bluuebunny/crossref_metadata_2025 huggingface dataset:
Southward extrusion of Tibetan crust and its effect on Himalayan tectonics
Authors: Hodges K. V. , Hurtado J. M. , Whipple K. X.
Published: February 2004

The Tibetan Plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during India‐Asia collision, and the contrast in potential energy between the Plateau and its surroundings strongly influences the modern tectonics of south Asia. The distribution of potential energy anomalies across the region, derived from geopotential models, indicates that the Himalayan front is the optimal location for focused dissipation of excess energy stored in the Plateau. The modern pattern of deformation and erosion in the Himalaya provides an efficient mechanism for such dissipation, and a review of the Neogene geological evolution of southern Tibet and the Himalaya shows that this mechanism has been operational for at least the past 20 million years. This persistence of deformational and erosional style suggests to us that orogens, like other complex systems, can evolve toward “steady state” configurations maintained by the continuous flow of energy. The capacity of erogenic systems to self‐organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far‐field plate interactions.
Southeast Baffin volcanic margin and the North American‐Greenland plate separation
Authors: Geoffroy Laurent , Callot Jean‐Paul , Scaillet Stéphane , Skuce Andy , Gélard J. P. , Ravilly M. , Angelier J. , Bonin B. , Cayet C. , Perrot K. , Lepvrier C.
Published: February 2004

Plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at Moho level and extruded as thick piles of seaward dipping lava formations. The Disko‐Svartenhuk area in west Greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward. We present a quantitative tectonic study of this margin, which we tentatively restored in its geodynamic position during the different stages of plate separation between Greenland and North America. Our structural data are constrained with recently published 39Ar/40Ar and new and coherent 40K/40Ar geochronology in dikes of different orientations. The first‐order structure is that of a tectonically‐driven seaward crustal flexure linked to definitive plate breakup between Greenland and Baffin Island during the Eocene, coeval with the formation of the upper part of the exposed seaward dipping volcanic prism. This flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults. This across‐strike structure is correlated to a fundamental along‐strike segmentation with the three “segments” adopting a “zigzag” strike. There is a clear increase of extensional strain at the extremities of the segments. Eocene extension trended N060 on average in the northern and southern segments but was NW trending in the central NE trending Nugssuaq segment. We discuss the interpretation of such an extension perpendicular to the different margin segments. From a regional point of view the N060 extension is distinct in orientation from the approximately N‐S trend of plate separation between North America and Greenland in the Baffin Bay during the Eocene. However, the extension recorded in the margin is nearly perpendicular to the obliquely spreading Eocene accretion axis in the Baffin Bay. This latter point suggests a mantle or ridge control of the development of the margin over regional far‐field lithospheric stress models.
Horizontal tectonic deformation geometries in a late Archaean sedimentary sequence, Belingwe greenstone belt, Zimbabwe
Authors: Hofmann Axel , Dirks Paul H. G. M. , Jelsma Heilke A.
Published: February 2004

In the Belingwe greenstone belt of Zimbabwe, structural evidence from the circa 2.65 Ga old sedimentary Cheshire Formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence. The Cheshire Formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin. The earliest deformational structures formed during a syndepositional to postdepositional, thin‐skinned thrusting event (D1) that affected poorly consolidated sediments and is recorded in bedding‐parallel ductile shear zones, boudins, folds, and block‐in‐matrix structures. D1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units. Syntectonic sulphide mineralization and silicification of mainly fine‐grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations. Stratigraphic units were locally duplicated along the D1 thrust faults, including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange. The elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport. Deposition of the Cheshire Formation took place in an asymmetric, foreland‐type basin contemporaneously with thrusting. Soon after deposition the formation was incorporated into the thrust stack. Subsequent deformation events include tight upright folding, gentle cross‐folding, and dextral strike‐slip faulting of the greenstone succession. Evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the Belingwe greenstone belt.
Fluvial aggradation in Vedder River: Testing a one‐dimensional sedimentation model
Authors: Ferguson R. I. , Church M. , Weatherly H.
Published: February 2004

