bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of electromagnetic wave scattering by ionospheric irregularities
Authors: Ho Antony Y. , Kuo S. P. , Lee M. C.
Published: February 2004

A quasi‐particle approach is used to analyze the effect of bottomside sinusoidal irregularities on the propagation of beacon satellite signals through the ionosphere. In this work, the radio wave is treated as a distribution of quasi‐particles described by a Wigner distribution function, and the irregularities are modeled as a two‐dimensional density modulation on an uniform background plasma. The collision of quasi‐particles with the irregularities modifies the quasi‐particle distribution and gives rise to the wave‐scattering phenomenon. The multiple scattering process is generally considered in this deterministic analysis of radio wave scattering off the ionospheric density irregularities. The analysis shows that this two‐dimensional density grating effectively modulates the intensity of the beacon satellite signals. This spatial modulation of the wave intensity is converted into time modulation due to the drift of the ionospheric irregularities, which then results in the scintillation of the beacon satellite signals. The modulation is due mainly to the transverse (to the wave propagation) grating. The results of the analysis agree well with the existing experimental data on the scintillation of Atmosphere Explorer satellites signal (Basu et al., 1986). The model is further based on to predict the scintillation of radio signals for their long distance propagation. For a fixed scintillation index (S4) the predicted propagation distance (d) versus the wave frequency (ƒ) follows a power law, namely, d∝ƒK, where k ≈ 2.
Propagation characteristics of the ELF emissions observed by the satellite Akebono in the magnetic equatorial region
Authors: Kasahara Yoshiya , Kenmochi Hideki , Kimura Iwane
Published: February 2004

Emissions with their frequencies below 100 Hz are often observed by the Akebono satellite in the vicinity of the geomagnetic equatorial plane. These ELF emissions are classified into two types: One is an ion cyclotron wave below the local proton cyclotron frequency and the other is assumed to be magnetosonic wave observed not only below but also above the local proton cyclotron frequency. The wave normal directions of the latter type of emissions were estimated by using the wave distribution function method. It is found that the emissions are propagating with their wave normal direction nearly perpendicular to the meridian plane. The propagation characteristics of these emissions are also examined by ray tracing including the effects of ions. The ray tracing study clarified that the wave can propagate around the plasmapause because of the trapping effect of density gradient. In this paper we propose that these emissions are generated just outside the plasmapause and are propagating around the plasmapause with their wave normal nearly perpendicular to the geomagnetic field line and to the meridian plane.
Application of the measured equation of invariance to solve scattering problems involving a penetrable medium
Authors: Hong Wei , Liu YaoWu , Mei Kenneth K.
Published: February 2004

The measured equation of invariance (MEI) is a simple technique used to derive finite difference type local equations at mesh boundaries, where the conventional finite difference approach fails (Mei et al., 1992, 1994). Conventionally, finite difference or finite element meshes span from boundary to boundary, or to any surface where an absorbing boundary condition can be simulated. It is demonstrated that the MEI technique can be used to terminate meshes very close to the object boundary and still strictly preserves the sparsity of the finite difference equations. It results in dramatic savings in computing time and memory needs. In an earlier paper (Mei et al., 1992) it was shown that this new method can be applied to general boundary geometries including both convex and concave metal surfaces. In this paper we shall show that the MEI can be applied also to material discontinuities. The method of MEI is based on the postulate that the boundary equations, which govern the scattered fields, are independent of the incident fields. That postulate has been shown to be true for metal scatterers. In a penetrable medium the situation is quite different; in fact, inside the medium the separation of the incident and scattered fields is not a simple matter. Normally, the total field has to be calculated inside the medium, thus violating the postulate. Therefore we cannot use the MEI inside a penetrable medium. The space inside the medium has to be filled by finite difference or finite element meshes. This paper shows how to apply the MEI in this situation and concludes that the CPU time required for the MEI method is comparable to that of the method of moments (MOM), when MOM is based on the boundary integral method, i.e., if the scatterer is homogeneous. However, for inhomogeneous penetrable scatterer the MEI method definitely is preferable to the MOM.
Application of beam simulation to scattering at low grazing angles: 1. Methodology and validation
Authors: Ngo Hoc D. , Rino Charles L.
Published: February 2004

Numerical simulations of rough surface scattering at near‐grazing incidence require very large surfaces (≳500λ). Conventional methods of exact solutions require the inversion of a very large matrix, which can exceed the memory and speed capabilities of even modern supercomputers. The beam simulation method proposed by Saillard and Maystre circumvents this problem by decomposing the large incident beam into narrower subbeams and then synthesizing the large beam by coherent superposition. The radius of these narrower subbeams is determined by the local interaction distance on the surface, which is found to increase with incidence angle, ultimately forcing a single beam in the limit of strict grazing incidence. This paper demonstrates that this technique gives essentially the same results as can be obtained by the method of moments and can handle surfaces as large as 1000λ for grazing incidence angle as low as 10°.
Finite element grid truncation schemes based on the measured equation of invariance
Authors: Wright D. B. , Cangellaris A. C.
Published: February 2004

