bluuebunny/crossref_metadata_2025 huggingface dataset:
Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model
Authors: Bi Daohua , Budd William F. , Hirst Anthony C. , Wu Xingren
Published: February 2004

This study investigates the long‐term behaviour of the thermohaline circulation (THC) in the CSIRO climate model, under a scenario of transient increase of atmospheric (equivalent) CO2 concentration followed by a perpetual stabilisation at triple the initial level (3 × CO2). The North Atlantic Deep Water Formation (NADWF) declines substantially and the Antarctic Bottom Water Formation (AABWF) essentially ceases by the time of CO2 tripling. During the subsequent millennium of stable 3 × CO2, NADWF recovers slightly but the AABWF shows no sign of returning and the residual deep overturning dies away. Accelerating the convergence to equilibrium of the deep ocean under the 3 × CO2 condition, the global THC eventually reaches a near‐stable state with the entire ocean warming by about 7°C, NADWF fully recovered and AABWF partly re‐established. This result shows a possible new quasi‐equilibrium of the ocean under long‐term global warming induced by the anthropogenic CO2 increase.
Titan's smile and collar: HST Observations of seasonal change 1994–2000
Authors: Lorenz R. D. , Young E. F. , Lemmon M. T.
Published: February 2004

We observed Titan with the Hubble Space Telescope in November 2000 using the Wide‐Field Planetary Camera (WFPC2) and the Space Telescope Imaging Spectrograph (STIS). Considerable change is apparent between these and observations in 1994‐1997 : in particular the North‐South asymmetry at visible wavelengths has reversed, with the southern hemisphere brighter, as during the Voyager epoch. The asymmetry shows considerable variation with wavelength in the near‐infrared : in the 889nm methane band the asymmetry (which formerly resembled a ‘smile’) has reversed in the last 3 years, while at 953nm the southern limb is still bright. A south polar collar, dark at ultraviolet wavelengths, is apparent some five years after equinox.
First atmospheric profile measurements of UV/visible O4 absorption band intensities: Implications for the spectroscopy, and the formation enthalpy of the O2‐O2 dimer
Authors: Pfeilsticker K. , Bösch H. , Camy‐Peyret C. , Fitzenberger R. , Harder H. , Osterkamp H.
Published: February 2004

The first atmospheric profiles of the ultraviolet/visible (UV/vis) absorption bands of the collision complex O2‐O2, or O4 in brief, are reported. The O4 absorption profiles are inferred from direct Sun spectra observed from the LPMA/DOAS (Laboratoire Physique Moléculaire et Application/Differential Optical Absorption Spectroscopy) balloon gondola. Seven O4 absorption bands ‐ centered at ∼360.7, 380.2, 446.7, 477.1, 532.2, 577.2, and 630.0 nm ‐ are investigated for atmospheric pressures (p) ranging from ∼500 hPa to ∼40 hPa and temperatures (T) ranging from 203 K to 250 K. For the encountered atmospheric conditions, it is found that, (a) the band shapes do not change with T and p and (b) the peak collision pair absorption intensities (αi) concurrently increase with decreasing T (by about 11 % over a ΔT=50 K). That result is in agreement with previous laboratory O4 studies mostly conducted at high O2 partial pressures (up to several hundred bars). Furthermore, by reasonably assuming that the O4 absorption cross sections are T‐independent, the inferred T‐dependence of αi(T) suggests a thermally averaged enthalpy change < ΔH > = −(1207±83) J/Mol involved in the formation of O4. Our inferred ΔH is in reasonable agreement with the orientation and spin averaged O4 well depth De(O4) (= ‐(1130±80) J/Mol) measured in a recent O2‐O2 collision experiment, when accounting for the rovibrational energy change during O4 formation (186 J/Mol).
Climate logging with a new rapid optical technique at Siple Dome
Authors: Bay Ryan C. , Price P. Buford , Clow Gary D. , Gow A. J.
Published: February 2004

The dust logger design is based on a decade of experience in the use of light sources to measure optical properties of deep Antarctic ice. Light is emitted at the top of the instrument by side‐directed LEDs, scattered or absorbed by dust in the ice surrounding the borehole, and collected in a downhole‐pointing photomultiplier tube (PMT) a meter below. With this method the ice is sampled at ambient pressure in a much larger volume than is the case in a core study, and the entire length can be logged in one day. In ice in which scattering is dominated by bubbles, the absorption from dust impurities is perceived as a drop in signal, whereas in bubble‐free ice the scattering from dust increases the light collected. We report on results obtained in Siple Dome Hole A in December 2000. The instrument measured increases in dust concentration extending over many meters during glacial maxima, as well as narrow spikes due to ∼ 1 cm thick ash and dust bands of volcanic origin. Monte Carlo simulation is employed to clarify data analysis and predict the capabilities of future designs.
Equable paleogene climates: The result of a stable, positive Arctic Oscillation?
Authors: Sewall Jacob O. , Sloan Lisa Cirbus
Published: February 2004

