bluuebunny/crossref_metadata_2025 huggingface dataset:
Uniform asymptotic solution for electromagnetic reflection and diffraction of an arbitrary Gaussian beam by a smooth surface with an edge
Authors: Chou Hsi‐Tseng , Pathak Prabhakar H.
Published: February 2004

A closed form solution is obtained to describe, in a physically appealing manner, the reflection and diffraction of a general astigmatic Gaussian beam which is incident on an arbitrary smooth, electrically large, slowly varying curved, perfectly conducting screen (or reflector). This closed form solution is obtained via an asymptotic evaluation of the radiation integral for the fields scattered from the reflector, to within the physical optics approximation that remains valid for the present situation. The analysis developed here is particularly well suited for the fast analysis of electrically large reflector antennas by representing the feed illumination by a relatively small set of Gaussian beams launched from the feed plane. Each of these Gaussian beams after being launched undergoes reflection and diffraction at the reflector; the expressions for the reflected and diffracted fields are developed in this paper and utilized by Chou [1996] to compute the radiation pattern of large reflector antennas in a matter of a few seconds as compared to the conventional numerical physical optics integral method which takes hours on the same computer.
Ionospheric sounding in support of over‐the‐horizon radar
Authors: Reinisch B. W. , Haines D. M. , Bibl K. , Galkin I. , Huang X. , Kitrosser D. F. , Sales G. S. , Scali J. L.
Published: February 2004

Precise coordinate registration for HF over‐the‐horizon (OTH) radar applications requires accurate knowledge of the ionospheric structure. In the mid‐1980s Digisonde 256 systems were deployed in the American sector to provide this information from strategically located sites via telephone lines to the user. The mid‐1990s saw the development of a new advanced system, the Digisonde portable sounder, or DPS, now being deployed in Australia in support of the Australian OTH radar system. A summary of the new features provided by the DPS is as follows: low radio frequency power (300 W); narrow transmission bandwidth; advanced automatic scaling; and control and data access via the Internet. The availability of real‐time electron density profiles as function of time from a network of stations makes it possible to calculate the three‐dimensional electron density distribution in the region of interest using Fourier transform techniques. The resulting density maps are the basis for the OTH radar coordinate registration. The DPS uses Doppler interferometry to determine the development of ionospheric irregularities.
Comparing model and measured rainfall rates obtained from a combination of remotely sensed and in situ observations
Authors: Hardaker P. J. , Holt A. R. , Goddard J. W. F.
Published: February 2004

The Olympus program was designed to develop new telecommunications technology in space and develop new market applications. This study analyzes attenuation data from two of the Olympus satellite beacons, those operating at 19.77 and 12.502 GHz. It is shown that the difference in the two attenuations provides estimates of rainfall rate that are in close agreement with rain gauge and distrometer measurements at the ground receiver site and along the propagation path. Modeling demonstrates that the attenuation difference using 19.77 and 12.502 GHz is a useful estimator of rainfall because it is largely independent of drop shape and size distribution, making it linearly related to rainfall rate. Attenuation along the propagation path due to the melting layer or ice introduces inaccuracies into estimates of rainfall. However, this difficulty would be eliminated if the method were to be used on terrestrial line‐of‐site links. Thus attenuation difference provides a means of improving the quality of rainfall estimates when used together with gauges and radar observations.
The EISCAT Svalbard radar: A case study in modern incoherent scatter radar system design
Authors: Wannberg G. , Wolf I. , Vanhainen L.‐G. , Koskenniemi K. , Röttger J. , Postila M. , Markkanen J. , Jacobsen R. , Stenberg A. , Larsen R. , Eliassen S. , Heck S. , Huuskonen A.
Published: February 2004

The EISCAT (European incoherent scatter) Svalbard radar (ESR) was officially inaugurated on August 22, 1996. This event marked the successful completion on schedule of the first phase of the EISCAT Svalbard radar project. In contrast to previous incoherent scatter radars, the ESR system design was adapted to make use of commercial off‐the‐shelf TV transmitter hardware, thereby reducing design risk, lead times, and cost to a minimum. Commercial hardware is also used in the digital signal processing system. Control and monitoring are performed by distributed, networked VME systems. Thanks to modern reflector antenna design methods and extreme efforts to reduce the receiver noise contribution, the system noise temperature is only 70 K, thus making the ESR about 30% faster than the much more powerful EISCAT UHF radar in F region experiments! Once the transmitter power is increased to 1 MW, it will become about 2–3 times faster than the UHF radar. State‐of‐the‐art exciter and receiver hardware has been developed in‐house to accommodate the special requirements introduced by operating the radar at the exceptionally high duty cycle of 25%. The RF waveform is generated by a system based on four switchable direct digital synthesizers. Continuous monitoring of the transmitted RF waveform by the receiver system allows removal of klystron‐induced spurious Doppler effects from the data. Intermediate‐frequency sampling at 7.5 MHz is employed, followed by fully digital channel separation, signal detection, and postdetection filtering in six parallel receiver channels. Radar codes for both E and F layer observation have been designed and perfected. So far, more than 40 hours of good quality ionospheric data have been collected and analyzed in terms of plasma parameters. While the tragic loss of the Cluster mission suddenly changed the plans and dispositions of a majority of the ESR user community, the radar has still been in high demand since its inauguration. It is now being operated by EISCAT staff on a campaign basis, to provide ground‐based support data for a number of other magnetospheric satellites, notably Polar and FAST, and will be opened to the EISCAT user community for special program operations later in 1997.
Reception of ELF signals at antipodal distances
Authors: Fraser‐Smith Antony C. , Bannister Peter R.
Published: February 2004

