bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling modern methane emissions from natural wetlands: 1. Model description and results
Authors: Walter Bernadette P. , Heimann Martin , Matthews Elaine
Published: February 2004

Methane is an important greenhouse gas which contributes about 22% to the present greenhouse effect. Natural wetlands currently constitute the biggest methane source and were the major source in preindustrial times. Wetland emissions depend highly on the climate, i.e., on soil temperature and water table. To investigate the response of methane emissions from natural wetlands to climate variations, a process‐based model that derives methane emissions from natural wetlands as a function of soil temperature, water table, and net primary productivity is used. For its application on the global scale, global data sets for all model parameters are generated. In addition, a simple hydrologic model is developed in order to simulate the position of the water table in wetlands. The hydrologic model is tested against data from different wetland sites, and the sensitivity of the hydrologic model to changes in precipitation is examined. The global methane‐hydrology model constitutes a tool to study temporal and spatial variations in methane emissions from natural wetlands. The model is applied using high‐frequency atmospheric forcing fields from ECMWF reanalyses of the period from 1982 to 1993. We calculate global annual methane emissions from wetlands to be 260 Tg yr−1. Twenty‐five percent of these methane emissions originate from wetlands north of 30°N. Only 60% of the produced methane is emitted, while the rest is reoxidized. A comparison of zonal integrals of simulated global wetland emissions and results obtained by an inverse modeling approach shows good agreement. In a test with data from two wetlands the seasonality of simulated and observed methane emissions agrees well.
Background aerosol concentration derived from the atmospheric electric conductivity measurements made over the Indian Ocean during INDOEX
Authors: Kamra A. K. , Murugavel P. , Pawar S. D. , Gopalakrishnan V.
Published: February 2004

Measurements of the atmospheric electric conductivity on board ORV Sagarkanya during her three cruises over the Indian Ocean (17°N to 20°S, 57°E to 79°E) during the periods of December to March 1996–1997, 1998, and 1999 are reported. The results show that the values of atmospheric conductivity over the southern hemisphere are 2 to 3 times ofthat over the northern Indian Ocean and the north‐to‐south gradients extend up to the Intertropical Convergence Zone (ITCZ) and have large interseasonal and intraseasonal variations. The values of electric conductivity have been used to calculate the aerosol concentrations. The latitudinal variations of the aerosol concentration have been observed to have positive gradients from the Indian coastline to the ITCZ, and the gradients are different during the three cruises. The aerosol concentrations attain their pristine level only at 15°–20°S in this season. Because of the large interseasonal variability of the aerosol concentration observed over the northern Indian Ocean, it is concluded that estimating any secular change in the background aerosol pollution may be a futile exercise in this area.
Vertical profiles of stratus cloud spectral optical parameters derived from airborne radiation measurements
Authors: Melnikova I. N. , Mikhailov V. V.
Published: February 2004

Formulas that express the optical thickness and the single scattering albedo (or volume scattering and absorption coefficients) through hemispherical irradiance inside optically thick and horizontally homogeneous extended cloud are derived for shortwave spectral ranges. A cloud with vertical heterogeneity and a multilayer cloud system are considered. The evaluation of uncertainties and applicability limits for these formulas is carried out. The formulas are applied to the interpretation of spectral radiation measurements taken during two aircraft field experiments. The spectral dependence of these optical parameters for different cloud sublayers is found and presented.
Changes in the global atmospheric methane budget over the last decades inferred from 13C and D isotopic analysis of Antarctic firn air
Authors: Bräunlich Maya , Aballain Olivier , Marik Thomas , Jöckel Patrick , Brenninkmeijer Carl A. M. , Chappellaz Jerome , Barnola Jean‐Marc , Mulvaney Robert , Sturges William T.
Published: February 2004

The atmospheric trend of methane isotopic ratios since the mid‐20th century has been reconstructed from Antarctic firn air. High volume air samples were extracted at several depth levels at two sites in East Antarctica. Methane concentration and its 13C/12C and D/H ratios were determined by gas chromatography, mass spectrometry, and infrared spectroscopy. A firn air transport model was applied to reconstruct past atmospheric trends in methane and its isotopic composition. By subsequent application of an atmospheric model, changes in methane sources and OH sink compatible with the past atmospheric trends are explored. In step with increasing methane mixing ratios, δ13C increased by ∼1.7‰ over the last 50 years. These changes mainly reflect a shift in relative source strength toward the heavier anthropogenic methane source, such as biomass burning and methane of nonbiological origin. The δD (CH4) showed a period of decline between the 1950s and 1975, followed by a gradual increase of 0.55‰/yr, also toward the heavier anthropogenic source. Dependent on possible changes in the OH sink, to which δD of methane is very sensitive, the inferred isotopic trends of δ13C and δD over the last 50 years constrain the relationship between natural and anthropogenic sources over the last century. The observed δD minimum around 1975 suggests that the slowing down in the methane source growth took place during this period.
Multiyear accumulation and temperature history near the North Greenland Ice Core Project site, north central Greenland
Authors: Shuman C. A. , Bromwich D. H. , Kipfstuhl J. , Schwager M.
Published: February 2004

