelhar L. W.
Published: February 2004

A stochastic analysis of the transient characteristics of sorption at field scales shows that sorption macrokinetics arise as a result of aquifer physical and chemical heterogeneities. Analytical expressions for the time evolution of the field scale retardation factor and longitudinal macrodispersivity for a reactive solute are obtained and compared with previous theoretical findings. Sorption macrokinetics are characterized by a timescale related to the average solute velocity, mean retardation factor, and variability correlation scale. A comparison of this timescale with the intragrain diffusion characteristic timescale provides a quantitative criterion to determine the relative importance of intragrain processes and aquifer physical and chemical heterogeneities on the characteristic field scale features of reactive solute plumes: retardation and dispersion. Calculations performed for the Borden field site in Ontario, Canada, show that the macrokinetics timescale is potentially much larger than the intragrain diffusion characteristic time, rendering natural heterogeneities as the controlling mechanism in the transient development of reactive plumes in the field. Analytically derived expressions for spatial moments of a reactive plume are evaluated to obtain results for the mass in solution, center of mass displacement, and longitudinal second moment. These results compare favorably with numerical simulations, but a comparison with field measurements is limited at this time owing to a lack of reliable data.
Influence of Specific Surface Area on Transport of Sorbing Solutes in Fractures: An Experimental Analysis
Authors: Wels Christoph , Smith Leslie , Vandergraaf T. T.
Published: February 2004

Experimental evidence is presented on the influence of specific surface area on the transport of the sorbing tracer strontium in fractures with uniform, but differing aperture. Specific surface area is defined as the ratio of the fracture surface area to the volume of mobile water in the fracture. Static sorption experiments on granite coupons suggest hysteresis in the sorption process, showing higher surface distribution coefficients for desorption than for sorption. Strontium was subject to significantly greater dispersion than the nonreactive tracer tritium. This enhanced dispersion is believed to be the result of chemical heterogeneity at the mineral grain scale, hysteresis in sorption, and limited transverse mixing across the fracture aperture. The influence of fracture aperture on retardation is much greater than predicted by the commonly used definition of the surface retardation factor. Strontium retardation was approximately an order of magnitude greater in a smaller‐aperture fracture (450 µm, Ra ∼ 45) than in a large‐aperture fracture (780 µm, Ra ∼ 3.5). We hypothesize that hysteresis in sorption, in conjunction with limited transverse mixing across the aperture, caused the apparent increase in sorption strength (Ka) with a decrease in fracture aperture.
Operational Validation and Intercomparison of Different Types of Hydrological Models
Authors: Refsgaard Jens Christian , Knudsen Jesper
Published: February 2004

A theoretical framework for model validation, based on the methodology originally proposed by Klemes [1985, 1986], is presented. It includes a hierarchial validation testing scheme for model application to runoff prediction in gauged and ungauged catchments subject to stationary and nonstationary climate conditions. A case study on validation and intercomparison of three different models on three catchments in Zimbabwe is described. The three models represent a lumped conceptual modeling system (NAM), a distributed physically based system (MIKE SHE), and an intermediate approach (WATBAL). It is concluded that all models performed equally well when at least 1 year's data were available for calibration, while the distributed models performed marginally better for cases where no calibration was allowed.
Steady Flow Toward Wells in Heterogeneous Formations: Mean Head and Equivalent Conductivity
Authors: Indelman P. , Fiori A. , Dagan G.
Published: February 2004

