bluuebunny/crossref_metadata_2025 huggingface dataset:
The angular correlation function of waves scattered from randomly distributed cylinders
Authors: Kuga Yasuo , Zhang Guifu , Yea Ji‐Hae , Ishimaru Akira
Published: February 2004

The correlation effect of waves scattered from random media has attracted considerable interest in recent years. In particular, the angular correlation function (ACF) of waves scattered from rough surfaces and volumes has been studied extensively, and the characteristics of the angular memory effect (AME), which was first described by Feng et al. [1988], were reported for different surface conditions. Applications of AME, including height profiling in interferometric synthetic aperture radar (INSAR) and detection of buried objects, have been proposed and studied by researchers using numerical and experimental techniques. However, most of the recent studies of AME have been confined to the area of waves scattered from rough surfaces. In this paper we will investigate the angular memory signature of volume scatterers. Numerical and experimental studies on the ACF of waves scattered from randomly distributed cylinders are presented at millimeter‐wave frequencies (80–105 GHz). It has been found that the angular width of ACF for volume scattering is narrower and shorter than that for random rough surfaces.
Comparison between numerical and experimental near‐field evaluation of a DCS1800 mobile telephone
Authors: Bahr Achim , Pan Sheng‐Gen , Beck Thomas , Kästle Ralf , Schmid Thomas , Kuster Niels
Published: February 2004

The objective of this study was to compare the results of the two most commonly used techniques for near‐field and specific absorption rate (SAR) evaluations. The comparison was performed by independently assessing the near‐field of a commercially available DCS1800 cellular phone in two different laboratories. The measurements were conducted at the Swiss Federal Institute of Technology with the near‐field scanner DASY2 equipped with specialized near‐field E and H field probes, while the numerical evaluation was performed using the finite difference time domain technique at the Institute for Mobile and Satellite Communications. The close near‐fields in free space, inside a flat phantom filled with tissue simulating liquid and inside complex and homogeneous head phantoms, were compared. The quantitative results of most test configurations tallied well. Although the current distribution of the simulation was only slightly different from that of the physical phone, differences of the spatial peak absorption were greater than > 2 dB in some positions of the phone with respect to the scatterer. This demonstrates the great sensitivity of the spatial peak SAR on the current distribution on the device both in terms of magnitude and geometrical distributions. For numerical compliance tests this places high requirements on the numerical phone model with respect to the accuracy of representing the near‐field RF characteristics of the phone.
Three‐dimensional computerized ionospheric tomography using volumetric constraints
Authors: Biswas Chaitali , Na Helen
Published: February 2004

A new algorithm for the reconstruction of three‐dimensional images of ionospheric electron density using volumetric a priori information is presented in this paper. This method relies upon the total electron content (TEC) data measured by an ionospheric tomography system to derive necessary distribution‐related information from a priori ionospheric images for projection domain and image domain volumetric correction processes. It involves extraction of shape information from a priori ionospheric images on an extremely localized basis to simultaneously satisfy measured TEC data and the need for coherent, convergent reconstruction guidance, necessary for the limited angle tomography system. The algorithm uses three‐dimensional polar geometry for computerized ionospheric tomography (CIT) which permits direct use of slant TEC data without the interpolations that are necessary for conventional two‐dimensional imaging of the ionosphere. Reconstructions based on data recorded in two recent CIT campaigns are also presented.
Applying wire antenna theory to cored search coils
Authors: Kaplan B. Z. , Suissa U.
Published: February 2004

It seems conceivable that the magnetic field pattern around ferromagnetic objects of high permeability is similar to the electric field pattern around conductive objects, provided that the shapes of the magnetic and electric objects are identical. As a result, we expect the mmf behavior in the gap between two ferromagnetic limbs immersed in a magnetic field to be similar to the open‐circuit voltage of a short electric dipole antenna immersed in a related electric field. Experimental setups have been developed in order to demonstrate the affinity between the electric and magnetic cases exists both in uniform and nonuniform surrounding field (electric and magnetic). The experimental results conform closely with the results obtained by the theoretical evaluation. We have thus discovered that the true counterpart of a short electric dipole is a geometrically similar magnetic structure, where ferromagnetic limbs of high permeability replace the conductive limbs of the short dipole. This is contrary to the majority of the literature on the subject, which regards a small wire loop as consisting of the entity dual to a short electric dipole. The present results enable us to treat practical magnetic sensors for ELF and ULF in terms of electric antenna theory. It is also shown that the present developments are of value in interpreting the results of measurements performed by employing ELF and ULF cored search coils in probing nonuniform magnetic fields.
Millimeter‐wave radiometry of deciduous trees at low‐depression angle
Authors: Wikner David , Dahlstrom Robert
Published: February 2004

