bluuebunny/crossref_metadata_2025 huggingface dataset:
Constrained state formulation for the stochastic control of multireservoir systems
Authors: Fletcher S. G. , Ponnambalam K.
Published: February 2004

A new formulation is presented for the analysis of multireservoir systems, based on the development of first and second moment expressions for the stochastic storage state variables. These expressions give explicit consideration to the maximum and minimum storage bounds in the reservoir system, a feature not incorporated in some existing formulations based on traditional control theory. Using this analysis, expected values of the storage states, variances of storage, release policies, reliability levels, and failure probabilities (useful information in the context of reservoir operations and design) can be obtained from a nonlinear programming solution. The approach does not involve any discretization of the system variables. The results for the means and standard deviations of the storage states for a multireservoir system compare favorably with those obtained from simulation.
Effect of landscape form on export of dissolved organic carbon, iron, and phosphorus from forested stream catchments
Authors: Dillon P. J. , Molot L. A.
Published: February 2004

Predicting the effects of climate change and atmospheric deposition on water quality requires predicting the effects of landscape form on export of substances downstream. In this paper, we present dissolved organic carbon (DOC), total phosphorus (TP), and iron (Fe) export data (1980–1992) for 20 relatively undisturbed, forested catchments draining into seven lakes in central Ontario and develop regression models of chemical export as functions of landscape composition. The extent of wetlands was correlated with export of DOC and TP; the proportion of the catchment covered by peatlands accounted for 78% of the variance in a regression model of long‐term average DOC export and 76% of the variance in a model of color “export.” Peatland coverage and Fe export together explained 76% of the variance in a long‐term average TP export model, which is consistent with published experimental evidence that Fe facilitates P complexation with DOC in surface waters. The long‐term average Fe export model was not significant when all 20 catchments were included. However, Fe export from the 14 catchments with thin tills was a function only of peatland coverage (R2 = 0.71), suggesting that Fe export is dependent to a large extent upon either the export of organic material or the reducing conditions. The long‐term export models worked well when export of a substance was dominated by peatlands but was not very sensitive to the influence of mineral soils. The long‐term average TP/DOC ratio was remarkably constant among most whole‐lake catchments, ranging from 1.4 to 2.0 mg P/g C, the exception being the catchment of anthropogenically acidified Plastic Lake with a ratio of 0.8. Fe export to Plastic lake was also enriched relative to TP export compared with the other lakes. Therefore TP export to Plastic Lake may be limited by some mechanism related to acidification.
Linear and Monte Carlo uncertainty analysis for subsurface contaminant transport simulation
Authors: James April L. , Oldenburg Curtis M.
Published: February 2004

Transport simulation can be used to predict subsurface contaminant distributions and the effectiveness of site remediation technologies. Because of the uncertainty inherent in subsurface transport prediction, an integral part of predictive modeling is uncertainty analysis. We have investigated the uncertainty of simulated trichloroethylene (TCE) concentrations due to parameter uncertainty and variation in conceptual model. The transport simulation is performed with T2VOC, a three‐dimensional integral finite difference code for three‐phase (gas, aqueous, non‐aqueous phase liquid), three‐component (air, water, volatile organic compound), nonisothermal subsurface flow. Our modeling is based on an actual site and considers a three‐dimensional system with a thick vadose zone (25 m) into which 35,000 kg of TCE was disposed of in surface trenches over 10 years. Subsurface transport involves TCE moving in the vadose and saturated zones from the source trench toward a nearby residence and includes the processes of advection, diffusion, and adsorption over extended distances and timescales. Uncertainties in the calculated concentrations due to variance in the major transport parameters are quantified using the inverse modeling code ITOUGH2. ITOUGH2 calculates uncertainty in the T2VOC simulation by both first‐order, second‐moment (FOSM) and Monte Carlo analyses. Significant uncertainty in simulated TCE concentrations at a site of potential human exposure is observed owing to uncertainty in permeability, porosity, diffusivity, chemical solubility, and adsorption within a single conceptual model. For the case study considered the linear FOSM analysis generally captures the uncertainty range calculated by the more accurate Monte Carlo method. Calculations also show that significant output uncertainty is introduced by conceptual model variation. Because human exposure and health risk depend strongly on contaminant concentrations, risk assessment and remediation selection based on transport simulation is meaningful only if the analysis includes quantitative estimates of transport simulation uncertainty.
Fractional Brownian motion and fractional Gaussian noise in subsurface hydrology: A review, presentation of fundamental properties, and extensions
Authors: Molz F. J. , Liu H. H. , Szulga J.
Published: February 2004