The Early Paleogene is recognized as a particularly warm interval in Earth's history. Paleogene proxy climate indicators suggest warm polar and mid‐latitude continental interior temperatures, and a reduced latitudinal temperature gradient. Most researchers believe that Early Paleogene climate was driven by forcing fields that act globally (e.g. greenhouse gases). However, modeling work based on this hypothesis has failed to reproduce Paleogene climate as indicated by proxy data. Quite possibly, an ameliorating influence acting directly at the poles would more effectively warm high latitudes, provide an additional heat source to mid‐latitude continental interiors, and reduce the latitudinal temperature gradient. Here we present a hypothesis based on the positive phase of the modern Arctic Oscillation; in short, that prolonged low pressure over the Arctic Ocean would have warmed mid‐latitude continental interiors and drastically reduced the Arctic Ocean's ice cover, thus producing conditions consistent with proxy climate indicators for the Paleogene greenhouse interval.
methyl bromide loss rate constants in the north Pacific Ocean
Authors: Tokarczyk Ryszard , Goodwin Kelly D. , Saltzman Eric S.
Published: February 2004

The degradation rate constant of CH3Br in the North Pacific Ocean was measured in surface seawater between September and October 1999, using a stable isotope (13CH3Br) incubation technique. Total degradation rate constants ranged from 0.02 to 0.43 d−1, decreasing in colder waters as a result of the temperature‐dependence of chemical losses. Biological rate constants ranged from 0.01 to 0.20 d−1. In subtropical waters (13‐20°N), biological loss rate constants were small compared to chemical loss rate constants. North of Hawaii, biological processes played an increasingly significant role in CH3Br degradation. In subpolar waters (40‐58°N), biological losses dominated the removal of methyl bromide. Comparison of the measured loss rate constants with surface water CH3Br concentrations suggest that the CH3Br production rate is higher in warm, low latitude waters than in cold subpolar waters at this time of year. Diel studies revealed a midday maximum in biological degradation of methyl bromide.
Temperature trends in regions affected by increasing aridity/humidity
Authors: Jones Philip D. , Reid Phillip A.
Published: February 2004

A paper in 1991 claimed that regions affected by desertification experience warming trends relative to neighbouring areas. To assess this, an index of aridity/humidity based on the ratio of annual precipitation to annual potential evapotranspiration totals (P/PET) is developed. This index is used to define regions experiencing increases (and those where the increase is statistically significant) in aridity and humidity. We also consider regions always arid (average values of P/PET <0.5) and always humid (P/PET >2.0). Trends of average annual and summer surface air temperature are then calculated for regions in the various aridity/humidity categories and compared to most of the rest of the world's land areas equatorward of 60°. The results indicate that most of the differences in trends between categories are not statistically significant.
Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes
Authors: Layer Paul W. , Gardner James E.
Published: February 2004

Excess argon in plagioclase crystals from young (<4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isotopes in understanding magmatic processes. 40Ar/39Ar step‐heating identifies two distinct argon reservoirs in the plagioclase; a high Cl/K, low Ca/K reservoir with atmospheric 40Ar/36Ar and a low Cl/K, higher Ca/K reservoir with variable 40Ar/36Ar. The first is probably glass, whereas the second is “true” plagioclase. Felsic dacite plagioclases have little or no excess argon, but those from more mafic layers have significantly higher 40Ar/36Ar, indicating a non‐atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent with either xenocrystic or restitic origin for plagioclase. The magma chamber exhibits long‐term open‐system degassing behavior punctuated by short‐term fluctuations from influx of high 40Ar/36Ar basalt. The relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase.
Seasonal variability of the backscattering coefficient in the Mediterranean Sea based on satellite SeaWiFS imagery
Authors: Loisel H. , Bosc E. , Stramski D. , Oubelkheir K. , Deschamps P.‐Y.
Published: February 2004

The annual progression of the particulate backscattering coefficient, bbp, within the whole Mediterranean sea is examined from satellite remote sensing and inverse modeling for the first time. This study reveals that bbp is roughly distributed uniformly over the Mediterranean that differs from the well known east‐west gradient in terms of chlorophyll concentration, Chl. In contrast to Chl, bbp is also weakly variable along the seasons, and presents different extrema. These observations are attributed to the presence of high concentrations of submicrometer particles such as detrital particles, heterotrophic bacteria and also possibly mineral particles from aeolian inputs. The interpretation of bbp in terms of particulate organic carbon concentration, POC, is discussed, and a relationship between bbp and POC is developed. Application of our algorithm to SeaWiFS data indicates that the satellite estimates are in good agreement with surface POC measurements.
A long‐term record of carbonyl sulfide (COS) in two hemispheres from firn air measurements
Authors: Sturges William T. , Penkett Stuart A. , Barnola Jean‐Marc , Chappellaz Jerome , Atlas Elliot , Stroud Verity
Published: February 2004

