bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of Momentum and Heat Fluxes Using Dissipation and Flux‐Variance Methods in the Unstable Surface Layer
Authors: Hsieh Cheng‐I , Katul Gabriel G. , Schieldge John , Sigmon John , Knoerr Kenneth R.
Published: February 2004

Dissipation and flux‐variance methods, derived from the turbulent kinetic energy and temperature variance budget equations in conjunction with Monin‐Obukov similarity theory, were used to estimate surface fluxes of momentum and sensible heat. To examine the performance of these two methods, direct eddy correlation measurements were carried out above a nonuniform grass‐covered forest clearing in Durham, North Carolina. The dissipation method sensible heat flux predictions were in good agreement with eddy correlation measurements. Also, the flux‐variance method reproduced the measured sensible heat flux well following an adjustment to the similarity constant. However, the momentum flux (or friction velocity) estimated by the dissipation and flux‐variance methods were both inferior to those for sensible heat flux. The data from this experiment indicated that the above two methods are sensitive to the dimensionless wind shear (ϕm) and temperature standard deviation (ϕθ) functions. On the basis of dimensional analysis and the temperature variance budget equation a new dissipation approach for estimating sensible heat flux was derived. The similarity constant for this new approach was shown to be around 1.6 for uniform surfaces and from the data of this experiment.
A Deterministic Geometric Representation of Temporal Rainfall: Results for a Storm in Boston
Authors: Puente Carlos E. , Obregón Nelson
Published: February 2004

The use of a deterministic fractal‐multifractal (FM) representation to model high‐resolution rainfall time series via projections of fractal interpolating functions weighed by multifractal measures is reported. It is shown that the intrinsic shape and variability of an 8‐hour Boston storm recorded every 15 s on October 25, 1980, may be encoded wholistically, employing the fractal geometric methodology. It is illustrated that the FM methodology provides very faithful descriptions of both major trends and small (noisy) fluctuations for this storm, resulting in preservation of not only classical statistical characteristics of the records but also multifractal and chaotic properties present in them. These results, and those for other storms, suggest that a stochastic framework for rainfall may be bypassed in favor of a deterministic representation based on projections.
The Impact of Head Gradient Transients on Transport in Heterogeneous Formations: Application to the Borden Site
Authors: Bellin Alberto , Dagan Gedeon , Rubin Yoram
Published: February 2004

A three‐dimensional interpretation of the Borden Site experiment is proposed with the aid of a recently developed stochastic model that incorporates transiency of the piezometric head gradient. The behavior of the second‐order central transverse plume moments is analyzed with the aim of explaining the underprediction of experimental results by existing steady state models. The model assumes uniformity in space, but time varying mean head gradient, stationary and anisotropic log conductivity, and a first‐order approximation in the log conductivity variance. The solution for the trajectory covariances, assumed to be equal to the plume spatial second moments under ergodic conditions, is evaluated with the aid of a few quadratures. An analysis of the parameters and plume spatial moments found in the literature precedes application of the model. It is found that unsteadiness leads to an increase in the transverse, horizontal, second moment compared with the one based on a steady state flow model. Still, application of Borden Site data leads to values lower than the ones inferred from concentration measurements. We conclude that unsteadiness of the mean head gradient does not fully explain the magnitude of observed transverse spreading. However, the impact of transients on spreading is significant in the transverse direction, and the definition of a Fickian transverse dispersion coefficient may not be a simple task for transport occurring under natural flow conditions.
Models of the water retention curve for soils with a fractal pore size distribution
Authors: Perrier Edith , Rieu Michel , Sposito Garrison , de Marsily Ghislain
Published: February 2004

The relationship between water content and water potential for a soil is termed its water retention curve. This basic hydraulic property is closely related to the soil pore size distribution, for which it serves as a conventional method of measurement. In this paper a general model of the water retention curve is derived for soils whose pore size distribution is fractal in the sense of the Mandelbrot number‐size distribution. This model, which contains two adjustable parameters (the fractal dimension and the upper limiting value of the fractal porosity) is shown to include other fractal approaches to the water retention curve as special cases. Application of the general model to a number of published data sets covering a broad range of soil texture indicated that unique, independent values of the two adjustable parameters may be difficult to obtain by statistical analysis of water retention data for a given soil. Discrimination among different fractal approaches thus will require water retention data of high density and precision.
Sources of moisture for rainfall in West Africa
Authors: Gong Cuiling , Eltahir Elfatih
Published: February 2004

