bluuebunny/crossref_metadata_2025 huggingface dataset:
Exact analytical expressions for the piezometric profile and water exchange between stream and groundwater during and after a uniform rise of the stream level
Authors: Boufadel Michel C. , Peridier Vallorie
Published: October 2002

We consider an archetypical problem relevant to a confined aquifer in contact with a stream. The model problem consists of an idealized one‐dimensional region 0 ≤ x ≤ L, where the left boundary at x = 0 is held at a fixed piezometric head h0, and the right boundary's piezometric head at x = L is increased from hL to h0 at a constant rate. Exact solutions for all times, all points in the aquifer, and for any possible constant rate of change of the right boundary piezometric head are presented for the piezometric head and the instantaneous flow rate. An exact expression for the exchange volume at the groundwater/stream interface for an arbitrary time is also provided. This expression shows that there is a specific critical rising rate of the stream level above which the net exchange volume is into the aquifer and below which it is out of the aquifer. The solution shows that regardless of the rise rate, a certain water volume, inversely proportional to the rise rate, enters the aquifer.
Influence of regional setting on the interaction between shallow lakes and aquifers
Authors: Smith Anthony J. , Townley Lloyd R.
Published: October 2002

Interaction between surface water and groundwater depends on the position of a lake within the regional flow system. The approach in this paper is to nest a local‐scale two‐dimensional (2‐D) lake aquifer model within a 1‐D regional aquifer model to examine the effect of the regional setting on surface water–groundwater interaction. This extends previous steady state flow modeling, which is presented in earlier publications. The results indicate that the type of regional setting and position of the lake within that setting impose constraints on the near field flow geometry. Only nine of 39 previously identified flow regimes are possible in the regional flow system considered. Flow‐through lakes are dominant and can occur anywhere within the regional flow. Recharge and discharge lakes are more likely to occur near the groundwater divide where regional hydraulic gradients are flattest and can be counteracted by local forcing in and near the lake.
Relationships between gas‐liquid interfacial surface area, liquid saturation, and light transmission in variably saturated porous media
Authors: Niemet Michael R. , Rockhold Mark L. , Weisbrod Noam , Selker John S.
Published: October 2002

Liquid saturation and gas‐liquid interfacial area are important parameters for evaluating the transport and fate of contaminants in unsaturated subsurface environments. Recent findings indicate that interfacial surface area controls the relative degree of transmitted light in laboratory systems containing translucent porous media. Equations are derived to estimate the specific gas‐liquid interfacial area from the area under the primary‐drainage branch of the Seff‐h characteristic curve as parameterized using common water retention functions. The total area under the curve provides the maximum available specific gas‐liquid interfacial area available at residual saturation, which can be incorporated into the relationship to determine the gas‐liquid interfacial area at intermediate degrees of saturation via light transmission. Experimental results, and analysis of external data sets, support these findings. Closed‐form relationships are presented as enhancements to a recent method for determination of liquid saturations above residual using light transmission. A physically based model is developed and tested for the quantification of liquid contents below residual saturation.
A vadose zone water fluxmeter with divergence control
Authors: Gee G. W. , Ward A. L. , Caldwell T. G. , Ritter J. C.
Published: October 2002

Unsaturated water flux densities are needed to quantify water and contaminant transfer within the vadose zone. However, water flux densities are seldom measured directly and often are predicted with uncertainties of an order or magnitude or more. A water fluxmeter was designed, constructed, and tested to directly measure drainage fluxes in field soils. The fluxmeter was designed to minimize divergence. It concentrates flow into a narrow sensing region filled with a fiberglass wick. The wick applies suction, proportional to its length, and passively drains the meter. The meter can be installed in an augured borehole at almost any depth below the root zone. Water flux through the meter is measured with a self‐calibrating tipping bucket, with a sensitivity of ∼4 mL tip−1. For our meter this is equivalent to detection limit of ∼0.1 mm. Passive‐wick devices previously have not properly corrected for flow divergence. Laboratory measurements supported predictions of a two‐dimensional (2‐D) numerical model, which showed that control of the collector height H and knowledge of soil hydraulic properties are required for improving divergence control, particularly at fluxes below 1000 mm yr−1. The water fluxmeter is simple in concept, is inexpensive, and has the capability of providing continuous and reliable monitoring of unsaturated water fluxes ranging from less than 1 mm yr−1 to more than 1000 mm yr−1.
Runoff generation in a steep, soil‐mantled landscape
Authors: Montgomery David R. , Dietrich William E.
Published: October 2002

