bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of oxygen transport on the areal hypolimnetic oxygen deficit
Authors: Stauffer Robert E.
Published: February 2008

Using a modified flux gradient technique, transmetalimnetic oxygen transport was evaluated in Lakes Mendota and Delavan, two eutrophic Wisconsin basins featuring moderate depth (17 < zmax < 25 m), good wind exposure, and classic clinograde oxygen profiles. The biweekly fluxes intensified between mid‐May and late July 1972, mainly on account of seasonal steepening of the metalimnetic O2 gradients. Fluxes also increased sharply during windy intervals characterized by accelerated transmetalimnetic heat fluxes. The fluxes across the lower epilimnetic boundary exceed those across the lower metalimnetic boundary; this differential is consistent with the metalimnion's role as an oxidation zone in these turbid lakes. The vertical fluxes are a large but variable (over time) fraction of the net areal oxygen deficit in both lakes, thus complicating statistical analysis of seasonal and intergenerational trends in lake productivity using the areal hypolimnetic oxygen deficit method. Even after correcting for the effects of O2 transport, the deficit in Lake Mendota exceeds that in Delavan following the onset of hypolimnetic anoxia in the shallower lake. This difference likely results, in part, from transmetalimnetic fluxes of reducing substances (CH4, NH3, etc.) into the epilimnion.
Modeling of evapotranspiration from homogenous soils
Authors: Markar M. S. , Mein Russell G.
Published: February 2008

A sink term to simulate the removal of water by transpiration from the soil profile has been added to the moisture flow model of Clapp (1982). The resultant model gives results comparable to those obtained from a finite difference solution of the Richards equation modified for transpiration, but uses only about 5% of the computer time. Example applications show that the model is capable of predicting evapotranspiration rates from homogenous soils without using fitted parameters.
Long‐term pollutant degradation in the unsaturated zone with stochastic rainfall infiltration
Authors: Small Mitchell J. , Mular John R.
Published: February 2008

Given the discharge of a reactive material to the land surface, the fraction of the contaminant which reaches the underlying aquifer is uncertain due to variations in the rate of precipitation and infiltration. A stochastic conceptual framework is developed to estimate this variation. The framework relates the pollutant leachate fraction to the long‐term rate of infiltration and derives the probability of a given leacheate fraction from the probability of obtaining the associated infiltration rate. Application of the methodology is illustrated for the case where cumulative infiltration is represented by a compound Poisson process and pollutant fate is determined for one‐dimensional advective‐dispersive transport with linear equilibrium adsorption and first‐order chemical decay. The derived probability model for this case compares favorably to results from Monte Carlo simulations of a numerical transport model. Applications and limitations of the methodology with more sophisticated transport, reaction, and stochastic infiltration models are considered.
Analysis of event‐based precipitation data with a view toward modeling
Authors: Guttorp Peter
Published: February 2008

Using data from two stations in the MAP3S acid precipitation monitoring network, some aspects of the marked point process of rainfall events are investigated and are used to illustrate some potentially useful data analytic techniques. We find no statistically significant evidence against a Poisson process model of storm arrivals, observed with a dead time corresponding to event duration. In particular, we look at the relation between amounts and the history of the process of events and the relation between events at two different stations. Events that occur close to previous events tend to have smaller than average precipitation amounts. We discuss some implications for parametric models of precipitation processes.
Modeling solute segregation during freezing of peatland waters
Authors: Kadlec Robert H. , Li Xiang‐Ming , Cotten Gerald B.
Published: February 2008

Freezing of the shallow water in a peatland causes the downward movement of solutes. Field and laboratory data demonstrate that a considerable portion of the solutes are driven into the top soil from the overlying water by freezing. Such solute redistribution phenomena in peatlands are of interest for establishing the geochronology of deposits and determining the nature of pollutant burial. A mathematical model has been developed to describe the solute segregation processes at the freezing front in both overlying water and interstitial water in the porous peat and solute transport in the unfrozen water. An algorithm has been developed to solve this nonlinear moving interface problem. The solute concentrations in ice and water phases are reproduced by the model. Computer simulation results provide good predictions of independent experimental data.
Turbidity current simulation in a diverging channel
Authors: Akiyama Juichiro , Stefan Heinz G.
Published: February 2008

Spatially varied turbidity currents with erosion and deposition of sediment occurring in slightly diverging‐sloping channels with a rectangular cross section are simulated. The model consists of cross section integrated equations for conservation of volume and mass, x momentum equation, and empirical relationships for water entrainment and sediment entrainment. The continuity equation for the sediment bed is coupled with the equations for the flow in order to simulate the change of bed elevation due to erosion and deposition of turbidity currents. The gradually varied flow equations are solved. The simulations show (1) underflows in diverging channels do not reach a normal condition, (2) three different types of turbidity currents can be simulated, (3) turbidity current behavior is sensitive to particle size, water viscosity, channel slope, as well as channel diverging angle when channel aspect ratio is small.
Hydrologic control of aluminum chemistry in an acidic headwater stream
Authors: Lawrence G. B. , Driscoll C. T. , Fuller R. D.
Published: February 2008

