bluuebunny/crossref_metadata_2025 huggingface dataset:
Relative sensitivity of full‐wave and half‐wave detectors in radiometry
Authors: Emerson D. T. , Thompson A. R.
Published: May 2003

The sensitivity in the measurement of Gaussian noise power using a half‐wave detector can suffer degradation relative to that of a full‐wave detector, in cases where the bandwidth of the input waveform is not small compared to the center frequency. There are two physical mechanisms that suggest a basis for this effect. The half‐wave detector responds to only half of the input waveform, resulting in possible loss of information. Also, in the half‐wave detector a component of the input spectrum feeds through the diode and adds to the noise that is subsequently filtered by averaging at the detector output. This feedthrough component does not occur in full‐wave detectors. The phenomena are examined using numerical simulation. The results for both linear and square‐law characteristics are considered, and formulas and graphs for estimating the magnitudes of the effects are given.
Selecting an optimal configuration for the Soil Moisture and Ocean Salinity mission
Authors: Waldteufel Philippe , Boutin Jacqueline , Kerr Yann
Published: May 2003

The Soil Moisture and Ocean Salinity (SMOS) mission is aimed at monitoring, globally, surface soil moisture and sea surface salinity from radiometric L‐band observations. The SMOS radiometer relies upon a two‐dimensional (2‐D) synthetic aperture concept in order to achieve satisfactory spatial resolution performances for a minimal cost in terms of payload mass and volume. Counterparts of this advantage are reduced radiometric sensitivity and increased complexity. The performances expected from SMOS, in terms of measurement accuracy, spatial resolution, and revisit time, depend on many parameters, among which several are crucial for assessing the payload and mission configurations. Most prominent among those configuration parameters are the flight altitude, the length of the interferometer arms, the spacing between radiating elements, and the tilt angle of the antenna plane. Their selection has to be optimized, so as to satisfy both scientific requirements and main technical constraints. This paper describes the way the optimization was carried out during the SMOS phase A. After assessing the main drivers on instrument configuration from the science requirements, the goal was to find an optimal trade‐off, minimizing technical challenges while fulfilling the science objectives. It was found that, even though salinity retrievals are the most challenging, soil moisture retrievals were the most demanding in terms of mission definition and that a configuration exists to satisfy the required retrieval accuracies. The obtained configuration was then checked against ocean salinity retrievals and found satisfactory.
Measured channel parameters for the disturbed wide‐bandwidth HF channel
Authors: Nissen Christopher A. , Bello Phillip A.
Published: May 2003

The wideband HF (WBHF) channel impulse response measurements presented here were taken over a high‐latitude auroral path using the MITRE experimental WBHF test facility. A nominal instantaneous bandwidth of 1 MHz was used for the majority of the tests. These tests took place from 14 March through 1 April 1992 over a path from Sondrestrom AFB, Greenland, to Bedford, Massachusetts, a distance of approximately 3,100 km. The measurements were made using a direct‐sequence pseudo‐noise (DSPN) channel probe with a (±180°) chip modulation and chip rate of 1024 kchips/second. Complex channel impulse response values at increments of 500 nanoseconds in delay were derived by correlating the received signal with a set of local DSPN references uniformly spaced in delay over a delay range of 2 ms. Each of these channel impulse response values was sampled in time at a rate of up to 62.5 samples per second, resulting in the ability to observe (one‐sided) Doppler spectra and shifts of up to 31.25 Hz. From these measurements, the channel scattering function, rms (2σ) Doppler spread, rms (2σ) Delay spread, Doppler shift, and rms (2σ) spread factor were calculated. In addition, for the first time, the impulse response correlation along the delay axis was calculated. These latter measurements support the use of an uncorrelated scattering hypothesis in modeling the disturbed WBHF channel.
Full Spectrum Inversion of radio occultation signals
Authors: Jensen Arne Skov , Lohmann Martin S. , Benzon Hans‐Henrik , Nielsen Alan Steen
Published: May 2003

Temperature, pressure, and humidity profiles of the Earth's atmosphere can be derived through the radio occultation technique. This technique is based on the Doppler shift imposed, by the atmosphere, on a signal emitted from a GNSS satellite and received by a low orbiting satellite. The method is very accurate with a temperature accuracy of 1°K, when both frequencies in the GPS system are used. However, difficulties arise when the signal consists of multiple frequencies generated by multipath phenomena in the atmosphere. We demonstrate that, in general, it is possible to determine the arrival times of the different frequency components in a radio occultation signal simply as the derivatives of the phases of the conjugated Fourier spectrum of the entire occultation signal. Based on this property, a novel Full Spectrum Inversion technique for radio occultation sounding capable of disentangling multiple rays in multipath regions is presented. As the entire signal is used in the Fourier transform, a high spatial resolution in the Doppler frequency, and hence in the retrieved temperature, pressure, and humidity profiles, can be achieved. The method is conceptual and computational simple and thus easy to implement. The performance of the Full Spectrum Inversion is demonstrated by applying the technique to simulated signals generated by solving Helmholtz equation with use of the multiple phase‐screen technique. Excellent agreement is found between computed bending angle profiles and corresponding solutions to the Abel integral.
Paleozoic tectonic evolution of the northern Xinjiang, China: Geochemical and geochronological constraints from the ophiolites
Authors: Wang Zhihong , Sun Shu , Li Jiliang , Hou Quanlin , Qin Kezhang , Xiao Wenjiao , Hao Jie
Published: May 2003

