bluuebunny/crossref_metadata_2025 huggingface dataset:
Tropospheric phase calibration in millimeter interferometry
Authors: Carilli C. L. , Holdaway M. A.
Published: September 2002

We review millimeter interferometric phase variations caused by variations in the precipitable water vapor content of the troposphere, and we discuss techniques proposed to correct for these variations. We present observations with the Very Large Array (VLA) at 22 and 43 GHz designed to test these techniques. We find that both the fast switching and paired array calibration techniques are effective at reducing tropospheric phase noise for radio interferometers. In both cases, the residual rms phase fluctuations after correction are independent of baseline length for b > beff. These techniques allow for diffraction‐limited imaging of faint sources on arbitrarily long baselines at millimeter wavelengths. We consider the technique of tropospheric phase correction using a measurement of the precipitable water vapor content of the troposphere via a radiometric measurement of the brightness temperature of the atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz for the millimeter array (MMA) and to 120 mK for the VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz at the MMA, assuming that the astronomical receivers are used for radiometry. This increases to 2000 for an uncooled system. The stability requirement is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radiometric phase corrections also requires knowledge of the tropospheric parameters and models to an accuracy of a few percent. It may be possible to perform an “empirically calibrated” radiometric phase correction, in which the relationship between fluctuations in brightness temperature differences and fluctuations in interferometric phases is calibrated by observing a strong celestial calibrator at regular intervals. A number of questions remain concerning this technique, including the following: (1) Over what timescale and distance will this technique allow for radiometric phase corrections when switching between the source and the calibrator? (2) How often will calibration of the TrmsB – Ørms relationship be required?
Shielding by a house from the electric field of a power line
Authors: King Ronold W. P.
Published: September 2002

The claim that “a typical house shields about 90% of electric fields from outside” is examined when the house is located near a high‐voltage 50‐ to 60‐Hz transmission line. Calculated electric fields near such a line are used. In order to provide an accurate analysis, the usual house shape is approximated first by a hemisphere, then by a cylinder, and finally by a box‐like shape on the conducting earth. It is shown that when actual materials and wall thicknesses are used, the electric field in the house is 99–100% of the incident field, so that there is no shielding. The electric field scattered into the house by an iron plumbing vent that extends from the ground to above the roof is found to be significant near the pipe. Scattered fields from plumbing and heating pipes and electric wiring are significant only within a meter or less. It is concluded that the superposition of incident and scattered fields from pipes and wires in the walls yields widely varying fields at different points but that the average exposure of persons living inside the house is not greatly reduced below exposure to the unmodified field of the power line.
Estimation of the average contrast of a buried object
Authors: Budko Neil V. , van den Berg Peter M.
Published: September 2002

The problem of estimation of the average contrast of a buried object is formulated as a nonlinear inverse scattering problem. An effective scattering model of reduced dimensionality is applied in order to make the inversion scheme numerically fast and well‐posed. It is shown that a good estimate of the object's average constitution is obtained if L1‐type metric is employed and the two‐media background is replaced with the average homogeneous one. The effective scattering model is used in combination with Synthetic aperture radar (SAR) imaging, adding significant value to conventional SAR images.
Blind, high‐resolution, space‐time separation of multipaths in an ionospheric propagation
Authors: Chenu‐Tournier M. , Larzabal P. , Barbot J. P. , Grouffaud J. , Ferreol A.
Published: September 2002

The ionospheric radio electrical transmissions have multiple paths due to the inhomogeneity of the propagation medium, that is, the ionospheric layers. Tactical applications such as radiolocation and radiocommunications systems need blind, high‐resolution identification of multipath channels. This work concerns the separation of the ionospheric paths and is based on recent work done on blind deconvolution which can estimate the impulse responses of a propagation channel. In this way, on the basis of a parametric model of the paths, we propose a blind, spatiotemporal identification of the propagation channel. The parameters that characterize the propagation model are the directions of arrivals (DOA) θ, time delays τ, and complex gains α (also called fading). We propose an algorithm that can both estimate the multipath parameters and test them on real life data. This new method needs fewer snapshots than other methods recently proposed, and thus can monitor more quickly varying channels. Moreover, compared to recent work we have relaxed the problem of making successive estimates of the impulse responses. The proposed method can also identify more paths than the number of sensors. An extension of the algorithm will be presented by including polarization diversity and thus increases the resolution. The proposed methods are illustrated on experimental data.
Nonlinear inversion of a buried object in transverse electric scattering
Authors: Kooij B. J. , Lambert M. , Lesselier D.
Published: September 2002