Carbonyl sulfide (COS) was measured in polar firn air from one Arctic and two Antarctic locations. The air samples represent atmospheric composition from the early to mid‐20th century up to the present day. This provides the longest record to date of atmospheric COS. Southern Hemispheric (SH) concentrations appear to have been almost constant at 482±13 ppt over this period, apart from a slight rise in the earliest part of the record. Northern Hemispheric (NH) concentrations also showed relatively little variation with a mean of 525±17 ppt. Over the last ten years, however, NH concentrations appear to have declined by about 8±5%. Such a decline might be due to decreased carbon disulfide (CS2) emissions by the viscose‐rayon industry. The absence of any large trend in COS concentrations over the last fifty or more years argues against COS being the origin of reported increases in stratospheric sulfate aerosol.
Solar irradiance and climate forcing in the near future
Authors: Lean J. L.
Published: February 2004

Annual levels of the Sun's irradiance are estimated until 2018 using parameterizations of the 10.7 cm radio flux. Peak irradiance during solar cycle 24 occurs in 2010, with levels comparable to or slightly lower than prior maxima in 2000, 1989 and 1981. Minima occur in 2006 and 2016. Future irradiance trends underlying the 11‐year cycle, which are assumed not to exceed those in historical reconstructions during the past 350 years, are estimated to be less than ±0.4 Wm−2 per decade for total irradiance, ±0.01 Wm−2 per decade for UV radiation at 295–310 nm and ±0.04 Wm−2 per decade for UV radiation at 200–295 nm. When activity cycles and longer‐term trend scenarios are combined, total solar irradiance forcing of climate between cycle minima in 1996 and 2016 is in the range ±0.1 Wm−2. For comparison, the forecast net anthropogenic climate forcing over this 22‐year period is in the range 0.5 to 0.9 Wm−2.
Miocene to Holocene exhumation of metamorphic crustal wedges in the NW Himalaya: Evidence for tectonic extrusion coupled to fluvial erosion
Authors: Vannay Jean‐Claude , Grasemann Bernhard , Rahn Meinert , Frank Wolfgang , Carter Andrew , Baudraz Vincent , Cosca Mike
Published: February 2004

The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high‐grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland‐directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para‐ and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present‐day regional seismicity, and by hydrothermal circulation linked to elevated near‐surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans‐Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal‐scale tectono‐thermal reworking and efficient erosion along major river systems.
Dissolution of atmospheric iron in seawater
Authors: Bonnet Sophie , Guieu Cécile
Published: February 2004

In vitro dissolution experiments were conducted in seawater in order to quantify the partitioning between dissolved and particulate phases of iron associated to Saharan dust and urban particles. The percentage of iron released was very low (0.05 to 2.2%) depending on the particulate load, the aerosol source and the contact time. This percentage decreased with the amount of particles introduced following a power law, indicating that the process could be modeled. Indeed, these dissolution processes were a function of the lability of iron in relation to the particle source, the particle size and adsorption processes. According to these experiments, the dissolved iron induced in a 10 m mixed layer by Saharan events of various magnitudes ranges from 0.07 to 1 nM. Since these concentrations are in agreement with the natural iron requirements for the phytoplankton, such inputs can profoundly influence the primary production, especially in oligotrophic conditions.
Effect of snow cover on threshold wind velocity of dust outbreak
Authors: Kurosaki Yasunori , Mikami Masao
Published: February 2004

This study proposes parameters to measure the effect of snow cover on the threshold wind velocity of dust outbreak for dust models. We used statistics of snow cover, wind velocity and dust outbreaks in East Asia for the months of March and April for the years from 1988 to 2003. Dust events frequently occur in that region in those months, although snow and melting snow cover a broad area, making it difficult to know the distribution of dust from East Asia. Our equation, presented in this paper, shows the degree to which snow cover affects dust outbreaks. To validate this equation, we checked the correlation between frequency of strong winds and dust outbreaks. Our results showed that this correlation is improved when the effect of snow cover is considered. We discuss the effects of soil wetness, vegetation, and errors due to the subgrid heterogeneity of snow coverage as well.
Radio and multiwavelength evidence of coronal loop eruption in a flare–coronal mass ejection event on 15 April 1998
Authors: Huang Guang‐Li
Published: February 2004

