bluuebunny/crossref_metadata_2025 huggingface dataset:
Laboratory method for investigating permeability upscaling
Authors: Tidwell Vincent C. , Wilson John L.
Published: February 2004

The purpose of this work is to describe, evaluate, and demonstrate a laboratory‐based method for physically investigating permeability upscaling. The method makes use of a simple instrument, the gas permeameter, to acquire rapid, precise, and nondestructive permeability measurements from heterogeneous blocks of dry rock. Critical to investigating permeability upscaling is the ability to acquire data at multiple sample supports subject to consistent boundary conditions and flow geometry. Such measurements, spanning almost 4 orders of magnitude on a per volume basis, are made with the gas permeameter by simply varying the size of the permeameter tip seal. The precision and consistency of measurements made in this way were evaluated using a suite of data collected from blocks of three relatively homogeneous materials: Berea Sandstone and two synthetic rocks. Results suggest that measurement error is small (approximately ±1% of the measured permeability) and consistent, and measurements made at different sample supports are free from systematic bias. To demonstrate the ability of this method to measure and quantify upscaling processes, limited data sets were collected with four different‐sized tip seals from the Berea Sandstone block. Analysis reveals distinct and consistent trends diagnostic of permeability upscaling relating the sample mean (increased), variance (decreased), and semivariogram to increasing sample support.
Relationship between fluvial bounding surfaces and the permeability correlation structure
Authors: Davis J. Matthew , Wilson John L. , Phillips Fred M. , Gotkowitz Madeline B.
Published: February 2004

Results of three meter‐scale outcrop studies suggest a quantifiable relationship between fluvial bounding surfaces and the permeability correlation structure. Mapping of bounding surfaces and in situ measurements of permeability were obtained in three types of deposits: a low‐energy channel deposit, a paleosol, and a high‐energy channel deposit. For the low‐energy channel deposit only second‐order surfaces separating lithofacies were mapped. The variogram exhibits anisotropic exponential behavior, and an analysis of variance (ANOVA) suggests that the within‐facies variation dominates over between‐facies variation. The paleosol contained no internal bounding surfaces and exhibited a “bellshaped” variogram consistent with a smoothly varying style of heterogeneity. Both firstand second‐order bounding surfaces were mapped in the high‐energy channel deposit. Results of the ANOVA suggest that the between‐facies variation dominates over the within‐facies variation. A model of spatial correlation using superposition of independent processes acting over different scales provides a good fit to the data. These three studies suggest that fluvial bounding surfaces provide a geological basis for conceptualizing and modeling heterogeneity in alluvial deposits.
The moments of the breakthrough curves of instantaneously and kinetically sorbing solutes in heterogeneous geologic media: Prediction and parameter inference from field measurements
Authors: Rubin Yoram , Cushey Mark A. , Wilson Amy
Published: February 2004

This paper presents a concise methodology for estimating the moments of the breakthrough curves for tracers and reactive solutes in heterogeneous aquifers. Under some conditions these are also the temporal or travel time moments between a source and a given destination downstream. The temporal moments of tracers as well as instantaneously or kinetically sorbing solutes, characterized by linear isotherms, are expressed in terms of a few parameters which characterize the chemical reactions and the spatial distribution and correlation structure of the hydraulic conductivity. The chemical reaction parameters are assumed to be homogeneous. The estimated moments can also be made conditional to field measurements. Applications for the case of uniform mean flow are presented, but the general approach can be applied for other flow regimes such as injection‐pumping well doublets. Physical and chemical nonequilibrium processes are represented by mobile‐immobile domains and two‐site models, respectively. The estimated temporal moments can be used for both predictive purposes as well as for interpretation of field experiments. These two objectives are pursued in this paper. A significant advantage of the solution is that it does not require the assumptions of Gaussianity or log Gaussianity of the travel times. Throughout the discussion the combined and relative effects of the mass transfer and kinetic parameters and the spatial variability of the conductivity on the travel time moments are evaluated.
Stochastic analysis of adsorbing solute transport in three‐dimensional, heterogeneous, unsaturated soils
Authors: Yang Jinzhong , Zhang Renduo , Wu Jinquan , Allen Myron B.
Published: February 2004

We analyze adsorbing solute transport in heterogeneous three‐dimensional unsaturated soils using a first‐order perturbation method. Both the unsaturated hydraulic conductivity and water content are treated as statistically anisotropic and stationary random space functions. The water flow is assumed to be steady state. Solute adsorption is described by a linear equilibrium isotherm with a spatially variable random adsorption coefficient. The macrodispersivity depends on the statistical properties of hydraulic and chemical parameters and on the distance traveled by the solute plume. The longitudinal macrodispersivity increases with the solute travel distance and approaches an asymptotic value at long travel distances. The transverse macrodispersivity increases with travel distance for short distances and then drops to zero. The effect of water content variability on macrodispersivity can be neglected only for a soil near saturation. Compared with effects of hydraulic gradient and suction head variabilities, the effects of soil conductivity parameters on macrodispersivity are dominant at high water content or long travel distance. Heterogeneous adsorption has a larger impact on longitudinal macrodispersivity than on the transverse one. Macrodispersivity increases as mean water content decreases and as mean retardation factor increases. The correlation between the adsorption coefficient and saturated hydraulic conductivity plays an important role in the determination of macrodispersivity.
A model for discharge in spring‐dominated streams and implications for the transmissivity and recharge of quaternary volcanics in the Oregon Cascades
Authors: Manga Michael
Published: February 2004