A method for reconstructing the material properties of a buried bounded inhomogeneous object from measured scattered field data at the surface of the Earth's interface is presented. This work extends the method previously developed for the homogeneous transverse electric (TE) case to the more complicated case of a buried object. In the TE case, the magnetic field is polarized along the axis of an inhomogeneous cylinder of arbitrary cross section, and the corresponding integral equation contains derivatives of both the background Green's function and the field. The nonlinear inversion, however, can be formulated from an electric field integral equation for the two transversal components of the electric field. The integrand is a product of the background Green's function, the contrast, and the electric field vector; however, in the case of a buried object the background Green's function is the one pertaining to a two‐media configuration. The derivatives are operative outside the integral. In this paper the latter formulation will be taken as the point of departure to develop a nonlinear inversion scheme based upon the modified gradient method. In the inversion scheme the positivity of the material parameters has to be ensured in order to obtain a convergent inversion scheme. Numerical results are presented in which the reconstruction of the contrast is shown and compared in the case of TE excitation as well as transverse magnetic (TM) excitation.
Efficient spectral evaluation of mutual coupling between planar antennas
Authors: Garino Massimo Sereno , Vecchi Giuseppe , Orefice Mario
Published: September 2002

This work deals with the evaluation of mutual coupling in arrays of apertures or printed radiators and other similar problems. Two analytical‐based numerical integration techniques will be described, for the efficient evaluation of the spectral reaction integrals between two arbitrary functions on separated domains in a general stratified medium. These techniques employ novel schemes of contour deformation. The spectral domain formulation is used, and integration is carried out in Cartesian coordinates, taking theχaxis along the direction adjoining the centers of the two basis functions involved. In both techniques thekχcontour is deformed along the negative imaginary axis in order to turn the oscillating shift exponential into a decaying term. A robust scheme is obtained, with yet additional advantages in the case where many coupling terms at different distances are to be evaluated (as in alignments within arrays). Validation against published data and an application example are presented, along with a discussion of the numerical performances.
Investigation of mode interaction on planar dielectric waveguides with loss and gain
Authors: Hanson George W. , Yakovlev Alexander B.
Published: September 2002

On lossless isotropic planar waveguides the discrete proper modes of propagation form independent transverse electric and transverse magnetic sets such that there is no mode coupling or interaction between modes. In the event of material loss or gain, mode interactions are possible, leading to a complicated spectrum and apparent nonuniqueness of the modes. In this paper we analyze for the first time the cause of these modal interactions by studying the simplest canonical planar waveguide which exhibits these effects, the symmetric‐slab waveguide. We show that mode interactions are due to the migration of complex‐frequency‐plane branch points associated with specific wave phenomena, with varying loss or gain. As these singularities move near the real‐frequency axis they influence the modal behavior for time‐harmonic (real‐valued) frequencies, crossing the real axis at some critical value of loss or gain. It is shown that as time‐harmonic frequency varies, passing above, below, or through these branch points results in different modal behavior. Passing above or below, and near to, the branch point yields mode coupling behavior, while passing through the branch point results in modal degeneracy. The result of this branch point migration is that the association of a particular mode with a certain branch of the dispersion function depends not only on the value of material loss or gain, but also on the order in which physical parameters of the problem are varied. Three different branch point types are identified and discussed, which leads to an understanding of the relevant wave phenomena and to a method for organizing the mode spectrum in a consistent and unique manner. While many of the observations described here are based on careful numerical analysis of the transverse magnetic modes existing on a certain symmetric‐slab waveguide, the described phenomena are reasonably expected to be generally found in other open dielectric waveguiding structures.
A new model for the structure function of integrated water vapor in turbulence
Authors: Bobak Justin P. , Ruf Christopher S.
Published: September 2002

Turbulent fluctuation of integrated water vapor in the troposphere is one of the major noise sources in radio interferometry. Processed integrated water vapor estimates from microwave radiometers colocated with interferometers have been used to set bounds on this uncertainty. The bound has been in the form of a calculated structure function, which is a measure of temporal or spatial decorrelation of fluctuations. In this paper a new model is presented for the estimation of the structure function in the absence of radiometer measurements. Using this model, the structure function can be estimated using measurements or estimates of a limited number of meteorological parameters. These parameters include boundary layer depth, surface heat and humidity fluxes, entrainment humidity flux, average virtual potential temperature in the boundary layer, and geostrophic wind speed. These parameters can be found or estimated from radiosonde and surface eddy correlation system data. The model is based on a framework of turbulence meteorology and provides excellent agreement when compared with state‐of‐the‐art atmospheric turbulence simulations. Results of preliminary comparisons with ground truth show some excellent agreement, as well as some problems. The performance of the new model exceeds that of one current model.
The Tatshenshini shear zone (new) in southwestern Yukon, Canada: Comparison with the Coast shear zone in British Columbia and southeastern Alaska and implications regarding the Shakwak suture
Authors: Lowey Grant W.
Published: September 2002