Measurements of 82‐Hz radio signals from a Russian ELF transmitter located on the Kola Peninsula are described. The measurements were made at several locations around the world, including Dunedin, New Zealand, and Arrival Heights, Antarctica, which are close to the antipodal point for the transmitter. This is the first time man‐made ELF signals have been observed over such long distances, and their clear reception makes possible a comparison of the theoretically predicted and measured amplitudes near the antipode. The agreement is excellent.
Surface‐based radiometric observations of water vapor and cloud liquid in the temperate zone and in the tropics
Authors: Snider J. B. , Hazen D. A.
Published: February 2004

Observations of water vapor and cloud liquid made with surface‐based microwave radiometers operating near 20, 23, 31, and 90 GHz are described. Measurements were made at several continental locations in the United States, an island and ship in the northern Atlantic Ocean, and an island and ship in the tropical western Pacific Ocean. The instrumentation and statistical inversion technique employed to retrieve vapor and liquid values from radiometric brightnesses are reviewed. Various aspects of the water vapor and cloud liquid data are compared as a function of geographic location. The diurnal and seasonal variability of water vapor, cloud liquid, cloud fraction, and infrared sky brightness temperature observed during a 1‐year period at an island in the North Atlantic Ocean are also presented.
Measurements of Doppler and multipath spread on oblique high‐latitude HF paths and their use in characterizing data modem performance
Authors: Angling Matthew J. , Cannon Paul S. , Davies Nigel C. , Willink Tricia J. , Jodalen Vivianne , Lundborg Bengt
Published: February 2004

Measurements of Doppler spread, multipath spread, and signal‐to‐noise ratio have been made on four high‐latitude high‐frequency (HF) communications paths. The measurement system and analysis techniques are outlined, and an analysis of the data pertinent to the design of robust HF data modems is presented. A summary of the spreads that are exceeded for 5% of time is presented for each path. Doppler spreads range from 2 to 55 Hz, while multipath spreads range from 1 to 11 ms. Physical interpretations of the data are made, and the data are related to the measured performance characteristics of an HF data modem to estimate the modem availability on the paths considered. When there is mode support, availabilities range from 64% to 100% for a signal‐to‐noise ratio of 0 dB, although the data indicate that the availabilities can generally be increased by optimizing frequency selection.
Global maps of microwave land surface emissivities: Potential for land surface characterization
Authors: Prigent Catherine , Rossow William B. , Matthews Elaine
Published: February 2004

Microwave land surface emissivities are derived from special sensor microwave imager (SSM/I) observations. Cloud‐free SSM/I observations are first isolated with the help of collocated visible/infrared satellite observations (International Satellite Cloud Climatology Project (ISCCP) data). Then, the cloud‐free atmospheric contribution is calculated from an estimate of the local atmospheric temperature‐humidity profile (National Centers for Environmental Prediction (NCEP) analyses). Finally, with the surface skin temperature derived from IR observations (ISCCP estimate), the surface emissivity is calculated for all the SSM/I channels. The method is applied to the SSM/I data for the globe for October 1991. Correspondences between geographical patterns of the emissivities and surface characteristics are briefly analyzed to investigate the potential of microwave emissivities to monitor vegetation phenology and surface properties at regional and continental scales.
Remote sensing of integrated cloud liquid water: Development of algorithms and quality control
Authors: Gérard Élisabeth , Eymard Laurence
Published: February 2004