This paper presents a comparison of two independent methods of estimating subseasonal accumulation across the interior of Greenland. These methods, high‐resolution snow pit studies and atmospheric modeling, have differing spatial and temporal resolution, but both can estimate net accumulation for subseasonal and shorter periods. The snow pit approach is based on a documented relationship between high‐resolution snow pit profiles of oxygen stable isotope ratio (δ18O) and multiyear Special Sensor Microwave/Imager (SSM/I) 37‐GHz brightness temperature records. Comparison of SSM/I data to profiles obtained during the 1995 Alfred Wegener Institut North Greenland Traverse field season shows that δ18O data from snow in north central Greenland are a reliable, high‐resolution temperature proxy. This enables determination of accumulation amount, rate, and timing from approximately July 1991 through June 1995 across this 220‐km‐long transect of the ice sheet. Precipitation estimates derived from early modeling based on European Centre for Medium‐Range Weather Forecasts data show a similar average seasonal pattern but a diminished magnitude of accumulation (∼56%) for these sites. The slope of the multiyearTversus δ correlation was evaluated for each site on the basis of the observed and calculated temperature history from the nearby North Greenland Ice core Project (NGRIP) site automatic weather station. These data should assist interpretation of the paleoclimatic record in the NGRIP deep core.
El Niño‐Southern Oscillation (ENSO) influence on a Sajama volcano glacier (Bolivia) from 1963 to 1998 as seen from Landsat data and aerial photography
Authors: Arnaud Yves , Muller Frédéric , Vuille Mathias , Ribstein Pierre
Published: February 2004

Sajama volcano, located in the Bolivian Altiplano, is the southernmost tropical glacier and, owing to its situation, approximately 100 km east of the Pacific coast, is well suited to study the El Niño‐Southern Oscillation phenomenon. Landsat data from 1972 to 1998 and a 1963 aerial photograph are used to monitor the snow line fluctuations on a selected part of Sajama volcano. We assume that a few months after the rainy season, the snow line is representative of the previous rainy season, if no recent snowfall has occurred. By observing precipitation from the stations surrounding Sajama volcano and by verifying snow presence on surrounding summits, we detect images with recent snowfall likely to disturb the climatic significance of the snow line. A snow line evolution model takes into account the different image acquisition dates and adjusts the snow line elevation accordingly for the middle of the dry season. A progressive rise of the snow line elevation is observed from 1963 to 1998 with a sustained rise from 1984 to 1990. The snow line altitude is related to the Southern Oscillation Index. Even after the high precipitation of the 1996–1997 wet season, the following El Niño 1997–1998 leads to a substantial rise of the snow line. The snow line elevation is related primarily to the total rainy season precipitation and to a lesser degree to the maximum monthly mean temperature of the warmest month, thus confirming a greater snow line sensitivity to precipitation than to temperature.
A preliminary three‐dimensional global model study of atmospheric methyl chloride distributions
Authors: Lee‐Taylor J. M. , Brasseur G. P. , Yokouchi Y.
Published: February 2004

A global three‐dimensional atmospheric model of methyl chloride (CH3Cl) is presented. When incorporating known terrestrial and oceanic source terms, the tropospheric budget of CH3Cl is unbalanced. We show that a reduction in the atmospheric CH3Cl loss rate could account for the net budget discrepancy but fails to reproduce the observed latitudinal distribution. We find that observed mixing ratios and latitudinal distributions can be reproduced by addition of a tropical terrestrial CH3Cl source of 2330–2430 Gg yr−1 combined with a 50% reduction in the southeastern Asian biomass burning contribution. This is equivalent to a net source of 3800–3900 Gg yr−1, slightly higher than previously estimated. The magnitude of additional emissions required to match observations is sensitive to their latitudinal distribution. We successfully simulate tropical land‐based observations best when the added source is increased at the coasts relative to inland areas. Mixing ratios at remote sites are relatively insensitive to the finer details of the source parameterization.
Small‐scale gravity waves near the mesopause observed by four all‐sky airglow imagers
Authors: Ejiri M. K. , Shiokawa K. , Ogawa T. , Nakamura T. , Maekawa R. , Tsuda T. , Kubota M.
Published: February 2004