The objective of this study is to identify the sources of moisture for rainfall in west Africa. A model of precipitation recycling is developed and applied to the region of west Africa to obtain quantitative estimates of the moisture contributed by local evaporation as well as the moisture contributed by the zonal and meridional fluxes from the surrounding regions. We estimated the recycling ratio for the entire region by specifying three subregions where evaporation is treated as the source of moisture: west Africa, central Africa, and the tropical Atlantic Ocean. We find that evaporation from the tropical Atlantic Ocean, west Africa, and central Africa contribute about 23, 27, and 17% of rainfall in west Africa, respectively. Moisture fluxes from the tropical Atlantic are almost in phase with rainfall in west Africa. However, we find that moisture supply from central Africa is strongly regulated and limited by the westerly flow associated with the monsoon circulation. Hence the large‐scale monsoon circulation is not only the main forcing of rainfall over west Africa, but the dynamics of this circulation exert significant control on where the moisture comes from.
Effect of land use changes on sediment transport in Goodwin Creek
Authors: Kuhnle Roger A. , Bingner Ronald L. , Foster George R. , Grissinger Earl H.
Published: February 2004

The Goodwin Creek Research Watershed (21.3 km2) is located in the north central part of Mississippi in the bluff hills just east of the Mississippi River floodplain. Land use on the watershed has been surveyed annually and the percentage of cultivated land has decreased from 26% in 1982 to 12% in 1990. During this 9‐year period the concentration of fines (<0.062 mm) in Goodwin Creek have decreased by 62%, concentrations of sand (0.062–2.0 mm) have decreased by 66%, and concentrations of gravel (>2.0 mm) have decreased by 39%. The decrease in the percentage of cultivated land affects the sediment budget of the watershed in two ways. A source of readily eroded sediment is removed, and the energy of the flowing water available to erode and transport sediment is reduced. The reduced flow in the channels from the decrease in cultivated land in the watershed was probably the main cause for the lower transport rates of sand and gravel.
Analytical Integration of the Kinematic Equation for Runoff on a Plane Under Constant Rainfall Rate and Smith and Parlange Infiltration
Authors: Giráldez J. V. , Woolhiser D. A.
Published: February 2004

An analytical solution to the kinematic wave approximation of the Saint‐Venant equations describing surface water flow with infiltration is presented. Adopting the Smith and Parlange formulation for the ponding time and the postponding infiltration rate, a change of variable among time and infiltration rate allow the analytical integration of the characteristic equations when the flow rate is expressed as the second power of the water depth. In the case of any other real exponent a simple numerical integration method may be used for the computation of the characteristic curves. An illustration is made to compare the solutions for different values of the exponent, as well as to assess the accuracy of some numerical schemes.
Accounting‐Induced Distortion in Public Enterprise Pricing
Authors: Moncur James E. T. , Pollock Richard L.
Published: February 2004

Municipal water utilities commonly aim to set prices at average cost. Because of various omissions and owing to inflation, unadjusted accounting data understate the economic costs of fixed assets and thus generate inefficiently low prices and high consumption rates for the output of these enterprises. We investigate the nature and extent of undercosting and underpricing for a group of large urban water utilities in the United States. Economic costs appear to be significantly greater than the corresponding accounting measures for the cases studied.
On Channel Network Fractal Properties: A Case of Study of the Hutt River Basin, New Zealand
Authors: Nikora Vladimir , Ibbitt Richard , Shankar Ude
Published: February 2004

The paper considers river networks as three‐dimensional self‐affine fractal objects. The Hutt River basin (New Zealand∥ was selected for detailed analysis on the basis of a digital elevation model (DEM). To characterize network properties quantitatively we used three scaling exponents in the relationships l ∝ ℒυl, wℒυw, and h ∝ ℒυh where l, w, h are some characteristic longitudinal, transversal, and vertical scales of a channel network; ℒ is the total length of channel network in three‐dimensional space; and υl, υw, and υh are the self‐affine scaling exponents. We determined υl, υw, and υh using Lp ∝ Aβ, ℒp ∝ Aϵ, and S ∝ A−θ, where Lp is the length of the projection of the longest river channel on the horizontal plane, ℒp is the total length of channel network projection on the horizontal plane, A is the catchment area, and S is the local slope. An approximate relationship υh ≈ υl − θ(υl + υw) is derived which connects the main scaling exponents. For two New Zealand rivers, we found υl=0.60 and υw=0.40. On the basis of simple considerations, we estimated a range of possible values of υh from 0.1 to 0.5 with 0.2 for the case study. The slope‐area‐elevation relation introduced by Willgoose [1994] was applied to interpret data concerning υh. The influence of threshold area (TA) values on the scaling properties of channel networks is shown to be small, and double scaling relationships are suggested for connecting the physical scaling of channel networks with scaling caused by threshold effect.
Effect of saltating sediment load on the determination of the mean velocity of overland flow
Authors: Li Gang , Abrahams Athol D.
Published: February 2004