Scale and slope dependence of hydrologic response are investigated for two channel network source areas (unchanneled valleys) in the Oregon Coast Range. Observations of response to both natural and applied precipitation reveal that runoff occurred as subsurface flow in which water passed through partially saturated soil, into the shallow fractured bedrock, to emerge as subsurface partial source areas near the channel head. The two dominant approaches to modeling subsurface flow in steep topography, routing of Darcy or fracture flow and the hydrologic similarity approximation of TOPMODEL, respectively predict either a strong slope dependence or no slope dependence to timescales of subsurface runoff generation. Compilation of data from our Coos Bay study sites with observations reported previously elsewhere indicates weak area dependence but no slope dependence in the lag‐to‐peak and discharge recession constants. This finding supports the interpretation that patterns of antecedent soil moisture and vadose zone characteristics control response times of runoff generation by subsurface storm flow. As slope should influence lateral flow routing once subsurface saturation develops, we conclude that the hydrologic response of steep catchments appears to be insensitive to slope because the controlling timescale is that of the vertical unsaturated flow.
Models of soil moisture dynamics in ecohydrology: A comparative study
Authors: Guswa Andrew J. , Celia M. A. , Rodriguez‐Iturbe I.
Published: October 2002

An accurate description of plant ecology requires an understanding of the interplay between precipitation, infiltration, and evapotranspiration. A simple model for soil moisture dynamics, which does not resolve spatial variations in saturation, facilitates analytical expressions of soil and plant behavior as functions of climate, soil, and vegetation characteristics. Proper application of such a model requires knowledge of the conditions under which the underlying simplifications are appropriate. To address this issue, we compare predictions of evapotranspiration and root zone saturation over a growing season from a simple bucket‐filling model to those from a more complex, vertically resolved model. Dimensionless groups of key parameters measure the quality of the match between the models. For a climate, soil, and woody plant characteristic of an African savanna the predictions of the two models are quite similar if the plant can extract water from locally wet regions to make up for roots in dry portions of the soil column; if not, the match is poor.
Groundwater seepage into northern San Francisco Bay: Implications for dissolved metals budgets
Authors: Spinelli Glenn A. , Fisher Andrew T. , Wheat C. Geoffrey , Tryon Michael D. , Brown Kevin M. , Flegal A. Russell
Published: October 2002

Nonconservative excesses of dissolved metals in northern San Francisco Bay indicate that there are internal sources of metals within the bay. We quantified groundwater seepage and bioirrigation rates in this area to determine their roles in transporting dissolved metals from benthic sediments to surface waters. We deployed seepage meters and collected sediment, pore water, and bottom water samples at three sites. We determined seepage rates from seepage meters and modeled the transport of water through the sediment using pore water data to constrain rates of diffusion, advection, and bioirrigation. A groundwater flow model incorporating sediment physical properties and local topography constrains more regional seepage estimates. The seepage meters indicate upflow rates from 7 to 56 cm yr−1 in March and April 1999 with some large (≤50 cm yr−1) daily fluctuations that greatly exceed predictions based on sediment physical properties and tidally induced pore pressure variations. During this period, results from modeling pore water chemical data are consistent with a small bioirrigation rate (<1.5 × 10−7 s−1) relative to values determined for southern San Francisco Bay, and an average groundwater upwelling speed of 15 cm yr−1. The speed and direction of flow changed throughout the year, with best fits to the data ranging from 20 cm yr−1 upflow to 34 cm yr−1 downflow and averaging 4 cm yr−1 upflow. Confidence intervals (95%) are about ±10 cm yr−1 for this method, yet the range of acceptable seepage rates for temporally successive periods only overlap in one of four cases, suggesting that temporal variability can be discerned from potential artifacts. Groundwater flow modeling suggests that the seepage rates determined at our sites represent ∼45% of the average seepage rate for the area, applying one consolidation and permeability relationship to all sediments. If we apply these approximations to all of northern San Francisco Bay, benthic fluxes of dissolved metals to the surface waters could account for a relatively large amount (≤60%) of the unknown sources of dissolved cobalt and a relatively small amount (≤4%) of the unknown sources of dissolved silver, cadmium, copper, nickel, and zinc. More focused groundwater discharge or elevated metals concentrations are required to have a larger impact on trace element budgets in this setting.
Partial entrainment of gravel bars during floods
Authors: Konrad Christopher P. , Booth Derek B. , Burges Stephen J. , Montgomery David R.
Published: October 2002