Recent studies have shown that fractional Brownian motion (fBm) and fractional Gaussian noise (fGn) are useful in characterizing subsurface heterogeneities in addition to geophysical time series. Although these studies have led to a fairly good understanding of some aspects of fBm/fGn, a comprehensive introduction to these stochastic, fractal functions is still lacking in the subsurface hydrology literature. In this paper, efforts have been made to define fBm/fGn and present a development of their mathematical properties in a direct yet rigorous manner. Use of the spectral representation theorem allows one to derive spectral representations for fBm/fGn even though these functions do not have classical Fourier transforms. The discrete and truncated forms of these representations have served as a basis for synthetic generation of fBm/fGn. The discrete spectral representations are developed and various implications discussed. In particular, it is shown that a discrete form of the fBm spectral representation is equivalent to the well known Weierstrass‐Mandelbrot random fractal function. Although the full implications are beyond the scope of the present paper, it is observed that discrete spectral representations of fBm constitute stationary processes even though fBm is nonstationary. A new and general spectral density function is introduced for construction of complicated, anisotropic, (3‐D) fractals, including those characterized by vertical fGn and horizontal fBm. Such fractals are useful for modeling anisotropic subsurface heterogeneities but cannot be generated with existing schemes. Finally, some basic properties of fractional Lévy motion and concepts of universal multifractals, which can be considered as generalizations of fBm/fGn, are reviewed briefly.
A generalized power function for the subsurface transmissivity profile in TOPMODEL
Authors: Duan Jinfan , Miller Norman L.
Published: February 2004

TOPMODEL, a simple physically based conceptual model, has been widely applied to various water resource investigations. Previous studies of the hydraulic transmissivity function with depth indicate that an exponential function alone may not be appropriate. Recent work suggests that a first‐order hyperbolic function can provide improved simulations of base flow recession in the context of TOPMODEL. We present a generalized power function for a range of possible cases and apply this function to the TOPMODEL concepts. The specific function that is most applicable is location dependent.
An analysis of the effects of spatial variability of soil and soil moisture on runoff
Authors: Merz Bruno , Plate Erich J.
Published: February 2004

Hydrological variables and processes usually exhibit a large spatial variability. Often this variability includes aspects of organization and randomness. Because any hydrological modeling has to deal with the question of spatial variability, methods that quantify the effects of spatial variability are valuable. Moreover, it is important to identify the situations where the spatial variability can be reduced (e.g., by using an “effective” value). For a small and well‐instrumented catchment in a loess area in southwest Germany effects of spatial variability of the initial soil moisture and soil hydraulic properties on the runoff are investigated. The analysis is performed with a process‐oriented rainfall runoff model. It is shown that organization in spatial patterns of soil moisture and soil properties may have a dominant influence on the catchment runoff. The simulations suggest that spatial variability can result in a complex, event dependent, behavior. It cannot be expected that a model with inputs based on mean parameters or mean initial conditions leads to mean outputs for heterogeneous fields. The analysis of different events shows the changing influence of spatial variability on the runoff with changing storm size. For very small and for large events spatial variability plays a negligible role. A large influence is found for medium‐sized events.
Hydrological and chemical estimates of the water balance of a closed‐basin lake in north central Minnesota
Authors: LaBaugh James W. , Winter Thomas C. , Rosenberry Donald O. , Schuster Paul F. , Reddy Michael M. , Aiken George R.
Published: February 2004

Chemical mass balances for sodium, magnesium, chloride, dissolved organic carbon, and oxygen 18 were used to estimate groundwater seepage to and from Williams Lake, Minnesota, over a 15‐month period, from April 1991 through June 1992. Groundwater seepage to the lake and seepage from the lake to groundwater were determined independently using a flow net approach using data from water table wells installed as part of the study. Hydrogeological analysis indicated groundwater seepage to the lake accounted for 74% of annual water input to the lake; the remainder came from atmospheric precipitation, as determined from a gage in the watershed and from nearby National Weather Service gages. Seepage from the lake accounted for 69% of annual water losses from the lake; the remainder was removed by evaporation, as determined by the energy budget method. Calculated annual water loss exceeded calculated annual water gain, and this imbalance was double the value of the independently measured decrease in lake volume. Seepage to the lake determined from oxygen 18 was larger (79% of annual water input) than that determined from the flow net approach and made the difference between calculated annual water gain and loss consistent with the independently measured decrease in lake volume. Although the net difference between volume of seepage to the lake and volume of seepage from the lake was 1% of average lake volume, movement of water into and out of the lake by seepage represented an annual exchange of groundwater with the lake equal to 26–27% of lake volume. Estimates of seepage to the lake from sodium, magnesium, chloride, and dissolved organic carbon did not agree with the values determined from flow net approach or oxygen 18. These results indicated the importance of using a combination of hydrogeological and chemical approaches to define volume of seepage to and from Williams Lake and identify uncertainties in chemical fluxes.
Correlation structure of flow variables for steady flow toward a well with application to highly anisotropic heterogeneous formations
Authors: Fiori A. , Indelman P. , Dagan G.
Published: February 2004