We consider steady flow of water in a confined aquifer toward a fully penetrating well of radius rw (Figure 1). The hydraulic conductivity K is modeled as a three‐dimensional stationary random space function. The two‐point covariance of Y = In (K/KG) is of axisymmetric anisotropy, with I and Iυ, the horizontal and vertical integral scales, respectively, and KG, the geometric mean of K. Unlike previous studies which assumed constant flux, the well boundary condition is of given constant head (Figure 1). The aim of the study is to derive the mean head 〈H〉 and the mean specific discharge 〈q〉 as functions of the radial coordinate r and of the parameters σy2, e = I/Iυ and rw/I. An approximate solution is obtained at first‐order in σy2, by replacing the well by a line source of strength proportional to K and by assuming ergodicity, i.e., equivalence between
,
, space averages over the vertical, and 〈H〉 〈q〉, ensemble means. An equivalent conductivity Keq is defined as the fictitious one of a homogeneous aquifer which conveys the same discharge Q as the actual one, for the given head Hw in the well and a given head
in a piezometer at distance r from the well. This definition corresponds to the transmissivity determined in a pumping test by an observer that measures Hw,
, andQ. The main result of the study is the relationship (19) Keq = KA(1 − λ) + Kefuλ, where KA is the conductivity arithmetic mean and Kefu is the effective conductivity for mean uniform flow in the horizontal direction in the same aquifer. The weight coefficient λ < 1 is derived explicitly in terms of two quadratures and is a function of e, rw/I and r/I . Hence Keq unlike Kefu, is not a property of the medium solely. For rw/I < 0.2 and forr/I > 10, λ has the simple approximate expression λ* = ln (r/I)/ In )r/rw). Near the well, λ ≅ 0 and Keq ≅ KA, which is easily understood, since for rw/I ≪ 1 the formation behaves locally like a stratified one. In contrast, far from the well λ ≅ 1 and Keq ≅ Kefu the flow being slowly varying there. Since KA > Kefu, our result indicates that the transmissivity is overestimated in a pumping test in a steady state and it decreases with the distance from the well. However, the difference between KA and Kefu is small for highly anisotropic formations for which e ≪ 1 . A nonlocal effective conductivity, which depends only on the heterogeneous structure, is derived in Appendix A along the lines of Indelman and Abramovich [1994].
A Simulation Model for Meandering Rivers
Authors: Sun Tao , Meakin Paul , Jøssang Torstein , Schwarz Klaus
Published: February 2004

A computer model for the dynamics of meandering rivers has been used to study the interplay between the migrating river and the changing sedimentary environment created by the meandering river itself. The model is based on the theory of Ikeda et al. [1981] and is closely related to that proposed by Howard [1983]. Coarser sands, which are often associated with high credibility, are deposited in the point bars formed when the river migrates away from its former bank. Fine‐grained material eventually fills the oxbow lakes created by cutoff processes and forms erosion‐resistant plugs. In the simulations, geometric forms of individual meanders observed in different natural sedimentary environments have been reproduced by changing the credibility of the corresponding sedimentary materials, such as point bar deposits, flood plain deposits, and oxbow lake deposits. The simulations indicate that the typical meander wavelength is determined mainly by hydraulic factors such as the flow in the channel and the inclination of the underlying flood plain and is independent of the difference in the credibilities of sedimentary deposits. The computational approach permits exploration of long‐term changes in the floodplain geology, mediated by the meandering river. As an initial demonstration, the formation of meander belts is investigated using the model. The results suggest that a meander belt will be formed by a river's own cutoff loops only if the characteristic time of deposition and solidification of an oxbow lake is longer than the typical time that it takes the river to migrate downstream over the distance of a meander‐loop wavelength.
Principles of Early Development of Karst Conduits Under Natural and Man‐Made Conditions Revealed by Mathematical Analysis of Numerical Models
Authors: Dreybrodt Wolfgang
Published: February 2004

Numerical models of the enlargement of primary fissures in limestone by calcite aggressive water show a complex behavior. If the lengths of the fractures are large and hydraulic heads are low, as is the case in nature, dissolution rates at the exit of the channel determine its development by causing a slow increase of water flow, which after a long gestation time by positive feedback accelerates dramatically within a short time span. Mathematical analysis of simplified approximations yields an analytical expression for the breakthrough time, when this happens, in excellent agreement with the results of a numerical model. This expression quantifies the geometrical, hydraulic, and chemical parameters determining such karst processes. If the lengths of the enlarging channels are small, but hydraulic heads are high, as is the case for artificial hydraulic structures such as dams, it is the widening at the entrance of the flow path which determines the enlargement of the conduit. Within the lifetime of the dam this can cause serious water losses. This can also be explained by mathematical analysis of simplified approximations which yield an analytical threshold condition from which the safety of a dam can be judged. Thus in both cases the dynamic processes of karstification are revealed to gain a deeper understanding of the early development of karst systems. As a further important result, one finds that minimum conditions, below which karstification cannot develop, do not exist.
Analyzing the discharge regime of a large tropical river through remote sensing, ground‐based climatic data, and modeling
Authors: Vörösmarty Charles J. , Willmott Cort J. , Choudhury Bhaskar J. , Schloss Annette L. , Stearns Timothy K. , Robeson Scott M. , Dorman Timothy J.
Published: February 2004