A typical flare–coronal mass ejection (CME) event is studied at the initial time and position of the flare and CME. The photospheric magnetic configuration consists of two main polarities (dipolar structure) with some emerged new polarities, similar to a quadrupolar structure, during the event. The coronal loops in soft X‐ray and EUV images show the signature of the loop interaction, i.e., the storage of the magnetic energy. The radio bursts at 1–2 GHz were observed with fast frequency drifts with bidirectional drift pairs, which may provide a direct signature of upward and downward particle acceleration from the reconnection site, followed by the bulk energy release of the flare. After that, the soft X‐ray and EUV loops were suddenly opened and closed again in 10 min. The open time is around the maximum phase of the flare as well as the start time of the associated CME. Meanwhile, the polarization sense of the microwave bursts suddenly reversed at 2.6–3.8 GHz, which might have been caused by the fast variation of coronal loops or the magnetic field. The high‐time‐resolution (8 ms) data show the microwave bursts at 2.6–3.8 GHz, with slow frequency drifts and zebra structures in the initial phase of the CMEs or shock formation. Hence it is suggested that flares and CMEs may be triggered simultaneously by some MHD instabilities or by reconnection in a favorable magnetic configuration to release the free magnetic energy in two different ways.
Proterozoic diabase sills of northern Ontario: Magnetic properties and history
Authors: Middleton R. S. , Borradaile G. J. , Baker D. , Lucas K.
Published: February 2004

Magnetic fabrics show that a series of massive diabase sills (“Logan sills”) possess flow fabrics away from the midcontinental Proterozoic rift system, near its triple junction on the north shore of Lake Superior. Their intense remanences are commonly antiparallel to the geomagnetic field, requiring some knowledge of natural remanent magnetization in order to interpret of ground magnetic surveys. Thermal demagnetization, low‐temperature demagnetization, and the combined techniques yield differently oriented characteristic remanence vectors in some of these sills. Although the commonly reported “Keweenawan” reversed paleofield direction is verified by most A components, the orientation of vectors within sites and through the thickness of sills is large and may be attributed to reversals, excursions, and secular variation. B components are mostly down, but their orientation distribution is poorly concentrated. Low‐temperature demagnetization, prior to thermal demagnetization, tends to improve the consistency of vectors defined in subsequent thermal demagnetization. Moreover, in some cases, three to five cycles of low‐temperature demagnetization alone produces the same result as the more time‐consuming thermal demagnetization. Some within‐site scatter of the A components is large, and some anomalous specimen directions and alternate polarities may be associated with platinum group mineralization. Whereas the grand mean direction for all sites lies on the available apparent polar wander path between 1100 and 1300 Ma, the mean paleopole locations differ at the 95% level for different demagnetization treatments of specimens from the same sites.
A compaction correction for the paleomagnetism of the Nanaimo Group sedimentary rocks: Implications for the Baja British Columbia hypothesis
Authors: Kim BangYeon , Kodama Kenneth P.
Published: February 2004

A paleomagnetic and magnetic anisotropy study of the Late Cretaceous Northumberland formation on Hornby Island, British Columbia, was conducted to determine if burial compaction could have caused its anomalously shallow inclinations. The shallow Nanaimo Group inclinations have been used to support the Baja British Columbia (Baja BC) model of continental dynamics in which superterranes were transported thousands of kilometers northward along the active North American continental margin in the Cretaceous. A mean of the site means from the Northumberland formation is D = 5.8°, I = 50.9°, α95 = 7.9° (N = 8). Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence (AAR) were measured to identify and correct for any inclination shallowing caused by depositional/compactional processes. Both concretion and fine‐grained rock samples had typical depositional/compactional AMS and AAR fabrics with bedding perpendicular minimum principal axes. A revised correction equation was used to account for the effects of either triaxial magnetic fabrics or small angular deviations of principal axes from the bedding plane. The average inclination was increased by 9.6° after the compaction correction (D = 6.1°, I = 60.5°, α95 = 7.1°, N = 8), and the mean inclination of concretion and fine‐grained rock sites was significantly steeper than Ward et al.'s [1997] result (I = 42.5°) placing Hornby Island and the Insular superterrane at a paleolatitude of 41°N in the Late Cretaceous. This paleolatitude indicates that Baja BC did translate northward since the Late Cretaceous by about 1600 km but not by the 3500 km previously indicated.
Modeling the circulation of Western Bank on the Scotian Shelf through sequential application of a variational algorithm and a nonlinear model
Authors: Panteleev G. G. , de Young B. , Luneva M. , Reiss C. , Semenov E. V.
Published: February 2004

A new approach is presented for the modeling of the quasi‐stationary circulation through the sequential application of a variational algorithm and a nonlinear diagnostic model. The model loop begins with data assimilation of temperature, salinity, surface elevation, and velocity data in a simplified geostrophic model in which transport and continuity equations are treated as weak constraints. The temperature/salinity fields and balanced open boundary conditions points are then used as input to a nonlinear primitive equation model, which employs a turbulent closure scheme. The nonlinear model is then run to produce a diagnostic flow field. A radiation open boundary condition is applied at the outflow points of the open boundaries. These two steps are organized into an iteration cycle by using output from the nonlinear model as input to the variational model. The proposed approach combines the advantages of variational data assimilation in simplified models with a complicated fully nonlinear primitive equation model. We apply the approach to Western Bank on the Scotian Shelf. Comparisons with observed current from Western Bank, in September and October 1998, show that the sequential application of the variational approach and the fully nonlinear model allow determination of the quasi‐stationary circulation whose agreement with the observations is ≈10–30% better than circulation determined from the variational or the nonlinear model alone. Our calculations of the cross‐shelf transport across Western Bank show that it varies from 0.20 Sv to 0.35 Sv over a 2‐week period. The combined models also allow us to determine the character of the circulation over the Bank, the role of wind forcing and the implications for resident biological populations.
Relationship between vegetation coverage and spring dust storms over northern China
Authors: Zou Xukai K. , Zhai Panmao M.
Published: February 2004