The study, a continuation of that of Indelman et al. [1996], aims at deriving the second‐order moments of flow variables such as hydraulic head, its gradient, and the specific discharge for steady flow toward a fully penetrating well in a confined heterogeneous aquifer. The log conductivity Y=ln K is modeled as a three‐dimensional stationary function of Gaussian correlation of anisotropy ratio e. By using first‐order approximations in σ2Y and e, we derive the variance and the vertical integral scale of the piezometric head H, of its radial gradient Er and of the radial component of the specific discharge qr. Owing to the nonuniformity of the average flow, these quantities are functions of the distance from the well. It is shown that the variances of the head σ2H and of its gradient σ2Er, as well as the crossvariance σE,Y between Er and Y vanish at the well, whereas the discharge variance σ2qr tends to the product between the log conductivity variance σ2Y and the squared mean discharge 〈qr〉2. This behavior pertains to a stratified formation surrounding the well. Far from the well (≈75 horizontal Y integral scales I) the head variance approaches a constant value. For r ≥ 10I the moments σ2Er, σ2qr and σErY tend to the corresponding values for uniform flow but with the local mean head gradient replacing the constant one. The head vertical integral scale grows indefinitely with r, whereas the vertical integral scale of the flux is larger by one log conductivity vertical scale than the one prevailing in uniform flow. This latter property is explained by the presence of the source line, which increases the correlations in the vertical direction. The present results may be used in identifying the log conductivity statistical parameters from flowmeter velocity measurements in piezometers surrounding pumping or injecting wells.
Frequency distribution of water and solute transport properties derived from Pan sampler data
Authors: Boll Jan , Selker John S. , Shalit Gil , Steenhuis Tammo S.
Published: February 2004

Modeling of water and solute movement requires knowledge of the nature of the spatial distribution of transport parameters. Only a few of the field experiments reported in the literature contained enough measurements to discriminate statistically between lognormal and normal distributions. To obtain statistically significant data sets, six field experiments at four different sites were performed. Different degrees of macropore and matrix flow occurred at each site. In each experiment a solute pulse was added followed by artificial or natural rainfall. Sixteen thousand spatial distributed fluxes and solute concentrations were collected with wick and gravity samplers. Spatial distributions of solute velocity, dispersion coefficient, water flux, and solute concentration were determined over different timescales ranging from 1 hour to the duration of the experiment. A chi‐square test was used to discriminate between the type of frequency distributions. The spatially distributed water and solute transport parameters when averaged over the experimental period were found to fit the lognormal distribution when macropore flow dominates. Otherwise, when only matrix flow occurs a normal distribution fitted the data better. Under no‐till cultivation, hourly concentration and water flux are lognormally distributed, while tillage makes the tracer concentration to be normally distributed. Spatial frequency distributions of daily solute concentration change in time: Concentrations were normally distributed when the bulk of the solute broke through with the highest concentrations and lognormally distributed in the beginning and end of the experiment. Daily water flux was found to be lognormally distributed throughout the experiment, but the distribution varied between water applications: Shortly after water application, when wick and gravity pan samplers collected water predominantly from macropores and normally distributed at later times when mostly matrix pores were sampled with wick pan samplers.
High‐resolution Monte Carlo simulation of flow and conservative transport in heterogeneous porous media: 2. Transport results
Authors: Naff R. L. , Haley D. F. , Sudicky E. A.
Published: February 2004

In this, the second of two papers concerned with the use of numerical simulation to examine flow and transport parameters in heterogeneous porous media via Monte Carlo methods, results from the transport aspect of these simulations are reported on. Transport simulations contained herein assume a finite pulse input of conservative tracer, and the numerical technique endeavors to realistically simulate tracer spreading as the cloud moves through a heterogeneous medium. Medium heterogeneity is limited to the hydraulic conductivity field, and generation of this field assumes that the hydraulic‐conductivity process is second‐order stationary. Methods of estimating cloud moments, and the interpretation of these moments, are discussed. Techniques for estimation of large‐time macrodispersivities from cloud second‐moment data, and for the approximation of the standard errors associated with these macrodispersivities, are also presented. These moment and macrodispersivity estimation techniques were applied to tracer clouds resulting from transport scenarios generated by specific Monte Carlo simulations. Where feasible, moments and macrodispersivities resulting from the Monte Carlo simulations are compared with first‐ and second‐order perturbation analyses. Some limited results concerning the possible ergodic nature of these simulations, and the presence of non‐Gaussian behavior of the mean cloud, are reported on as well.
High‐resolution Monte Carlo simulation of flow and conservative transport in heterogeneous porous media: 1. Methodology and flow results
Authors: Naff R. L. , Haley D. F. , Sudicky E. A.
Published: February 2004

In this, the first of two papers concerned with the use of numerical simulation to examine flow and transport parameters in heterogeneous porous media via Monte Carlo methods, various aspects of the modelling effort are examined. In particular, the need to save on core memory causes one to use only specific realizations that have certain initial characteristics; in effect, these transport simulations are conditioned by these characteristics. Also, the need to independently estimate length scales for the generated fields is discussed. The statistical uniformity of the flow field is investigated by plotting the variance of the seepage velocity for vector components in thex,y, andzdirections. Finally, specific features of the velocity field itself are illuminated in this first paper. In particular, these data give one the opportunity to investigate the effective hydraulic conductivity in a flow field which is approximately statistically uniform; comparisons are made with first‐ and second‐order perturbation analyses. The mean cloud velocity is examined to ascertain whether it is identical to the mean seepage velocity of the model. Finally, the variance in the cloud centroid velocity is examined for the effect of source size and differing strengths of local transverse dispersion.
Contaminant mass transfer during gas‐phase transport in unsaturated porous media
Authors: Popovičová Jarmila , Brusseau Mark L.
Published: February 2004