Algorithms are developed to infer integrated cloud liquid water path (LWP) over the oceans from spaceborne and ground‐based passive microwave measurements. These algorithms are built from simulated observations, which are calculated with a radiative transfer model applied to a set of about 10,000 atmospheric profiles obtained from the European Centre for Medium‐Range Weather Forecasts forecast model. In this model the liquid water content is computed from a prognostic cloud scheme. A multilinear regression is applied to functions of simulated brightness temperatures (log linear form) and LWP to derive the algorithm coefficients. The retrieval accuracy based on the regression analysis including instrumental noise is 0.0257 and 0.0345 kg m−2 for the DMSP special sensor microwave imager (SSM/I) and the ERSl along‐track scanning radiometer/microwave (ATSR/M), respectively, and 0.0308 kg m−2 for the ground‐based radiometer. It is shown that the log linear form is adequate to transform the nonlinear problem into a quasi‐linear problem for LWP below 0.8 kg m−2. The coherence of the global approach is verified through the validation of total precipitable water (TPW) algorithms developed in a way similar to LWP algorithms. The LWP retrievals from the algorithm for the ground‐based radiometer are in good agreement with retrievals from airborne measurements performed in the vicinity of the radiometer. A coherence test is performed for ATSR/M, benefiting from the coincident infrared images obtained from an infrared radiometer (ATSR/IR) aboard the same platform to select clear‐air areas. Regardless of the slight mean bias of the inferred LWP due to inaccurate calibration, there is no anomalous dependency upon latitude, i.e., upon high water vapor contents in the tropics and strong winds in the high latitudes. The results of the algorithm for SSM/I are compared with a Meteosat cloud classification. When the classification detects the ocean surface, the algorithm systematically retrieves contents close to zero. The retrievals for other classes (i.e., low stratiform clouds, medium clouds) are consistent with the Meteosat data; retrievals in the presence of low stratiform clouds appear more realistic than values provided by some already published algorithms. It is also shown that up to 0.8 kg m−2 the log linear regression approach has a quality of the same order as a variational method, which requires much more computation time.
New model‐based Bayesian inversion algorithm for the retrieval of wet troposphere path delay from radiometric measurements
Authors: Keihm S. J. , Marsh K. A.
Published: February 2004

A new inversion algorithm has been developed for the retrieval of tropospheric wet path delay from microwave radiometer measurements. The algorithm is based on the principle of maximum probability and uses Bayes' theorem to determine the most probable state vector (comprised of discrete height profiles of temperature and water vapor density) for a given set of measurements. The solution probabilities depend both on conformity to apriori statistics and a minimization of residuals between measured observables and observables computed from candidate state vectors. The new algorithm has been compared with standard statistical inversion techniques using simulations based on radiosonde, lidar, and acoustic sounding measurements of atmospheric temperature and water vapor profiles. For clear conditions the results indicate that the nonlinear algorithm produces a factor of 3–5 improvement in path delay retrieval performance, relative to a nonstratified statistical algorithm, when the observational system includes both a water vapor radiometer and a microwave temperature profiler and measurement errors are minimized. The new algorithm is shown to be most useful when applied to radiometric measurements of path delay fluctuations over minute to multihour timescales. Recommended applications include tropospheric calibration systems for radio science experiments such as very long baseline interferometry astrometry and the planned Cassini gravitational wave experiment.
An anisotropic model of the melting layer
Authors: D'Amico Michele M. G. , Holt Anthony R. , Capsoni Carlo
Published: February 2004

There is current interest in the effects of the melting layer on the propagation of electromagnetic waves, in the radar, remote sensing, and telecommunication communities. In this work a microphysical model of the melting layer is presented for the prediction of the electromagnetic behavior of this important region of the atmosphere. The model is anisotropic to be able to take into account the depolarization effects. The model is also tested against Doppler, dual‐frequency measurements, and dual‐polarization measurements collected by modern multiparameter radar systems.
Ionospheric electron density profiles obtained with the Global Positioning System: Results from the GPS/MET experiment
Authors: Hajj George A. , Romans Larry J.
Published: February 2004

The Global Positioning System Meteorology (GPS/MET) experiment, which placed a GPS receiver in a low‐Earth orbit tracking GPS satellites setting behind the Earth's limb, has collected data from several thousands of occultations since its launch in April 1995. This experiment demonstrated for the first time the use of GPS in obtaining profiles of electron density and other geophysical variables such as temperature, pressure, and water vapor in the lower atmosphere. This paper discusses some of the effects of the ionosphere, such as bending and scintillation, on the GPS signal during occultation. It also presents a set of ionospheric profiles obtained from GPS/MET using the Abel inversion technique, and compares these profiles with ones obtained from the parameterized ionospheric model (PIM) and with ionosonde and incoherent scatter radar measurements. Statistical comparison of NmF2 values obtained from GPS/MET profiles and nearby ionosondes indicates that they agree to about ∼20% (1‐sigma) in a fractional sense. The high vertical resolution, characteristic of the occultation geometry, is reflected in the GPS/MET profiles which reveal ionospheric structures of very small vertical scales such as the sporadic E.
Thermomechanical evolution of a ductile duplex
Authors: Dunlap W. J. , Hirth G. , Teyssier C.
Published: February 2004

