bluuebunny/crossref_metadata_2025 huggingface dataset:
A search for nitrates in Martian meteorites
Authors: Grady Monica M. , Wright I. P. , Pillinger C. T.
Published: February 2004

Martian atmospheric nitrogen is highly enriched in 15N; nitrates formed by interaction of the atmosphere with the Martian regolith should therefore also be characterized by an elevated δ15N value. A search has been made for nitrates in two Martian meteorites, in order to determine the extent of possible regolith‐atmosphere interaction. Shock‐produced glass from the Elephant Moraine (EET) A79001 shergottite (E 1,149) and a water‐soluble extract from Nakhla were analyzed by Fourier transform infrared (FTIR) spectroscopy and stepped combustion‐stable isotope mass spectrometry. FTIR of both meteorites had features at 1375 cm−1 and 1630 cm−1, consistent with nitrates. On account of their low thermal stability, nitrates break down at temperatures below 600°C; in this temperature range, E1,149 yielded ∼1250 ppb nitrogen with δ15N −8±5‰. If this nitrogen is from a nitrate, then it cannot be distinguished from terrestrial salts by its isotopic composition. The water‐soluble extract from Nakhla also released nitrogen at low temperatures, approximately 17 ppb with δ15N approximately −11±4‰. Since Nakhla is an observed “fall”, this is unlikely to be a terrestrial weathering product. Nitrates apparently occur in E 1,149 and Nakhla, but in very low abundance, and their origin is unclear. The isotopic composition of the salts, which is within the range of that proposed for Martian magmatic volatiles, is far removed from that of nitrogen in the present‐day Martian atmosphere. If the nitrates are Martian in origin, they did not form in recent times from reactions involving atmospheric gases. Rather, the nitrates could be the result of an earlier episode of atmospheric interaction with the regolith, or with implantation of magmatic volatiles introduced during degassing.
Comment on “The bromine content of micrometeorites: Arguments for stratospheric contamination” by F. J. M. Rietmeijer
Authors: Bohsung Jörg , Arndt Peter , Jessberger Elmar K.
Published: February 2004

Previous spectral analyses have given evidence of collisionally excited Jovian and (at times) Saturnian H2 Werner bands being absorbed by hydrocarbons at the shortest wavelengths along the auroral ovals, and of a longitudinal dependence of this absorption in the Jovian aurorae. This “color ratio” has been used to estimate the energy of the primary particles. In such estimates, particles are generally assumed to penetrate vertically into the atmosphere. However, the precipitating particle angular distribution is unknown, and a model developed for a diffuse aurora by Prangé and Elkhamsi (1991), for instance, predicts quite different possible distributions. We consider here the influence of the angular distribution used in the model, and show that distributions peaking far from vertical may increase the energy derived from a given color ratio by as much as a factor of 3. We discuss previous interpretations of the color ratio longitudinal modulation (variation of the auroral atmosphere structure, or of the incident particle energy) in view of the subsequent increase in energy input. We argue that an interpretation in terms of energy variations only is not consistent with the energy available in the magnetosphere if the aurorae are diffuse, and we discuss this finding in the context of recent Hubble Space Telescope images.
Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years
Authors: Frank Martin , Eckhardt Jörg‐Detlef , Eisenhauer Anton , Kubik Peter W. , Dittrich‐Hannen Beate , Segl Monika , Mangini Augusto
Published: February 2004

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high‐productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.
Waxing (and Waning) lyrical on hiatuses: Eocene ‐ Quaternary Indian Ocean hiatuses as proxy indicators of water mass production
Authors: Ramsay Anthony T. S. , Sykes Tim J. S. , Kidd Robert B.
Published: February 2004

The temporal distributions of Cenozoic hiatuses in the Indian Ocean are reconsidered in the light of a new time framework developed by the Paleoceanographic Indian Ocean Synthesis. On the basis of the temporal distribution of hiatus terminations, eight 3 m.y. time slices were selected for plotting the paleogeographical distribution of hiatus terminations. These distributions define areas of potential hiatus development, which formed along flow paths of proto‐Antarctic Bottom Water (an analogue of modern Antarctic Bottom Water) and proto‐Antarctic Intermediate Water (an analogue of modern Antarctic Intermediate Water). Expansion and contraction of these areas suggested changes in the volume of these water masses, which modified the distibution patterns of erosive deepwater currents and corrosive bottom water. Consequently, the spatial and bathymetric distribution of hiatus formation was altered. Age‐depth analysis of hiatuses from sites within post‐late Eocene areas of potential hiatus development was used, in conjunction with models of the behavior of unconformity boundaries within fluctuating water masses, to construct waxing/waning curves for proto‐Antarctic Bottom Water and proto‐Antarctic Intermediate Water. These curves define volume changes in the production histories of these water masses which were related to the post‐late Eocene glacial history of Antarctica. Since the late Oligocene, the proto‐Antarctic Bottom Water waned during intervals of increased glacial intensity, whereas the proto‐Antarctic Intermediate Water waxed during these intervals. These relationships reversed during intervals of reduced glacial intensity, including intervals where there is no evidence for Antarctic glaciation. This conflicts with interpretations made by other workers. A comparison is made between the production histories of proto‐Antarctric Bottom Water and Northern Component Water (an analogue of North Atlantic Deep Water) based on ∂13C for the Miocene to Pliocene.
Monsoon upwelling effects on test size of some planktonic foraminiferal species from the Oman Margin, Arabian Sea
Authors: Naidu Pothuri Divakar , Malmgren Björn A.
Published: February 2004