Personnel from the Army Research Laboratory conducted experiments during the summer and early fall of 1996 to measure the low‐depression angle brightness temperatures of deciduous trees at 94 GHz. Changes in the signatures of five different deciduous trees were recorded over several months and related to infrared temperature, sky temperature, and leaf water content. Distinct differences were found in the brightness temperatures of the deciduous trees measured. The results of these measurements are presented and a good parametric fit to the data is made, showing the consistency of canopy emissivity and reflectivity over the measurement period. In addition, suggestions are made as to how future measurements could benefit from wideband, bistatic canopy cross‐section data.
Mutual coupling effects on antenna radiation pattern: An experimental study applied to interferometric radiometers
Authors: Camps A. , Torres F. , Corbella I. , Bará J. , de Paco P.
Published: February 2004

Antenna pattern mismatches are one of the most important error sources in planned Earth‐observation interferometric radiometers. From a low Earth orbit, the wide field of view, about ±30°, leads to the use of antennas with a large beam. In addition, antennas must be closely spaced to avoid, or at least minimize, aliasing effects in the formation of the synthetic brightness temperature images. The accuracy demanded of these systems requires the precise knowledge of all the antenna radiation voltage patterns (amplitude and phase), which may differ from their theoretical values due to mechanical and electrical tolerances in the manufacturing process and which can change due to the proximity of other structures, i.e., other antennas of the array or the mechanical support. Two approaches are found in the literature to interpret the impact of antenna mutual coupling on the performance of an interferometric radiometer: (1) a modification of the antenna voltage pattern and (2) a mixing of the cross correlations measured between the signals collected by the antennas. The main contribution of the present work is a detailed theoretical analysis of the impact of mutual coupling effects showing the equivalence between both approaches. Theoretical results are corroborated with a set of experimental measurements with two kinds of antennas. Theoretical and experimental results can be used in the design of the antennas of interferometric radiometers in order to predict the impact of mutual coupling on the system's performance and point out the importance of an accurate antenna pattern characterization.
Birth and decay of an intracontinental magmatic swell: Early Cretaceous tectonics of southern Israel
Authors: Gvirtzman Zohar , Garfunkel Zvi , Gvirtzman Gedaliahu
Published: February 2004

The Early Cretaceous history of southern Israel is an example of the formation, growth, and decay of an intracontinental swell associated with magmatism. In this paper we document and quantify this process and analyze its geodynamic mechanism. The magmatism and the uplifting continued for about 10 Myr and produced a 150 to 200‐km‐wide dome, from whose crest nearly 1000 m of sediments were removed. Shortly after the magmatism ended, a period of cooling and subsidence began, and sediments started to cover the swell's margins. However, this subsidence was much smaller than the initial uplift and did not bury the swell's crest, which remained elevated for an additional 10 Myr. The fact that most of the uplift remained indicates that the major cause for uplifting was the permanent buoyancy effect of intrusions. Transient effects such as heating at the base of the lithosphere and dynamic support of the asthenosphere may have also occurred, but their contribution to the local uplifting was minor. The distinction between temporal and permanent effects is possible because of the long‐term history provided by the stratigraphic record (a full life cycle of a fossil swell). In currently active swells this distinction is much more complicated, and from this point of view this case provides unique information. The tectonostratigraphic analysis we present illustrates how geological observations can be used to calculate tectonic uplift and to estimate accordingly the volume of magmatism that produced it. Taking into account the sea level rise during uplift and considering the topographic relief that remained after the erosion, the calculated tectonic uplift amounts to about 500 m or 40% higher than the uplift inferred only from the amount of erosion. Thermal‐mechanical modeling indicates that this uplift can be explained by intrusions with a volume equivalent to a 4 to 5‐km‐thick sill, consistent with other geological and geophysical observations.
Crustal collapse, mantle upwelling, and Cenozoic extension in the North American Cordillera
Authors: Liu Mian , Shen Yunqing
Published: February 2004

Gravitational collapse has been suggested as the major cause of Cenozoic extension in the North American Cordillera and many other orogenic belts. Although both crustal thickening and mantle upwelling may have contributed to the Cordilleran extension, previous models of gravitational collapse have focused on the former; the cause of mantle upwelling and its relationship to crustal collapse remain obscure. Here we attempt to address the question of whether gravitational collapse of an overthickened crust could induce major mantle upwelling and whole‐lithosphere extension. Thermal‐rheological calculations indicate that crustal collapse may decouple from the mantle lithosphere, because the extensional forces arising from an overthickened crust are limited to the crust, while the rheology of continental lithosphere is intrinsically stratified. Even when the mantle lithosphere is mechanically coupled to the crust, thermomechanical modeling indicates that strain is localized in the weak lower crust during crustal collapse, and no significant (<10 km) thinning of the mantle lithosphere may be induced at the absence of extensional forces from plate boundaries. Crustal collapse of the Sevier‐Laramide orogen seems adequate to account for much of the mid‐Tertiary extension in the Cordillera, including formation of many core complexes, but it is unlikely to have been the major cause of the more recent basin‐and‐range extension. We suggest that a strong pulse of mantle upwelling in the mid‐Tertiary, as indicated by the “ignimbrite flare‐up,” may have triggered basin‐and‐range extension by weakening the lithosphere and providing excess gravitational potential energy. The cause of mantle upwelling remains uncertain, but the continued extension and volcanism since mid‐Miocene in the northern Basin and Range province favor an active mantle upwelling with internal convective heating.
Crustal structure of the Himalayan orogen at ∼90° east longitude from Project INDEPTH deep reflection profiles
Authors: Hauck M. L. , Nelson K. D. , Brown L. D. , Zhao Wenjin , Ross A. R.
Published: February 2004

