bluuebunny/crossref_metadata_2025 huggingface dataset:
Mountain polar stratospheric cloud measurements by Ground Based FTIR Solar Absorption Spectroscopy
Authors: Höpfner Michael , Blumenstock Thomas , Hase Frank , Zimmermann Alexandra , Flentje Harald , Fueglistaler Stefan
Published: February 2004

An analysis of mountain polar stratospheric cloud observations by solar absorption FTIR from Kiruna/Sweden was performed and compared with co‐located airborne lidar measurements. The infrared cloud spectral signature and optical depth indicated water ice particles compatible with the high backscatter and depolarization ratio of the lidar signal. Retrieval of mean particle properties from the FTIR spectrum depend on the assumed width of the particle size distribution and range from 2.0 to 1.1 µm radius and from 1.9 to 5.1 cm−3 number density. The volume density is well determined with 63.8–60.2 µm³/cm³ equivalent to about 2.5 ppmv of condensed water at 20 hPa. Simulations of lidar backscatter ratio based on these results are consistent with the lidar observations.
Evaporation changes over the contiguous United States and the former USSR: A reassessment
Authors: Golubev Valentin S. , Lawrimore Jay H. , Groisman Pavel Ya. , Speranskaya Nina A. , Zhuravin Sergey A. , Menne Matthew J. , Peterson Thomas C. , Malone Robert W.
Published: February 2004

Observed decreases in pan evaporation over most of the United States and the former USSR during the post‐WWII period, if interpreted as a decrease in actual evaporation, are at odds with increases in temperature and precipitation over many regions of these two countries. Using parallel observations of actual and pan evaporation at six Russian, one Latvian, and one U.S. experimental sites, we recalibrate trends in pan evaporation to make them more representative of actual evaporation changes. After applying this transformation, pan evaporation time series over southern Russia and most of the United States reveal an increasing trend in actual evaporation during the past forty years.
Effects of solar wind density on auroral electrojets
Authors: Shue J.‐H. , Kamide Y.
Published: February 2004

During the magnetic cloud event of January 10–11, 1997, several density enhancements in the solar wind were observed while other solar wind parameters were relatively constant. By using this fortuitous opportunity, the effects of the solar wind density on the intensity of the auroral electrojets are examined. It is found that a positive relationship exists between the solar wind density and the intensity of the auroral electrojets. The relationship is found to be much weaker for northward interplanetary magnetic field (IMF) than for southward IMF. It is also shown that when the IMF is directed southward, the density effect is dominant in the westward electro jet, while it is most effective in the eastward electro jet when the IMF is directed northward.
Numerical simulation of three‐dimensional, time‐averaged flow structure at river channel confluences
Authors: Bradbrook K. F. , Lane S. N. , Richards K. S.
Published: February 2004

Current confluence research emphasizes three broad controls on flow structure generation: (1) planform curvature; (2) topographic steering; and (3) anisotropic turbulence associated with flow separation and shear layer dynamics. The relative importance of these processes in explaining observed flow structures is controversial, a situation that may be related to the fact that different investigators have examined different confluence configurations. This paper uses a three‐dimensional numerical model, with a fully elliptic solution, a free surface treatment, and a turbulence model based on a renormalized group (RNG) to help to provide a physically based explanation of the controls upon flow structure generation for both a laboratory (rectangular) and a field confluence (the confluence of the Kaskaskia River and Copper Slough) and to identify the particular conditions under which particular flow structures are observed. Results suggest that an analogy with back‐to‐back meanders is possible for symmetrical configurations but that there will be progressive divergence from this state as confluence asymmetry increases. In asymmetric situations a dual‐cell structure may be limited to the immediate vicinity of the junction because of the effects of streamline curvature and topographic steering. These differences can be explained by consideration of the dynamic pressure field, which may be specific to each confluence configuration. As such, this study partially reconciles differing views over what controls time‐averaged flow structures in river channel confluences, although further research into the interaction of these processes with instantaneous velocity fluctuations is required.
Separating the natural and anthropogenic components of spring flood pH decline: A method for areas that are not chronically acidified
Authors: Bishop Kevin H. , Laudon Hjalmar , Köhler Stephan
Published: February 2004

The quantitative distinction of the natural and anthropogenic components of episodic pH decline is important but problematic. This paper presents the Boreal Dilution Model as a means of separating the natural effects of acid neutralizing capacity (ANC) dilution and organic acids from those of acid deposition on stream pH during spring flood or other flow episodes in areas that are not chronically acidified. Key model assumptions are that base flow ANC reflects preindustrial ANC, that the contemporary dissolved organic carbon is natural, and that dilution of base cations during spring flood is a measure of natural ANC dilution. The model could help elucidate the natural variation of spring flood chemistry, as well as the response to changes in acidification pressures. An example of the model's application to one catchment is presented. This example, together with the correlation between winter SO42− deposition and modeled anthropogenic ANC depression during spring flood in some 27 episodes from 18 catchments, is presented to assess the model's plausibility.
Surface flux estimation using radiometric temperature: A dual‐temperature‐difference method to minimize measurement errors
Authors: Norman J. M. , Kustas W. P. , Prueger J. H. , Diak G. R.
Published: February 2004