On the basis of normalized difference vegetation index (NDVI) data from 1982 to 2001 and dust storm observations in China the relationship between vegetation and spring dust storms over northern China is discussed. The results show that poor vegetation coverage in northern China is one important factor for the frequent occurrence of spring dust storms. In addition, vegetation cover plays an important role in interannual variations of dust storms. In general, a negative correlation is noted between vegetation coverage and occurrence of dust storms in northern China for spring during the period 1982–2001. The correlation coefficient between vegetation coverage and areas affected by dust storms is −0.59, which is statistically meaningful at 99% confidence level. The sharp decrease of spring vegetation coverage in recent years is one of the major contributors to frequent spring dust storms over northern China specifically during 2000 and 2001. A negative correlation is especially significant in the eastern part of northern China, mainly in central and eastern Inner Mongolia. When vegetation decreases (increases), the occurrence of dust storms increases (decreases). Furthermore, statistics show that abundant vegetation in previous seasons could help reduce dust storms in the coming spring. The effect of prior summer vegetation on the variation of spring dust storms is particularly evident in the central and eastern part of northern China. Because of the presence of little to no vegetation in the desert areas of northwest China the variation in occurrence of spring dust storms seems unrelated to the vegetation.
Asteroid 5535 Annefrank size, shape, and orientation: Stardust first results
Authors: Duxbury Thomas C. , Newburn Ray L. , Acton Charles H. , Carranza Eric , McElrath Timothy P. , Ryan Robert E. , Synnott Stephen P. , You T. Han , Brownlee Donald E. , Cheuvront Allan R. , Adams William R. , Toro‐Allen Scott L. , Freund Sandra , Gilliland Kevin V. , Irish Kelly J. , Love Charles R. , McAllister J. Greg , Mumaw Susan J. , Oliver Thomas H. , Perkins David E.
Published: February 2004

The NASA Discovery Stardust spacecraft flew by the main belt asteroid 5535 Annefrank at a distance of 3100 km and a speed of 7.4 km/s in November 2002 to test the encounter sequence developed for its primary science target, the comet 81P/Wild 2. During this testing, over 70 images of Annefrank were obtained, taken over a phase angle range from 40 to 140 deg. This viewing showed that Annefrank was at least 6.6 × 5.0 × 3.4 km in size (diameters), with its shortest dimension normal to its orbit plane. Annefrank is highly angular, with flat appearing surfaces, possibly planes formed when it was fractured off of a larger parent body. For the limited part of the surface seen, Annefrank resembles a triangular prism for the main body, with smaller, rounder bodies, possibly accreted through contact.
Monitoring an underground steam injection process using electrical resistance tomography
Authors: Ramirez A. , Daily W. , LaBrecque D. , Owen E. , Chesnut D.
Published: February 2004

We used electrical resistance tomography (ERT) to map the subsurface distribution of a steam flood as a function of time as part of a prototype environmental restoration process performed by the Dynamic Underground Stripping Project. We evaluated the capability of ERT to monitor changes in the soil resistivity during the steam injection process using a dipole‐dipole measurement technique to measure the bulk electrical resistivity distribution in the soil mass. The injected steam caused changes in the soil's resistivity because the steam displaced some of the native pore water, increased the pore water and soil temperatures and changed the ionic content of the pore water. We could detect the effects of steam invasion by mapping changes in the soil resistivity as a function of space and time. The ERT tomographs are compared with induction well logs, formation temperature logs and lithologic logs. These comparisons suggest that the ERT tomographs mapped the formation regions invaded by the steam flood. The data also suggest that steam invasion was limited in vertical extent to a gravel horizon at depth of approximately 43 m. The tomographs show that with time, the steam invasion zone extended laterally to all areas monitored by the ERT technique.
A fully coupled model for water flow and airflow in deformable porous media
Authors: Schrefler B. A. , Xiaoyong Zhan
Published: February 2004

A fully coupled model is developed to simulate the slow transient phenomena (consolidation) involving flow of water and air in deforming porous media. The model is of the Biot type and incorporates the capillary pressure relationship. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The temporal discretization error, iteration error and stability error are evaluated. The model is validated with respect to a documented experiment on semisaturated soil behavior. Other examples involving an air storage problem in an aquifer and a flexible footing resting on a semisaturated soil are also presented.
The dipole flow test: A new single‐borehole test for aquifer characterization
Authors: Kabala Z. J.
Published: February 2004