Where overland flow velocity is measured using dye or salt tracing, the mean velocity is often determined by multiplying the velocity of the leading edge of the tracer plume by a correction factor α. Flume experiments show that a varies with both Reynolds number and sediment load. For sediment‐free flow over a sand‐covered bed, α is less than the theoretical value of 0.67 in laminar flow and increases rapidly with Reynolds number in transitional flow and more slowly with Reynolds number in turbulent flow. For sediment‐laden flow, α decreases as sediment load increases in transitional and turbulent flows. Saltating sediment extracts momentum from the flow, causing velocities near the bed to decrease and α to decrease. A multiple‐regression equation is developed which can be used to predict the value of α from Reynolds number and sediment load in transitional and turbulent overland flows.
The (mis)behavior of behavior analysis storage estimates
Authors: Pretto Paul B. , Chiew Francis H. S. , McMahon Thomas A. , Vogel Richard M. , Stedinger Jery R.
Published: February 2004

This paper investigates the dependence of estimates of reservoir storage capacity derived using behavior analysis on the length of inflow sequence used for overyear reservoir simulation. It has generally been assumed that simulation using behavior analysis, which incorporates a given probability of failure, will always give steady state estimates of the storage capacity (apart from the effects of the initial reservoir condition). The results reported here show that it may take sequence lengths as much as 1000 years or more for the mean of the distribution of storage capacity estimates to approach a stationary value. For some cases with high draft and high inflow variability, we show that a significant swing of the mean storage estimates from an initial downward bias into an upward bias occurs before their convergence to a stationary level. However, the median storage estimates always showed downward bias which sometimes decreased very slowly with increasing sequence length. We provide explanations for these observations and discuss some of the implications on the choice of inflow sequence length in determining reservoir storage capacities using behavior analysis.
Effects of solution mining of salt on wetland hydrology as inferred from tree rings
Authors: Yanosky Thomas M. , Kappel William M.
Published: February 2004

Radial growth and concentrations of selected elements within rings were studied in white pine (Pinus strobus) trees from a wetland in central New York approximately 5 km north of a salt‐solution mining field that operated from 1889 to 1988. Trees seemingly document three sequential episodes of mine‐induced alterations of groundwater discharge irrigating the wetland during the 100‐year period. The radial growth of trees established before the onset of mining declined abruptly in the early 1890s and remained suppressed until about 1960, as did growth of numerous other trees that became established after the onset of mining. Suppressed pre‐1960 radial growth coincided with the interval that surface water was injected into the saltbeds, suggesting that losses of injected water to the bedrock and/or unconsolidated deposits increased groundwater flow into the wetland. An abrupt and sustained enhancement of radial growth beginning about 1960 indicates that the wetland became drier, and thus more conducive to tree growth, when injection practices were discontinued in the late 1950s despite the continued pumping of brine. Following the cessation of mining in the late 1980s, head pressures again increased in the upper valley, driving chloride‐enriched flow northward along regional bedding‐plane fractures and into the wetland. Large concentrations of chloride were detected within the most recently formed rings of some trees. As the result of chloride‐enriched irrigation, the radial growth of some trees declined, and some trees died. Thus trees have preserved evidence of a century of hydrologic alterations, unobtainable by other means, where the effects of brine mining have not been documented previously.
Operating rules for multireservoir systems
Authors: Oliveira Rodrigo , Loucks Daniel P.
Published: February 2004

Multireservoir operating policies are usually defined by rules that specify either individual reservoir desired (target) storage volumes or desired (target) releases based on the time of year and the existing total storage volume in all reservoirs. This paper focuses on the use of genetic search algorithms to derive these multireservoir operating policies. The genetic algorithms use real‐valued vectors containing information needed to define both system release and individual reservoir storage volume targets as functions of total storage in each of multiple within‐year periods. Elitism, arithmetic crossover, mutation, and “en bloc” replacement are used in the algorithms to generate successive sets of possible operating policies. Each policy is then evaluated using simulation to compute a performance index for a given flow series. The better performing policies are then used as a basis for generating new sets of possible policies. The process of improved policy generation and evaluation is repeated until no further improvement in performance is obtained. The proposed algorithm is applied to example reservoir systems used for water supply and hydropower.
Stochastic analysis of concentration measurements in the transport experiment at Twin Lake Site
Authors: Dagan G. , Indelman P. , Moltyaner G.
Published: February 2004