This study was conducted to investigate the relative effects of physical heterogeneity, gas‐liquid mass transfer, and rate‐limited sorption on the gas‐phase transport of contaminants in idealized unsaturated homogeneous and heterogeneous porous media. The transport of methane in the unsaturated homogeneous porous medium was ideal, whereas that of trichloroethene and benzene was nonideal, governed by rate‐limited diffusive mass transfer in immobile water and by rate‐limited sorption/desorption. Transport of both methane and trichloroethene through the unsaturated heterogeneous porous medium was nonideal. Gas‐phase mass transfer between unsaturated advective and nonadvective domains caused nonideal transport of methane. Trichloroethene nonideality was due to a combination of gas‐phase mass transfer between advective and nonadvective domains, diffusion within immobile water, and rate‐limited sorption/desorption. The transport of trichloroethene through the heterogeneous porous medium was predicted by use of a multiprocess mass transfer model, wherein all parameters were estimated independently.
Enhancing water quality in hydropower system operations
Authors: Hayes Donald F. , Labadie John W. , Sanders Thomas G. , Brown Jackson K.
Published: February 2004

The quality of impounded waters often degrades over time because of thermal stratification, sediment oxygen demands, and accumulation of pollutants. Consequently, reservoir releases impact water quality in tailwaters, channels, and other downstream water bodies. Low dissolved oxygen (DO) concentrations in the Cumberland River below Old Hickory dam result from stratification of upstream reservoirs and seasonally low release rates. Operational changes in upstream hydropower reservoirs may be one method to increase DO levels without substantially impacting existing project purposes. A water quality model of the upper Cumberland basin is integrated into an optimal control algorithm to evaluate water quality improvement opportunities through operational modifications. The integrated water quantity/quality model maximizes hydropower revenues, subject to various flow and headwater operational restrictions for satisfying multiple project purposes, as well as maintenance of water quality targets. Optimal daily reservoir release policies are determined for the summer drawdown period which increase DO concentrations under stratification conditions with minimal impact on hydropower production and other project purposes.

Appendixes A–D available with entire article on microfiche. Order by mail from AGU, 2000 Florida Ave., N.W., Washington, DC 20009 or by phone at 800‐966‐2481; $2.50. Document W97‐003. Payment must accompany order.
A conceptual model of the soil water retention curve
Authors: Assouline S. , Tessier D. , Bruand A.
Published: February 2004

A conceptual model based on the assumption that soil structure evolves from a uniform random fragmentation process is proposed to define the water retention function. The fragmentation process determines the particle size distribution of the soil. The transformation of particles volumes into pore volumes via a power function and the adoption of the capillarity equation lead to an expression for the water retention curve. This expression presents two fitting parameters only. The proposed model is tested on water retention data sets of 12 soils representing a wide range of soil textures, from sand to clay. The agreement between the fitted curves and the measured data is very good. The performances of the model are also compared with those of the two‐parameter models of van Genuchten [1980] and Russo [1988] for the water retention function. In general, the proposed model exhibits increased flexibility and improves the fit at both the high and the low water contents range.
On constraining the predictions of a distributed model: The incorporation of fuzzy estimates of saturated areas into the calibration process
Authors: Franks Stewart W. , Gineste Philippe , Beven Keith J. , Merot Philippe
Published: February 2004

Distributed hydrological models are generally overparameterized, resulting in the possibility of multiple parameterizations from many areas of the parameter space providing acceptable fits to observed data. In this study, TOPMODEL parameterizations are conditioned on discharges, and then further conditioned on estimates of saturated areas derived from ERS‐I synthetic aperture radar (SAR) images combined with the In (α/tan β) topographic index, and compared to ground truth saturation measurements made in one small subcatchment. The uncertainty associated with the catchment‐wide predictions of saturated area is explicitly incorporated into the conditioning through the weighting of estimates within a fuzzy set framework. The predictive uncertainty associated with the parameterizations is then assessed using the generalized likelihood uncertainty estimation (GLUE) methodology. It is shown that despite the uncertainty in the predictions of saturated area the methodology can reject many previously acceptable parameterizations with the consequence of a marked reduction in the acceptable range of a catchment average transmissivity parameter and of improved predictions of some discharge events.
Impacts of climate variability on the operational forecast and management of the Upper Des Moines River Basin
Authors: Georgakakos Aris P. , Yao Huaming , Mullusky Mary G. , Georgakakos Konstantine P.
Published: February 2004