Variation in upwelling may be monitored using various micropaleontological indices, such as the relative abundance or accumulation rate of the planktonic foraminiferal species Globigerina bulloides or mass accumulation rate of planktonic foraminifera. We have analyzed the relationships between the upwelling history and mean test size variations in the planktonic foraminiferal species Globigerinoides ruber, Globigerinita glutinata, G. bulloides, and Neogloboquadrina dutertrei through the last 19,000 years in Ocean Drilling Program hole 723 A from the Oman Margin, Arabian Sea. The study reveals that these species were larger on average between 11 and 5 ka, which coincides with a period of strong upwelling. On the other hand, these species showed smaller mean test sizes during the last glacial period and late Holocene when upwelling was weaker. Mass accumulation rates of these species are linearly proportional to their mean test sizes, indicating that both mean size and productivity of individual species are related to the upwelling intensity. Therefore we propose that the size variations in G. ruber, G. glutinata, G. bulloides, and N. dutertrei can be used as an additional micropaleontological index of upwelling intensity in the Arabian Sea.
Empirical models of the magnetospheric magnetic field
Authors: Jordan C. E.
Published: February 2004

Space scientists have a wide variety of models to choose from for their studies of the Earth's auroral phenomenon, radiation belts, magnetotail, magnetopause, etc. Different types of models are suited for different purposes. At this time, there is no one model to suit everyone's needs. In this paper a general overview of magnetospheric modeling is given, along with a more detailed discussion of several empirical models which are widely used. These models are composed of representations of the Earth's main internal field (basically a dipolar field), plus external field contributions due to ring currents (carried by the particles in the Van Allen radiation belts), magnetopause currents (the boundary surface between the Earth's magnetic field and the interplanetary magnetic field carried by the solar wind), and tail currents (carried by particles in the neutral sheet of the magnetotail). As in many disciplines, models are frequently chosen for convenience rather than for their intrinsic properties. Here a summary of the properties of the most commonly used models is given. Discussion of MHD models is beyond the scope of this paper. The empirical models presented here are the Mead‐Fair‐field, Olson‐Pfitzer tilt‐dependent (1977), Tsyganenko‐Usmanov, Tsyganenko (1987), Olson‐Pfitzer dynamic (1988), Tsyganenko (1989), and Hilmer‐Voigt models. The derivations, agreement with quiet time and storm time data from the two satellite programs SCATHA and CRRES, and computation requirements of these models are compared.
Transport of reactive contaminants in heterogeneous porous media
Authors: Brusseau Mark L.
Published: February 2004

The potential for human activities to adversely affect the environment has become of increasing concern during the past two decades. Concomitantly, the transport and fate of contaminants in subsurface systems has become one of the major research areas in the environmental, hydrological, and Earth sciences. An understanding of how contaminants move in the subsurface is needed to evaluate the probability of contaminants associated with a chemical spill reaching an aquifer and contaminating groundwater. This knowledge is also required to develop and evaluate methods for cleaning up contaminated soils and aquifers. Just as importantly, knowledge of contaminant transport and fate is necessary to design “pollution prevention” strategies. A tremendous body of literature on contaminant transport has been generated in response to these needs. This literature consists primarily of results obtained by theoretical, experimental, and mathematical modeling based investigations and, to a much lesser extent, field experiments. This paper consists of a brief review of some of the major aspects associated with the transport of reactive contaminants in heterogeneous subsurface environments. It begins with a review of basic concepts related to contaminant transport, followed by a discussion of the results obtained from some of the few well‐controlled field experiments designed to investigate transport of reactive contaminants in the subsurface. Some of the major factors controlling contaminant transport will then be discussed, followed by a review of conceptual and mathematical approaches used to represent those factors in mathematical models. A brief overview of future needs and opportunities in contaminant transport will close the discussion.
Predicted and measured total electron content at both peaks of the equatorial anomaly
Authors: Ezquer Rodolfo G. , de Adler Nieves Ortiz , Heredia Teresita
Published: February 2004

