bluuebunny/crossref_metadata_2025 huggingface dataset:
Bayesian flood frequency analysis with paleohydrologic bound data
Authors: O'Connell Daniel R. H. , Ostenaa Dean A. , Levish Daniel R. , Klinger Ralph E.
Published: October 2002

It is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge, historical, and paleohydrologic bound information in Bayesian flood frequency analyses. Estimates of primary posterior modes for common three‐parameter frequency distributions are constructed using simulated annealing and the simplex method. Parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration. Bayesian flood frequency analyses with annual peak discharge, historical, and paleohydrologic bound data for the Santa Ynez River, California, and the Big Lost River, Idaho, demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long‐term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities.
Multiple‐objective optimization of drinking water production strategies using a genetic algorithm
Authors: Vink Kees , Schot Paul
Published: October 2002

Finding a strategy that allows economically efficient drinking water production at minimal environmental cost is often a complex task. A systematic trade‐off among the costs and benefits of possible strategies is required for determining the optimal production configuration. Such a trade‐off involves the handling of interdependent and nonlinear relations for drawdown‐related objective categories like damage to wetland vegetation, agricultural yield depression, reduction of river base flow rates, and soil subsidence. We developed a method for multiple‐objective optimization of drinking water production by combining Busacker and Gowen's [1961] “minimum cost flow” procedure for optimal use of the transport network with a genetic algorithm (GA) for optimization of other impacts. The performance of the GA was compared with analytically determined solutions of a series of hypothetical case studies. Pareto‐optimality and uniqueness of solutions proved to be effective fitness criteria for identifying trade‐off curves with the GA.
Asymptotic properties of the Eulerian truncation approximation: Analysis of the perfectly stratified transport problem
Authors: Li Shu‐Guang , McLaughlin Dennis
Published: October 2002

It is well known that stochastic groundwater problems are difficult to solve without making approximations of one kind or another. A popular approximation used in both subsurface flow and transport applications is the so‐called “Eulerian truncation.” This approximation, which relies on a perturbation expansion of the governing equation, neglects certain terms involving products of small fluctuations. Several recent publications [e.g., Dagan and Neuman, 1991; Neuman and Orr, 1993; Neuman, 1993] question the validity of Eulerian truncation, arguing that it is inconsistent because neglected terms are of the same order as those retained. In this paper we analyze the asymptotic properties of the Eulerian truncation solution, using a Taylor series approach which clearly reveals the properties of different order approximations. We show that the Eulerian truncation does, in fact, generate an asymptotic ensemble mean expansion for the perfectly stratified example considered by Dagan and Neuman [1991]. Moreover, we show that the Eulerian truncation solution to this problem is identical to the well‐known cumulant expansion approximation, which is also based on an asymptotic expansion.
An objective approach to regional wastewater treatment planning
Authors: Wang C. G. , Jamieson D. G.
Published: October 2002

This research program was aimed at developing an objective methodology for water quality management on a river basin scale. To that end, a mathematical model has been formulated to determine the best configuration of wastewater treatment plants consistent with either fixed‐emission standards or prescribed river quality objectives. It will, of course, be appreciated that the latter case is considerably more difficult since this involves not only site selection but also waste load allocation. In the case of waste load allocation it was first necessary to use a process‐based river water quality simulation model to predict the impact of different combinations of effluent discharge standards on the river. For reasons of computational efficiency an artificial neural network was employed to replicate the process‐based model, which was then used in conjunction with a genetic algorithm to determine both the best sites and individual effluent discharge standards, subject to meeting the required river water quality. The overall model has been applied to the upper Thames basin in southern England, initially for site selection alone and then for site selection with waste load allocation. The results show that the genetic algorithm performs well for both options, thereby providing an efficient means of planning wastewater treatment on a regional basis.
Material transport from different sources in a network of streams through a catchment
Authors: Gupta A. , Cvetkovic V.
Published: October 2002

A probabilistic model for material transport in a stream network is developed based on geomorphology of the subcatchment and water transit time distribution. A tracer may be a point source or a source distributed over the entire catchment. The tracer particles are transported by advection through streams of different order and also diffuse and react chemically with the sediments lying at the bottom of the streams. Stochastic analysis of travel time is complicated by microscopic exchange processes, which act to retain the tracer in the surrounding medium. These include kinetically controlled exchange with the storage zones, diffusion into the bed sediment, and linear equilibrium sorption. In addition, degradation delays the downstream tracer movement. The sensitivity analysis indicates that for 50% mass arrival the mean arrival time is increased by 3 times for a change in mass transfer parameter, χ*, by 10 times. This increase is further pronounced for higher mass arrivals and higher χ*. The results on a specific application example show that a mere doubling of the uncertain value of diffusive mass transfer rate in the bed sediment reduces the probability that 25% of solute mass arrives at the outlet by ≈90%. The high sensitivity of the probability of the solute mass arrival at the outlet to the uncertain diffusive mass transfer rate implies uncertainty also in predictions of the solute transport process. Thus it can be concluded that the correct estimation of mass transfer rate in the bed sediments within a catchment plays an important role in field‐scale estimation of transport parameters.
A note on the effective parameters of the convection‐dispersion equation
Authors: Russo David
Published: October 2002