A procedure to identify the parameters characterizing flow and transport in heterogeneous aquifers with the aid of concentration measurements in tracer field experiments is developed. Unlike previous studies, which employed the measured plume spatial moments at different times and their theoretical expressions, we rely here on breakthrough curves and temporal moments in order to analyze the field tests at Chalk River Site. In these experiments, breakthrough curves of a radioactive tracer were measured continuously at a large number of points in parallel control planes. We derive theoretical expressions of the temporal moments of the breakthrough curves by the same Lagrangian approach that was used previously for spatial moment. We assume a stationary random velocity field of constant mean and relate it to the axisymmetric log conductivity covariance by a first‐order approximation in the variance, with neglect of the effect of pore‐scale dispersion. The final theoretical results relate the temporal moments to U (the mean velocity), σY2 (the log conductivity variance), IYh (the horizontal integral scale), and b(e) (a function of the anisotropy ratio e=IYυ/IYh). By assuming ergodicity, we identify the temporal moments at the Chalk River Site experiment from measured breakthrough curves. With the aid of the theoretical results and by a best fit we could estimate U, σY2, IYh, and IYh/b(e). An attempt to identify the vertical and transverse integral scales from temporal‐spatial moments in the control planes was not successful. We took advantage of the dense measurements of breakthrough curves along vertical transects (at intervals of 1 cm) in order to identify the experimental concentration two‐point covariance. We derived a simplified theoretical expression for its dependence on the log conductivity vertical integral scale IYυ, which was identified by a best fit with experimental results. This procedure, applied for the first time to analysis of field tests, led to the identification of the estimates of σY2 and IYh, as well as of their variances of estimation.
Comparison of annual maximum series and partial duration series methods for modeling extreme hydrologic events: 2. Regional modeling
Authors: Madsen Henrik , Pearson Charles P. , Rosbjerg Dan
Published: February 2004

Two regional estimation schemes, based on, respectively, partial duration series (PDS) and annual maximum series (AMS), are compared. The PDS model assumes a generalized Pareto (GP) distribution for modeling threshold exceedances corresponding to a generalized extreme value (GEV) distribution for annual maxima. First, the accuracy of PDS/GP and AMS/GEV regional index‐floodT‐year event estimators are compared using Monte Carlo simulations. For estimation in typical regions assuming a realistic degree of heterogeneity, the PDS/GP index‐flood model is more efficient. The regional PDS and AMS procedures are subsequently applied to flood records from 48 catchments in New Zealand. To identify homogeneous groupings of catchments, a split‐sample regionalization approach based on catchment characteristics is adopted. The defined groups are more homogeneous for PDS data than for AMS data; a two‐way grouping based on annual average rainfall is sufficient to attain homogeneity for PDS, whereas a further partitioning is necessary for AMS. In determination of the regional parent distribution usingL‐ moment ratio diagrams, PDS data, in contrast to AMS data, provide an unambiguous interpretation, supporting a GP distribution.
Generalized least squares and empirical bayes estimation in regional partial duration series index‐flood modeling
Authors: Madsen Henrik , Rosbjerg Dan
Published: February 2004

A regional estimation procedure that combines the index‐flood concept with an empirical Bayes method for inferring regional information is introduced. The model is based on the partial duration series approach with generalized Pareto (GP) distributed exceedances. The prior information of the model parameters is inferred from regional data using generalized least squares (GLS) regression. Two different BayesianT‐year event estimators are introduced: a linear estimator that requires only some moments of the prior distributions to be specified and a parametric estimator that is based on specified families of prior distributions. The regional method is applied to flood records from 48 New Zealand catchments. In the case of a strongly heterogeneous intersite correlation structure, the GLS procedure provides a more efficient estimate of the regional GP shape parameter as compared to the usually applied weighted regional average. If intersite dependence is ignored, the uncertainty of the regional estimator may be seriously underestimated and erroneous conclusions with respect to regional homogeneity may be drawn. The GLS procedure is shown to provide a general framework for a reliable evaluation of parameter uncertainty as well as for an objective appraisal of regional homogeneity. A comparison of the two different BayesianT‐year event estimators reveals that generally the simple linear estimator is adequate.
The influence of physical heterogeneity on microbial degradation and distribution in porous media
Authors: Murphy Ellyn M. , Ginn Timothy R. , Chilakapati Ashokkumar , Resch C. Thomas , Phillips Jerry L. , Wietsma Thomas W. , Spadoni Christina M.
Published: February 2004

Intermediate‐scale experiments (meter‐long, two‐dimensional flow cell) were performed with aerobic biodegradation of benzoate substrate in physically heterogeneous (bimodal inclusive) media. Clastic heterogeneities were represented in a quasi‐two‐dimensional field, with low‐conductivity inclusions embedded in a high‐conductivity sandy matrix. The two media had similar pore‐scale dispersivities but the conductivity ratio (∼1∶50) incurred macrodispersive spreading in the longitudinal direction. The high‐conductivity sand was uniformly inoculated with Pseudomonas cepacia sp., and a pulse input of substrate and chloride ion tracer were evaluated. Degradation and growth were oxygen‐limited under nonlinear dual‐Monod kinetics and controlled by spatial and temporal variations in nutrient flux. The low‐conductivity inclusions created regions of slow transport that prolonged the dual availability of both oxygen and substrate, which in turn enhanced microbial growth in these regions. Bacterial detachment was significant, and the fivefold increase in biomass due to growth was entirely accounted for in the aqueous effluent which displayed a complicated nonlinear breakthrough curve. High‐resolution deterministic modeling was applied to simulate the intermediate‐scale experiment, with parameters of the relevant constitutive relations calibrated independently through batch and small‐scale column experiments. Parameter fitting to match flow cell data was avoided. This approach was taken in order both to test the predictive modeling capability as it would necessarily be used in a field application and to avoid the a priori assumption that all relevant processes were adequately represented in the respective constitutive theories. Analyses of the fit between the independently calibrated model and the flow cell data were then used to isolate processes for further experimental study. This iterative experimental/modeling approach identified processes that contributed (surprisingly) to biodegradation in heterogeneous media and yet are not currently incorporated in most mathematical models: (1) buoyancy effects associated with very small solution density variations, amplified in heterogeneous media, and (2) dynamic biological processes associated with growth, namely, endogenous respiration, cell division partitioning to the aqueous phase, and active adhesion/detachment that are strongly coupled to the transport of dissolved nutrients or microorganisms.
Chemical transfer from soil solution to surface runoff
Authors: Zhang X. C. , Norton D. , Nearing M. A.
Published: February 2004