A new local method for finite difference/finite element (FD/FE) mesh truncation in the frequency domain is investigated. The method is based on the measured equation of invariance (MEI) concept recently proposed for the numerical solution of electromagnetic wave scattering by perfectly conducting targets in unbounded regions. An MEI is a numerically derived discrete, linear equation which relates the field at a given boundary node to the field values at neighboring nodes. The extension of the MEI‐based mesh truncation method to treat the case of penetrable scatterers is discussed. A new approach that employs distributions of multipoles to expedite the generation of the MEIs is proposed. This new approach is compared to an earlier one based on the direct application of Huygen's principle. Numerical results are presented for time harmonic scattering by various two‐dimensional targets. Since the derivation of the MEI is not based on any far‐field assumptions, the mesh truncation condition can be applied just a few cells away from the scatterer's boundary, and a computationally efficient and accurate FD/FE grid truncation can be achieved.
Electromagnetic fields in bone repair and adaptation
Authors: McLeod Kenneth J. , Rubin Clinton T. , Donahue Henry J.
Published: February 2004

The treatment of delayed union of bone fractures has served for the past 20 years as the principal testing ground for determining whether nonionizing electromagnetic fields can have any substantial, long‐term effects in clinical medicine. Recent double‐blinded clinical trials have confirmed the significance of the reported effects on bone healing and have led to the suggestion that electromagnetic fields may also be useful in the treatment of other orthopedic problems such as fresh fractures, stabilization of prosthetic implants, or even the prevention or treatment of osteoporosis. However, the design of appropriate treatment regimens for these new applications would be greatly facilitated if it were understood how the biological cells within bone tissue sense these low‐frequency, and remarkably low level, electromagnetic fields. Here we address the engineering and physical science aspects of this problem. We review the characteristics of clinically used electromagnetic fields and discuss which components of these fields may actually be responsible for altering the activity of the bone cells. We then consider several physical mechanisms which have been proposed to explain how the cells within the bone or fracture tissue detect this field component.
An asymptotic technique for computing the electric field associated with a traveling wave current filament above Earth
Authors: Budihardjo A. , Dvorak S. L. , Mechaik M. M.
Published: February 2004

Asymptotic approximations are obtained for the electric field associated with a traveling wave current filament above Earth. A first‐order asymptotic solution is obtained by applying the steepest descent technique to the direct and reflected fields separately. The first‐order solution is used together with a recurrence relationship to obtain a second‐order asymptotic approximation for the total field. Superposition of the fields due to two semiinfinite wires is used to find the field associated with a finite length traveling wave current filament above Earth. The fields which are computed using the asymptotic approximations are compared with the “true” fields which are obtained by numerical integration of the two‐dimensional Sommerfeld integral. The second‐order asymptotic approximation is found to yield accurate results for all observation points located away from the source.
Local time and seasonal variations in the precipitation of energetic electrons from the inner radiation belt by cyclotron resonance with waves from powerful VLF transmitters
Authors: Datlowe Dayton W. , Imhof William L. , Fishman Gerald J. , Finger Mark H.
Published: February 2004

Powerful ground‐based VLF transmitters resonantly scatter ∼100‐keV electrons from the inner radiation belt onto trajectories from which they precipitate into the atmosphere as they drift eastward. To obtain a better understanding of the underlying wave‐particle mechanisms, we have studied energetic electron data from the Burst and Transient Source Experiment (BATSE) on NASA's Compton Gamma Ray Observatory (GRO) satellite. Wave‐particle interactions are detected when the satellite traverses a cloud of energetic electrons which have been scattered into quasi‐trapped trajectories by VLF waves. The previously reported seasonal variation in the frequency of occurrence of cyclotron resonance interaction is confirmed with the continuous coverage provided by GRO. These data also provide the first detailed measurements of the frequency of occurrence of the quasi‐trapped electron fluxes as a function of local time at the transmitter. The variations are more complex than just a day‐night effect, and the precipitation is most often seen in the predawn hours. As with the seasonal changes, the local time variations are governed by the transmission of VLF waves through the ionosphere.
Scattering of a Gaussian beam by an anisotropic material coated conducting circular cylinder
Authors: Wu Xin Bao , Ren Wei
Published: February 2004