Nocturnal airglow images obtained at the Shigaraki middle and upper atmosphere (MU) observatory (34.9°N, 136.1°E), Japan, were analyzed to study the three‐dimensional structure of small‐scale gravity waves in the mesopause region. Airglow images from the near‐infrared OH (layer height of ∼86 km) and O2(0, 1) (∼94 km) bands and the visible O I (∼96 km) and Na (∼90 km) lines were obtained simultaneously by using four all‐sky cooled charge‐coupled device imagers with a high time resolution of 0.5–2.0 min. A clear wave packet (horizontal wavelength of 60–70 km) propagating downward was observed for 2000–2030 LT on January 28, 1998. The downward motion of the wave packet is identified from a comparison of the edge location of the packet at the four airglow layers. The observed waves became faint first in O I (the highest altitude) and last in OH (the lower altitude), suggesting also that the wave packet passed through each of the airglow layers downward. From the simultaneous wind observation by the MU radar, we conclude that the observed downward propagating wave packet was generated by wave reflection at higher altitudes in the background wind field that was opposite to the horizontal k vector of the waves. For the same event, we estimate the vertical wavelength of the waves to be ∼5 km or 20–30 km by comparing the wave phases observed in the four airglow layers. The vertical wavelengths estimated from the dispersion relation are consistent with the latter value.
Introduction to the special section: Photolysis and radiolysis of outer solar system ices (PROSSI)
Authors: Domingue Deborah , Allamandola Louis
Published: February 2004

Ices are an integral component of many extraterrestrial objects, from grains in the interstellar medium (ISM) to comets and planetary surfaces. Their physical properties and the chemistry of their behavior are affected by many different processes, among the most prevalent being photolysis and radiolysis. Both laboratory and theoretical studies have focused on understanding how these two processes affect the structure and chemistry of ices. In order to bring together the various scientific communities studying solar system ices (from those involved in laboratory and theoretical studies to those who observe ices in their various habitats) and access the current understanding of our knowledge base, the Johns Hopkins University/Applied Physics Laboratory hosted a 3‐day NASA‐sponsored workshop on photolysis and radiolysis of outer solar system ices (PROSSI) in March of 2000.
Ionospheric reconstruction using Faraday rotation data: A new technique
Authors: Ganguly S. , Brown A. , DasGupta A. , Ray S.
Published: February 2004

Measurements of total electron content of the ionosphere by recording group delay or differential phase of transionospheric satellite signals have been used for reconstruction of ionospheric profiles. Integrated electron content measurements contain little information of ionospheric height distribution and require precise observations. These measurements are difficult for VHF and higher frequencies and require reasonably sophisticated instruments. Faraday rotation measurements, on the other hand, require relatively simple instruments and were widely used throughout the world. Faraday rotation of HF, VHF, and lower UHF band signals contains an additional term, e.g., a known function of the magnetic field. The functional form depends on the ray path and orientation of the magnetic field and in general is a complex quantity. Since the magnetic field distribution in the Earth's ionosphere is known with very high precision, this information can be used to obtain improved reconstruction of the ionospheric profiles. The magnetic field components have a strong dependence with the height, and this provides additional information regarding the ionospheric height retrieval. We have performed simulations using these techniques, and we present results using both simulated data as well as Faraday rotation data measured from several stations. We compare the sensitivities and reconstruction of ionospheric profiles using group delay and Faraday rotation techniques and demonstrate the improvements with the use of Faraday rotation data. The most difficult (in terms of radio propagation) regions, i.e., those near the equatorial regions, are simulated, and data from several stations, i.e., those near Calcutta, India, as well as those from several stations in Africa, are used. For the first time, we present the results of the reconstruction of the ionosphere using both simulated and observed Faraday rotation data. Large amounts of Faraday rotation data have been acquired during the 1960s and 1970s. These data contain a history of the ionosphere. The present technique would allow retrieval of ionospheric profiles over extended regions and could allow inferences to be drawn regarding the long‐term changes in the ionosphere. The technique can also be incorporated in future satellite beacons, and the combination of group delay and Faraday rotation techniques will allow us to obtain superior reconstructions.
Synthrusting metamorphism, cooling, and erosion of the Himalayan Kathmandu Complex, Nepal
Authors: Johnson M. R. W. , Oliver G. J. H. , Parrish R. R. , Johnson S. P.
Published: February 2004

In this paper we tackle some of the outstanding problems of the Himalaya, in particular the external zone in the Kathmandu Complex, using an integrated approach involving field mapping, microstructure, thermobarometry, and geochronology. The result is a new model showing the evolution of one major Main Central Thrust: therefore we refute suggestions that the Kathmandu Complex is a klippe or separate thrust sheet. Compared to the Main Central Thrust sheet in the High Himalaya, the Kathmandu Complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the High Himalaya, fairly near the leading edge. Unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the Main Central Thrust wedge was ∼40 km thick on the northern side of the Kathmandu Complex and <20 km thick on the southern margin. Initiation of the Main Central Thrust occurred at ∼22 Ma, possibly during the closing stages of peak amphibolite facies metamorphism; slip at elevated temperature (500°–600°C) continued until ∼14 Ma. This is a little longer than has previously been proposed. In marked contrast to the famous inverted metamorphism on the Main Central Thrust in the High Himalaya, the metamorphic zonal scheme in the Kathmandu Complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base. These mylonites postdate the high‐grade regional amphibolite metamorphism and give an illusion of inverted metamorphism. A likely reason for the contrast is that the Main Central Thrust cut up section toward the foreland and therefore at Kathmandu, carries high levels in the metamorphic structure. Our model involves reactivation of the Main Central Thrust at 7–8 Ma as inferred from published monazite and mica ages, but because the Kathmandu rocks show no evidence for high‐temperature reactivation at this time, we presume that the late reactivation involved only the internal High Himalaya zone while the Main Central Thrust was inactive in the external Kathmandu zone. We attempt to quantify rates of cooling, exhumation and thrusting during time period 22 Ma to the present.
Numerical modeling of Cenozoic stress patterns in the mid‐Norwegian margin and the northern North Sea
Authors: Pascal Christophe , Gabrielsen Roy H.
Published: February 2004

