bluuebunny/crossref_metadata_2025 huggingface dataset:
Identifying scale‐dependent models: The case of overland flow at the hillslope scale
Authors: van Loon E. Emiel , Keesman Karel J.
Published: September 2002

The hydrological problem to correctly partition rainfall at various scales may require scale‐dependent models. This study introduces a new method to identify scale‐ dependent state space models in a hydrological context. In addition, it investigates relations between model structure, resolution, and uncertainty for the case of overland flow from a hillslope. The main strengths of the method are the ability to evaluate a very large set of candidate models, general applicability, the possibility of interpreting identified models in a physical sense, and computational simplicity. Weaknesses are the possibility complex models that result, computation time required, and the relatively poor fit of models compared to methods with more strict assumptions on errors. For these reasons the method is mainly suitable as a first stage in an identification procedure, leading to the selection of a limited number of candidate models. For the case of overland flow at the hillslope scale it was found that model structure, resolution, and uncertainty are closely related. When compared to the total set of calibrated models, the fittest (best) models are characterized by a small average parameter uncertainty and a constant number of parameters. However, marked structural model differences exist for equally fit models at different spatial and temporal resolutions. Considering the whole spectrum of spatial and temporal resolutions covered by calibrated models, the fittest models are found in a narrow range.
Growing season water balance at a boreal jack pine forest
Authors: Moore Kathleen E. , Fitzjarrald David R. , Sakai Ricardo K. , Freedman Jeffrey M.
Published: September 2002

Measurements of energy and CO2 fluxes were made over the growing seasons of 1994 and 1996 in a northern jack pine forest as part of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Simultaneous measurements of soil water potential and content, rainfall, leaf wetness, and air specific humidity by our group and others allowed us to construct a complete water balance at this site for the 1994 season. The longer‐term (spanning weeks) rate of loss of soil moisture in the upper 0.25 m of soil matched the total evapotranspiration, measured by eddy covariance. Evapotranspiration (measured as QE, latent heat flux) was just 24% of the net radiation, a result that has been found in many boreal forest types, regardless of the canopy coverage. Low canopy conductances (typically 2–4 mm s−1) probably represent an adaptation to the extremely low soil moistures and poor nutrient status of the site. Net radiation was the best single‐variable predictor of evapotranspiration, having a correlation coefficient of 0.8 with QE for the 1994 season. Afternoons with sustained (>4 continuous hours) cloudless conditions resulted in water stress detectable as reduced QE relative to what would be predicted from the net radiation alone. The open canopy at our site promoted the role of the lichen‐covered surface in the overall water vapor exchange; subcanopy QE was 26% of the total.
Influence of various water quality sampling strategies on load estimates for small streams
Authors: Robertson Dale M. , Roerish Eric D.
Published: September 2002

Extensive streamflow and water quality data from eight small streams were systematically subsampled to represent various water‐quality sampling strategies. The subsampled data were then used to determine the accuracy and precision of annual load estimates generated by means of a regression approach (typically used for big rivers) and to determine the most effective sampling strategy for small streams. Estimation of annual loads by regression was imprecise regardless of the sampling strategy used; for the most effective strategy, median absolute errors were ∼30% based on the load estimated with an integration method and all available data, if a regression approach is used with daily average streamflow. The most effective sampling strategy depends on the length of the study. For 1‐year studies, fixed‐period monthly sampling supplemented by storm chasing was the most effective strategy. For studies of 2 or more years, fixed‐period semimonthly sampling resulted in not only the least biased but also the most precise loads. Additional high‐flow samples, typically collected to help define the relation between high streamflow and high loads, result in imprecise, overestimated annual loads if these samples are consistently collected early in high‐flow events.
Controls on nitrogen flux in alpine/subalpine watersheds of Colorado
Authors: Campbell Donald H. , Baron Jill S. , Tonnessen Kathy A. , Brooks Paul D. , Schuster Paul F.
Published: September 2002