A model for discharge in spring‐fed streams is described and applied to streams in the Oregon Cascades. These streams are assumed to be fed by an unconfined aquifer composed predominantly of quaternary basalts and basaltic andesites. The model is based on the unsteady Boussinesq equation and is characterized by a single parameter, a normalized length scale of the aquifer. Flow in a runoff‐dominated stream is used as a proxy for the time‐dependent groundwater recharge. Four spring‐dominated streams with 50 years of daily discharge records are studied, and modeling efficiencies are in the range of 0.76–0.89. The effective transmissivity is found to be approximately proportional to the length scale of the aquifers. Groundwater recharge rates are in the range of 66–127 cm/yr and 40–73% of the mean annual precipitation.
The determinants of perceptions of fairness in the allocation of water to multiple uses
Authors: Syme Geoffrey J. , Nancarrow Blair E.
Published: February 2004

Five water allocation case studies were examined from four states in Australia. Issues varied from allocation between irrigators to that of 12 competing uses. The attitudinal and philosophical determinants of the fairness of the decision making for case studies separately and collectively were determined for a water‐literate community. Agreement with general philosophical attitudes, rated importance of philosophical principles in decision making in each case study, and attitudes towards localized issues could, in combination, clearly discriminate people's differential perceptions of the fairness. It was also found that there was consistency in the importance placed on philosophical principles across the case studies. These principles can be used as a basis for empirical community evaluation of the fairness of decision making in water allocation.
Locating groundwater discharge in large lakes using bottom sediment electrical conductivity mapping
Authors: Harvey F. Edwin , Lee David R. , Rudolph David L. , Frape Shaun K.
Published: February 2004

Groundwater‐surface water studies that use conventional near‐shore piezometers and /or seepage meters are impractical in larger, areal extensive lakes, as they require exorbitant numbers of instruments to quantify groundwater discharge zones. In smaller lakes an electrical conductivity mapping method has proven useful in mapping groundwater discharge zones. The technique identifies groundwater discharge by measuring variations in sediment pore water electrical conductivity and reduces the number of instruments necessary to quantify inflow, thereby lowering instrumentation costs and increasing a study's efficiency. This study sought to determine the technique's applicability in larger lakes. Thus the method was tested within the Hamilton Harbour, at the western end of Lake Ontario. This study found systematic variations between nearshore and offshore sediments and identified three anomalous zones that were thought to represent groundwater inflow. Onshore and offshore piezometers were used to verify the presence of upward gradients and elevated electrical conductivities. The sediment probe survey provided qualitative maps of areas of elevated electrical conductivity indicative of groundwater discharge and allowed a fairly extensive shoreline to be mapped quickly and economically. Survey results guided the installation of nearshore piezometers to discharge zones, eliminating the inefficiency of more conventional “hit or miss” point source installation approaches. This research demonstrated that the sediment probe was a valuable tool for studying groundwater inputs into large lakes.
Estimation of evapotranspiration at the landscape scale: A fuzzy disaggregation approach
Authors: Franks Stewart W. , Beven Keith J.
Published: February 2004

This paper presents a methodology for the representation of variability in land surface fluxes across a given domain. Landsat thematic mapper (TM) data are employed to estimate the pixel‐scale variability of the energy partition at the time of a TM overpass. Multiple realizations of the TOPUP soil vegetation‐atmosphere transfer (SVAT) model are run deductively to define a space of model responses over a longer period of time. This allows the models to be classified into different functional types. The uncertain pixel estimates are then used to condition or map the estimates of the local landscape fluxes into the space of model functional types using fuzzy measures. The identified fuzzy weights may then be employed to derive time series of the mean areal latent heat flux and quantiles to represent the range of the flux variability. This scheme has the advantages that the resulting model is simple enough to be used directly as a representation of a heterogeneous land surface in an atmospheric model and that the fuzzy weights may be updated with additional data.
Predicting interrill erodibility factor from measured infiltration rate
Authors: Ben‐Hur M. , Agassi M.
Published: February 2004