Numerical modeling of Cenozoic stress patterns in the northern North Sea and the mid‐Norwegian margin is presented, and the sense of potential slip along major fault planes belonging to the two areas is restored. We assume that the main regional source of stresses is the Atlantic ridge push as demonstrated by previous studies. Furthermore, we also assume a nearly consistent NW‐SE strike for the far‐field stress from continental breakup between Greenland and Norway (earliest Eocene) to present day. First, we applied the commercial two‐dimensional distinct element method (UDEC) to simulate Cenozoic stress and displacement patterns in the study area. Variations in rheology and major fault zones were introduced into the model. The Møre‐Trøndelag Fault Complex and its inferred continuation into the Shetland Platform forms the major mechanical discontinuity in the model. Second, we used the SORTAN method, developed at the University of Paris VI, to predict the sense of potential slip along major fault planes. The input for the SORTAN model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the UDEC modeling. Our results show that the Møre‐Trøndelag Fault Complex and its inferred continuation into the Shetland Platform act as a weak fault zone. This fault zone divides the study area into two different stress provinces: the continental margin and the northern North Sea. This result agrees well with the observed differences in Cenozoic structural evolution of the two areas. Compressive structures are observed along the continental margin, whereas relative tectonic quiescence characterizes the northern North Sea during the Tertiary. The restored stress patterns in the northern North Sea and the mid‐Norwegian margin also agree well with the observed present‐day stress configuration. Our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools (UDEC and SORTAN). As a result, it is demonstrated that oblique‐slip motions are mainly expected, in particular, strike‐slip and reverse dip‐slip faulting are simulated.
Pollution source identification in heterogeneous porous media
Authors: Atmadja Juliana , Bagtzoglou Amvrossios C.
Published: February 2004

The reliable assessment of hazards or risks arising from water contamination problems and the design of efficient and effective techniques to mitigate these problems require the capability to predict the behavior of chemical contaminants in flowing water. Most attempts at quantifying contaminant transport have relied on a solution of some form of the advection‐dispersion‐reaction equation. In this paper, the Backward Beam Equation (BBE) method is studied and enhanced to solve the Advection‐Dispersion Equation (ADE) within a contaminant source identification context. Even though the BBE has been applied successfully to parabolic problems before, it has never been applied to solving the ADE with heterogeneous parameters. The BBE employed in this work is capable of recovering the time history and spatial distribution of a groundwater contaminant plume from measurements of its current position. Using examples involving deterministic heterogeneous dispersion coefficients, we show that the method is robust enough to handle heterogeneous parameters. By altering the method, to produce a hybrid between a marching and a jury method called the Marching‐Jury Backward Beam Equation (MJBBE), we were also able to make the problem practical to solve.
Optimal replacement policy for single pipes in water distribution networks
Authors: Luong Huynh T. , Nagarur Nagen N.
Published: February 2004

In the actual operation of a distribution network, failure can occur in any component of the network, such as pumps, valves, junctions, and pipes. When a component is experiencing failure, the question raised is whether to replace or repair it. For the case of a pipe in the distribution network, which is one of the most frequently subject‐to‐failure components, there are some aspects that still remain unresolved in relation to practical operations of a maintenance program. In this research paper, a mathematical model is developed which aims to support a decision to repair or replace a main pipe in the state of failure. The objective of the model is to maximize the long‐run availability of the pipe under some budget constraints. A semi‐Markov process is used to depict the behavior of the pipe, and replacement ages of the pipe in each of its deteriorating stages are taken as the decision variables. The original nonlinear problem resulting from model formulation is converted to a linear problem by some simple transformations, and then numerical experiments are conducted to illustrate the applicability of the proposed model.
A robust rainfall‐runoff transfer model
Authors: Capkun G. , Davison A. C. , Musy A.
Published: February 2004