Data from the regulated 14,000 km2 upper Des Moines River basin and a coupled forecast‐control model are used to study the sensitivity of flow forecasts and reservoir management to climatic variability over scales ranging from daily to interdecadal. Robust coupled forecast‐control methodologies are employed to minimize reservoir system sensitivity to climate variability and change. Large‐scale hydrologic‐hydraulic prediction models, models for forecast uncertainty, and models for reservoir control are the building blocks of the methodology. The case study concerns the 833.8 × 106 m3 Saylorville reservoir on the upper Des Moines River. The reservoir is operated by the U.S. Corps of Engineers for flood control, low‐flow augmentation, and water supply. The total record of 64 years of daily data is divided into three periods, each with distinct characteristics of atmospheric forcing. For each climatic period the coupled forecast‐control methodology is simulated with a maximum forecast lead time of 4 months and daily resolution. For comparison, the results of operation using current reservoir control practices were obtained for the historical periods of study. Large differences are found to exist between the probabilistic long‐term predictions of the forecast component when using warm or cool and wet or dry initial conditions in the spring and late summer. Using ensemble input corresponding to warm or cool and wet or dry years increases these differences. Current reservoir management practices cannot accommodate historical climate variability. Substantial gain in resilience to climate variability is shown to result when the reservoir is operated by a control scheme which uses reliable forecasts and accounts for their uncertainty. This study shows that such coupled forecast‐control decision systems can mitigate adverse effects of climatic forcing on regional water resources.
Precipitation and dissolution of reactive solutes in fractures
Authors: Dijk Peter , Berkowitz Brian
Published: February 2004

The precipitation and dissolution of reactive solutes, transported under the action of fully developed laminar flow in saturated fractures, is analyzed assuming an irreversible first‐order kinetic surface reaction for one component. Equations describing solute transport, precipitation and dissolution, and the evolution of fracture aperture were approximated and solved using combined analytical and numerical techniques; dimensionless transport parameters incorporated into the solutions were estimated from data available in the literature. Fractures with initially flat, linearly constricted, and sinusoidal apertures were investigated. The initial fracture geometry and the solute saturation content of the inflowing fluid have a profound effect on the reaction processes. The results show that the evolution of the solute transport and fracture geometry can be adequately described by the Damköhler and Péclet numbers. Two extreme transport regimes were identified: relatively uniform evolution of fracture apertures and nonuniform evolution of fracture apertures restricted to the inlet region of fractures. In the case of precipitation with half‐life times of the order of seconds to years and with fluid residence times of the order of minutes to days, the time for a fracture to close completely is of the order of days to millions of years. This is consistent with the order of magnitude of hydrogeological timescales. In the model the process of dissolution is the inverse of precipitation, although the combined solute transport and reaction processes are irreversible. These results and the applied dimensionless analysis can be used as a basis for the development of more complex models of reactive solute transport, precipitation, and dissolution in saturated fractured media.
Coupling transport and biodegradation of toluene and trichloroethylene in unsaturated soils
Authors: El‐Farhan Y. H. , Scow K. M. , de Jonge L. W. , Rolston D. E. , Moldrup P.
Published: February 2004

Trichloroethylene (TCE), a common groundwater pollutant generally resistant to aerobic biodegradation, can be cometabolized in the presence of another compound such as toluene. The coupled transport and biodegradation of TCE and toluene was investigated and modeled in laboratory soil columns. Toluene biodegradation was linked to microbial growth using Monod kinetics, while TCE degradation was described using Michaelis‐Menten kinetics modified to account for changing enzyme levels. Biodegradation of TCE was modeled as a mass fraction of the toluene degradation rate. Both growth and decay were incorporated into the equations to model microbial population dynamics. With the exception of the initial biomass, a single set of parameters to describe both degradation functions was obtained from independent soil batch experiments. Physical parameters were obtained from sterile soil columns. The initial biomass declined from the inlet to the outlet side of the chamber. Toluene was fully degraded in the soil column with the majority occurring closest to the inlet chamber. A substantial amount of TCE was not degraded because it diffused faster and was transformed at a lower rate than toluene.
Using neural networks to assess the influence of changing seasonal climates in modifying discharge, dissolved organic carbon, and nitrogen export in eastern Canadian rivers
Authors: Clair Thomas A. , Ehrman James M.
Published: February 2004

Changes in both air temperature and precipitation are expected as atmospheric CO2 increases because of anthropogenic activities. These changes will not be evenly distributed across seasons because of the complexity of climate patterns. Using climate, hydrology, and water chemistry data, we investigated how changes in the seasonal climates of the Atlantic Provinces of Canada would change both hydrology and geochemical cycling of 14 river basins influenced by wetlands. We calculated monthly discharge (Q) and dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) exports from 1983 to 1992 and developed three neural network models relating climate variability to hydrologic and geochemical fluxes from the basins. We then identified a series of potential temperature/precipitation scenarios which we applied to the models. We then studied how changes in climate regimes would affect water discharge and aquatic dissolved carbon and nitrogen released from these basins. Our results predict a complex series of possible outcomes for water and elemental fluxes. Warming or cooling in each season combined with potential changes in precipitation will cause completely different outcomes as snowpack melting dominates winter and early spring conditions and evapotranspiration controls summer and fall. A warming winter would cause less snow storage and more runoff, with little evapotranspiration. Warmer summers, on the other hand, will be influenced by evapotranspiration, and water flows will be lower. Carbon and nitrogen exports from basins generally mirror those of Q. Our results suggest that major hydrological changes in basins may lead to significant ecological and water resource impacts, especially in the spring and early summer.
Stochastic analysis of flow and transport in unsaturated heterogeneous porous formation: Effects of variability in water saturation
Authors: Russo David
Published: February 2004