Direct measurement of an interrill erodibility factor (Ki) is costly and time intensive. As Ki and the final infiltration rate (FIR) under seal formation are both affected by aggregate breakdown at the soil surface, it was hypothesized that the Ki and FIR values are correlative. FIR and soil‐loss values of 53 soils, measured in several different laboratory rainfall simulators, were investigated. The slope factor (Sf) of smectitic soils was higher than that of nonsmectitic soils at slope angle (θ) > 9%. The equation Sf = exp (−0.68 + 8.28 sin θ) defines significantly (r2 = 0.94) the Sf for smectitic soil. For the various soils the FIR and Ki values were correlative and fitted significantly the Ki = a −b In(FIR) model; a and b are empirical coefficients. This model was found applicable for a wide range of rain intensities (34–68 mm h−1). However, an increase of the raindrop kinetic energy from <11.6 to 22.3 J mm−1 m−2 increased the absolute values of the coefficients, a and b. The FIR‐Ki model differed for smectitic and nonsmectitic soils; at a given FIR, the smectitic soils had a higher Ki value than the nonsmectitic soils.
Transient flow in bounded randomly heterogeneous domains: 1. Exact conditional moment equations and recursive approximations
Authors: Tartakovsky Daniel M. , Neuman Shlomo P.
Published: February 2004

We consider the effect of measuring randomly varying local hydraulic conductivities K(x) on one's ability to predict transient flow within bounded domains, driven by random sources, initial head, and boundary conditions. Our aim is to allow optimum unbiased prediction of local hydraulic heads h(x, t) and Darcy fluxes q(x, t) by means of their ensemble moments, 〈h(x, t)〉c and 〈q(x, t)〉c conditioned on measurements of K(x). We show that these predictors satisfy a compact deterministic flow equation which contains a space‐time integrodifferential “residual flux” term. This term renders 〈q(x, t)〉c nonlocal and non‐Darcian so that the concept of effective hydraulic conductivity looses meaning in all but a few special cases. Instead, the residual flux contains kernels that constitute nonlocal parameters in space‐time that are additionally conditional on hydraulic conductivity data and thus nonunique. The kernels include symmetric and nonsymmetric second‐rank tensors as well as vectors. We also develop nonlocal equations for second conditional moments of head and flux which constitute measures of predictive uncertainty. The nonlocal expressions cannot be evaluated directly without either a closure approximation or high‐resolution conditional Monte Carlo simulation. To render our theory workable, we develop recursive closure approximations for the moment equations through expansion in powers of a small parameter which represents the standard estimation error of natural log K(x). These approximations are valid to arbitrary order for either mildly heterogeneous or well‐conditioned strongly heterogeneous media. They allow, in principle, evaluating the conditional moments numerically on relatively coarse grids, without upscaling, by standard methods such as finite elements.
A diffusion‐based interpretation of tetrachloroethene and trichloroethene concentration profiles in a groundwater aquitard
Authors: Ball William P. , Liu Chongxuan , Xia Guoshou , Young Dirk F.
Published: February 2004

Analysis of subsurface soil cores from the site of a field‐scale groundwater remediation experiment at Dover Air Force Base, Delaware, has revealed that tetrachloroethene (PCE) and trichloroethene (TCE) contamination extends into an aquitard underlying a groundwater aquifer. The site location is well downgradient of the locations of contaminant release, and the aquitard contamination is believed to have begun when contaminated groundwater first arrived in the overlying aquifer. Using independent estimates of sorption and diffusion properties in the aquitard layers, mathematical modeling based on diffusion in laminate slabs has been used to make inferences regarding the historical concentration conditions in the overlying aquifer. The results suggest that plume arrival occurred within the last two decades, with some important differences in the inferred TCE and PCE plume histories. The diffusion model was also applied toward predicting future aquitard concentrations and fluxes under scenarios based on the current condition as a starting point and hypothesized conditions of future groundwater cleanup. The results demonstrate how aquitard sampling and diffusion modeling can provide essential information relevant to forensic analysis, risk assessment, and subsurface cleanup.
Groundwater head sampling based on stochastic analysis
Authors: Ahn Hosung , Salas Jose D.
Published: February 2004

The problem of determining a uniform sampling time interval for monitoring serially correlated groundwater heads is the main subject discussed here. The problem is approached by stochastic time series analysis and modeling. An autoregressive integrated moving average model is assumed to fit the underlying series. Given that groundwater head data can be sampled at different time intervals and that the same stochastic model must represent the time series regardless of the sampling timescale, the parameters of the underlying model for the series sampled at a given arbitrary time interval h are obtained as a function of h and as a function of the model parameters for the series sampled at a unit time interval. This is accomplished by linking the derived variances and autocovariances at the two sampling scales. The derived equations and the sampling design procedure are tested and illustrated using the groundwater head data of Collier County, Florida.
Measurement of porous media component content and heterogeneity using gamma ray tomography
Authors: Hsieh H. T. , Brown Glenn O. , Stone Marvin L. , Lucero Dan A.
Published: February 2004

