bluuebunny/crossref_metadata_2025 huggingface dataset:
On self‐similarity and self‐affinity of drainage basins
Authors: Nikora Vladimir I.
Published: February 2004

An approach is proposed which allows consideration from common positions the self‐similar and self‐affine properties of river drainage basins and which introduces respective characteristics of their fractal structure. Horton's (1945) model has been assumed as the basis of this study. It is shown that self‐similarity of river drainage basins is characterized by the self‐similarity dimension DS = ln RB/ln RL. In the case of self‐affinity, two scaling indices DL = ln RB/ln RL and Dw = ln RB/ln (RA/RL) are required, the combinations of which determine the Hurst H = DL/DW index (characterizing the degree of drainage basins self‐affinity) and the lacunary dimension DG = 2DLDW/(DL + DW) (characterizing the degree of lacunarity or noncompactness of drainage basins). On the basis of analysis of published data from rivers in the United States, Italy, Russia, and Romania, the conclusion is drawn that river drainage basins are generally noncompact (DG < 2) and self‐affine (H < 1) fractal objects. Besides, for a quantitative description of river drainage basins, a new parameter is introduced, which characterizes the degree to which a large catchment is filled with smaller ones.
A numerical approach to the analysis and classification of channel network patterns
Authors: Ichoku Charles , Chorowicz Jean
Published: February 2004

A large number of samples of visually classified channel networks are taken from some published works. The samples are representative of the following pattern types: dendritic, parallel, rectangular, trellis, and pinnate. By means of computer techniques, essential parameters of elements of these pattern samples are determined. These include lengths, directions, and degrees of curvedness and meandering of channel segments, as well as confluence angles. The parameters are used to determine pattern attributes such as density, texture, parallelism, rectangularity, and bifurcation ratios. Thresholds are generated for the attributes and used in the construction of classification models for the five pattern types studied. The processes and models are coded in a computer program for use in the automated classification of numerically valued channel networks. After classification the image is displayed with each individual network bearing a color which shows the pattern type to which it has been assigned.
The effect of concentration‐dependent surface tension on the flaw of water and transport of dissolved organic compounds: A pressure head‐based formulation and numerical model
Authors: Smith James E. , Gillham Robert W.
Published: February 2004

Unsaturated flow and solute transport models in isothermal porous media generally assume surface tension at the air‐water interface to be constant. Yet many organic compounds, including a number of environmental interest, reduce surface tension in proportion to their aqueous phase concentrations. An isothermal saturated‐unsaturated flow and transport model has been developed that incorporates variable surface tension caused by varying solute concentration. The model was based on the pressure head form of the unsaturated flow equation and was solved using finite element methods. The extreme nonlinearities inherent in the problem necessitated spatial discretization on the scale of millimeters and initial temporal discretization on the scale of milliseconds. A comparison of simulations with and without varying surface tension showed that changes in surface tension could have significant effects on pressure head, water content, water flux, and solute flux. Implications relative to multiphase flow models, unsaturated zone contamination, and remedial technologies are discussed.
A water use model for locating the boreal/deciduous forest ecotone in eastern North America
Authors: Arris Lelani L. , Eagleson Peter S.
Published: February 2004

A model is developed for predicting the location of major vegetation ecotones in eastern North America. The model is based upon the interaction of plant physiological characteristics, climate, and soil in order to be useful as an interactive boundary condition in atmospheric general circulation models. The model represents the relative competitive ability of different vegetation types by their annual net primary productivity as expressed through water use. At any given location, the vegetation type with the highest productivity is assumed to be dominant. Ecotones are located where competitive dominance shifts from one vegetation type to another. Productivity is computed as a function of annual evapotranspiration, which is a function of the length of the growing season, photosynthetic capacity, potential evapotranspiration, and soil moisture availability, among other things. When considering the coniferous boreal/deciduous forest ecotone, it is found that inherent physiological differences between conifers and deciduous trees lead to differences in productivity which are related primarily to temperature. The model predicts a reversal of deciduous/coniferous productive dominance at the latitude of the observed ecotone; that is, within the transition zone of mixed forest. The productivity model does not adequately explain the existence of the deciduous/southern pine ecotone. Soil chemistry apparently plays a role in the determination of this boundary.
On the generation of instabilities in variable density flow
Authors: Schincariol Robert A. , Schwartz Franklin W. , Mendoza Carl A.
Published: February 2004