We focus on the analysis of the effective parameters of the one‐dimensional convection‐dispersion equation (CDE), estimated from the depth distribution of the solute concentration at a given observation time. This situation should be distinguished from that in which the effective parameters of the one‐dimensional CDE are estimated from the temporal distribution of the solute concentration at a given arbitrary soil depth. In this latter case, both the mean and the variance of the solute travel time probability density function increase linearly with travel distance, and the resultant effective solute velocity, Ve, and dispersion coefficient, De, are constant, independent of travel distance. Results of the present analysis suggest that under both first‐type and third‐type upper boundary conditions, the resultant Veand De are time dependent. Consequently, the effective dispersivity, λe = De/Ve, increases with travel time or distance as the solute body first invades the flow system and tends to approach a constant value (i.e., Fickian behavior) after the solute body has traveled a distance of few tens of pore‐scale dispersivity. These findings, which stem from the Lagrangian nature of the dispersion phenomenon, are consistent with the results of first‐order analyses in saturated and partially saturated heterogeneous formations, based on the theory of dispersion by continuous motions. The findings presented in this technical note have important implications regarding the estimation of parameters of the one‐dimensional CDE from the depth distribution of the solute concentration at a given observation time, and the subsequent prediction of solute movement.
Indirect estimation of soil thermal properties and water flux using heat pulse probe measurements: Geometry and dispersion effects
Authors: Hopmans Jan W. , Šimunek Jirka , Bristow Keith L.
Published: October 2002

Traditionally, analytical solutions for heat transport in soils have been used in combination with heat pulse probe (HPP) measurements to estimate soil thermal properties. Although the analytical method has resulted in accurate estimation of soil thermal properties, we suggest that parameter estimation using inverse modeling (IM) provides new and unique opportunities for soil thermal characterization. Moreover, we show that the IM approach provides accurate estimation of soil water flux density in both unsaturated and saturated soil conditions for a wider range of water velocities than originally thought possible. Specifically, we show that accurate soil water velocity is obtained, simultaneously with soil thermal properties, if heat dispersion is included in the heat transport equation. The requirement for including heat dispersivity depends on the value of the newly defined dimensionless Keith Jirka Jan (KJJ) number, which is equal to the ratio of thermal dispersion to thermal conductivity. For example, when KJJ > 1, ignoring thermal dispersivity leads to errors in the water flux density which can exceed 10%. By including thermal dispersivity, water flow velocities were accurately determined for water flux densities ranging from 1.0 to >10 m d−1. We also demonstrate the general application of inverse modeling to estimate soil thermal properties and their functional dependence on volumetric water content in a separate numerical experiment. We suggest that inverse modeling of HPP temperature data may allow simultaneous estimation of soil water retention (when combined with matric potential measurements) and unsaturated hydraulic conductivity (through water flux estimation) from simple laboratory experiments.
Upscaled flow and transport properties for heterogeneous unsaturated media
Authors: Khaleel Raziuddin , Yeh T.‐C. Jim , Lu Zhiming
Published: October 2002

To represent a heterogeneous unsaturated medium by its homogeneous equivalent, stochastic theory‐based analytical formulas and numerical Monte Carlo simulations are used to obtain upscaled (effective) flow and transport properties. The Monte Carlo experiments simulate steady state flow and transient transport of nonreactive solute in two‐dimensional heterogeneous media. Constitutive relations for unsaturated media at the mesh‐size scale are based on the van Genuchten‐Mualem relationships. A unit‐mean‐gradient approach is used to derive upscaled properties for flow parallel and perpendicular to bedding. Upscaled moisture retention and unsaturated conductivities (K) and longitudinal dispersivities are obtained by simulating steady gravity drainage conditions for a series of applied infiltration rates, characteristic of relatively dry conditions in coarse‐textured sediments. Macrodispersivities are calculated on the basis of spatial moments of the ensemble‐mean plume. Results show that the calculated effective unsaturated K based on the analytical formulas compare well with those based on Monte Carlo simulations. The macroscopic anisotropy (ratio of K parallel to bedding to K perpendicular to bedding) increases with increasing tension, although the increase is rather mild. The longitudinal macrodispersivity for the equivalent homogeneous medium also increases with increasing tension. A comparison of numerical results with stochastic solutions suggests that the computed dispersivities are of the same order of magnitude at low tensions. At higher tensions the dispersivity estimates deviate significantly. Nonetheless, both numerical and analytical results show that the longitudinal dispersivities for flow parallel to bedding are higher than those for flow perpendicular to bedding. In addition, our results suggest that the Fickian regime is reached much earlier for cases with flow perpendicular to bedding than for those with flow parallel to bedding.
Estimating nonpoint source pollution: An application of a sequential entropy filter
Authors: Kaplan Jonathan D. , Howitt Richard E.
Published: October 2002