Sediment routing models which simulate the coevolution of river long profile and bed grain size distributions have been used to investigate downstream fining above base level, channel response to rectification, and other disequilibrium situations at a variety of timescales. We extend one such model (SEDROUT [Hoey and Ferguson, 1994]) to deal with sand as well as gravel and test its ability to simulate aggradation and downstream fining in a well‐documented Canadian river. Previous tests of this and similar models have been largely restricted to comparing observed and simulated gradients of downstream fining, but it is not obvious when to make the comparison in a time‐dependent model with uncertain initial state and no equilibrium except in the very long term. We discuss and apply a more rigorous set of test criteria and address issues of defining initial conditions and time to test. The model's varying sensitivity to different boundary conditions and parameters indicates key data constraints on the testability and predictive accuracy of any such model. We also consider the adequacy of one‐dimensional calculations in channels with variable width and present initial results of attempts to allow for this.
The Lagrangian concentration approach for determining dilution in aquifer transport: Theoretical analysis and comparison with field experiments
Authors: Fiori A.
Published: February 2004

Transport of nonreactive solutes in aquifers of random permeability is considered. The local concentration C, which is a random function of space and time, can be modeled through 〈C〉, the ensemble mean, and σC2, the variance. Because of the irregular spatial distribution of the concentration, which is a consequence of the spreading and dilution of the solute plume, the uncertainty in the local concentration prediction can be very large, and the mean concentration 〈C〉 may not accurately predict the concentration distribution in the field, in particular, the peak concentration. The aim of the present paper is to quantify dilution through the peak concentration by introducing a quantity denoted as Lagrangian concentration (CL). According to the proposed methodology the combined effects of dispersion, which is ruled by large‐scale advection, and dilution, which is a consequence of pore‐scale dispersion, are analyzed by following each Darcy‐scale solute particle constituting the initial plume by focusing on the mass exchanges between the examined particle and the surrounding ones. After definition and derivation of CL and its statistical moments, illustrative examples are shown for a small solute injection. Then the comparison of the theoretical results with the Borden and Cape Cod experimental findings shows that the Lagrangian concentration gives a reasonable prediction of the peak concentration: conversely, the often employed mean concentration 〈C〉 underestimates the maximum concentration. Furthermore, the fluctuations of CL are also smaller than those for C, as indicated by the analysis of the variance. The results are of some importance for applications in which the environmental regulations are formulated in terms of maximal admissible local concentrations.
Impact of exchange flows on wetland flushing
Authors: Andradóttir Hrund Ó. , Nepf Heidi M.
Published: February 2004

The flushing of littoral regions is governed by barotropic river flows,QR, and baroclinic exchange flows, ΔQ. This note presents field observations of two different flushing regimes in a shallow wetland that borders a lake. In spring, when river flows are high, the wetland circulation is river‐ or jet‐dominated, ΔQ/QR< 1, and the river short‐ circuits through the wetland in a much shorter time than the nominal residence time. During summer low flows, however, the wetland circulation is dominated by exchange flows, ΔQ/QR> 1, that vary both on diurnal and synoptic (10–20 days) timescales in response to differential heating and cooling between the wetland and lake and to wind. These exchange flows can enhance wetland flushing by a factor of 10 relative to river flushing. A one‐dimensional decoupled heat and flow model represents diurnal exchange flows in this system well and may be used to assess the importance of exchange flows in other systems.
Bayesian Estimation of change points using the general linear model
Authors: Rasmussen Peter
Published: February 2004

Bayesian analysis is applied to the general linear model to develop a framework for studying different types of change in the mean value of time series and linear regressions. The output of the Bayesian analysis is the posterior distribution of change point location and amplitude. This information provides a rational and relatively objective basis for making decisions as to where to locate a change point. Several examples of hydrological applications are presented to demonstrate the utility of the methodology.
Field investigation of three‐dimensional flow structure at stream confluences: 1. Thermal mixing and time‐averaged velocities
Authors: Rhoads Bruce L. , Sukhodolov Alexander N.
Published: February 2004

Stream confluences are characterized by complex patterns of three‐dimensional fluid motion. This paper examines the three‐dimensional time‐averaged flow structure at three concordant‐bed confluences in east central Illinois. Two of the junctions have symmetrical planforms, whereas the other has an asymmetrical planform. Similarities among the sites include (1) pronounced convergence of flow at the upstream end of the confluences, (2) a region of stagnated fluid near the upstream junction corner, (3) a well‐defined thermal mixing interface between the converging flows that persists downstream of each confluence, (4) a downstream velocity field characterized by two zones of maximum velocity separated by an intervening region of low velocity centered on the mixing interface, (5) convective acceleration of flow within the mixing interface leading to increasing uniformity of the downstream velocity field in the downstream direction, and (6) lateral deflection of flow by the dominant tributary. Prominent helical motion occurs at the asymmetrical confluence, whereas weak helicity is detectable only at one of the two symmetrical confluences. The downstream persistence of a well‐defined mixing interface at the two symmetrical confluences and the disruption of this interface at the asymmetrical confluence suggest that helical motion enhances patterns of mixing at confluences.
Field investigation of three‐dimensional flow structure at stream confluences: 2. Turbulence
Authors: Sukhodolov Alexander N. , Rhoads Bruce L.
Published: February 2004