High‐altitude watersheds in the Front Range of Colorado show symptoms of advanced stages of nitrogen excess, despite having less nitrogen in atmospheric deposition than other regions where watersheds retain nitrogen. In two alpine/subalpine subbasins of the Loch Vale watershed, atmospheric deposition of NO3− plus NH4+ was 3.2–5.5 kg N ha−1, and watershed export was 1.8–3.9 kg N ha−1 for water years 1992–1997. Annual N export increased in years with greater input of N, but most of the additional N was retained in the watershed, indicating that parts of the ecosystem are nitrogen‐limited. Dissolved inorganic nitrogen (DIN) concentrations were greatest in subsurface water of talus landscapes, where mineralization and nitrification augment high rates of atmospheric deposition of N. Tundra landscapes had moderately high DIN concentrations, whereas forest and wetland landscapes had low concentrations, indicating little export of nitrogen from these landscapes. Between the two subbasins the catchment of Icy Brook had greater retention of nitrogen than that of Andrews Creek because of landscape and hydrologic characteristics that favor greater N assimilation in both the terrestrial and aquatic ecosystems. These results suggest that export of N from alpine/subalpine watersheds is caused by a combination of direct flushing of N from atmospheric deposition and release of N from ecosystem biogeochemical processes (N cycling). Sensitivity of alpine ecosystems in the western United States to atmospheric deposition of N is a function of landscape heterogeneity, hydrologic flow paths, and climatic extremes that limit primary productivity and microbial activity, which, in turn, control retention and release of nitrogen. Conceptual and mechanistic models of N excess that have been developed for forested ecosystems need to be modified in order to predict the response of alpine ecosystems to future changes in climate and atmospheric deposition of N.
Glass casts of rock fracture surfaces: A new tool for studying flow and transport
Authors: Wan Jiamin , Tokunaga Tetsu K. , Orr Thomas R. , O'Neill Jim , Connors Robert W.
Published: September 2002

A method was developed for fabricating transparent glass casts of fractured rock pairs. These glass casts provide reproduction of surface topography and roughness of natural fractures, optical clarity, and representative wettabilities of mineral surfaces. The glass casts are an improved tool for studying flow and transport in fractures compared to previous approaches. The surface of a clean glass fracture has a contact angle near zero, and more importantly, the surface wettability can be intentionally altered with chemical treatment. Glass casts of rock fracture surfaces may be used for visually and quantitatively studying various physical, chemical, and microbial processes occurring in rock fractures, especially when multiple fluid phases are involved.
Identifying the conditions amenable to the determination of solute concentrations with time domain reflectometry
Authors: Ferré P. A. , Rudolph D. L. , Kachanoski R. G.
Published: September 2002

On the basis of an analysis of the weighting of the bulk electrical conductivity along time domain reflectivity (TDR) probes we show theoretical limitations to the measurement of solute concentrations with TDR. Simple example calculations demonstrate that there will only be a unique relationship between the TDR‐measured electrical conductivity and the average solute concentration in the pore water under one of two conditions. First, if the water content is spatially uniform throughout the sample volume of the probe, TDR may be used to determine solute concentrations. Second, if the water content is spatially variable but the spatial distribution is temporally constant and the solute concentration is spatially uniform throughout the probe's sample volume, solute concentrations can be inferred from the electrical conductivity response. Further complications arise in soils with spatially variable porosities or surface electrical conductivities, making TDR unsuited to determining solute concentrations even if one of these conditions is met.
Transient salt transport modeling of shallow brine beneath a freshwater lake, the Sea of Galilee, Israel
Authors: Hurwitz Shaul , Lyakhovsky Vladimir , Gvirtzman Haim
Published: September 2002

During a lake highstand phase in the late Pleistocene the former saline Lake Lisan covered the topographic depression of Kinarot Basin currently occupied by the freshwater lake, Sea of Galilee. It was hypothesized that during this period, the dense saline waters of Lake Lisan percolated into the sediment. The recession of the saline lake from the basin and the rapid formation of a freshwater lake triggered solute transport from the sediment into the lake. A one‐dimensional numerical model of solute transport that considers sediment compaction was developed to simulate chloride transport from the sediment into the lake. Simulation results were compared with measured chloride concentration profiles in sediment cores. On the basis of a sensitivity analysis, results are in agreement with the hypothesis that Lake Lisan solutes are currently discharged into the Sea of Galilee. The calculated upward water velocity in the sediment ranges between 9 and 22 mm yr−1.
Relations between basin characteristics and stream water chemistry in alpine/subalpine basins in Rocky Mountain National Park, Colorado
Authors: Clow David W. , Sueker Julie K.
Published: September 2002