Interfacial or fingering instabilities have been studied recently in relation to contamination problems where a more dense plume is enclosed by and is moving along in a body of less dense fluid. Instabilities can play an important role in the mixing or dispersion process. Through the use of a variable density flow and transport code, we were able to study how the style of interfacial perturbation controls the pattern of instability development. Whether initial perturbations grow or decay depends mainly on the wavelength of the perturbing function. A critical perturbation wavelength must be exceeded for a perturbation to grow; otherwise the perturbation simply decays. Our work confirms earlier analyses that suggest that all stratified systems are inherently unstable, given some spectrum of the perturbing waves that exceed the critical wavelength. By implication, Rayleigh number stability criteria are inappropriate for evaluating the dense plume problem. Our study also demonstrates how numerical errors in a mass transport code can serve as a perturbing function and lead to the development of instabilities. However, these instabilities are not physically realistic and are essentially uncontrollable because their character depends on the extent to which numerical errors develop, as evidenced by the grid Peclet and Courant numbers.
Chloride migration in heterogeneous soil: 1. Experimental methodology and results
Authors: Sassner Mona , Jensen Karsten H. , Destouni Georgia
Published: February 2004

The statistics of chloride transport through 29 undisturbed soil monoliths (20 cm in diameter and 1 m long, sampled from a field plot of 15×175 m2) were evaluated in a controlled laboratory experiment. Although the field soil (loamy sand) was relatively homogeneous with regard to texture, the individual monoliths showed large and irregular variability in their soil characteristics and in their flow and transport properties. The distribution of the specific discharge of water could be quantified by a bimodal distribution; the horizontal correlation length for the specific discharge of water was estimated to about 10 m. The ratio between the arrival time of the peak in chloride concentration and the water residence time given by the measured flow parameters indicated a mobile water content that was smaller than the measured water content. The large variability in hydraulic conductivity in both the vertical and horizontal direction, and the agreement between the present hydraulic conductivity measurements and earlier measurements in the field area, indicate that the partition of the measured water content between mobile and relatively immobile water was an effect of the soil structure rather than a boundary effect.
Colloid transport in unsaturated porous media
Authors: Wan Jiamin , Wilson John L.
Published: February 2004

This paper explores the significance of the gas‐water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (1) a completely water‐saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2 μm), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas‐water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two‐dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water‐saturated laboratory column experiments and related theory.
Estimation of effective aerodynamic roughness of Walnut Gulch watershed with laser altimeter measurements
Authors: Menenti Massimo , Ritchie Jerry C.
Published: February 2004

A new method to estimate the effective aerodynamic roughness of a complex landscape is presented. High‐resolution elevation profiles measured with an airborne laser altimeter have been used to compute geometrical parameters of three landscape elements of the Walnut Gulch experimental watershed, Arizona. Mean crop height, its standard deviation, and the frequency distribution (as a function of penetration depth) of plant elements hit by the laser beam have been calculated for short segments (1 m) measured over intervening grass between shrubs. Longer segments have been applied to obtain the mean height and spacing of taller shrubs and trees. Finally, laser profiles covering the entire watershed gave the mean amplitude and wavelength of hillocks and ridges. The aerodynamic roughness length of the intervening grass is estimated using the ratio of standard deviation to vegetation height, corrected for instrument noise, times mean height. An effective roughness length which parameterizes the total stress due to grass and taller shrubs and trees is calculated first. Finally, a watershed‐scale effective roughness length is calculated combining the amplitude and wavelength of hillocks and ridges with the roughness length obtained in the previous step.
Use of ground‐based remotely sensed data for surface energy balance evaluation of a semiarid rangeland
Authors: Moran M. S. , Kustas W. P. , Vidal A. , Stannard D. I. , Blanford J. H. , Nichols W. D.
Published: February 2004

An interdisciplinary field experiment was conducted to study the water and energy balance of a semiarid rangeland watershed in southeast Arizona during the summer of 1990. Two subwatersheds, one grass dominated and the other shrub dominated, were selected for intensive study with ground‐based remote sensing systems and hydrometeorological instrumentation. Surface energy balance was evaluated at both sites using direct and indirect measurements of the turbulent fluxes (eddy correlation, variance, and Bowen ratio methods) and using an aerodynamic approach based on remote measurements of surface reflectance and temperature and conventional meteorological information. Estimates of net radiant flux density (Rn), derived from measurements of air temperature, incoming solar radiation, and surface temperature and radiance compared well with values measured using a net radiometer (mean absolute difference (MAD) ≃ 50 W/m2 over a range from 115 to 670 W/m2). Soil heat flux density (G) was estimated using a relation between G/Rn and a spectral vegetation index computed from the red and near‐infrared surface reflectance. These G estimates compared well with conventional measurements of G using buried soil heat flux plates (MAD ≃ 20 W/m2 over a range from −13 to 213 W/m2). In order to account for the effects of sparse vegetation, semiempirical adjustments to the single‐layer bulk aerodynamic resistance approach were required for evaluation of sensible heat flux density (H). This yielded differences between measurements and remote estimates of H of approximately 33 W/m2 over a range from 13 to 303 W/m2. The resulting estimates of latent heat flux density, LE, were of the same magnitude and trend as measured values; however, a significant scatter was still observed: MAD ≃ 40 W/m2 over a range from 0 to 340 W/m2. Because LE was solved as a residual, there was a cumulative effect of errors associated with remote estimates of Rn, G, and H.
Preface [to special section on Monsoon '90 Multidisciplinary Experiment]
Authors: Kustas W. P. , Goodrich D. C.
Published: February 2004