Stream confluences are among the most highly turbulent locations in fluvial systems. This paper examines the three‐dimensional structure of turbulence at three stream confluences in east central Illinois. The analysis focuses on the characteristics of turbulence both within the shear layer and in the ambient flow. Results show that at the upstream end of each confluence the shear layer occupies a limited portion of the flow cross‐sectional area, but turbulence kinetic energy within this layer is 2–3 times greater than the turbulence kinetic energy of the ambient flow, which has turbulence characteristics similar to those for flow in straight channels. Turbulence within the shear layer can be characterized as quasi‐two‐dimensional in the sense that large‐scale turbulence generated by transverse shear is predominantly two dimensional, whereas small‐scale turbulence associated with bed friction is three dimensional. Spectral analysis suggests that the structure of fluid motion within the shear layer differs for confluences with symmetrical versus asymmetrical planforms. The shear layer dissipates rapidly as flow enters the downstream channel, even though a well‐defined mixing interface persists at downstream locations.
Water flow within a fault in altered nonwelded tuff
Authors: Salve Rohit , Oldenburg Curtis M.
Published: February 2004

In situ flow experiments were conducted at Alcove 4 in the Exploratory Studies Facility at Yucca Mountain to investigate flow dynamics in a small normal fault located in zeolitically and argillically altered tuffs of the nonwelded rocks of the Paintbrush group. Water was released at constant head into a packed‐off interval straddling the fault within a horizontal borehole. Changes in water‐intake rate into the interval were monitored along with saturation in the formation using boreholes installed with electrical resistivity probes. For water released into the fault the water‐intake rate gradually fell from ∼200 to 50 mL min−1 over a period of 41 hours of cumulative release time spread over 17 days of testing. Resistance changes recorded by electrical resistivity probes in matrix rock on either side of the fault interval suggest that flow in the fault spread out laterally in the fault plane. The time for the wetting front to travel l m in the fault was initially ∼0.15 days. Subsequent tests in the fault showed the response time to be dependent on wetting history, with a faster response observed in a wetter system. Data analysis using the numerical code TOUGH2/ITOUGH2 reveals that a simple decline in permeability caused by simulated clay swelling can explain the experimental observations.
The derivation of areal reduction factor of storm rainfall from its scaling properties
Authors: De Michele Carlo , Kottegoda Nathabandu T. , Rosso Renzo
Published: February 2004

We present a method of modeling the areal reduction factor (ARF) of storm rainfall. The ARF is widely used in reducing point rainfall to obtain areal average values for the same duration, probability of exceedance, and specified area. The concepts of scaling and multiscaling, developed in recent years, provide a powerful framework for studying spatial and temporal variability of hydrological processes. It is our view that ARF must reflect the scaling properties of rainfall in space and time. We develop a simple statistical approach to the ARF of extreme storm rainfall based on the scaling properties of the underlying process in space and time. We derive the scaling relations of mean rainfall intensity over an area A and for a duration T using the concepts of dynamic scaling and statistical self‐affinity. A new physically based formula for the ARF is then obtained. Applications are made to observations from the metropolitan area of Milan, Italy, and to data in the United Kingdom, as given in the Natural Environmental Research Council Flood Studies Report. These studies indicate that storm rates in space and time are scaling for extreme events, and hence this concept is shown to provide a useful practical approach to the evaluation of design storms for specified areas.
Valuing water quality advisories and beach amenities in the Great Lakes
Authors: Murray Chris , Sohngen Brent , Pendleton Linwood
Published: February 2004

This paper presents estimates of the value of reducing beach advisories in Great Lakes beaches located along Lake Erie's shoreline in Ohio. Given the potential health consequences associated with swimming in contaminated water, health officials have made more effort in recent years to ensure that information about beach advisories is available through the media and other sources. A recent survey of over 800 single‐day beach visitors during the summer of 1998 found that nearly two thirds of visitors take advantage of this information when making decisions about their beach trips. When accounting for this information in a travel cost model, we find that the average (across all visitors) seasonal benefits of reducing one advisory is approximately $28 per visitor. Individuals who use the media in advance of trips gain less, approximately $24 per year, while those who use only signs posted at the beach would gain more, $38 per year.
Long‐Term Climate Database, Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Hanson C. L. , Marks D. , Van Vactor S. S.
Published: February 2004