An analytical solution of Gaussian beam scattering by a conducting circular cylinder coated with an anisotropic material is formulated in terms of the wave functions for anisotropic media. This solution is formally similar to the solution of Gaussian beam scattering by a conducting elliptical cylinder with an isotropic dielectric coating. In the anisotropic dielectric coating and in the exterior free space region, the fields are expanded by the newly developed wave functions for anisotropic media and the well‐known wave functions for isotropic media, respectively. Numerical results are presented for the reference purpose. The effects of the spot size and the location of the waist of the beam, and the electrical size of the anisotropic‐coated conducting circular cylinder on the scattered field are discussed.
Like‐ and cross‐polarized transmission scatter cross sections for random rough surfaces: Full wave solutions
Authors: Bahar Ezekiel , Lee Bom Son , Huang Guorong , Kubik Robert D.
Published: February 2004

In this work full wave expressions are derived for the incoherent diffuse scatter cross sections (per unit area) for waves transmitted across rough surfaces. Both the like‐ and cross‐polarized cross sections are considered since the surfaces are rough in two dimensions. The full wave solutions are compared with the small perturbation solutions as well as with the physical/geometrical solutions. It is shown that when the heights and slopes are of the same order of smallness, the full wave solutions are in agreement with the small perturbation solution, while in the high‐frequency limit the full wave solution is in agreement with physical/geometrical optics solutions, provided that specular (stationary phase) points exist on the surface. Several illustrative examples are considered and the refractive index is assumed to be larger or smaller than unity. The critical angle for total internal reflection has a significant impact on diffuse scattering in the nonspecular direction. Since the full wave solutions account for specular point scattering as well as Bragg type scattering in a uniform, self‐consistent manner, it is not necessary to adopt the two‐scale model for the rough surfaces.
High‐frequency ducting in the high‐latitude bottomside F region
Authors: James H. G.
Published: February 2004

High‐frequency radio wave ducting in cylindrical field‐aligned depletions of electron density in the high‐latitude bottomside F region has been explored using a three‐dimensional geometric optics treatment. The ray paths thus found are different from the two‐dimensional rays predicted under the common assumption of vertical propagation through a horizontally stratified density distribution. In addition to the nearly specular reflections where the wave frequency is close to the cutoff frequency, the model predicts oblique reflections with caustics at other locations. The technique has been applied to two ground‐based radio techniques which may be practically affected by ducting at high latitudes: ionospheric modification and bottomside sounding. In ionospheric modification, oblique reflections focus rays, producing plasma nonlinearities at different locations in the irregularity. Ray tracing is used to determine where the European incoherent scatter (EISCAT) ground‐based radars would detect parametric decay instability heating by the EISCAT Heating. The possibility that ducts are responsible for complete traces in ionograms from ground‐based ionosondes has been examined. It is found that ducts can return more or less complete traces from a considerably greater solid angle than what is obtained during reflection from a horizontally stratified F layer. In comparison with the ionogram trace for horizontal stratification, the ducted traces exhibit some broadening (i.e., spread) and are at somewhat greater apparent heights.
The high‐frequency environment at the ROTHR Amchitka radar site
Authors: McNeal George D.
Published: February 2004

The successful performance of a high‐frequency (HF) over‐the‐horizon radar (OTHR) is dependent upon the environment in which the radar must operate. This study investigates measured power levels of noise and signal characteristics in the HF environment of an operational OTHR system located in Amchitka, Alaska. These measurements were acquired by the Navy's AN/TPS‐71 radar (ROTHR) spectrum monitor and represent a new source of HF environmental measurements for noise characteristics, channel availability, and expected duration of various channel widths as a function of frequency, time of day, and season of year. The analysis and results presented are based on HF environmental measurements obtained over the 5–28 MHz frequency band by a receiver with a very low noise figure, an efficient antenna, high dynamic range, and which was seldom internally noise limited. The analysis of this data has provided information on power levels associated with noise and HF users in the geographic region of the radar. Clear channel availability results are based on a threshold level defined by empirical distribution functions (EDF) of the measured power levels from the ROTHR site in Amchitka, Alaska.
A Fourier domain technique for ionospheric tomography
Authors: Na H. , Shen J. , Lee H.
Published: February 2004

Ionospheric tomography is an emerging technique for two‐dimensional reconstruction of the electron density distribution in the ionosphere. One of the fundamental difficulties with this technique, currently, is the limited resolving capability caused by the physical constraints of the data acquisition scheme. The angles of the signal propagation paths are restricted to a certain angular coverage because each ground station can only make observations over a limited range. As a result, the corresponding projection angles of the data are severely limited. This effect creates a cone‐shaped region in the spatial‐frequency domain, where spectral content is not available. Because of the incomplete information content of the data, uniqueness of the solution to the inverse problem does not exist. In this paper a new Fourier domain extrapolation technique is presented which utilizes the information contained in the known region of the spectrum to estimate the frequency samples in the missing cone area. Unlike conventional two‐dimensional extrapolation techniques, this method uses bidirectional extrapolation to maximize the amount of known information in each extrapolation step. By fully utilizing the information of the measured data, the resulting image provides a more accurate estimation of the electron density distribution.
Subdaily northern hemisphere ionospheric maps using an extensive network of GPS receivers
Authors: Wilson Brian D. , Mannucci Anthony J. , Edwards Charles D.
Published: February 2004