The influence of hydrologic processes on Al chemistry was investigated along an elevational gradient in a headwater stream at the Hubbard Brook Experimental Forest. Relationships between streamflow and concentrations of inorganic and organic Al, H+, and dissolved organic carbon varied within the watershed. At high elevations, increased streamflow was associated with reduced surface water acidity and decreased inorganic Al concentrations. At low elevations, however, increased streamflow was associated with increases in stream acidity and concentrations of inorganic Al. Changes in soil flow paths and variations in source areas both appear to regulate stream chemistry. Flow through upper soil horizons under high‐flow conditions appears to be the controlling hydrologic influence on stream chemistry at the high‐elevation site. At the low‐elevation site, contributions of flow from the more acidic, upper region of the watershed during high‐flow conditions appears to be the major hydrologic influence on stream chemistry. During low‐flow conditions the upper reaches of the stream become dry, so that there is no contribution to flow from this part of the watershed. Stream solutions appeared to be closest to equilibrium with respect to natural gibbsite under high‐ and low‐flow conditions, with no indication that changes in flow introduced disequilibrium conditions. Efforts to incorporate hydrologic effects into surface water acidification models must address the relationship between runoff generation mechanisms and spatial variations in watershed biogeochemistry.
Stream chemistry in the New Jersey pinelands: The influence of precipitation and watershed disturbance
Authors: Morgan Mark D. , Good Ralph E.
Published: February 2008

Bulk precipitation chemistry in the New Jersey Pinelands from July 1984 to July 1986 was influenced by both marine and continental air masses. The marine air masses were less acidic and dominated by Cl−, Na+, and Mg+2 while the continental air masses were high in ions typical of acid rain in the northeastern United States (SO4−2, H+, and NO3−). Over the same period, Pinelands streams draining undisturbed watersheds were acidic and very low in dissolved substances. The dominant ions were Na+ and Cl−, followed by SO4−2, Mg+2, H+, and Ca+2. Streams draining watersheds disturbed by residential and agricultural development exhibited elevated concentration of Ca+2, Mg+2, K+, SO4−2, and NO3−, and lower H+. The altered levels of each of these ions was directly related to inputs from human activities and illustrated the sensitivity of Pinelands ecosystems to disturbance. Using Cl− as a tracer, enrichment factors were calculated for the major solutes. In undisturbed streams the enrichment factors showed that Na+ and K+ were passed through the system and that H+, Ca+2, NH4+, NO3−, and SO4−2 were retained. Of the measured materials only Mg+2 was exported from undisturbed streams, although previous work has shown that iron, aluminum, and organic matter are also exported. The retention of SO4−2 was probably related to sulfate reduction in the extensive wetland systems of the Pinelands. These data strongly support earlier work that suggested sediment weathering in the Pinelands contributes few of the measured ionic constituents to stream discharge. Undisturbed Pinelands stream chemistry appears to be highly dependent on precipitation input and is modified primarily by biological activity within this ecosystem.
A continuum approach for modeling mass transport in fractured media
Authors: Schwartz Franklin W. , Smith Leslie
Published: February 2008

A continuum approach has been developed for modeling mass transport in fractured rocks. It involves a new application of a particle‐tracking method in which physical transport is simulated in terms of velocity and the variations in velocity. Statistics describing the particle motion come from observing the actual pattern of particle motion in a discrete subdomain. This subdomain is a small but representative piece of the much larger continuum. In effect, particle motion in the continuum is forced to mimic that in the discrete subdomain. The model successfully duplicates patterns of anisotropic dispersion predicted by de Josselin de Jong (1972) and breakthrough curves simulated with a discrete fracture model. Applications demonstrate that the de Josselin de Jong approach for estimating dispersivities for idealized networks cannot generally be applied to networks formed from sets of finite, irregularly spaced fractures. In addition, this study reinforces indications from earlier studies about the complexity of dispersion in fractured media. Factors that contribute to spreading in the major and minor principal directions change with the orientation of the network relative to the mean hydraulic gradient.
Unsaturated seepage and subterranean holes: Conspectus, and exclusion problem for circular cylindrical cavities
Authors: Philip J. R. , Knight J. H. , Waechter R. T.
Published: February 2008

This paper introduces the general theory of water exclusion from, or entry into, subterranean holes from steady uniform downward unsaturated seepage. Buried holes serve as obstacles to the flow and so increase water pressure at parts of the hole surface. When downward seepage is fast enough and/or the hole is large enough, water pressure increases to the point where a seepage surface forms and water enters the hole. Contrary to the conventional picture drawn from capillary statics, hydrodynamics shows that the larger the hole the more vulnerable it is to water entry. Cavity shape is important also. Applications include optimal design of configurations of tunnels and underground repositories (e.g., for nuclear wastes) against entry of seepage water. The theory also embraces the disturbance of seepage flows by buried impermeable obstacles such as stones and structures. The quasi‐linear exclusion problem for circular cylindrical cavities is solved. Both exact solutions and simple asymptotic results are found, and graphs and tables presented. The moisture field about the cavity exhibits upstream and downstream stagnation points and retarded regions: “roof‐drip lobes” in which the moisture content and downward flow velocity are augmented by water essentially deflected from the cavity roof, and the “dry shadow” region of reduced moisture content and flow velocity, essentially sheltered by the cavity. A practical consequence is that we can establish, for any given combination of downward seepage velocity, cavity radius, saturated hydraulic conductivity K1, and sorptive number α, whether or not seepage water will enter a cylindrical cavity.
Regional scale transport in a Karst Aquifer: 1. Component separation of spring flow hydrographs
Authors: Dreiss Shirley J.
Published: February 2008