Surface temperature serves as a key boundary condition that defines the partitioning of surface radiation into sensible and latent heat fluxes. Surface brightness temperature measurements from satellites offer the unique possibility of mapping surface heat fluxes at regional scales. Because uncertainties in satellite measurements of surface radiometric temperature arise from atmospheric corrections, surface emissivity, and instrument calibrations, a number of studies have found significant discrepancies between modeled and measured heat fluxes when using radiometric temperature. Recent research efforts have overcome these uncertainties and in addition have accounted for the difference between radiometric and aerodynamic temperature by considering soil and vegetative‐canopy aerodynamic resistances. The major remaining obstacle to using satellite data for regional heat flux estimation is inadequate density of near‐surface air temperature observations. In this paper we describe a simple, operational, double‐difference approach for relating surface sensible heat flux to remote observations of surface brightness temperature, vegetative cover and type, and measurements of near‐surface wind speed and air temperature from the synoptic weather network. A double difference of the time rate of change in radiometric and air temperature observations is related to heat flux. This double‐difference approach reduces both the errors associated with deriving a radiometric temperature and with defining meteorological quantities at large scales. The scheme is simpler than other recent approaches because it requires minimal ground‐based data and does not require modeling boundary layer development. The utility of this scheme is tested with ground‐based radiometric temperature observations from several arid and subhumid climates with a wide range of vegetative cover and meteorological conditions.
Regional rainfall depth‐duration‐frequency equations for Canada
Authors: Alila Younes
Published: February 2004

The geographical variation of short‐duration rainfall extremes in Canada is first evaluated in terms of depth‐duration and depth‐frequency ratios. Depth‐duration ratios, defined as the ratios of the t‐min to the 60‐min rainfall depths of the same return period, are found to be independent of return period and geographical location for any storm duration of less than 60 min. However, for storms of longer durations, depth‐duration ratios are found to depend on both the return period and geographical location indexed by the at‐site mean annual precipitation. Depth‐frequency ratios, defined as the ratios of the T‐year to the 10‐year rainfall depths of the same storm duration, are also found to depend on the return period and geographical location. Generalized expressions of depth‐duration and depth‐frequency ratios are then combined to develop regional depth‐duration‐frequency equations. A split sampling experiment has verified that the proposed equations reproduce satisfactorily the design storms at long‐term record stations in different hydrologic zones. The proposed equations represent a viable alternative to current interpolation procedures as they eliminate the need for isoline maps of both mean and standard deviation of annual rainfall maxima for various durations.
Inverse modeling for locating dense nonaqueous pools in groundwater under steady flow conditions
Authors: Sciortino Antonella , Harmon Thomas C. , Yeh William W‐G.
Published: February 2004

In this work we develop an inverse modeling procedure to identify the location and the dimensions of a single‐component dense nonaqueous phase liquid (DNAPL) pool in a saturated porous medium under steady flow conditions. The inverse problem is formulated as a least squares minimization problem and solved by a search procedure based on the Levenberg‐Marquardt method. Model output is calculated by an existing three‐dimensional analytical model describing the transport of solute from a dissolving distributed noise upon the forward model‐generated concentration field. We further test the algorithm's ability to predict the location and size of a DNAPL pool placed in a controlled three‐dimensional bench‐scale experiment. In this case we apply the Levenberg‐Marquardt algorithm to the minimization of three types of residuals: ordinary residuals, weighted residuals with weights equal to the square of the inverse of the observations, and weighted residuals with weights obtained by adding a constant term to the observed concentrations. The results are sensitive to the location of the observation wells and to the type of residuals minimized. In general, better results in terms of pool location and dimensions were obtained by the minimization of weighted residuals with weights obtained by adding a constant term to the observed concentrations. The results also indicate that the inverse problem is nonunique and nonconvex even in the absence of observation errors. Finally, the sensitivity of the inverse modeling scheme to transport parameter uncertainty was addressed. The inverse solution was found to be extremely sensitive to errors in the dispersion coefficients and relatively insensitive to errors in the mass transfer coefficient.
Evaluation of continuous distribution models for rate‐limited solute adsorption to geologic media
Authors: Saiers James E. , Tao Gujin
Published: February 2004