Relations between stream water chemistry and topographic, vegetative, and geologic characteristics of basins were evaluated for nine alpine/subalpine basins in Rocky Mountain National Park, Colorado, to identify controlling parameters and to better understand processes governing patterns in stream water chemistry. Fractional amounts of steep slopes (≥30°), unvegetated terrain, and young surficial debris within each basin were positively correlated to each other. These terrain features, which commonly occur on steep valley side slopes underlain by talus, were negatively correlated with concentrations of base cations, silica, and alkalinity and were positively correlated with nitrate, acidity, and runoff. These relations might result from the short residence times of water and limited soil development in the talus environment, which limit chemical weathering and nitrogen uptake. Steep, unvegetated terrains also tend to promote high Ca/Na ratios in stream water, probably because physical weathering rates in those areas are high. Physical weathering exposes fresh bedrock that contains interstitial calcite, which weathers relatively quickly. The fractional amounts of subalpine meadow and, to a lesser extent, old surficial debris in the basins were positively correlated to concentrations of weathering products and were negatively correlated to nitrate and acidity. These relations may reflect more opportunities for silicate weathering and nitrogen uptake in the lower‐energy environments of the valley floor, where soils are finer‐grained, older, and better developed and slopes are relatively flat. These results indicate that in alpine/subalpine basins, slope, vegetation (or lack thereof), and distribution and age of surficial materials are interrelated and can have major effects on stream water chemistry.
Cumulative losses of buried volatile organic chemicals
Authors: Robinson N. I.
Published: September 2002

New analytical expressions are presented for cumulative losses of the Jury et al. [1983] behavior assessment type of model for volatile organic chemicals buried in soil vadose zones. These expressions are for the soil‐surface cumulative volatilization flux and subsurface cumulative degradation and mass loss. At the soil surface a flux boundary condition is used which includes a nonzero, atmospheric volatile concentration. Numerical results for time‐dependent surface flux and cumulative quantities are presented for this boundary condition.
The value of water levels in water‐based recreation: A pooled revealed preference/contingent behavior model
Authors: Eiswerth Mark E. , Englin Jeffrey , Fadali Elizabeth , Shaw W. Douglass
Published: September 2002

In this paper we present estimated recreation values for preventing a decline in water levels at, and even the total loss of, a large western lake that is drying up. We use a Poisson version of the count data travel cost model; however, in addition to and in combination with revealed preference (RP) data, we employ contingent behavior (CB) responses to hypothetical questions on alternative water levels and number of trips. The pooled model used allows for tests of differences between results using RP and CB data. This particular pooled RP/CB approach has not to our knowledge previously been applied to examine the values of alternative water quantities in water‐based recreation.
Specific yield for a two‐dimensional flow
Authors: Tritscher Peter , Read W. Wayne , Broadbridge Philip
Published: September 2002

We investigate the systematic secular spatial variation of specific yield. As a vehicle for this analysis we consider a canonical unconfined aquifer consisting of a porous zone whose cross section is a simple long rectangle. The hydraulic conductivity in the unsaturated zone is modeled by the quasi‐linear approximation. We find that locally the specific yield may be strongly influenced by the water table depth and mildly dependent on the recharge rate if that rate is high. For the simple geometry considered, a lateral component of flow has been found to have an insignificant effect on the local specific yield and that a model that assumes locally purely vertical flow to the given phreatic surface provides a more‐than‐adequate estimate of the specific yield. For the overall yield of an aquifer we find that the simplest model, wherein the flow through the soil is neglected, i.e., the model with static water and horizontal phreatic surface, provides a reasonable indication of the actual specific yield for most infiltration rates and aquifer dimensions. However, if the infiltration rate is high or the aquifer is particularly long, then the yield obtained from an assumed purely vertical flow, presupposing that the phreatic depth is accurately known, gives an excellent estimate of the actual specific yield.
A virtual node method for handling well bore boundary conditions in modeling multiphase flow in porous and fractured media
Authors: Wu Yu‐Shu
Published: September 2002

How to treat well boundary conditions is a difficult issue when formulating and coding a multiphase numerical reservoir simulator. The difficulty arises because the partial differential equation governing multiphase subsurface flow is of a mixed parabolic‐hyperbolic type. Maximum changes in primary variables and mass/heat fluxes occur at boundaries or well nodes. Consequently, these well nodes with small volume tend to be singular, leading to computational convergence problems. The conventional method of well treatment in geothermal or oil reservoir simulators is to use a sink/source term approach and to distribute flow rates by a potential or mobility allocation scheme for a multilayered well. However, this traditional method cannot handle a backflow problem, which may occur in a multilayered well in heterogeneous formations. This paper presents a “virtual node” method to handle a well bore either as a single node or several computational nodes screened and connected to many neighboring nodes for a multilayered well. The well bore can be vertical, inclined, or horizontal, and the well borehole node is treated in the same way as any other nonwell node for flow calculations. The solution at the well is then obtained by solving mass balance equations for the well node. It is shown that the new method provides a natural, physically consistent, and numerically efficient approach to handling well flow problems. Implementation of this new method for a three‐phase flow reservoir simulator is discussed, and three examples are provided.
Stochastic analysis of transient flow in unsaturated heterogeneous soils
Authors: Foussereau X. , Graham W. D. , Rao P. S. C.
Published: September 2002