The northern Xinjiang of China is a composite orogenic belt in the south central part of the Altaids formed by subduction‐accretion mainly during the Paleozoic. Geochronologic and geochemical data from the Paleozoic ophiolites and associated assemblages are reported here to place constrains on the tectonic evolution of this region and to reconstruct its history, using a model of subduction‐accretion plus large‐scale map‐view strike‐slip faulting. Geologic and geochronologic review indicates that the Paleozoic ophiolites and associated assemblages of the western and eastern Junggar regions are tectonically related, occurring as a pattern with the early Paleozoic Tangbale‐Mayile‐Hongguleleng‐Aermantai complexes in the center, the late Paleozoic Darbut‐Karamaili and the northern Tianshan complexes in the south, and the late Paleozoic Kekesentao‐Qiaoxiahala‐Kuerti complexes in the north. Available geochemical data suggest that the Paleozoic ophiolites of the northern Xinjiang are mainly oceanic fragments of these three types: mid‐ocean ridge basalt, oceanic island basalts, and island arc basalts. The Paleozoic subduction‐accretion complexes grew mainly along different segments of the Kipchak arc, and in associated with migration of magmatic arc. The Paleozoic ophiolite‐associated sedimentary sequences and subduction‐related volcanic rocks exhibit an evolution history from intraoceanic (early Paleozoic) to marginal settings (late Paleozoic), implying coeval strike‐slip stacking with subduction‐accretion processes, and by which to transport the respective complexes together and further to amalgamate with neighboring units of the Altaids. Subduction‐accretion processes terminated in the mid‐Carboniferous in this region. In the late Permian‐early Triassic, a counterclockwise rotation of fault blocks in a giant sinistral strike‐slip and extension domain between Altay and Tianshan, lastly generated present configuration of the Paleozoic subduction‐accretion complexes in the northern Xinjiang.
Tectonic and denudational history of the Rennick Graben (North Victoria Land): Implications for the evolution of rifting between East and West Antarctica
Authors: Rossetti Federico , Lisker Frank , Storti Fabrizio , Läufer Andreas L.
Published: May 2003

The tectonic depression of the Rennick Graben (northern Victoria Land, Antarctica) is a key element for deciphering the Cenozoic separation history between East and West Antarctica and evolution of the West Antarctic rift system. The geometry, kinematics and timing of the boundary faults of the Rennick Graben are presented, on the basis of a detailed structural study of the major fault segments combined with apatite fission track data. The structural architecture of the Rennick Graben is controlled by distributed and partitioned right‐lateral strike‐slip faulting. Fission track data indicate that the last significant denudation stage of the Rennick Graben shoulders started from Eocene times, with increasing cooling and denudation since 40–50 Ma. A direct structural connection is then established between Cenozoic denudation and the activation of the dextral strike‐slip regime in north Victoria Land during the Cenozoic. In this framework, the Rennick Graben is interpreted as a localized tectonic depression generated as a displacement accommodation area of the regionally sized dextral strike‐slip shearing. The results of this study (1) invalidate the commonly assumed link between Mesozoic Ross Sea rifting and opening of the Rennick Graben; (2) argue against orthogonal rifting models for the Cenozoic evolution of the Ross Sea region; and (3) provide constraints to the modalities and timing of the separation between the East Antarctic craton and the West Antarctica extensional province.
Emplacement of an Archean gneiss dome, northern Ontario, Canada: Inflation inferred from magnetic fabrics
Authors: Borradaile G. J. , Gauthier D.
Published: May 2003

Previous field structural studies of the Ash Bay gneiss dome revealed an upward convex foliation with an integral east‐west lineation defined by a quartzo‐felspathic matrix. Unfortunately, the amphibolite facies gneisses lack traditional indicators of microfabric evolution, such as kinematic indicators, so that it is difficult to relate the foliation's orientation distribution to an emplacement mechanism for the dome. Fortunately, anisotropy of magnetic susceptibility isolates a distinct microfabric because of the preferred orientation of high‐susceptibility, accessory magnetite. The magnetite subfabric developed late in the metamorphic crystallization history, and its magnetic foliation and magnetic lineation are similarly oriented to the S‐L fabric of the quartz‐feldspar matrix. However, the magnetite foliation dips less steeply and forms a less convex domal surface than the field schistosity. The metamorphic schistosity and the magnetite foliation thus isolate the domal deflection at different stages, confirming the inflation and upward motion of the Ash Bay diapir during the closing stages of amphibolite facies metamorphism.
An analytical solution of the three‐component transport equation with application to third‐order transport
Authors: Montas Hubert J.
Published: May 2003