Tomographic images of porous media are complex distributions of linear attenuation coefficients that reflect the combined effects of scanning spatial resolution, photon statistical measurement errors, and true material densities. We address how the true voxel‐scale attenuation distribution and measurement errors are convoluted to yield measured density frequency distributions. A deconvolution algorithm is demonstrated that uses the measured density frequency distributions and known photon statistical errors to quantify average cross‐section volume contents of pure components and a mixed‐component phase. The mixed‐component phase represents regions where components are intertwined or varied in spaces smaller than the scanning resolution. This approach is applied to a complex core of the Culebra Dolomite Member of the Rustler Formation collected at the Waste Isolation Pilot Plant, near Carlsbad, New Mexico. The methodology provides a quantitative measure of the volume content of gypsum, dolomite, and mixed‐components, and heterogeneity in the sample.
Stochastic forcing of sediment routing and storage in channel networks
Authors: Benda Lee , Dunne Thomas
Published: February 2004

The stochastic field of sediment supply to the channel network of a drainage basin depends on the large‐scale interactions among climatically driven processes such as forest fire and rainstorms, topography, channel network topology, and basin scale. During infrequent periods of intense erosion, large volumes of colluvium are concentrated in parts of a channel network, particularly near tributary junctions. The rivers carry bed material and wash load downstream from these storage sites at different rates. The bed material travels slowly, creating transient patterns of sediment transport, sediment storage, and channel morphology along the channel network. As the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion, interference at tributary junctions, and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion, which converts bed material to wash load. We investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215‐km2 drainage basin within the Oregon Coast Range over a simulated 3000‐year period with a climate typical of the late Holocene. We used field measurements and a simulation model to illustrate interactions between the major controls on large‐scale processes functioning over long periods of time in complex drainage basins.
Stochastic forcing of sediment supply to channel networks from landsliding and debris flow
Authors: Benda Lee , Dunne Thomas
Published: February 2004

Sediment influx to channel networks is stochastically driven by rainstorms and other perturbations, which are discrete in time and space and which occur on a landscape with its own spatial variability in topography, colluvium properties, and state of recovery from previous disturbances. The resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin. The regime varies systematically with basin area. We describe how the stochastic sediment supply is generated by climatic, topographic, geotechnical, and biotic controls that vary between regions. The general principle is illustrated through application to a landscape where sediment is supplied by mass wasting, and the forcing variables are deterministic thickening of colluvium, random sequences of root‐destroying wildfires, and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength. Landslides stop in channels or convert to scouring debris flows, depending on the nature of the low‐order channel network. Sediment accumulates within these channels for centuries before being transferred downstream by debris flows. Time series of sediment supply, transport, and storage vary with basin scale for any combination of climatic, topographic, and geotechnical controls. In a companion paper [Benda and Dunne, this issue] we use simulations of timing, volumes, and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks.
Experimental investigations for trapping oxygen gas in saturated porous media for in situ bioremediation
Authors: Fry V. A. , Selker J. S. , Gorelick S. M.
Published: February 2004

Oxygen is often the rate‐limiting factor in aerobic in situ bioremediation. This paper investigates the degree to which air or oxygen gas can be emplaced into the pore space of saturated porous media and provide a significant mass of oxygen. Column experiments were performed to test three emplacement methods: direct gas injection, injection of water supersaturated with gas, and injection of a hydrogen peroxide solution. The direct gas injection method fills 14–17% of the pore space with trapped gas. Water supersaturated with gas fills 18–27% of the pore space with a trapped gas phase, and hydrogen peroxide solution injections emplaces trapped gas in 17–55% of the pore space. In addition to supplying oxygen, gas entrapment causes a decrease in hydraulic conductivity which could be an advantage by decreasing the flow of contaminants offsite. The relative hydraulic conductivity of porous media with a trapped gas volume of 14–55% was 0.62–0.05.
Optimal estimation of residual non–aqueous phase liquid saturations using partitioning tracer concentration data
Authors: James Andrew I. , Graham Wendy D. , Hatfield Kirk , Rao P. S. C. , Annable Mike D.
Published: February 2004

Stochastic methods are applied to the analysis of partitioning and nonpartitioning tracer breakthrough data to obtain optimal estimates of the spatial distribution of subsurface residual non–aqueous phase liquid (NAPL). Uncertainty in the transport of the partitioning tracer is assumed to result from small‐scale spatial variations in a steady state velocity field as well as spatial variations in NAPL saturation. In contrast, uncertainty in the transport of the nonpartitioning tracer is assumed to be due solely to the velocity variations. Partial differential equations for the covariances and cross cpvariances between the partitioning tracer temporal moments, nonpartitioning tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields are derived assuming steady flow in an infinite domain [Gelhar, 1993] and the advection‐dispersion equation for temporal moment transport [Harvey and Gorelick, 1995]. These equations are solved using a finite difference technique. The resulting covariance matrices are incorporated into a conditioning algorithm which provides optimal estimates of the tracer temporal moments, residual NAPL saturation, pore water velocity, and hydraulic conductivity fields given available measurements of any of these random fields. The algorithm was tested on a synthetically generated data set, patterned after the partitioning tracer test conducted at Hill AFB by Annable et al. [1997]. Results show that the algorithm successfully estimates major features of the random NAPL distribution. The performance of the algorithm, as indicated by analysis of the “true” estimation errors, is consistent with the theoretical estimation errors predicted by the conditioning algorithm.
Higher‐order corrections to the flow velocity covariance tensor, revisited
Authors: Deng F.‐W. , Cushman J. H.
Published: February 2004