Ionospheric total electron content (TEC) data derived from dual‐frequency Global Positioning System (GPS) signals from 30 globally distributed network sites are fit to a simple ionospheric shell model, yielding a map of the ionosphere in the northern hemisphere every 12 hours during the January 1–15, 1993 period, as well as values for the satellite and receiver instrumental biases. Root‐mean‐square (RMS) residuals of 2–3 TEC units are observed over the 20°–80° latitude band. Various systematic errors affecting the TEC estimates are discussed. The capability of using these global maps to produce ionospheric calibrations for sites at which no GPS data are available is also investigated.
Ionospheric model validation at VLF and LF
Authors: Ferguson Jerry A.
Published: February 2004

A reliable knowledge of radio signal amplitude and phase characteristics is required to design and maintain communications and navigational circuits at VLF and LF. The ability to accurately calculate signal levels as a function of frequency, position, and time is of considerable importance in achieving reliable assessment of communication coverage. Detailed computer models based on multiple mode waveguide theory have been developed. These models have been found to produce good comparisons between measurements and calculations of signal variations as a function of propagation distance. However, results can be very sensitive to the ionospheric inputs to these computer models. This paper has two purposes. The first is to present the results of a systematic comparison of a set of measurements of signal strength from various transmitters over a number of propagation paths using a simple model of the ionosphere. The variation of the parameters of this simple model with basic propagation parameters is examined. The second purpose is to examine the built‐in version of this simple model of the ionosphere as implemented in the Long Wave Propagation Capability. This model is found to adequately represent a set of in‐flight signal strength measurements. It is also clear that there is still room for improvements in this ionospheric model.
Large‐scale deformation related to the collision of the Aleutian Arc with Kamchatka
Authors: Geist Eric L. , Scholl David W.
Published: February 2004

The far western Aleutian Island Arc is actively colliding with Kamchatka. Westward motion of the Aleutian Arc is brought about by the tangential relative motion of the Pacific plate transferred to major, right‐lateral shear zones north and south of the arc. Early geologic mapping of Cape Kamchatka (a promontory of Kamchatka along strike with the Aleutian Arc) revealed many similarities to the geology of the Aleutian Islands. Later studies support the notion that Cape Kamchatka is the farthest west Aleutian “island” and that it has been accreted to Kamchatka by the process of arc‐continent collision. Deformation associated with the collision onshore Kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rocks directly west of Cape Kamchatka. The trend of the thrust faults is concave toward the collision zone, indicating a radial distribution of maximum horizontal compressive stress. Offshore, major crustal faults trend either oblique to the Kamchatka margin or parallel to major Aleutian shear zones. These offshore faults are complex, accommodating both strike‐slip and thrust displacements as documented by focal mechanisms and seismic reflection data. Earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost Aleutian shear zone and to the west by an apparent aseismic front. Analysis of focal mechanisms in the region indicate that the present‐day arc‐continent “contact zone” is located directly east of Cape Kamchatka. In modeling the dynamics of the collision zone using thin viscous sheet theory, the rheological parameters are only partially constrained to values of n (the effective power law exponent) ≥ 3 and Ar (the Argand number) ≤ 30. These values are consistent with a forearc thermal profile of Kamchatka, previously determined from heat flow modeling. The thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence, not only of compressive stresses resulting from the west directed collision, but also of sediment‐induced coupling of the subducting Pacific plate.
Thermochronologic constraints on post‐Paleozoic tectonic evolution of the central Transantarctic Mountains, Antarctica
Authors: Fitzgerald Paul G.
Published: February 2004

Built upon the roots of a compressive orogenic belt of late Proterozoic‐early Paleozoic age and once adjacent to North America, the present‐day Transantarctic Mountains (TAM) represent a rift flank, resulting from episodic uplift in the Cretaceous and Cenozoic. Fault blocks are discernible in present‐day topography and subglacial morphology. Fission track results give information on differential block movement (uplift and denudation) and are important in constraining models for the uplift of the range. Apatite fission track thermochronology on samples collected from the central TAM record a complex thermotectonic history for this region over the past 350 m.y. Apatite ages in the Miller Range vary from ∼250 to ∼350 Ma and are from an exhumed apatite partial annealing zone formed following cooling of Cambro‐Ordovician granitoids. A period of Cretaceous denudation (≲2 km), beginning at ∼115 Ma, is recorded at Moody Nunatak on the inland side of the TAM. Near the coast, samples along the Beardmore Glacier record rapid cooling indicative of denudation initiated in the early Cenozoic (∼50 Ma). The amount of uplift ∼70 km inland of the coast in the Queen Alexandra Range since the early Cenozoic is ∼7 km, with the likelihood of an additional ∼3 km at the coast. Eastward facing topographic escarpments in the Queen Alexandra Range mark the likely position of steeply dipping normal faults, which offset the apatite ages. Apatite ages on the east side of the Beardmore Glacier mouth are generally younger (average 27 Ma) than on the west side (average 33 Ma), reflecting greater denudation. Assumptions made regarding the use of an assumed paleogeothermal gradient are tested with available geologic evidence. The fission track data neither conflict with nor confirm paleobotanical evidence from the Sirius Group in the central TAM which suggests significant surface uplift (2–3 km) of the TAM since the Pliocene. Results build upon the available fission track database along the TAM and emphasize the subtle variability of uplift along the TAM due to episodic uplift involving differential block movements.
Roles of plutonism, midcrustal flow, tectonic rafting, and horizontal collapse in shaping the Miocene strain field of the Lake Mead area, Nevada and Arizona
Authors: Anderson R. Ernest , Barnhard Theodore P. , Snee Lawrence W.
Published: February 2004