An explicit analytical solution is derived and illustrated for multidimensional advective‐dispersive‐reactive transport in systems with three distinct velocities coupled by first‐order interactions or reactions. The solution is an exponentially scaled space‐time convolution of an advective‐dispersive‐reactive kernel with a coupled advective transport kernel. Explicit forms are provided for both kernels. The solution process for the coupled advective kernel uses one‐sided two‐dimensional Laplace transforms and introduces two constraints into the transport problem which are to be relaxed in future work. The behavior of the solution is illustrated with a third‐order approximation of single species transport in a heterogeneous aquifer. Results indicate that the early time behavior of the third‐order system is that of noninteracting advective‐dispersive‐reactive transport. The large time mean behavior is that of a single advective‐dispersive‐reactive transport equation with constant effective parameters. Results further demonstrate that the third‐order approximation provides accurate predictions of the mean and uncertainty of concentration distributions calculated from detailed two‐dimensional transport simulations at all times.
Unstable density‐driven flow in heterogeneous porous media: A stochastic study of the Elder [1967b] “short heater” problem
Authors: Prasad Awadhesh , Simmons Craig T.
Published: May 2003

The situation of a dense fluid overlying a lighter one is potentially unstable and under certain conditions may result in fingers of dense contaminant freely convecting downward. Free convection causes increased contaminant transport over larger distances and over shorter timescales than is possible by diffusion alone. Unlike free convection in homogenous porous media, free convection in heterogeneous porous media has received relatively little attention. In this study, a well‐understood problem of transient free convection in homogeneous porous media, theElder [1967b] “short heater” problem has been modified to study the effects of heterogeneity in permeability distributions on solute transport processes using a stochastic framework. A set of measurable indicator characteristics including solute fluxes, solute present, center of gravity and finger penetration depth are used in the quantitative analysis of output. Heterogeneity in the permeability distribution provides both the triggering mechanism for the onset of instabilities and also controls their subsequent growth and/or decay. Results show that (1) an increase in the standard deviation of the log permeability field results in a greater degree of instability at earlier times but promotes stability at later times, (2) an increase in the horizontal correlation length of the log permeability field creates laterally extensive low‐permeability zones that dissipate upward and downward motion needed to maintain convection and therefore causes a reduction in the degree of instability, (3) a greater degree of heterogeneity causes greater uncertainty in predictions, and (4) traditional predictive methods such as the Rayleigh number (based upon an average permeability) do not generally work in their application to heterogeneous systems. Probability of exceedence analysis also demonstrates that analyses based upon homogeneous assumptions will typically underestimate, often quite significantly, the value of key measurable characteristics.
Hydraulic boundary conditions as a controlling factor in karst genesis: A numerical modeling study on artesian conduit development in gypsum
Authors: Birk Steffen , Liedl Rudolf , Sauter Martin , Teutsch Georg
Published: May 2003

A coupled continuum‐pipe flow model is employed to simulate the development of karst conduits in an artesian setting, where aggressive water flows upward from an insoluble aquifer into a gypsum unit. Speleogenetic processes and controlling parameters are identified first for single conduit development. It is demonstrated that transient flow from the conduit into the surrounding fissured system of the rock enhances solutional conduit enlargement. Moreover, single conduit development is found to be highly sensitive to variations in initial diameter and hydraulic gradient. Simulating the solutional enlargement of pipe networks reveals that these two factors determine the resulting conduit pattern. Structural preferences are found to be essential for horizontal conduit development, otherwise flow paths are solely enlarged in vertical direction following the hydraulic gradient. Hydraulic gradients that increase with time can limit the extension or completely inhibit the development of horizontal caves even if lateral conduit development was structurally preferred.
A kinetic model for bacterial Fe(III) oxide reduction in batch cultures
Authors: Hacherl Eric L. , Kosson David S. , Cowan Robert M.
Published: May 2003

A model has been developed describing the microbial reduction of solid‐phase electron acceptors (Fe(III) oxides) as well as dissolved electron acceptors (chelated Fe(III) or organic electron shuttles) in Shewanella alga BrY. The model utilized a multiple‐substrate, Monod kinetics formulation. The Monod description of solid Fe(III) reduction requires a normalization of surface Fe concentration to biomass concentration in order to describe the “bioavailable” Fe(III) concentration. The model also contains provisions for irreversible sorption of Fe(II) to Fe(III) oxide surfaces and for the precipitation of Fe(III) carbonates. The loss of bioavailable Fe(III) due to sorption of Fe(II) was found to be minor, even for highly sorptive amorphous Fe(III) oxyhydroxides. However, the final extent of microbial reduction is very sensitive to the rate of siderite precipitation, assuming that siderite precipitation could partially occlude Fe(III) surface sites. The use of a multisubstrate Monod kinetics model enabled an evaluation of the effects of electron shuttles on solid Fe(III) reduction. Because the electron shuttle is recycled, very small additions can greatly increase the overall rate of solid Fe(III) reduction.
Density and pressure effects on the transport of gas phase chemicals in unsaturated porous media
Authors: Altevogt Andrew S. , Rolston Dennis E. , Venterea Rodney T.
Published: May 2003