Spatial patterns of bed material entrainment by floods were documented at seven gravel bars using arrays of metal washers (bed tags) placed in the streambed. The observed patterns were used to test a general stochastic model that bed material entrainment is a spatially independent, random process where the probability of entrainment is uniform over a gravel bar and a function of the peak dimensionless shear stress τ0* of the flood. The fraction of tags missing from a gravel bar during a flood, or partial entrainment, had an approximately normal distribution with respect to τ0* with a mean value (50% of the tags entrained) of 0.085 and standard deviation of 0.022 (root‐mean‐square error of 0.09). Variation in partial entrainment for a given τ0* demonstrated the effects of flow conditioning on bed strength, with lower values of partial entrainment after intermediate magnitude floods (0.065 < τ0*< 0.08) than after higher magnitude floods. Although the probability of bed material entrainment was approximately uniform over a gravel bar during individual floods and independent from flood to flood, regions of preferential stability and instability emerged at some bars over the course of a wet season. Deviations from spatially uniform and independent bed material entrainment were most pronounced for reaches with varied flow and in consecutive floods with small to intermediate magnitudes.
Tree species effects on stand transpiration in northern Wisconsin
Authors: Ewers B. E. , Mackay D. S. , Gower S. T. , Ahl D. E. , Burrows S. N. , Samanta S. S.
Published: October 2002

We quantified canopy transpiration (EC) using sap flux measurements representing the four major forest types (northern hardwoods, conifer, aspen/fir, and forested wetland) around the WLEF‐TV tall tower in northern Wisconsin. In order to scale individual sap flux measurements to EC, we quantified the amount of sapwood area per unit ground area and the spatial distribution of sap flux within trees. Contrary to our hypothesis that all tree species would have the same positive relationship between tree diameter and sapwood depth, white cedar and speckled alder, both wetland species, showed no relationship. We also hypothesized that the conifer trees would have a lower whole tree hydraulic conductance than deciduous trees. We actually discovered that white cedar had the highest hydraulic conductance. Our third hypothesis, that sapwood area per unit ground area would determine stand EC, was not rejected. The resulting average daily EC values over 53 days (23 June to 16 August 2000) from combining sap flux and sapwood area per unit ground area were 1.4, 0.8, 2.1, and 1.4 mm d−1 for conifer, northern hardwoods, aspen/fir, and forested wetland cover types, respectively. Average daily EC was only explained by an exponential saturation with daily average vapor pressure deficit.
Stochastic analysis of macrodispersion in gravity‐dominated flow through bimodal heterogeneous unsaturated formations
Authors: Russo David
Published: October 2002

First‐order analysis, based on a stochastic continuum presentation of the flow and a general Lagrangian description of the transport, was used to investigate the effects of a few characteristics of a bimodal, heterogeneous, partially saturated formation, on the principal components of the time‐dependent macrodispersion coefficient, under steady state, unsaturated flow conditions. Results of the present analysis show that generally, a bimodal distribution of the formation properties, characterized by a relatively complex spatial correlation structure, contributes to the variability in water velocity and, consequently, may considerably enhance solute spreading. This applies especially in formations in which (1) the correlation length scales and the variances of the soil properties associated with the embedded soil are much larger than those of the background soil, (2) the contrast between mean properties of the two subdomains is large, (3) mean water saturation is relatively small, and (4) the volume fraction of the flow domain occupied by the embedded soil is relatively large. One of the major findings of this study is the important role played by the formation parameter α under unsaturated flow conditions. Consequently, the macrodispersion in a bimodal, unsaturated formation may be larger than its counterpart in a saturated formation, especially when the degree of saturation decreases, when the volume fraction of the flow domain occupied by the embedded soil increases, when the disparity between the correlation length scales of the two subdomains is relatively small, and when the texture of the embedded soil is coarser than that of the background soil.
Point‐nonpoint nutrient trading in the Susquehanna River basin
Authors: Horan Richard D. , Shortle James S. , Abler David G.
Published: October 2002