A new single‐borehole measurement technique for confined aquifers, the dipole flow test, yields the vertical distributions of the horizontal hydraulic conductivity, the vertical hydraulic conductivity, and the specific storativity when applied to different borehole intervals. The test utilizes straddle packers to isolate two chambers in the borehole, pressure transducers to monitor drawdown in them, and a small pump to create a dipole flow pattern in the aquifer by pumping water at a constant rate from the aquifer into one chamber, transferring it within the well to the next chamber, and finally injecting it back to the aquifer. A mathematical model describing the drawdown in each chamber is derived for the transient as well as the steady state cases. The aquifer parameters may be estimated from data produced by the dipole flow test alone or in conjunction with conventional pumping tests. The dipole flow regime reaches a steady state relatively quickly, especially in well permeable aquifers. A robust computational methodology for estimating the aquifer parameters, suitable for automatization, is based on the Newton‐Raphson algorithm applied to a system of up to three nonlinear equations, each describing the well drawdown at a different judiciously chosen time. Due to the relatively small drawdown it invokes, the dipole flow test may be applicable to unconfined aquifers as well.
Transport of reacting solute in a one‐dimensional, chemically heterogeneous porous medium
Authors: Bosma Willem Jan P. , van der Zee Sjoerd E. A. T. M.
Published: February 2004

Reacting, nonlinearly adsorbing solute transport in chemically heterogeneous soils is studied. Assuming adsorption is adequately described with the Freundlich equation, random variation of the adsorption coefficient is assumed to describe the heterogeneity. In a homogeneous case, traveling wave fronts develop, characterized by a constant velocity and a constant front shape. Using the method of moments, an analytical expression is derived to describe the constant variance of the traveling wave front. Deviations from the analytical variance and velocity, both calculated with an average adsorption coefficient, show that column scale heterogeneity has significant effects on front spreading and front movement. Expected values of front velocity and variance are computed as averages of values of 600 randomly generated columns. The nonlinear process causes small deviations from the case with average parameters. The ensemble average concentration front, representing an average front for the flow domain, shows that three mechanisms are responsible for the front spreading. At early displacement times the front spreading is caused by the thickness of the individual traveling waves. Subsequently, the effect of the internal variation of the adsorption coefficient (column scale heterogeneity) increases, whereas at large displacement times the front spreading is dominated by the different retardation coefficients of the different columns. The latter effect causes the variance to increase in proportion to t2. An analytical approximation is derived for the ensemble average front, ignoring column scale heterogeneity.
Exact Scheffé‐type confidence intervals for output from groundwater flow models: 1. Use of hydrogeologic information
Authors: Cooley Richard L.
Published: February 2004

A new method is developed to efficiently compute exact Scheffé‐type confidence intervals for output (or other function of parameters) g(β) derived from a groundwater flow model. The method is general in that parameter uncertainty can be specified by any statistical distribution having a log probability density function (log pdf) that can be expanded in a Taylor series. However, for this study parameter uncertainty is specified by a statistical multivariate beta distribution that incorporates hydrogeologic information in the form of the investigator's best estimates of parameters and a grouping of random variables representing possible parameter values so that each group is defined by maximum and minimum bounds and an ordering according to increasing value. The new method forms the confidence intervals from maximum and minimum limits of g(β) on a contour of a linear combination of (1) the quadratic form for the parameters used by Cooley and Vecchia (1987) and (2) the log pdf for the multivariate beta distribution. Three example problems are used to compare characteristics of the confidence intervals for hydraulic head obtained using different weights for the linear combination. Different weights generally produced similar confidence intervals, whereas the method of Cooley and Vecchia (1987) often produced much larger confidence intervals.
A model for a moving interface in a layered coastal aquifer
Authors: Pistiner Arieh , Shapiro Michael
Published: February 2004

The framework of the present study is the development of a new approach to describe the displacement of fresh water by salt water in a layered coastal aquifer. The aquifer is composed of horizontal layers possessing different hydraulic conductivities, which are also different in the saltwater and the freshwater zones. A one‐dimensional analysis is performed, describing the displacement of the saltwater‐freshwater interface under the influence of freshwater flow toward the sea. Several analytical solutions are found revealing the effects of various parameters on the interface displacement process. On the basis of these solutions, several type curves for different freshwater saltwater hydraulic conductivity are drawn. The model is calibrated by matching a type curve with field data taken from an observation well and is further used to predict the evolution of the interface shape and position.
Simultaneous solution algorithm for channel network modeling
Authors: Choi G. W. , Molinas A.
Published: February 2004