Large fluctuations in Ca and Mg concentrations occur in the discharge of karst springs in southeastern Missouri after major storm events. Rapid flow of relatively dilute, storm‐derived water through solution conduits in the aquifer causes the fluctuations in chemistry. A cation balance for the discharge of one spring indicates that about 25% of the total spring flow is storm‐derived water. These storm‐derived contributions reach a maximum during the recession of storm responses in the spring flow hydrograph. At the peak of the responses, spring flow is composed primarily of prestorm water that has been displaced in the conduit system by the storm‐derived water. The observed chemical fluctuations and hydrograph components differ from those observed in streamflow where the time of the maximum dilution usually coincides with the time of peak discharge.
Estimation of spatial covariance structures by adjoint state maximum likelihood cross validation: 3. Application to hydrochemical and isotopic data
Authors: Samper F. Javier , Neuman Shlomo P.
Published: February 2008

Paper 3 of this three‐part series presents applications of our adjoint state maximum likelihood cross‐validation (ASMLCV) method to real data from aquifers. The Madrid basin in Spain serves as the source of information about 11 hydrochemical variables (pH, electrical conductivity, silica content, and the concentration of major ions) and two isotopes (oxygen 18 and carbon 14). Due to a lack of sufficient vertical resolution, our analysis is restricted to the horizontal plane. With the exception of oxygen 18 and silica, the variables appear to be free of a horizontal drift. No discernible directional effects are seen. All variables exhibit a large nugget effect which is indicative of background noise. We conclude that more detailed and careful sampling in three dimensions is required if groundwater quality information is to become less prone to such noise and thereby more useful in the context of quantitative hydrogeological analyses. Despite the existing noise, we are able to confirm geostatistically some (though not all) of the hypotheses advanced by others about hydrochemical evolution and isotope changes in the basin. The ability of ASMLCV to filter out spatial variations from part of the measurement noise is illustrated on carbon 14 data. The same data are also used to investigate the utility of model structure identification criteria in selecting the best among a set of alternative semivariogram models.
Description of periodic variation in parameters of hydrologic time series
Authors: Yevjevich Vujica , Harmancioglu Nilgun Bayraktar
Published: February 2008

The multivariate approach of marginal distributions and their sequential dependence models is often used for description of periodic‐stochastic hydrologic time series. This is a nonfunctional (or nonparametric) method of description of periodic variation in basic parameters. Usually, it requires a large number of estimated parameters. This often represents a lack of respect for the sound statistical principle of parsimony in the total number of model parameters. Two distortions result from this approach when new samples are generated by these models. First, some estimates vary less in generated samples than they should. Second, a tendency exists to generate new extremes at the times of historic extremes, with some of them more pronounced than the historic extremes. The fitted Fourier functions with a limited number of significant harmonics to the periodic variation of basic parameters, as the functional (or parametric) description of periodicity in time series, require a much smaller total number of parameters. They either avoid or minimize the distortions in generated samples.
A semiautomatic mesh generation algorithm for three‐dimensional groundwater tetrahedral finite element models
Authors: Tucciarelli Tullio
Published: February 2008

An algorithm for the semiautomatic generation of three‐dimensional groundwater tetrahedral finite element models is proposed. The first step is the representation of the plan of the aquifer by means of a triangular mesh. In the second step the code subdivides each triangular prism into many others according to the layers actually intersected by the prism. All the necessary information for the above said decomposition is obtained from the verticals subtended by the nodes of the plan mesh. The final tetrahedral decomposition generated by the code guarantees the conformity of the elements.
An inclusive infiltration equation for downward water entry into soil
Authors: Swartzendruber D. , Clague F. R.
Published: February 2008

A new infiltration equation, derived exactly from a recent quasi‐solution of the governing differential flow equation, is capable of expressing eleven previously published infiltration equations. On the basis of least squares fitting, the new equation in three‐parameter dimensionless form expressed each of the eleven previous equations within ±0.31% over all times. If only one parameter was retained and fitted in the dimensionless form, nine of the eleven equations were still expressed within ±2.5%. This one‐parameter dimensionless form implies the dimensional infiltration equation I = (S/A0)]1 − exp (−A0t½)] + Kt, which is therefore proposed as a realistic blend of rigor and utility for the complete range of time t(0≦t ≦∞), where I is the cumulative quality of water infiltrated, S is the sorptivity, A0 is a constant arising from the quasi‐solution, and K is the sated (satiated, near‐saturated) hydraulic conductivity. Because of its broad matching capability, this equation is deemed to hold considerable promise for describing and fitting experimental infiltration data, whether from the laboratory or the field.
Redox‐controlled multiple‐species reactive chemical transport: 1. Model development
Authors: Liu Chen Wuing , Narasimhan T. N.
Published: February 2008