International Deep Profiling of Tibet and the Himalaya (INDEPTH) deep seismic reflection profiles show that the Indian lithosphere is underthrusting the central Himalaya along a gently north dipping decollement that is traceable northward beneath the Tethyan belt to ∼28.6°N and to a depth of about 45 km. The decollement carries in its hanging wall a near‐crustal‐thickness slice of internally deformed Indian continental basement and cover represented in outcrop by the Greater Himalayan belt and structurally overlying Tethyan belt. Geometric relationships suggest that the decollement probably ramps downward beneath the northern Tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from Indian mantle lithosphere that presently underlies southern Tibet to the north. The North Himalayan anticlinorium (Kangmar dome) appears to be a large duplex ramp anticline in the hanging wall of the decollement. The South Tibetan Detachment appears to have been imaged in two areas. In the Wagye La area it appears to follow the Tethyan belt/Greater Himalayan belt contact northward in the subsurface and to have been folded over the Kangmar dome. In the Zherger La area it appears to have been active relatively recently as a ∼30° north dipping normal fault that cuts deeply into the Greater Himalayan belt allochthon. Palinspastic reconstruction indicates a minimum of about 326 km of shortening of the Indian basement across the Himalaya since the initiation of the Main Central Thrust. Hence Indian continental lithosphere, largely stripped of its overlying crust, can extend northward beneath the Tibetan plateau to at least 32°N, where earthquake seismological observations show that the properties of the upper mantle change markedly [e.g., McNamara et al., 1995]. Taken together, the seismic reflection and earthquake seismological observations lend support for the view that the Indian mantle lid mechanically underplates roughly the southern half of the Tibetan plateau. The INDEPTH reflection data do not show that Indian crust attached to this lithosphere extends north of southernmost Tibet (Kangmar dome), nor do they negate this possibility. Similarly, the reflection data do not yield direct evidence for or against lower‐crustal subduction beneath the Himalaya.
Postorogenic denudation along the late Paleozoic Ouachita trend, south central United States of America: Magnitude and timing constraints from apatite fission track data
Authors: Corrigan Jeff , Cervany Philip F. , Donelick Raymond , Bergman Steven C.
Published: February 2004

The magnitude and timing of synorogenic and postorogenic denudation along fold and thrust belts and their associated foreland basins, of interest because of both tectonic and economic implications, is generally poorly constrained. Along the late Paleozoic Ouachita trend, a thin veneer of Cretaceous strata is preserved above a low‐relief erosional surface that beveled the Ouachita orogen and adjacent foreland areas. This regional erosion surface provides a valuable constraint for interpreting new and previously published apatite fission track (AFT) data obtained from exposed structural highs along the Ouachita trend (Marathon, Llano, Arbuckle, and Benton uplifts). AFT data from sampled localities within the deformation belt (Marathon and Benton uplifts) exhibit younger ages and, generally, longer mean lengths than data from localities on the foreland side of the deformation front (Llano and Arbuckle uplifts). This observation suggests that erosion of the orogen, rather than its extensional collapse, was the primary mechanism responsible for flexural isostatic unloading of the foreland crust. In addition, all samples show evidence for mild reheating following their pre‐Cretaceous cooling history. Specifically, the lack of a significant population of >14.5‐μm tracks in all samples appears to require residence at temperatures of ≥55°±5°C after development of the sub‐Cretaceous erosional surface. This implies that ∼1000 m of Cretaceous‐Paleogene (?) strata were deposited across the entire Ouachita frontal trend and subsequently removed during later Tertiary time. This Tertiary denudation is interpreted to reflect the interplay between regional denudation and isostatic compensation in response to slow (∼10 m/m.y.) epeirogenic uplift of the southern midcontinent and a long‐term drop (∼200 m) in eustatic sealevel during this time.
Inverse and forward numerical modeling of trishear fault‐propagation folds
Authors: Allmendinger Richard W.
Published: February 2004