This paper develops a sequential entropy filter for disaggregating nonpoint sources from ambient data. A numerical simulation based on sediment loading is provided to illustrate the ability of the sequential entropy filter to recover the underlying parameters and optimally disaggregate ambient sediment load among nonpoint sources. In the process we show the equivalence of this sequential entropy filter with Bayes' theorem and, given this equivalence, argue that the sequential entropy filter is more applicable than traditional Bayesian estimators are when the parameter distributions are unknown or when the sample is undersized, which is typically the case when dealing with natural resource data.
Combined effect of interblock and interaggregate capillary cracks on the hydraulic conductivity of swelling clay soils
Authors: Chertkov V. Y. , Ravina I.
Published: October 2002

The cracks in swelling clay soils at sufficiently high water content can be presented as the superposition of interblock‐ and interaggregate‐capillary‐crack networks. The objective of this work is the generalization of an earlier proposed model describing the hydraulic conductivity of a soil matrix with only interaggregate capillary cracks. The generalized model describes the hydraulic conductivity of a soil with capillary cracks of both types. For the case of vertical hydraulic conductivity the generalization is based on the available results relative to the geometrical and hydraulic properties of capillary crack networks. The contribution of interblock capillary cracks to the hydraulic conductivity of a soil at a given pressure head changes with depth even for a given matrix structure, contrary to the contributions of interaggregate capillary cracks and soil matrix. For numerical estimates and validation of the model we use literature data on the variation of water content in different soil layers, evaporation, subsidence, and crack volume during a drying period in the course of a laboratory lysimeter experiment. The numerical estimates indicate the essential and even prevailing contribution of capillary cracks of both types to hydraulic conductivity at sufficiently small pressure heads and their negligible contribution to the water retention of clay soil. The validation is based on a comparison between two independent estimates of the summary hydraulic conductivity of the soil matrix and interaggregate capillary cracks. One of these estimates is found as a difference between the total hydraulic conductivity of the soil and the hydraulic conductivity of the interblock‐capillary‐crack network. Another estimate is found by the model describing the hydraulic conductivity of a soil matrix with only interaggregate capillary cracks. The results of the comparison validate the feasibility of the model showing that it is in agreement with the available data of the lysimeter experiment.
Stochastic water quality: Timing and option value of treatment
Authors: Conrad Jon M. , López Andrés
Published: October 2002

An option‐pricing model is developed to rank investments that might improve water quality. The model presumes that two investment options exist that have the potential to alter the stochastic drift of a pollutant. The investments have capital and operating costs and are irreversible once constructed. A stochastically evolving pollutant induces damage. The model provides a criterion for determining when it is optimal to adopt the investment with the highest option value. Option value, in this model, measures the expected present value of reduced damage, relative to doing nothing. If the investments are mutually exclusive, it is possible to obtain closed‐form solutions for the barriers which would trigger investment. If the investments can be sequentially adopted, a methodology is developed to calculate option values for all possible combinations of adoption dates. To illustrate the option‐pricing approach, a stylized analysis of investments to protect New York City's water supply is presented. Watershed management dominates filtration and, in the case of mutually exclusive investments, is initiated when the concentration of phosphorus reaches 22.80 μg/L.
New form of dispersion tensor for axisymmetric porous media with implementation in particle tracking
Authors: Lichtner Peter C. , Kelkar Sharad , Robinson Bruce
Published: October 2002

A general form of the dispersion tensor is derived for axisymmetric porous media involving four dispersivity coefficients corresponding to longitudinal and transverse dispersion in horizontal and vertical directions, defined as perpendicular and parallel to the axis of symmetry, respectively. The general form of the dispersion tensor provides for distinct vertical and horizontal longitudinal dispersivity values. Transverse dispersion is isotropic for flow parallel to the symmetry axis and anisotropic for flow perpendicular to the symmetry axis with distinct horizontal and vertical transverse dispersivities. The new form of the dispersion tensor is applied to several examples involving axisymmetric media utilizing particle tracking techniques and compared to the tensor proposed by Burnett and Frind [1987]. It is demonstrated that for the case of spatially variable flow the drift term ∇ · (ϕD)/ϕ must generally be included in the particle tracking algorithm to obtain accurate results.
A field‐based study of soil water and groundwater nitrate release in an Adirondack forested watershed
Authors: McHale Michael R. , McDonnell Jeffrey J. , Mitchell Myron J. , Cirmo Christopher P.
Published: October 2002