The Monsoon '90 multidisciplinary field campaign was conducted over the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona during June–September 1990. A primary objective of this combined ground, aircraft, and satellite campaign was to assess the feasibility of utilizing remotely sensed data coupled with water and energy balance modeling for large‐area estimates of fluxes in semiarid rangelands. The experimental period encompassed a variety of vegetation, soil moisture, and rainfall conditions characterized by large temporal and spatial gradients. This preface outlines experimental objectives, briefly discusses the field campaigns, summarizes initial observations, and provides an overview of articles that are a part of the Monsoon '90 special section.
Use of remote sensing and reference site measurements to estimate instantaneous surface energy balance components over a semiarid rangeland watershed
Authors: Humes K. S. , Kustas W. P. , Moran M. S.
Published: February 2004

A primary motivation for using remotely sensed data to estimate components of the surface energy balance is to quantify surface energy fluxes in a spatially distributed manner over various spatial scales. However, all models which utilize remotely sensed data to estimate surface fluxes also require input variables and parameters which cannot be estimated on a spatially distributed basis with remotely sensed data. In this analysis, data from the Monsoon '90 experiment were used to evaluate the limitations in spatially extending a relatively simple energy balance model with remotely sensed data over a semiarid rangeland watershed. Using one ground‐based meteorological and flux station as a reference site, aircraft‐based remotely sensed data (surface temperatures and reflectances) were used to compute energy balance components for seven other locations within the watershed. The results indicated that for clear sky conditions, all components of the surface energy balance could be estimated to within approximately the same level of uncertainty with which the fluxes were measured with ground‐based flux instrumentation. However, under partly cloudy conditions the variability in incoming solar radiation across the watershed significantly degraded the estimation of distributed values of net radiation (Rnet). If ground‐based estimates of incoming solar radiation are used to calculate Rnet from remotely sensed data, then the spatial extent over which that measurement is valid limits the area over which accurate spatially distributed values of Rnet can be estimated. Additionally, the results of sensitivity analyses indicate that the level of uncertainty to which the roughness length for momentum, or z0m, is typically known for spatially distributed values in an area of naturally variable vegetation can give rise to significant uncertainties in the estimation of sensible heat flux. For areas where the spatial variation in roughness parameters is of the order of several centimeters, the error associated with assuming constant values for the roughness length for momentum is similar in size to the errors associated with temperature variations of the order of several degrees. In order to utilize radiometric temperatures to reliably estimate spatially distributed values of sensible heat flux, techniques such as those explored by Menenti and Ritchie (this issue) are needed to provide spatially distributed information on surface roughness parameters.
Movements of water, solutes, and stable isotopes in the unsaturated zones of two sand plains in the upper Midwest
Authors: Komor Stephen C. , Emerson Douglas G.
Published: February 2004

Four month‐long field experiments investigated movements of water and solutes through unsaturated sand plains near Princeton, Minnesota, and Oakes, North Dakota. Atrazine and bromide were applied to bare soils and soils planted with corn. The field plots were irrigated according to local farming practices. At the end of each experiment, unsaturated soils were analyzed for atrazine and bromide concentrations and oxygen and hydrogen isotope compositions of soil water. Most soil water was affected by evaporation but groundwater beneath the plots had no evaporative isotopic signature. Therefore most recharge consisted of water that was unaffected by evaporation. Sources of such water may have included snowmelt, prolonged or high‐intensity rainfalls that were not interrupted by periods of drying, and water that moved through preferential flow paths. Preferential flow also was suggested by the detection of atrazine, deethylatrazine, and bromide in groundwater shortly after each application of irrigation water at Princeton and by isolated concentrations of atrazine and bromide in soil well below the main masses of chemicals at Oakes.
Spectral and bispectral analysis for single‐ and multiple‐input nonlinear phreatic aquifer systems
Authors: Jin Minghui , Duffy Christopher J.
Published: February 2004