A stochastic unsaturated water flow model is developed for heterogeneous soils subject to a transient flow regime. Equations are developed for a fully three‐dimensional soil profile, and results are presented for an example one‐dimensional problem. The model predicts the mean and covariance of the soil water content, Darcy's flux, and pore water velocity as a function of the boundary flux and saturated hydraulic conductivity statistics. The statistics of the pore water velocity can be used to predict solute transport in soils, as shown by Foussereau et al. [this issue]. Approximate flow‐related moment equations are solved analytically in the Laplace domain. Then, the analytical results are numerically inverted using a modified fast Fourier transform algorithm. The model predictions are compared to results obtained from Monte Carlo simulations for two different boundary flux patterns characteristic of humid climates and two different soil types (a fine sand and a sandy loam). Comparing the approximate solutions of the statistical moments to the outputs of the Monte Carlo simulations shows (1) the dominance of the boundary flux variability over that of the saturated conductivity on the overall prediction uncertainty, particularly at shallow depths, and (2) the good performance of the stochastic unsaturated flow model, particularly for fine‐textured soils subject to boundary fluxes with coefficients of variation up to ∼1.5. As the boundary flux coefficient of variation increases and the soil becomes coarser, the model performance deteriorates because the flow system becomes significantly more nonlinear.
Coherent modes in multiscale variability of streamflow over the United States
Authors: Saco Patricia , Kumar Praveen
Published: September 2002

Motivated by the need to understand large‐scale hydrologic response, significant research has been directed toward the identification of coherent regions using characteristics of streamflow variability. Typically, these regions are delineated using principal component analysis on streamflow. This method does not account for differences in temporal scales of fluctuations embedded in the time series. To capture this, we use wavelet spectral analysis. Wavelet spectra from the specific streamflow series are obtained for outflow binned at 3°‐length segments along the border of the conterminous United States. Rotated principal component analysis is performed on the wavelet spectra to obtain clusters of segments that exhibit similar distribution of variability across scales. Three physically distinct modes explain over 89% of the variability. Two of the modes identified are associated with high variability at seasonal scales, and the third is associated with high variability at small timescales. The runoff generation mechanisms underlying the observed modes of multiscale variability of various regions are also discussed. Each of these coherent modes of multiscale variability indicate the existence of regions with similar scales of fluctuations that are located geographically apart, as well as regions located geographically close with dissimilar scales of fluctuations.
Influence of bank vegetation on alluvial channel patterns
Authors: Millar Robert G.
Published: September 2002

Numerous studies have suggested the importance of bank vegetation as a control of channel patterns; however, to date there is no conclusive evidence that vegetation does represent a significant control. An analytical model is developed in order to assess the influence of bank vegetation on channel patterns of alluvial gravel‐bed rivers. Three channel types are considered: meandering, wandering, and braided. Bank vegetation effects are quantified in terms of a friction angle ϕ′. A new theoretical meandering‐braiding transition criterion is formulated that includes ϕ′, median grain diameter D50, and bank‐full discharge Q. The theoretical relation is tested against field data from 137 rivers and successfully discriminates between meandering and braided rivers. Wandering rivers show greater scatter. It is concluded that bank vegetation, as expressed in terms of ϕ′, does exert significant and quantifiable control on alluvial channel patterns. A simple planform stability diagram is developed to determine the sensitivity of gravel‐bed rivers to changes in bank vegetation.
An advective‐dispersive stream tube approach for the transfer of conservative‐tracer data to reactive transport
Authors: Cirpka Olaf A. , Kitanidis Peter K.
Published: September 2002

Conservative‐tracer data are used for the parameterization of mixing‐controlled reactive transport. Temporal moments of the tracer breakthrough curve integrated over the outflow boundary of the domain yield the average velocity and the path‐averaged macrodispersion coefficient. On the basis of this information alone, no distinction is possible between spreading and mixing of the tracer. Analyzing the temporal moments of breakthrough curves locally obtained at single points in the domain gives additional information about the dilution of the tracer. In an accompanying paper [Cirpka and Kitanidis, this issue] we derive an apparent Péclet number of mixing Pea from local temporal moments. Assuming that Pea is constant over the cross section of the outflow boundary, a corrected probability density function of arrival times is determined. By interpreting the spatially integrated breakthrough curve as the result of advective‐ dispersive transport in independent stream tubes with identical Péclet number but differing seepage velocity, it is possible to transfer results of conservative transport to the transport of interacting compounds for cases in which mixing of the compounds is influenced significantly by local‐scale dispersion. This is an improvement to the stochastic‐convective model of Simmons et al. [1995] for the transfer of integrated tracer data to reactive transport. The approach is applied to the hypothetical case of a bimolecular reaction in a heterogeneous two‐dimensional aquifer.
Estimating recharge in a tropical Karst Aquifer
Authors: Jones Ian C. , Banner Jay L. , Humphrey John D.
Published: September 2002