Recently the authors presented an accurate (in the variance of fluctuating log conductivity), second‐order solution to the steady, saturated flow problem. The longitudinal, second spatial moment was evaluated numerically on the basis of the perturbation solution, and it was found that second‐order corrections to the head field had little effect on this moment. Here we use the same theory and evaluate the second, transverse spatial moment. We find, in agreement with others, that our theory suggests that the second‐order head corrections, as well as the nonlinear velocity corrections, are important.
Air entrapment effects on infiltration rate and flow instability
Authors: Wang Zhi , Feyen Jan , van Genuchten Martinus Th. , Nielsen Donald R.
Published: February 2004

Experiments were conducted to quantify the effects of entrapped air on water infiltration into a loamy sand. Transparent three‐dimensional (3‐D) and 2‐D columns were used for experiments carried out for two infiltration conditions: (1) when air was free to move ahead of the wetting front and leave the bottom of the column (air draining) and (2) when air was confined ahead of the wetting front and hence could escape only through the soil surface (air confining). The measurement setup was composed of a tension‐pressure infiltrometer, an air flowmeter, water manometers, and video‐picture cameras. We applied both positive and negative water pressures at the soil surface and measured the simultaneous changes in the rates of water inflow and air outflow, the air pressure ahead of the wetting front, and the dynamic behavior and advance of the wetting front. The air pressure ahead of the wetting front for the air‐confining condition was generally found to increase with time rather than reaching a constant level, as observed in other studies by other researchers. The air pressure fluctuated locally because of air escaping from the soil surface. On the basis of an analysis of the results we present two empirical equations to predict the maximum air pressure at which air begins to erupt from the soil surface and to predict the minimum air pressure at which air eruption stops. We found that the infiltration rate was always equal to, and controlled by, the rate of air outflow. The infiltration rate varied inversely with the air pressure ahead of the wetting front and with the ponding depth at the soil surface. The infiltration rate fluctuated with time rather than undergoing changes in a three‐stage process, as is often characterized in the literature. The volume of residual entrapped air in the air‐confining condition increased 7% on average, and the infiltration rate decreased threefold to tenfold as compared to the air‐draining condition. Finally, it was shown that the air‐confining infiltration flow is fingered and unstable, consistent with the predictions of an existing theory.
Evaluation of chloride and pesticide transport in a fractured clayey till using large undisturbed columns and numerical modeling
Authors: Jørgensen Peter R. , McKay Larry D. , Spliid Niels H.
Published: February 2004

Saturated groundwater flow and tracer experiments using fluorescent dye, chloride, and the herbicides mecoprop and simazine were carried out in the laboratory using three large‐diameter (0.5 m) undisturbed columns of fractured clayey till. Hydraulic conductivity of the columns ranged from 10−5 m/s in the shallowest column (1 m dept)) to 10−7 m/s in the deepest column (4 m depth) and were similar to field‐measured values for these deposits. Results of the tracer experiments are consistent with a conceptual model of advective transport along the fractures combined with diffusion into the fine‐grained matrix between the fractures. Arrival of the chloride tracer in the effluent was highly retarded relative to fracture flow velocities calculated on the basis of the cubic law and measured values of fracture spacing and hydraulic conductivity. The herbicides were more strongly retarded than the chloride at low flow rates, but at higher flow rates the herbicides arrived with the chloride, indicating the influence of nonequilibrium sorption of the herbicides to fracture walls and the matrix solids. The columns were dismantled following the tracer experiments and mapping under UV light showed that nearly all of the visible, weathered fractures (and the few root holes in the case of the shallowest sample) were active transport pathways, with the dye appearing mainly on the fracture surfaces and as a “rim” in the adjacent matrix. Concentration profiles measured perpendicular to the fracture surfaces showed that the herbicides had also moved into the matrix, apparently by diffusion. Simulations of solute transport with a discrete fracture flow/matrix diffusion model showed that the simulations could be “fit” to the data if all of the visible fractures were hydraulically active, but could not be fit if all or most of the flow was channelled through just the primary fractures (defined by prominent oxidation stains). Simulations with an equivalent porous media (EPM) model could not fit the data using the measured total porosity as the effective porosity. The simulations could likely be fit with a smaller value of effective porosity, but this would limit applicability to field situations because fitted effective porosity is expected to change with physical scale and residence time of the solute in the soil.
A neural network approach to multiobjective optimization for water quality management in a river basin
Authors: Wen Ching‐Gung , Lee Chih‐Sheng
Published: February 2004