Three soils were exposed to three consecutive simulated rains under dry, wet, and water table conditions with gypsum as a tracer placed at a 5‐mm depth to evaluate the extent and nature of chemical transfer from soil solution to runoff and to determine the effective depth of the mixing zone. No electrolyte release was detected in runoff during the dry run for any of the soils. Electrical conductivity decreased exponentially with time during the initial stages of the wet and water table runs. Results were consistent with the complete mixing concept, but they also suggest that the assumption of no chemical transfer into the mixing zone from below should be modified. The effective mixing depth appears to be less than 3–4 mm. Two timescale processes were identified. The fast rate process, driven by raindrop impact and confined to the mixing zone, causes an exponential depletion of chemicals from that zone. The slow rate process, dominated by molecular diffusion and mechanical dispersion, describes chemical transfer to the mixing zone from below. The identification of the two processes indicates that the fast rate process is adequate for approximating chemical loss under free drainage conditions, while otherwise the slow rate process must be considered.
Variations of δ13C in the surface soil organic carbon pool
Authors: Bird M. I. , Pousai P.
Published: February 2004

This study examines the regional distribution of the stable isotopes of organic carbon in the surface soils (SOC) of a variety of biomes including forests, savannas, and grasslands. A transect through tropical/subtropical biomes in northern Australia demonstrates that forest and grassland soils exhibit comparatively small variations in δ13C value on both local and regional scales. Savanna soil δ13C values exhibit extreme variability at all spatial scales with samples separated by only a few meters differing by up to 6.6‰, and a total range of values for savanna samples from −15.9 to −26.6‰. Forest surface SOC has an average δ13C value of −28.4±0.7‰ (1σ), while tropical grasslands (C4‐dominated) have an average δ13C value of −15.5±0.8‰ (1σ) and temperate grasslands (C3‐dominated) −26.0±l.l‰ (1σ). Despite extreme variability between savanna samples, there is a consistent relationship between δ13C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low δ13C values and high SOC contents, and tropical grasslands with high δ13C values and low SOC contents. The relationship suggests that an integrated regional δ13C value for SOC is a useful proxy for terrestrial carbon storage. River sediment δ13C values from the transect region reflect the δ13C values obtained for the regional soils, with a bias toward the C3 end‐member. Size‐fractionated “average” soils from a variety of biomes suggest that little isotopic fractionation accompanies degradation but that in mixed C3/C4 biomes, C3‐derived carbon is preferentially incorporated into the coarse size fractions, while C4‐derived carbon is preferentially added to the fine size fractions.
Nitrate leaching in coniferous forest ecosystems: The European Field‐Scale Manipulation Experiments NITREX (Nitrogen Saturation Experiments) and EXMAN (Experimental Manipulation of Forest Ecosystems)
Authors: Tietema Albert , Beier Claus , de Visser Pieter H. B. , Emmett Bridget A. , Gundersen Per , Kjønaas O. Janne , Koopmans Chris J.
Published: February 2004

The results of two European field‐scale manipulation projects (NITREX (nitrogen saturation experiments) and EXMAN (experimental manipulation of forest ecosystems)) were used to evaluate the effect of ecosystem disturbance on nitrate leaching in coniferous forest ecosystems. The first principle component (PC1) of a principle component analysis explained 85% of the variation in nitrate leaching between the 12 sites. This PC1 consisted of nitrogen concentrations and fluxes in the ecosystem and was interpreted as an indicator of N status. Nitrate leaching responded rapidly to manipulation of nitrogen deposition, especially in sites with ambient high nitrate leaching. This rapid response could be explained partly by an immediate hydrological response of increased drainage. However, results of field‐scale 15N tracer experiments indicated that microbial processes in the organic layer had changed after a few years of changed N deposition. In sites with already significant nitrate leaching, irrigation caused a large increase in nitrate leaching due to enhanced mineralization. Combined fertilization and irrigation had only a limited effect on nitrate leaching in nitrogen‐limited sites, whereas in nitrogen‐saturated sites, nitrate leaching was significantly increased. The hypothesized nitrate pulse as a result of rewetting after drought did not occur in any of the sites. We conclude that the effect of disturbance on nitrate leaching depends on the N status of the ecosystem: in sites that are nitrogen‐saturated, nitrate leaching is very sensitive to disturbance.
Methane emissions from wetlands in the zone of discontinuous permafrost: Fort Simpson, Northwest Territories, Canada
Authors: Liblik L. K. , Moore T. R. , Bubier J. L. , Robinson S. D.
Published: February 2004