Unique constraints on seasonal and spatial variations in recharge to the Pleistocene limestone aquifer of Barbados are obtained from the analysis of oxygen isotopic compositions of groundwater and rainwater. Conventional methods of estimating recharge are based on groundwater chloride variations, coastal groundwater discharge, and potential evapotranspiration. These methods typically yield estimates of recharge for Barbados that range from 9% to 20% of average annual rainfall, with significant uncertainties that arise from poorly constrained model input parameters. Owing to the low relief and tropical climate of Barbados, variations in rainwater and groundwater δ18O values are primarily influenced by the amount of rainfall, with negligible temperature or altitude effects. Composite monthly rainwater δ18O values are inversely related to rainfall, while groundwater δ18O values show little seasonal variability. Rainwater δ18O values are equivalent to groundwater values only at the peak of the wet season. By using mass balance, the difference between groundwater and weighted‐mean rainwater δ18O values gives recharge values. These values are in general agreement with estimates by conventional methods (10–20%) and provide unique additional information including the following: (1) Recharge is restricted to the wettest 1–3 months of the year, and (2) there is less recharge at higher elevations. The effective shift in δ18O values between contemporaneous rainwater and groundwater via recharge is a useful tool for estimating temporal and spatial variability in recharge and must be considered in paleoclimatic studies where climate inferences are based on groundwater δ18O values preserved in the geologic record.
Role of vegetation dynamics in enhancing the low‐frequency variability of the Sahel rainfall
Authors: Wang Guiling , Eltahir Elfatih A. B.
Published: September 2002

Rainfall observations in the Sahel region of West Africa show significant variability at the timescale of decades. Here we explore the mechanisms of this low‐frequency variability using a coupled biosphere‐atmosphere model which includes explicit representation of vegetation dynamics. By forcing the model with the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1898–1997, numerical experiments on the climate variability of West Africa have been carried out. The results of these experiments suggest that vegetation dynamics is a significant process in shaping the natural variability of the Sahel rainfall. The response of the regional climate system to large‐scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere acts to enhance the low‐frequency rainfall variability. The regional climate system over West Africa has several climate regimes coexisting under the current precessional forcing. Climate transitions between different regimes act as another mechanism contributing to the low‐frequency rainfall variability. Climate persistence at one regime and climate transition toward another collectively compose a distinct type of multidecadal variability.
Coral microatolls from the central Pacific record Late Holocene El Niño
Authors: Woodroffe Colin D. , Gagan Michael K.
Published: September 2002

Microatolls are discoid corals that have grown laterally because vertical growth is constrained by exposure at lowest tides. We demonstrate that a modern reef‐flat Porites microatoll from Christmas (Kiritimati) Island preserves an oxygen isotope record of substantial sea surface temperature variations related to El Niño‐Southern Oscillation (ENSO). We also show that a late Holocene fossil microatoll from the centre of the island contains interannual oxygen isotope variations over an approximate 20‐year period. Three pronounced negative isotope anomalies attributed to warm El Niño events are superimposed on an annual cycle. El Niño events similar to those seen in recent decades appear to have been a feature of mid‐Pacific climate in the late Holocene. Analysis of further microatolls offers a source of pre‐instrumental El Niño tropical climate data. It may be possible to extend the NINO‐C proxy to indicate intensity and frequency of ENSO over the past three millennia.
Alfvénic turbulence in the polar wind: A statistical study on cross helicity and residual energy variations
Authors: Bavassano B. , Pietropaolo E. , Bruno R.
Published: September 2002

A study of MHD turbulence properties in the polar wind has been performed with Ulysses data. The parameters under examination are the normalized cross helicity and the normalized residual energy. The correlation coefficient between velocity and magnetic field fluctuation vectors has also been computed. Both southern and northern phases of full immersion in the high‐latitude fast wind have been examined. Observations during the short low‐latitude phase around the Ulysses perihelion have been used as a term of comparison. Our results indicate that in the region up to about 2 AU the turbulence evolution leads to a decrease of cross helicity and Alfvénic correlation when distance increases. Farther out no appreciable variation is observed. It is concluded that in the high‐latitude heliosphere the MHD turbulence maintains a clear Alfvénic character even at large distances. As regards the residual energy, a clear imbalance in favor of the magnetic fluctuations is everywhere present. This imbalance becomes even more pronounced for increasing distance inside 2 AU, due to turbulence evolution. Other effects lead to an inversion of this trend in the outer region. It is noteworthy that the polar turbulence does not appear too different from that observed inside the trailing edge of low‐latitude fast streams.
On the location of trapped particle populations in quadrupole magnetospheres
Authors: Vogt J. , Glassmeier K. H.
Published: September 2002