A new neural network‐based multiobjective optimization of water quality management for water pollution control and river basin planning is presented. Past research on water quality management problems has shown that traditional multiobjective decision making does not provide an adequate solution since it depends directly on the decision maker's (DM's) preferences, which may not be clearly defined. In order to overcome the DM's preferences problem this study uses a neural network algorithm to form a model for the solution of the multiobjective problems of water quality management in a river basin. Using the backpropagation algorithm of feedforward neural networks, a multiobjective programming model can simulate the DM's preferences, providing direct help to the analysts involved in real applications. Before describing the details of the multiobjective optimization problem, neural networks are proposed which can be used to predict the DM's preference structures. To demonstrate the procedures and the performance of the neural network‐based approach, the case of the Tou‐Chen River Basin in Taiwan is selected for analysis and discussion.
Characterizing the heterogeneity and correlation of perchloroethene sorption and hydraulic conductivity using a facies‐based approach
Authors: Allen‐King Richelle M. , Halket Richard M. , Gaylord David R. , Robin Michel J. L.
Published: February 2004

A facies‐based approach was used to delineate both geochemical and physical aquifer property heterogeneity. Specifically, the parameters considered were the sorption coefficient (Kd) for a model organic contaminant and hydraulic conductivity (K) inferred from the particle size distribution. Subfacies‐sized samples were analyzed for both tetrachloroethene Kd and K in a vertical core of the Borden aquifer. Both parameters were correlated with the six sedimentary facies identified within the vertical core. As a consequence of their relations to sedimentary facies, significant positive correlations between ln K and ln Kd which were different from the core‐scale correlation and different from each other were identified for some facies. Fades‐specific correlations generally resulted in improved statistics (i.e., higher correlation coefficients and reduced confidence intervals) compared to the core‐scale correlation. The results suggest that a facies‐based approach can be useful in characterizing the heterogeneity of the transport properties of sedimentary aquifers.
Thermal and hydrologic dynamics of the active layer at a continuous permafrost site (Taymyr Peninsula, Siberia)
Authors: Boike Julia , Roth Kurt , Overduin Pier Paul
Published: February 2004

The ground thermal and hydrologic regimes of a site located in the continuous permafrost landscape of Taymyr Peninsula, northern Siberia, were studied in 1994 and 1995. The aim was to quantify the seasonal fluxes of water and heat in the active layer from spring thaw to fall freeze‐back. Liquid water content was measured in frozen and unfrozen soils using time domain reflectometry. Liquid water was present in frozen soil at temperatures down to −12°C, and its volumetric fraction increased with temperature before melting occurred. The ground thermal regime during spring thaw and fall freeze‐back was dominated by latent heat fluxes that stabilized soil temperatures at 0°C for extended periods. The transfer of large amounts of latent heat released by freeze‐back in the fall suggested convective heat transfer mechanisms. Seasonal fluxes of heat and water were well described using a simple zero‐dimensional model of water and energy balance. The dominant heat sinks during the spring and summer were sensible and latent heat fluxes into the atmosphere. During fall freeze‐back the dominant heat source was phase change. The soil heterogeneity strongly impacted hydrologic and thermal processes in the active layer. Two direct consequences were the development of preferential flowpaths and the preferential freezing of the profile during freeze‐back.
Analytical solutions to hillslope subsurface storm flow and saturation overland flow
Authors: Fan Ying , Bras Rafael L.
Published: February 2004

We present the analytical solutions to a hillslope‐based formulation of subsurface storm flow and saturation overland flow. Here a hillslope is defined as the area between two streamlines originating from two ends of a channel link or from the channel head and ending at the ridgelines. The three‐dimensional soil mantle of a hillslope is reduced to a one‐dimensional profile along which the soil moisture and discharge are modeled. Continuity and a kinematic form of Darcy's law lead to quasi‐linear wave equations solvable with the method of characteristics. Examples are given to illustrate the ability and limitations of the solutions in capturing the essential runoff behavior of different slope types.
simulating carbon dynamics along the Boreal Forest Transect Case Study (BFTCS) in central Canada: 2. Sensitivity to climate change
Authors: Peng Changhui , Apps Michael J.
Published: February 2004

The effects of climate change and doubling atmospheric CO2on carbon dynamics of the boreal forest in the area of the Boreal Forest Transect Case Study in central Canada were investigated using the process‐based plant‐soil model CENTURY 4.0. The results presented here suggest that (1) across the transect climate change would result in increased total carbon in vegetation biomass but decreased overall carbon in soil; (2) increased atmospheric CO2concentration under current climatic patterns would result in increased total carbon in vegetation and in soil organic matter; and (3) combined climate change and elevated CO2would increase both net primary productivity and decomposition rates relative to the current climate condition, but their combined action would be a reduction of soil carbon losses relative to those due to climate change alone. The interactive effects of climate change and elevated CO2, however, are not a simple additive combination of the individual responses. The responses to climate change and elevated CO2vary across the climate gradient from southern to northern sites on the transect. The present simulations indicate that the northern sites are more sensitive to climate change than the southern sites are, but these simulations do not consider likely changes in the disturbance regime or changes in forest species distribution.
Terrestrial sedimentation and the carbon cycle: Coupling weathering and erosion to carbon burial
Authors: Stallard Robert F.
Published: February 2004