We develop two mathematical models for coupled solute transport and nonlinear adsorption to geologic media. Both models quantify nonuniformity in mass transfer rates with parameters that vary according to a gamma (γ) probability density function. The multisite model, which describes surface‐reaction controls on adsorption kinetics, solves equations for a second‐order rate law. The adsorption rate coefficient of the kinetics equation is correlated with a γ‐distributed desorption rate coefficient. The mobile‐immobile model is based on the assumption that rate limitations are diffusion controlled and account for a γ distribution in solute exchange rates between zones of mobile and immobile water. Solute adsorption in the immobile zone is quantified with the nonlinear expression for the Langmuir equilibrium isotherm. We test the models against results of column experiments on the transport of hydroxyatrazine (HA), a persistent contaminant produced from the degradation of atrazine. We find that the experimental data are matched more closely by calculations of the multisite model than by calculations of the mobile‐immobile model, suggesting that HA adsorption can be understood best as a kinetics reaction with a solid phase composed of binding sites with a broad distribution in adsorption and desorption energies.
Kaolinite exchange between a stream and streambed: Laboratory experiments and validation of a colloid transport model
Authors: Packman Aaron I. , Brooks Norman H. , Morgan James J.
Published: February 2004

Experiments were conducted in a recirculating flume to elucidate the fundamental physical and chemical processes which control the stream‐subsurface exchange of colloids. Results are presented on the rate of exchange of colloids (kaolinite clay) and a conservative solute (lithium) from a stream to a sand streambed covered by stationary bed forms (dunes, ripples). Kaolinite and lithium were added to the recirculating stream, and their exchange with the bed was observed over time. Kaolinite was observed to be much more extensively trapped in the streambed than lithium owing to nonconservative processes. By the end of most experiments, essentially all added kaolinite was taken up by the streambed. The observed exchange rates can be explained by analyzing the solute and particle fluxes through the stream‐subsurface interface and the physicochemical interactions between transported kaolinite and the bed sediment. The colloid pumping model predicts particle exchange based on pumping hydraulics, particle settling in the bed, and filtration by the bed sediments. Observed colloid and solute exchanges were successfully predicted by the process‐based models without the use of fitting coefficients. Hydraulic parameters measured in the flume and particle parameters measured in separate experiments were used as model inputs. The successful prediction of experimental results validates the modeling approach of combining a fundamental hydraulic exchange model with a physicochemical model for colloid transport and filtration in the streambed. Further, because colloid transport behavior was interpreted in terms of basic exchange and trapping processes, the results of this study are expected to be directly applicable to the analysis of fine sediment dynamics in natural streams.
A physicochemical model for colloid exchange between a stream and a sand streambed with bed forms
Authors: Packman Aaron I. , Brooks Norman H. , Morgan James J.
Published: February 2004

Fine sediment exchange between a stream and the surrounding subsurface influences downstream contaminant transport and stream ecology. Fundamental models for this exchange were developed on the basis of (1) the hydraulics of bed form‐driven advective pore water flow and (2) subsurface colloid transport processes. First, a model was developed to predict the advective flow induced in a sand bed by stream flow over bedforms. The resulting “pumping” exchange rate was calculated based on the streamflow conditions, bed form geometry, and bed depth. The pumping exchange of suspended sediment was then calculated by superimposing advective transport and particle settling in the bed and including the effect of physicochemical filtration by bed sediment. The filtration coefficient approach was used to predict the reduction in the concentration of transported particles. Both settling and filtration cause colloids to be trapped in stream beds, producing a higher net exchange rate relative to conservative solutes. When transported particles are completely trapped in a single pass through the bed, the exchange calculation is simplified because only the particle flux to the bed must be considered. In this case, the net exchange rate may be adequately represented by an effective piston velocity (flux/concentration) or loss rate to the bed in the advection‐dispersion equation for the stream. Solute and colloid exchanges are predicted by the models without the use of fitting coefficients; only measurable hydraulic and particle parameters were used as model inputs. Simulations are presented which show the effect of stream parameters, settling, and filtration on net particle exchange. This fundamental approach to modeling stream‐subsurface exchange potentially has great utility for understanding and predicting the transport and fate of reactive substances in streams.
Precipitation uncertainty processor for probabilistic river stage forecasting
Authors: Kelly Karen S. , Krzysztofowicz Roman
Published: February 2004