This paper presents a simultaneous solution algorithm for one‐dimensional unsteady flow routing through channel networks. This direct solution algorithm is based on the double‐sweep method and utilizes separate recursion equations for continuity, momentum and energy equations for each of the individual components of channel networks. Through separate recursion equations for each of the components, the new algorithm converts the channel network problem into a single‐channel problem. The new algorithm is utilized in conjunction with a linearized unsteady flow model using full dynamic flow equations. The required computer storage for the coefficient matrix of the whole system is reduced significantly from the 2N × 2N matrix to a 2N × 4 matrix, where N is the number of cross sections used in the computation of flow variables in the channel network. The algorithm presented in this paper provides an efficient and accurate modeling of unsteady flow events through channel networks.
Stochastic modeling of macrodispersion for solute transport in a heterogeneous unsaturated porous formation
Authors: Russo David
Published: February 2004

A framework for the prediction of solute spread in partially saturated heterogeneous porous formation is presented by combining a general Lagrangian formulation, relating the spatial moments of the solute body to the velocity field, with the stochastic theory of Yeh et al. (1985a, b) for steady unsaturated flow, relating the statistical moments of the velocity field to properties of the heterogenous formation. The relevant formation properties are log saturated conductivity, log Ks, and the pore size distribution parameter, α, which, in turn, are viewed as independent, random spatial functions. First‐order approximations of the longitudinal and transverse components of the velocity covariance function are derived for unidirectional, vertical mean flow in partially saturated, heterogeneous porous formations of three‐dimensional structure. Neglecting pore scale dispersion, and by using basic, kinematical relationships to relate the displacement to the velocity field, the effect of mean water saturation, and of statistical parameters of porous formation properties on longitudingal and transverse components of the time‐dependent displacement covariance tensor X, is evaluated. Results of the analyses show that solute spread increases as water saturation decreases. In the case where the correlation scale of α was small, as compared with that of log Ks, however, the longitudinal component of X is essentially robust to water saturation, suggesting that in this case, vadose zone transport might be amenable to an analysis similar to that for saturated groundwater transport.
Comparison of recreation use values among alternative reservoir water level management scenarios
Authors: Cordell H. Ken , Bergstrom John C.
Published: February 2004

Throughout the United States, reservoirs are managed for multiple uses, including hydropower, stream flow regulation, flood control, and recreation. Water level drawdowns for hydropower, stream flow regulation, and flood control often reduce the suitability of reservoirs for water‐based recreation. The gain in aggregate economic use value of outdoor recreation under three alternative water level management scenarios was measured for four reservoirs in western North Carolina as part of an interagency policy analysis. Use values were estimated using a contingent valuation survey and expert panel data. The basic question addressed by this study was whether the value recreational users place on higher water levels held longer into the summer and fall is significantly greater than the value of using these reservoirs as they were managed at the time of this study. Maintaining high water levels for longer periods during the summer and fall was found to result in considerable gains in estimated recreational benefits. While not a primary objective of this study, having these estimates provided us an opportunity to compare increased recreational benefits with the value the Tennessee Valley Authority estimated for the reduced production of electricity that would result if the lakes were managed to hold reservoir levels higher, longer into the year.
The influence of nonuniform transmissivity and storativity on drawdown
Authors: Oliver Dean S.
Published: February 2004

Although the existence of spatial variability in hydrogeological parameters is widely recognized, most analyses of pumping test data are performed under the assumption that the aquifer is adequately characterized by a few parameters, for example, constant transmissivity, constant storativity, distance to a barrier, and well skin. Even if the possibility of variability is recognized, pumping tests are often thought to investigate the region between a pumping well and an observation well. Recently, however, investigations of the sensitivity of pumping test data to radially symmetric nonuniform flow properties have shown that the drawdown at an observation well is insensitive to transmissivity and storativity of the region between the pumping well and the observation well. In this paper I use a perturbation approach to derive the Fréchet derivatives and kernels for the effect of two‐dimensional areal variations in transmissivity and storativity on drawdown at an observation well. Fréchet derivatives and kernels are related to sensitivity coefficients, but their definition is independent of zonation. Examination of the Fréchet kernels at large distances from the pumping and observation wells shows that the area of investigation of a pumping test is bounded by an ellipse that encloses both wells. This study also shows that observation well drawdown is relatively sensitive to near‐well transmissivity variation, especially if the nonuniformity is not radially symmetric about the well.
Prediction of steady state flow in nonuniform geologic media by conditional moments: Exact nonlocal formalism, effective conductivities, and weak approximation
Authors: Neuman Shlomo P. , Orr Shlomo
Published: February 2004