In general, stochastic partial differential equations and/or boundary conditions describing flow in phreatic aquifers are nonlinear. This paper uses second‐order perturbation to approximate the response of a phreatic aquifer system subject to multiple random inputs. The approach is equivalent to assuming quadratic nonlinearities in the original nonlinear phreatic aquifer system. Using frequency domain representations, the resulting model is decomposed into linear and nonlinear (quadratic) frequency response functions (FRF). For Gaussian input, the response of a nonlinear phreatic aquifer system is non‐Gaussian and is typically skewed. Thus the variance spectrum is insufficient to describe the response process. The bispectrum, defined as the Fourier transform of the third‐order cumulant sequence, represents the distribution of skewness with frequency and further serves as a measure of the nonlinearity of the system. Given input processes of aquifer recharge and stream stage fluctuations, the linear and quadratic FRF are used to construct the spectrum and bispectrum of the response process. The role of nonlinearity in a stream‐aquifer system is examined.
Parameter identification for a runoff model for forest roads
Authors: Luce Charles H. , Cundy Terrance W.
Published: February 2004

Rainfall simulation is a commonly used approach for studying runoff and erosion from forest roads, and a method is needed to estimate infiltration parameters from these experiments. We used two algorithms, the Simplex and Shuffled Complex Evolution, to estimate parameters for a physically based infiltration and overland flow model. Each algorithm was tested by estimating parameters for 92 field‐measured hydrographs from forest roads. Nine of the field‐measured hydrographs allowed us to further test whether estimated parameters could be extended to other antecedent conditions and plot sizes. The results demonstrate (1) the physically based model is able to estimate hydrographs from forest roads, (2) the two algorithms find unique parameter sets in spite of an error surface that suggests identifiability problems between the hydraulic conductivity and pressure parameters, (3) the two algorithms converged to the same parameter values, and (4) that parameters estimated for one antecedent condition and plot size can be extended to others with reasonably small error.
Integrated embedding optimization applied to Salt Lake valley aquifers
Authors: Gharbi A. , Peralta R. C.
Published: February 2004

The embedding optimization modeling approach is adapted to aid sustainable groundwater quantity and quality management of complex nonlinear multilayer aquifers. Implicit block‐centered finite difference approximations of the quasi three‐dimensional unsteady flow equation and Galerkin finite element approximations of the two‐dimensional advection‐dispersion transport equation are embedded directly as constraints in the model. Also used are nonlinear constraints describing river‐aquifer interflow, evapotranspiration, and vertical flow reduction due to unconfinement. These circumvent use of large numbers of integer variables. The use of both linear and nonlinear formulations in a cyclical manner reduces execution time and improves confidence in solution optimality. The methodology is demonstrated for Salt Lake valley where groundwater quantity and quality management are needed, the proportion of pumping cells and cells needing head constraint is large, and many flows are described by discrete nonlinear or piece wise linear functions.
Vertical mixing of coarse particles in gravel bed rivers: A kinematic model
Authors: Hassan Marwan A. , Church Michael
Published: February 2004

Burial of transported material in gravel bed rivers was examined using several hundred magnetically tagged particles. Burial depth of individual particles varies with no simple relation to particle size. Distributions of burial depth of particles were examined by considering observed burial depth of individual size groups of all mobile particles and of all tagged particles (including static ones). “Simple” (single‐peak) flow events yielded exponential distributions of burial depth of mobile particles. Distributions with secondary peaks were obtained from large and multipeak events. Season long results were skew peaked with a tendency toward uniformity. An m‐layer model is developed to account for the variation of frequency of burial depth of particles, based on the assumptions that the burial depth of particles in a simple event is exponentially distributed and the proportion of particles which change vertical position from a given layer is constant from event to event. The model is elaborated for varying mixing rates, varying scour depths, and variable behavior of different size classes. After many events the material is uniformly distributed throughout all layers, in conformity with field experience.
Riverbed heat conduction prediction
Authors: Hondzo Midhat , Stefan Heinz G.
Published: February 2004

Heat flux between river water and riverbed contributes significantly to river water heat content and river temperature, especially in shallow rivers. A numerical model of heat conduction in a riverbed was verified with detailed temperatures measurements in a riverbed. Three different methods, the heat budget method, the gradient method, and an inverse method, were used alternatively for the estimation of the riverbed heat flux. Propagation of uncertainty in the riverbed temperature model was studied using a vector state space perturbation method. Output uncertainty was estimated by random perturbations of the effective thermal conductivity from the main value. The coefficient of variation of effective thermal conductivity was 0.1. Temperatures and heat fluxes in the riverbed were affected by the uncertain thermal conductivity.
Simulating the influence of lake position on groundwater fluxes
Authors: Cheng Xiangxue , Anderson Mary P.
Published: February 2004

Groundwater flow around three hypothetical flow‐through lakes, located in the upper, middle, and lower portions of a watershed, was numerically simulated in cross section under steady state and transient conditions. Results showed that (1) groundwater fluxes into and out of the lake located in the lowest position were higher and more stable than for lakes farther upgradient, and (2) periods of intense groundwater recharge caused flow reversals and the formation of a stagnation point downgradient of the uppermost lake, thereby causing a net increase in groundwater inflow. These simulation results provide guidance on monitoring network design, including the frequency of monitoring and/or sampling.
Digital Elevation Model Networks (DEMON): A model of flow over hillslopes for computation of contributing and dispersal areas
Authors: Costa‐Cabral Mariza C. , Burges Stephen J.
Published: February 2004