The precipitation uncertainty processor (PUP) is a component of the Bayesian forecasting system which produces a short‐term probabilistic river stage forecast (PRSF) based on a probabilistic quantitative precipitation forecast (PQPF). The task of the PUP is to process a probability distribution of the total precipitation amount through a deterministic hydrologic model (of any complexity) into a probability distribution of the model river stage. An analytic‐numerical PUP is developed based on the theory of response functions and empirical data simulated from the operational forecast system of the National Weather Service for a 1430 km2 headwater basin. The PUP outputs a five‐parameter two‐piece Weibull distribution of the model river stage. The corresponding response function is a two‐piece power function. Structural properties of the PUP are investigated empirically, including the deterministic equivalence principle: Under certain conditions a deterministic forecast of the temporal disaggregation of the total precipitation amount is equivalent to a probabilistic forecast. This considerably simplifies the PQPF, without affecting the optimality of the PRSF.
Application of a continuous‐flow centrifugation method for solute transport in disturbed, unsaturated sediments and illustration of mobile‐immobile water
Authors: Gamerdinger A. P. , Kaplan D. I.
Published: February 2004

A centrifuge method was adapted for evaluating solute transport during steady state flow. Solute breakthrough curves were obtained on three sediment types at average water contents ranging from 14 to 84% moisture saturation. Interruptions in flow for sampling, a potential limitation of the method, had no apparent effect on breakthrough curves for the tracers and conditions evaluated. Compaction was observed for a silt loam sediment but was not observed for fine and coarse sand sediments. Regions of immobile water developed with decreasing moisture saturation; these results extend the range of previous reports. The continuum from a single to a multiregion flow domain was illustrated, and a modeling approach for estimating dispersion and mobile‐immobile water parameters was applied. The fraction of immobile water ranged from 48 to 80%; solute velocity in the mobile domain was up to fivefold greater than the average pore water velocity.
Multidimensional quasi‐linear steady infiltration toward a shallow water table
Authors: Basha H. A.
Published: February 2004

The present work explores further a class of analytical solutions of moisture movement in unsaturated porous media, characterized by an exponential dependence of the hydraulic conductivity on water pressure. The Green's function method is used to derive a general analytical model pertaining to multidimensional steady infiltration to a shallow water table with arbitrary boundary conditions and arbitrary root‐uptake forcing functions and for various simple source geometry. The general solution is expressed in integral form from which particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, and root‐uptake in the presence of a shallow water table can be easily deduced from the general analytical model. New solutions include the one‐dimensional infiltration subject to discrete extraction functions, the two‐dimensional infiltration from a strip source with a uniform exponential and a periodic flux distribution, and the three‐dimensional infiltration from a rectangular source at the land surface. Particular solutions have also been obtained for infiltration from a periodic series of point, line, and strip sources. Algebraic equations for the parameter estimation of the hydraulic conductivity function were also derived from the simple one‐dimensional solution. The proposed model offers the analyst significant flexibility in deriving results and analyzing infiltration phenomena of practical interest and provides a basis upon which parameter estimation can be effected.
A stochastic approach to modeling the role of rainfall variability in drainage basin evolution
Authors: Tucker Gregory E. , Bras Rafael L.
Published: February 2004

We develop a simple stochastic theory for erosion and sediment transport, based on the Poisson pulse rainfall model, in order to analyze how variability in rainfall and runoff influences drainage basin evolution. Two cases are considered: sediment transport by runoff in rills and channels and particle detachment from bedrock or cohesive soils. Analytical and numerical results show that under some circumstances, rainfall variability can have an impact equal to or greater than that of mean rainfall amount. The predicted sensitivity to rainfall variability is greatest when (1) thresholds for runoff generation and/or particle detachment are significant and/or (2) erosion and transport are strong nonlinear functions of discharge. In general, sediment transport capacity is predicted to increase with increasing rainfall variability. Depending on the degree of nonlinearity, particle detachment capacity may either increase or decrease with increasing rainfall variability. These findings underscore the critical importance of hydrogeomorphic thresholds and other sources of nonlinearity in process dynamics. The morphologic consequences of rainfall variability are illustrated by incorporating the pulse rainfall theory into a landscape simulation model. The simulation results imply that, all else being equal, catchments experiencing a shift toward greater climate variability will tend to have (1) higher erosion rates, (2) higher drainage density (because of increased runoff erosion efficiency), and ultimately (3) reduced relief. The stochastic approach provides a useful method for incorporating physically meaningful climate data within the current generation of landscape evolution models.
Conservation equations governing hillslope responses: Exploring the physical basis of water balance
Authors: Reggiani Paolo , Sivapalan Murugesu , Hassanizadeh S. Majid
Published: February 2004