A 33 year (1964–1996), climatic database has been developed for three climate stations on the Reynolds Creek Experimental Watershed located near the north end of the Owyhee Mountains in southwest Idaho. The longest records (1964–1996) are for daily maximum and minimum air temperature. The length of record for other weather elements that include hourly air temperature, relative humidity, dew point temperature, vapor pressure, solar radiation, wind speed and direction, barometric pressure, and daily class A pan evaporation varies, but, in general, the record is from 1974–1996. These data can be accessed from the U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center database through the anonymous ftp site: ftp.nwrc.ars.usda.gov.
Long‐Term Stream Discharge and Suspended‐Sediment Database, Reynolds Creek Experimental Watershed, Idaho, United States
Authors: Pierson Frederick B. , Slaughter Charles W. , Cram Zane K.
Published: February 2004

The U.S. Department of Agriculture, Agricultural Research Service, Northwest Watershed Research Center initiated a stream discharge and suspended‐sediment research program at Reynolds Creek Experimental Watershed in the early 1960s. Continuous discharge measurements began at two sites in 1963, at three additional sites in 1964, and at eight additional sites in subsequent years. Contributing areas to these gauging stations range from 1.03 to >23,822 ha, selected to represent the broad range of environmental settings found across northwestern rangelands. Quality‐controlled, validated breakpoint and hourly stream discharge data sets are available for these 13 sites for the period 1963 through 1996 (or for a subset of that time for some sites). Suspended‐sediment data are available for three gauging stations (high elevation, middle elevation, and low elevation). All data are available on the Northwest Watershed Research Center anonymous ftp site (ftp.nwrc.ars.usda.gov).
Downscaling of radio brightness measurements for soil moisture estimation: A four‐dimensional variational data assimilation approach
Authors: Reichle Rolf H. , Entekhabi Dara , McLaughlin Dennis B.
Published: February 2004

This paper investigates the feasibility of estimating large‐scale soil moisture profiles and related land surface variables from 1.4 GHz (L‐band) passive microwave measurements, using variational data assimilation. Our four‐dimensional assimilation algorithm takes into account both model and measurement uncertainties and provides dynamically consistent interpolation and extrapolation of remote sensing data over space and time. The land surface hydrologic model which forms the heart of the variational algorithm was expressly designed for data assimilation purposes. This model captures key physical processes while remaining computationally efficient. We test our algorithm with a series of synthetic experiments based on the Southern Great Plains 1997 Hydrology Experiment. These experiments provide insights about three issues that are crucial to the design of an operational soil moisture assimilation system. Our first synthetic experiment shows that soil moisture can be satisfactorily estimated at scales finer than the resolution of the brightness images. This downscaling experiment indicates that brightness images with a resolution of tens of kilometers can yield soil moisture profile estimates on a scale of a few kilometers, provided that micrometeorological, soil texture, and land cover inputs are available at the finer scale. In our second synthetic experiment we show that adequate soil moisture estimates can be obtained even if quantitative precipitation data are not available. Model error terms estimated from radio brightness measurements are able to account in an aggregate way for the effects of precipitation events. In our third experiment we show that reductions in estimation performance resulting from a decrease in the length of the assimilation time interval are offset by a substantial improvement in computational efficiency.
Hyporheic exchange of solutes and colloids with moving bed forms
Authors: Packman Aaron I. , Brooks Norman H.
Published: February 2004

Stream‐subsurface exchange provides the opportunity for stream‐borne substances to interact with streambed sediments in the subsurface hyporheic mixing zone. The downstream transport of both solutes and colloids can be substantially affected by this exchange, with significant implications for contaminant transport and stream ecology. Several previous studies have demonstrated that bed form–induced advective flows (pumping) and scour/deposition of bed sediments (turnover) will often be the dominant processes controlling local exchange with the streambed. A new model is presented for combined turnover and pumping exchange due to relatively fast‐moving bed forms, i.e., when turnover dominates the exchange in the upper part of the bed where active bed sediment transport occurs. While turnover rapidly mixes the upper layer of the bed, advective pumping produces exchange with the deeper, unsecured region of the subsurface. The net exchange due to these processes was analyzed using fundamental hydraulic principles: the initial exchange was calculated using an existing geometric model for turnover, and then the later exchange was determined by analyzing the advective flow induced under the moving bed form field. The exchange of colloidal particles due to moving bed forms was also modeled by considering the further effects of particle settling and filtration in the subsurface. Experiments were conducted in a recirculating flume to evaluate solute (conservative Li+) and colloid (kaolinite) exchange with a sand bed. The solute and colloid exchange models performed well for fast‐moving bed forms, but underpredicted the colloid exchanges observed with lower rates of bed sediment transport. For very slowly moving bed forms it was found that turnover could be completely neglected, and observed colloid exchanges were represented well by a pure pumping model. In the intermediate case where turnover and pumping rates are similar, water carried into the bed by turnover is immediately released by pumping, and vice versa. Thus, while this work further elucidated the basic processes controlling solute and colloid exchange with a bed covered by bed forms and provided a fundamental model for exchange due to fast‐moving bed forms, exchange in the intermediate case where turnover and pumping tend to compete can only be bounded by current models.
Travel times of nonlocal dispersion and their geoelectric approximation in Nevada's fractured welded tuffs
Authors: Purvance David T.
Published: February 2004