A redox‐controlled multiple‐species, multidimensional reactive chemical transport model, DYNAMIX, is developed. The model includes advection, diffusion‐dispersion, transport of oxygen, oxidation‐reduction and, as a consequence, acid‐base reactions, aqueous complexation, precipitation‐dissolution, and kinetic mineral dissolution. A partial equilibrium condition is incorporated into the model which accounts for both thermodynamic equilibrium and kinetic chemical interaction between aqueous and solid phases. The model solves for aqueous and mineral phase compositions as function of time and space. A correct mineral assemblage is automatically searched for based on the principle of minimized Gibbs free energy. The mass of chemicals is conserved in both aqueous and solid phases. A two‐step dynamic mixing algorithm is used to solve the coupled transport and reaction equations. At each time step, the transport equation is first solved by the explicit difference method. The chemical equilibrium submodel is then called to calculate the distribution of chemical species under thermodynamic partial equilibrium conditions. By splitting the transport and reaction equations, the two‐step method significantly reduces computational time and memory storage requirements, which allows DYNAMIX to efficiently handle two‐ or three‐dimensional problems.
On the main channel length‐magnitude formula for random networks: A solution to Moon's conjecture
Authors: Waymire Ed
Published: February 2008

This is a technical note which solves Moon's conjecture for the exact asymptotic formula for the expected length of the longest path in Shreve's random network for large magnitude (source number).
Evaluation of electromagnetic tomography to map in situ water in heated welded tuff
Authors: Daily William , Ramirez Abelardo
Published: February 2008

An experiment was conducted in the G tunnel complex at the Nevada Test Site to evaluate geotomography as a possible candidate for in situ monitoring of moisture content near a heater in densely welded tuff. Alterant tomographs of electromagnetic permittivity at 200 MHz were made of the densely welded tuff in two planes near a heater which was used to simulate the thermal load of a nuclear waste container. When the 1‐kW heater was turned on, the tomographs indicated a rapid and strong drying adjacent to the heater. Moisture loss was not symmetric about the heater but seemed to be strongly influenced by heterogeneity in the rock mass. The linear character of many tomographic features and their spatial correlation with fractures mapped in boreholes are evidence that drying was most rapid along certain fractures. When the heater was turned off an increase in moisture content occurred around the heater and along the dry fractures. However, this process is much slower and the magnitude of the moisture increase much smaller than the changes observed during heating of the rock. The interpretation of the tomographs is preliminary until they can be processed without the restrictive assumption of straight ray paths for the signals through the highly heterogeneous rock mass. However, from these preliminary results we conclude that geotomography can provide useful moisture content information near a heater in densely welded tuff. This conclusion is based on the fact that the experiment results and their interpretation are self consistent and in good agreement with behavior expected of the heated rock mass.
Natural and anthropogenic components in bulk precipitation at Blidö (Archipelago of Stockholm)
Authors: Boström Kurt , Boström Birgitta , Andersson Per
Published: February 2008

Analyses of 37 precipitates from Blidö in the archipelago of Stockholm collected 1984–1987 reveal distinct seasonal variations. Particularly, P and K but also Mn, Ba, Ca, and Mg are enriched in rain during the early growth season, whereas Na is enriched in rains at the end of the year due to storm activity at sea. Semiconservative mixing models involving seawater (SW), plant matter (PLM), terrigenous matter (TM), and coal emissions (CEM) suggest that PLM supplies 99–20% of P, K, Mn, Ba, Ca, Sr, and Mg, SW explains 99–35% of Na and Mg, and TM the major fraction of Al, Fe, Ti, and Si. However, 90–65% of S and Zn are in excesses compared to the models: probably these elements arrive in volatile anthropogenic phases. These mixing models explain the complex sources of rain components better than conventional enrichment factors. The results appear representative for Northern Europe, but other areas of the world probably show different input ratios of PLM, SW, etc.
Extreme precipitation events generated by periodic processes
Authors: Ballerini Rocco , Waylen Peter
Published: February 2008

A procedure for the estimation of large quantile daily precipitation totals is developed. The technique draws on the statistical properties of extremes generated by processes with periodic variances. Forty‐one years of records at Homestead, Florida, are used to calibrate the model and are incorporated into the simulation procedure that compares the proposed technique to standard methods of analyzing annual series. Results indicate that the procedure yields reasonable estimates of large quantile precipitation and performs as reliably, or more so, than the standard techniques. The physical and theoretical bases of the procedure are maintained throughout the study.
A numerical investigation of the conjugate gradient method as applied to three‐dimensional groundwater flow problems in randomly heterogeneous porous media
Authors: Meyer Philip D. , Valocchi Albert J. , Ashby Steven F. , Saylor Paul E.
Published: February 2008