A unifying approach for the derivation of watershed‐scale conservation equations governing hydrologic responses has recently been introduced. The approach is based on space‐time averaging of the corresponding point‐scale conservation equations over an averaging region called the representative elementary watershed (REW). The conservation equations are supplemented by constitutive relationships needed for the closure of various mass and momentum exchange terms. In this paper, we present a summary of the resulting equations for a somewhat simpler problem and show how these equations can be employed to model the long‐term water balance of a single, hypothetical REW. The governing equations are formulated in terms of an average saturation and a flow velocity of the unsaturated zone soils and the average thickness of the saturated zone soils. The equations are coupled ordinary differential equations and are nonlinear. Their solution is carried out simultaneously for a variety of combinations of watershed geometry, soil type, and atmospheric forcing. We show how a similarity analysis of the governing equations can lead to a general classification of REWs in terms of meaningful dimensionless similarity variables. In addition, we investigate the long‐term water balance and show that the governing equations are able to provide a realistic picture of the impact of changing climate, soil, and topographic controls on long‐term water balance.
Experiments on headcut growth and migration in concentrated flows typical of upland areas
Authors: Bennett Sean J. , Alonso Carlos V. , Prasad Shyam N. , Römkens Mathias J. M.
Published: February 2004

Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well‐developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.
Analyzing latent heat fluxes of coniferous forests with fuzzy logic
Authors: van Wijk M. T. , Bouten W.
Published: February 2004

Fuzzy logic (FL) was applied to simulate latent heat fluxes of six coniferous forests. The goal was to test the application possibilities of fuzzy logic and to evaluate the information that could be derived from it. The data set consisted of five forests of the Euroflux project and a set of the Douglas fir Speulderbos, 1 year before and 1 year after a thinning. A set of models was compared to their performance and interpretability: a general FL model, FL models for the 2 years of the Speulderbos, and a forest growth model parameterized for Douglas fir. The performance of the FL models was close to optimal. The rules of the FL models made objective model comparison possible and showed a change in the sensitivity to vapor pressure deficit of the Speulderbos. Fuzzy logic proved to be a very promising method for top down modeling of forest gas exchange, systematic model evaluation, and model simplification.
Modified total variation methods for three‐dimensional electrical resistance tomography inverse problems
Authors: Yu Mengchun , Dougherty David E.
Published: February 2004

Images from geophysical surveys and data inversion can be valuable information for hydrogeologic site characterization and monitoring. This paper focuses on solving nonlinear electrical resistance tomography (ERT) inverse problems; the methods are applicable to other inverse problems, such as groundwater flow. The total variation (TV) method is used to constrain ERT inverse solutions. The classic TV functions are modified to obtain differentiability with respect to parameters, which allows the use of gradient search optimization methods. The modified total variation (mTV) method constrains ERT inverse solutions effectively. Sometimes the mTV constraints cause important parameter features to be smoothed out. To eliminate excessive smoothing, two successive partial variation relaxation (SPVR) methods are devised for use with the mTV method. Computational experiments demonstrate that mTV works well in identifying very sharp discontinuities and smoothly distributed parameters and that SPVR improves mTV solutions for both problem types. The mTV and SPVR methods are applied to synthetic layered systems with and without measurement error, and a comparison of results for a large three‐dimensional ERT problem with block anomaly presented by Zhang et al. [1995] demonstrates the superiority of the mTV and SPVR methods. A final example examines a smoothly varying case and explores the impact of two computational parameters introduced in the development of mTV and SPVR. The example shows also that SPVR can be used directly to invert data and not necessarily as an adjunct to mTV.
Tracer transport in fractured crystalline rock: Evidence of nondiffusive breakthrough tailing
Authors: Becker Matthew W. , Shapiro Allen M.
Published: February 2004

Extended tailing of tracer breakthrough is often observed in pulse injection tracer tests conducted in fractured geologic media. This behavior has been attributed to diffusive exchange of tracer between mobile fluids traveling through channels in fractures and relatively stagnant fluid between fluid channels, along fracture walls, or within the bulk matrix. We present a field example where tracer breakthrough tailing apparently results from nondiffusive transport. Tracer tests were conducted in a fractured crystalline rock using both a convergent and weak dipole injection and pumping scheme. Deuterated water, bromide, and pentafluorobenzoic acid were selected as tracers for their wide range in molecular diffusivity. The late time behavior of the normalized breakthrough curves were consistent for all tracers, even when the pumping rate was changed. The lack of separation between tracers of varying diffusivity indicates that strong breakthrough tailing in fractured geologic media may be caused by advective transport processes. This finding has implications for the interpretation of tracer tests designed to measure matrix diffusion in situ and the prediction of contaminant transport in fractured rock.
Dissolution of a well‐defined trichloroethylene pool in saturated porous media: Experimental design and aquifer characterization
Authors: Chrysikopoulos Constantinos V. , Lee Kenneth Y. , Harmon Thomas C.
Published: February 2004