The density of gas phase contaminants may be responsible for several important transport phenomena in porous media. One‐dimensional laboratory experiments were conducted to explore the transport of a dense gas (Freon‐113) through an air‐dry sand. Gas densities were measured and fluxes were estimated during transport through a column filled with oso‐flaco sand. Significant differences in density profiles and fluxes were observed for the three primary flow directions (horizontal, vertically upward, and vertically downward) at high source densities. Estimates indicate that pressure gradients of up to 20 Pa/m measured in the first 2.5 cm of the column were possibly due to the nonequimolar diffusion of Freon and air. Simulated Freon densities from numerical models based on the standard Darcy‐Fickian transport equation did not compare well against measured density data. Density profiles generated by the model differed from the data by up to 400%. Numerical simulations indicated that slip flow may be significant relative to Darcy advective flow, but the slip phenomenon did not account for the discrepancy between model simulations and data. Further research and equation development will be necessary in order to ascertain why the standard theory does not adequately describe the diffusive and advective transport processes for dense gases.
Fluid flow in synthetic rough‐walled fractures: Navier‐Stokes, Stokes, and local cubic law simulations
Authors: Brush David J. , Thomson Neil R.
Published: May 2003

The results of three‐dimensional Navier‐Stokes (NS) and Stokes simulations and two‐dimensional local cubic law (LCL) simulations of fluid flow through single rough‐walled fractures are presented. Synthetic rough‐walled fractures were created by combining random fields of aperture and the mean wall topography or midsurface, which quantifies undulation about the fracture plane. A finite volume formulation of the LCL that incorporates geometric corrections for fracture undulation is presented. Simulations of fluid flow through planar fractures with sinusoidal variation in aperture were compared to published results. The rough‐walled fracture simulations demonstrated that the total flow rates predicted by the corrected LCL were within 10% of those predicted by the Stokes equations for all the fractures examined in this work. Differences between the NS and Stokes simulations clearly demonstrated that inertial forces can significantly influence the internal flow field within a fracture and the total flow rate across a fracture. By limiting the total flow rate differences between the NS and Stokes simulations, constraints for three kinematic parameters were determined. For all the fractures presented in this work, the corrected LCL was determined to be an acceptable approximation to the NS equations, provided that the kinematic and geometric constraints were met.
Flexible automated parameterization of hydrologic models using fuzzy logic
Authors: Samanta Sudeep , Mackay D. Scott
Published: May 2003

Recent developments in model calibration suggest that information obtained from calibration is inherently uncertain in nature. Therefore identification of optimum parameter values is often highly nonspecific. A calibration framework using fuzzy logic is presented to deal with such uncertain information. An application of this technique to calibrate the streamflow of a hydrologic submodel embedded within an ecosystem simulation model demonstrates that objective estimates of parameter values and the range of model output associated with a failure to identify a unique solution can be obtained with suitable choices of objective functions. An iterative refinement in parameter estimates through a process of elimination was possible by incorporating multiple objective functions in calibration, thereby reducing the range of parameter values that capture the streamflow response. It is shown that objective function tradeoffs can lead to suboptimal solutions using the process of elimination without an automated procedure for reevaluation. Owing to its computational simplicity and flexibility this framework could be extended into a nonmonotonic system for automated parameter estimation.
Comparison of energy balance modeling schemes using microwave‐derived soil moisture and radiometric surface temperature
Authors: Kustas William P. , Bindlish Rajat , French Andrew N. , Schmugge Tom J.
Published: May 2003