Fault‐propagation folds commonly display footwall synclines as well as changes in stratigraphic thickness and dip on their forelimbs, features that cannot easily be explained by simple parallel kink fold kinematics. An alternative kinematic model, trishear, can explain these observations, as well as a variety of other features which have long intrigued structural geologists. Trishear has received little attention until recently, in part because it must be applied numerically rather than graphically. A new computer program has been developed to analyze trishear and hybrid trishear‐fault‐bend fold deformation. Trishear fold shape can vary considerably by changing the apical angle of the trishear zone and/or the propagation to slip ratio (P/S) during the evolution of the structure. Breakouts, anticlinal and synclinal ramps, and inversion structures can also be modeled, tracking the kinematics with growth strata. Strain within trishear zones can be used to predict fracture orientations throughout the structures as demonstrated by comparison with analog clay models. Also presented is a method for inverting data on real structures for a best fit trishear model by performing a grid search over a six‐parameter space (ramp angle, trishear apical angle, displacement, P/S, and X and Y positions of the fault tip line). The inversion is performed by restoring a key bed to a planar orientation by least squares regression. Because trishear provides a bulk kinematic description of a deforming zone, it is complementary to, rather than competing with, other kinematic models.
Tracer advection by steady groundwater flow in a stratified aquifer
Authors: Sposito Garrison , Weeks Scott W.
Published: February 2004

The perfectly stratified aquifer has often been investigated as a simple, tractable model for exploring new theoretical issues in subsurface hydrology. Adopting this approach, we show that steady groundwater flows in the perfectly stratified aquifer are always confined to a set of nonintersecting permanent surfaces, on which both streamlines and vorticity lines lie. This foliation of the flow domain exists as well for steady groundwater flows in any isotropic, spatially heterogeneous aquifer. In the present model example it is a direct consequence of the existence of a stream function. We then demonstrate that tracer plume advection by steady groundwater flow in a perfectly stratified aquifer is never ergodic, regardless of the initial size of the tracer plume. This nonergodicity, which holds also for tracer advection in any isotropic, spatially heterogenous aquifer, implies that stochastic theories of purely advective tracer plume movement err in assuming ergodic behavior to simplify probabilistic calculations of plume spatial concentration moments.
Relation between ERS‐1 synthetic aperture radar data and measurements of surface roughness and moisture content of rocky soils in a semiarid rangeland
Authors: Sano Edson E. , Huete Alfredo R. , Troufleau Denis , Moran M. Susan , Vidal Alain
Published: February 2004

Surface roughness and soil moisture content control the distribution of rainfall into runoff, evapotranspiration, and infiltration. Satellite radar data have the potential to provide spatial and multitemporal estimates of these variables, depending upon the sensor configuration and field condition. The relation between the European Remote Sensing Satellite (ERS‐1) synthetic aperture radar (SAR) data and measurements of surface roughness and moisture content of rocky soils in a semiarid rangeland in southeast Arizona was analyzed in this study. A dry and a wet season C band SAR image were acquired and corrected for topographic effects. Field soil roughness and moisture content data were obtained from 47 sampling sites. An intensive soil moisture sampling campaign was also conducted at three sites to determine the number of samples necessary to estimate soil moisture content with 10% accuracy. Dry and wet season SAR data were found to be correlated (r2=0.80 and 0.59, respectively) with root‐mean‐square (RMS) height measurements, while SAR data from the wet season image were poorly correlated with soil moisture. The results indicated that C band SAR data are promising for estimation of surface roughness in semiarid rangelands. However, they are less promising for soil moisture estimation, unless the effects of soil roughness and vegetation are removed. The acquisition of an adequate number of soil moisture samples to obtain representative soil moisture measurements is also a key issue in the validation of soil moisture retrieval from SAR data. In the study area, at least 17 samples per hectare were needed to obtain soil moisture estimates with 10% accuracy.
Regional frequency analysis methods for evaluating changes in hydrologic extremes
Authors: Bradley A. Allen
Published: February 2004

A common assumption in frequency analysis is that hydrologic extremes (floods or heavy precipitation) are generated by a random process. This implies that natural climatic variability does not change the distribution of extreme events. A regional frequency analysis approach is proposed to test the hypothesis of randomness over secular timescales. Observed regional occurrences of extreme events are compared to those from a random process. Significant departures may indicate nonrandomness due to climatic variability. Application of the approach to a region in the Southern Plains indicates nonrandomness in annual maximum precipitation.
Dispersive transport of ions in column experiments: An explanation of long‐tailed profiles
Authors: Hatano Y. , Hatano N.
Published: February 2004

We present a novel microscopic model of sorption and convection of ions in heterogeneous media. Our model is based on an analogy to electron transport in a semiconductor. A new feature of our model is a power law random distribution of the adsorption time of ions. Diverging standard deviation of the distribution function yields anomalous ion transport. We show that this anomalous transport explains a concentration profile with a long tail that has been observed in column experiments. We successfully fit recent experimental data. Finally, we propose new experiments by which we can check the validity of our model.
Capillary barrier performance in heterogeneous porous media
Authors: Ho Clifford K. , Webb Stephen W.
Published: February 2004