This paper describes a simple and flexible new approach to rainfall‐runoff transfer modeling, which jointly models the mean and the variance structure of runoff. Its mean is taken to be a linear autoregressive combination of present and previous rainfall and previous runoff, while its variance also depends on rainfall history. Inference for our model is performed using classical likelihood methods, and also by the more robust technique of quasi‐likelihood, presupposing no particular distribution for runoff. The model has been developed on the Biorde catchment in Switzerland and calibrated and validated for 12 small Swiss catchments, with promising results.
Pore‐scale modeling of biological clogging due to aggregate expansion: A material mechanics approach
Authors: Dupin Hubert J. , Kitanidis Peter K. , McCarty Perry L.
Published: February 2004

Whereas most previous studies of biomass growth and biological clogging consider continuous biofilms, we investigate how the growth of biomass in the form of aggregates affects the permeability and the transport properties of porous media. This paper presents modeling of processes in a single pore, and a companion paper [Dupin et al., this issue] describes modeling over a network of pores. Each pore (channel) is seeded with initial biomass that consumes an electron donor and an electron acceptor according to dual Monod kinetics. Biomass is modeled as a continuous uniform isotropic hyperelastic material, whose expansion and deformation are governed by material mechanics stress‐strain relations, unlike traditional approaches that use ad hoc empirical schemes. The Stokes flow, the advection‐diffusion‐reaction mass transport, and the biomass deformation partial differential equations are solved using finite elements. The solute transport problem is made more computationally efficient by controlling the time step discretization. Results from a simulation illustrate the methodology.
Population index flood method for regional frequency analysis
Authors: Sveinsson Oli G. B. , Boes Duane C. , Salas Jose D.
Published: February 2004

Regional frequency analyses based on index flood procedures have been used within the hydrologic community since 1960. It appears that when the index flood method was first suggested, the index flood was taken to be the at‐site population mean, which, in turn, in the last two or three decades, has been estimated by the at‐site sample mean. The objectives of this paper are to investigate the consequences of replacing a population characteristic with its sample counterpart and to propose an analytically correct regional model dubbed as the population index flood (PIF) method. In this method the homogeneity of the region is embedded in the structure of the parameter space of the underlying distribution model. Simulation experiments are conducted to test the proposed PIF method based on the generalized extreme value distribution with parameters estimated using the method of maximum likelihood (MLE) and the method of probability‐ weighted moments (PWM). Furthermore, in the simulation experiments the PIF method is compared with theHosking and Wallis[1997] regional estimation scheme (HW scheme). Comparing among all index flood methods investigated herein, the PIF method with parameters estimated using MLE provides the best overall results for the 0.95 and the 0.99 quantites in terms of both bias and root‐mean‐square error for moderate to sufficiently large sample sizes, but for the 0.995 quantile the HW scheme seems to perform best for the investigated sample sizes.
Effect of buoyancy ratio on the development of double‐diffusive finger convection in a Hele‐Shaw Cell
Authors: Cooper Clay A. , Glass Robert J. , Tyler Scott W.
Published: February 2004

We consider the evolution of double‐diffusive finger convection for a two‐ solute (salt‐sucrose) system in a Hele‐Shaw cell. A high‐resolution, full‐field, light transmission technique was used to study the development of the instability that resulted from layering a lighter sucrose solution over a denser salt solution. The buoyancy ratio (Rρ), which is a ratio of fluid density contributions by the two solutes and defines the degree of system disequilibrium, was varied systematically from conditions that were nearly stable (Rρ = 2.8) to those that were moderately unstable (Rρ = 1.4). In all experiments, fingers are found to form continuously throughout time from a finger “generation” zone that straddles the location of the initial interface between solutions. At low Rρ, fingers develop rapidly, merge with adjacent fingers, and grow far beyond the finger generation zone through a series of finger “conduits.” In the higher Rρ experiments, fingers are slower to evolve, do not interact as dynamically, and do not grow far beyond the generation zone. Solute mass fluxes at low Rρ quickly reach a constant many times greater than that of a purely diffusive system; at high Rρ, mass fluxes decay as t−½ and behave diffusively but with effective diffusion coefficients much greater than those for molecular diffusion.
On the applicability of analytical methods for estimating solute travel time statistics in nonuniform groundwater flow
Authors: Destouni Georgia , Simic Eva , Graham Wendy
Published: February 2004

Groundwater recharge from the unsaturated zone results in a nonuniform mean groundwater flow. We report closed‐form expressions for the mean and variance of advective solute travel time under such nonuniform flow conditions. These expressions are derived directly from velocity statistics, based on an analytical direct evaluation method (DEM). An alternative, indirect evaluation method (IEM) has previously been used for deriving and evaluating the cumulative distribution function of solute travel time in nonuniform mean flow, through its relation to an assumed Gaussian probability density function of solute displacement. For negative groundwater recharge we argue that neither of the two analytical methods DEM and IEM can be considered reliable. For positive groundwater recharge we show that (1) the DEM results reported here remain as reliable for nonuniform as for uniform mean groundwater flow; (2) an assumed lognormal cumulative distribution function based on the DEM‐derived travel time moments is consistent with the IEM‐derived solute travel time cumulative distribution function (cdf), which is in turn consistent with numerical simulation results reported in the literature; and (3) the detailed differences between the DEM‐based lognormal cdf and the IEM‐derived cdf for nonuniform flow are such that the former (DEM) may exhibit even better agreement with reported numerical simulation results than the latter (IEM).
Geographic Database, Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Seyfried M. , Harris R. , Marks D. , Jacob B.
Published: February 2004