Field‐scale modeling of three‐dimensional groundwater flow in randomly heterogeneous porous media is considered. The system of linear equations resulting from the finite difference discretization of this problem may easily involve more than a million unknowns. Problems of such magnitude can only be solved practically on supercomputers and require an efficient iterative solution method that is well suited to the particular computer architecture being used. The preconditioned conjugate gradient method is highly efficient for this groundwater flow problem, and by using an appropriate preconditioning matrix the method can be adapted to different computers. The numerical software package CgCode enhances this adaptability; the user of CgCode must only provide subroutines for preconditioning and matrix‐vector multiplication. These subroutines should be tailored to the specific problem and machine architecture. Numerical results demonstrating the efficiency of polynomial preconditioning for the conjugate gradient method on both a vector machine (Cray X‐MP/48) and a vector‐parallel machine (Alliant FX/8) are presented.
Lake acidification studies: The role of input uncertainty in long‐term predictions
Authors: Georgakakos Konstantine P. , Valle‐Filho Gilberto M. , Nikolaidis Nikolaos P. , Schnoor Jerald L.
Published: February 2008

An assessment of the importance of input uncertainty in long‐term predictions of lake acidification is presented. Mathematical models that simulate the behavior of hydrologic catchments under acid deposition require input data of precipitation, temperature, and deposition chemistry. In long‐term studies of lake acidification it is necessary to hypothesize a scenario of future input to the mathematical models. The present study indicates that uncertainty in precipitation amount and acid concentrations can be very important in the characterization of future lake alkalinity levels. It also suggests that the natural day‐to‐day variability present in precipitation and other weather variables significantly affects long‐term predictions of lake alkalinity in watersheds with short response time, even more than wet and dry weather cycles do.
Field scale transport of bromide in an unsaturated soil: 2. Dispersion modeling
Authors: Butters Greg L. , Jury William A.
Published: February 2008

The solute concentrations measured in the field experiment of G. L. Butters et al. (this issue) are used to compare two models of vadose zone solute transport: the deterministic one‐dimensional convection‐dispersion model, which represents solute transport far from the source of solute entry, and the stochastic‐convective lognormal transfer function model, which represents solute transport near the source. The stochastic‐convective model provided an excellent representation of the spreading of the solute pulse to a depth of 3 m after calibration at 0.3 m. Conversely, the deterministic model dramatically underpredicted solute spreading beyond 0.3 m after calibration. An analysis of the area‐averaged solute concentration revealed a nearly linear scale effect in the dispersivity to a depth of at least 14.8 m. A change in the growth pattern of dispersion observed in the breakthrough curve at 4.5 m was attributed to a soil texture change near 3 m, which caused the apparent dispersivity of the pulse to decrease between 3.0 and 4.5 m, after which it increased significantly between 4.5 m and the final profile sampling between 0 and 25 m.
The economics of dam failure: Another look at catastrophic losses
Authors: Cochrane Hal
Published: February 2008

This paper revises the framework for incorporating the risk of catastrophic failure in the evaluation of new and existing high‐hazard dams. The paper demonstrates that by including both losses to structures and lost flood control benefits, the current method of computing risk costs introduces some measure of double counting. Several rebuilding scenarios are discussed and related to the loss of flood control benefits.
Sampling properties of parameter estimators for a storm field rainfall model
Authors: Krajewski Witold F. , Smith James A.
Published: February 2008

A statistical model of rainfall fields has been developed. The model parameters can be estimated from radar and rain gage data. The radar data are used only to estimate the spatial features of the model. The rain gage data are used to estimate the magnitude of rainfall. The parameter estimators are based on the method of moments and are shown to be consistent and asymptotically normal. To investigate the small sample properties of the estimators a Monte Carlo simulation experiment has been conducted. The results indicate that for certain combinations of the true rainfall field parameters the estimation procedure leads to biased results. The model has an attractive feature in that a simple statistic can be precomputed which indicates the feasibility of the model to represent a given rainfall data set. In case the statistic indicates infeasibility of the model it is difficult to distinguish whether the proposed model is not appropriate or the data sample is too small.
An improved algorithm for hydropower optimization
Authors: Reznicek K. K. , Simonovic S. P.
Published: February 2008

A new algorithm named energy management by successive linear programming (EMSLP) was developed to solve the optimization problem of the hydropower system operation. The EMSLP algorithm has two iteration levels: at the first level a stable solution is sought, and at the second the interior of the feasible region is searched to improve the objective function whenever its value decreases. The EMSLP algorithm has been tested using the Manitoba Hydro system data applied to a single reservoir system. To evaluate the performance of the algorithm the comparison has been made with the results obtained by the energy management and maintenance analysis (EMMA) program used in the Manitoba Hydro practice. The paper describes the EMSLP algorithm and presents the results of the comparison with EMMA.
Simulation of biodegradable organic contaminants in groundwater: 2. Plume behavior in uniform and random flow fields
Authors: MacQuarrie K. T. B. , Sudicky E. A.
Published: February 2008