We examined the influence of water table position, temperature, vegetation, and CH4 storage on CH4 flux at 22 wetland sites located in the zone of discontinuous permafrost, near Fort Simpson, Northwest Territories. Sites ranged from frozen peat plateaux and palsas with collapse scars to fens and bogs and ponds. Mean summer CH4 emissions ranged from −1.3 to 255 mg m−2 d−1, and the variation among sites was best explained by mean water table position (r2 = 0.62, n = 19). Large CH4 fluxes (>30 mg m−2 d−1) and high water tables occurred in ponds and graminoid sites, whereas small fluxes (<30 mg m−2 d−1) and low water tables occur in shrub and woody sites. These relationships were upheld across several northern boreal regions, based on water table depth and broad ecological criteria. CH4 turnover times were generated by comparing surficial fluxes and storage within the saturated portion of the peat profile. Graminoid sites had CH4 turnover times from 1 to 4 days, whereas shrub or woody sites had longer turnover times, ranging from weeks to years. We estimated an overall flux of ∼18 mg m−2 d−1 for the landscape around Fort Simpson. This CH4 flux may increase with the onset of global warming, owing to the fragility of permafrost, resulting in the formation of collapse scars with high CH4 emission rates.
The contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric carbon dioxide
Authors: Randerson James T. , Thompson Matthew V. , Conway Thomas J. , Fung Inez Y. , Field Christopher B.
Published: February 2004

We characterized decadal changes in the amplitude and shape of the seasonal cycle of atmospheric CO2 with three kinds of analysis. First, we calculated the trends in the seasonal cycle of measured atmospheric CO2 at observation stations in the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostic Laboratory network. Second, we assessed the impact of terrestrial ecosystems in various localities on the mean seasonal cycle of CO2 at observation stations using the Carnegie‐Ames‐Stanford Approach terrestrial biosphere model and the Goddard Institute for Space Studies (GISS) atmospheric tracer transport model. Third, we used the GISS tracer model to quantify the contribution of terrestrial sources and sinks to trends in the seasonal cycle of atmospheric CO2 for the period 1961–1990, specifically examining the effects of biomass burning, emissions from fossil fuel combustion, and regional increases in net primary production (NPP). Our analysis supports results from previous studies that indicate a significant positive increase in the amplitude of the seasonal cycle of CO2 at Arctic and subarctic observation stations. For stations north of 55°N the amplitude increased at a mean rate of 0.66% yr−1 from 1981 to 1995. From the analysis of ecosystem impacts on the mean seasonal cycle we find that tundra, boreal forest, and other northern ecosystems are responsible for most of the seasonal variation in CO2 at stations north of 55°N. The effects of tropical biomass burning on trends in the seasonal cycle are minimal at these stations, probably because of strong vertical convection in equatorial regions. From 1981 to 1990, fossil fuel emissions contributed a trend of 0.20% yr−1 to the seasonal cycle amplitude at Mauna Loa and less than 0.10% yr−1 at stations north of 55°N. To match the observed amplitude increases at Arctic and subarctic stations with NPP increases, we find that north of 30°N a 1.7 Pg C yr−1 terrestrial sink would be required. In contrast, over regions south of 30°N, even large NPP increases and accompanying terrestrial sinks would be insufficient to account for the increase in high‐latitude amplitudes.
Domestic biomass combustion and associated atmospheric emissions in West Africa
Authors: Brocard Delphine , Lacaux Jean‐Pierre , Eva Hugh
Published: February 2004

Biofuel is the main source of energy for cooking and heating in Africa. In order to estimate the consumption of this resource at a regional level, a database with a spatial resolution of 1° latitude by 1° longitude of the distribution of the amounts of fuel wood and charcoal annually burned in West Africa has been derived. Chemical emission factors for fuel wood, for charcoal burning, and for charcoal fabrication measured during two field experiments are then used in conjunction with this database to produce a second 1° latitude by 1° longitude database of the emissions due to domestic fires for the region. A comparison of these emissions from domestic fires with those of savanna fires, the dominant form of biomass burning in tropical Africa, shows that the relative contribution of the wood fuel (i.e. fuel wood and charcoal) combustion is important for CH4(46%), CO (42%), and nonmethane hydrocarbons (NMHC) (44%), less so for CO2(32%). This source of biomass burning has a different spatial and temporal distribution than that of savanna fires and represents an atmospheric background noise throughout the year, whereas the savanna fires occur during a limited season.
Mid‐Holocene land‐surface conditions in northern Africa and the Arabian Peninsula: A data set for the analysis of biogeophysical feedbacks in the climate system
Authors: Hoelzmann P. , Jolly D. , Harrison S. P. , Laarif F. , Bonnefille R. , Pachur H.‐J.
Published: February 2004