The thermomechanical evolution of a midcrustal ductile duplex in central Australia has been reconstructed through space and time using 40Ar/39Ar thermochronology, flow stress estimates, cross‐sectional restoration of dislocation creep microstructures, and microstructural and structural analysis. A critical aspect of this analysis is the identification of populations of white micas in quartzite mylonites that have neocrystallized below their closure temperature and which record the time when ductile deformation ceased. In dating these micas the mylonitic microstructures have effectively been dated. The time‐temperature history of the duplex has been constrained through multidomain thermal modeling of K‐feldspar 40Ar/39Ar data. The modeling demonstrates that a temperature gradient existed across the duplex during its formation. The concept of microstructural continuity during ductile deformation has great potential for elucidating the kinematic evolution of ductile duplexes. Mapping of the deformation mechanisms and recrystallized grain sizes of quartzites deformed under greenschist facies conditions has been used to evaluate tectonic offsets that occurred after microstructural freezing. This analysis shows that the duplex formed as a forward propagating thrust system accommodating ∼60 km of convergence between the upper and lower plates of the megathrust, with a significant fraction of the displacement occurring after microstructural freezing. Finally, using the data as input to published flow laws for quartz aggregates provides a strain rate history for the duplex. Although uncertainties are clearly large, the timing of highest‐estimated strain rates during duplex evolution does, indeed, correlate with the highest rates of convergence between the upper and lower plates of the megathrust system (according to regional cooling history studies) and with coeval sedimentation in adjoining molasse basins.
40Ar/39Ar evidence for Middle Proterozoic (1300–1500 Ma) slow cooling of the southern Black Hills, South Dakota, midcontinent, North America: Implications for Early Proterozoic P‐T evolution and posttectonic magmatism
Authors: Holm Daniel K. , Dahl Peter S. , Lux Daniel R.
Published: February 2004

40Ar/39Ar total gas and plateau dates from moscovite and biotite in the southern Black Hills, South Dakota, provide evidence for a period of Middle Proterozoic slow cooling. Early Proterozoic (1600–1650 Ma) mica dates were obtained from metasedimentary rocks located in a synformal structure between the Harney Peak and Bear Mountain domes and also south of Bear Mountain. Metamorphic rocks from the dome areas and undeformed samples of the ∼1710 Ma Harney Peak Granite (HPG) yield Middle Proterozoic mica dates (∼1270–1500 Ma). Two samples collected between the synform and Bear Mountain dome yield intermediate total gas mica dates of ∼1550 Ma. We suggest two end‐member interpretations to explain the map pattern of cooling ages: (1) subhorizontal slow cooling of an area which exhibits variation in mica Ar retention intervals or (2) mild folding of a Middle Proterozoic (∼1500 Ma) ∼300°C isotherm. According to the second interpretation, the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure (and downwarping of relatively cold rocks) during a period of approximately east‐west contraction and slow uplift during the Middle Proterozoic. The mica data, together with hornblende data from the Black Hills published elsewhere, indicate that the ambient country‐rock temperature at the 3–4 kbar depth of emplacement of the HPG was between 350°C and 500°C, suggesting that the average upper crustal geothermal gradient was 25°–40°C/km prior to intrusion. The thermochronologic data suggest HPG emplacement was followed by a ∼200 m.y. period of stability and tectonic quiescence with little uplift. We propose that crust thickened during the Early Proterozoic was uplifted and erosionally(?) thinned prior to ∼1710 Ma and that the HPG magma was emplaced into isostatically stable crust of relatively normal thickness. We speculate that uplift and crustal thinning prior to HPG intrusion was the result of differential thinning of the subcrustal lithosphere beneath the Black Hills. If so, this process would have also caused an increase in mantle heat flux across the Moho and triggered vapor‐absent melting of biotite to produce the HPG magma. This scenario for posttectonic granite generation is supported, in part, by the fact that in the whole of the Black Hills, the HPG is spatially associated with the deepest exposed Early Proterozoic country rock.
Supracrustal intraplate thickening of Variscan basement due to Alpine foreland compression: Results from the superdeep well KTB (Bohemian Massif, Germany)
Authors: Zulauf G. , Duyster J.
Published: February 2004

Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle‐ductile and brittle deformation during late‐ and post‐Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth‐dependent variations of metamorphic conditions of the post‐Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle‐ductile boundary layer of quartz‐bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle‐ductile boundary layer is prone to form a long‐lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.
Tectonic inheritance, crustal architecture, and contrasting structural styles in the Venezuela Andes
Authors: Colletta Bernard , Roure François , de Toni Bruno , Loureiro Daniel , Passalacqua Herminio , Gou Yves
Published: February 2004