A Two‐Source (soil + vegetation) Energy Balance (TSEB) modeling scheme has been developed to use either microwave‐derived near‐surface soil moisture (TSEBSM) or radiometric surface temperature (TSEBTR) as the key remotely sensed surface boundary condition for computing spatially distributed heat fluxes. Output of the surface heat fluxes from both two‐source schemes have been validated using tower‐ and aircraft‐based flux observations. However, these observations rarely provide the necessary spatial information for evaluating heat flux patterns produced by spatially based models. By collecting microwave and radiometric surface temperature observations concurrently during the Southern Great Plains 1997 (SGP97) experiment conducted in Oklahoma, USA, heat flux estimates by the two modeling schemes were compared on a pixel‐by‐pixel basis. This provided a unique opportunity for evaluating the consistency in spatial patterns of the heat fluxes. Comparisons with radiometric surface temperature observations helped to elucidate factors contributing to discrepancies between TSEBSM and TSEBTR output, because the TSEBSM modeling scheme computes an effective surface temperature. Results from the heat flux comparisons and simulated versus observed surface temperatures suggested revisions to TSEBSM parameterizations are needed to better constrain flux predictions from the soil and vegetation. When the revisions are made, TSEBSM accommodates a wider range of environmental conditions. The revisions involve an adjustment to the soil evaporation algorithm for differential drying of the near‐surface soil layer and adopting the Priestley–Taylor coefficient estimated from the TSEBTR model. It was also found that areas with high fractional vegetative cover conditions, TSEBTR estimates of energy partitioning between sensible and latent heat flux at the soil surface (expressed in terms of the soil Bowen ratio, BOS), were uncorrelated to the remotely sensed near‐surface soil moisture. This contributed to inconsistencies in BOS patterns estimated by TSEBTR during a dry down period. A ∼20% change in the maximum fractional vegetation cover estimated using the remote‐sensing‐based algorithm is shown to dramatically impact BOS values estimated by TSEBTR for the densely vegetated areas while having little effect on TSEBSM‐derived values. This result suggests that under certain environmental conditions, energy balance partitioning at the soil surface over densely vegetated areas may be tenuous using the TSEBTR scheme.
Colloid mobilization and transport within unsaturated porous media under transient‐flow conditions
Authors: Saiers James E. , Lenhart John J.
Published: May 2003

Contaminant‐scavenging colloids are mobilized in the vadose zone during infiltration events, characterized by transient flow regimes. In the research reported here we develop a mathematical model in order to investigate the influences of flow transients on colloid mobilization in unsaturated media. The model solves coupled equations for unsteady pore water flow, colloid release, advective‐dispersive colloid transport, and redeposition of pore water colloids. The immobile‐phase colloid population is discretized into a series of compartments, each of which are assigned a value of critical moisture content (Θcr) according to a piecewise linear density function. A compartment is activated when the model‐calculated moisture content exceeds the Θcr for the compartment, whereupon colloids are released at a rate proportional to the product of the pore water velocity and the colloid concentration within the compartment. We fit solutions of the model equations to data on the mobilization and transport of silica colloids within columns of unsaturated quartz sand. The model accurately reproduces measured pulse‐type colloid releases induced by successive step‐change increases in flow rate and moisture content. This work illuminates the important role of flow transients in colloid mobilization and, through the derivation of a model that couples pore water flow characteristics with colloid mass transfer kinetics, provides a means for quantifying the phenomenon.
Fault detection for salinity sensors in the Columbia estuary
Authors: Archer Cynthia , Baptista Antonio , Leen Todd K.
Published: May 2003

Sensors deployed in the Columbia River estuary gather information on physical dynamics and changes in estuary habitat. Of these sensors, conductivity sensors are particularly susceptible to biofouling, which gradually degrades sensor response and corrupts critical data. Several weeks may pass before degradation is visibly detected. Since the onset time of biofouling is unknown, an indeterminate amount of measurement data is corrupted. To speed detection and minimize data loss, we develop automatic biofouling detectors based on machine learning approaches for these conductivity sensors. We demonstrate that our detectors identify biofouling at least as reliably as human experts. In addition, these detectors provide accurate estimates of biofouling onset time. Real‐time detectors installed during the summer of 2001 produced no false alarms yet detected all episodes of sensor degradation before the field staff.
Use of hydraulic head to estimate volumetric gas content and ebullition flux in northern peatlands
Authors: Rosenberry Donald O. , Glaser Paul H. , Siegel Donald I. , Weeks Edwin P.
Published: May 2003

Hydraulic head was overpressured at middepth in a 4.2‐m thick raised bog in the Glacial Lake Agassiz peatlands of northern Minnesota, and fluctuated in response to atmospheric pressure. Barometric efficiency (BE), determined by calculating ratios of change in hydraulic head to change in atmospheric pressure, ranged from 0.05 to 0.15 during July through November of both 1997 and 1998. The overpressuring and a BE response were caused by free‐phase gas contained primarily in the center of the peat column between two or more semielastic, semiconfining layers of more competent peat. Two methods were used to determine the volume of gas bubbles contained in the peat, one using the degree of overpressuring in the middepth of the peat, and the other relating BE to specific yield of the shallow peat. The volume of gas calculated from the overpressuring method averaged 9%, assuming that the gas was distributed over a 2‐m thick overpressured interval. The volume of gas using the BE method averaged 13%. Temporal changes in overpressuring and in BE indicate that the volume of gaseous‐phase gas also changed with time, most likely because of rapid degassing (ebullition) that allowed sudden loss of gas to the atmosphere. Estimates of gas released during the largest ebullition events ranged from 0.3 to 0.7 mol m−2 d−1. These ebullition events may contribute a significant source of methane and carbon dioxide to the atmosphere that has so far largely gone unmeasured by gas‐flux chambers or tower‐mounted sensors.
Time‐dependent transport in heterogeneous formations of bimodal structures: 1. The model
Authors: Dagan G. , Fiori A.
Published: May 2003