The Lake Mead area contains a well‐exposed record of highly contrasting Neogene structural development that includes major magmatism and extension in its southern part and large‐magnitude lateral translations (to 65 km) of amagmatic structural blocks in its northern part. These highly contrasting parts are joined at the Hamblin Bay fault, a major crustal boundary that forms the south margin of the Lake Mead fault system and the north margin of the lower Colorado River extensional corridor. Intrusive rocks adjacent to this crustal boundary on the south record major crustal rifting normal to an 350° axis. They also display steep‐axis disharmonic folds that formed by post‐magmatic extension‐related horizontal collapse of highly weakened crust. The rocks directly north of the crustal boundary not only display large lateral translations, but they also are cut by fault systems suggestive of diverse paleostress conditions and formed into diversely oriented coeval folds collectively producing a complex Neogene deformation field that defies interpretation in terms of a uniform system of remote stresses. Because structures forming this complex deformation field range from outcrop scale to 65 km of lateral displacement and can not be uniformly partitioned according to relative age, we prefer tectonic models based on a protracted uniform dynamic process or super‐posed dynamic processes to models based on time‐varying states of stress. We suggest that the large lateral displacements and extreme structural complexity record synextensional rafting of structural blocks atop a flowing undermass and concomitant contraction of the zone of flowage. Structural blocks north of the main zone of flowage (the Great Basin sector of the Basin and Range) moved southward and occluded to the block south of the zone (the Colorado River extensional corridor) as a large west‐ward widening wedge of upper crust was carried to the west–southwest toward California on the flowing undermass. Where occlusion is almost complete in the northern Black Mountains, north–south shortening is estimated at 55 km. The narrow zone of strong rifting, plutonism, crustal softening, and horizontal collapse that developed at the north margin of the Colorado River extensional corridor is interpreted to result from contact with the zone of crustal flowage.
Tectonics of short‐lived intra‐arc basins in the arc‐continent collision terrane of the Coastal Range, eastern Taiwan
Authors: Huang Chi‐Yue , Yuan Peter B. , Song Sheng‐Rong , Lin Ching‐Weei , Wang Chengsung , Chen Min‐Te , Shyu Chen‐Tien , Karp Boris
Published: February 2004

The Coastal Range in eastern Taiwan was originated from an oblique collision between the Luzon volcanic arc and Asian continent since the late Neogene. In this collision terrane, two intra‐arc basins, the Pliocene Chingpu and Pleistocene Chengkung basins, were developed on the eastern part of the Neogene Chimei and Chengkuangao volcanic islands, respectively, prior to their accretion to eastern Taiwan. The tectonic evolution of these Neogene volcanic islands and associated intra‐arc basins is reconstructed by stratigraphic and sedimentological analysis, igneous rock geochemistry, and comparison with observations in modern collision zone in the regions off southeastern Taiwan. In the Coastal Range, the intra‐arc basin sequences are 1.5–10 km wide and 40 km long, comparable in size to their modern analogues in the active collision zone. The basin axis trends subparallel to the volcanic ridge. In both basins, deepwater flysch overlies shallow marine reef carbonates, which in turn rest on volcanic basement, indicating rapid arc collapse (minimum rate of 1 km/m.y.) soon after the arc‐continent collision. The arc collapse occurred earlier in the north (Chimei, between 5.1 and 3.5 Ma) and later in the south (Chengkuangao, between 2.9 and 1.8 Ma), in concert with a southward propagation of the oblique collision. Sedimentation ended about 2 Ma and 1 Ma in the Chingpu and Chengkung basins, respectively, coeval with rotation of the Neogene volcanic islands. This suggests that the rotation inverted the intra‐arc basin into thrusting, uplifting, and final emergence. Thus the duration of sedimentation for the intra‐arc basins spanned only about 0.8–3.1 m.y. On the basis of land geology, offshore observations, and a clay model experiment simulating oblique arc‐continent collision, a model for the intra‐arc basin evolution in eastern Taiwan is proposed. During the collision, strike‐slip faults would have been developed in the eastern part of volcanic islands to induce transtension movements, thus forming pull‐apart, intra‐arc basins on the collapsed volcanic island. This mechanism is believed to be responsible for the formation of the Pliocene Chingpu and Pleistocene Chengkung basins as well as the present‐day offshore intra‐arc basins found on the Lutao and Lanhsu volcanic islands. The two intra‐arc basins on Lutao and Lanhsu are predicted to be short lived. As collision continues, these two basins, together with their underlying northern part of the Luzon arc, will be rotated, thrust, and uplifted in the next 1 m.y. and, finally, will become part of the southern extension of the Coastal Range.
One‐dimensional infiltration with moving finite elements and improved soil water diffusivity
Authors: Cox Christopher L. , Jones Walter F. , Quisenberry Virgil L. , Yo Frans
Published: February 2004