Surface and subsurface data image contrasting structural styles for the two flanks of the Venezuela Andes. In the north, a flexural basin developed in Neogene times between the Andes and the Lake Maracaibo. North verging thrusts are mainly detached in the pre‐Cretaceous substratum and form a deeply buried antiformal stack. Secondary décollement levels occur both in the Upper Cretaceous and Tertiary strata, accounting for the passive roof thrust of a conventional frontal triangle zone. In the south, the Barinas basin hardly compares with a flexural basin. It is largely dominated by both north and south verging basement‐involving structures. Paleogene normal faults are locally inverted, and Caribbean nappes are frequently reactivated or refolded by younger oblique Neogene Andean structures. Seismic profiles in this area also attest to the strong Neogene structural inversion of Upper Jurassic‐Lower Cretaceous grabens. Hercynian, or more likely Early Paleozoic structures account for local preexisting crustal heterogeneities, reactivated during both the Tethyan rifting and the Andean deformations. Two trans‐Andean regional profiles have been balanced and constrained by an inversion of the gravimetric data. They imply a progressive deepening of the northern Moho and a south dipping subduction of the infracontinental lithospheric mantle of the Maracaibo block. The shortening for both sections averages 60 km. Palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the Andes and minimize the possible effects of a progressive right‐lateral escape of the Maracaibo block with respect to stable South America along the Bocono Fault. Strain partitioning during Neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary, strike‐slip motion in the allochthon along the Bocono Fault, and an asymmetric subduction (wedging) at depth.
Estudio Sísmico de la Corteza Ibérica Norte 3.3: A seismic image of the Variscan crust in the hinterland of the NW Iberian Massif
Authors: Ayarza Puy , Catalán José R. Martínez , Gallart Josep , Pulgar Javier A. , Dañobeitia Juan J.
Published: February 2004

An offshore vertical incidence reflection seismic study with simultaneous on‐land wide‐angle recording has been conducted, as part of the Estudio Sismico de la Corteza Ibérica Norte (ESCIN) Project, in the NW Iberian Variscan Belt, an area deeply affected by Variscan and Alpine deformation episodes. Line ESCIN‐3.3 was recorded across strike of the main Variscan structures, transecting two of the zones in which the NW Iberian Massif has been subdivided. This study intends to correlate the seismic features observed in the near‐vertical reflection profile with the velocity model deduced from the wide‐angle data, with the well‐known geological structure of the Variscan upper crust, and with some other geophysical features described on the basis of offshore and onshore experiments carried out in the area. The main results can be summarized as follows: (1) identification of two important sedimentary basins in the shallow part of the marine seismic section, related with the Mesozoic extension that gave rise to the opening of the Bay of Biscay, (2) correlation of a number of subhorizontal and west dipping intermediate depth reflections with Variscan compressional structures, (3) interpretation of a local set of strong reflections at 6 s (two‐way travel time) as lower crust emplaced at an anomalous shallow level (thought to image the northward prolongation of a body with high P wave velocity and high magnetic susceptibility, described on land underneath a late Variscan antiformal structure, the Lugo Dome), and (4) recognition of two controversial highly reflective subhorizontal bands, located at 7–9 and 11–12 s, respectively. The shallowest of them appears to have a lower crustal P wave velocity and to be separated from the deepest one, probably with similar characteristics, by mantle P wave velocity material. On this basis, a duplication of the lower continental crust is inferred. The deepest of the bands could either represent the remnants of a late Paleozoic crustal root or an Alpine underthrusting of the lower continental crust.
Gravity constraints on lithosphere flexure and the structure of the late Paleozoic Ouachita orogen in Arkansas and Oklahoma, south central North America
Authors: Harry Dennis L. , Mickus Kevin L.
Published: February 2004

Spectral analysis of Bouguer gravity anomalies in western central Arkansas and eastern Oklahoma indicates that the thickness of the crust in the Ouachita fold and thrust belt increases from 38 km in the western Ouachitas to 44 km in the eastern Ouachitas. The change in crustal thickness occurs near the western end of the Broken Bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1.8×1024 N m in the western Ouachitas to 5.0×1023 N m in the eastern Ouachitas. The flexural rigidity in the western Ouachitas is similar to values determined in the Appalachian fold and thrust belt and coincides with the depth of the 450°C isotherm predicted by conductive cooling models for the thermal evolution of the early Paleozoic southern Laurentian rifted continental margin. The thick crust in the eastern Ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age. The thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the Paleozoic continental margin. A layered density model derived from the gravity data shows that strata interpreted to be deformed Ouachita facies rocks are thickest in the eastern Ouachitas and are consistent with a greater amount of shortening in the central thrust belt in Arkansas as compared to Oklahoma. The opposite relationship is observed in the frontal Ouachita province, where shortening appears greater in Oklahoma. The cross‐strike changes in the locus of shortening, crustal thickness, flexural rigidity, and the inferred transition from rift to transform segments of the early Paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the Benton uplift. Flexural modeling indicates that the load required to produce the observed Bouguer gravity low in the Arkoma foreland basin trends parallel to the Benton and Broken Bow uplifts but is located 114 to 276 km farther south. In the western Ouachitas, the position of the load coincides with the northern edge of the Sabine uplift and is interpreted to mark the southern extent of Ouachita facies rocks that were emplaced on the Laurentian continental margin and/or attached remnant oceanic crust.
Hydraulics and erosion in eroding rills
Authors: Nearing M. A. , Norton L. D. , Bulgakov D. A. , Larionov G. A. , West L. T. , Dontsova K. M.
Published: February 2004