The behavior of dissolved organic contaminants in groundwater is often difficult to interpret in field settings because of the complex interaction of physical, chemical, and biological processes. The organic solute transport model presented by MacQuarrie et al. (this issue) is used to examine plume migration in a shallow aquifer where dissolved oxygen is the sole electron acceptor. In uniform groundwater flow a plume originating from a high‐concentration source will experience more spreading and slower normalized mass loss than a plume from a lower initial concentration source because dissolved oxygen is more quickly depleted. Low background dissolved oxygen concentrations also cause organic mass loss to decrease, while the initial size of the microbial population has little effect. Large groundwater velocities produce increases in the rate of organic solute mass loss because of increased mechanical mixing of the organic plume with oxygenated groundwater. Because sorbed organic mass is unavailable for biodegradation, increasing the retardation factor of an organic solute causes slower mass loss. For easily biodegraded organics the mass loss is only weakly dependent on the kinetic biodegradation parameters because the amount of mixing of the organic and dissolved oxygen controls the biodegradation process. This implies that for these types of compounds the kinetic parameters do not need to be known with precision. For all simulations the rate of mass loss for the organic plumes decreased with time because the concentrations of organic and dissolved oxygen decrease with time. In complex‐structured flow fields caused by heterogeneity, the bulk velocity, position, and rate of spreading of an organic plume is strongly dependent on local‐scale transport parameters such as the groundwater velocity in individual beds. Because an organic plume emanating from a small to moderate sized source does not evolve in a manner similar to a conservative plume in heterogeneous materials due to a decrease in the size of the plume, it is suggested that average transport parameters such as macrodispersivity may not be applicable to such organic plume transport. If this is the case, then reliable predictions concerning the ultimate fate of organics in groundwater exemplifies the need for obtaining knowledge about the bedding features and degree of heterogeneity of the host geologic materials.
Cold water injection into single‐ and two‐phase geothermal reservoirs
Authors: Garg S. K. , Pritchett J. W.
Published: February 2008

Approximate analytical solutions are derived for cold water injection into single‐ and two‐phase “porous medium” geothermal reservoirs. A numerical geothermal reservoir simulator is used to verify the applicability of the analytical solutions for pressure transient analysis. In accordance with these numerical results the analytical solutions indicate that the pressure buildup behavior during injection into either a single‐phase liquid or two‐phase reservoir is governed by the kinematic viscosity of the cold injected fluid. The falloff pressures after cold water injection into a single‐phase liquid reservoir (except for very early shut‐in times) are primarily controlled by the kinematic viscosity of the in situ (hot) reservoir fluid. For two‐phase reservoirs, however, the falloff behavior is considerably more complex and cannot be fully described by the analytical solution developed in this paper.
Stochastic interpolation of rainfall data from rain gages and radar using cokriging: 1. Design of experiments
Authors: Seo Dong‐Jun , Krajewski Witold F. , Bowles David S.
Published: February 2008

Cokriging is used to merge rain gage measurements and radar rainfall data. The cokriging estimators included are ordinary, universal, and disjunctive. To evaluate the estimators, two simulation experiments are performed. The first experiment assumes that high‐quality radar rainfall fields are ground truth rainfall fields. From each ground truth rainfall field, multiple combinations of rain gage measurement field and radar rainfall field are artificially generated with varying gage network density and error characteristics of radar rainfall. The second experiment uses a stochastic space‐time rainfall model to generate assumed ground truth rainfall fields of various characteristics. Due to the sparsity of rain gage measurements, the second‐order statistics required for cokriging can only be estimated with large uncertainty. The adverse effects of this uncertainty, and the point sampling error of rain gage measurements are explicitly assessed by cokriging the ground truth rainfall data and the radar rainfall data with near perfectly known second‐order statistics.
Asymptotic expansion for steady state overland flow
Authors: Parlance J.‐Y. , Hogarth W. , Sander G. , Rose C. , Haverkamp R. , Surin A. , Brutsaert W.
Published: February 2008

The full Saint Venant equations of overland flow on a plane are often replaced by simpler models. The errors in the kinematic and the diffusion models are estimated by comparing their predictions with the exact numerical solution of the Saint Venant equations under steady state conditions. It is shown that the two approximate models can have significant errors even for critical flow and fairly large kinematic wave numbers. When the kinematic approximation is inaccurate, the improvement of the diffusion approximation seems modest. For the same level of mathematical complexity as the diffusion approximation, a far more accurate approximation is proposed when the kinematic wave number is large. It is then possible to split the solution of the Saint Venant equation in two regions, one near the downstream end of the plane and the other covering most of the plane.
Stochastic differential equation models of erratic infiltration
Authors: Serrano Sergio E.
Published: February 2008