This technical note argues the importance of incorporating nonlocal dispersion effects when modeling contaminant travel times. Using the transport theory ofDi Fedenco and Neuman[1998], expressions for nonlocal contaminant travel times and breakthrough are developed. They show that nonlocal travel times are significantly faster than their local (Fickian) counterpart and that nonlocal breakthrough occurs significantly earlier than Fickian. As an illustrative example, nonlocal breakthrough curves for Nevada's fractured welded tuffs, a common rock unit found at the U.S. Department of Energy's Yucca Mountain Project, are approximated using the electrical‐hydraulic conductivity correlation theory and measurements ofPurvance and Andricevic[2000a, 2000b]. At the 5 km compliance point the approximation shows that nonlocal contaminant breakthrough occurs ∼5 times earlier than what is predicted by the Department of Energy's Fickian model.
Interparticle collision of natural sediment grains in water
Authors: Schmeeckle Mark W. , Nelson Jonathan M. , Pitlick John , Bennett James P.
Published: February 2004

Elastohydrodynamic theory and measurements of particle impacts on an inclined glass plane in water are used to investigate the mechanics of interparticle collisions in sediment‐transporting flows. A collision Stokes number is proposed as a measure of the momentum of an interparticle collision versus the viscous pressure force in the interstitial gap between colliding particles. The viscous pressure force opposes motion of the particles on approach and rebound. A Stokes number of between 39 and 105 is estimated as the critical range below which particle impacts are completely viscously damped and above which impacts are partially elastic. The critical Stokes number is shown to roughly coincide with the Bagnold number transition between macroviscous and grain inertial debris flows and the transition between damped and partially elastic bed load transport saltation impacts. The nonspherical nature of natural particles significantly alters the motion of the center of mass after a partially elastic collision. The normal to the point of contact between the particles does not necessarily go through the center of mass. Thus normal rebound of the center of mass may not occur. A model of particle motion after rebound for particles of arbitrary shape, conserving both linear and angular momentum, is proposed.
Use of a landscape simulator in the validation of the SIBERIA Catchment Evolution Model: Declining equilibrium landforms
Authors: Hancock Greg , Willgoose Garry
Published: February 2004

This paper presents a study to test the ability of the current generation of landscape evolution models to correctly predict landscape form from measured erosion processes. Landscapes generated by the SIBERIA landscape evolution model were compared to experimental model landscapes in declining equilibrium. The model simulations were compared using geomorphologically and hydrologically significant parameters. Comparisons of the simulated landscapes with the experimental landscapes were carried out using the hypsometric curve, width function, cumulative area distribution, and area‐slope relationship. These comparisons demonstrate that SIBERIA can correctly simulate the experimental model landscape at declining equilibrium. The simulation showed sensitivity to the spatial distribution of the rainfall, particularly with respect to hypsometry. Using the correct measured distribution of rainfall was necessary rather than using a spatially uniform rainfall distribution. The results also highlighted the importance of digital terrain map (DTM) error in deriving geomorphic statistics. The observed landform had a consistently larger width function than the simulation. Only when the simulations were corrupted with elevation errors with statistical properties of the errors in the experimental landscape DTM, as measured by photogrammetry, did the observed and simulated width functions match.
Stability analysis of the unsaturated water flow equation: 1. Mathematical derivation
Authors: Du X. , Yao T. , Stone W. D. , Hendrickx J. M. H.
Published: February 2004