Flow of uniform mean velocity U takes place in a heterogeneous medium made up from a matrix of conductivity K0 and inclusions of a different conductivity K. The inclusions of given shape are implanted at random and independently in the medium, without overlapping. The aim of the study is to derive simple, approximate solutions of advective transport of solutes in such heterogeneous formations for arbitrary permeability ratio κ = K/K0 and inclusions volume fraction n. Transport is characterized by the spatial moments, which in turn are equal to the one particle trajectory statistical moments for ergodic plumes. The flow and transport problems are solved for isotropic media, for circular (2D) and spherical (3D) inclusions by using the model of composite inclusions of Hashin and Shtrikman [1962]. The results tend to the dilute limit analyzed in the past by Eames and Bush [1999] for n = o(1). Asymptotic, analytical results are derived for weak heterogeneity (κ ≃ 1); they coincide with those of Rubin [1995] for a similar structure. Similarly, simple asymptotic expressions of the macrodispersivity are derived for low‐permeability inclusions, κ = o(1). The theoretical developments are applied to effectively computing trajectories moments in part 2 [Fiori and Dagan, 2003].
Time‐dependent transport in heterogeneous formations of bimodal structures: 2. Results
Authors: Fiori A. , Dagan G.
Published: May 2003

The theoretical results of part 1 [Dagan and Fiori, 2003] for modeling time‐dependent, advective transport of a conservative solute in porous formations of bimodal structure are applied to illustrate the behavior of a few trajectory statistical moments as function of time, of the permeability contrast κ, and of the inclusions volume density n. The computations are carried out for circular (2D) and spherical (3D) inclusions to represent isotropic media. Advective transport is solved by studying the distortion of a thin plume, linear (2D) or planar (3D), normal to the mean velocity U and moving through a single inclusion. The deformation of the plume is determined from the residual trajectories of solute particles that are derived numerically by a quadrature. The longitudinal macrodispersivity is defined by αL(t; n, κ) = (2U)−1dX11/dt, where X11 is the trajectories second moment in the mean flow direction. The general behavior of the time‐dependent longitudinal dispersivity αL(t; n, κ) and, in particular, its constant, large time limit are examined. The tendency of αL to the “Fickian” limit with time depends strongly on the conductivity contrast; in particular, for low permeable inclusions (κ ≪ 1) it may be extremely slow. It is shown that the first‐order approximation in the conductivity contrast κ is of limited validity (0.3 < κ < 2). The transverse moment X22 tends asymptotically to a constant value. The analysis of the trajectory high order moments shows that the probability density function (pdf) of the solute trajectories tends to normality at large time. Similar to the “Fickian” limit the normal distribution may be reached at very large time in presence of low conductivity inclusions, with the pdf characterized by significant tailing for the trailing part of the pdf.
Groundwater dynamics along a hillslope: A test of the steady state hypothesis
Authors: Seibert Jan , Bishop Kevin , Rodhe Allan , McDonnell Jeffrey J.
Published: May 2003

Appropriate conceptual simplifications and assumptions are a central issue for hydrological modeling, especially when those models serve as the foundation for more complex hydrochemical or ecological models. A common and often unexamined assumption in conceptual modeling is that the relation between groundwater levels and runoff can be described as a succession of steady state conditions. This results in a single‐valued, monotonic function between the groundwater levels and runoff. Consequently, the simulated rise and fall in groundwater levels always follow the dynamics of runoff. We tested this assumption with an analysis of detailed groundwater level data along two opposing hillslopes along a stream reach in a Swedish till catchment at Svartberget. Groundwater levels in areas close to the stream followed the dynamics of the runoff. The correlation between groundwater level and runoff decreased markedly for wells farther than approximately 40 m from the stream. The levels were often independent of streamflow: Upslope area groundwater could be rising when riparian groundwater and runoff were falling, and vice versa. There was a high degree of correlation between groundwater levels at similar distances from the stream. The median Spearman rank correlation between wells within 35 m from the stream was 0.86 and for wells located more than 60 m from the stream was 0.96. This indicated that there is a common hydrological pattern even in the upslope area that can be identified and modeled. Despite the widespread acceptance of the steady state assumption previously in this and other study catchments, our study shows that it is not valid for the investigated hillslope site. If the divergence from steady state, with potential ramifications for other processes such as runoff chemistry, is common, then it will be worthwhile to reconsider the appropriate range of applicability for the steady state hypothesis, and the alternatives to that hypothesis.
Transient well‐type flows in heterogenous formations
Authors: Indelman Peter
Published: May 2003