A problem of significant interest to environmental scientists is the flow of water and solutes through the vadose zone. The partial differential equations which govern this flow are typically time‐dependent and nonlinear. Valid solutions to these equations require (1) accurate relationships between various coefficients and variables on which they depend (e.g., coefficient of diffusivity and water content) and (2) sophisticated numerical methods which can handle complexities such as sharp moving fronts. In cases where coefficients are not known explicitly, curve‐fitting techniques are needed to smooth out scattered experimental data. Nonlinear coefficients can then be calculated. A constrained least squares spline fit is compared to empirical function fits which have appeared recently. Then, a state‐of‐the‐art numerical technique is used to accurately model transient flow through unsaturated homogeneous soils. The moving finite element method of Miller and colleagues is an adaptive approach in the sense that the grid moves so that nodes are concentrated where they are most needed. As a result, better accuracy is achieved with fewer nodes than are required for standard fixed‐grid methods. Petzold's robust Gear‐type solver DASSL is used for time‐integration. Numerical results are compared to experimental data. Mass balance errors are neglible, and accurate solutions are obtained at all time steps. Though only one‐dimensional problems are considered here, the numerical approach generalizes to heterogeneous media and problems in higher dimensions.
A finite element model for simulating runoff and soil erosion from mechanically treated agricultural lands: 1. Governing equations and solutions
Authors: Sharda V. N. , Singh Sita Ram
Published: February 2004

A finite element model simulating runoff and soil erosion from agricultural lands is developed. The computational efficiency and stability of various numerical schemes used for time integration are critically examined employing L2 and Chebycheff (Ghebyshev) norms. Predictor‐corrector and fully implicit schemes are found to give the least values of norms, thereby permitting larger time steps. A finite element solution of the one‐dimensional Richards equation with a sink term simulates rain infiltration and soil moisture balance in cropped fields. A criterion to ensure stability and convergence of the solution is suggested. A finite element solution of the sediment continuity equation in conjunction with a fully implicit scheme for time integration and Yalin's equation for sediment transport capacity is developed to simulate soil erosion. The potential of the model to reasonably simulate runoff and soil erosion is demonstrated by comparing the finite element solutions with the analytical solutions under simplified configurations and with experimental data.
Hybrid finite analytic solution of lateral subsurface flow
Authors: Pi Zhanzhong , Hjelmfelt Allen T.
Published: February 2004

A hybrid finite analytic method, a numerical technique, was used to solve lateral subsurface storm flow described by the extended Dupuit‐Forchheimer equation. In subregions the local linearized one‐dimensional Dupuit‐Forchheimer equation was solved analytically in space and discretized in time by a simple difference formula. This is called a hybrid finite analytic approach. The system of algebraic equations obtained by the hybrid finite analytic approach can approximately preserve the overall nonlinear effect because the term (∂H/∂X)2 and the coefficient of diffusion are treated as constants only in the local regions. A four‐point numerical formula can provide results that are stable and acceptably accurate without complex calculation and without small time steps. The comparison between steady state profiles of the water table provided by this study with profiles provided by previous investigations shows that the method performs very well in the analysis of lateral subsurface storm flow.
Simulation of streamflow in a macroscale watershed using general circulation model data
Authors: Kite G. W. , Dalton A. , Dion K.
Published: February 2004