This study demonstrates the potential for applying passive microwave satellite sensor data to infer the discharge dynamics of large river systems using the main stem Amazon as a test case. The methodology combines (1) interpolated ground‐based meteorological station data, (2) horizontally and vertically polarized temperature differences (HVPTD) from the 37‐GHz scanning multichannel microwave radiometer (SMMR) aboard the Nimbus 7 satellite, and (3) a calibrated water balance/water transport model (WBM/WTM). Monthly HVPTD values at 0.25° (latitude by longitude) resolution were resampled spatially and temporally to produce an enhanced HVPTD time series at 0.5° resolution for the period May 1979 through February 1985. Enhanced HVPTD values were regressed against monthly discharge derived from the WBM/WTM for each of 40 grid cells along the main stem over a calibration period from May 1979 to February 1983 to provide a spatially contiguous estimate of time‐varying discharge. HVPTD‐estimated flows generated for a validation period from March 1983 to February 1985 were found to be in good agreement with both observed arid modeled discharges over a 1400‐km section of the main stem Amazon. This span of river is bounded downstream by a region of tidal influence and upstream by low sensor response associated with dense forest canopy. Both the WBM/WTM and HVPTD‐derived flow rates reflect the significant impact of the 1982–1983 El Niño‐;Southern Oscillation (ENSO) event on water balances within the drainage basin.
Predictive Methods for Real‐Time Control of Flood Operation of a Multireservoir System: Methodology and Comparative Study
Authors: Niewiadomska‐Szynkiewicz Ewa , Malinowski Krzysztof , Karbowski Andrzej
Published: February 2004

Predictive methods for real‐time flood operation of water systems consisting of reservoirs located in parallel on tributaries to the main river are presented and discussed. The aspect of conflicting individual goals of the local decision units and other objectives important from an overall point of view is taken into account. The particular attention is focused on hierarchical control structure which provides framework for organization of an on‐line reservoir management problem. The important factor involved in flood control the uncertainty with respect to future inflows is taken into consideration. A case study of the upper Vistula river basin system in the southern part of Poland is presented. Simulation results based on 11 historical floods are briefly described and discussed.
One‐Dimensional Virus Transport in Porous Media With Time‐Dependent Inactivation Rate Coefficients
Authors: Sim Youn , Chrysikopoulos Constantinos V.
Published: February 2004

A model for virus transport in one‐dimensional, homogeneous, saturated porous media is developed, accounting for virus sorption and inactivation of liquid phase and adsorbed viruses with different time dependent rate coefficients. The virus inactivation process is represented by a pseudo first‐order rate expression. The pseudo first‐order approximation is shown to simulate available experimental data from three virus inactivation batch studies better than the frequently employed constant rate inactivation model. Model simulations indicated that the pseudo first‐order approximation, compared to the constant inactivation, leads to extended survival of viruses and, consequently, more distant migration. Results from a parameter sensitivity analysis demonstrated that estimation of pseudo first‐order inactivation rate coefficients from field observations requires data collection near the source of virus contamination during initial stages of virus transport.
A Geostatistical Inverse Method for Variably Saturated Flow in the Vadose Zone
Authors: Yeh T.‐C. Jim , Zhang Jinqi
Published: February 2004

A geostatistical inverse technique utilizing both primary and secondary information is developed to estimate conditional means of unsaturated hydraulic conductivity parameters (saturated hydraulic conductivity and pore size distribution parameters) in the vadose zone. Measurements of saturated hydraulic conductivity and pore size distribution parameters are considered as the primary information, while measurements of steady state flow processes (soil‐water pressure head and degree of saturation) are regarded as the secondary information. This inverse approach relies on the classical linear predictor (cokriging) theory and takes the advantage of the spatial cross correlation between the soil‐water pressure head and each of the following: degree of saturation, saturated hydraulic conductivity, and a pore size distribution parameter. Using an approximate perturbation solution for steady, variably saturated flow under general boundary conditions, the cross covariances between the primary and secondary information are derived. The approximate solution is formulated on the basis of a first‐order Taylor series expansion of a discretized finite element equation. The sensitivity matrix in the solution is evaluated by an adjoint state sensitivity approach for flow in heterogeneous media under variably saturated conditions. Through several numerical examples the inverse model demonstrates its ability to improve the estimates of the spatial distribution of saturated hydraulic conductivity and pore size distribution parameters using the secondary information.
Specification of Sediment Maintenance Flows for a Large Gravel‐Bed River
Authors: Wilcock Peter R. , Kondolf G. Mathias , Matthews W. V. Graham , Barta Alan F.
Published: February 2004