First‐order analysis, based on a stochastic continuum presentation of the flow and a general Lagrangian description of the transport, was used to investigate the effect of the variability in water saturation on the flow and transport in a heterogeneous, partially saturated porous formation. Results of the analyses show that for a given mean water saturation, the variability in water saturation contributes to the variability in the velocity and, concurrently, enhances solute spreading. This applies especially in formations in which (1) the length scale of the heterogeneity in the direction perpendicular to the mean flow is large compared with its counterpart in the direction parallel to the mean flow; (2) the macroscopic capillary length scale is small compared with the length scale of the heterogeneity in the direction parallel to the mean flow; (3) the variability in the soil parameter log α is not small compared with the variability in log Ks; (4) the fluctuations in log α are negatively correlated with those of log Ks; (5) mean water saturation is relatively small; and (6) the transverse components of the mean head gradient are not small compared with its longitudinal component. Another finding of the present study suggests that the longitudinal components of the velocity and the displacement covariance tensors are much more sensitive to the variability in water saturation than the respective transverse components. For the special case in which the mean flow is aligned along the vertical axis only, the aforementioned transverse components are independent of the spatial variability of water saturation and are influenced only by its mean value. Results of the analysis of the concentration spatial moments suggest that the average cloud centroid will be delayed compared with the mean velocity of the fluid. This stems from the fact that the fluid velocity in the variably saturated formation is not divergence‐free and its fluctuations are negatively correlated with those of water saturation.
Transient climate change and net ecosystem production of the terrestrial biosphere
Authors: Xiao X. , Melillo J. M. , Kicklighter D. W. , McGuire A. D. , Prinn R. G. , Wang C. , Stone P. H. , Sokolov A.
Published: February 2004

In this sensitivity study, we have applied the Terrestrial Ecosystem Model ((TEM) version 4.1) to examine the responses of terrestrial ecosystems to transient changes in atmospheric CO2 concentration and climate in the 21st century at the scales of the globe, biomes, latitudinal gradient, and economic regions. Three predictions of transient change in climate and atmospheric CO2 concentration in the 21st century from the Integrated Global System Model developed at Massachusetts Institute of Technology were used. The TEM estimates a global annual net ecosystem production (NEP) of about 0.8 Pg C yr−1 in 1990. Global annual NEP in 2100 increases by about 2.6 Pg C yr−1 for the HHL (higher CO2 emissions and temperature increases), 1.8 Pg C yr−1 for the RRR (reference CO2 emissions and temperature increases), and 0.5 Pg C yr−1 for the LLH (lower CO2 emissions and temperature increases) climate change predictions. The boreal and tropical evergreen forests account for a large portion of the increased global annual NEP. Latitudinal distribution of total annual NEP along 0.5°‐resolution latitudinal bands shifts significantly from the tropics to the northern middle and high latitudes over time. The potential CO2 uptake over the period of 1990–2100 differs substantially among the 12 economic regions of the world. As we used potential mature natural vegetation in the global extrapolation of TEM, these NEP estimates represent the potential CO2 uptake or the upper bound for long‐term carbon sequestration by the terrestrial biosphere. This sensitivity study shows that the temporal dynamics and spatial distribution of carbon, nitrogen, and water fluxes of terrestrial ecosystems are very sensitive to the magnitudes and paths of transient changes in atmospheric CO2 concentration and climate in the 21st century.
The calcite lysocline as a constraint on glacial/interglacial low‐latitude production changes
Authors: Sigman Daniel M. , McCorkle Daniel C. , Martin William R.
Published: February 2004

We investigate the response of the calcite lysocline to changes in the export production of the low‐latitude surface ocean (the combined equatorial, tropical, and subtropical regions). We employ different CaCO3 throughput schemes in a time‐dependent ocean carbon cycle model to separate the CaCO3 production/lysocline balance from the other components of the model response and to estimate the effect of dissolution driven by organic matter decomposition in surface sediments. With this model, we carry out three experiments based on previous hypotheses for the cause of glacial/interglacial atmospheric CO2 variations: (1) a simple increase in low‐latitude production, (2) a decrease in its CaCO3/organic carbon rain ratio, and (3) an increase in the depth of organic carbon regeneration. None of these changes, when taken alone, can lower atmospheric CO2 to glacial levels without violating observations of the glacial lysocline depth. If a low‐latitude production increase were accompanied by a decrease in the CaCO3/organic carbon rain ratio, then these simultaneous changes could lower atmospheric CO2 to observed glacial levels without causing a significant change in the ocean‐average depth of the lysocline. However, even with these simultaneous changes, the required increase in production is 50% or more of the modern rate of production.
Does atmospheric CO2 police the rate of chemical weathering?
Authors: Broecker Wallace S. , Sanyal Abhijit
Published: February 2004