The total electron content (TEC) of the ionosphere is a parameter of great importance for the systems which use transionospheric radiowaves and for the checking of the validity of ionospheric models. It is known that the highest TEC values in the world occur at the peaks of the equatorial anomaly (EA) regions at ∼15° either side from the magnetic equator. In this paper, TEC measurements obtained with Faraday technique at three stations placed near both peaks of the EA are used to check the validity of three ionospheric models to predict TEC in these regions. The TEC data of the southern peak of the EA were obtained at Tucumán (26.9°S, 294.6°E) in 1982, and those corresponding to the northern peak were Obtained at Palehua (21.4°N, 203.2°E) in 1978 and Delhi (28.6°N, 77.2°E) in 1968. Ionosonde data from Tucumán, Delhi, and Maui, (20.8°N, 203.5°E) are also used. Two widely used models are considered, namely: the international reference ionosphere and the semiempirical low‐latitude ionospheric model. The third model considered is a Chapman layer with scale height equal to atomic oxygen scale height (CHOEA). In general, the models underestimate TEC during daylight hours at Tucumán suggesting that the real electron density profiles are larger in electron density magnitudes than those given by the models. For Palehua and Delhi the models' predictions are better than those for Tucumán, and a very good agreement among calculated and measured TEC values at Palehua for some months is observed. Taking into account the simplicity of the TEC calculus with CHOEA, this model would be an adequate alternative to predict TEC at stations placed near the northern crest of the EA, at least for the considered solar activity. This model allows also to calculate TEC at the southern crest of the EA with a similar degree of accuracy of those values obtained with the other models.
Examples of vertical velocity comparison from collocated VHF and UHF profilers
Authors: McAfee J. R. , Gage K. S. , Strauch R. G.
Published: February 2004

Examples of vertical velocity measurements by the VHF and UHF profilers at Platteville, Colorado, are presented. These show excellent agreement on timescales of 1 min, suggesting that both systems are measuring true vertical velocities.
The application of recursive aggregate T‐matrix algorithm in the Monte Carlo simulations of the extinction rate of random distribution of particles
Authors: Lu C. C. , Chew W. C. , Tsang L.
Published: February 2004

The recursive T ‐matrix algorithm is applied to Monte Carlo simulations of multiple scattering by random distribution of dielectric spheres. The method is a fast algorithm for calculating the exact solution of Maxwell's equations for a large number of scattering objects. The extinction rate is calculated by averaging over many realizations. The computed results are compared with those from analytic approximations, namely, the quasi‐crystalline approximation.
A simple, convenient formalism for electron density profiles
Authors: Fox Matthew W.
Published: February 2004

Chapman‐type layer shapes have been a foundation in ionospheric physics since the formalism was first proposed, due to both a simple functional form and an underlying physical basis in solar production. In this report, a new means for utilizing these convenient layer shapes is described for the specification of electron density profiles, wherein both the topside and bottomside are approximated by Chapman layers of varying scale heights. Permitting the scale heights to vary is essentially a mathematical means of accounting for the changes in layer shapes from the purely solar‐produced value to changes produced in the presence of diffusive transport, while retaining the overall numerical convenience of the Chapman formalism. This profile model has first been applied to a sample of electron density profiles obtained at the Millstone Hill, Massachusetts, incoherent scatter radar, to establish credibility of the profile formalism with respect to real profiles. Next, the formalism was applied to a database of theoretical ionospheres from the Phillips Laboratory Ionospheric Model, to develop a model along the lines of the Semiempirical Low‐latitude Ionospheric Model that is (1) more authentic to the original model runs, (2) more accurate in terms of total electron content, and (3) applicable over a wider latitude range.
A new frequency‐modulated continuous wave radar for studying planetary boundary layer morphology
Authors: Eaton Frank D. , McLaughlin Scott A. , Hines John R.
Published: February 2004

This paper describes a new generation frequency‐modulated continuous wave (FMCW) radar using state‐of‐the‐art electronics and computerized data processing that greatly enhance the use of the radar as a practical tool for atmospheric research. The system senses at high resolution (∼2‐m range and 12 s for obtaining each profile), has ultrasensitivity (< −165 dBm), and is accurately calibrated for the refractive index structure parameter (C2n). The authors present salient features, discuss the calibration procedure, and present and discuss examples of various types of fine detail wave and frontal activity, boundary layer convection, and a light winter storm sensed by the radar over the last 2 years. The authors also show associated data from radiosonde and tower‐mounted sensors that are relevant to the episodal events shown from the FMCW radar observations. Since the FMCW radar technique can resolve and sense individual insects, these point targets are shown to act as tracers and enhance flow visualization.
Tomographic reconstruction of the ionosphere over North America with comparisons to ground‐based radar
Authors: Pakula W. A. , Fougere P. F. , Klobuchar J. A. , Kuenzler H. J. , Buonsanto M. J. , Roth J. M. , Foster J. C. , Sheehan R. E.
Published: February 2004