There is considerable interest in the use of pollution trading between point and nonpoint sources to improve the cost‐effectiveness of water pollution control but little literature to guide the design of trading systems involving nonpoint sources. Expanding on prior theoretical work, this paper provides empirical evidence about design and performance issues for two types of trading systems that would allow nutrient trading among and between point and nonpoint sources in the Susquehanna River basin in Pennsylvania.
Influence of spatial resolution on simulated streamflow in a macroscale hydrologic model
Authors: Haddeland Ingjerd , Matheussen Bernt V. , Lettenmaier Dennis P.
Published: October 2002

The sensitivity to spatial scale of runoff produced by the variable infiltration capacity (VIC) macroscale hydrologic model is investigated by implementing the model over the Columbia and Arkansas‐Red River basins at spatial resolutions from one‐eighth to 2° latitude by longitude. At lower resolutions the meteorological, topographical, and land cover data at the highest spatial resolution are averaged spatially. Simulated mean annual streamflow at lower resolutions is as much as 18 and 12% lower for the Arkansas‐Red and Columbia River basins, respectively, as compared to the one‐eighth degree implementation. When the VIC model subgrid parameterization for spatial precipitation variability is implemented, the model's sensitivity to spatial scale decreases slightly in the Arkansas‐Red River basin, where total runoff at lower resolutions decreases by up to 14% relative to the high‐resolution runs. In the Columbia River basin, total runoff is only 4% lower at 2° resolution than at one‐eighth degree resolution when precipitation variation with elevation is reparameterized at lower resolutions in a manner that replicates mean annual precipitation in the corresponding higher‐resolution cells.
Multicomponent reactive transport modeling in variably saturated porous media using a generalized formulation for kinetically controlled reactions
Authors: Mayer K. Ulrich , Frind Emil O. , Blowes David W.
Published: October 2002

A generalized formulation for kinetically controlled reactions has been developed and incorporated into a multicomponent reactive transport model to facilitate the investigation of a large variety of problems involving inorganic and organic chemicals in variably saturated media. The general kinetic formulation includes intra‐aqueous and dissolution‐precipitation reactions in addition to geochemical equilibrium expressions for hydrolysis, aqueous complexation, oxidation‐reduction, ion exchange, surface complexation, and gas dissolution‐exsolution reactions. The generalized approach allows consideration of fractional order terms with respect to any dissolved species in terms of species activities or in terms of total concentrations, which facilitates the incorporation of a variety of experimentally derived rate expressions. Monod and inhibition terms can be used to describe microbially mediated reactions or to limit the reaction progress of inorganic reactions. Dissolution‐precipitation reactions can be described as surface‐controlled or transport‐controlled reactions. The formulation also facilitates the consideration of any number of parallel reaction pathways, and reactions can be treated as irreversible or reversible processes. Two groundwater contamination scenarios, both set in variably saturated media but with significantly different geochemical reaction networks, are investigated and demonstrate the advantage of the generalized approach. The first problem focuses on a hypothetical case study of the natural attenuation of organic contaminants undergoing dissolution, volatilization, and biodegradation in an unconfined aquifer overlaid by unsaturated sediments. The second problem addresses the generation of acid mine drainage in the unsaturated zone of a tailings impoundment at the Nickel Rim Mine Site near Sudbury, Ontario, and subsequent reactive transport in the saturated portion of the tailings.
Nonideal equilibrium dissolution of trichloroethene from a decane‐based nonaqueous phase liquid mixture: Experimental and modeling investigation
Authors: McCray John E. , Dugan Pamela J.
Published: October 2002