The Tatshenshini shear zone extends from Klukshu Lake in southwestern Yukon, southward to the Yukon‐British Columbia border. Deformation in the shear zone ranges from hundreds of meters to a few kilometers in width and is characterized by protomylonitic and mylonitic turbidites belonging to the Dezadeash Formation (Jurassic‐Cretaceous) that are overlain by protomylonitic granodiorite of the Ruby Range Batholith (Late Cretaceous‐Tertiary). The Dezadeash Formation‐Ruby Range Batholith contact, bedding, and deformation and magmatic fabrics within the Tatshenshini shear zone are all coplanar, striking northwest‐southeast and dipping moderately to the northeast. Kinematic indicators from the Dezadeash Formation and Ruby Range Batholith record a top‐to‐the‐southwest sense of shear. Collision of the Insular superterrane with the Intermontane superterrane by the mid‐Cretaceous resulted in the eastward directed underplating of Yukon‐Tanana terrane‐equivalent rocks by the Dezadeash Formation and the formation of the Tatshenshini shear zone. The Tatshenshini shear zone preceded and controlled emplacement of the Ruby Range Batholith, but magmatism, ending ∼ 58–56 Myr ago, outlasted deformation. The results of this work support the correlation of the Ruby Range Batholith with the adjacent Great Tonalite Sill in southeastern Alaska. It also suggests a linkage between the Tatshenshini shear zone and the Coast shear zone in southeastern Alaska and along which the Great Tonalite Sill was emplaced. The Tatshenshini shear zone and the Coast shear zone represent the coda of long‐lived tectonism along the Shakwak suture, the accretionary boundary between the Insular and Intermontane superterranes.
Interplay of plutonism and regional deformation in an obliquely convergent arc, southern Coast Belt, British Columbia
Authors: Brown E. H. , Talbot J. L. , McClelland W. C. , Feltman J. A. , Lapen T. J. , Bennett J. D. , Hettinga M. A. , Troost M. L. , Alvarez K. M. , Calvert A. T.
Published: September 2002

The Coast Plutonic Complex is an extensive zone of continental growth that formed along the Mesozoic convergent margin of northwestern North America. The orogeny creating this belt involved terrane accretion and assembly, massive upward transfer and emplacement of sial in the form of batholiths constituting a magmatic arc, and transformation of broad tracts of sedimentary and volcanic rocks into crystalline continental crust, all operating in more or less the same space and time. The mechanisms and interplay of these orogenic processes are well displayed in the Harrison Lake area of the southern Coast Belt, British Columbia. Great structural relief across the area exhibits a present‐day architecture defined by thin, thrust‐stacked terrane sheets and early concordant pluton sheets folded on a macroscopic scale, all truncated by oblique dextral‐reverse faults and overlain by later floored plutons. Construction of this complex began with terrane assembly on orogen‐normal thrusts during a lull in plutonism in the Early Cretaceous. Low‐grade metamorphism during this event indicates only modest crustal thickening. Subsequent plutons intruded into the assembled terranes appear to be composites of sheets. Early pluton sheets are now steeply dipping due to folding but were likely intruded as horizontal bodies. Large ovoid post folding plutons are mostly subhorizontal floored bodies, at least in part sheeted. These plutons are underlain by Barrovian mineralogic aureoles that indicate downward vertical displacement of 10 km or more during plutonism, suggesting pluton emplacement by vertical inflation. Magmatic fabrics in these bodies, and the discordant relation of plutons to regional structures, preclude emplacement in active fault zones. Penetrative strain aureoles flanking plutons are mostly limited to zones a few hundred meters wide, and regional tectonic structures are widely preserved. Tectonic deformation of the arc is characterized by contraction and strike‐slip, not orogen‐normal extension. Plutons played a greater role than terrane accretion in causing crustal thickening and high‐grade metamorphism.
Flexural rift flank uplift at the Rio Grande rift, New Mexico
Authors: Brown C. David , Phillips Roger J.
Published: September 2002

Like other Cenozoic continental rifts, the Rio Grande rift in Colorado and New Mexico exhibits prominent flanking uplifts. Several driving stresses and thermal‐mechanical processes have been proposed to explain the origin of rift flank relief, which can be modeled to infer lithospheric structure. Although we have identified multiple uplift styles at the Rio Grande rift, only one range—the Sacramento Mountains—is attributable to flexural upwarping of the lithosphere, the process most suitable for geodynamic modeling and interpretation. We demonstrate that two common assumptions in such modeling potentially introduce serious errors. First, presuming only one mechanism acts to uplift the flanks is inappropriate; various forces influence flank topography at different depths and wavelengths and no single one is dominant. Second, the end‐member boundary conditions of complete mechanical continuity or discontinuity (broken plate) at the range‐bounding normal fault are, in general, not applicable at rift flanks. We examine alternative analytic plate flexure solutions by comparing them to finite element models of footwall flexure at a normal fault in a two‐dimensional elastic plate undergoing extension. These simulations indicate that broken plate fits to rift flanks underestimate the plate thickness unless the uplift is large (at least ∼1 km), which promotes decoupling between the hanging wall and footwall. If denudation dominates the flank unloading, as may commonly be the case, the best‐fit broken plate thickness error can be even greater. Our flexural analysis of the Sacramento Mountains suggests that the Pecos River Valley originated as a flexural down warp adjacent to the rift flank. Sensitivity tests of least‐squares fits to the Sacramento Mountains imply typical plate thickness errors of <20%, although in extreme cases the combined errors may be ∼50%. The average effective elastic lithosphere thickness is ∼23 km. We find that elastic‐plastic models of rift flank flexure are unable to provide meaningful constraints on the thermal structure of continental lithosphere.
The tectonic evolution of the Kohistan‐Karakoram collision belt along the Karakoram Highway transect, north Pakistan
Authors: Searle M. P. , Khan M. Asif , Fraser J. E. , Gough S. J. , Jan M. Qasim
Published: September 2002