Data collection for the first ground‐based ionospheric tomography campaign in North America was conducted over a 48‐hour period in mid‐November 1991. The data consist of records of ionospheric total electron content (TEC) from a number of passes of the U. S. Navy Navigation Satellite System spacecraft over a chain of ground‐based receiving stations. Data collection and reduction techniques are discussed; these include the determination of absolute TEC from the different phase advances induced by the ionosphere in each component of the dual‐frequency spacecraft signal. The use of tomographic methods to reconstruct ionospheric electron densities over a two‐dimensional (2‐D) region of the Earth's ionosphere at a number of different times is demonstrated. Specifically, two distinct tomographic methods, the algebraic reconstruction technique and a maximum entropy method, are used to mathematically invert the records of TEC. The resulting 2‐D contour “maps” of ionospheric electron density are then compared to similar maps produced by the Millstone Hill incoherent backscatter radar facility located at Westford, Massachusetts. Both qualitative and quantitative measures of agreement among the different reconstructions and the radar maps are presented. The behavior of the ionosphere over the course of the experiment is discussed.
Magnetic fabric of Pleistocene clays from the Tyrrhenian arc; A magnetic lineation induced in the final stage of the middle Pleistocene compressive event
Authors: Scheepers P. J. J. , Langereis C. G.
Published: February 2004

We present the results of an analysis of the anisotropy of magnetic susceptibility (AMS) in early Pleistocene clay sediments from the southern Apennines and its foreland (Apulia). Measurements were performed on samples from different tectonic units; the Apulian Unit, the Bradano Unit, and the Apenninic belt. The AMS is characterized by a dominant magnetic foliation parallel to the bedding plane in all cases, pointing to a primary sedimentary magnetic fabric. No lineation is found in the clays from the Apulian Unit, whereas the inner part of the Bradano Unit and the Apenninic belt show a clear NW–SE trending magnetic lineation which is perpendicular to the main compressive stresses known for this area. There is a very consistent WNW‐ESE trending magnetic lineation for the clays from the southern Apennines and the Calabrian block to the south. Correction of this lineation with the established tectonic rotations shows that the lineation is clearly of postrotational origin and reflects the latest stages of deformation. The lineation is linked to middle and late Pleistocene tectonics: its alignment occurred in the final stages of the compressive event affecting the Tyrrhenian arc system. For the Sant'Arcangelo basin in the southern Apennines, anomalous trends of magnetic lineation (compressive stress) were established.
Precise U‐Pb ages for Grenvillian and pre‐Grenvillian thrusting of Proterozoic and Archean metamorphic assemblages in the Grenville Front tectonic zone, Canada
Authors: Krogh T. E.
Published: February 2004

While the position of the Grenville Front is placed at a structural feature associated with the onset of high‐grade rocks when approached from the north, the time of first metamorphism in these rocks is, in general, pre‐Grenvillian and highly variable. At Killarney, amphibolite to granulite grade rocks were metamorphosed, lineated, and foliated to their present “front‐parallel” configuration mainly by 1453±6 Ma. North of North Bay, amphibolite grade migmatites have been subjected to a pre‐2650 Ma metamorphism. Granulite grade gneisses south of the Grenville Front near Val d’Or record a 2640 Ma metamorphic event. South of Chibougamau, mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 Ma. In central Labrador a 1650 Ma foliated granite is present south of the Grenville Front, and at least some of the banding is this old. At the Atlantic coast, granulite facies metamorphism in front‐parallel gneisses predates a 1790–1662 Ma deformation. In contrast, each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies NW directed thrusting or crustal loading. Near Killarney on Georgian Bay, titanite records an episode of 977±2 Ma partial resetting, the degree of which increases with distance into the Grenville Province. Here Grenvillian deformation, where present, is manifest as feldspar‐porphyroclastic fabric with strong SE plunging lineations. Near North Bay, Archean pegmatites in the Grenville Front zone were similarly deformed at 988±10 Ma, and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987±2 Ma. A 991±4 Ma cooling age for titanite in this melt is analytically identical to the zircon growth age, hence rapid cooling occurred. South of Chibougamau, new zircons grew in high‐pressure coronite in response to tectonic loading at 995±6 Ma, and in Central Labrador, melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995±2 Ma. Titanite in the granitic host rock has an analytically identical age of 992±4 Ma, again indicating rapid cooling. This second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651±5 Ma to 988±5 Ma two‐age mixing line. Grenville resetting at about 1037±19 Ma in the Grenville Front zone on the Labrador coast is indicated, but major thrusting is pre‐1662 Ma. U‐Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U‐Pb systems, high‐pressure assemblages, local melting, and rapid cooling. Crustal loading and shearing, with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling, are required.
Tectonic evolution of the High Himalaya in Upper Lahul (NW Himalaya, India)
Authors: Vannay Jean‐Claude , Steck Albrecht
Published: February 2004