This paper examines the linkages between the carbon cycle and sedimentary processes on land. Available data suggest that sedimentation on land can bury vast quantities of organic carbon, roughly 1015 g C yr−1. To evaluate the relative roles of various classes of processes in the burial of carbon on land, terrestrial sedimentation was modeled as a series of 864 scenarios. Each scenario represents a unique choice of intensities for seven classes of processes and two different global wetland distributions. Comparison was made with presumed preagricultural conditions. The classes of processes were divided into two major component parts: clastic sedimentation of soil‐derived carbon and organic sedimentation of autochthonous carbon. For clastic sedimentation, masses of sediment were considered for burial as reservoir sediment, lake sediment, and combined colluvium, alluvium, and aeolian deposits. When the ensemble of models is examined, the human‐induced burial of 0.6‐1.5·1015 g yr−1 of carbon on land is entirely plausible. This sink reaches its maximum strength between 30° and 50° N. Paddy lands stand out as a type of land use that warrants future study, but the many faces of rice agriculture limit generalization. In an extreme scenario, paddy lands alone could be made to bury about 1·1015 g C yr−1. Arguing that terrestrial sedimentation processes could be much of the sink for the so called “missing carbon” is reasonable. Such a hypothesis, however, requires major redesign of how the carbon cycle is modeled. Unlike ecosystem processes that are amenable to satellite monitoring and parallel modeling, many aspects of terrestrial sedimentation are hidden from space.
Seasonal dynamics and biological productivity in the Arabian Sea Euphotic Zone as simulated by a three‐dimensional ecosystem model
Authors: Ryabchenko V. A. , Gorchakov V. A. , Fasham M. J. R.
Published: February 2004

A nitrogen‐based, seven‐component ecosystem model of Fasham [1993] type has been coupled to a three‐dimensional quasi‐geostrophic ocean general circulation model and applied to a simulation of the seasonal variability of physical and ecosystem variables in the northwestern Indian Ocean. The comparison of the solution with available data (seasonal changes in surface chlorophyll at certain key positions from the climatological coastal zone color scanner (CZCS) database, satellite‐derived annual primary production, and data from cruises) has highlighted many agreements but also some disagreements between the model and observations. In particular, the model produces an entrainment bloom in May‐June in the central Arabian Sea for which there is no evidence in the CZCS data and does not reproduce the observed August‐September bloom in the northern Arabian Sea. In the latter case an analysis showed that zooplankton grazing control prevents the development of the model bloom. A comparison of our solution with the results obtained by McCreary et al. [1996] using a 2.5‐layer physical‐ecosystem model showed that, despite numerous differences between the models, the period and location for most blooms in these solutions coincide closely. The model reproduced the differences in primary productivity between the northeast and southwest monsoon periods and showed that the model phytoplankton in the main upwelling areas were only seriously nutrient limited during the spring intermonsoon period. For large parts of the year the model predicts that phytoplankton production is closely coupled to zooplankton grazing with blooms only occurring when there are rapid changes in phytoplankton growth rate due either to the entrainment of nitrate into the mixed layer or decreased light limitation when the mixed layer is shallowing thereby allowing the phytoplankton to escape from grazing control. The predicted particle flux from the euphotic zone was of the order of 40% of the primary production and lagged the latter quantity by 8‐10 days. This time lag meant that the correlation between daily values of the two quantities was low, and this may partly explain the difficulties of finding good correlations between observations of these two quantities.
Phosphorus accumulation in marine sediments and the oceanic phosphorus cycle
Authors: Delaney M. L.
Published: February 2004

Ideas about key factors in the oceanic mass balance of dissolved, reactive phosphate have changed substantially. I present an integrated overview of these here, with an emphasis on evaluating the burial sinks for P and defining areas needing further research. The major source of reactive P to the ocean is river input. Reactive P is delivered to the oceanic sediment‐water interface primarily in particulate organic matter. P scavenged by hydrothermal iron‐rich oxyhydroxide particles, with uptake in proportion to deep water phosphate concentrations, represents a substantially smaller flux to the sediment‐water interface. Diagenetic transformations are important influences on the form of reactive P burial in marine sediments. P burial occurs with organic carbon burial and as P associated with iron‐rich oxyhydroxide particles and coatings. Formation of authigenic P‐rich phases, presumably apatite, at the expense of organic P and oxide‐associated P, is significant in open ocean marine sediments. The authigenic P sink may represent a substantially larger portion of the sedimentary burial than indicated by previous estimates focused on P burial in organic‐rich continental margin sediments.
Importance of suspended participates in riverine delivery of bioavailable nitrogen to coastal zones
Authors: Mayer Lawrence M. , Keil Richard G. , Macko Stephen A. , Joye Samantha B. , Ruttenberg Kathleen C. , Aller Robert C.
Published: February 2004