Populations of trapped particles in Earth's magnetic field are intimately linked to prominent magnetospheric phenomena, for example, to the radiation belts and the ring current. Particles are forced to bounce between both hemispheres because field lines converge from the magnetic equatorial region toward the poles owing to the dominating dipolar component. The scenario may change essentially if the so‐called paleomagnetosphere is considered, that is, the terrestrial magrietosphere during a polarity reversal period. This report deals with particle trapping in general field configurations and concentrates on the quadrupole case as a potential scenario for paleomagnetospheric studies. By using the tensor representation of the quadrupole field, we show that its topology is controlled by a single “shape” parameter which is a measure for the relative strength of quadrupole tensor components, or equivalently, coefficients in the spherical harmonics expansion of the potential. In order to locate centers of particle bounce motion the concept of a magnetic equatorial surface is extended to a more general type of “trapping center surface” by means of geometrical criteria such as B · ∇ B = 0. The quadrupole case yields two such surfaces. As does the field line topology, those surfaces and the resulting drift orbits vary strongly with the quadrupole shape parameter.
A climatology of middle and upper atmosphere radar observations of thermospheric winds
Authors: Kawamura S. , Otsuka Y. , Zhang S.‐R. , Fukao S. , Oliver W. L.
Published: September 2002

Shigaraki middle and upper atmosphere (MU) radar observations of horizontal thermospheric winds in the magnetic meridian plane over the period September 1986 to September 1996 are reported as climatological averages in the form of time‐of‐day variations for several combinations of seasonal and solar activity conditions and are compared with winds predicted by the horizontal wind model (HWM) and with winds measured at Saint Santin and Millstone Hill. The dominant feature of the MU wind behavior is its mean diurnal variation of northward flow by day and southward flow by night, with the nighttime wind smoothly approaching and receding from a midnight maximum, while the daytime wind tends to show two peaks, a strong one in the early daylight hours and a weak one in the afternoon‐evening. HWM shows the same unimodal nighttime and bimodal daytime behavior, but the HWM pattern is shifted about 2 hours later in time. The amplitude of the diurnal harmonic decreases from 78 m/s at solar minimum to 45 m/s at solar maximum, while HWM shows a corresponding increase from 53 to 62 m/s. The diurnal amplitude is remarkably stable with season but is superposed on a steady wind of 41 m/s southward in summer, 15 m/s northward in winter, and midway between these limits at the equinoxes. HWM shows a symmetric pattern of 30 m/s southward in summer and 30 m/s northward in winter. Ion drag appears to be the main regulator of wind speed, and the seasonal wind patterns have a profound effect on the seasonal behavior of the ionosphere.
Methodology for global geodetic time series estimation: A new tool for geodynamics
Authors: Davies Philip , Blewitt Geoffrey
Published: September 2002

A method of automatically combining geodetic network solutions to produce station coordinate time series with realistic computed errors has been developed and tested and is being applied on a weekly basis to Global Positioning System (GPS) global and regional networks of the International GPS Service. Our techniques include modified Helmert blocking, stochastic modeling to minimize frame bias, Monte Carlo simulation, variance component estimation, and multiparameter data snooping. An 18‐month time series evaluation of 150 globally distributed stations demonstrates that our combined weekly solution is more complete, precise, and reliable than any contributing solution. Our method of attaching regional networks without perturbing the global network solution, rather than combining normal equations, improves the quality measures. The median RMS of station position residuals with respect to a constant velocity model is 2.4 mm in latitude, 3.0 mm in longitude, and 7.2 mm in height. Our solution has since been incorporated into the reference frame ITRF96 (International Terrestrial Reference Frame 1996), showing a RMS coordinate difference of 5.4 mm, the lowest of all contributing solutions. As an independent test, the RMS difference with the ITRF94 is 4.5 mm in horizontal and 8.1 mm in height. As a second external test, the station velocity solution was used to estimate plate tectonic Euler vectors, which were then compared with the NUVEL‐1A model and found to differ at a level consistent with the computed errors. Given a few more years of data, our error model predicts solutions that will be sufficiently precise to rigorously test NUVEL‐1A or its successors.
Geochemical evidence for a magmatic CO2degassing event at Mammoth Mountain, California, September–December 1997
Authors: McGee K. A. , Gerlach T. M. , Kessler R. , Doukas M. P.
Published: September 2002