Reservoir releases may be specified to flush interstitial fine sediment from gravel beds in the river downstream. Choice of an effective flow depends on trade‐offs among discharge, flow duration, and pool dredging as they determine rates of bed mobilization, sand removal, and gravel loss. A basis for evaluating these trade‐offs is developed with an approximate method appropriate to the sparse data typically available. Sand and gravel transport are represented with rating curves. Approximate methods are introduced for estimating effective gravel entrainment, subsurface sand supply, and pool sediment trapping. These are combined in a sand routing algorithm to evaluate flushing alternatives for the Trinity River, California. A sediment maintenance flow of moderate size, just sufficient to entrain the bed surface over the duration of the release, limits gravel loss and maximizes sand trapping by pools. Larger discharges produce more fines removal but at the cost of greater gravel loss and reduced selective transport of fines. Dredged pools increase sand removal efficiency by providing multiple exits from the channel and minimize gravel loss if dredged sediment is screened and gravel returned to the river.
Contributions of Water‐Rock Interactions to the Composition of Groundwater in Areas with a Sizeable Anthropogenic Input: A Case Study of the Waters of the Fundão Area, Central Portugal
Authors: Pacheco Fernando , van der Weijden Cornelis H.
Published: February 2004

We used a combination of a grouping algorithm and a weathering algorithm to assess the contributions made by chemical weathering and anthropogenic inputs to the groundwater composition in a granitoid area. The first algorithm is based on the mathematical concept of equivalent relations between objects and is used to find groups of water samples. Using the grouping algorithm, we identified groups with similar chemistries in a set of data relating to the water composition in 160 springs and wells in the Fundão area (Portugal). The second algorithm is based on stoichiometries, mass, and charge balances in weathering reactions and is used to relate the water composition of each identified group to water‐mineral interactions in the area. Background information on the petrology and mineralogy of the area allowed us to choose the most realistic water‐mineral interactions. We also had information about the use and composition of fertilizers in this agricultural area. In applying the weathering algorithm we considered all dissolved silica and bicarbonate to be produced by chemical weathering and all dissolved chloride, sulphate, and nitrate to be derived from other sources, mainly from fertilizers. The anthropogenic contributions to the water chemistries in the area are high. Despite the high background concentrations derived from pollution, we were able to assess the contribution of weathering to the water chemistries. We obtained realistic results with the two algorithms, supported by the fact that the spatial distribution of the samples belonging to the various groups shows a good correlation with the geology and petrology of the area.
Fractional Coverage of Rainfall Over a Grid: Analyses of NEXRAD Data Over the Southern Plains
Authors: Fan Ying , Wood Eric E. , Baeck Mary Lynn , Smith James A.
Published: February 2004

An important parameter in land surface hydrology is the fractional area of a model grid receiving rainfall when rain is observed. Using NEXRAD (Next Generation Weather Radar) hourly rainfall estimates for the southern Plains, we examined the temporal variability in this quantity and tested the important assumptions of the threshold method. The following conclusions are reached. First, seasonal/diurnal variations explain less than 18% of the variance at all spatial scales examined; an efficient method of computing fractional coverage must consider the large event‐scale variability. Second, stationarity in conditional distributions of spatial rainfall, a key assumption in the threshold method, is not warranted at the event scale for small grids or at the seasonal/diurnal scale for large grids. Third, biases are introduced by the high correlation between the parameter and the independent variable of the threshold method. Fourth, the spatial conditional mean rain rate is higher than its temporal counterpart for all cases examined; the ergodicity assumption overpredicts the fractional coverage. Two simple methods were proposed to solve the above problems. Results of an intercomparison showed that they can modestly improve the threshold method.
Analysis of nitrate in near‐surface aquifers in the midcontinental United States: An application of the Inverse Hyperbolic Sine Tobit Model
Authors: Yen Steven T. , Liu Shiping , Kolpin Dana W.
Published: February 2004