Rills often act as sediment sources and the dominant sediment and water transport mechanism for hillslopes. Six experiments were conducted on two soils and a uniform sand using three experimental methodologies. The results of this study challenge the assumption often used in hydrologic and erosion models that relationships derived for sheet flow or larger channel flow are applicable to actively eroding rills. Velocity did not vary with slope, and Reynolds number was not a consistent predictor of hydraulic friction. This result was due to interactions of slope gradient, flow rate, erosion, and the formation of rill roughness, bed structures, and head cuts. A relationship for rill flow velocities was proposed. Stream power was found to be a consistent and appropriate predictor for unit sediment load for the entire data set, while other hydraulic variables were not. The data for stream power and sediment load fit the form of a logistic curve (r2 = 0.93), which is promising relative to recently proposed erosion models which are based on probabilistic particle threshold theory.
Scaling of random fields by means of truncated power variograms and associated spectra
Authors: Di Federico Vittorio , Neuman Shlomo P.
Published: February 2004

An interpretation is offered for the observation that the log hydraulic conductivity of geologic media often appears to be statistically homogeneous but with variance and integral scale which grow with domain size. We first demonstrate that the power (semi)variogram and associated spectra of random fields, having homogeneous isotropic increments, can be constructed as weighted integrals from zero to infinity (an infinite hierarchy) of exponential or Gaussian variograms and spectra of mutually uncorrelated homogeneous isotropic fields (modes). We then analyze the effect of filtering out (truncating) high‐ and low‐frequency modes from this infinite hierarchy in the real and spectral domains. A low‐frequency cutoff renders the truncated hierarchy homogeneous with an autocovariance function that varies monotonically with separation distance in a manner not too dissimilar than that of its constituent modes. The integral scales of the lowest‐ and highest‐frequency modes (cutoffs) are related, respectively, to the length scales of the sampling window (domain) and data support (sample volume). Taking each relationship to be one of proportionality renders our expressions for the integral scale and variance of a truncated field dependent on window and support scales in a manner consistent with observations. The traditional approach of truncating power spectral densities yields autocovariance functions that oscillate about zero with finite (in one and two dimensions) or vanishing (in one dimension) integral scales. Our hierarchical theory allows bridging across scales at a specific locale, by calibrating a truncated variogram model to data observed on a given support in one domain and predicting the autocovariance structure of the corresponding multiscale field in domains that are either smaller or larger. One may also venture (we suspect with less predictive power) to bridge across both domain scales and locales by adopting generalized variogram parameters derived on the basis of juxtaposed hydraulic and tracer data from many sites.
How certain are visitors of their economic values of river recreation: An evaluation using repeated questioning and revealed preference
Authors: Loomis John B. , González‐Cabán Armando
Published: February 2004

We test the robustness of visitor dichotomous choice willingness to pay (WTP) for maintaining instream flow by challenging respondent's affirmative answers. This is followed by rephrasing the per trip WTP question into an annual dichotomous choice WTP question and reasking the question. About 5% of visitors revised their “yes” answer to “no.” Estimated WTP changed from $12.81 per trip to $11.96 per trip. Using the method of convolutions, this is an insignificant difference. To further evaluate the robustness of contingent valuation method estimated WTP, these values are compared to WTP derived from the revealed preference travel cost model of the same visitors. No statistical difference was found.
Process controls on regional flood frequency: Coefficient of variation and basin scale
Authors: Blöschl Günter , Sivapalan Murugesu
Published: February 2004