The Kohistan arc terrane comprises an intra‐oceanic island arc of Cretaceous age separating the Indian plate to the south from the Karakoram (Asian) plate to the north within the Indus suture zone of north Pakistan. The intra‐oceanic arc volcanics (Chalt, Dras Group) were built on a foundation of dominantly mid‐ocean ridge basalt (MORB)‐related amphibolites of the Kamila Group. The subarc magma chamber is represented by multiple intrusions of a huge gabbro‐norite complex (Chilas complex), which includes some ultramafic assemblages of residual mantle harzburgite and dunite, layered cumulates, and hornblendites cut by late stage dikes of hornblende + plagioclase pegmatites. The Chilas complex norites intrude the Gilgit metasediments of lower amphibolite and greenschist facies in northern Kohistan, which also form xenolithic roof pendants within the top of the Chilas complex. Along the southern margin of Kohistan, Jijal and Sapat complex ultramafics (dunites, harzburgites and websterites) form remnant suprasubduction zone ophiolitic mantle rocks along the hanging wall of the Main Mantle Thrust, the Cretaceous obduction plane along which Kohistan was emplaced onto Indian plate rocks. Garnet granulites of the Jijal complex, formed at 12–14 kbars, represent original magmatic lower crustal rocks subducted to depths of at least 45 km and metamorphosed during high‐pressure and high‐temperature subduction of earlier arc‐related rocks. Obduction of the Sapat ophiolite and Kohistan arc occurred between ∼75 and 55 Ma.

The closure of the Shyok suture zone separating Kohistan from the Karakoram plate must have occurred prior to 75 Ma, the age of the Jutal basic dikes which crosscut the closure‐related fabrics, mainly late north directed backthrusting in the lower Hunza valley. Andean‐type granitoid (gabbrodiorite‐granodiorite‐granite) emplacement along the Kohistan‐Ladakh batholith ended at the time of India‐Asia collision, ∼ 60–50 Myr ago. Postcollisional crustal thickening along the Karakoram led to multiple episodes of metamorphism from latest Cretaceous and throughout the Tertiary. Sillimanite grade metamorphism in Hunza was actually pre‐India‐Asia collision and may have resulted from the earlier Kohistan collision. Localized and sporadic crustal melting episodes across northern Kohistan (Indus confluence and Parri granite sheets) and the southern Karakoram (Hunza dikes and Sumayar and Mango Gusar leucogranites) occurred from 51 to 9 Ma and culminated in the huge Baltoro monzogranite‐leucogranite intrusion 25–21 Myr ago. A vast network of leucogranitic and pegmatitite dikes containing gem quality aquamarine + muscovite ± tourmaline ± garnet ± biotite quartz are younger than 5 Ma and form the final phase of intrusion in the Haramosh area and parts of the southern Karakoram area.
Generalized covariance functions associated with the Laplace Equation and Their use in interpolation and inverse problems
Authors: Kitanidis Peter K.
Published: September 2002

This work examines which generalized covariance function when used in the stochastic approach produces the flattest possible estimate of an unknown function that is consistent with the data. Such an estimate is the plainest possible continuous function, thus in a sense eliminating details that are irrelevant or unsupported by data. The answer is found from the solution of the following variational problem: Determine the function that reproduces the data, has the smallest gradient (in the square norm sense), and has a gradient that vanishes at large distances from the observations. The generalized covariance functions are shown to be the Green's functions for the free‐space Laplace equation: the linear distance, in one dimension; the logarithmic distance in two dimensions; and the inverse distance in three dimensions. It is demonstrated that they are appropriate covariance functions for intrinsic random fields, a modification is proposed to facilitate numerical implementation, and a couple of examples are presented to illustrate the applicability of the methodology.
On the effect of digital elevation model accuracy on hydrology and geomorphology
Authors: Walker Jeffrey P. , Willgoose Garry R.
Published: September 2002

This study compares published cartometric and photogrammetric digital elevation models (DEMs) of various grid spacings with a ground truth data set, obtained by ground survey, and studies the implications of these differences on key hydrologic statistics. Inferred catchment sizes and stream networks from published DEMs were found to be significantly different than those from the ground truth in most instances. Furthermore, the width functions and cumulative area relationships determined from the published DEMs were found to fall consistently outside the 90% confidence limits determined from the ground truth for more than 60% of the relationship, suggesting that these hydrologic properties are poorly estimated from published DEMs. However, the slope‐area relationships determined from published DEMs were found to be less sensitive to catchment shape, size, and stream network, with the relationship falling outside the 90% confidence limits for less than 40% of the relationship for all catchments identified from the published DEMs. A published relationship linking the horizontal resolution with the vertical accuracy of the DEM was tested, predicting a horizontal resolution of about 10 m for the published DEMs tested.
Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow
Authors: Davis J. Matthew
Published: September 2002