Recent time series soil CO2concentration data from monitoring stations in the vicinity of Mammoth Mountain, California, reveal strong evidence for a magmatic degassing event during the fall of 1997 lasting more than 2 months. Two sensors at Horseshoe Lake first recorded the episode on September 23, 1997, followed 10 days later by a sensor on the north flank of Mammoth Mountain. Direct degassing from shallow intruding magma seems an implausible cause of the degassing event, since the gas released at Horseshoe Lake continued to be cold and barren of other magmatic gases, except for He. We suggest that an increase in compressional strain on the area south of Mammoth Mountain driven by movement of major fault blocks in Long Valley caldera may have triggered an episode of increased degassing by squeezing additional accumulated CO2from a shallow gas reservoir to the surface along faults and other structures where it could be detected by the CO2monitoring network. Recharge of the gas reservoir by CO2emanating from the deep intrusions that probably triggered deep long‐period earthquakes may also have contributed to the degassing event. The nature of CO2discharge at the soil‐air interface is influenced by the porous character of High Sierra soils and by meteorological processes. Solar insolation is the primary source of energy for the Earth atmosphere and plays a significant role in most diurnal processes at the Earth surface. Data from this study suggest that external forcing due largely to local orographic winds influences the fine structure of the recorded CO2signals.
Empirical relationships between temperature, pressure, and MgO content in olivine and pyroxene saturated liquid
Authors: Sugawara Toru
Published: September 2002

From a large number of experimental data on olivine‐liquid and pyroxene‐liquid equilibria, it is revealed that there is a linear relationship between the mole percent of MgO in olivine and/or pyroxene saturated liquid, temperature and pressure. The effect of dissolved water content on the linear relationship has also been taken into account based on recently reported experimental data in the hydrous systems. The linear equation allows one to calculate the temperature from the MgO content and pressure or the MgO in the liquid from the temperature and pressure. The standard deviations of calculated temperature and MgO content of the anhydrous liquid are about ±30°C and within 3 mol%, respectively. The standard deviation of the temperature is the same or less than those of previously reported geothermometers. This geothermometer has the advantage that it is unnecessary to know the FeO content of the liquid or oxygen fugacity.
Crustal structure and seismicity distribution adjacent to the Pacific and North America plate boundary in southern California
Authors: Hauksson Egill
Published: September 2002

New three‐dimensional (3‐D) VP and VP/VS models are determined for southern California using P and S‐P travel times from local earthquakes and controlled sources. These models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the Pacific‐North America plate boundary. The models extend from the U.S.‐Mexico border in the south to the southernmost Coast Ranges and Sierra Nevada in the north and have a 15‐km horizontal grid spacing and an average vertical grid spacing of 4 km, down to 22 km depth. The heterogeneity of the crustal structure as imaged by VP and VP/VS models is larger within the Pacific plate than the North American plate. Similarly, the relocated seismicity deepens and shows more complex 3‐D distribution in areas of the Pacific plate exhibiting compressional tectonics. The models reflect mapped changes in the lithology across major geological terranes such as the Mojave Desert, the Peninsular Ranges, and the Transverse Ranges. The interface between the shallow Mono of the Continental Borderland and the deep Moho of onshore California forms a broad zone to the north beneath the western Transverse Ranges, Ventura basin, and the Los Angeles basin and a narrow zone to the south, along the Peninsular Ranges. The near‐surface increase in velocity, from the surface to up to 8 km depth, is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins. At midcrustal depths a rapid increase in VP is imaged beneath the sediments of the large sedimentary basins, while beneath the adjacent mountain ranges the increase is small or absent.
Lithospheric assembly and modification of the SE Canadian Shield: Abitibi‐Grenville teleseismic experiment
Authors: Rondenay Stéphane , Bostock Michael G. , Hearn Thomas M. , White Donald J. , Ellis Robert M.
Published: September 2002

This paper presents the results of a joint Lithoprobe‐Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Québec‐Ontario border. Data from a 600‐km‐long, N‐S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low‐velocity, NW‐SE striking corridor that crosses the southern portion of the line at latitude 46°N and lies between 50 and 300 km depth. Multievent S K S‐splitting results yield an average delay time of 0.57±0.22 s and a direction of fast polarization of N93°E±18°, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low‐velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45°N and 46°N, which is followed by an abrupt thinning to 35 km at 46.6°N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre‐Grenvillian (Archean‐Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low‐velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian‐White Mountain‐New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes.
Evidence for local variations in the depth to the 410 km discontinuity beneath Albuquerque, New Mexico
Authors: Gurrola Harold , Minster J. Bernard
Published: September 2002