The coefficient of variation (CV) of maximum annual floods is examined to understand the effects of process controls and catchment size. A derived flood frequency model is used to interpret data from 489 catchments in Austria. At the core of process controls appears to be the interaction of catchment response time and storm duration, but the magnitude is not large, and often this interaction is hidden by other processes. The dependence of rainfall intensity and duration is clearly very important and reduces CV significantly. Increasing channel travel times with catchment scale tend to translate into decreasing CVs with area for small catchments while they tend to translate into increasing CVs with area for larger catchments. Nonlinear runoff processes, including threshold effects, is the main mechanism for increasing CV. They give rise to complex patterns in the relationship between CV and area. Base flow has been used as a surrogate for a number of processes, such as seasonality of streamflow. It always decreases CV and, in particular, leads to a significant decrease of CV with area. Both the observed tendency of CV to decrease with area and the scatter in the data are the result of a complex interplay of a number of processes which allows various alternative interpretations. Depending on which processes dominate under a particular hydrologic regime, different patterns arise. It appears that the explanations of the relationship between CV and catchment scale suggested in the literature are too simplistic. The case is made for using the concept of hydrologic regimes and process studies of the type presented here to help delineate homogeneous regions for regional flood frequency analyses in a physically consistent way.
Digital elevation analysis for distributed hydrological modeling: Reducing scale dependence in effective hydraulic conductivity values
Authors: Saulnier Georges‐Marie , Beven Keith , Obled Charles
Published: February 2004

The recent widespread availability of digital terrain data has made automatic procedures for topographic analyses popular. Previous studies have shown that hydrological models and their effective parameter values are dependent on the resolution of the elevation grid. This paper examines the analysis of raster elevation data within the topography‐based model, TOPMODEL, framework. It is demonstrated that the algorithm used in processing channel pixels in calculating the topographic index k=ln(a/tanβ) can have a dramatic effect on the sensitivity of effective parameter values to the grid size. Suggestions are made for calculating the topographic index of channel pixels, consistent with the TOPMODEL assumptions, that strongly decrease the sensitivity of the calibrated effective hydraulic conductivity values to grid size.
The first‐order reliability method of predicting cumulative mass flux in heterogeneous porous formations
Authors: Skaggs Todd H. , Barry D. A.
Published: February 2004

Previous studies have proposed the first‐order reliability method (FORM) as an approach to quantitative stochastic analysis of subsurface transport. Most of these considered only simple analytical models of transport in homogeneous media. Studies that looked at more‐complex, heterogeneous systems found FORM to be computationally demanding and were inconclusive as to the accuracy of the method. Here we show that FORM is poorly suited for computing point concentration cumulative distribution functions (cdfs) except in the case of a constant or monotonically increasing solute source. FORM is better equipped to predict transport in terms of the cumulative mass flux across a control surface. As a demonstration, we use FORM to estimate the cumulative mass flux cdf in two‐dimensional, random porous media. Adjoint sensitivity theory is employed to minimize the computational burden. In addition, properties of the conductivity covariance and distribution are exploited to improve efficiency. FORM required eight times less CPU time than Monte Carlo simulation to generate the results presented. The accuracy of FORM is found to be minimally affected by the size of the initial solute body and the solute travel distance. However, the accuracy is significantly influenced by the degree of heterogeneity, providing an accurate estimate of the cdf when there is mild heterogeneity (σInK = 0.5) but a less accurate estimate when there is stronger heterogeneity (σ1nK = 1.0).
Derived flood frequency distribution for negatively correlated rainfall intensity and duration
Authors: Kurothe R. S. , Goel N. K. , Mathur B. S.
Published: February 2004

Derived flood frequency distributions (DFFD) are relatively new in the field of hydrology. These models use rainfall and catchment characteristics for deriving the flood frequency distribution. The DFFD models developed so far use joint probability density function of exponentially distributed rainfall intensity and duration and consider these variables as independent of each other. In reality this may not be true and these variables may be correlated as well. In the present paper a physically based flood frequency model has been developed for negatively correlated rainfall intensity and duration. In the model, infiltration losses are represented by the Φ index. The geomorphoclimatic instantaneous unit hydrograph (GcIUH) is used as the effective rainfall‐runoff model.
Multiphase flow modeling of a crude‐oil spill site with a bimodal permeability distribution
Authors: Dillard Leslie A. , Essaid Hedeff I. , Herkelrath William N.
Published: February 2004

Fluid saturation, particle‐size distribution, and porosity measurements were obtained from 269 core samples collected from six boreholes along a 90‐m transect at a subregion of a crude‐oil spill site, the north pool, near Bemidji, Minnesota. The oil saturation data, collected 11 years after the spill, showed an irregularly shaped oil body that appeared to be affected by sediment spatial variability. The particle‐size distribution data were used to estimate the permeability (k) and retention curves for each sample. An additional 344 k estimates were obtained from samples previously collected at the north pool. The 613 k estimates were distributed bimodal lognormally with the two population distributions corresponding to the two predominant lithologies: a coarse glacial outwash deposit and fine‐grained interbedded lenses. A two‐step geostatistical approach was used to generate a conditioned realization of k representing the bimodal heterogeneity. A cross‐sectional multiphase flow model was used to simulate the flow of oil and water in the presence of air along the north pool transect for an 11‐year period. The inclusion of a representation of the bimodal aquifer heterogeneity was crucial for reproduction of general features of the observed oil body. If the bimodal heterogeneity was characterized, hysteresis did not have to be incorporated into the model because a hysteretic effect was produced by the sediment spatial variability. By revising the relative permeability functional relation, an improved reproduction of the observed oil saturation distribution was achieved. The inclusion of water table fluctuations in the model did not significantly affect the simulated oil saturation distribution.
Periodic stationarity conditions for periodic autoregressive moving average processes as eigenvalue problems
Authors: Ula Taylan A. , Smadi Abdullah A.
Published: February 2004