A unique three‐dimensional bench‐scale model aquifer is designed and constructed to carry out dense nonaqueous phase liquid (DNAPL) pool dissolution experiments. The model aquifer consists of a rectangular glass tank with internal dimensions 150.0 cm length, 21.6 cm width, and 40.0 cm height. The formation of a well‐defined circular pool with a perfectly flat pool‐water interface is obtained by a bottom plate with a precise cutout to contain the DNAPL. The aquifer is packed with a well‐characterized relatively uniform sand. A conservative tracer is employed for the determination of the longitudinal and transverse aquifer dispersivities. The dissolution studies are conducted using a circular trichloroethylene (TCE) pool. The sorption characteristics of TCE onto the aquifer sand are independently determined from a flow‐through column experiment. Steady state dissolved TCE concentrations at specific downstream locations within the aquifer are collected under three different interstitial velocities. An appropriate overall mass transfer coefficient is determined from each data set. The data collected in this study are useful for the validation of numerical and analytical DNAPL pool dissolution models.
Solute flux approach to transport through spatially nonstationary flow in porous media
Authors: Zhang Dongxiao , Andricevic Roko , Sun Alexander Y. , Hu Xiaolong , He Guowei
Published: February 2004

A theoretical framework for solute flux through spatially nonstationary flows in porous media is presented. The flow nonstationarity may stem from medium nonstationarity (e.g., the presence of distinct geological layers, zones, or facies), finite domain boundaries, and/or fluid pumping and injecting. This work provides an approach for studying solute transport in multiscale media, where random heterogeneities exist at some small scale while deterministic geological structures and patterns can be prescribed at some larger scale. In such a flow field the solute flux depends on solute travel time and transverse displacement at a fixed control plane. The solute flux statistics (mean and variance) are derived using the Lagrangian framework and are expressed in terms of the probability density functions (PDFs) of the particle travel time and transverse displacement. These PDFs are given with the statistical moments derived based on nonstationary Eulerian velocity moments. The general approach is illustrated with some examples of conservative and reactive solute transport in stationary and nonstationary flow fields. It is found based on these examples that medium nonstationarities (or multiscale structures and heterogeneities) have a strong impact on predicting solute flux across a control plane and on the corresponding prediction uncertainty. In particular, the behavior of solute flux moments strongly depends on the configuration of nonstationary medium features and the source dimension and location. The developed nonstationary approach may result in non‐Gaussian (multiple modal) yet realistic behaviors for solute flux moments in the presence of flow nonstationarities, while these non‐Gaussian behaviors may not be reproduced with a traditional stationary approach.
Nonideal transport of reactive solutes in heterogeneous porous media: 6. Microscopic and macroscopic approaches for incorporating heterogeneous rate‐limited mass transfer
Authors: Li Zhen , Brusseau Mark L.
Published: February 2004

Two major approaches have been used to incorporate heterogeneous rate‐limited mass transfer into mathematical models for solute transport. One focuses on processes operative at the microscopic scale and associated grain‐scale heterogeneity, while the other stresses the macroscopic variability of the medium and the field‐scale behavior of solute transport. In this paper, we examine the conceptual framework and model formulation of these two approaches in an attempt to evaluate potential commonality. Numerical solvers are developed for both sets of governing equations, and the performance of these two models is tested for two systems, each incorporating one of two types of mass transfer mechanisms. The results show that despite differences in conceptualization and formulation, the models produce comparable behavior for smaller‐scale systems. However, greater deviations are observed at larger scales. This suggests that caution should be exercised when using mathematical modeling for elucidating the specific processes that may be influencing reactive‐solute transport for a given system. We also evaluate the impact of microscopic‐scale mass transfer heterogeneity on field‐scale transport in systems for which hydraulic conductivity is spatially variable. The results show that inclusion of locally heterogeneous mass transfer does not appear to significantly influence the mean transport behavior for systems with field‐scale heterogeneity. However, it does appear to influence low‐concentration tailing. For simulations of reactive transport over extended distances, models with locally heterogeneous mass transfer may “preserve” the nonequilibrium effects associated with rate‐limited mass transfer better than models incorporating locally uniform mass transfer when both pore‐scale and field‐scale heterogeneity coexist.
Stochastic model for posttillage soil pore space evolution
Authors: Or Dani , Leij Feike J. , Snyder Victor , Ghezzehei Teamrat A.
Published: February 2004