This paper provides a theoretical stability analysis of gradual wetting fronts based on perturbation analysis. A traveling wave solution of the one‐dimensional vertical flow Richards' equation is used as the basic flow on which three‐dimensional perturbations are introduced. By locally linearizing the diffusivity form of the three‐dimensional Richards' equation a linear partial differential equation is obtained which governs the perturbation variables. The stability of each point at the wetting front is considered in a local coordinate system. The analysis of this perturbation equation at these points of the wetting front provides not only the relationship between the finger sizes and the nonponding infiltration rates at the soil surface but also the traveling speeds of the fingers rooted from these points. Once a perturbation is introduced at some point on the wetting front, there are three possibilities for the development of the perturbation. (1) The perturbation will monotonically decline with time; in this case, no fingers will form and the system is stable. (2) The perturbation does not decline with time, but its downward velocity is less than that of the stable basic wetting front; thus the distribution layer will gradually cover the fingers and the system will become stable. (3) The perturbation will increase with time and have a downward velocity greater than that of the stable wetting front; in this case, the finger will persistently grow in front of the stable wetting front and the system will become unstable. This analysis can be applied to an unsaturated homogeneous soil profile with uniform initial water content for the prediction of instability and for the estimation of finger characteristics over a wide range of infiltration rates.
Hydraulic properties of two‐dimensional random fracture networks following a power law length distribution: 1. Effective connectivity
Authors: de Dreuzy Jean‐Raynald , Davy Philippe , Bour Olivier
Published: February 2004

Natural fracture networks involve a very broad range of fractures of variable lengths and apertures, modeled, in general, by a power law length distribution and a lognormal aperture distribution. The objective of this two‐part paper is to characterize the permeability variations as well as the relevant flow structure of two‐dimensional isotropic models of fracture networks as determined by the fracture length and aperture distributions and by the other parameters of the model (such as density and scale). In this paper we study the sole influence of the fracture length distribution on permeability by assigning the same aperture to all fractures. In the following paper [de Dreuzy et al., this issue] we study the more general case of networks in which fractures have both length and aperture distributions. Theoretical and numerical studies show that the hydraulic properties of power law length fracture networks can be classified into three types of simplified model. If a power law length distributionn(l) ∼l−ais used in the network design, the classical percolation model based on a population of small fractures is applicable for a power law exponentahigher than 3. Foralower than 2, on the contrary, the applicable model is the one made up of the largest fractures of the network. Between these two limits, i.e., forain the range 2–3, neither of the previous simplified models can be applied so that a simplified two‐scale structure is proposed. For this latter model the crossover scale is the classical correlation length, defined in the percolation theory, above which networks can be homogenized and below which networks have a multipath, multisegment structure. Moreover, the determination of the effective fracture length range, within which fractures significantly contribute to flow, corroborates the relevance of the previous models and clarifies their geometrical characteristics. Finally, whatever the exponenta, the sole significant scale effect is a decrease of the equivalent permeability for networks below or at percolation threshold.
Generalized probability weighted moments: Application to the generalized Pareto Distribution
Authors: Rasmussen Peter F.
Published: February 2004

Probability weighted moments (PWMs) are widely used in hydrology for estimating parameters of flood distributions. The classical PWM approach considers moments of the typeE[XFj] (or, alternatively,E[X(1 −F)k]), wherej(ork) takes values 0, 1, or 2 depending on the number of parameters to be estimated. The classical approach is here compared with an extended class of PWMs that does not restrictjorkto be small nonnegative integers. Estimation based on the extended class of PWMs is named the generalized method of PWMs to distinguish it from the classical procedure. To illustrate the method, we consider estimation of quantiles in the generalized Pareto distribution and demonstrate that substantial gain in estimation accuracy can be obtained by using generalized PWMs.
Sensible heat flux estimation by flux variance and half‐order time derivative methods
Authors: Wesson Karen H. , Katul Gabriel , Lai Chun‐Ta
Published: February 2004

This study is the first to contrast two similarity theory methods, the flux variance and the half‐order time derivative, over a wide range of atmospheric stability and surface roughness conditions. These two methods were selected because they require only single‐level temperature measurement to estimate sensible heat flux. The data used were collected over bare soil, a grass‐covered forest clearing, and an even‐aged pine forest. For all three sites the flux variance method estimated the sensible heat flux relatively well for unstable atmospheric conditions. The half‐order time derivative method was found to be sensitive to the parameterization of the eddy diffusivity, especially for the grass and bare soil field sites. Overall, the flux variance method was able to reproduce the measured sensible heat flux with greater accuracy than the half‐order time derivative methods for the three experiment sites.
A direct method for the identification of the permeability field of an anisotropic porous medium
Authors: Zhan Lang , Yortsos Yanis C.
Published: February 2004