The determination of periodic stationarity conditions for periodic autoregressive moving average (PARMA) processes is a prerequisite to their analysis. Means of obtaining these conditions in analytically simple forms are sought. It is shown that periodic stationarity conditions for univariate and multivariate PARMA processes can always be reduced to eigenvalue problems, which are computationally and analytically easier to deal with. Two different lumpings of the periodic process are considered along this line. The first is the common w‐span lumping over all w periods. The second lumping considered is the p‐span lumping of the pth order periodic autoregressive process over p periods, which is based on a recently introduced lumping technique. It is shown that p‐span lumping may yield the periodic stationarity conditions in an analytically simpler form as compared to w‐span lumping when p < w.
Infiltration from a surface point source and drip irrigation: 1. The midpoint soil water pressure
Authors: Revol P. , Vauclin M. , Vachaud G. , Clothier B. E.
Published: February 2004

Bresler [1978] proposed a procedure for drip irrigation design which is focused on the midpoint soil water pressure hc. We present a practical field test of this approach in order to evaluate the validity of the underlying assumptions. The simulated hc values were obtained from Raats' [1971] steady state theory for 32 points in the field where the hydraulic conductivity parameters Ks and αwere measured. The hc values were measured at the same locations during microirrigation of a maize crop. Measured hc's appear to be lower than the simulated ones, especially late in the season. The measured spatial variability in hc appeared to be higher than the simulated ones. This could well have been caused by root uptake activity, which is not considered in the analysis, as well as by the large but typical drippers spacing of d = 1.00 m. Thus the tensiometers could have been beyond the practical limit of wetting. Consequences for design and management are important. For design, even if a high hc value is chosen, there is no real guarantee that the wetting would be effective at the midpoint. For irrigation management, tensiometer placement too far from the dripper would lead to overirrigation, so for a large dripper spacing d, the midpoint placement is not judicious.
Hydrograph separations in an arctic watershed using mixing model and graphical techniques
Authors: McNamara James P. , Kane Douglas L. , Hinzman Larry D.
Published: February 2004

Storm hydrographs in the Upper Kuparuk River basin (142 km2) in northern Alaska were separated into source components using a mixing model and by recession analysis. In non‐Arctic regions, storm flow is commonly dominated by old water, that is, water that existed in the basin before the storm. We suspected that this may not be true in Arctic regions where permafrost diminishes subsurface storage capacity. Streamflow during the snowmelt period was nearly all new water. However, all summer storms were dominated by old water. Storms in a neighboring basin were dominated by new water but much less than was the snowmelt event. Thus a large increase in old water contributions occurred following the snowmelt period. This increase continued moderately through the summer in 1994 but not in 1995. We credit the seasonal changes in old water contributions to increased subsurface storage capacity due to thawing of the active layer.
A parametric rule for planning and management of multiple‐reservoir systems
Authors: Nalbantis I. , Koutsoyiannis D.
Published: February 2004

A parametric rule for multireservoir system operation is formulated and tested. It is a generalization of the well‐known space rule of simultaneously accounting for various system operating goals, in addition to the standard goal of avoiding unnecessary spills, including avoiding leakage losses, avoiding conveyance problems, taking into account the impacts of the reservoir system topology, and assuring satisfaction of secondary uses. Theoretical values of the rule's parameters for each one of these isolated goals are derived. In practice, parameters are evaluated to optimize one or more objective functions selected by the user. The rule is embedded in a simulation model so that optimization requires repeated simulations of the system operation with specific values of the parameters each time. The rule is tested on the case of the multireservoir water supply system of the city of Athens, Greece, which is driven by all of the operating goals listed above. Two problems at the system design level are tackled. First, the total release from the system is maximized for a selected level of failure probability. Second, the annual operating cost is minimized for given levels of water demand and failure probability. A detailed simulation model is used in the case study. Sensitivity analysis of the rule's parameters revealed a subset of insensitive parameters that allowed for rule simplification. Finally, the rule is validated through comparison with a number of heuristic rules also applied to the test case.

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