Tillage operations disrupt surface layers of agricultural soils, creating a loosened structure with a substantial proportion of interaggregate porosity that enhances liquid and gaseous exchange properties favorable for plant growth. Unfortunately, such desirable soil tilth is structurally unstable and is susceptible to change by subsequent wetting and drying processes and other mechanical stresses that reduce total porosity and modify pore size distribution (PSD). Ability to model posttillage dynamics of soil pore space and concurrent changes in hydraulic properties is important for realistic predictions of transport processes through this surface layer. We propose a stochastic modeling framework that couples the probabilistic nature of pore space distributions with physically based soil deformation models using the Fokker‐Planck equation (FPE) formalism. Three important features of soil pore space evolution are addressed: (1) reduction of the total porosity, (2) reduction of mean pore radius, and (3) changes in the variance of the PSD. The proposed framework may be used to provide input to hydrological models concerning temporal variations in near‐surface soil hydraulic properties. In a preliminary investigation of this approach we link a previously proposed mechanistic model of soil aggregate coalescence to the stochastic FPE framework to determine the FPE coefficients. An illustrative example is presented which describes changes in interaggregate pore size due to wetting‐drying cycles and the resulting effects on dynamics of the soil water characteristic curve and hydraulic conductivity functions.
Intermediate‐scale experiments and numerical simulations of transport under radial flow in a two‐dimensional heterogeneous porous medium
Authors: Chao Hsin‐Chia , Rajaram Harihar , Illangasekare Tissa
Published: February 2004

Estimated field dispersivities from uniform and nonuniform flow tracer tests are often compared without distinction, yet the difference in the flow field is important. The difference is investigated here based on intermediate‐scale experiments and numerical simulations in two‐dimensional heterogeneous porous media. The scale dependence of dispersivities estimated from radial flow tracer tests in two‐dimensional heterogeneous porous media and the variability of these estimates are quantified. The results presented in this paper demonstrate the difference between the scale dependence inferred from uniform flow and radial flow tracer experiments in the same random hydraulic conductivity realization. In particular, dispersivities estimated using type‐curve matching from radial flow tracer experiments continue to exhibit a scale dependence, even at scales where an asymptotic constant dispersivity value applies for transport in a uniform mean flow. The discrepancy between the behavior of transport in uniform and radial flows is partly due to the converging nature of the radial flow and, more importantly, due to the small source size involved in forced‐gradient tracer tests. There is substantial variability in dispersivity values estimated from different injection points at the same radial distance from the pumping well. In the range of σlnK = 1.0 to 2.5 the coefficient of variation of the dispersivity estimated at the same radial distance approaches a value of about 1.0 at radial distances much larger than the correlation scale.
Regional interdisciplinary paleoflood approach to assess extreme flood potential
Authors: Jarrett Robert D. , Tomlinson Edward M.
Published: February 2004

In the past decade, there has been a growing interest of dam safety officials to incorporate a risk‐based analysis for design‐flood hydrology. Extreme or rare floods, with probabilities in the range of about 10−3 to 10−7 chance of occurrence per year, are of continuing interest to the hydrologic and engineering communities for purposes of planning and design of structures such as dams [National Research Council, 1988]. The National Research Council stresses that as much information as possible about floods needs to be used for evaluation of the risk and consequences of any decision. A regional interdisciplinary paleoflood approach was developed to assist dam safety officials and floodplain managers in their assessments of the risk of large floods. The interdisciplinary components included documenting maximum paleofloods and a regional analyses of contemporary extreme rainfall and flood data to complement a site‐specific probable maximum precipitation study [Tomlinson and Solak, 1997]. The cost‐effective approach, which can be used in many other hydrometeorologic settings, was applied to Elkhead Reservoir in Elkhead Creek (531 km2) in northwestern Colorado; the regional study area was 10,900 km2. Paleoflood data using bouldery flood deposits and noninundation surfaces for 88 streams were used to document maximum flood discharges that have occurred during the Holocene. Several relative dating methods were used to determine the age of paleoflood deposits and noninundation surfaces. No evidence of substantial flooding was found in the study area. The maximum paleoflood of 135 m3 s−1 for Elkhead Creek is about 13% of the site‐specific probable maximum flood of 1020 m3 s−1. Flood‐frequency relations using the expected moments algorithm, which better incorporates paleoflood data, were developed to assess the risk of extreme floods. Envelope curves encompassing maximum rainfall (181 sites) and floods (218 sites) were developed for northwestern Colorado to help define maximum contemporary and Holocene flooding in Elkhead Creek and in a regional frequency context. Study results for Elkhead Reservoir were accepted by the Colorado State Engineer for dam safety certification.
Low stream nitrate concentrations associated with oak forests on the Allegheny high plateau of Pennsylvania
Authors: Lewis Gregory P. , Likens Gene E.
Published: February 2004