A nonnormal and heteroscedastic Tobit model is used to determine the primary factors that affect nitrate concentrations in near‐surface aquifers, using data from the U.S. Geological Survey collected in 1991. Both normality and homoscedasticity of errors are rejected, justifying the use of a nonnormal and heteroscedastic model. The following factors are found to have significant impacts on nitrate concentrations in groundwater: well screen interval, depth to top of aquifers, percentages of urban residential, forest land, and pasture within 3.2 km, dissolved oxygen concentration level, and presence of a chemical facility and feedlot. The effects of explanatory variables on nitrate concentration are explored further by calculating elasticities. Dissolved oxygen concentration level has more notable effects on nitrate concentrations in groundwater than other variables.
Solute Transport in Soils Under Conditions of Variable Flow Velocities
Authors: Ma Liwang , Selim H. M.
Published: February 2004

Temporal and spatial variabilities of flow distribution significantly influence solute transport in soils. This laboratory study was designed to investigate the effects of temporal variation in flow velocity on pesticide transport in soils. Two pesticides, metribuzin (weakly adsorbed) and atrazine (moderately adsorbed), were chosen along with the following two soils: Cecil (<2 mm) and Sharkey (2–4 mm). Several tritium pulses were introduced into packed soil columns (15 or 30 cm in length) under different flow velocities to obtain velocity‐dependent dispersion coefficients (D). Subsequently, several atrazine and metribuzin pulses were introduced under conditions of constant and variable velocities. For each experiment, changes in flow velocity were stepwise using a piston flow pump and were carried out during pulse application and leaching. For constant and variable flow velocity experiments, approximately similar pulse volumes and average flow velocities were maintained. Values of D versus pore water velocity (ν) from tritium breakthrough curves (BTCs) were well described using a linear equation for both soils. Identical BTCs for metribuzin were observed under conditions of constant or variable flow velocities in the Cecil soil column. However, metribuzin transport in the Sharkey soil was significantly influenced by velocity variations. Atrazine transport in the Sharkey soil was also significantly affected by variations in flow velocity. We further examined the error when an average rather than actual velocity distribution was used in BTC representation. The resulting experimental BTCs (concentration versus velocity‐averaged pore volume) exhibited early arrival and the appearance of multiple peaks. Moreover, predictions of such BTCs based on the convective‐dispersive equation were not successful. We concluded that actual water velocity distributions should be used in BTC representation, and, whenever possible, the use of an average velocity should be avoided.
Instabilities in variable density flows: Stability and sensitivity analyses for homogeneous and heterogeneous media
Authors: Schincariol Robert A. , Schwartz Franklin W. , Mendoza Carl A.
Published: February 2004

This study improves our understanding of instability phenomena that may accompany the transport of dense plumes of dissolved contaminants. One major objective is to test how well analytic stability theory developed by List [1965] applies to the transport of dense plumes in both homogeneous and heterogeneous media. The data to test the prediction come from numerical model experiments in which instability growth is generated by perturbing the interface between fluids of differing density. Stability criteria, as determined by the transverse Rayleigh number, the ratio of transverse to longitudinal Rayleigh numbers, and the nondimensional wave number, compare very well with results observed in the numerical experiments for isotropic media. Comparisons involving correlated random fields were much less successful because plume stability is determined on a local basis as a function of the changing permeability field. Instabilities tend to dissipate in zones of lower permeability and grow in zones of higher permeability. Another objective of the study is to determine the factors that contribute to stability and instability in homogeneous and heterogeneous systems. Sensitivity analyses using a transport model within the framework of List's stability theory show that stability is promoted by low medium permeability, small density differences, and significant dispersion. In heterogeneous media, stability is promoted by increased correlation length scales and increased log permeability variance. Furthermore, the simulations illustrate the intimate relationship that exists between instability growth and decay and the heterogeneous nature of the permeability field. Thus stability criteria that do not incorporate characteristics of the permeability field will not be suitable for natural or field‐scale porous media.
Transfer of nonsorbing solutes to a streambed with bed forms: Laboratory experiments
Authors: Elliott Alexander H. , Brooks Norman H.
Published: February 2004