The Reynolds Creek Experimental Watershed (RCEW) exhibits spatial variability typical of the intermountain region. We provide a geographic database to provide continuous spatial coverage of landscape properties that may be useful for distributed hydrological modeling or other kinds of spatial analyses and to provide a spatial context for point measurements that have been part of the long‐term monitoring described in companion papers. All data are available as separate geographic information system (GIS) layers which can be selected independently according to need. The base map for all the RCEW GIS layers is a 30 m resolution digital elevation model. Data are available in either vector or raster format where appropriate via the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center anonymous ftp site ftp.nwrc.ars.usda.gov.
Identifying eroding and depositional reaches of valley by analysis of suspended sediment Transport in the Sacramento River, California
Authors: Singer Michael Bliss , Dunne Thomas
Published: February 2004

Spatial patterns in suspended sediment transport and storage along the Sacramento River were assessed by evaluating the suspended sediment budget for the main channel accounting for all tributaries and diversions. Time series analysis was employed to quantify the relationship between streamflow and suspended sediment concentration for gauging stations along the main channel and signature tributaries. Sediment concentration records (of 2‐yr duration) were extended using Box‐Jenkins transfer function models to calculate annual rates of suspended sediment discharge over a 32‐year period since dam construction on the Sacramento River. The suspended sediment budget was evaluated to identify reaches of net erosion or deposition. The results of the budget suggest the influence of tectonics and anthropogenic channel modification.
Effect of temperature on surface energy balance
Authors: Wang Jingfeng , Bras Rafael L.
Published: February 2004

The conventional energy balance equation at land surface representing the repartition of net radiation into latent, sensible, and ground heat fluxes was revised to acknowledge the fact that sensible heat flux is always measured at a certain height level within the atmosphere even though the other components are well defined at the surface. A correction factor defined as the ratio of surface (skin) temperature to air temperature associated with the measured sensible heat flux term should be included in the energy balance equation. A preliminary test using independent measurements of radiation and heat fluxes shows that the revision improves the energy balance at the surface.
Pore‐scale modeling of dissolution from variably distributed nonaqueous phase liquid blobs
Authors: Knutson Chad E. , Werth Charles J. , Valocchi Albert J.
Published: February 2004

Contamination of groundwater by nonaqueous phase liquids (NAPLs) is widely recognized as a serious environmental problem. Predicting the dissolution, fate, and transport of these organic chemicals in the subsurface is challenging because geological heterogeneity exists at numerous scales. To better understand heterogeneity at the pore scale, we use the lattice Boltzmann (LB) method to simulate water flow and solute transport from distributed NAPL blobs in a two‐dimensional porous media. The LB method approximates the momentum and mass transport equations at the pore scale, easily incorporating complex boundary conditions of the porous media. The effects of NAPL blob configuration and Peclet number (Pe) on steady state mass transfer are studied at 7% and 15% NAPL saturation. We find that the solute flux out of the simulated system decreases substantially as the transverse length over which NAPL blobs are distributed decreases; for example, the solute flux is reduced by a factor of 2 by confining the NAPL blobs to only half of the transverse length. Values of Sherwood numbers determined from our simulations are slightly less than values determined from previously published mass transfer correlations. Our results indicate that pore‐scale NAPL configuration significantly affects mass transfer and that correlations should be modified to account for it. We find that the dimensionless mass transfer coefficient increases with Pe for the values used in our simulations, where the rate of increase decreases with increasing Pe. We observe that much of the variability in computed mass transfer coefficients is accounted for by differences in the NAPL‐water interfacial area at high Pe. However, at lower Pe, variability remains due to NAPL configuration.
Laboratory study of chemical transport to wells within heterogeneous porous media
Authors: Silliman S. E.
Published: February 2004