Batch equilibrium solubility studies were conducted to examine the solubilization behavior of a chlorinated solvent, trichloroethene (TCE), from a fuel‐based nonaqueous phase liquid (NAPL) mixture. An alkane (n–decane) was used as a model compound because it is often a primary compound in jet fuel. The NAPL phase mole fractions of the chlorinated solvent in the mixture (XTCEN) that were investigated are typical of in situ values found at industrial and military waste sites (0.0001 ≥ XTCEN ≤ 0.1). The measured aqueous concentrations of TCE were essentially equal to the concentrations predicted with ideal dissolution theory (Raoult's law) at XTCEN values near 0.1. However, the ratio of the measured concentration to the ideal concentration, or the NAPL phase activity coefficient (γNTCE), increased nonlinearly as the XTCEN decreased. The γTCEN approached 6 at XTCEN = 0.0001. The UNIFAC method greatly underpredicts the γTCEN in this surrogate fuel. A NAPL‐mixture equilibrium‐dissolution model was developed that incorporates the observed nonideal dissolution. This model indicates that nonideal NAPL dissolution is 4 times faster than ideal dissolution for a hypothetical NAPL mixture with an initial XTCEN = 0.001. The magnitude of this effect becomes more important as the initial value of the XTCEN is decreased.
Modeling transport in fractured porous media with the random‐walk particle method: The transient activity range and the particle transfer probability
Authors: Pan Lehua , Bodvarsson Gudmundur S.
Published: October 2002

Multiscale features of transport processes in fractured porous media make numerical modeling a difficult task, both in conceptualization and computation. Modeling the mass transfer through the fracture‐matrix interface is one of the critical issues in the simulation of transport in a fractured porous medium. Because conventional dual‐continuum‐based numerical methods are unable to capture the transient features of the diffusion depth into the matrix (unless they assume a passive matrix medium), such methods will overestimate the transport of tracers through the fractures, especially for the cases with large fracture spacing, resulting in artificial early breakthroughs. We have developed a new method for calculating the particle transfer probability that can capture the transient features of diffusion depth into the matrix within the framework of the dual‐continuum random‐walk particle method by introducing a new concept of activity range of a particle within the matrix. Unlike the multiple‐continuum approach, the new dual‐continuum particle tracking method does not require using additional grid blocks to represent the matrix. It does not assume a passive matrix medium and can be applied to the cases where global water flow exists in both continua. The new method has been verified against analytical solutions for transport in the fracture‐matrix systems with various fracture spacing. The calculations of the breakthrough curves of radionuclides from a potential repository to the water table in Yucca Mountain demonstrate the effectiveness of the new method for simulating three‐dimensional, mountain‐scale transport in a heterogeneous, fractured porous medium under variably saturated conditions.
On a better understanding of hydraulic lift: A numerical study
Authors: Mendel M. , Hergarten S. , Neugebauer H. J.
Published: October 2002

Hydraulic lift is the transport of water from moist into drier soil layers through plant root systems: Plant roots sometimes do not only take up water but also release water if the soil is dry. Hydraulic lift has been shown for a relatively small number of species, but it is believed to be a more general phenomenon. We model numerically water uptake and two‐dimensional water transport through the soil and through the root system as coupled processes. Both water uptake and transport through the root system are considered to be hydraulic processes; osmotic effects are neglected. The model is capable of tracing hydraulic lift; the simulated amount of shifted water is consistent with experimental data. This supports the theory that hydraulic lift is a pure hydraulic process without an osmotic component. Furthermore, we discuss how far hydraulic lift could be an optimized strategy for the plant.
Bayesian methodology to stochastic capture zone determination: Conditioning on transmissivity measurements
Authors: Feyen L. , Ribeiro P. J. , De Smedt F. , Diggle P. J.
Published: October 2002

A methodology to determine the uncertainty associated with the delineation of well capture zones in heterogeneous aquifers is presented. The log transmissivity field is modeled as a random space function and the Bayesian paradigm accounts for the uncertainty that stems from the imperfect knowledge about the parameters of the stochastic model. Unknown parameters are treated as random quantities and characterized by a prior probability distribution. Log transmissivity measurements are incorporated into Bayes' theorem, updating the prior distribution and yielding posterior estimates of the mean value and the covariance parameters of the log transmissivity. Conditional simulations of the log transmissivity field are generated using samples from the posterior distribution of the parameters, yielding samples from the predictive distribution of the log transmissivity field. The uncertainty in the model parameters is propagated to the predictive uncertainty of the capture zone by solving numerically the groundwater flow equation, followed by a semianalytical particle‐tracking algorithm. The method is applied to a set of hypothetical flow fields for various sampling densities and assuming different levels of parameter uncertainty. Simulation results for all the sampling densities show no univocal relation between the predictive uncertainty of the capture zones and the level of parameter uncertainty. However, in general, the predictive uncertainty increases when parameter uncertainty is taken into account.
A nonparametric approach for representing interannual dependence in monthly streamflow sequences
Authors: Sharma Ashish , O'Neill Robert
Published: October 2002