General circulation models (GCMs) currently perform vertical water and energy balances at 20‐ or 30‐min time intervals for grid points 2°–4° apart but generally contain no information on the land‐phase transfer of water between grid points or within watersheds. As a result, they operate on an incomplete hydrological cycle. This study combines a hydrological model with a GCM for a macroscale watershed. A water balance was carried out at 12‐hour time intervals for a 10‐year period using the Canadian Climate Centre GCM II data set for grid points within and surrounding the 1.6×106 km2 Mackenzie River Basin in northwestern Canada. The water surpluses from each relevant grid point were accumulated to provide a simulated hydrograph at the outlet of the Mackenzie River. A hydrological model was calibrated and verified using 5 years of recorded climatological and hydrometric data as well as land cover data from classified National Oceanic and Atmospheric Administration advanced very high resolution radiometer images. The climatological outputs from the GCM (precipitation, temperature, and evaporation) were then used as inputs to the hydrological model, generating a second hydrograph for the Mackenzie River. The results show that using the hydrological model with the GCM data produces a better representation of the recorded flow regime. The study provides a means of verifying the performance of the GCM and is a first step in developing a continental‐scale hydrological model which will, ultimately, form a part of a full model of the global hydrological cycle.
New perspective on the convection‐diffusion‐dispersion equation
Authors: Ponce V. M. , Huston P. T.
Published: February 2004

The coefficients of the dimensionless partial differential equation of convection‐diffusion‐dispersion of flood waves are shown to be functions of the Froude and Vedernikov numbers only. The Froude number is the ratio of mean velocity to relative dynamic wave celerity. The Vedernikov number is the ratio of relative kinematic wave celerity to relative dynamic wave celerity. The third‐order convection‐diffusion‐dispersion equation can be used to analyze flood propagation problems where both diffusion and dispersion are deemed to be significant.
Gravel Saltation: 2. Modeling
Authors: Niño Yarko , García Marcelo
Published: February 2004

A Lagrangian model for the motion of a sediment particle in a water flow is proposed. The particle motion equation is averaged over flow turbulence, specialized to the case of coarse sediment particles saltating in water, and coupled with a stochastic model for particle collision with the bed. Model predictions of statistics of gravel saltation generally agree well with experimental observations. As an application of the saltation model, bed load transport rates are estimated from modeled mean saltation streamwise velocity and a dynamic friction coefficient. The results obtained compare favorably with typical bed load equations, but they overestimate experimental measurements of gravel transport, which would suggest that a Bagnoldean formulation for bed load transport of coarse material by saltating particles may not be valid.
Planning versus competitive rates of groundwater pumping
Authors: Brill Thomas C. , Burness H. Stuart
Published: February 2004

Groundwater pumping from a common property aquifer leads to pumping cost externalities which imply a divergence between competitive and socially optimal rates of groundwater pumping. Gisser and Sanchez (1980) show in a specific context that this qualitative distinction is quantitatively negligible. However, this result seems to defy the common consensus that in many aquifers, depletion will occur prematurely. Given these observations, serious questions arise concerning the role and scope of water management policy. We further explore this issue under alternative hydrologic/economic hypotheses and note scenarios under which divergence of competitive and socially optimal rates of water pumping is significant. We find that divergence increases with demand growth, declining well yields, and low social discount rates. The role and scope of policy are then inferred.
Similarity analysis of runoff generation processes in real‐world catchments
Authors: Larsen Jens E. , Sivapalan Murugesu , Coles Neil A. , Linnet Per E.
Published: February 2004

This paper addresses the question of similarity of runoff generation processes between catchments in the eastern wheat belt of Western Australia, and the use of dimensionless parameterizations to quantify this similarity. A spatially distributed rainfall‐runoff model, simulating runoff generation by both the infiltration excess (Horton type) and saturation excess (Dunne type) mechanisms, was developed for catchments in the region. Seven small experimental catchments, with field‐measured soil hydraulic properties and topography, were used in the study. Following on from the similarity theory developed by Sivapalan et al. (1987), a number of dimensionless similarity parameters were constructed using the field‐measured soil and topographic properties, a characteristic length scale, and a characteristic flow velocity. The objective was to determine whether the dominant runoff generation mechanism on a catchment could be reliably predicted by these similarity parameters. This was achieved through sensitivity analyses carried out with the rainfall‐runoff model. Two dimensionless parameters, K0* and ƒ*, were found to be critical for characterizing the similarity or dissimilarity of the runoff generation responses between the seven experimental catchments. Within the assumptions of the analysis, two catchments in the wheat belt region can be considered to be hydrologically similar, in terms of their runoff responses, if K0* and ƒ* are identical in both catchments. The dominant mechanism of runoff generation on any catchment can be reliably predicted, provided that the values of K0* and ƒ* are known. A partial quantification of the Dunne diagram (Dunne, 1978) for the wheat belt region, in terms of the infiltration excess and saturation excess mechanisms, was achieved by artificially varying K0* and ƒ* in the rainfall‐runoff model to explore the full range of possible runoff generation responses.
Determining the range of predictions of a groundwater model which arises from alternative calibrations
Authors: Brooks Roger J. , Lerner David N. , Tobias Andrew M.
Published: February 2004