Experiments were performed to determine the mechanisms and rates of solute exchange between a flowing stream and porous streambed. Mass balance, flow visualisation, and depth profiles of concentration were used to investigate the exchange of an inert tracer in a laboratory flume. The measured mass transfer was compared to the exchange predicted by models of exchange related to bed forms, which are presented in a companion paper. For the initial stages with stationary bed forms or slowly moving bed forms the net exchange was predicted satisfactorily and was dominated by bed form‐induced interstitial flows (pumping). In the initial stages with rapidly moving bed forms the exchange was dominated by scour/deposition as bed forms propagated (turnover). At later times the models of bed form‐related exchange significantly underpredicted the measured exchange. The additional exchange, and the exchange to a flat bed, may be related to bed inhomogeneity or irregular variations in pressure at the bed surface which are not related to bed forms. Such effects are likely to be more pronounced in the field situation.
On Constitutive Equations and Effective Stress Principles for Deformable, Double‐Porosity Media
Authors: Wang Herbert F. , Berryman James G.
Published: February 2004

Tuncay and Corapcioglu [1995] used volume averaging methods to derive an effective stress principle for the bulk volumetric strain in a deformable double‐porosity medium. The coefficients of the matrix pore pressure and fracture pore pressure in their equation for the effective stress are shown to be identical to those which can be obtained from the constitutive equation approach of Berryman and Wang [1995]. Representative values for a fractured Berea sandstone show that a change in pore pressure within matrix blocks produces about 10% of the volume change resulting from an equal but opposite change in confining pressure, whereas the same change in pore pressure within fractures is about 90% as effective. A similar result is true for Westerly granite.
A tracer study of the Floridan Aquifer in southeastern Georgia: Implications for groundwater flow and paleoclimate
Authors: Clark Jordan F. , Stute Martin , Schlosser Peter , Drenkard Stefan , Bonani Georges
Published: February 2004

Distributions of stable isotopes of water, radiocarbon, noble gases, and chloride (Cl) in groundwater from the Upper Floridan aquifer in southeastern Georgia suggest that down gradient of the Gulf Trough this aquifer contains waters representative of both regional and local groundwater flow systems. In this area, locally recharged waters tend to remain near the top of the aquifer and do not mix substantially with the regional groundwater flow system. Noble gas temperatures suggest that this region of Georgia was 4.0 ± 0.6°C cooler during the last glacial period (LGP). Similar temperature changes have been reported for southern Texas and northern New Mexico, suggesting that the southern United States cooled uniformly during the LGP. Stable isotopes of water became enriched down gradient from the recharge area. These enrichments which have been observed before appear to result from local influx of shallow groundwater into the regional aquifer system rather than representing a climate change signal. An inland gradient of the stable isotope composition (0.60 ± 0.14‰/100 km in δ18O) was found in young (Holocene) water. After correcting for the change in the stable isotope composition of the ocean during the LGP, water that was recharged during the LGP was found to be slightly depleted in stable isotopes relative to modern recharge (Δδ18O = 0.6 ± 0.4‰). Assuming the modern inland gradient, the change in δ18O is consistent with a shift in the coastline, which was caused by the lower sea level during the LGP.
The Lagrangian Stochastic Model for fetch and latent heat flux estimation above uniform and nonuniform terrain
Authors: Hsieh Cheng‐I , Katul Gabriel G. , Schieldge John , Sigmon John T. , Knoerr Kenneth K.
Published: February 2004

A Lagrangian stochastic model was used to estimate the fetch and latent heat flux above a nonuniform grass‐covered forest clearing site at the Duke Forest, in Durham, North Carolina, and an irrigated bare soil patch at the University of California in Davis. The latent heat flux predictions by the Lagrangian model compared well with eddy correlation flux measurements. In order to apply the Lagrangian model to a nonuniform grass‐covered forest clearing, the surface was treated as an imaginary “equivalent” uniform terrain subjected to surface roughness and turbulence statistics (i.e., mean, variance, and covariance) of velocities and scalars identical to those measured above the nonuniform terrain. At the irrigated bare soil site the equilibrium distance of the air from dry to wet was well defined, and its influence on the water vapor flux internal boundary layer was considered. In the Lagrangian model, five different schemes to account for inhomogeneous turbulent flows were compared in terms of estimating scalar fluxes. Our comparisons demonstrate that the five different schemes produce similar scalar fluxes despite the fact that some of them do not satisfy the well‐mixed criterion. Also, the analytical solution to the advection‐diffusion equation was used to predict the fetch and latent heat flux under neutral conditions and compared to the Lagrangian model.
A statistical model for extreme precipitation
Authors: Elíasson Jónas
Published: February 2004