The effects of heterogeneities on the performance of capillary barriers is investigated by numerically simulating three systems comprised of a fine soil layer overlying a coarse gravel layer with (1) homogeneous, (2) layered heterogeneous, and (3) random heterogeneous property fields. The amount of lateral diversion above the coarse layer under steady state infiltration conditions is compared among the simulations. Results indicate that the performance of capillary barriers can be significantly influenced by the spatial variability of hydraulic properties. In the layered heterogeneous systems, realizations with highly stratified regions within the fine layer performed best and resulted in localized capillary barriers that delayed breakthrough into the coarse layer. In contrast, realizations of the random heterogeneous system performed worst because of channeled flow that produced numerous localized regions of breakthrough into the coarse layer. Results of the homogeneous model were comparable to the mean results of the layered heterogeneous realizations, but homogeneous results underpredicted the frequency and amount of breakthrough for all realizations of the random heterogeneous system. These results indicate that homogeneous models can be used to estimate the average behavior of layered heterogeneous systems with reasonable accuracy. In addition, engineered capillary barriers may be improved through emplacement and packing methods that induce highly stratified features within the fine layer of a capillary barrier system.
Solute transport in highly heterogeneous aquifers
Authors: Salandin Paolo , Fiorotto Virgilio
Published: February 2004

The paper deals with the transport of nonreactive solute in heterogeneous formations with prescribed statistical properties of the hydraulic log conductivity Y=ln K. Available solutions obtained from perturbation methods are limited to first‐ and second‐order solutions, valid only for small values of the log conductivity variance σ2Y. Published numerical investigations give comparative results with finite values of σ2Y, but some discrepancies among the results generate doubts about the capability of numerical methods to capture high‐order effects for media with large variance values. When these large σ2Y values are encountered in natural formations, the related nonlinear effects could be significant in the velocity statistics and in the overall dispersion process. The nonlinearity consequences are here investigated in two‐dimensional isotropic porous media by the Monte Carlo technique coupled with a finite element analysis. The analysis includes σ2Y values from 0.05 to 4 enhancing the relevance of the nonlinear effects in the dispersion tensor solution. To dissipate the doubts related to the numerical approach, the accuracy of the solutions was defined by checking the influence of the factors which can affect the solution and by giving an estimation of the related errors. The numerical results confirm the validity of the first‐order and second‐order analyses when σ2Y → 0; the second‐order solution captures the nonlinear effects in a small range of log conductivity variance close to zero. The nonlinear terms neglected in the first‐order formulation for higher σ2Y values give (1) late travel time longitudinal dispersion values greater than the linear solution, (2) notably non‐Gaussian distribution of the Lagrangian velocity and particle displacements, and (3) travel times to approach the asymptotic Fickian regime longer than those obtained using the linear solution.
A note on a method for measuring the transport properties of a formation using a single well
Authors: Novakowski K. S. , Lapcevic P. A. , Voralek J. W. , Sudicky E. A.
Published: February 2004

For some subsurface investigations of contaminant transport, particularly those conducted in consolidated material, the costs related to well construction prohibit the installation of a comprehensive field of monitoring wells. To alleviate this problem for fractured, low‐porosity formations, a method for measuring transport properties using a single well was developed. The method involves the injection of fluid and tracer over a short duration which establishes a radial source condition in the formation. Following this the ambient flow is allowed to carry the tracer back through the injection well where tracer concentration is monitored passively, in situ. To interpret the experimental results, a numerical model was adapted to account for the mass balance of solute in the source/monitoring well during the injection and monitoring periods. The model accommodates advection‐dispersion, adsorption, decay, and matrix diffusion in a framework of fractures having a variety of geometries. To illustrate the use of the method, a field experiment was conducted using a single well which is intersected by a discrete horizontal fracture in a flat‐lying shale and limestone formation. Interpretation of the results agreed well with the interpretation of other tracer experiments conducted previously in the same fracture plane. This suggests that the method may yield defensible estimates of transport properties such as matrix porosity and groundwater velocity in geological formations that are expensive and difficult to characterize.
Diffusion theory for transport in porous media: Transition‐probability densities of diffusion processes corresponding to advection‐dispersion equations
Authors: LaBolle Eric M. , Quastel Jeremy , Fogg Graham E.
Published: February 2004

Local‐scale spatial averaging of pore‐scale advection‐diffusion equations in porous media leads to advection‐dispersion equations (ADEs). While often used to describe subsurface transport, ADEs may pose special problems in the context of diffusion theory. Standard diffusion theory applies only when characteristic coefficients, velocity, porosity, and dispersion tensor, are smooth functions of space. Subsurface porous‐material properties, however, naturally exhibit spatial variability. Transitions between material types are often abrupt rather than smooth, such as sand in contact with clay. In such composite porous media, characteristic coefficients in spatially averaged transport equations may be discontinuous. Although commonly called on to model transport in these cases, standard diffusion theory does not apply. Herein we develop diffusion theory for ADEs of transport in porous media. Derivation of ADEs from probabilistic assumptions yields (1) necessary conditions for convergence of diffusion processes to ADEs, even when coefficients are discontinuous, and (2) general probabilistic definitions of physical quantities, velocity, and dispersion tensor. As examples of how the new theory can be applied to theoretical and numerical problems of transport in porous media, we evaluate several random walk methods that have appeared in the water resources literature.
Long‐term planning methodology for water distribution system rehabilitation
Authors: Kleiner Yehuda , Adams Barry J. , Rogers J. Scott
Published: February 2004