A laboratory flow cell was used to study chemical transport to wells producing water from two realizations of a heterogeneous porous medium. The wells produced water from a confined aquifer which was otherwise governed by a mean uniform hydraulic gradient. For both realizations the aquifer was composed of a two‐dimensional, lognormally distributed, second‐order stationary, exponentially correlated conductivity field. For the first realization, response was monitored at a single well 45 integral scales from the inflow boundary. Four wells were used in the second realization, with locations ranging from 10 to 45 integral scales from this boundary. The focus of this work was on the nature of the dispersion observed in the chemical arrival at the pumping well, and this work builds on an earlier study in which it was shown that results under mean uniform flow were relatively consistent with stochastic theories for fluid flow and chemical transport under mean uniform flow. Consistent results were obtained across the two realizations for the wells located 45 integral scales from the boundary as the breakthrough curves demonstrated an increase in dispersive behavior with an increase in the ratio of the pumping rate to the regional flow, consistent with existing theory for finite‐sized sources. In contrast, results for the well located closest to the source (10 integral scales) demonstrated variability in the timing and shape of the breakthrough curves that did not correlate with the ratio of pumping at the well to regional flow. The results indicate that the shape and timing of a breakthrough curve at a well producing within a regional flow field may be strongly dependent on the distance of the well from the source. Further, the parameters (e.g., dispersivity) obtained from analysis of the breakthrough curve are shown to be functions of the ratio of pumping rate at the well to the regional flux.
Accuracy of frequency domain analysis scenarios for the determination of complex dielectric permittivity
Authors: Huisman J. A. , Bouten W. , Vrugt J. A. , Ferré P. A.
Published: February 2004

Frequency domain analysis of time domain reflectometry waveforms has been shown to be useful for more accurate water content determination, water content determination in saline soils, and determination of such difficult to measure soil properties as specific surface area and soil solution conductivity. Earlier frequency domain analysis approaches to determine frequency‐dependent dielectric properties of soils have used a variety of methods. In this paper, these methods for the determination of dielectric permittivity were compared using the Shuffled Complex Evolution Metropolis algorithm (SCEM‐UA). SCEM‐UA is a global optimization method that allows the simultaneous determination of optimal Debye parameters, which describe the dielectric permittivity as a function of frequency, and their confidence intervals. The analysis of numerically generated measurements with added instrumental noise showed that analysis of network analyzer measurements in the frequency domain potentially has the highest accuracy for determination of dielectric permittivity. Furthermore, the analysis of time domain reflectometry waveforms in the time domain was found to be more accurate than analysis of these waveforms in the frequency domain. Analysis of real network analyzer measurements in the time and frequency domain showed that both analysis scenarios allowed reasonably accurate estimates of the Debye parameters with the SCEM‐UA algorithm, even when the true value of a parameter falls beyond the limits of the frequency bandwidth. However, frequency domain analysis of ethanol measurements showed that results were susceptible to model errors caused by nonideal probe behavior. These errors were larger for three‐wire probes than for seven‐wire probes. This study shows that the accuracy of the dielectric permittivity determination can be improved by reducing the model error. This can be achieved by the use of more accurate models, such as multiscatter functions, and by using more advanced probes, such as coaxial cells. The results also imply that future research on dielectric properties of soils should focus more on the use of network analyzers instead of cable testers, since model errors are more obvious in the frequency domain. The SCEM‐UA algorithm proved to be a valuable tool in frequency domain analysis because reported problems with parameter identification and initialization of the optimization are circumvented with this global optimization algorithm.
SABER observations of mesospheric temperatures and comparisons with falling sphere measurements taken during the 2002 summer MaCWAVE campaign
Authors: Mertens Christopher J. , Schmidlin Francis J. , Goldberg Richard A. , Remsberg Ellis E. , Pesnell W. Dean , Russell James M. , Mlynczak Martin G. , López‐Puertas Manuel , Wintersteiner Peter P. , Picard Richard H. , Winick Jeremy R. , Gordley Larry L.
Published: February 2004

The SABER instrument was launched onboard the TIMED satellite in December 2001. Vertical profiles of kinetic temperature (Tk) are derived from broadband measurements of CO2 15 μm limb emission, in combination with measurements of CO2 4.3 μm limb emission used to derive CO2 volume mixing ratio (vmr). Infrared emission from the CO2 ro‐vibrational bands are in non‐local thermodynamic equilibrium (non‐LTE) in the mesosphere and lower thermosphere (MLT), requiring new radiation transfer and retrieval methods. In this paper we focus on Tk and show some of the first SABER observations of MLT Tk and compare SABER Tk profiles with rocket falling sphere (FS) measurements taken during the 2002 summer MaCWAVE campaign at Andøya, Norway (69°N, 16°E). The comparisons are very encouraging and demonstrate a significant advance in satellite remote sensing of MLT limb emission and the ability to retrieve Tk under extreme non‐LTE conditions.
Temporal variations in frost‐free season in the United States: 1895–2000
Authors: Kunkel Kenneth E. , Easterling David R. , Hubbard Kenneth , Redmond Kelly
Published: February 2004

A newly available data set of daily temperature observations was used to study the temporal variability of the frost‐free season, based on an inclusive 0°C threshold, for 1895–2000 in the conterminous United States. A national average time series of the length of the frost‐free season is characterized by 3 distinct regimes. The period prior to 1930 was notable for decreasing frost‐free season length from 1895 to a minimum around 1910, followed by a marked increase in length of about 1 week from 1910 to 1930. During 1930–1980, frost‐free season length was near the period average with relatively little decadal‐scale variability. Since 1980, frost‐free season length has increased by about 1 week. The national average increase in frost‐free season length from the beginning to the end of the 20th Century is about 2 weeks. Frost‐free season length has increased much more in the western U.S. than in the eastern U.S.
Temporal variation of total gaseous mercury in the air of Guiyang, China
Authors: Feng Xinbin , Shang Lihai , Wang Shaofeng , Tang Shunlin , Zheng Wei
Published: February 2004