Large changes in the extent of northern subtropical arid regions during the Holocene are attributed to orbitally forced variations in monsoon strength and have been implicated in the regulation of atmospheric trace gas concentrations on millenial timescales. Models that omit biogeophysical feedback, however, are unable to account for the full magnitude of African monsoon amplification and extension during the early to middle Holocene (˜9500–5000 years B.P.). A data set describing land‐surface conditions 6000 years B.P. on a 1° × 1° grid across northern Africa and the Arabian Peninsula has been prepared from published maps and other sources of palaeoenvironmental data, with the primary aim of providing a realistic lower boundary condition for atmospheric general circulation model experiments similar to those performed in the Palaeoclimate Modelling Intercomparison Project. The data set includes information on the percentage of each grid cell occupied by specific vegetation types (steppe, savanna, xerophytic woods/scrub, tropical deciduous forest, and tropical montane evergreen forest), open water (lakes), and wetlands, plus information on the flow direction of major drainage channels for use in large‐scale palaeohydrological modeling. The data set is available in digital form by anonymous ftp.
Relationship between ecosystem productivity and photosynthetically active radiation for northern peatlands
Authors: Frolking S. E. , Bubier J. L. , Moore T. R. , Ball T. , Bellisario L. M. , Bhardwaj A. , Carroll P. , Crill P. M. , Lafleur P. M. , McCaughey J. H. , Roulet N. T. , Suyker A. E. , Verma S. B. , Waddington J. M. , Whiting G. J.
Published: February 2004

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy‐correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE‐PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE‐PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.
Aircraft sulfur emissions and the formation of visible contrails
Authors: Brown R. C. , Miake‐Lye R. C. , Anderson M. R. , Kolb C. E.
Published: February 2004

Contrail formation in the exhaust plume of the ATTAS aircraft engine burning fuels with 2 ppmm and 266 ppmm sulfur has been studied using an aerosol dynamics model coupled to a 2‐dimensional, axisymmetric flow code. For both the low and high sulfur fuels, the model predicted approximately 35% of the available water condenses within 200 m downstream of the exhaust exit. However, particle size distributions for the low sulfur plume are broader and extend to larger sizes. Model results indicate that, for the engine and flight conditions treated, sulfuric acid is a viable soot activating agent when the fuel sulfur mass loading is reduced to 2 ppmm and that differences in the contrail particle size distribution for sulfur mass loadings between 2 ppmm and 266 ppmm would be difficult to detect.
Transmittance of a cloud is wavelength‐dependent in the UV‐range: Physical interpretation
Authors: Kylling A. , Albold A. , Seckmeyer G.
Published: February 2004

In the recently published GRL paper by Seckmeyer et al. [1996] an example of a cloud that has a wavelength dependent effect on the surface UV‐radiation is given. Through careful and accurate radiative transfer modelling, the present paper aims to give a physical interpretation of the reported wavelength dependence of this particular cloud. The modelling shows that the transmission of the cloud alone does not vary significantly with wavelength in the UV. However, the cloud gives a wavelength dependent effect in the surface UV‐radiation due to radiation scattered upwards from the cloud and then scattered downwards again, effectively trying to make it through the cloud more than once. The number of photons this happens to is a function of the wavelength dependent Rayleigh scattering and ozone absorption cross sections.
Formaldehyde production in clean marine air
Authors: Ayers G. P. , Gillett R. W. , Granek H. , de Serves C. , Cox R. A.
Published: February 2004

The atmospheric mixing ratio of formaldehyde (HCHO) was measured continuously in clean marine air at the Australian Baseline Station at Cape Grim between mid November and mid December 1993. A diurnal cycle in mixing ratio was observed, consistent in amplitude with the expected photochemical source of this species. However the absolute values of the HCHO mixing ratio were higher than expected if the major source of HCHO under clean, low‐NOx conditions is photolysis of methyl hydroperoxide (CH3OOH) derived from oxidation of methane by OH radicals. Possible explanations for elevated HCHO levels are considered. One sufficient to explain the observed HCHO levels is that reaction between hydroperoxy (HO2) and methylhydroperoxy (CH3O2) radicals may not proceed with 100% efficiency to form CH3OOH, but may have an additional branch yielding HCHO in clean marine conditions. There is some evidence from laboratory studies consistent with this proposal.
The seismic cycle in southern California: Precursor or response?
Authors: Jones Lucile M. , Hauksson Egill
Published: February 2004