The most expensive component of a water supply system is the distribution network. Deterioration due to aging and stress causes increased operation and maintenance costs, water losses, reduction in the quality of service, and reduction in the quality of water supplied. In this paper an approach is proposed in which the water distribution network economics and hydraulic capacity are analyzed simultaneously over a predefined analysis period while the deterioration over time of both the structural integrity and the hydraulic capacity of every pipe in the system is explicitly considered. The cost associated with each pipe in the network is calculated as the present value of an infinite stream of costs. In Kleiner et al. [this issue] a methodology is presented to implement this approach into a decision support system that facilitates the identification of an optimal rehabilitation strategy.
A space‐time rainfall disaggregation model adapted to Sahelian Mesoscale Convective Complexes
Authors: Lebel Thierry , Braud Isabelle , Creutin Jean‐Dominique
Published: February 2004

A model adapted to the disaggregation of rainfields associated to Sahelian mesoscale convective complexes (MCCs) at small time (≈5 min) and space (1–10 km) scales is presented. Spatial and temporal disaggregation have been decoupled in order to simplify the problem. The average rain depth over the study area for each storm is first disaggregated in space by using the turning band method, a geostatistical technique able to generate spatially correlated fields, with a known covariance function. The method has been adapted in order to simulate non‐Gaussian rainfall fields including in particular a probability of zero rainfall. The movement of the rain system is described by mapping the starting time of the rain event as the convective front moves through the domain. Finally, at each location, the temporal disaggregation of the storm rain depth is obtained by using a standard hyetogram model reproducing the typical sequence of a convective front followed by a stratiform trail. In its present form the only input of the disaggregation model is the average rain depth over the domain which can be provided either by a general circulation model output or a satellite estimate. To perform the various steps of the spatial and temporal disaggregation, some knowledge on the climatology of the zone is needed. Over the Sahel, this knowledge is provided by the EPSAT‐Niger data set, which contains 170 mesoscale convective systems (MCSs) observed over a 4‐year period by a dense recording raingauge network and a weather radar in the Niamey area. Half of these MCSs were identified as MCCs. The analysis of the MCCs' data set allowed the derivation of a probability distribution and a covariance function of the point storm rain depth. It also allowed the calculation of statistics for the average speed and direction of storm displacement and the inference of parameters describing the standard hyetograms. A complete example of the use of the model is presented showing that it produces realistic rainfall fields. Perspectives for further development and validation of the model are given in the conclusion.
Assessment of hydrologic parameter uncertainty and the worth of multiresponse data
Authors: Kuczera George , Mroczkowski Marek
Published: February 2004

Conceptual catchment models with more than four or five parameters calibrated to streamflow data often have poorly identified parameters. This study reassesses the role of the computationally efficient multinormal approximation to parameter uncertainty and considers the worth of multiresponse data to improve identifiability. A case study involving the nine‐parameter CATPRO model presents three findings. First, when an overparameterized model is calibrated to streamflow data, the parameter covariance matrix can help identify the poorly defined parameters and provide insight about the structural reasons for poor identifiability. Second, when multiresponse data are available to calibrate the catchment model, the multinormal approximation may provide an adequate description of parameter uncertainty. Third, augmenting streamflow data with other response time series data may not reduce parameter uncertainty. Augmenting streamflow data with groundwater level data did little to reduce the uncertainty in the poorly defined CATPRO parameters, whereas augmenting with stream salinity data substantially reduced parameter uncertainty. This suggests the worth of multiresponse data should, where possible, be assessed a priori.
Study of spatial scaling in braided river patterns using synthetic aperture radar imagery
Authors: Nykanen Deborah K. , Foufoula‐Georgiou Efi , Sapozhnikov Victor B.
Published: February 2004

Synthetic aperture radar (SAR) imagery offers an appealing way of remotely monitoring the complex and rapidly changing forms of braided rivers. These rivers are often found in scarcely inhabited regions and are so dynamic that in situ measurements are almost impossible. In this paper, SAR imagery was used to extract braided river patterns such that their spatial scaling characteristics could be studied. From analysis of several reaches of a braided river in Alaska (the Tanana River), self‐affine spatial scaling of the river patterns was found to be present under different flow rates and in different seasons when the river was undisturbed (free of external topographic controls). In regions where predominant geologic controls (i.e., mountains) or predominant flow paths (several tens of times the size of the other channels) were present, no spatial scaling was found. When scaling was found, the values of the anisotropic scaling exponents vx and vy had very similar values to those found by Sapozhnikov and Foufoula‐Georgiou [1996] from traced and digitized aerial photographs of several braided rivers.
Solute transport in three‐dimensional heterogeneous media with a Gaussian covariance of log hydraulic conductivity
Authors: Zhang You‐Kuan , Federico Vittorio Di
Published: February 2004