The Upper Lahul region in the NW Himalaya is located in the transition zone between the High Himalayan Crystalline (HHC) to the SW and the Tethyan Zone sedimentary series to the NE. The tectonic evolution of these domains during the Himalayan Orogeny is the consequence of a succession of five deformation events. An early Dl phase corresponds to synmetamorphic, NE verging folding. This deformation created the Tandi Syncline, which consists of Permian to Jurassic Tethyan metasediments cropping out in the core of a large‐scale synformal fold within the HHC paragneiss. This tectonic event is interpreted as related to a NE directed nappe stacking (Shikar Beh Nappe), probably during the late Eocene to the early Oligocène. A subsequent D2a phase caused SW verging folding in the HHC. This deformation is interpreted as contemporaneous with late Oligocene to early Miocene SW directed thrusting along the Main Central Thrust. In the Tethyan Zone, a D2b phase is marked by a decollement thrust, a system of reverse faults, and gentle folds, associated with SW directed tectonic movements. This deformation is related to an imbricate structure, characteristic of a shallow structural level, and developed in the frontal part of a nappe affecting the Tethyan Zone units of SE Zanskar (Nyimaling‐Tsarap Nappe). A later D3 phase generated the Chandra Dextral Shear Zone (CDSZ), a large‐scale, ductile, dextral strike‐slip shear zone, located in the transition zone between the HHC and the Tethyan Himalaya. The CDSZ most likely represents a part of a system of early Miocene extensional and/or dextral, strike‐slip shear zones observed at the HHC‐Tethyan Zone contact along the entire Himalaya. A final D4 phase induced large‐scale doming and NE verging back folding.
The Limón, Costa Rica earthquake of April 22, 1991: Back arc thrusting and collisional tectonics in a subduction environment
Authors: Suárez Gerardo , Pardo Mario , Domínguez Jaime , Ponce Lautaro , Montero Walter , Boschini Ileana , Rojas Wilfredo
Published: February 2004

On April 22, 1991, a large earthquake (Mw = 7.7) occurred along the Caribbean coast of Costa Rica and western Panama. The rupture area of the fault mapped from the aftershocks is 45×85 km2;. The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Limón and decreasing gradually toward the southeast was observed along the Caribbean. Northwest of Puerto Limón no significant coastal uplift was observed. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Limón earthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thrust and strike‐slip faults that lie in the eastern piedmont of the Talamanca Cordillera. The historical seismicity indicates that the Caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7.0. On April 26, 1916, another earthquake (Ms = 6.9) took place in the same region. Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is approximately 0.8 cm/yr, compared with a value of 0.4 to 0.8 cm/yr along the Pacific subduction zone, depending on the estimated width of the seismogenic zone. Therefore a large fraction of the relative plate motion between the Cocos and Caribbean plates (9.8 cm/yr) appears to be taken up by crustal deformation in the back arc. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ridge with the subduction zone. The absence of a Wadati‐Benioff zone where the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Peninsula; this is supported by the Pliocene gap of volcanism present in Costa Rica. Thus the predicted relative motion between the Cocos and Caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zone of active crustal shortening and underthrusting in the back arc. This type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment.
Accuracy of mixed and control volume finite element approximations to Darcy velocity and related quantities
Authors: Durlofsky Louis J.
Published: February 2004

The accuracy of the Darcy velocity, flux, and stream function computed from lowest‐order, triangle‐based, control volume and mixed finite element approximations to the two‐dimensional pressure equation is considered. The control volume finite element method, similar to integrated finite difference methods and analogous to the interpolation of Galerkin finite element results over “control volumes,” is shown to yield a conservative velocity field and smooth streamlines. The streamlines and fluxes through the system computed with the control volume finite element approach are compared to those computed from the mixed finite element method, which approximates the pressure and velocity variables separately. It is shown that for systems with only moderate degrees of heterogeneity, the control volume finite element method is the more computationally efficient alternative; i.e., it provides more accurate flow results for a given number of unknowns. For more variable or discontinuous permeability fields, by contrast, such as sand/shale systems, the mixed finite element method is shown to approximate flow variables more accurately and more realistically than the control volume method with the same number of unknowns.
Temporal and spatial changes of terminal electron‐accepting processes in a petroleum hydrocarbon‐contaminated aquifer and the significance for contaminant biodegradation
Authors: Vroblesky Don A. , Chapelle Francis H.
Published: February 2004