In a recent publication [Zhan and Yortsos, 2000] we proposed a direct method for the inversion of the permeability field of an isotropic porous medium based on the analysis of the displacement of a passive tracer. By monitoring the displacement front at successive time intervals (e.g., using a tomographic method), the permeability can be directly obtained from the solution of a nonlinear boundary value problem. In this paper we extend this approach to the case when the porous medium is anisotropic. When the principal axes of anisotropy are known and fixed, a procedure is proposed in which the tracer is injected two (or three) consecutive times along the two (or three) principal directions (for the case of a two‐dimensional (2‐D) (or three‐dimensional) problem). It is shown that the diagonal components can be obtained from the solution of two (or three) coupled boundary value problems involving the experimentally obtained fields of arrival times. Numerical examples show that the method works well when the permeability variation is not very sharp (e.g., for correlated distributions). When the permeability tensor is full and the principal axes vary in space, a procedure involving the injection in three different directions (for the case of a 2‐D problem) is also described.
A borehole flowmeter investigation of small‐scale hydraulic conductivity variation in the Biscayne Aquifer, Florida
Authors: Genereux David , Guardiario Jose
Published: February 2004

Geostatistical analysis of closely spaced borehole flowmeter measurements was used to estimate the variance (2.53) and vertical and horizontal correlation lengths of ln K(0.57 and 7.3 m) in the Biscayne Aquifer, a limestone aquifer critical for Florida's water supply and for Everglades restoration efforts. The variance and correlation lengths of the Biscayne Aquifer are similar to some of the values for unconsolidated siliciclastic sediments (especially those at Columbus, Mississippi∥. The larger λh, for the Biscayne Aquifer (7.3 m) is thought to be due at least in part to the lower lateral variability of the carbonate platform depositional environment, compared to the fluvial environments in which the siliciclastic sediments were deposited. An improved down hole packer would allow for data with finer vertical resolution; the current system is adequate for work inside well screens but cannot adequately seal many spots in open, irregular rock boreholes. Research in rock presents additional logistical difficulties but is important for addressing fundamental questions about solute transport in a wider range of geological media, beyond unconsolidated siliciclastic deposits.
Scaling gridded river networks for macroscale hydrology: Development, analysis, and control of error
Authors: Fekete Balázs M. , Vörösmarty Charles J. , Lammers Richard B.
Published: February 2004

A simple and robust river network scaling algorithm (NSA) is presented to rescale fine‐resolution networks to any coarser resolution. The algorithm was tested over the Danube River basin and the European continent. Coarse‐resolution networks, at 2.5, 5, 10, and 30 min resolutions, were derived from higher‐resolution gridded networks using NSA and geomorphometric attributes, such as river order, shape index, and width function. These parameters were calculated and compared at each resolution. Simple scaling relationships were found to predict decreasing river lengths with coarser‐resolution data. This relationship can be used to correct river length as a function of grid resolution. The length‐corrected width functions of the major river basins in Europe were compared at different resolutions to assess river network performance. The discretization error in representing basin area and river lengths at coarser resolutions were analyzed, and simple relationships were found to calculate the minimum number of grid cells needed to maintain the catchment area and length within a desired level of accuracy. This relationship among geomorphological characteristics, such as shape index and width function (derived from gridded networks at different resolutions), suggests that a minimum of 200–300 grid cells is necessary to maintain the geomorphological characteristics of the river networks with sufficient accuracy.
Modern isotope climatology of Russia: A first assessment
Authors: Kurita Naoyuki , Yoshida Naohiro , Inoue Gen , Chayanova Eleonora A.
Published: February 2004

The spatial and temporal variation in the stable isotopic composition of precipitation collected at 13 monitoring stations across Russia between 1996 and 2000 was determined. The results show that eastward moisture transport over the continent generates a tendency toward more negative isotopic content farther inland throughout the year. This negative isotopic gradient can be explained by the gradual rain‐out of moist, oceanic air masses, which are transported inland by westerly winds. In summer, however, the isotopic gradient is less clear, because of additional moisture that is supplied from land surfaces. The isotopic pattern of summer precipitation is less sensitive to moisture content but is largely influenced by the original moisture. In Siberia, more than half of the moisture that forms summer precipitation originates from land surfaces; thus the isotopic content of precipitation in this region is controlled mainly by the contribution of recycled water; e.g., the proportion of water that is recycled (recycling ratio) and its isotopic composition. Comparisons of the observed summer isotopic content of precipitation and the calculated recycling ratio from National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis data show that about 20% of the total variability of isotopic content during summer can be linked to the recycling ratio. About 45% of the summer isotopic variability cannot be explained by either temperature, which is used as an indicator of moisture content, or the recycling ratio. This remaining variability may be linked to the isotopic variability of the recycled water that falls as observed precipitation. It may be that the isotopic content of recycled water varies in space and time and the isotopic distribution in summer precipitation reveals details in this feature; thus it might be possible to deduce information about the interaction between land and atmosphere in the hydrologic cycle from the isotopic content of precipitation.
A rational function approach for estimating mean annual evapotranspiration
Authors: Zhang L. , Hickel K. , Dawes W. R. , Chiew F. H. S. , Western A. W. , Briggs P. R.
Published: February 2004