Laboratory and field infiltration data exhibit a degree of erratic variability usually associated with measurement errors and uncertainties in the phenomenon of unsaturated porous media flow. Traditionally, these uncertainties are ignored and averaged soil characteristic curves are used in the inverse and direct modeling problems. However it is desirable to develop models capable of reproducing the inherent variability of soil moisture in order to study the erratic physics of flow at the laboratory level and to reproduce infiltration data in natural watersheds. In the present article, two exploratory models are tested as to their ability to replicate the erratic variability of experimental horizontal infiltration data. The first is based on the full partial differential equation of infiltration, and the second on the Boltzmann‐reduced differential equation. Both models are subject to a space or a time and space random soil‐water diffusivity defined as uncertainty term. The solution of the equations is presented, statistical properties of the water content function are described and verification of the models is conducted. Both models satisfactorily reproduced the statistical properties of the experimental data. While the first model easily relates to real space and time variables, the second required less computer time. As an application of the methodology, a third model is introduced as a new approach to predict vertical infiltration in hysteretic soils in natural watersheds. For this purpose, the effect of time variability of point rainfall is represented as a shot noise process, the hysteretic loops resulting from the natural wetting and drying cycles generate a correlated random soil‐water diffusivity process, and a solution of the vertical infiltration equation is presented along with statistical properties of the water content.
A Lagrangian‐Eulerian Method with zoomable hidden fine‐mesh approach to solving advection‐dispersion equations
Authors: Yeh G. T.
Published: February 2008

A Lagrangian‐Eulerian method with zoomable hidden fine‐mesh approach (LEZOOM), that can be adapted with either finite element or finite difference methods, is used to solve the advection dispersion equation. The approach is based on automatic adaptation of zooming a hidden fine mesh in regions where the sharp front is located. Application of LEZOOM to four bench mark problems indicates that it can handle the advection‐dispersion/diffusion problems with mesh Peclet numbers ranging from 0 to ∞ and with mesh Courant numbers well in excess of 1. Difficulties that can be resolved with LEZOOM include numerical dispersion, oscillations, the clipping of peaks, and the effect of grid orientation. Nonuniform grid as well as spatial temporally variable flow pose no problems with LEZOOM. Both initial and boundary value problems can be solved accurately with LEZOOM. It is shown that although the mixed Lagrangian‐Eulerian (LE) approach (LEZOOM without zooming) also produces excessive numerical dispersion as the upstream finite element (UFE) method, the LE approach is superior to the UFE method.
Sediment‐water interaction in a small stream: Adsorption of 137Cs by bed load sediments
Authors: Cerling Thure E. , Morrison Stan J. , Sobocinski Robert W. , Larsen Ingvar L.
Published: February 2008

In this paper we describe the interaction between dissolved solutes and stationary bed load sediments in White Oak Creek, east Tennessee. Irreversible adsorption of 137Cs by bed load sediments of White Oak Creek, a small gravel‐bottomed riffle‐and‐pool stream, was intensively studied to characterize sediment‐water interaction. Adsorption and desorption measurements in stream water show that adsorption is essentially irreversible, with no measurable desorption taking place during 6 years of exposure to stream water. The irreversible adsorption rate constant for 137Cs for 2‐ to 4‐mm gravel is 2 × 10−3 s−1. The adsorption rate constant is directly proportional to surface area and is related to mineralogy so that the adsorption rate constant for shale is much higher than for chert or limestone (<1 × 10−3 s−1). Newly deposited, uncontaminated sediment quickly becomes contaminated by 137Cs in White Oak Creek. Even though 137Cs is strongly adsorbed, measurable contamination to 20 cm depth in 64 days indicates that significant underflow takes place and results in the exchange of about 40 pore volumes of water per day. The measured adsorption rate constant is used to estimate a vertical dispersivity for underflow of about 10−1 cm2 s−1. Vertical dispersivity is probably induced by horizontal flow and is not directly related to net vertical flow. Such a high amount of sediment‐water interaction results in much greater contamination of sediments by highly adsorptive ions than is possible without significant underflow and exchange of pore fluids.
Toward empirical estimation of the total value of protecting rivers
Authors: Sanders Larry D. , Walsh Richard G. , Loomis John B.
Published: February 2008

The purpose of this paper is to develop and apply a procedure to estimate a statistical demand function for the protection of rivers in the Rocky Mountains of Colorado. Other states and nations around the world face a similar problem of estimating how much they can afford to pay for the protection of rivers. The results suggest that in addition to the direct consumption benefits of onsite recreation, total value includes offsite consumption of the flow of information about these activities and resources consumed as preservation benefits. A sample of the general population of the state reports a willingness to pay rather than forego both types of utility. We recommended that offsite values be added to the value of onsite recreation use to determine the total value of rivers to society.
Finite Hydraulic Conductivity Effects on Optimal Groundwater Pumping Rates
Authors: Zimmerman W. R.
Published: February 2008

The effect of a finite hydraulic conductivity on the optimal pumping rate from a closed aquifer is examined. A methodology is presented which determines the optimal pumping rate of a dynamic groundwater model with complex state and control functions. It is shown that the optimal pumping rate can be substantially lower when the hydraulic conductivity is small enough to result in a significant cone of depression.
Error Analysis and Stochastic Differentiability in Subsurface Flow Modeling
Authors: Loaiciga Hugo A. , Mariño Miguel A.
Published: February 2008