Measurements of dissolved hydrogen and other biologically active solutes in groundwater from a shallow petroleum hydrocarbon‐contaminated aquifer indicate that the distribution of microbial terminal electron‐accepting processes (TEAPs), such as methanogenesis, sulfate reduction, and ferric iron (Fe (III)) reduction, is highly dynamic in both time and space. Delivery of sulfate to methanogenic zones by infiltrating recharge or lateral transport can result in a TEAP shift from methanogenesis to sulfate reduction. Conversely, lack of recharge and consumption of available sulfate can result in a shift from sulfate reduction to methanogenesis. Temporal shifts between sulfate and Fe (III) reduction were also observed. Time lags associated with TEAP shifts ranged from less than 10 days to about
months. The relation between TEAP and biodegradation rates of a variety of organic compounds indicate that biodegradation rates of petroleum hydrocarbons probably vary temporally and spatially in a contaminated aquifer.
A simple energy budget algorithm for the snowmelt runoff model
Authors: Kustas William P. , Rango Albert , Uijlenhoet Remko
Published: February 2004

The snowmelt runoff model (SRM) uses a degree‐day approach for melting snow in a basin. A simple radiation component was combined with the degree‐day approach (restricted degree‐day method) in an effort to improve estimates of snowmelt and reduce the need to adjust the melt factor over the ablation season. A daily energy balance model was formulated that requires not only the input of radiation but also measurements of daily wind speed, air temperature, and relative humidity. The three approaches for computing snowmelt, namely, the degree‐day, restricted degree‐day, and daily energy balance model were tested at the local scale by comparing melt rates with lysimeter outflow measurements. Because radiation measurements are not often available, a simple model for simulating shortwave and longwave components of the radiation balance that requires minimal information (i.e., daily cloud cover estimates, air temperature, and relative humidity) was developed. It was found that clouds and their effects on daily insolation at the surface can produce significant differences between measured and model estimates. In the comparisons of snowmelt estimates with the lysimeter outflow, the restricted degree‐day method yielded melt rates that were in better agreement with the observed outflow than the degree‐day method and were practically the same as estimates given by the energy balance model. A sensitivity analysis of runoff generated with SRM using as input the local snowmelt computations given by the three models and measured outflow from the lysimeter was performed for a basin. A comparison of the synthetic hydrographs for the basin suggests that a radiation‐based snowmelt factor may improve runoff predictions at the basin scale.
Gravity‐driven infiltration instability in initially dry nonhorizontal fractures
Authors: Nicholl M. J. , Glass R. J. , Wheatcraft S. W.
Published: February 2004

Experimental evidence demonstrating gravity‐driven wetting front instability in an initially dry natural fracture is presented. An experimental approach is developed using a transparent analog rough‐walled fracture to explore gravity‐driven instability. Three different boundary conditions were observed to produce unstable fronts in the analog fracture: application of fluid at less than the imbibition capacity, inversion of a density‐stratified system, and redistribution of flow at the cessation of stable infiltration. The redistribution boundary condition (analogous to the cessation of ponded infiltration) is considered in a series of systematic experiments. Gravitational gradient and magnitude of the fluid input were varied during experimentation. Qualitative observations imply that finger development is strongly correlated to the structure of the imbibition front at the onset of flow redistribution. Measurements of fingertip velocity are used to develop a first‐order relationship with fingertip length. Measured finger width is compared to theoretical predictions based on linear stability theory.
A flood routing Muskingum type simulation and forecasting model based on level data alone
Authors: Franchini Marco , Lamberti Paolo
Published: February 2004

While the use of remote hydrometers for measuring the level in water courses is both economical and widespread, the same cannot be said for cross section and channel profile measurements and, even less, for rating curves at the measuring cross sections, all of which are more often than not incomplete, out of date, and unreliable. The mass of data involved in level measurements alone induces a degree of perplexity in those who try to use them, for example, for flood event simulations or the construction of forecasting models which are not purely statistical. This paper proposes a method which uses recorded level data alone to construct a simulation model and a forecasting model, both of them characterized by an extremely simple structure that can be used on any pocket calculator. These models, referring to a river reach bounded by two measuring sections, furnish the downstream levels, where the upstream levels are known, and the downstream level at time t + Δt*, where the upstream and downstream levels are known at time t, respectively. The numerical applications performed show that while the simulation model is somewhat penalized by the simplifications adopted, giving not consistently satisfactory results on validation, the forecasting model generated good results in all the cases examined and seems reliable.
Estimating storm areal average rainfall intensity in field experiments
Authors: Peters‐Lidard Christa D. , Wood Eric F.
Published: February 2004