Oriented carbonate concretions appear to provide a powerful new tool in the evaluation of flow through heterogeneous aquifers. Calcite cement forms elongate bodies of indurated rock that have distinct and mappable spatial orientations. We have mapped ∼200 of these concretion orientations as well as the hydrogeologic characteristics of the host sediments in outcrops of the Pliocene‐Pleistocene Sierra Ladrones Formation in central New Mexico. The average azimuthal concretion orientations is 150°, which is consistent with the southeasterly groundwater flow suggested by past and present basin geometry. Using a first‐order approximation, the dimensionless transverse velocity is estimated from the orientation data. The spatial correlation structure of the dimensionless transverse velocity is distinctly different in the two units investigated. However, both are consistent with existing stochastic models. The differences between the units and the similarities with theoretical models suggest that concretion orientations can be used to study how fluids flow through heterogeneous systems over scales ranging from centimeters to tens of kilometers.
Potential relative future effects of sulfur and nitrogen deposition on lake chemistry in the Adirondack Mountains, United States
Authors: Church M. Robbins , Van Sickle John
Published: September 2002

Leaching of atmospherically deposited nitrogen from forested watersheds can acidify lakes and streams. Using a modified version of the Model of Acidification of Groundwater in Catchments, we made computer simulations of such effects for 36 lake catchments in the Adirondack Mountains of New York State. Our simulations bound the potential changes in regional‐scale surface water acidification that might occur there over the next 50 years across broad scenarios of both nitrogen and sulfur deposition as well as ranges of times to watershed nitrogen saturation. Model projections indicated that nitrogen deposition may play a more important role in future acidification than has been previously considered and may rival or exceed potential effects of sulfur deposition, depending on how rapidly watersheds might become saturated with atmospherically deposited nitrogen.
A general method for obtaining analytical expressions for the first‐order velocity covariance in heterogeneous porous media
Authors: Hsu Kuo‐Chin
Published: September 2002

Velocity covariance of seepage flow is the essential quantity in understanding the uncertainty of groundwater flow and contaminant transport in heterogeneous porous media. Nevertheless, a closed‐form analytical expression is available only for first‐order analysis. These available expressions are limited to some specific correlation functions of natural log hydraulic conductivity. The present paper develops a general method for obtaining the first‐order velocity covariance for any statistically isotropic log‐hydraulic conductivity correlation function. The general analytical expressions are derived for both two and three dimensions. The results are demonstrated for several frequently used correlation functions.
Estimation of surface heat flux and an index of soil moisture using adjoint‐state surface energy balance
Authors: Castelli Fabio , Entekhabi Dara , Caporali Enrica
Published: September 2002

Estimates of surface heat flux and an index of the surface control over evaporation are made based on radiometric observations of ground temperature. A variational data assimilation approach is used to include surface energy balance in the estimation procedure as a physical constraint (the adjoint technique). This technique formulates the estimation problem as a minimization of ground temperature forecast misfits against observations. The surface energy balance equation is incorporated as a physical constraint. Applications to the First International Satellite Land Surface Climatology Project Field Experiment (FIFE) are presented. The procedure estimates of the surface control over latent heat flux match those values based on independent latent heat flux measurements. The estimates of surface heat flux, when compared with measurements, have a root‐mean‐square error of 20 W m−2. The need to discriminate between soil wetness and aerodynamic contributions to the surface control over evaporation is recognized.
The Edwards Aquifer Water Resource Conflict: USDA Farm Program resource‐use incentives?
Authors: Schaible Glenn D. , McCarl Bruce A. , Lacewell Ronald D.
Published: September 2002

This paper summarizes economic and hydrological analyses of the impacts of the 1990 and 1996 U.S. Department of Agriculture (USDA) farm programs on irrigation water withdrawals from the Edwards Aquifer in south central Texas and on aquifer‐dependent spring flows that support threatened and endangered species. Economic modeling, a regional producer behavioral survey, as well as institutional and farm characteristic analyses are used to examine likely irrigation water‐use impacts. Hydrologie modeling is used to examine spring flow effects. Study results show that 1990 USDA commodity programs caused producers to require less irrigation water, in turn increasing rather than decreasing aquifer spring flows. Market economic factors are the dominant criteria influencing producer irrigation decisions. Farm‐tenure arrangements and aquifer management responsibilities of the Edwards Aquifer Authority indicate that the 1996 Farm Act's PFC payment program will not cause an increase in irrigation withdrawals. Broader actions such as long‐term water supply enhancement/conservation programs, dry‐year water‐use reduction incentives and water markets all provide tools for Edwards water‐use conflict resolution. USDA farm programs do not apparently play a material part in the total debate.
Submarine groundwater discharge and associated chemical input to a coastal sea
Authors: Li L. , Barry D. A. , Stagnitti F. , Parlange J.‐Y.
Published: September 2002