The estimation of risks associated with water management plans requires generation of synthetic streamflow sequences. The mathematical algorithms used to generate these sequences at monthly timescales are found lacking in two main respects: inability in preserving dependence attributes particularly at large (seasonal to interannual) time lags and a poor representation of observed distributional characteristics, in particular, representation of strong asymmetry or multimodality in the probability density function. Proposed here is an alternative that naturally incorporates both observed dependence and distributional attributes in the generated sequences. Use of a nonparametric framework provides an effective means for representing the observed probability distribution, while the use of a “variable kernel” ensures accurate modeling of streamflow data sets that contain a substantial number of zero‐flow values. A careful selection of prior flows imparts the appropriate short‐term memory, while use of an “aggregate” flow variable allows representation of interannual dependence. The nonparametric simulation model is applied to monthly flows from the Beaver River near Beaver, Utah, and the Burrendong dam inflows, New South Wales, Australia.
Geochemical study of arsenic release mechanisms in the Bengal Basin groundwater
Authors: Dowling Carolyn B. , Poreda Robert J. , Basu Asish R. , Peters Scott L. , Aggarwal Pradeep K.
Published: October 2002

To investigate arsenic mobility in the Bengal Basin groundwater, we sampled water wells and sediments throughout the region. There are strong correlations among high levels of dissolved arsenic and iron, ammonia, and methane, especially in samples from a single site (Laxmipur). No linkage is seen between As and agricultural tracers such as phosphate. The association of As and Fe occurs because arsenic strongly adsorbs onto FeOOH particles in river water. They flocculate with other fine‐grained particles at the freshwater/saltwater transition zone. Subsequent bacterially mediated reduction of FeOOH in the clay releases the adsorbed arsenic. Weathering of As‐bearing mica plays a significant role in the As budget. The “correlated” presence of As, CH4, and NH4 in water supply wells is the result of diffusion out of organic‐rich clay into the more permeable zones. Arsenic is mainly released from recent sediments at <50 m depth deposited in the GBR floodplain as sea level rose throughout the Holocene.
Consistency of patterns in concentration‐discharge plots
Authors: Chanat Jeffrey G. , Rice Karen C. , Hornberger George M.
Published: October 2002

Concentration‐discharge (c‐Q) plots have been used to infer how flow components such as event water, soil water, and groundwater mix to produce the observed episodic hydrochemical response of small catchments. Because c‐Q plots are based only on observed streamflow and solute concentration, their interpretation requires assumptions about the relative volume, hydrograph timing, and solute concentration of the streamflow end‐members. Evans and Davies [1998] present a taxonomy of c‐Q loops resulting from three‐component conservative mixing. Their analysis, based on a fixed template of end‐member hydrograph volume, timing, and concentration, suggests a unique relationship between c‐Q loop form and the rank order of end‐member concentrations. Many catchments exhibit variability in component contributions to storm flow in response to antecedent conditions or rainfall characteristics, but the effects of such variation on c‐Q relationships have not been studied systematically. Starting with a “baseline” condition similar to that assumed by Evans and Davies [1998], we use a simple computer model to characterize the variability in c‐Q plot patterns resulting from variation in end‐member volume, timing, and solute concentration. Variability in these three factors can result in more than one c‐Q loop shape for a given rank order of end‐member solute concentrations. The number of resulting hysteresis patterns and their relative frequency depends on the rank order of solute concentrations and on their separation in absolute value. In ambiguous cases the c‐Q loop shape is determined by the relative “prominence” of the event water versus soil water components. This “prominence” is broadly defined as a capacity to influence the total streamflow concentration and may result from a combination of end‐member volume, timing, or concentration. The modeling results indicate that plausible hydrological variability in field situations can confound the interpretation of c‐Q plots, even when fundamental end‐member mixing assumptions are satisfied.
A stochastic approach to the evaluation of residence times
Authors: Maran Stefano
Published: October 2002