A case is made that in the absence of an effective feedback control on the rate of delivery of CaO to the oceans, the CO2 content of the Earth's atmosphere would have wandered over a large range threatening life either by overheating or by carbon dioxide starvation. In this paper, we defend the suggestion by Walker et al. [1981] that control is exerted by the interaction between the CO2 content of the atmosphere and the continental weathering rates. We contend that in spite of the arguments raised against it [Raymo and Ruddiman, 1992; Edmond and Huh, 1997] the CO2‐ chemical weathering feedback is the dominant mechanism that stabilizes the atmospheric carbon dioxide content.
Net community production of dissolved organic carbon
Authors: Hansell Dennis A. , Carlson Craig A.
Published: February 2004

Each year large amounts of carbon, with a residence time of months, accumulate in the surface layer of the ocean as semilabile dissolved organic carbon (DOC). This material is transported long distances, contributing to the interhemispheric transfer and deep ocean export of carbon. The fraction of net community production resulting in the accumulation of semilabile DOC is estimated here by mass balance during periods of net phytoplankton production in three diverse environments: the Ross Sea polynya, the Equatorial Pacific Ocean, and the Sargasso Sea. In the eutrophic systems of the Ross Sea and the Equatorial Pacific, net DOC production generally fell between 10 and 20% of net community production. For the 1995 spring bloom in the Sargasso Sea, net DOC production was 59‐70% of the net community production. Net DOC production was maximal during the period of deep convective overturn of the water column, indicating linkage between the processes. Following the Sargasso Sea spring bloom and into the summer period, net DOC production was nil over the upper 250 m so that net DOC production was reduced to ∼8% of net community production on a seasonal timescale. Consideration of the various types of production regimes in the ocean indicates that the global net production of semilabile DOC is ∼17% of global new production. Regions of the world's oceans with the greatest contributions to global net community production, such as equatorial and coastal upwelling areas, contribute most to the global production of semilabile DOC.
Methane oxidation by termite mounds estimated by the carbon isotopic composition of methane
Authors: Sugimoto Atsuko , Inoue Tetsushi , Kirtibutr Nit , Abe Takuya
Published: February 2004

Emission rates and carbon isotope ratios of CH4, emitted by workers of termites, and of CH4, emitted from their mounds, were observed in a dry evergreen forest in Thailand to estimate the proportion of CH4oxidized during emission through the mound. The δ13C of CH4emitted from a termite mound (−70.9 to −82.4‰) was higher than that of CH4emitted by workers in the mound (−85.4 to −97. l‰). Using a fractionation factor (a = 0.987) for oxidation of CH4which was obtained in the incubation experiment, an emission factor defined as (CH4emitted from a termite mound/CH4produced by termites) was calculated. The emission factor obtained in each termite mound was nearly zero forMacrotermes(fungus‐growing termites), of which the nest has a thick soil wall and subterrannean termites, and 0.17 to 0.47 for Termitinae (small‐mound‐making termites). Global CH4emission by termites was estimated on the basis of the CH4emission rates by workers and termite biomass with the emission factors. The calculated result was 1.5 to 7.4 Tg/y (0.3 to 1.3% of total source), which is considerably smaller than the estimate by theIPCC[1994].
Soil Carbon stocks and their rates of accumulation and loss in a boreal forest landscape
Authors: Rapalee Gloria , Trumbore Susan E. , Davidson Eric A. , Harden Jennifer W. , Veldhuis Hugo
Published: February 2004

Boreal forests and wetlands are thought to be significant carbon sinks, and they could become net C sources as the Earth warms. Most of the C of boreal forest ecosystems is stored in the moss layer and in the soil. The objective of this study was to estimate soil C stocks (including moss layers) and rates of accumulation and loss for a 733 km2 area of the BOReal Ecosystem‐Atmosphere Study site in northern Manitoba, using data from smaller‐scale intensive field studies. A simple process‐based model developed from measurements of soil C inventories and radiocarbon was used to relate soil C storage and dynamics to soil drainage and forest stand age. Soil C stocks covary with soil drainage class, with the largest C stocks occurring in poorly drained sites. Estimated rates of soil C accumulation or loss are sensitive to the estimated decomposition constants for the large pool of deep soil C, and improved understanding of deep soil C decomposition is needed. While the upper moss layers regrow and accumulate C after fires, the deep C dynamics vary across the landscape, from a small net sink to a significant source. Estimated net soil C accumulation, averaged for the entire 733 km2 area, was 20 g C m−2 yr−1 (28 g C m−2 yr−1 accumulation in surface mosses offset by 8 g C m−2 yr−1 lost from deep C pools) in a year with no fire. Most of the C accumulated in poorly and very poorly drained soils (peatlands and wetlands). Burning of the moss layer in only 1% of uplands would offset the C stored in the remaining 99% of the area. Significant interannual variability in C storage is expected because of the irregular occurrence of fire in space and time. The effects of climate change and management on fire frequency and on decomposition of immense deep soil C stocks are key to understanding future C budgets in boreal forests.
Seasonal patterns and controls on net ecosystem CO2 exchange in a boreal peatland complex
Authors: Bubier Jill L. , Crill Patrick M. , Moore Tim R. , Savage Kathleen , Varner Ruth K.
Published: February 2004