We stack data from the three‐component broadband seismic stations at Piñon Flat, California (PFO); Ontario, Canada (RSON); and Albuquerque, New Mexico (ANMO) using the velocity spectrum stacking method. By doing so, we estimate the transition zone thickness (TZT) beneath southern California and Ontario to be 236 and 263 km, respectively. Beneath ANMO we‐find a bimodal distribution in our estimates of the TZT. The transition zone west of ANMO appears to be similar in thickness to that beneath PFO (232 km). The transition zone east of ANMO is 253 km, similar to that estimated for RSON. We conclude that surface tectonics across the Rio Grande Rift are closely related to the transition zone structure beneath it and that the rift, which separates the region of active tectonism from the stable High Plains of eastern New Mexico, is also associated with a boundary in transition zone structure. Most of this 20 km change in TZT appears to be due to topography on the 410 km discontinuity. This implies that beneath New Mexico, lateral variations in temperature do not cross the transition zone and that the 410 km discontinuity reflects surface tectonics more closely than does the 660 km discontinuity.
Evidence for the location and cause of large crustal earthquakes in Japan
Authors: Zhao Dapeng , Ochi Fumiko , Hasegawa Akira , Yamamoto Akira
Published: September 2002

Large crustal earthquakes (magnitude 5.7–8.0, depth 0–20 km) during a period of 115 years from 1885 to 1999 in Japan occurred in or around zones of low seismic velocity revealed by seismic tomography. The low‐velocity zones may represent weak sections of the seismogenic crust. The crustal weakening is considered to be closely related to the subduction of the oceanic Pacific and Philippine Sea plates in this region. Along the volcanic front and in back‐arc areas the crustal weakening may be caused by active volcanoes and magma chambers. In the forearc areas, fluids are detected in the earthquake source zones, which may have contributed to the crustal weakening and the rupture nucleation. The fluids may be related to the dehydration of the subducting Philippine Sea slab. These results indicate that large crustal earthquakes do not strike anywhere, but only anomalous areas that may be detected with geophysical methods.
Textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios
Authors: Holzheid Astrid , Schmitz Mark D. , Grove Timothy L.
Published: September 2002

The influence of the coexistence of silicate and iron sulfide melts on the wetting of mantle olivine by iron sulfide melt is investigated. The dihedral angle between iron alloy and olivine is unaffected by the presence of silicate melt at 1.5 GPa and 1370–1410°C and is much greater than the maximum allowable dihedral angle for melt connectivity at low melt fractions. In experimental charges where silicate and sulfide melt occupy the same melt pocket, the sulfide melt adheres to olivine grain faces rather than forming discrete droplets. The relative interfacial energy between iron sulfide and basaltic silicate melt is lower than that of iron sulfide with olivine, but the contrast is not sufficient for basaltic silicate melt to wet FeS‐olivine grain boundaries. This result implies that very large silicate melt fractions are necessary to mobilize iron sulfide melts in partially molten silicate phase assemblages. Therefore segregation of Fe‐rich metallic melts by porous flow at low melt fraction is not a likely process for metal separation. The experiments also investigated the influence of the presence of a thermal gradient on sulfide melt migration. Although the sulfide melt can not migrate through the olivine network by porous flow, evidence in the experiments indicates mobility of sulfide through the interconnected silicate melt network. The sulfide melt pools in contact with silicate melt at the hot, top part of the experimental charges grow over time at the expense of the silicate melt pools in the cooler part of the charge. This points to a diffusive transport mechanism for local sulfide redistribution. We review literature data on metal‐olivine wetting angles and find that the metal composition has a dominant influence on the wetting behavior of a metallic liquid on an olivine matrix. Iron‐rich metallic liquid compositions are characterized by large dihedral angles (>90°). The metal‐olivine dihedral angle decreases with increasing amounts of dissolved light element (S, C, O). For light element contents in excess of ∼59 at %, the dihedral angle approaches 60°, and the wetting behavior of a metallic liquid is similar to that of a silicate liquid.
Resonant behavior of saturated porous media
Authors: Pan Yan , Horne Roland N.
Published: September 2002

The resonant behavior of a porous medium composed of periodic channel structures was studied. The natural frequencies of the dry pore system were obtained by theoretical analysis. The resonant frequencies of the saturated porous medium were calculated by numerical simulation, which considered the two‐phase coupling effects of fluid and solid. The numerical results showed that most of the resonant frequencies of the saturated porous medium match with the natural frequencies of the corresponding dry porous medium but that some of the resonant frequencies are different from those of the solid matrix depending on the porosity of the pore system. Both the theoretical study and the numerical study suggest that the resonant frequencies of a porous medium are functions of the porosity or the fracture density.