Stream nitrate (NO3−) concentrations were 2.6–7.0 times lower in watersheds in which red oak (Quercus rubra) stands were present than in other forested watersheds in northwestern Pennsylvania. Also, seasonal variations in NO3− concentrations were smaller in watersheds with oak stands than in watersheds without oak stands. Stream water concentrations of calcium, magnesium, potassium, sodium, chloride, and sulfate did not differ significantly between the two groups of watersheds. The variability in NO3− concentrations among streams did not appear to be due to differences in atmospheric nitrogen deposition among watersheds. Low stream NO3− concentrations occurred even if only 10–20% of a watershed was covered by oak stands. We suggest that oaks, rather than directly controlling NO3− losses, instead merely occur on soils which have low NO3− losses.
Numerical simulation of floodplain hydrology
Authors: Bates P. D. , Stewart M. D. , Desitter A. , Anderson M. G. , Renaud J.‐P. , Smith J. A.
Published: February 2004

Despite the extensive research into hillslope and channel interactions in headwater catchments, surprisingly little attention has been paid to such processes in lowland rivers. In particular, previous studies have not addressed the influence of hillslope contributions and have concentrated solely on in‐bank floods rather than more complex out‐of‐bank cases. Accordingly, we combine field monitoring and numerical modeling to study hillslope, floodplain, and channel interactions for a lowland river. Piezometric, precipitation, and river stage data were used to parameterize and test a new two‐dimensional finite element model of saturated‐unsaturated flow applied to two vertically aligned cross sections through a lowland floodplain. Data for two major out‐of‐bank flood events were simulated which appeared to show the presence of a significant unsaturated zone extending up to 5 m below the surface. The model simulated reasonably well the pressure head field that was recorded at a number of piezometers located internal to the computational domain on each transect, and we conclude that floodplain hydrology is predominately a two‐dimensional (lateral∥ process. Three‐dimensional (down reach) flow effects would seem to become more significant at the beginning and end of each event. The simulations also showed that the unsaturated zone remained close to saturation at all times and that it was not significant in terms of the floodplain hydrology. Examination of velocity vector patterns showed the formation of a strong groundwater ridge within the floodplain. This led to the development of strong velocities directed toward hillslope areas as the inundation front approached the hillslope/floodplain junction. This suggests that surface water may move into hillslope areas adjoining the floodplain during major floods. Thus the extent of the hyporheic zone may be larger than previously thought.
Hydrologic processes and peak discharge response to forest removal, regrowth, and roads in 10 small experimental basins, Western Cascades, Oregon
Authors: Jones J. A.
Published: February 2004

The magnitude, seasonality, and duration of peak discharge responses to forest removal and regrowth and roads in 10 pairs of experimental basins in the western Cascade Range of Oregon are consistent with fundamental water balance and routing concepts in hydrology. Hypothesized effects of forestry treatments on evapotranspiration, cloud water interception, snowpack dynamics, and subsurface flow interception vary predictably by season, geographic setting, amount of forest canopy removal, stage of canopy regrowth, and arrangement of roads in the basin. Posttreatment responses of selected subpopulations of matched peak discharge events were examined over 10‐ to 34‐year posttreatment periods in treated‐control basin pairs in a range of geographic settings. Changes in evapotranspiration associated with forest canopy removal and regrowth apparently accounted for significant increases (31–116%) in peak discharges during the first postharvest decade in 8 of 10 treated basins, but the events that were affected were small (<0.22‐ or 0.28‐year return periods) and occurred in the fall (September–November), when soils are in moisture deficit, rather than in spring (March–May), when soils are in moisture surplus. For a given amount of forest canopy removal, initial increases in small, fall events were greater in drier basins than wetter basins, and increases tended to disappear as forest canopies regrew. Changes in cloud water interception apparently offset changes in evapotranspiration in two partially cut basins. Changes in snowpack dynamics apparently accounted for significant increases (25–31%) in winter rain‐on‐snow events, but other types of winter events did not change, in four of five basins at the H. J. Andrews Experimental Forest. Changes in subsurface flow interception apparently accounted for significant increases (13–36%) in large (>1‐year return period) events in seven of eight basins with roads, and, controlling for geographic location, the magnitude of increases was related to the density of midslope roads.
Effects of nearshore recharge on groundwater interactions with a lake in mantled karst terrain
Authors: Lee Terrie M.
Published: February 2004

The recharge and discharge of groundwater were investigated for a lake basin in the mantled karst terrain of central Florida to determine the relative importance of transient groundwater inflow to the lake water budget. Variably saturated groundwater flow modeling simulated water table responses observed beneath two hillsides radiating outward from the groundwater flow‐through lake. Modeling results indicated that transient water table mounding and groundwater flow reversals in the nearshore region following large daily rainfall events generated most of the net groundwater inflow to the lake. Simulated daily groundwater inflow was greatest following water table mounding near the lake, not following subsequent peaks in the water level of upper basin wells. Transient mounding generated net groundwater inflow to the lake, that is, groundwater inflow in excess of the outflow occurring through the deeper lake bottom. The timing of the modeled net groundwater inflow agreed with an independent lake water budget; however, the quantity was considerably less than the budget‐derived value.