Nitrate (NO3−) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO3− and the mechanisms that deliver NO3− to the stream to test two hypotheses: (1) Soil water NO3− concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO3− concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO3− concentrations before storms (range of 1.1–5.1 μmol L−1 throughout the study). High soil water NO3− concentrations (up to 136 μmol L−1) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end‐member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near‐stream wetland groundwater and event water contributed little to streamflow during most conditions. Near‐stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h−1) during the summer and fall. Highest stream water NO3− concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using δ18O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO3− release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO3− during both base flow and storms.
Quasi‐linear steady infiltration from a sloping land surface
Authors: Basha H. A.
Published: October 2002

The present work derives a general analytical solution to the problem of steady unsaturated flow in hillslopes. The geometry of the domain consists of either a semi‐infinite medium or a finite area contained between two parallel sloping planes with a prescribed flux boundary condition at the top and at the bottom. The top boundary condition models rainfall infiltration while the bottom boundary condition models either an impermeable or a semipermeable underlying layer if the leakage flux is known a priori. Particular solutions for point, line, strip, and rectangle source configurations at the sloping land surface are presented. The numerical results for the strip and rectangle source solution show the effect of the slope, impermeable base, and soil type on the distribution of the pressure and flux in the domain. In particular, the three‐dimensional rectangular source solution displays the interaction between the slope, soil characteristics, and the point of maximum potential at which the saturation of the soil begins.
Multicriteria parameter estimation for models of stream chemical composition
Authors: Meixner T. , Bastidas L. A. , Gupta H. V. , Bales R. C.
Published: October 2002

The inability to develop an accurate and precise parameter estimation method for catchment hydrochemical models has been a persistent problem. We investigate the use of multicriteria calibration techniques and the selection of the criteria as a first step in solving this problem. We applied a multicriteria search algorithm to the Alpine Hydrochemical Model (AHM) of the Emerald Lake watershed, Sequoia National Park, California. A total of 21 chemical and hydrologic criteria were available for determining model performance. Four subsets of these criteria were selected for the multicriteria analysis using three different methods. The first set used the four least correlated observations of stream chemical composition. A second set of criteria was determined by using the four species with the least correlated root mean square error criteria values (ρ < 0.05 for each pair). Finally, two sets were chosen using the results of a multicriteria sensitivity analysis. The most accurate and precise results were observed using criteria selected using results from a sensitivity analysis, with the correlation analyses being a poor method for selecting criteria. This set of criteria emphasizes attributes of the model structure, the observations, and our understanding of the processes influencing watershed hydrology and water quality. Also, our results gave improved estimates of several hydrologic and biogeochemical processes in addition to identifying a flaw in the current representation of mineral weathering within the AHM, as applied to the Emerald Lake watershed.
Seasonal variations in the evapotranspiration of a transitional tropical forest of Mato Grosso, Brazil
Authors: Vourlitis George L. , Filho Nicolau Priante , Hayashi Mauro M. S. , de S. Nogueira José , Caseiro Fernando T. , Campelo José Holanda
Published: October 2002

The seasonal pattern of evapotranspiration (expressed as latent heat flux Qe) for a 28‐ to 30‐m‐tall tropical transitional (ecotonal) forest was quantified over an annual cycle using eddy covariance measurement and micrometeorological estimation techniques. The study was conducted near the city of Sinop, in northern Mato Grosso, Brazil, which is located within the ecotone of tropical rain forest and savanna (cerrado). Although the majority of net radiation (Q*) was consumed by Qe (50–90%), seasonal variations in Qe were large and positively correlated with precipitation. Total daily Qe for the dry season (June–September) was on average 6.0 MJ m−2 d−1, while daily Qe for the transition (October–November and April–May) and wet (December–March) season periods were 7.5 and 10.0 MJ m−2 d−1, respectively. The seasonal variation in midday (0900–1500 LT) surface conductance (gs) was also positively correlated with precipitation. Analysis of the “decoupling factor” (Ω) indicated that the forest was strongly coupled to the atmosphere (Ω = 0.1–0.3) over the dry season and transition periods, suggesting that Qe was under relatively strong stomatal control. Although rainfall during the study period was above the long‐term (30‐year) average, our results indicate that the seasonal dynamics of Qe for the tropical transitional forest were more comparable to tropical savanna than to rain forest.
Experimental design and model parameter estimation for locating a dissolving dense nonaqueous phase liquid pool in groundwater
Authors: Sciortino Antonella , Harmon Thomas C. , Yeh William W‐G.
Published: October 2002