Current algorithms for computing contributing areas from a rectangular grid digital elevation model (DEM) use the flow‐routing model of O'Callaghan and Mark (1984), which has two major restrictions: (1) flow which originates over a two‐dimensional pixel is treated as a point source (nondimensional) and is projected downslope by a line (one dimensional) (Moore and Grayson, 1991), and (2) the flow direction in each pixel is restricted to eight possibilities. We show that large errors in the computed contributing areas result for any terrain topography: divergent, convergent, or planar. We present a new model, called digital elevation model networks (DEMON), which avoids the above problems by representing flow in two dimensions and directed by aspect. DEMON allows computation of both contributing and dispersal areas. DEMON offers the main advantage of contour‐based models (e.g., Moore et al., 1988), the representation of varying flow width over nonplanar topography, while having the convenience of using rectangular grid DEMs.
Mass balance models of Ekman Transport and nutrient fluxes in coastal upwelling zones
Authors: Jewell Paul W.
Published: February 2004

The nutrient cycles of coastal upwelling zones are studied with simple mass balance models of Ekman transport, longshore transport, surface productivity, and dissolved phosphorous. The models are constrained with data from the Peru, northwest Africa, and Oregon upwelling systems. The onshore‐offshore mass balance model agrees with published Ekman transport, surface productivity, and nutrient data as well as hypothesized nutrient f‐ratios for highly productive coastal settings. The onshore‐offshore model suggests that increased primary productivity in glacial‐era coastal upwelling zones was not a linear function of Ekman transport, but instead was probably dependent on the physical and chemical dynamics of a specific setting. In the Peru upwelling system, longshore equatorward surface currents and poleward undercurrents produce positive surface nutrient gradients in the equatorward direction and relatively constant gradients in subsurface waters. Longshore nutrient gradients off northwest Africa are positive in the equatorward direction for both surface and subsurface waters. These observations are consistent with the conceptual model of surface and subsurface currents which are moving toward the equator and continually being upgraded by the offshore flux of nutrients. The northwest Africa and Peru data are not consistent with the longshore nutrient model of Redfield et al. (1963).
Modeling methane emissions from rice paddies
Authors: Cao Mingkui , Dent J. B. , Heal O. W.
Published: February 2004

The quantification of methane (CH4) emissions from global rice paddies is still highly uncertain. The extrapolation of CH4 release rates from point measurements to regional level or empirically correlating methane emission rates with a few factors (e.g., temperature and rice primary production) is unlikely to yield reliable results. Reducing the uncertainties in estimates of current CH4 emission and predicting its future change require a process model to simulate CH4 emissions from various paddyland environments. CH4 emission is an ecosystem process closely coupled to plant growth and soil organic matter decomposition. In this study a methane emission model was developed based on supplies of carbon substrate for methanogens by rice primary production and soil organic matter degradation, direct environmental controls on methanogenesis, and the balance between CH4 production and consumption by methanotrophic oxidation. A validation of the model indicated its reasonable ability to calculate CH4 emission rates and their seasonal variations. The model, when coupled with supporting data sets, would complement spatial and dynamic analysis of CH4 emissions from rice paddies in the framework of climate‐plant‐soil interactions.
Three‐dimensional currents of electrodynamic tethers obtained from laboratory models
Authors: Urrutia J. M. , Stenzel R. L. , Rousculp C. L.
Published: February 2004

Magnetic probe measurements in three dimensions (>15,000 positions) and time in a large laboratory plasma (ne ≥1011 cm−3, kTe ≥1 eV, B0 = 20 G, 1 m diam. × 2.5 m length) reveal the plasma currents J = ∇ × B/μ0 excited by a pulsed (Δt = 100 ns), stationary, tethered pair of electrodes (≈ 1 cm diam., 20 cm spacing ⟂ B0). The plasma currents for a moving, dc‐current carrying electrodynamic tether are obtained by a superposition of delayed pulses emitted at successively displaced tether positions. The transient plasma currents are carried by low‐frequency whistlers instead of Alfvén waves and form a 3D wing structure but no long phantom loop due to cross‐field Hall current shunting.
Mapping bedrock beneath glacial till using CDP seismic reflection methods
Authors: Keiswetter Dean , Black Ross , Steeples Don
Published: February 2004