This paper presents a theoretical model of flow and chemical transport processes in subterranean estuaries (unconfined brackish groundwater aquifers at the ocean‐land interface). The model shows that groundwater circulation and oscillating flow, caused by wave setup and tide, may constitute up to 96% of submarine groundwater discharge (SGWD) compared with 4% due to the net groundwater discharge. While these local flow processes do not change the total amount of land‐derived chemical input to the ocean over a long period (e.g., yearly), they induce fluctuations of the chemical transfer rate as the aquifer undergoes saltwater intrusion. This may result in a substantial increase in chemical fluxes to the ocean over a short period (e.g., monthly and by a factor of 20 above the averaged level), imposing a possible threat to the marine environment. These results are essentially consistent with the experimental findings of Moore [1996] and have important implications for coastal resources management.
Coupling groundwater and riparian vegetation models to assess effects of reservoir releases
Authors: Springer Abraham E. , Wright Julie M. , Shafroth Patrick B. , Stromberg Juliet C. , Patten Duncan T.
Published: September 2002

Although riparian areas in the arid southwestern United States are critical for maintaining species diversity, their extent and health have been declining since Euro‐American settlement. The purpose of this study was to develop a methodology to evaluate the potential for riparian vegetation restoration and groundwater recharge. A numerical groundwater flow model was coupled with a conceptual riparian vegetation model to predict hydrologic conditions favorable to maintaining riparian vegetation downstream of a reservoir. A Geographic Information System (GIS) was used for this one‐way coupling. Constant and seasonally varying releases from the dam were simulated using volumes anticipated to be permitted by a regional water supplier. Simulations indicated that seasonally variable releases would produce surface flow 5.4–8.5 km below the dam in a previously dry reach. Using depth to groundwater simulations from the numerical flow model with conceptual models of depths to water necessary for maintenance of riparian vegetation, the GIS analysis predicted a 5‐ to 6.5‐fold increase in the area capable of sustaining riparian vegetation.
Saturated flow in a single fracture: evaluation of the Reynolds Equation in measured aperture fields
Authors: Nicholl M. J. , Rajaram H. , Glass R. J. , Detwiler R.
Published: September 2002

Fracture transmissivity and detailed aperture fields are measured in analog fractures specifically designed to evaluate the utility of the Reynolds equation. We employ a light transmission technique with well‐defined accuracy (∼1% error) to measure aperture fields at high spatial resolution (∼0.015 cm). A Hele‐Shaw cell is used to confirm our approach by demonstrating agreement between experimental transmissivity, simulated transmissivity on the measured aperture field, and the parallel plate law. In the two rough‐walled analog fractures considered, the discrepancy between the experimental and numerical estimates of fracture transmissivity was sufficiently large (∼22–47%) to exclude numerical and experimental errors (<2%) as a source. We conclude that the three‐dimensional character of the flow field is important for fully describing fluid flow in the two rough‐walled fractures considered and that the approach of depth averaging inherent in the formulation of the Reynolds equation is inadequate. We also explore the effects of spatial resolution, aperture measurement technique, and alternative definitions for link transmissivities in the finite difference formulation, including some that contain corrections for tortuosity perpendicular to the mean fracture plane and Stokes flow. Various formulations for link transmissivity are shown to converge at high resolution (∼1/5 the spatial correlation length) in our smoothly varying fracture. At coarser resolutions the solution becomes increasingly sensitive to definition of link transmissivity and measurement technique. Aperture measurements that integrate over individual grid blocks were less sensitive to measurement scale and definition of link transmissivity than point sampling techniques.
On a test case for density‐dependent groundwater flow and solute transport models: The Salt Lake Problem
Authors: Simmons Craig T. , Narayan Kumar A. , Wooding Robin A.
Published: September 2002

Like any other computer model, density‐dependent mathematical groundwater models must be “verified” to ascertain that they accurately represent the physics implied by a governing set of equations. Very few test cases for density‐dependent groundwater numerical models exist. As such, there is still a need for new and more robust tests of these modeling codes. In this paper a numerical model of an idealized evaporating salt lake is produced using the two‐dimensional density‐dependent saturated‐unsaturated transport (SUTRA) model, the results of which are compared with an equivalent laboratory Hele‐Shaw cell system developed by Wooding et al. [1997a, b]. Evaporation results in dense brine overlying less dense fluid, which is hydrodynamically unstable and leads to downward convection of salt fingers or plumes. A comparison of experimental and numerical plume growth shows good spatial and temporal agreement. The numerically generated plume pattern is sensitive to changes in random noise level applied just below the evaporation surface that serves as a trigger for the growth of instabilities. Experimental plume patterns were best matched with a noise level corresponding to 1% of the total salinity difference between boundary layer and background fluid concentrations at saturation. In a second comparison the stream‐function‐based finite difference model described by Wooding et al., [1997a, b] which differs significantly in principle from SUTRA is shown, after revision, to give good spatial and temporal agreement with experimental results. This test for density‐dependent groundwater models appears to be the most comprehensive and detailed available to date.
On the spatial and temporal links between vegetation, climate, and soil moisture
Authors: Rodriguez‐Iturbe I. , D'Odorico P. , Porporato A. , Ridolfi L.
Published: September 2002