This paper investigates the experimental design problem in the context of estimating the location and dimensions of a dissolving dense nonaqueous phase liquid (DNAPL) pool in an aquifer. The design problem deals with the selection of the number and location of observation wells that minimizes the parameter uncertainty while minimizing installation and sampling costs. The solution of this multiobjective optimization problem is achieved by solving a series of combinatorial optimization problems in which, for each imposed budget constraint, the optimal combination of wells is found. The combinatorial optimization problems are solved by a genetic algorithm for two different design strategies: one where the prior estimates are not updated and one where the parameter updating is performed at the end of each design stage (sequential design). In the latter case, parameter updating is obtained by executing an inverse model based on concentration values collected at the locations specified by the previous optimal design. The algorithms are tested for a controlled, bench‐scale pool dissolution experiment. The results demonstrate that the sequential design performs better in terms of final parameter estimates. Three optimality criteria associated with experimental design for model parameter estimation are compared: the A optimality, D optimality, and E optimality. Among the three criteria, the A and E optimalities produce better estimates of the pool location and size, which is justified by appreciably different variances associated with the parameters and their high degree of correlation.
Hydrogeologic assessment of in situ natural attenuation in a controlled field experiment
Authors: Devlin J. F. , McMaster M. , Barker J. F.
Published: October 2002

An experiment to investigate the natural attenuation of three volatile organic compounds, toluene, carbon tetrachloride, and tetrachloroethene (∼1–10 mg L−1) was performed in a 3 m deep, sandy aquifer isolated within a 24 m long, 2 m wide, three‐sided sheet pile alleyway (hereafter referred to as the gate). A constant flow was maintained in the test volume by pumping a well at the closed end of the gate at 130 mL min−1. The test compounds were introduced to the aquifer using diffusive emitters installed inside 25 cm diameter wells located at the open end of the gate. Monitoring was performed by sampling along six multilevel fences (consisting of 12 sampling points each) ranging in distance from 1 to 22 m from the source wells. A bromide tracer experiment established that there were no significant hydraulic leaks, nor was there any continuous channeling through the gate. Degradation of the test compounds was assessed by mass balance calculations between fences located 1 and 7 m from the source, and the results were compared with degradation rate estimates from snapshot analyses and the analysis of fluxes. There was reasonably good agreement between rates estimated by these different methods. Toluene degraded with a half‐life of 58–62 days, carbon tetrachloride degraded with a half‐life of ∼11–13 days, and tetrachloroethene degraded too slowly for a reliable estimate of rate to be made. Transformation products identified in the gate included acetate, possibly from toluene degradation, chloroform, trichloroethene, and cis‐1,2, dichloroethene. The latter two compounds only appeared in trace quantities and could not be assessed for continuing degradation. However, chloroform degradation was assessed with the snapshot data and using the flux estimates and was found to degrade with a half‐life in the range of 10–34 days. No additional chlorinated methanes were detected in the gate, suggesting that the carbon tetrachloride was completely dechlorinated by natural processes within 10 m of the source wells. This experiment demonstrated that degradation of chlorinated solvents occurs naturally at the Borden site but that the ethenes are more resistant to biodegradation than the methanes. In addition, the flux calculations were found to be the most robust in terms of estimating degradation rates.
Using groundwater temperature data to constrain parameter estimation in a groundwater flow model of a wetland system
Authors: Bravo Hector R. , Jiang Feng , Hunt Randall J.
Published: October 2002

Parameter estimation is a powerful way to calibrate models. While head data alone are often insufficient to estimate unique parameters due to model nonuniqueness, flow‐and‐heat‐transport modeling can constrain estimation and allow simultaneous estimation of boundary fluxes and hydraulic conductivity. In this work, synthetic and field models that did not converge when head data were used did converge when head and temperature were used. Furthermore, frequency domain analyses of head and temperature data allowed selection of appropriate modeling timescales. Inflows in the Wilton, Wisconsin, wetlands could be estimated over periods such as a growing season and over periods of a few days when heads were nearly steady and groundwater temperature varied during the day. While this methodology is computationally more demanding than traditional head calibration, the results gained are unobtainable using the traditional approach. These results suggest that temperature can efficiently supplement head data in systems where accurate flux calibration targets are unavailable.
The connectivity of two‐dimensional networks of spatially correlated fractures
Authors: Manzocchi Tom
Published: October 2002

Continuum percolation thresholds for fracture systems are based on models with random fracture locations, resulting in uniformly clustered systems in which all connections are fracture intersections. Fractures in natural systems can be either clustered or anticlustered, and a significant portion of the connectivity is achieved through fracture abutments or splays. Analytical and numerical methods are combined to define critical properties of isotropic networks of constant length lines as a function of fracture clustering and of the ratio of abutments to intersections. A second suite of numerical results indicates that the influence of these parameters varies as a function of the fracture length distribution. A preliminary predictor of critical density of anisotropic nonrandom systems with lognormal length distributions is derived and applied to a suite of natural fracture systems. Predicted connectivities are closer to the observed connectivities when the predictor is used than when connectivity is estimated from the random threshold.
Controls of mass recovery rates in single‐well injection‐withdrawal tracer tests with a single‐porosity, heterogeneous conceptualization
Authors: Altman Susan J. , Meigs Lucy C. , Jones Toya L. , McKenna Sean A.
Published: October 2002