We measured seasonal patterns of net ecosystem exchange (NEE) of CO2 in a diverse peatland complex underlain by discontinuous permafrost in northern Manitoba, Canada, as part of the Boreal Ecosystems Atmosphere Study (BOREAS). Study sites spanned the full range of peatland trophic and moisture gradients found in boreal environments from bog (pH 3.9) to rich fen (pH 7.2). During midseason (July‐August, 1996), highest rates of NEE and respiration followed the trophic sequence of bog (5.4 to −3.9 μmol CO2 m−2 s−1) < poor fen (6.3 to −6.5 μmol CO2 m−2 s−1) < intermediate fen (10.5 to −7.8 μmol CO2 m−2 s−1) < rich fen (14.9 to −8.7 μmol CO2m−2 s−1). The sequence changed during spring (May‐June) and fall (September‐October) when ericaceous shrub (e.g., Chamaedaphne calyculata) bogs and sedge (Carex spp.) communities in poor to intermediate fens had higher maximum CO2 fixation rates than deciduous shrub‐dominated (Salix spp. and Betula spp.) rich fens. Timing of snowmelt and differential rates of peat surface thaw in microtopographic hummocks and hollows controlled the onset of carbon uptake in spring. Maximum photosynthesis and respiration were closely correlated throughout the growing season with a ratio of approximately 1/3 ecosystem respiration to maximum carbon uptake at all sites across the trophic gradient. Soil temperatures above the water table and timing of surface thaw and freeze‐up in the spring and fall were more important to net CO2 exchange than deep soil warming. This close coupling of maximum CO2 uptake and respiration to easily measurable variables, such as trophic status, peat temperature, and water table, will improve models of wetland carbon exchange. Although trophic status, aboveground net primary productivity, and surface temperatures were more important than water level in predicting respiration on a daily basis, the mean position of the water table was a good predictor (r2 = 0.63) of mean respiration rates across the range of plant community and moisture gradients. Q10 values ranged from 3.0 to 4.1 from bog to rich fen, but when normalized by above ground vascular plant biomass, the Q10 for all sites was 3.3.
Teleseismic body‐wave analysis of the 9 October, 1995 (Mw = 8.0), Colima‐Jalisco, Mexico Earthquake, and its largest foreshock and aftershock
Authors: Escobedo David , Pacheco Javier F. , Suárez Gerardo
Published: February 2004

The source process of the October 9, 1995, Colima‐Jalisco, Mexico, earthquake (Mw=8.0), and its largest foreshock and aftershock were determined from teleseismic body waves, using a least‐squares inversion scheme. The three events are shallow‐dipping, thrust‐fault earthquakes, in agreement with the relative plate motions for Rivera‐North America and Cocos‐North America plate boundaries. Neither the foreshock nor the largest aftershock show significant differences in focal mechanism nor centroidal depth with respect to the average mechanism of the mainshock. The rupture of the mainshock began near the foreshock hypocenter and propagated, with a variable rupture velocity, towards the NW. The source‐time function for this event shows four distinct episodes of energy release. About 50% of the moment was released at the northern end of the rupture, between 90 and 110 km from the epicenter. This rupture geometry is in agreement with crustal deformation observed inland using GPS measurements. However, the scalar seismic moment of the main shock obtained in this study (Mo=1.84×1020) is smaller with respect to the lower frequency CMT results reported by Harvard (Mo=1.15×1021)
Growth of ice crystals in a precipitating contrail
Authors: Heymsfield Andrew J. , Lawson R. Paul , Sachse G. W.
Published: February 2004

This study examines how jet aircraft contrails develop precipitation trails, using data collected on 12 May, 1996 during SUCCESS. The DC‐8 sampled the precontrail conditions, produced a contrail largely in clear air at −52°C, and sampled the contrail and developing trails for over an hour. The environment was highly ice‐supersaturated, reaching nearly water saturation in some locations. Inside the contrail core, almost all ice particles remained small (∼1 to 10 µm) due to high crystal concentrations (∼10¹ to 10² cm−3) which reduced the vapor density to saturation. Mixing of moist environmental air and vapor‐depleted contrail air produced localized regions of supersaturation along the contrail periphery, where crystals grew to several hundred microns at about 0.1 µm s−1 These particles could then fall from the contrail into the vapor‐rich, undepleted, supersaturated environment below. As heavier crystals left the contrail, others moved into the regions of ice supersaturation. Precipitation trails developed as this process continued over time.