The impact of climate fluctuations can be observed in the dynamics of vegetation and most particularly in the sensitive environment of savannas. In this paper we present a model for the local competition for soil moisture among neighboring vegetation. The initial condition for the model is a random field where at each point the soil moisture is the mean water content when there are no spatial interactions between sites. The mean soil moisture values account for stochasticity of climate and losses from evapotranspiration and leakage which depend on the existing water content. A spatial dynamics is then implemented based on the explicit minimization of the global water stress over the region. This approach explains the coexistence of herbaceous and woody plants in savannas as well as the changes in canopy density that have been documented in the southwest of the United States as a function of regional climatic fluctuations.
Daily phosphorus variation in a mountain stream
Authors: Hatch Lorin K. , Reuter John E. , Goldman Charles R.
Published: September 2002

Monthly diel monitoring studies for phosphorus content were conducted (1995–1996 period) for multiple stations on Incline Creek, a mountain stream in the Lake Tahoe basin (California‐Nevada). Large discharge and particulate P (PP) concentration fluctuations occurred during June in the early evening as snowmelt from higher elevations arrived at the lower stream reaches. June diel dissolved organic P (DOP) concentrations steadily increased, while soluble reactive P (SRP) concentrations remained constant. June diel PP concentrations associated with sand‐sized particles (PPsand: >63 μm) exhibited a clockwise hysteresis, indicating possible sediment source depletion on a daily timescale. June diel PP associated with silt‐ and clay‐sized particles (PPSC: >0.45 μm and <63 μm) exhibited counterclockwise hysteresis behavior, suggesting a potential groundwater contribution to PPSC. PPSC comprised the majority of PP concentration, except during high‐discharge events when PPsand concentration was dominant. Areal PP loading, specifically PPsand, appears to originate primarily from the lower eastern branch of Incline Creek during the spring snowmelt season. Possible sources include a ski resort/parking lot and a golf course. DOP and SRP areal loads were greatest from the undeveloped upper subwatershed, suggesting that natural factors such as slope are influencing loading of small‐sized P fractions.
Cation transport and partitioning during a field test of electroosmosis
Authors: Chen Jiann‐Long , Al‐Abed Souhail R. , Bryndzia L. Taras , Murdoch Larry
Published: September 2002

Field experiments were conducted to evaluate the effects of soil properties, such as the cation exchange capacity and mineral content, on pH, soluble ion concentrations, and electrical conductivity during electroosmosis in a silty clay soil. The soil is composed mainly of quartz (60%) and smectite/illite (20–29%). The measured cation exchange capacity of the test site soil ranged from 11 to 22 cmol kg−1, which was consistent with the contribution from the clay contents. The exchangeable cations and reserve acidity measurements suggest that the majority of protons that were generated at the anode by electrolysis not only exchanged the base cations but also were adsorbed on the soil minerals. Protons also reacted with the soil minerals and caused the release of dissolved Al3+, which was then exchanged and adsorbed by the soil minerals. The exchange and adsorption of proton and Al3+ resulted in the decrease of the exchangeable base cations and ion concentrations of the pore fluid. Sorption (exchange and adsorption) and reaction significantly retarded the transport of protons. The apparent retardation factor for protons calculated from the movement of the pH front was of the order of 1000, whereas retardation factors calculated by assuming a Langmuir isotherm ranged from 30 to 6700 in the pH range of 3.6–4.8. The electrical conductivity of the soil also was influenced and dominated by the sorption and reaction of protons. In the region close to the anode where protons were sorbed, the electrical conductivity at the end of the test decreased to 0.37 of the initial value despite the fact that the proton concentration in pore fluid increased by 1–2 orders of magnitude. The results of this study show that soil properties, such as cation exchange capacity and mineral content, have a pronounced effect on solution chemistry and hence the utility of electroosmosis.
Sensitivity of a high‐elevation rocky mountain watershed to altered climate and CO2
Authors: Baron Jill S. , Hartman Melannie D. , Band L. E. , Lammers R. B.
Published: September 2002

We explored the hydrologic and ecological responses of a headwater mountain catchment, Loch Vale watershed, to climate change and doubling of atmospheric CO2 scenarios using the Regional Hydro‐Ecological Simulation System (RHESSys). A slight (2°C) cooling, comparable to conditions observed over the past 40 years, led to greater snowpack and slightly less runoff, evaporation, transpiration, and plant productivity. An increase of 2°C yielded the opposite response, but model output for an increase of 4°C showed dramatic changes in timing of hydrologic responses. The snowpack was reduced by 50%, and runoff and soil water increased and occurred 4–5 weeks earlier with 4°C warming. Alpine tundra photosynthetic rates responded more to warmer and wetter conditions than subalpine forest, but subalpine forest showed a greater response to doubling of atmospheric CO2 than tundra. Even though water use efficiency increased with the double CO2 scenario, this had little effect on basin‐wide runoff because the catchment is largely unvegetated. Changes in winter and spring climate conditions were more important to hydrologic and vegetation dynamics than changes that occurred during summer.
Eulerian solutions of O(σNv) for the stochastic transport problem for conservative tracers coupled with O(σ4ƒ) solutions for the flow problem in an infinite domain
Authors: Hu Bill X. , Hassan Ahmed E. , Cushman John H.
Published: September 2002