A single‐well injection‐withdrawal (SWIW) test is evaluated as a tool to assess a single‐porosity conceptualization for a transport system. We have shown that there are cases in which matrix diffusion cannot be proven definitively from the results of SWIW tests owing to uncertainties of aquifer properties. Drift during the resting phase of the test can carry a plume to areas where the flux is lower during the withdrawal phase, leading to decreased mass recovery rates. Results of sensitivity studies on multiple, stochastically generated, heterogeneous transmissivity fields indicate that variance in transmissivity has a strong effect on drift and consequently mass recovery rates. In addition to impacting drift directly, the regional gradient has the potential to influence mass recovery rates by interfering with the pumping‐induced fluxes toward the well during withdrawal. The Kolmogorov‐Smirnov test is used to evaluate whether simulated data using a single‐porosity conceptualization with plume drift significantly differ from SWIW tracer test data from a fractured dolomite unit at the Waste Isolation Pilot Plant. Given the uncertainty in the variance of transmissivity, it is unlikely that a single‐porosity conceptualization can match the field data. Matrix diffusion is a likely explanation for the observed gradual mass recovery rates.
Numerical modeling of subgrid heterogeneity in two phase flow simulations
Authors: Efendiev Y. , Durlofsky L. J.
Published: October 2002

A coarse‐scale model governing two‐phase viscous dominated flow in heterogeneous porous media is presented. The coarse‐scale water transport equation (or saturation equation) is derived from a volume average of the underlying fine‐scale saturation equation. A key component of the coarse‐scale description is the subgrid model, which appears in the formulation as a nonlinear, nonlocal term in the coarse‐scale saturation equation. This subgrid model captures the effects of the unresolved (fluctuating) components of saturation and velocity. An approximation is introduced which allows the nonlocal effect to be estimated using local fine‐scale information (computed in a preprocessing step) coupled with global coarse‐scale quantities, thus avoiding the need for any global fine‐scale calculations. Results using the new model are presented for a variety of heterogeneous permeability fields in two dimensions over a range of fluid viscosity ratios. In all cases considered, computations using the new model are significantly more accurate, relative to the reference fine‐scale results, than simulations in which subgrid effects are neglected. Further extensions of the formulation, which would enable the modeling of realistic systems, are discussed.
Determination of changes in streamflow variance by means of a wavelet‐based test
Authors: Cahill Anthony T.
Published: October 2002

Changes in the variance of streamflow over time may possibly be caused by land cover changes or climate changes. Knowledge of these possible changes in the time series structure is desirable for validation of modeling efforts; detection is not always straightforward, however. We present the use of a wavelet‐based statistical test for the detection of changes in the variance of a time series applied to records of streamflow from 12 rivers in the United States from the period 1954–1999. The test is able to show during which time periods and at what wavelet scales changes in the variance are taking place. It is found that the major similarities in variance change for all the streamflow time series exist when the largest time domain wavelet is used and also when a wavelet of the second largest frequency scale is used. Sampling considerations prevent exact localization of the time and frequency of the change(s). Results of the test indicate that for the data analyzed in this study a change in the variance of the streamflow time series most likely did take place over the time period examined.
Modeling biodegradation processes in porous media by the finite cell method
Authors: Sun Ne‐Zheng
Published: October 2002

In this paper, the finite cell method (FCM) presented by Sun [1999] is extended to the two‐dimensional case and used to simulate complicated biodegradation processes in porous media. FCM is based on the direct simulation of various physical, chemical, and biological phenomena rather than solving advection‐dispersion‐reaction equations. The developed two‐dimensional FCM is able to deal with heterogeneous structures and variable flow fields. Two test problems show that FCM can produce more accurate solutions than other numerical methods. It can keep both local and global mass conservation and avoid numerical dispersion even when the Peclet number becomes infinite. The biodegradation model formulated in this paper consists of five partial differential equations with five unknown concentrations when kinetic mass exchange and rate‐limited attachment of microorganisms are considered. Using FCM to solve this problem, we define two sets of cells, the mobile fluid cells and the immobile solid cells. A mobile fluid cell may contain dissolved substrate, suspended microorganisms, and electron acceptor components, while an immobile solid cell may contain adsorbed substrate and attached biomass. When a mobile cell meets an immobile cell on its way, mass exchanges may occur between them and reactions may occur within them. By directly simulating these processes, all the five unknown concentrations can be obtained explicitly and simultaneously. A numerical example is given that shows the FCM solution of a biodegradation process in a randomly heterogeneous porous media. We expect that the FCM can serve as not only a powerful simulation tool but also a useful tool for mechanism study.
Solute transport at fracture intersections
Authors: Mourzenko V. V. , Yousefian F. , Kolbah B. , Thovert J.‐F. , Adler P. M.
Published: October 2002