This paper is a case history demonstrating the applicability of the common depth point (CDP) seismic reflection method to image bedrock beneath glacial till in northwestern Iowa. Reflections from the base of the 40‐m thick glacial till are clearly observable on field files at around 45 to 50 ms two‐way traveltime and possess a dominant frequency of around 100 Hz. The bedrock reflection is confirmed by drill data. The seismic data are of sufficient quality to detect local bedrock topographic changes and to interpret discontinuities along the till‐bedrock interface. Finite‐difference synthetic seismograms substantiate the interpreted reflections and the diffraction signatures from faults observed on the field files. At some locations along the seismic line, intra‐till reflections are apparent on the field files. These Intra‐till features are on the order of tens of meters in length along the line traverse and reflections from them are not enhanced by common depth point processing. Intra‐till reflections could be indicative of gravels or other alluvial materials that may serve as local aquifers.
Freja observatons of correlated small‐scale density depletions and enhanced lower hybrid waves
Authors: Eriksson A. I. , Holback B. , Dovner P. O. , Boström R. , Holmgren G. , André M. , Eliasson L. , Kintner P. M.
Published: February 2004

Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small‐scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.
Latitudinal and season variations of water vapour in the middle atmosphere
Authors: Munro R. , Rodgers C. D.
Published: February 2004

The data from the B2 channel of the Stratospheric and Mesospheric Sounder (SAMS), flown on Nimbus 7, were intended to provide water vapour cross‐sections ranging from 45°S—65°N in latitude and from 30–65 km in altitude. However, anomalous cross‐sections obtained by Mutlow [1984] suggest that the in‐flight transmittance behaviour of the SAMS B2 channel was different from that expected. As a result a two‐stage retrieval process has been developed to obtain both an improved estimate of the transmittance behaviour of the channel, and the required water vapour cross‐sections. The water vapour cross‐sections obtained over the three‐year period from January 1979–December 1981, clearly show the presence of a mid to high latitude summer maximum in water vapour at ∼5‐10 mb (∼30–35 km), a poleward decrease in volume mixing ratios below ∼10 mb (∼30 km) in the winter hemisphere, a secondary maximum in the summer hemisphere at ∼0.1 mb (∼65 km) in the lower mesosphere, and a less distinct equatorial minimum at ∼10 mb (∼30 km). However, the latter has not been shown to be statistically significant in this analysis.
New composite reflectance spectra of Mars from 0.4 to 3.14 μm
Authors: Mustard John F. , Bell James F.
Published: February 2004

To verify the quality and accuracy of Phobos‐2 ISM spectra of Mars, we have performed comparisons with telescopic data obtained contemporaneously and which have spatial and spectral overlap with the ISM measurements. We find general agreement between the independently‐calibrated data sets in terms of spectral shape and absorption band strength and position in the 0.77 to 0.93 μm overlap region. The telescopic data were scaled to the ISM reflectance values in this overlap region to yield new bright and dark region composite reflectance spectra from 0.40 to 3.14 μm. We ascribe the increase in band depth and overall spectral heterogeneity of the ISM data relative to previous telescopic observations to the more than order of magnitude increase in spatial resolution achieved by ISM from Martian orbit. Further increases in spatial resolution from future orbital imaging spectroscopic instruments can be expected to reveal even greater degrees of compositional and mineralogic diversity on Mars.
Infrared spectral measurements (450–2500 cm−1) of shuttle‐induced optical contamination
Authors: Zhou D. K. , Pendleton W. R. , Bingham G. E. , Steed A. J. , Dean D. A.
Published: February 2004

The first shuttle‐based infrared spectral measurements in the region beyond 800 nm were made during the STS‐39 mission. A cryogenic Michelson interferometer in the CIRRIS‐1A payload yielded high quality atmospheric IR spectra in the earthlimb observing mode. In addition, observations in the very high earthlimb and in the bay‐to‐space mode provided important opportunities to assess optical contamination. These observations have revealed spectrally‐extended H2O and NO emissions as dominant in the region 450–2500 cm−1 under quiescent conditions. The viewing conditions, spectral and “spatial” (limb scan) distributions, and temporal behavior of the H2O emissions have resulted in their classification as shuttle‐induced “cloud glow” optical contamination.
Suppression of multiple reflections in GLORIA sidescan sonar imagery
Authors: Le Bas Timothy P. , Mason David C.
Published: February 2004

The GLORIA side‐scan sonar system is an invaluable tool for studying the texture of the sea floor. A substantial set of computer algorithms now exist for radiometric and geometric correction of the raw GLORIA imagery. One type of artefact not presently removed is bright lines parallel to the ship track due to multiple reflections between the seabed and the sea surface. These are common where the sea floor is flat and can lead to misinterpretation. A method is presented here to suppress multiple reflections in GLORIA and other side‐scan sonar imagery. As a by‐product of the suppression method a value for the seabed backscattering coefficient can be calculated, giving some extra insight regarding sea‐floor type for sedimentary areas.
Modal fields: A new method for characterization of random seismic velocity heterogeneity
Authors: Goff John A. , Holliger Klaus , Levander Alan
Published: February 2004