An O(σvN) Eulerian stochastic theory [Cushman and Hu, 1997] for conservative transport is coupled with an O(σf4) Eulerian solution to the flow problem. The theory provides a self‐consistent recursive solution to the closure problem associated with Eulerian methods. The stochastic concentration is given to arbitrary order in σv, the variance of fluctuating velocity. The balance law for mean concentration is not required in the analysis. Closed forms for mean concentration are given up to O(σv4). The mean concentration is transformed to wave‐vector and frequency domain via fast Fourier transform to calculate numerically the mean concentration as well as its spatial moments. The results show that the first‐order solution in σv2 is equivalent to the nonlocal theory ofDeng et al. [1993]. Second‐order corrections to flow and transport equations slightly decrease the second longitudinal moment but significantly increase the second transverse moment, which is consistent with Hsu et al.'s [1996] results. The influence of second‐order corrections to skewness is not clear. The second longitudinal moment obtained from the second‐order correction agrees with the Monte Carlo result, but second‐order results for the second transverse moment and skewness significantly differ from those given in Monte Carlo simulations. Coupling the transport correction models with the velocity covariance generated through Monte Carlo simulation gives second transverse spatial moments that are very close to Monte Carlo simulations, which suggests that the correction to flow is more important than the correction to transport. The results also bring into question the accuracy of Monte Carlo simulations for flow.
Timescales for migration of atmospherically derived sulphate through an alpine/subalpine watershed, Loch Vale Colorado
Authors: Michel Robert L. , Campbell Don , Clow Dave , Turk John T.
Published: September 2002

Sulphur 35, a cosmogenically produced radioisotope with a short half‐life (87 days), was measured in snowpack during 1993–1997 and at four locations within the Loch Vale watershed during 1995–1997. The four sites include the two main drainages in the watershed, Andrews Creek and Icy Brook, a small south facing catchment flowing into Andrews Creek (Andrews Spring 1), and a similar north facing catchment flowing out of a scree field into Icy Brook (Spring 19). Concentrations ranged from a high of almost 50 mBq/L for a sample from Spring 19 in June 1996 to a concentration near the detection limit for a sample from Andrews Creek in April 1997. Sulphur 35 concentrations were normalized to sulphate (as mBq/mg SO4−2) and were decay‐corrected to a Julian day of 90 (April 1) for each year. Snowpack had the highest 35S concentration with an average concentration of 53 mBq/mg SO4−2. Concentrations in the streams were much lower, even when corrected for decay relative to JD 90. The large 35S concentrations found in Spring 19 were the result of increases in concentration due to sublimation and/or evapotranspiration and were lower than snowpack when normalized to sulphate. Using 35S concentrations found in snowpack as of JD 90 as a beginning concentration, the fraction of sulphate in streamflow that was derived from atmospheric deposition within the prior water year was estimated. For Icy Brook and Andrews Creek the fraction of the sulphate in streamflow derived from that year's snowpack and precipitation was low prior to the beginning of the main spring melt, reached a maximum during the period of maximum flow, and decreased as the summer progressed. A calculation of the seasonal flux indicated that about 40% of the sulphate that flowed out of the watershed was derived from atmospheric sulphate deposited during the previous year. This suggests that more than half of the sulphate deposited in the watershed by atmospheric processes during the previous year was removed during the following summer. Thus sulphate retention in alpine watersheds like Loch Vale is very limited, and changes in sulphate deposition should be quickly reflected in stream chemistry.
Projecting U.S. freshwater withdrawals
Authors: Brown Thomas C.
Published: September 2002

As past attempts to forecast water use have shown, predicting the future is fraught with difficulty. Yet a lesser objective, to extend past trends into the future, can offer a useful look at what may lie ahead. Relying on U.S. Geological Survey water use data for the period 1960–1995, this paper projects U.S. water use based on trends in basic water use factors. Those trends are largely encouraging. Over the past 35 years, withdrawals in industry and at thermoelectric plants have steadily dropped per unit of output, withdrawals per acre have dropped in some irrigated regions, and per capita domestic withdrawals, after rising steadily from 1960 to 1990, dropped in 1995. If these trends continue, aggregate withdrawals in the United States over the next 40 years will stay within 10% of the 1995 level, despite economic growth and an expected 41% increase in population. This projection is in contrast to most previous projections of U.S. water use, which did not adequately account for future improvements in water use efficiency. Important qualifications to these projections are that some regions of the United States, especially the southeast, will experience above average increases and that the projections are for the average year and thus underestimate demands during dry years.