Total nitrogen (TN) loadings in riverine sediments and their coastal depocenters were compared for 11 river systems worldwide to assess the potential impact of riverine particulates on coastal nitrogen budgets. Strong relationships between sediment specific surface area and TN allow these impacts to be estimated without the intense sampling normally required to achieve such budgets. About half of the systems showed higher nitrogen loadings in the riverine sediments than those from the coastal depocenter. In spite of uncertainties, these comparisons indicate that large, turbid rivers, such as the Amazon, Huanghe, and the Mississippi, deliver sediments that in turn release significant or major fractions of the total riverine nitrogen delivery. Riverine particulates must therefore be considered an essential factor in watershed nutrient loading to coastal ecosystems and may affect delivered nutrient ratios as well as total nutrient loading. The relative importance of particulate versus dissolved delivery has decreased over recent decades in the Mississippi as a result of damming and fertilizer use in the watershed.
Comparison of nitrogen monoxide emissions from several African tropical ecosystems and influence of season and fire
Authors: Serça D. , Delmas R. , Le Roux X. , Parsons D. A. B. , Scholes M. C. , Abbadie L. , Lensi R. , Ronce O. , Labroue L.
Published: February 2004

NO emission rates from soils were measured for twelve major African ecosystems in four countries (Congo, Niger, Ivory Coast, and South Africa) and within four major phytogeographic domains: the Guineo‐Congolese, Guinean, Sahelian, and Zambezian domains. Measurements were performed during wet and/or dry seasons. All the measurements were made with the same dynamic chamber device, which allowed true comparisons to be made. This study showed that emission rates strongly differed between ecosystems and exhibited a marked temporal variability. Ecosystem effect was highly significant during both the dry and wet seasons. Emission rates were low (<0.6 ng NO‐N m−2s−1) inHyparrheniaandLoudetiasavannas of the Guinean or Guineo‐Congolese domains. Intermediate NO fluxes were obtained in rain forest and gallery forest ecosystems, in a broad‐leafed savanna and in a seasonally wetted grassland (sandy soil) of the Zambezian domain, and in a dry fallow savanna of the Sahelian domain. Emission rates were maximum (>7 ng NO‐N m−2s−1) in a seasonally wetted grassland (site 2) and in particular sites subjected to various disturbances, for example soil fauna activity (termite mounds) or past human disturbance (Acaciapatches‐settlement site). Microbial activity potentials (i.e., carbon mineralization, nitrification, denitrification, and total net N mineralization) were determined for most of the soils where NO fluxes were measured. In some sites, these potential activities were useful to identify the major processes controlling NO emission rates. Denitrification potential was very low and could not explain substantial NO fluxes from broad‐ and fine‐leafed savannas andHyperiheliasavannas of the Zambezian domain. Very low potentials of both nitrification and denitrification could be related to the low NO fluxes for the three Guinean savanna sites studied. NO fluxes were significantly higher during the wet season than the dry season in both savanna and forest ecosystems. Emission rates in savanna ecosystems were significantly increased within a few hours after fire. The measurements presented here provide a unique, consistent database which can be used to further analyze the processes involved in the spatial and temporal variations of NO emissions.
Carbon dioxide fluxes in a northern fen during a hot, dry summer
Authors: Schreader Cheryl P. , Rouse Wayne R. , Griffis Timothy J. , Boudreau L. Dale , Blanken P. D.
Published: February 2004

Atmospheric gradient techniques were used to measure the net ecosystem exchange of CO2 for a subarctic sedge fen near Churchill, Manitoba, during the summer of 1994. This was the second driest and wannest summer since 1943. The mean daily temperature was 2°C above average, and the rainfall was 55% below normal. More than half of the rain fell after the main growth period. The fen landscape comprises hummocks and hollows. Equilibrium retention storage occurs at an average standing water depth of 80 mm above the hollow bottoms (water table reference is O). During the summer of 1994 the average water table position at −117 mm resided well below the zero equilibrium retention depth. Periodically this decreased to −265 mm, well below a 30‐year average depth of −70 mm. During the full summer period, measurements indicate that the fen was a source of CO2. Only during a relatively short period of most active photosynthesis in midseason was there a small net CO2 uptake. A deep and warm soil aerobic layer promoted a large respiration flux, and this exceeded the photosynthetic CO2 uptake of the stressed sedge community. Diurnally, changes in surface temperature and incident solar radiation can explain most of the changes in the net CO2 exchange. It is hypothesized that in 1994 photosynthesis was significantly decreased and the respiration loss enhanced by the hot, dry conditions. If this hypothesis is correct, by analogy, climate warming would need to be accompanied by a substantial rainfall increase to maintain a condition of net CO2 gain to this peatland.