The statistical distribution of 1‐day (one reading in 24 hours) and 24‐hour (several readings in 24 hours) annual maxima is considered and a transformed extreme value type 1 distribution function (TDF) that includes a probable maximum (PM) value is suggested. The distribution function fits standardized annual maximum station values from Iceland and Washington State very well. A generalized distribution function, derived from the TDF, is suggested. To use it, two local parameters have to be known; the 5‐year event, M5, that must be picked from a map and a slope factor, Ci, that is a function of the coefficient of variation. The variation of Ci between independent observation stations is assumed to be random, and guidelines on how a Ci may be selected are discussed. The generalized distribution function is used to calculate quantile estimates and a local probable maximum precipitation (PMP). Regional PMPs can be calculated by maximizing this value. Two independent sets of Ci's compare favorably: (1) a statistical set compiled from the British Natural Environment Resource Council (NERC) PMP envelope and )2) a meteorological set calculated from U.S. National Weather Service estimates of PMP for Washington State. The regional PMP estimates calculated from the generalized distribution also compare favorably with the NERC PMPs, except that the estimates for low M5 produce up to 33% lower PMPs. This difference may be explained by a number of factors that are also discussed.
A model for numerical simulating of evaporation from bare saline soil
Authors: Yakirevich Alexander , Berliner Pedro , Sorek Shaul
Published: February 2004

A mathematical model for the computation of evaporation from bare saline soils is presented. Moisture flow and heat transport equations in the partially saturated zone of the soil are coupled at the soil surface with a lower atmosphere boundary layer model. The transport model considers multicomponent salt migration. The flow equation accounts for osmotic flux due to the presence of ion species in the soil solution. Nonlinear one‐dimensional equations were solved using fully implicit finite difference schemes. The soil part of the model was tested using experimental data of water and solute redistribution under temperature gradients in sealed soil columns. In order to demonstrate the effect of soil salinity on the evaporation, the problem of water infiltration into soil with initially low water content and its subsequent evaporation was considered. Results show a significant effect of osmotic pressure gradients on predicted evaporation.
An analytical solution technique for one‐dimensional, steady vertical water flow in layered soils
Authors: Rockhold Mark L. , Simmons Carver S. , Fayer Michael J.
Published: February 2004

An analytical solution technique was developed for one‐dimensional, steady vertical water flow in variably saturated, layered soils with arbitrary hydraulic properties. The solution technique is based on the exact integral solution for the Gardner exponential hydraulic conductivity function. The exact solution is extended for use with arbitrary hydraulic property functions, or measured K(h) data, by approximating ln K(h) with piecewise‐linear curve segments and integrating analytically, segment by segment. The resulting analytical solution technique is accurate, computationally efficient, and applicable to unsaturated and/or saturated conditions. Several application examples are presented, including a comparison with earlier experimental results.
Experimental investigation of the stability boundary for double‐diffusive finger convection in a Hele‐Shaw cell
Authors: Cooper Clay A. , Glass Robert J. , Tyler Scott W.
Published: February 2004

Double‐diffusive convection may be an important transport phenomenon in subsurface porous media and fractures. The classic linear stability analysis derived for a porous medium with two components stratified such that each affects the vertical density gradient in an opposing manner predicts double‐diffusive finger instability to occur when Rs1 + Rs2 ≥ Rsc, where Rs1 and Rs2 are the Rayleigh numbers of the faster and slower diffusing components, respectively, and Rsc is a critical value dependent upon the boundary conditions (0 ≤ Rsc ≤ 4π2). For cases where Rsc/|Rs1| ≪ 1, the above result can be simplified to −Rρ < 1/т, where Rρ is the buoyancy ratio of the fluid and т is the ratio of diffusivities (0 < т < 1). We experimentally tested the applicability of both stability criteria for situations where a narrow transition zone exists bounded above and below by constant concentrations and within a domain of uniform permeability. Experiments were conducted in a Hele‐Shaw cell using a digital imaging technique which provided pixel‐scale (∼0.2 mm) resolution of the evolving concentration field during convection. Within experimental error, our experiments support both criteria within their predicted ranges of applicability.