The analytical solutions for the velocity covariance, uij one particle displacement covariance Xij, and the macrodispersivity tensor αij defined as (0.5/μ)(dXij/dt), were derived in three‐dimensional heterogeneous media. A Gaussian covariance function (GCF) of logarithmic hydraulic conductivity, log K, was used, assuming uniform mean flow and first‐order approximation in log‐conductivity variance, where μ is the magnitude of the mean flow velocity μ. Based on these solutions, the time‐dependent ensemble averages of the second spatial moments, Zij ≡ 〈Aij〉 − Aij∥0) = Xij − Rij and the effective dispersivity tensor γij defined as (0.5/μ)(d〈Aij〉/dt, were evaluated for a finite line source either normal or parallel to μ, where Aij(0) is the initial value of the second spatial moment of a plume, Aij and Rij is the plume centroid covariance. The results obtained in this study were compared with previous results for an exponential covariance function (ECF). It was found that in a stationary log K field the spreading of a solute plume depends not only on the variance and integral scale of the log K field but also on the shape of its covariance function. The more strongly correlated the hydraulic conductivities at short separation distances are, the faster Zii and γii grow at early time. Also, the earlier that γii approaches its asymptote or peak, and the higher the peak is, the larger the negative transverse dispersivity. More importantly, the ergodic limits for GCF are reached faster than those for ECF, as the initial size of a plume increases. The ergodic limit X11 for GCF is slightly larger than that for ECF, but X22 and X33 are significantly smaller than those for ECF even though the asymptotic αii for GCF is the same with that for ECF.
The Tarrawarra Data Set: Soil moisture patterns, soil characteristics, and hydrological flux measurements
Authors: Western Andrew W. , Grayson Rodger B.
Published: February 2004

Experiments investigating the spatial variability of soil moisture conducted in the 10.5 ha Tarrawarra catchment, southeastern Australia, are described. The resulting data include high‐resolution soil moisture maps (over 10,000 point measurements at up to 2060 sites), information from 125 soil cores, over 1000 soil moisture profiles from 20 sites, 2500 water level measurements from 74 piezometers, surface roughness and vegetation measurements, meteorological and hydrological flux measurements, and topographic survey data. These experiments required a major commitment of resources including 250 person days in the field, with a further 100 person days in the laboratory preparing for field trips and checking and collating data. These data are available on the World Wide Web (http://www.civag.unimelb.edu.au/data/).
Seepage shedding by parabolic capillary barriers and cavities
Authors: Philip J. R.
Published: February 2004

The 1989 analysis of seepage exclusion from underground cavities established the parabolic and paraboloidal geometries as the most efficient. That work is extended here to an analysis of seepage into the cavity. The cavity results carry over to maximal capillary barriers of macroscopically coarse material such as smooth gravel. The analysis is extended further to the conventional submaximal capillary barrier of a coarser soil underlying finer soil. The shedding efficiency of the submaximal barrier is always less than that of the maximal barrier, and in worst case scenarios, poorly designed submaximal barriers actually concentrate seepage into the region they are supposed to protect. The 1989 work, supplemented by the present study, provides a detailed physical basis for the recent proposal that the most efficient geometry for capillary barriers is parabolic. In some (but not all) circumstances barriers are ineffective if their horizontal dimensions exceed a few sorptive lengths.
Determination of fracture storativity in hard rocks using high‐pressure injection testing
Authors: Rutqvist Jonny , Noorishad Jahan , Tsang Chin‐Fu , Stephansson Ove
Published: February 2004

The determination of the storage capacity of fractures in crystalline rocks by means of hydraulic injection tests is studied by coupled hydromechanical finite element simulations. The results verify that the storage is related to the fracture opening, which is dependent on the combined stiffness of the fracture and the ambient rock mass. In most practical cases the storage is entirely controlled by the normal stiffness of the fractures. The strong coupling to the fracture opening implies that the storage capacity can be estimated from the pressure dependency of the fracture aperture in a high‐pressure injection test. Such high‐pressure injection tests can be conducted in addition to a conventional low‐pressure test to independently determine the storativity of the fracture. This provides an additional validation of the evaluated storativity, which implies not only that the value is more accurately assessed but also that other hydraulic properties can be determined more unambiguously. The method of high‐pressure injection testing is applied in field experiments to deep fractures in granitic rocks at two sites, and its usefulness is demonstrated in an analysis of the field data.
A nonlinear regression model for weekly stream temperatures
Authors: Mohseni Omid , Stefan Heinz G. , Erickson Troy R.
Published: February 2004