Estimates of areal mean precipitation intensity derived from rain gages are commonly used to assess the performance of rainfall radars and satellite rainfall retrieval algorithms. Areal mean precipitation time series collected during short‐duration climate field studies are also used as inputs to water and energy balance models which simulate land‐atmosphere interactions during the experiments. In two recent field experiments (1987 First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) and the Multisensor Airborne Campaign for Hydrology 1990 (MAC‐HYDRO '90)) designed to investigate the climatic signatures of land‐surface forcings and to test airborne sensors, rain gages were placed over the watersheds of interest. These gages provide the sole means for estimating storm precipitation over these areas, and the gage densities present during these experiments indicate that there is a large uncertainty in estimating areal mean precipitation intensity for single storm events. Using a theoretical model of time‐ and area‐averaged space‐ time rainfall and a model rainfall generator, the error structure of areal mean precipitation intensity is studied for storms statistically similar to those observed in the FIFE and MAC‐HYDRO field experiments. Comparisons of the error versus gage density trade‐off curves to those calculated using the storm observations show that the rainfall simulator can provide good estimates of the expected measurement error given only the expected intensity, coefficient of variation, and rain cell diameter or correlation length scale, and that these errors can quickly become very large (in excess of 20%) for certain storms measured with a network whose size is below a “critical” gage density. Because the mean storm rainfall error is particularly sensitive to the correlation length, it is important that future field experiments include radar and/or dense rain gage networks capable of accurately characterizing the rainstorm spatial and temporal correlation structure.
Estimation of in situ unsaturated soil hydraulic functions from scaled cumulative drainage data
Authors: Eching S. O. , Hopmans J. W. , Wallender W. W.
Published: February 2004

Simulation of water flow and transport processes in soils rely on field representative soil hydraulic functions. The linear variability concept in combination with the inverse technique was used to estimate in situ soil hydraulic properties in a 32‐ha field. Measured cumulative drainage curves were scaled yielding scaling factors. Subsequently, the drainage and moisture content distribution of the scaled reference profile were input to a numerical model to optimize the soil water retention and hydraulic conductivity curves for the reference soil profile by inverse solution of the scaled Richards equation. Field hydraulic functions for each location were computed from the reference curves and scaling factors. In addition, undisturbed soil cores taken from 0.3‐m and 0.6‐m depths at 44 locations were used to determine soil texture, and soil water retention and hydraulic conductivity curves in the laboratory using the multistep outflow technique. These hydraulic functions were scaled using the simultaneous scaling technique. The reference field hydraulic functions compared well with those determined from the soil cores taken from the 0.6‐m depth. In situ saturated hydraulic conductivity variability was one order of magnitude less than that of the soil cores.
Cosmogenic 36Cl accumulation in unstable landforms: 2. Simulations and measurements on eroding moraines
Authors: Zreda Marek G. , Phillips Fred M. , Elmore David
Published: February 2004

Cosmogenic 36Cl surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger than that of the analytical methods employed. This excessive boulder‐to‐boulder variability, progressively more profound with increasing age of landforms, is due to removal of soil and gradual exposure of boulders at the surface. In our gradual exposure model, boulders are initially buried in moraine matrix. With time, erosion lowers the moraine surface and the boulders are gradually exposed to cosmic rays. Because the cosmic ray intensity changes with depth, the boulders are subjected to variable production rates of the cosmogenic 36Cl. Initial depth of boulders and their chemical composition are variable, which results in different amounts of the accumulated cosmogenic 36Cl and thus different apparent ages of boulders. The shape of the resulting distribution of the apparent ages and the coefficient of variation depend on the erosion depth, while the first moment is a function of the true surface age and the erosion depth. These properties of the apparent age distributions permit calculation of the surface age, the erosion depth, and also the average erosion rate. We tested the model calculations using 26 boulders from a late Pleistocene moraine at Bishop Creek, Sierra Nevada, California. The set exhibited a bimodal distribution of the 36Cl surface exposure ages. We interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial processes other than soil removal. The 10 samples that constitute the older mode produced a distribution which closely matches the modeled distribution calculated using an age of 85 kyr and erosion depth of 570 g cm−2. This age is the same as an independent estimate obtained from cation ratio studies, and the calculated erosion depth is very close to the erosion depth of 600 g cm−2 based on a simple analytical model of soil erosion. These results indicate that our statistical model adequately describes effects of soil erosion on accumulation of cosmogenic 36Cl. The approach can be used to simultaneously obtain the true landform age and the erosion rate from apparent 36Cl ages and thereifore may help in evaluation of surface exposure ages of eroding landforms.
A numerical study of variable density flow and mixing in porous media
Authors: Fan Yin , Kahawita René
Published: February 2004