A numerical study of three‐dimensional solute transport at fracture intersections by using a particle tracking technique is presented. Two models of orthogonal fracture intersection are considered, namely, two parallel‐walled channels and two rough‐walled Gaussian fractures. The fluid velocity is calculated by solving the three‐dimensional Stokes equation with no‐slip boundary condition at the solid wall. Examples of individual trajectories of particles are first given in order to illustrate the main features of the phenomenon. Solute mass partitioning between outgoing fracture branches is considered for various transport regimes, characterized by the local Péclet number, and for various ratios of the flow rates in the intersecting channels. Generally speaking, it can be said that at dominant diffusion the influence of the flow rates ratio is weak, while it is important in the opposite situation. Validity of the classical models of solute mixing, stream tube routing, and perfect mixing is analyzed by comparing their predictions with the numerical data. Preliminary recommendations are made for the use of these results in large‐scale modeling.
Trace gas responses of tropical forest and pasture soils to N and P fertilization
Authors: Steudler Paul A. , Garcia‐Montiel Diana C. , Piccolo Marisa C. , Neill Christopher , Melillo Jerry M. , Feigl Brigitte J. , Cerri Carlos C.
Published: October 2002

We measured the responses of nitric oxide (NO), nitrous oxide (N2O) and carbon dioxide (CO2) to nitrogen (N) and/or phosphorus (P) fertilization in a mature moist tropical forest and an 11‐year‐old pasture in the Brazilian Amazon. Nitrogen was applied in two forms, ammonium (NH4+) and nitrate (NO3−). In the forest, NO emissions increased by 4 to 9 times the controls in the NH4+ amended plots. Nitrous oxide emissions showed a small response only in the NH4+ amended plots. In the pasture, NO emissions during the first 7 days after fertilization with either form of N were about twice those in the control plots. Nitrous oxide emissions increased more than 18 times the controls in the NO3− amended plots 1 day after fertilization. The estimated yields of total nitrogen oxide loss from the forest were between 0.2 and 1.6% of the applied nitrogen, predominately as NO. Pasture yields were greater, up to 2.8% of the applied nitrogen, predominately as N2O. In the context of Rondônia and other regions in the Amazon Basin, pasture management practices are changing to include increased use of fertilizer, particularly in older pastures that have lower NO and N2O emissions than the original intact forests. This may lead to large short‐term releases of N2O and alter the future N2O emissions from the Basin.
Modeling the impact of hydrological changes on nitrate transport in the Mississippi River Basin from 1955 to 1994
Authors: Donner Simon D. , Coe Michael T. , Lenters John D. , Twine Tracy E. , Foley Jonathan A.
Published: October 2002

The export of nitrate by the Mississippi River to the Gulf of Mexico has tripled since the 1950s primarily due to an increase in agricultural fertilizer application and hydrological changes. Here we have adapted two physically based models, the Integrated Biosphere Simulator (IBIS) terrestrial ecosystem model and the Hydrological Routing Algorithm (HYDRA) hydrological transport model, to simulate the nitrate export in the Mississippi River system and isolate the role of hydrological processes in the observed increase and interannual variability in nitrate export. Using an empirical nitrate input algorithm based on constant land cover and variability in runoff, the modeling system is able to represent much of the spatial and interannual variability in aquatic nitrate export. The results indicate that about a quarter of the sharp increase in nitrate export from 1966 to 1994 was due to an increase in runoff across the basin. This illustrates the pivotal role of hydrology and climate in the balance between storage of nitrate in the terrestrial system and leaching.
Inhibition experiments on nitrous oxide emission from paddy soils
Authors: Xu Xingkai , Boeckx Pascal , Zhou Likai , Van Cleemput Oswald
Published: October 2002

Rice fields using nitrogen‐based fertilizers play an important role in the global N2O budget. However, our knowledge is still limited with regard to the mechanisms affecting the N2O emission and to the measures that can reduce the emission. This paper reports a study of N2O emission from paddy soils. The effects of urea, hydroquinone (HQ, a urease inhibitor), and dicyandiamide (DCD, a nitrification inhibitor) have been studied in pot experiments with and without rice plants and with and without addition of wheat straw. With no wheat straw amendment, all treatments with inhibitors, especially with HQ + DCD, had a much smaller N2O emission during the rice growing period than the urea treatment, whereas a substantially increased N2O emission was observed from a rice‐free soil with inhibitors. The N2O emission from the rice‐planted soil was exponentially positive correlated with the NO3−‐N concentration in the rice aboveground biomass. By comparing the total N2O emission from the rice‐free soil and from the rice‐planted soil, we found that urea application alone might induce an apparent plant‐mediated N2O emission, being 0.39 ± 0.08% of the applied urea N. Wheat straw incorporated into the flooded surface layer soil could increase the plant‐mediated N2O emission significantly. However, application of HQ + DCD could reduce this emission (0.27 ± 0.08% of the applied urea N, compared with 0.89 ± 0.18% in the urea treatment). It also reduced the N2O emission from the rice‐free soil and from the rice‐planted soil. Stepwise regression analysis indicates that denitrification in the flooded surface layer soil was the main source of N2O emission from this wetland rice cultivation, particularly when wheat straw was added. A significantly nonlinear negative relation was found between the N2O emission and the CH4emission when no wheat straw was added, but it was hard to quantify this trade‐off relation when wheat straw was incorporated into the flooded surface layer soil.
Wet deposition of trace elements and radon daughter systematics in the South and equatorial Atlantic atmosphere
Authors: Kim Guebuem , Church Thomas M.
Published: October 2002