In the stochastic analysis of steady aquifer flow, the log hydraulic conductivity is the random “input” and the piezometric potential is the random “output.” Their joint behavior is governed by a differential equation that, in view of the random nature of its dependent (piezometric potential) and independent (log hydraulic conductivity) variables, represents a stochastic differential equation. The analysis of the distributional properties of the piezometric potential field involves the product of the gradients of log hydraulic conductivity and of the piezometric potential. Previous research in this field assumes that such product of gradients is small in some sense. This paper derives a closed‐form expression for the standard deviation, and hence the order of magnitude, of the product of the random gradients of log hydraulic conductivity and of piezometric potential. It was found in this research that for statistically homogeneous log hydraulic conductivity fields (1) the product of the random gradients may or may not have a zero mean, depending on whether the specific discharge is a constant or a random quantity, respectively; (2) under joint normality of the log hydraulic conductivity and the piezometric potential fields, their random gradients are statistically independent if the specific discharge is constant but are dependent when the specific discharge is random; (3) the standard deviation of the product of random gradients is proportional to the variance of log‐hydraulic conductivity times a term involving three quantities: the covariance of the piezometric potential, the covariance of the log hydraulic conductivity, and the cross covariance of the latter two fields; (4) a necessary and sufficient condition for the smallness of the product of random gradients is that the second derivatives of the covariance of the log hydraulic conductivity, the covariance of the piezometric potential, and the cross covariance of the latter two random fields be finite and that the variance of log hydraulic conductivity be much less than one. This paper also reviews some fundamental principles on the stochastic analysis of random fields and their importance to the modeling of log hydraulic conductivity fields and to the analysis of subsurface flow. Specifically, the paper highlights the role of Gaussian distributional assumptions in deriving key results of stochastic groundwater flow via perturbation analysis.
Status report of stable and radioactive ion beam production at GANIL
Authors: Gaubert G. , Barué C. , Canet C. , Cornell J. C. , Dubois M. , Dupuis M. , Eleon C. , Flambard J. L. , Frigot R. , Jardin P. , Leboucher C. , Lecesne N. , Leherissier P. , Lemagnen F. , Leroy R. , Pacquet J. Y.
Published: February 2008

GANIL has been producing many stable and radioactive ion beams for nearly 25years. Constant progresses have been made in terms of intensity, stability, and reliability. The intensity for some stable metallic beams now exceeds or approaches the pμA level at an energy up to 95MeV∕u, e.g., 1.14pμA for S36 (65% enriched) at 77MeV∕u, 0.35pμA for Ni58 (63% enriched) at 74MeV∕u. Some recent results with Magnesocene using the metallic ions from volatile compounds method should also make possible the production of metallic beams with an intensity greater than 1pμA. This has still to be measured. The ISOL facility SPIRAL I has been in operation for almost six years. Up to now, 17 exotic He experiments have been done with 14 target/ion-source (TIS) units; 19 other experiments (with O, Ne, Ar, and Kr) have been achieved with 14 TISs. Statistics show a fairly good ratio of available beam time to scheduled beam time. The radioactive beams and available intensities are compiled in this report. Future developments on radioactive ion beam production are briefly presented, while more details will be discussed elsewhere at this conference.
Photoinduced insulator-to-metal transition in ZnO∕Mg0.15Zn0.85O heterostructures
Authors: Tsukazaki A. , Ohtomo A. , Nakano M. , Kawasaki M.
Published: February 2008

We report on the persistent photoconductivity accompanied with a steep insulator-to-metal transition at low temperatures in ZnO∕Mg0.15Zn0.85O heterostructures. The photoexcited electrons were confined in the ZnO adjacent to the Mg0.15Zn0.85O to form a two-dimensional electron gas (2DEG). The electron density was controlled either by the power or number of ultraviolet laser pulses (266nm wavelength) irradiated to the sample. The 2DEG exhibits Shubnikov-de Haas oscillation in magnetoresistance, whose oscillation periods coincide with the electron density evaluated by Hall effect measurements.
Application of dopant segregation to metal-germanium-metal photodetectors and its dark current suppression mechanism
Authors: Zang H. , Lee S. J. , Loh W. Y. , Wang J. , Yu M. B. , Lo G. Q. , Kwong D. L. , Cho B. J.
Published: February 2008

We report the mechanism of dopant-segregation (DS) technique as applied in metal-germanium-metal photodetectors (MGM-PDs) for dark current suppression. Photodetectors with various dopant-segregation strategies were designed, fabricated, and characterized. Results show that asymmetric MGM-PD, with n- and p-type dopants segregated separately in two NiGe electrodes, is the optimized scheme in terms of dark current and responsivity. It shows a dark current of 10−6A at −1V, which is two to three orders of magnitude lower than that of MGM-PD without DS. n-type dopant (As) segregation in NiGe barrier increases the hole Schottky barrier height to 0.5eV and, thus, plays a crucial role in dark current suppression.