A numerical study of a negatively buoyant plume intruding into a neutrally stratified porous medium has been undertaken using finite different methods. Of particular interest has been to ascertain whether the experimentally observed gravitational instabilities that form along the lower edge of the plume are reproduced in the numerical model. The model has been found to faithfully reproduce the mean flow as well as the gravitational instabilities in the intruding plume. A linear stability analysis has confirmed the fact that the negatively buoyant plume is in fact gravitationally unstable and that the stability depends on two parameters: a concentration Rayleigh number and a characteristic length scale which is dependent on the transverse dispersivity.
A semianalytical method of path line computation for transient finite‐difference groundwater flow models
Authors: Lu Ning
Published: February 2004

This paper presents a new method of semianalytical particle tracking analysis under transient conditions. To permit accurate particle tracking in transient flows, the particle velocity is interpolated linearly in both space and time coordinates within each finite difference cell and time step. By using the total derivative and this interpolation scheme, the particle velocity is derived and integrated analytically to obtain the particle's trajectory within each cell and time step. Errors are much smaller than in numerical integration schemes. Numerical experiments show that the transient semianalytical solutions are efficient and accurate.
Effects of rate‐limited desorption on the feasibility of in situ bioremediation
Authors: Fry V. A. , Istok J. D.
Published: February 2004

Remediation of contaminated groundwater aquifers can be extremely difficult when desorption of the contaminant from the solid phase to the aqueous phase is slow. The effects of desorption rates on the feasibility of in situ bioremediation are investigated using an analytical solution to the solute transport equation with rate‐limited desorption and first‐order decay. Sensitivity analyses are used to identify when desorption or degradation is the rate‐limiting process for bioremediation for the range of desorption rates, degradation rates, and equilibrium distribution coefficients reported in the literature. The results show that when the desorption coefficient is large relative to the degradation coefficient (degradation is rate limiting), the performance of in situ bioremediation compared to groundwater extraction is improved as the degradation rate increases. Degradation in the aqueous phase increases the concentration gradient between solid and aqueous phases, increasing the rate of desorption and thus the rate of remediation. However, when the desorption coefficient is small relative to the degradation coefficient (desorption is rate limiting), increasing the degradation rate any further will not improve the performance of in situ bioremediation. For sites that conform to the assumptions of the model and for which model parameters can be measured or estimated, the results can be used to perform a preliminary assessment of the feasibility of in situ bioremediation.
A Simple and Efficient Space Domain Implementation of the Turning Bands Method
Authors: Dietrich C. R.
Published: February 2004

Given a correlation functionc(x) withxinRn, the turning bands method (TBM( forn= 2 and 3 constructs realizations of ann‐dimensional and stationary processY∼N(O,c(x)) from appropriately summed line processes. Therefore an implementation of TBM calls naturally for fast and accurate generations of line realizations. These have generally been generated by a spectral approach since the Fourier transform of the line covariances is linked in a very simple fashion to then‐dimensional Fourier transform ofc(x). However, we show that in many instances a space domain implementation of TBM, with line realizations obtained from a circulant embedding method, has advantages over a spectral implementation in that without increase in computational costs (1) the line realizations display exactly the required covariance structure, (2) knowledge of the spectral density of the processYis not required, and (3) fine tuning of line process parameters is not needed.
Preventing pesticide contamination of groundwater while maximizing irrigated crop yield
Authors: Peralta R. C. , Hegazy M. A. , Musharrafieh G. R.
Published: February 2004

A simulation/optimization model is developed for maximizing irrigated crop yield while avoiding unacceptable pesticide leaching. The optimization model is designed to help managers prevent non‐point source contamination of shallow groundwater aquifers. It computes optimal irrigation amounts for given soil, crop, chemical, and weather data and irrigation frequencies. It directly computes the minimum irrigated crop yield reduction needed to prevent groundwater contamination. Constraint equations used in the model maintain a layered soil moisture volume balance; describe percolation, downward unsaturated zone solute transport and pesticide degradation; and limit the amount of pesticide reaching groundwater. Constraints are linear, piecewise linear, nonlinear, and exponential. The problem is solved using nonlinear programming optimization. The model is tested for different scenarios of irrigating corn. The modeling approach is promising as a tool to aid in the development of environmentally sound agricultural production practices. It allows direct estimation of trade‐offs between crop production and groundwater protection for different management approaches. More frequent irrigation tends to give better crop yield and reduce solute movement. Trade‐offs decrease with increasing irrigation frequency. More frequent irrigation reduces yield loss due to moisture stress and requires less water to fill the root zone to field capacity. This prevents the solute from moving to deeper soil layers. Yield‐environmental quality trade‐offs are smaller for deeper groundwater tables because deeper groundwater allows more time for chemical degradation.