To estimate weekly stream temperatures needed for fish habitat evaluation throughout an annual cycle, a four‐parameter, nonlinear function of weekly air temperatures was used. The regression function was developed separately for the warming season and the cooling season to take heat storage effects (hysteresis) into account. Regression functions were developed for stream temperatures recorded over a 3‐year period (1978–1980) at 584 U.S. Geological Survey (USGS) gaging stations in the contiguous United States. Representative air temperatures were obtained from the closest of 197 weather stations. The distance between a stream gaging station and the corresponding weather station was from 1.4 to 244 km. These distances did not have a significant effect on the goodness of fit. The regression model fitted the weekly stream temperatures at 573 stream gaging stations (98% of all records used) with a coefficient of determination larger than 0.7. For 491 records (84% of all gaging stations) the coefficient was >0.9. At 56 gaging stations (10% of all records used), estimated maximum stream temperatures were smaller than at least four weekly stream temperatures recorded for the period of study. Consequently, the model is deemed successfully applicable (with 99% confidence) to more than 89% of the stream gaging stations. The average coefficient of determination of the stream temperature projection for these stations is 0.93 ± 0.01.
A saturation degree‐dependent composite spheres model for describing the effective dielectric constant of unsaturated porous media
Authors: Friedman Shmulik P.
Published: February 2004

Understanding the relation between the effective dielectric constant (relative permittivity) of soils and their volumetric water content is important because measurements of the dielectric constant of soils by capacitance sensors, time domain reflectometry (TDR) probes, and remote‐sensing radar devices are used to determine their moisture content. In the present study a physical model is proposed which relates the effective dielectric constant of unsaturated mineral soils at TDR frequencies to their volumetric water content, porosity, and specific surface area. It is assumed that the solid, water, and air components form a mixture of composite spheres and that the radial order of the single‐phase concentric shells depends on the saturation degree of the medium. The dielectric constant of the aqueous phase is smaller than that of free water, because of interfacial solid‐liquid forces, and the dependence of this reduction on the moisture content and on the specific surface area is represented here via a general approximated relationship. The model prediction is based on readily available soil properties (porosity, specific surface area, or texture), and it does not require any calibration. Tests of the model predictions against measurements made here on 3 porous media and against published data for an additional 19 media resulted in a promising agreement, with the proposed model's predictions being better than those given by the commonly used empirical relations for mineral soils. It is believed that the model performance could be further improved by better representation of the saturation degree‐dependent, three‐phase configuration by considering nonspherical soil particles and by a more realistic incorporation of interfacial relaxation processes.
On the simulation of infiltration‐ and saturation‐excess runoff using radar‐based rainfall estimates: Effects of algorithm uncertainty and pixel aggregation
Authors: Winchell Michael , Gupta Hoshin Vijai , Sorooshian Soroosh
Published: February 2004

The effects of uncertainty in radar‐estimated precipitation input on simulated runoff generation from a medium‐sized (100‐km2) basin in northern Texas are investigated. The radar‐estimated rainfall was derived from Next Generation Weather Radar (NEXRAD) Level II base reflectivity data and was supplemented by ground‐based rain‐gauge data. Two types of uncertainty in the precipitation estimates are considered: (1) those arising from the transformation of reflectivity to rainfall rate and (2) those due to the spatial and temporal representation of the “true” rainfall field. The study explicitly differentiates between the response of simulated saturation‐excess runoff and infiltration‐excess runoff to these uncertainties. The results indicate that infiltration‐excess runoff generation is much more sensitive than saturation‐excess runoff generation to both types of precipitation uncertainty. Furthermore, significant reductions in infiltration‐excess runoff volume occur when the temporal and spatial resolution of the precipitation input is decreased. A method is developed to relate this storm‐dependent reduction in runoff volume to the spatial heterogeneity of the highest‐intensity rainfall periods during a storm.
An active fracture model for unsaturated flow and transport in fractured rocks
Authors: Liu H. H. , Doughty C. , Bodvarsson G. S.
Published: February 2004

The unsaturated zone at Yucca Mountain, a potential repository site of high‐level nuclear waste, is a complex hydrologic system in which a variety of important flow and transport processes is involved. To quantify these processes as accurately as possible is a theoretically challenging and practically important issue. In this study, we propose a new formulation for modeling flow and transport in unsaturated fractured rocks. The formulation is mainly based on a hypothesis that only a portion of connected fractures are active in conducting water. Analysis of the relevant data with the new formulation suggests that about 18–27% of the connected fractures in the Topopah Spring welded (TSw) unit (the potential repository unit) of Yucca Mountain are active under ambient conditions. The relatively high percentage of active fractures is consistent with field observations from a variety of sources. Sensitivity analyses are performed to investigate effects of the “activity” of connected fractures on flow and transport behavior in unsaturated rocks.