Geologically and petrophysically constrained synthetic random velocity fields are important tools for exploring (through the application of numerical codes) the seismic response of small‐scale lithospheric heterogeneities. Statistical and geophysical analysis of mid‐ and lower‐crustal exposures has demonstrated that the probability density function for some seismic velocity fields is likely to be discrete rather than continuous. We apply the term “modal” fields to describe fields of this sort. This letter details a methodology for generating synthetic modal fields which satisfy the von Kármán covariance function. In addition, we explore some of the mathematics of “modality”, and define a modality parameter which quantifies the variation between end members binary and continuous fields.
The role of subduction on the horizontal motions in the Tyrrhenian Basin: A numerical model
Authors: Giunchi Carlo , Gasperini Paolo , Sabadini Roberto , D'Agostino Gabriella
Published: February 2004

The horizontal motions in the Tyrrhenian basin and surrounding chains, induced by subduction along the Calabrian Arc and Northern Apennines, are analyzed by means of numerical models based on finite element techniques. The driving mechanism, in 2‐dimensional vertical cross‐sections perpendicular to the trench, is the slab‐pull due to the negative buoyancy of a stratified viscoelastic plate that models the Ionic oceanic lithosphere or Adriatic plate sinking in the upper mantle. For the modern tectonic setting of the area, we test the sensitivity of extension in the back‐arc and of roll‐back of the overriding plate to variations in the viscosity structure and in the geometry of subduction. Our results are compared with extensional velocities inferred from geology and SLR‐VLBI data.
A reconnaissance paleomagnetic study of Cretaceous rocks from central Cuba
Authors: Chauvin Annick , Bazhenov Mikhail L. , Beaudouin Thierry
Published: February 2004

We applied thermal demagnetization to 12 sites of mid‐ and Upper Cretaceous volcanics and sediments of the Zaza terrane (central Cuba). Consistent directions of low‐ and/or high‐temperature components were obtained from eight sites. The fold test performed on a short‐wave fold in the Upper Cretaceous Arimao Formation is negative, as well as the conglomerate test on lava boulders from the same formation, whereas site‐means for the entire data set are better grouped after tilt‐correction. We conclude that the entire Cretaceous succession of the Zaza terrane was remagnetized in the Campanian. The measured latitude is lower by 15°±6° with respect to the North American APWP indicating ca. 1600 ± 600 km northward displacement of the Zaza terrane since the Campanian. Also discordant is the measured declination implying 37°±11° counterclockwise rotation of the Zaza terrane.
Strain accumulation in the Shumagin Islands: Results of initial GPS measurements
Authors: Larson Kristine M. , Lisowski Michael
Published: February 2004

Deformation in the Shumagin seismic gap has been monitored with repeated trilateration (EDM) in the 1980–1987 interval and with the Global Positioning System (GPS) in the 1987–1991 interval. The geodetic network extends for 100‐km across the Shumagin Islands to the Alaska Peninsula. Results from the GPS surveys are consistent with those previously reported for the EDM surveys: we failed to detect significant strain accumulation in the N30°W direction of plate convergence. Using the method of simultaneous reduction for position and strain rates, we found the average rate of extension in the direction of plate convergence to be −25±25 nanostrain/yr (nstrain/yr) during the 1987–1991 interval of GPS surveys compared with −20±15 nstrain/yr during the 1981–1987 interval of complete EDM surveys. We found a marginally significant −26±12 nstrain/yr extension rate in the 1981–1991 interval covered by the combined EDM and GPS surveys. Strain rates are higher, but not significantly so, in the part of the network closest to the trench. Spatial variation in the deformation is observed in the 1980–1991 average station velocities, where three of the four stations closest to the trench have an arcward velocity of a few mm/yr. The observed strain rates are an order of magnitude lower than the −200 nstrain/yr rate predicted by dislocation models.
Chlorine chemistry and the potential for ozone depletion in the Arctic stratosphere in the winter of 1991/92
Authors: Müller R. , Peter Th. , Crutzen P. J. , Oelhaf H. , Adrian G. P. , Clarmann Th.V. , Wegner A. , Schmidt U. , Lary D.
Published: February 2004

We present an analysis of chlorine chemistry in the Arctic stratosphere during the winter of 1991/92 and assess its potential implications for ozone depletion. In accordance with observations of total organic chlorine, ClONO2 and HCl, box model results indicate the following: (1) An almost complete activation of chlorine during the cold winter period. (2) A possible contribution from the heterogeneous reaction HOCl + HCl and the gas‐phase reaction CH3O2 + ClO to the complete conversion of HCl to active chlorine. (3) A strong buildup of ClONO2 following PSC disappearance which remains the main chlorine reservoir for about a month, after which HCl becomes dominant. (4) Appreciable chemical ozone loss in the lower stratosphere inside the polar vortex is conceivable for the winter of 1991/92.
The Sudbury Structure: Constraints on its genesis from Lithoprobe results
Authors: Deutsch A. , Grieve R. A. F.
Published: February 2004