The problem of averaging transient flows by sources of a given head boundary condition in heterogeneous formations of random conductivity is investigated. The study generalizes the recently developed mathematical model of average transient nonuniform flow [Indelman, 1996;Tartakovsky and Neuman, 1998a]. The latter allows calculating the mean head for sources of flux boundary condition and, as such, is not applicable to modeling common well flows. To account for the head condition, the source term in the local flow model is modeled by a random function proportional to the hydraulic conductivity. The local flow equations are further averaged in order to determine the mean flow variables. It is shown that the effective conductivity cannot be defined for arbitrary initial head distributions. This precludes deriving the mathematical model of an average transient flow with the head boundary condition. However, the average flow equations are derived for the important particular case of a uniformly distributed initial head. The effective conductivity tensor is defined for an arbitrary heterogeneous formation and analyzed in detail for isotropic media. It is shown that at the initial stage of the transient flow, the effective conductivity is larger than the conductivity arithmetic mean. The fundamental solution of the average flow equations (mean Green function), corresponding to the mean head distribution due to the instantaneous injection of the unit mean amount of water through a point source, is derived at first order in the conductivity variance and is compared with the mean head for the flux boundary condition.
Interplay of stream‐subsurface exchange, clay particle deposition, and streambed evolution
Authors: Packman Aaron I. , MacKay Jeffrey S.
Published: May 2003

Hyporheic exchange (mixing of stream water with pore water beneath the stream) is responsible for the transport of many ecologically relevant substances across the stream‐subsurface interface. This process is dependent on both the streamflow conditions and sedimentary properties, but the complex nature of fluvial systems has presented a barrier to the development of mechanistic understanding of the dynamics of stream‐subsurface interactions. This work presents the results of a controlled study to examine in detail the relationship between stream‐subsurface exchange fluxes, delivery of suspended sediments to the hyporheic region, fine particle accumulation in the streambed, and alteration of sedimentary properties. Laboratory flume experiments were used to observe kaolinite clay deposition in a sand bed and the resulting alteration of hyporheic exchange fluxes. Solute and suspended sediment exchange with clean sand beds is predicted well by a fundamental model for bed form‐driven advective pumping exchange. However, substantial accumulation of clay in the bed causes an alteration of the pore water flow environment, which reduces both water flux across the stream‐subsurface interface and subsequent particle deposition. Measurement of bulk solute exchange and direct observation of clay accumulation in the bed both indicate that transported fine particles are preferentially removed near the stream‐subsurface interface. Clogging of inflow regions produces heterogeneous subsurface clay deposits even when the bed is initially homogeneous. This behavior is also predicted by the fundamental colloid pumping model. These results contradict the accepted view that suspended sediments will generally deposit in the deepest regions of the bed and thus clog the bed from the bottom upward. Our results indicate that clogging of the streambed surface will often isolate deeper regions of the bed from the streamflow, so that even relatively low amounts of suspended sediments can substantially degrade streambed habitat.
Prediction of runoff and soil moistures at the watershed scale: Effects of model complexity and parameter assignment
Authors: Downer Charles W. , Ogden Fred L.
Published: May 2003

The application of physically based hydrologic models implies they properly simulate processes at the computational scale. A chief criticism is that model predictions are compared only to discharge data. The physically based, hydrologic model CASC2D is reformulated such that soil moistures and fluxes can be computed using Richards' equation. The gridded surface subsurface hydrologic analysis (GSSHA) model is calibrated and verified against outlet discharge measurements during the growing season. The verified model is used to simulate an extended period during which measurements of soil moisture are available. Though soil moisture data are not used in the calibration and verification efforts, the model reproduces both the trends and the magnitude of soil moisture during the growing season. With additional formulation enhancements, soil moistures during the nongrowing season are also reproduced within a root‐mean‐square error of 0.1. However, more work is needed to understand the underprediction of runoff during the nongrowing season.
On the correlation structure of continuous and discrete point rainfall
Authors: Marani Marco
Published: May 2003

This paper proposes new analytical results based on the theory of stochastic processes that establish a theoretical dependence of point rainfall variance on the scale of observation under somewhat general assumptions. It is shown that power law scaling, commonly assumed by analyses and models of point rainfall, cannot hold over all scales of observation and that an “inner” regime exists for small sampling intervals where the variance is a quadratic function of interval length. A power law scaling regime is shown to exist for larger aggregation intervals, whose characteristics depend on the memory of the continuous rainfall process. The presence of a “transition” regime between the inner and the scaling regime is also shown and may explain the deviations from a power law scaling behavior in observed rainfall reported in the literature. Furthermore, the application of the theory to rainfall data from a representative variety of climate types shows that fractal and multifractal analysis techniques and models may indeed fail to capture observed rainfall properties over wide ranges of aggregation scales. Finally, correlation structures with “short‐” and “long‐”term memory are considered and different theoretical relationships expressing rainfall variance as a function of aggregation interval are derived and tested against rainfall observed at different locations with aggregation timescales ranging from 15 min to 2 days. The theoretical and observational analyses show that both finite and infinite memory rainfall processes may be observed in nature.
Penetration of liquid fingers into superheated fractured rock
Authors: Birkholzer Jens
Published: May 2003