Total gaseous mercury (TGM) concentrations in ambient air were monitored at an urban site in Guiyang from 23 November 2001 to 30 November 2002 using a high temporal resolution 5 min mercury vapor analyzer (Tekran 2537A). TGM concentrations follow lognormal frequency distribution pattern, and the mean TGM concentration at the measurement site is 8.40 ng m−3 on the basis of 1 year observation. TGM concentrations in Guiyang are significantly elevated compared to the continental global background values. Coal combustion from both industrial and domestic uses is estimated to be the primary atmospheric source. A seasonal distribution pattern of TGM with a descending order of winter, spring, fall, and summer was observed. The highest TGM concentration in winter is attributed to household heating using coal. A consistent diurnal distribution pattern of TGM was obtained among all seasons, and the nighttime TGM concentration is elevated compared to the daytime values. Meteorological conditions are responsible for the formation of diurnal TGM distribution pattern. Tremendous efforts are needed to reduce mercury emissions from coal combustion to decrease TGM concentrations in Guiyang.
Fractal relation of mainstream length to catchment area in river networks
Authors: Rosso Renzo , Bacchi Baldassare , La Barbera Paolo
Published: February 2004

Mandelbrot's (1982) hypothesis that river length is fractal has been recently substantiated by Hjelmfelt (1988) using eight rivers in Missouri. The fractal dimension of river length, d, is derived here from the Horton's laws of network composition. This results in a simple function of stream length and stream area ratios, that is, d = max (1, 2 log RL/log RA). Three case studies are reported showing this estimate to be coherent with measurements of d obtained from map analysis. The scaling properties of the network as a whole are also investigated, showing the fractal dimension of river network, D, to depend upon bifurcation and stream area ratios according to D = min (2, 2 log RB/log RA). These results provide a linkage between quantitative analysis of drainage network composition and scaling properties of river networks.
Radial flow in heterogeneous porous media: An analysis of specific discharge
Authors: Naff R. L.
Published: February 2004

A perturbation solution to three‐dimensional radial flow in heterogeneous porous media is reported on; the analytical solution is in terms of the first and second moments of the gradient in the radial direction and the specific discharge in this direction. The solution requires evaluation, numerically in the case of this paper, of some rather difficult integrals which compose its basic form. By proper adjustment of the statistical anisotropy of the medium, a solution for the two‐dimensional flow problem can be emulated; the two‐dimensional solution for the mean and variance in specific discharge resembles asymptotes reported earlier by another investigator. In general, the effective hydraulic conductivity, outside a few length scales of the well itself, is found to be nearly constant and dependent upon the value selected for the statistical anisotropy factor; the variance in specific discharge behaves nearly as the inverse square of the distance from the well in all cases.
The geostatistical characteristics of the borden aquifer
Authors: Woodbury Allan D. , Sudicky E. A.
Published: February 2004

A complete reexamination of Sudicky's (1986) field experiment for the geostatistical characterization of hydraulic conductivity at the Borden aquifer in Ontario, Canada is performed. The sampled data reveal that a number of outliers (low ln (K) values) are present in the data base. These low values cause difficulties in both variogram estimation and determining population statistics. The analysis shows that assuming either a normal distribution or exponential distribution for log conductivity is appropriate. The classical, Cressie/Hawkins and squared median of the absolute deviations (SMAD) estimators are used to compute experimental variograms. None of these estimators provides completely satisfactory variograms for the Borden data with the exception of the classical estimator with outliers removed from the data set. Theoretical exponential variogram parameters are determined from nonlinear (NL) estimation. Differences are obtained between NL fits and those of Sudicky (1986). For the classical‐screened estimated variogram, NL fits produce an ln (K) variance of 0.24, nugget of 0.07, and integral scales of 5.1 m horizontal and 0.21 m vertical along A–A′. For B–B′ these values are 0.37, 0.11, 8.3 and 0.34. The fitted parameter set for B–B′ data (horizontal and vertical) is statistically different than the parameter set determined for A–A′. We also evaluate a probabilistic form of Dagan's (1982, 1987) equations relating geostatistical parameters to a tracer cloud's spreading moments. The equations are evaluated using the parameter estimates and covariances determined from line A–A′ as input, with a velocity equal to 9.0 cm/day. The results are compared with actual values determined from the field test, but evaluated by both Freyberg (1986) and Rajaram and Gelhar (1988). The geostatistical parameters developed from this study produce an excellent fit to both sets of calculated plume moments when combined with Dagan's stochastic theory for predicting the spread of a tracer cloud.