The seismicity rate (M ≥ 3.0) in southern California shows two cycles with periods of high activity (90 events/year), from 1945–1952 and 1969–1992, and lower activity (60–70 events /year) from 1952–1969 and 1992‐present. Abrupt drops in the seismicity rate occur after the 1952 Kern County (M7.5) and the 1992 Landers (M7.3) earthquakes. The sudden increase in 1969 does not coincide with any major event but approximates the time needed to reaccumulate the seismic moment released in the 1952 earthquake. This temporal correlation with the preceding earthquake suggests that the seismic cycle (lower seismicity after a major earthquake and higher seismicity before the next major earthquake) should be interpreted as a response to the first earthquake rather than a precursor to the second. Southern California is now at a rate of seismicity as low as it experienced in the 1950s and 1960s.
Local time and altitude variation of equatorial thermosphere midnight density maximum (MDM): San Marco drag balance measurements
Authors: Arduini C. , Laneve G. , Herrero F. A.
Published: February 2004

We present the first study of the local time and altitude variation of the MDM, the density component of the equatorial midnight pressure bulge, an important feature in the nighttime motions of the ionosphere‐thermosphere system in the equatorial region. The neutral density data of the San Marco 3 (SM3) and San Marco 5 (SM5) satellites were averaged to obtain 24 hr density variations from April to December in 1971 and 1988. From these variations, the MDM amplitude as a function of altitude and local time was obtained at altitudes from 220 to 400 km at the geographic equator. We show the first evidence of downward phase propagation of the MDM together with a vertical structure not observed before, which may suggest an ionospheric interaction or perhaps a viscous dissipation effect. In 1971, the MDM propagates rather gradually from about 350 to 220 km in about two hours. In 1988, there appear two regions in which the MDM time shows little change with height with a discontinuous jump in between the two regions. The net effect is still a downward displacement of the MDM with local time. The results show significant seasonal and solar activity effects. During low solar activity periods (1971), the MDM occurs earlier in equinox than in solstice for all altitudes, consistent with the seasonal variation of the midnight temperature maximum (MTM). However, with higher solar activity (1988) and at altitudes below 340 km, the MDM occurs earlier in solstice than in equinox, raising new questions on solar activity effects on the relative phases of the tidal oscillations of the neutral density and the neutral temperature.
Climate forcing‐response relationships for greenhouse and shortwave radiative perturbations
Authors: Ramaswamy V. , Chen C.‐T.
Published: February 2004

The earth's climate system is subject to radiative forcings caused by perturbations in the infrared ‘greenhouse’ effect and absorbed solar energy. The forcings can be classified as being global in extent (e.g., increase of CO2) or spatially confined to the northern hemisphere midlatitudes (e.g., anthropogenic sulfate aerosols). Of central importance to climate change assessments are the characteristics of the global and latitudinal changes, and the forcing‐response relationships for different kinds of perturbations. Using a general circulation climate model with fixed cloud distributions and microphysical properties, we analyze the equilibrium climate responses to different perturbations representing global and spatially localized radiative forcings. The total climate feedback in the various experiments does not differ significantly, and the global‐mean climate sensitivity (ratio of the equilibrium global‐mean surface temperature change to the global‐mean imposed radiative forcing) behaves in a near‐invariant manner for both global and spatially confined forcings. However, relative to the global perturbation cases, forcings confined to the northern hemisphere midlatitudes exhibit a steepening of the meridional gradient of the temperature response in that hemisphere.
A dayside auroral energy deposition case study using the Polar Ultraviolet Imager
Authors: Brittnacher M. , Elsen R. , Parks G. , Chen L. , Germany G. , Spann J.
Published: February 2004

In this letter, we report preliminary results from a study of dayside auroral energy deposition during quiet times using global auroral images acquired by the Ultraviolet Imager experiment on the Polar spacecraft. Solar wind plasma and interplanetary magnetic field measurements from the Wind spacecraft and Kp values were used to characterize the state of the magnetosphere. The auroral oval was observed for a two‐hour period in spring and summer during relatively quiet times (KP ≃ 0 to 1). We find that, although the nightside energy deposition rate varied by an order of magnitude to as low as 1 × 1016 erg s−1 (1 gigawatt), the dayside was much less variable and remained between 4 and 10 × 1016 erg s−1.
A GPS estimate of relative motion between North and South America
Authors: Dixon Timothy H. , Mao Ailin
Published: February 2004

GPS velocity data are used to estimate the Euler vector describing rigid body motion of North America relative to South America. Assuming the boundary between the North and South American plates is located near the Fifteen Twenty fracture zone in the equatorial Atlantic, the Euler vector predicts extension across the Royal Trough up to 1 mm/yr, and convergence across the Barracuda Ridge at about 2 mm/yr, in agreement with geological estimates averaged over tens of millions of years. Further west, convergence between North and South America at rates up to 8 mm/yr may contribute to deformation of the Caribbean plate along its southwest boundary with South America.