Water infiltrating down a fracture in unsaturated rock experiences complex fluid flow and heat transfer phenomena when entering above‐boiling rock temperature regions. Such conditions are expected, for example, after emplacement of heat‐generating nuclear waste in underground repositories. A new efficient semianalytical method is proposed in this paper that simulates the flow processes of infiltration events subject to vigorous boiling from the adjacent hot rock. It is assumed that liquid flow forms in localized preferential flow paths and that infiltration events are typically short in duration but large in magnitude relative to the average net infiltration. The new solution scheme is applied to several test cases studying sensitivity to a variety of input parameters. Sample simulations are performed for conditions representative of the potential nuclear waste repository at Yucca Mountain, Nevada. A characteristic parameter is introduced that provides a quick estimate of the relative significance of boiling at a given location of interest.
The role of surface storage in a low‐gradient Arctic watershed
Authors: Bowling Laura C. , Kane Douglas L. , Gieck Robert E. , Hinzman Larry D. , Lettenmaier Dennis P.
Published: May 2003

The Arctic land surface water balance plays an important role in regulating the planetary heat balance and global ocean circulation. Lakes and wetlands are common features in the low‐gradient Putuligayuk River watershed in northern Alaska, with important implications for the annual water balance. Evapotranspiration exceeds precipitation over the summer, and there is a gradual reduction in wetland extent. Total inundated area derived from RADARSAT ScanSAR synthetic aperture radar images throughout 1999 and 2000 varied from 15 to 67 percent of the 471 km2 watershed. The hydrological system becomes disconnected within 2 weeks of snowmelt, and overland flow largely ceases. End‐of‐winter snow water equivalent and discharge during the melt period for 1999, 2000, and 2001 were used to estimate that between 30 and 37 mm (24–42 percent) of snow meltwater serves to recharge the evaporation deficit of the previous summer. The percent of snowmelt entering storage is dependent on the available surface storage.
Special relative permeability functions with analytical solutions for transient flow into unsaturated rock matrix
Authors: Wu Yu‐Shu , Pan Lehua
Published: May 2003

This paper presents a class of analytical solutions for transient flow into unsaturated rock matrix. These analytical solutions are derived using specially correlated, physically meaningful relative permeability and capillary functions. The transient flow processes in unsaturated rock matrix blocks are generally described by the Richards' equation. The analytical solutions describe the full transient behavior of flow into unsaturated matrix blocks and have proven (through various examples) to be useful for verifying numerical model results.
Diagnostic tools for mixing models of stream water chemistry
Authors: Hooper Richard P.
Published: May 2003

Mixing models provide a useful null hypothesis against which to evaluate processes controlling stream water chemical data. Because conservative mixing of end‐members with constant concentration is a linear process, a number of simple mathematical and multivariate statistical methods can be applied to this problem. Although mixing models have been most typically used in the context of mixing soil and groundwater end‐members, an extension of the mathematics of mixing models is presented that assesses the “fit” of a multivariate data set to a lower dimensional mixing subspace without the need for explicitly identified end‐members. Diagnostic tools are developed to determine the approximate rank of the data set and to assess lack of fit of the data. This permits identification of processes that violate the assumptions of the mixing model and can suggest the dominant processes controlling stream water chemical variation. These same diagnostic tools can be used to assess the fit of the chemistry of one site into the mixing subspace of a different site, thereby permitting an assessment of the consistency of controlling end‐members across sites. This technique is applied to a number of sites at the Panola Mountain Research Watershed located near Atlanta, Georgia.
Partial transport in a natural gravel bed channel
Authors: Haschenburger Judith K. , Wilcock Peter R.
Published: May 2003

Partial transport is documented in the gravel bed channel of Carnation Creek using magnetically tagged stones. For four flood peaks the active proportion of surface grains was used to map streambed areas into distinct units of three different levels of grain entrainment. In partially mobile regions of the bed, the active proportion of surface grains declines with grain size. As flow increases, areas of partial transport grow at the expense of inactive areas and fully active areas replace areas with partial mobility. Approximately 25–50% of the bed remained in a state of partial mobility during a flood with a 2‐year return period, indicating that inactive regions of the bed surface typically persist from year to year. During a flood with a 7‐year return period, surface grain entrainment was nearly complete, indicating that full mobilization of surface grains is not a frequent event.
An analytical solution representing groundwater–surface water interaction
Authors: Anderson Erik I.
Published: May 2003

An analytical solution is developed and used to examine groundwater–surface water interaction at streams; we consider two‐dimensional, steady groundwater flow in the vertical plane. The aquifer is idealized as an infinite strip, and the streambed is modeled as a horizontal equipotential. Segments of constant recharge are applied along the boundaries of the infinite strip. Explicit analytical solutions for the complex potential and complex specific discharge are developed using conformal mapping, the method of images, and boundary integral techniques. The expressions for the complex specific discharge may be used to identify and quantify the numerous groundwater flow regimes first identified by Nield et al. [1994]. We demonstrate this by evaluating the conditions required for transition from a flow‐through regime to a flow‐through regime with a zone of reverse flow.