A major element in constructing a groundwater model is choosing the parameter values. The traditional approach is to aim for a single best set of values. The parameters used in a model are effective rather than measurable, and this combined with the inherent uncertainties in the modeling process means that there are often many plausible sets of values. A single prediction obtained from a single set of parameter values is not appropriate, but rather the range in predictions from the alternative calibrations should be used. A method is presented for finding the best case and worst case predictions among the plausible parameter sets and is applied to a real case study. Widely different feasible parameter sets were found giving significantly different predictions.
Equivalent steady soil moisture profile and the time compression approximation in water balance modeling
Authors: Salvucci Guido Daniel , Entekhabi Dara
Published: February 2004

The definition of preevent soil moisture profile and time compression analysis are critical components in water balance models that are based on realistic infiltration/exfiltration relations and include profile redistribution of vadose zone moisture. In this paper, detailed analysis of these two fundamental components of water balance modeling is presented. Numerical integration of the governing equations for liquid moisture flow in the unsaturated zone is used in simulations designed to illustrate the role of temporal variability in the system. The simulations consist of forcing the surface of a one‐dimensional soil column, bounded at its base by a fixed water table, with the output of a stochastic event‐based model of precipitation and potential evaporation. The simulations are run until (and beyond when) an equilibrium condition is reached between the long‐term mean values of surface and bottom fluxes. For situations with a deep water table, two distinct zones develop: a near‐surface, highly unsteady zone, and a deeper, quasi‐steady zone. The equivalent steady moisture profile, i.e., the steady profile corresponding to the mean of the simulated column flow, is found to reasonably approximate the temporal mean, mean prestorm, and mean poststorm moisture profiles, particularly in the deeper zone. In the upper zone the equivalent steady profile forms a biased estimate of the temporal mean. The bias is shown, through perturbation analysis, to lead to overestimation of the wetness of the mean moisture profile. For shallow water tables the distinction between the two zones collapses. In this case both the equivalent steady profile and the mean profile are close to the hydrostatic profile. The simulations are also used to test the utility of the time compression approximation (TCA) in modeling surface fluxes under temporally variable initial conditions. We demonstrate that the use of the mean prestorm and poststorm event moisture profiles as initial conditions for the infiltration and exfiltration flux capacities does not produce major bias in TCA‐predicted surface fluxes. In this sense the TCA behaves as a linear operator. Furthermore, we demonstrate that the estimation of these mean initial condition profiles by the equivalent steady state soil moisture profile is an adequate approximation for determining mean land surface response to event‐based atmospheric forcing.
Modeling infiltration during complex rainfall sequences
Authors: Corradini Corrado , Melone Florisa , Smith Roger E.
Published: February 2004

An extension of the conceptual model earlier developed by Smith et al. (1993) is presented. Their basic model considered the problem of point infiltration during a storm consisting of two parts separated by a rainfall hiatus, with surface saturation and runoff occurring in each part. The model is here extended toward further generality, including the representation of a sequence of infiltration‐redistribution cycles with situations not leading to soil surface saturation, and rainfall periods of intensity less than the soil infiltration capacity. The model employs at most a two‐part profile for simulating the actual one. When the surface flux is not at capacity, it uses a slightly modified version of the Parlange et al. (1985) model for description of increases in the surface water content and the Smith et al. (1993) redistribution equation for decreases. Criteria for the development of compound profiles and for their reduction to single profiles are also incorporated. The extended model is tested by comparison with numerical solutions of Richards's equation, carried out for a variety of experiments upon two contrasting soils. The model applications yield very accurate results and support its use as part of a watershed hydrologic model.
Water movement in glass bead porous media: 1. Experiments of capillary rise and hysteresis
Authors: Lu T. X. , Biggar J. W. , Nielsen D. R.
Published: February 2004

Experimental observations of capillary rise and hysteresis of water or ethanol in glass beads are presented to improve our understanding of those physical processes in porous media. The results provide evidence that capillary rise into porous media cannot be fully explained by a model of cylinders. They further demonstrate that the “Ink bottle” model does not provide an adequate explanation of hysteresis. Glass beads serving as a model for ideal soil are enclosed in a rectangular glass chamber model. A TV camera associated with a microscope was used to record the processes of capillary rise and drainage. It is clearly shown during capillary rise that the fluid exhibits a “jump” behavior at the neck of the pores in an initially dry profile or at the bottom of the water film in an initially wet profile. Under an initially dry condition, the jump initiates at the particle with smallest diameter. The jump process continues to higher elevations until at equilibrium the surface tensile force is balanced by the hydrostatic force. The wetting front at that time is readily observed as flat and saturated. Under an initially wet condition, capillary rise occurs as a water film thickening process associated with the jump process. Trapped air behind the wetting front renders the wetting front irregular and unsaturated. The capillary rise into an initially wet porous medium can be higher than that into an initially dry profile. During the drying process, large surface areas associated with the gas‐liquid interface develop, allowing the porous medium to retain more water than during the wetting process at the same pressure. That mechanism explains better the hysteresis phenomenon in porous media in contrast to other mechanisms that now prevail.