We consider the effect of measuring randomly varying local hydraulic conductivities K(x) on one's ability to predict steady state flow within a bounded domain, driven by random source and boundary functions. More precisely, we consider the prediction of local hydraulic head h(x) and Darcy flux q(x) by means of their unbiased ensemble moments 〈h(x)〉κ and 〈q(x)〉κ conditioned on measurements of K(x). These predictors satisfy a deterministic flow equation in which 〈q(x)〉κ = −κ(x)∇〈h(x)〉κ + rκ(x), where κ(x) is a relatively smooth unbiased estimate of K(x) and rκ(x) is a “residual flux.” We derive a compact integral expression for rκ(x) which is rigorously valid for a broad class of K(x) fields, including fractals. It demonstrates that 〈q(x)〉κ is nonlocal and non‐Darcian so that an effective hydraulic conductivity does not generally exist. We show analytically that under uniform mean flow the effective conductivity may be a scalar, a symmetric or a nonsymmetric tensor, or a set of directional scalars which do not form a tensor. We demonstrate numerically that in two‐dimensional mean radial flow it may increase from the harmonic mean of K(x) near interior and boundary sources to the geometric mean far from such sources. For cases where rκ(x) can neither be expressed nor approximated by a local expression, we propose a weak (integral) approximation (closure) which appears to work well in media with pronounced heterogeneity and improves as the quantity and quality of K(x) measurements increase. The nonlocal deterministic flow equation can be solved numerically by standard methods; our theory shows clearly how the scale of grid discretization should relate to the scale, quantity, and quality of available data. After providing explicit approximations for the second moments of head and flux prediction errors, we conclude by discussing practical methods to compute κ(x) from noisy measurements of K(x) and to calculate required second moments of the associated estimation errors when K(x) is lognormal.
Effective water table depth to describe initial conditions prior to storm rainfall in humid regions
Authors: Troch Peter A. , De Troch François P. , Brutsaert Wilfried
Published: February 2004

A physically meaningful technique to determine the effective depth to the water table, as a measure of the initial storage capacity of a basin is developed. The estimation of the initial storage capacity prior to a given flood event is essential to obtain useful results from storm runoff prediction models based on saturation excess overland flow. It is shown how this effective depth to the water table can be related to streamflow measurements at the outlet of the basin. The analysis is based on Boussinesq's standard hydraulic groundwater theory. The main feature of the present formulation is that it allows the estimation of catchment‐scale parameters, namely the aquifer hydraulic conductivity and the average depth to the impervious layer. The estimation of these parameters is based on a drought flow analysis which is consistent with the hydraulic groundwater theory used to develop the described technique. This hydraulic theory is found to be applicable for a catchment under humid temperate climatic conditions, namely the Zwalm catchment situated in East‐Flanders, Belgium. The results of the proposed analysis are used to estimate the initial conditions in a partial area runoff generation model. It is shown that accurate estimates of total runoff volume are obtained without further calibration of the model.
Coupled withdrawal and sampling designs for groundwater supply models
Authors: Andricevic Roko
Published: February 2004

A coupled formulation of withdrawal and sampling designs for ground water supply models is presented. The withdrawal design is described as a discrete time optimal control problem and solved using a closed loop stochastic control (CLSC) method. Two main features of the CLSC method are the anticipation of the future observation program and decomposition of the objective function into the deterministic and stochastic part. The former characteristic indicates the necessity for coupling the withdrawal and sampling designs, while the later feature allows a decision maker to estimate the uncertainty in the objective function if certain withdrawal rates are applied. The sampling network is sequentially developed, with the design criterion defined as a sensitivity of the objective function stochastic part to the uncertainty in the hydraulic head distribution multiplied with the variance of the hydraulic head. The concept of minimizing the stochastic part of the objective function with respect to the hydraulic head uncertainty provides a convenient way to couple the withdrawal design objectives with the monitoring network development weighted with the magnitude of the prediction error in the hydraulic head distribution. The Bayesian concept of measurement conditioning is employed to sequentially adjust the withdrawal rates and sampling network development by accounting for the information conveyed in field observations. Between the sampling sessions the uncertainty in the hydraulic head prediction is evaluated using the first‐ and second‐moment analysis applied to the discretized ground water flow model. The uncertainty in the hydraulic head prediction is assumed to come from the natural uncertainty in the hydraulic conductivity and uncertainty in the boundary condition values and other external fluxes (e.g., leakages and recharge). A hypothetical example is included to demonstrate the application procedure and to illustrate the main features of the proposed coupled formulation.
Modeling infiltration for multistorm runoff events
Authors: Smith Roger E. , Corradini Corrado , Melone Florisa
Published: February 2004

We present a relatively simple analytical/conceptual model for rainfall infiltration during complex storms. It is an approximate but physically based model which can treat intervals of either no rain, low rain, or evaporation. The infiltration model is based on the very general three‐parameter analytic model of Parlange et al. (1982), extended to treat soils with very high initial water content. The redistribution model is based on profile extension with shape similarity. A wide range of soil types can be simulated. The model is tested by comparison with numerical solutions of Richards's equation carried out for a variety of events upon four selected soils. The model simulates the solution to Richards's equation quite accurately, provided basic soil retention relations are parametrically represented. It simulates redistribution particularly well for redistribution intervals up to 20 hours. The model usefulness in comparison with the common and simple approach which disregards soil water redistribution is also shown.