Atmospheric samples were collected aboard ship in the South and equatorial Atlantic (35°S–10°N) between 19 May and 20 June 1996. We measured 222Rn in air, 210Pb in aerosol, and trace elements (Fe, Mn, Zn, Pb, Cu, Cd, Ni, and Cr), 210Pb, and 210Po in precipitation samples. The large variation of 222Rn in air suggests a significant change in the incursion of continental air with time and latitude in the remote Atlantic. In the equatorial and subtropical Atlantic (20°S–10°N), 222Rn activity was lower but 210Pb/222Rn ratios were higher than those at higher latitudes. The higher 210Pb/222Rn ratios in the equatorial Atlantic appear to be due to prevailing trade easterly winds which transport a supported source of 210Pb in Saharan dust from the African Sahel. The enrichment of noncrustal trace elements in precipitation samples from the remote equatorial Atlantic was small on account of the remoteness from the continental emission regions and as a result of dilution with Saharan dust. The wet depositional fluxes of major crustal elements (Fe and Mn) were two‐ to three‐fold higher, while those of Cd and Zn were two‐ to ten‐fold lower, in the South and equatorial Atlantic relative to the western North Atlantic (Bermuda) or North Atlantic coast (Lewes, Delaware). Thus, dominant wet precipitation of Saharan dust in the Intertropical Convergence Zone (ITCZ) areas of the equatorial Atlantic appears to be a large potential source of micronutrients (i.e., Fe) to surface seawater.
High‐resolution Holocene N2O ice core record and its relationship with CH4 and CO2
Authors: Flückiger Jacqueline , Monnin Eric , Stauffer Bernhard , Schwander Jakob , Stocker Thomas F. , Chappellaz Jérôme , Raynaud Dominique , Barnola Jean‐Marc
Published: October 2002

Nitrous oxide (N2O) concentration records exist for the last 1000 years and for time periods of rapid climatic changes like the transition from the last glacial to today's interglacial and for one of the fast climate variations during the last ice age. Little is known, however, about possible N2O variations during the more stable climate of the present interglacial (Holocene) spanning the last 11 thousand years. Here we fill this gap with a high‐resolution N2O record measured along the European Project for Ice Coring in Antarctica (EPICA) Dome C Antarctic ice core. On the same ice we obtained high‐resolution methane and carbon dioxide records. This provides the unique opportunity to compare variations of the three most important greenhouse gases (after water vapor) without any uncertainty in their relative timing. The CO2 and CH4 records are in good agreement with previous measurements on other ice cores. The N2O concentration started to decrease in the early Holocene and reached minimum values around 8 ka (<260 ppbv) before a slow increase to its preindustrial concentration of ∼265 ppbv.
Rapid biologically mediated oxygen isotope exchange between water and phosphate
Authors: Paytan Adina , Kolodny Yehoshua , Neori Amir , Luz Boaz
Published: October 2002

In order to better constrain the rate of oxygen isotope exchange between water and phosphate via biochemical reactions a set of controlled experiments were conducted in 1988 at the Aquaculture Plant in Elat, Israel. Different species of algae and other organisms were grown in seawater tanks under controlled conditions, and the water temperature and oxygen isotopic composition (δ18Ow) were monitored. The oxygen isotopic composition of phosphate (δ18Op) in the organisms' food source, tissues, and the δ18Op of dissolved inorganic phosphate (DIP) were measured at different stages of the experiments. Results indicate that intracellular oxygen isotope exchange between phosphorus compounds and water is very rapid and occurs at all levels of the food chain. Through these reactions the soft tissue δ18Op values become 23–26‰ higher than δ18Ow, and δ18Op values of DIP become ∼20‰ higher than δ18Ow. No correlation between δ18Op values and either temperature or P concentrations in these experiments was observed. Our data imply that biogenic recycling and intracellular phosphorus turnover, which involves kinetic fractionation effects, are the major parameters controlling the δ18Op values of P compounds dissolved in aquatic systems. This information is fundamental to any application of δ18Op of dissolved organic or inorganic phosphate to quantify the dynamics of phosphorus cycling in aquatic systems.