Results of the LITHOPROBE transect provide additional corroboration of the impact model for the origin of the Sudbury Structure. High resolution seismic reflection data indicate that the Sudbury Igneous Complex (SIC) is indeed continuous under the Sudbury Basin. There is no indication of a feeder dike system or a previously assumed “hidden mafic mass”. All these observations strongly support the interpretation of the SIC as a coherent impact melt sheet. In addition, the seismic, and to a lesser extent magnetic and gravity, results point to a highly assymmetric structure to the SIC at depth, with significant tectonic disruption in the South Range. The present non‐circularity of the SIC is caused by post‐impact ductile deformation and thrusting due to northwest directed compression. Decompressing this strain pattern leads to an original diameter of the SIC between 60 and 80 km, which, in consequence, would extend the original diameter of the final crater up to a maximum of 280 km.
The global distribution of giant radiating dike swarms on Venus: Implications for the global stress state
Authors: Grosfils Eric B. , Head James W.
Published: February 2004

Magellan radar data of Venus reveal 163 large radial lineament systems composed of graben, fissure, and fracture elements. On the basis of their structure, plan view geometry, and volcanic associations, at least 72% are interpreted to have formed primarily through subsurface dike swarm emplacement, the remainder through uplift or a combination of these two mechanisms. The population of swarms is used to determine regional and global stress orientation. The stress configuration recorded from 330–210°E (Aphrodite Terra) is best explained by isostatic compensation of existing long wavelength topography or coupling between mantle flow and the lithosphere. The rest are correlated with concentrations of rifting and volcanism in the Beta‐Atla‐Themis region. The global stress field on Venus is different than that of Earth, where plate boundary forces dominate.
Stable isotopes in the basal silty ice preserved in the Greenland Ice Sheet at summit; environmental implications
Authors: Souchez R. , Tison J.‐L. , Lorrain R. , Lemmens M. , Janssens L. , Stievenard M. , Jouzel J. , Sveinbjörnsdottir A. , Johnsen S. J.
Published: February 2004

Modelling ice sheet behaviour in the context of climatic changes depends on initial and boundary conditions which can be better defined by studying the composition of basal ice. This study deals with basal ice reached by deep drilling at Summit in Central Greenland (GRIP core). The isotopic composition of this ice indicates that ice formed at the ground surface in the absence of the ice sheet largely contributed to its formation. The basal silty ice is a remnant of a growing stage of the ice sheet, possibly the original build up.
Transverse ion energization and wave emissions observed by the Freja satellite
Authors: André Mats , Norqvist Patrik , Vaivads Andris , Eliasson Lars , Norberg Olle , Eriksson Anders I. , Holback Bengt
Published: February 2004

Observations by the Freja satellite at altitudes of about 1700 km in the auroral zone sometimes show energization of both light and heavy ions to characteristic energies of up to about 50–100 eV. This ion heating is often associated with lower hybrid waves, and also with wave emissions at lower frequencies. No obvious decrease of electric field wave power below the proton gyrofrequency, reported by other spacecraft at similar altitudes, is observed in the regions of intense ion heating. Furthermore, localized density depletions associated with lower hybrid waves have not been detected in these regions. We present a preliminary model where waves observed during an event in the evening‐side auroral zone are used to explain the observed ion energies.
Fault parameters and tsunami excitation of the May 13, 1993, Shumagin Islands earthquake
Authors: Tanioka Yuichiro , Satake Kenji , Ruff Larry , González Frank
Published: February 2004

The Shumagin Islands earthquake of May 13, 1993, occurred in a previously identified seismic gap where a large subduction earthquake is expected. We analyzed long‐period surface waves and P waves recorded on the IRIS stations to estimate the fault parameters. The Centroid Moment Tensor solution shows that the focal mechanism is a thrust type with the strike parallel to the Aleutian trench. The seismic moment is 2.0×1019 Nm and the corresponding moment magnitude is 6.8. The Moment Tensor Rate Function inversion from P waves also yields a similar focal mechanism and seismic moment. In addition, this computation provides estimates of 10 s for the duration of the source time function and 35 km for the best point source depth. These seismological analyses indicate that the fault mechanism of the 1993 earthquake was as expected, but that the magnitude was too small to fill the gap. This earthquake did not generate a tsunami large enough to be observed at the Sand Point, Alaska tide gauge or at an ocean bottom pressure gauge, at distances of 100 and 300 km, respectively. Numerical tsunami simulations result in amplitudes at both stations that are within the background noise level. Additional numerical experiments also suggest that the small tsunami amplitudes are due to the location of the source area in the shallow shelf region.