Mean annual evapotranspiration from a catchment is determined largely by precipitation and potential evapotranspiration; characteristics of the catchment (e.g., soil, topography, etc.) play only a secondary role. It has been shown that the ratio of mean annual potential evapotranspiration to precipitation (referred as the index of dryness) can be used to estimate mean annual evapotranspiration by using one additional parameter. This study evaluates the effects of climatic and catchment characteristics on the partitioning of mean annual precipitation into evapotranspiration using a rational function approach, which was developed based on phenomenological considerations. Over 470 catchments worldwide with long‐term records of precipitation, potential evapotranspiration, and runoff were considered, and results show that model estimates of mean annual evapotranspiration agree well with observed evapotranspiration taken as the difference between precipitation and runoff. The mean absolute error between modeled and observed evapotranspiration was 54 mm, and the model was able to explain 89% of the variance with a slope of 1.00 through the origin. This indicates that the index of dryness is the most significant variable in determining mean annual evapotranspiration. Results also suggest that forested catchments tend to show higher evapotranspiration than grassed catchments and their evapotranspiration ratio (evapotranspiration divided by precipitation) is most sensitive to changes in catchment characteristics for regions with the index of dryness around 1.0. Additionally, a stepwise regression analysis was performed for over 270 Australian catchments where detailed information of vegetation cover, precipitation characteristics, catchment slopes, and plant available water capacity was available. It is shown that apart from the index of dryness, average storm depth, plant available water capacity, and storm arrival rate are also significant.
Empirical Bayes regionalization methods for spatial stochastic processes
Authors: Butcher Jonathan B. , Medina Miguel A. , Marin Carlos M.
Published: February 2004

Many geophysical properties can be described as spatial stochastic processes, including spatially correlated hydraulic conductivity fields. Use of regional data can potentially improve estimation of such processes. We consider the case in which observations at each of several sites are described by a general linear model, while the parameters of these models arise from a common regional distribution. Parametric empirical Bayes methods enable the determination of the parameters of the regional distribution via maximum likelihood. However, such methods have not been utilized for spatial stochastic processes. We develop the application of a simple iterative technique for maximum likelihood estimation of the regional parameters, and demonstrate its use with a common parameterization of the spatial covariance structure. Synthetic data tests show the potential for substantial reduction in estimation risk through use of such techniques.
Water price as a policy variable in managing urban water use: Tucson, Arizona
Authors: Martin William E. , Kulakowski Susan
Published: February 2004

An informal analysis of readily available time series data relevant to urban water use in Tucson, Arizona, is presented as an illustration of how a management‐oriented empirical analysis, structured with general knowledge of the results of formal econometric studies, can produce useful insights for making applied price policy. Precise estimates of urban water demand price and income elasticities are not required in order to achieve conservation‐oriented goals. In the Tucson experience, annual water price increases equal to the rate of inflation plus approximately the rate of change in real per capita income would have been necessary just to maintain constant rather than increasing water use. Given the actual price policy, it is not surprising that Tucson has not achieved its mandated conservation goal.
The efficiency function of detention reservoirs in urban drainage systems
Authors: Kessler A. , Diskin M. H.
Published: February 2004

Detention reservoirs in urban drainage systems are provided for reducing the peak flows downstream of the reservoir. The flow reduction is due to the storage volume of the reservoir in which the incoming flow is temporarily stored. The relationship between the peak reduction ratio (the peak outflow discharge to the peak inflow discharge) and the relative storage ratio (maximum volume stored in the reservoir to the volume of the inflow hydrograph) is evaluated in this study. This relationship, named the efficiency function, is derived by a numerical solution of the dimensionless routing equation. Using the three‐parameter gamma distribution to represent the inflow to the reservoir, the efficiency functions can be expressed by simple linear equations. Each equation depends on a single parameter whose value is determined by the reservoir configuration (depth‐storage relationship) and the type of the outlet device (e.g., spillway or culvert). The implementation of the efficiency function is most suitable for the preliminary design stage where a large number of alternative designs can be easily tested without performing the conventional routing procedure.