Residence times of conservative pollutants in a lake or a reservoir are studied without the usual approximation of constant inflow/outflow. The effects of a random discharge flushing out a reservoir are investigated with the techniques of stochastic differential equations. Stochastic properties of the concentration in the reservoir are derived from stochastic properties of discharge (moments, autocorrelation function, and probability distribution function), and some approximations are analyzed. The major results are two simple and usable corrections for the residence time in two limiting cases: when autocorrelation time of discharge is much shorter than reservoir residence time and, at the opposite, when it is much longer.
A novel three‐dimensional lattice Boltzmann model for solute transport in variably saturated porous media
Authors: Zhang Xiaoxian , Bengough A. Glyn , Deeks Lynda K. , Crawford John W. , Young Iain M.
Published: October 2002

This paper presents a lattice Boltzmann method for the advection and dispersion of solute in three‐dimensional variably saturated porous media. The proposed method is based on the BGK model and discretizes the particle velocity space with a cuboid lattice in which the particles move in 19 directions and 7 speeds. In the proposed model a directionally dependent relaxation time is introduced to produce the second‐order dispersion tensor, and a modification of the equilibrium distribution functions is given to model the solute in porous media where the volumetric water content varies over space and time. The concentration is calculated from a weighted summation of the particle distribution functions to ensure that the BGK collision does not result in mass change. The accuracy and consistency of the proposed model are verified against benchmark problems and the finite difference solution of solute transport in an unsaturated heterogeneous soil. The results show that the proposed model conserves mass perfectly and gives efficient and accurate solutions for both advection‐dominated and dispersion‐dominated problems.
Uncertainty analysis of transient unsaturated flow in bounded domain
Authors: Ferrante Marco , Brunone Bruno , Yeh T.‐C. Jim
Published: October 2002

This paper analyzes the head variance during transient flows in unsaturated porous media. The analysis is based on a first‐order Taylor series expansion of the discretized Richards' equation and stochastic conceptualization of hydrologic properties in space. Effects of boundary conditions and flow conditions (drying or wetting) were examined for different soil models. Results of the study indicate that a constant flux boundary condition affected the pressure head variance distribution close to the surface, originating a decrease of the values at early times during wetting scenarios. On the other hand, a constant head boundary condition strongly affected the variance profiles during both transient and steady flows, also far from the boundary.
A synthetic aperture radar–based model to assess historical changes in lowland floodplain hydroperiod
Authors: Townsend Philip A. , Foster Jane R.
Published: October 2002

The hydrology of riparian wetlands worldwide has been altered extensively owing to the construction and operation of dams. We developed a model for the Roanoke River floodplain (United States) to simulate flood extent and duration based on a power law correlation between inundation area A, as mapped from synthetic aperture radar (SAR) imagery, and river discharge Q. Model fit was 0.955 for the upper portion of the study area and 0.789 for the tidally influenced lower section. We then compared hydroperiod simulated for predam (1912–1949) and postdam (1965–1995) periods. Topographically wet areas are now flooded longer than before damming, and dry areas are now drier. Similarly, hydrologically wet years experience longer floods, whereas the driest years are drier. Most importantly, spring hydroperiod regimes are now wetter than prior to damming. Our results suggest that the intermediate zone of the hydrologic gradient has been squeezed to either wetter or drier conditions. The model presented represents a simple but effective empirical method to simulate hydroperiod regimes at the landscape scale in large lowland systems where the data necessary to develop more complex physical models are not available.
Modeling density‐driven flow and radionuclide transport at an underground nuclear test: Uncertainty analysis and effect of parameter correlation
Authors: Pohlmann Karl F. , Hassan Ahmed E. , Chapman Jenny B.
Published: October 2002

The effects of uncertainty associated with input parameters and conceptual groundwater flow and transport models are studied and evaluated for radionuclide transport modeling at the Milrow underground nuclear test site on Amchitka Island, Alaska. Two‐dimensional numerical simulations in conjunction with a multiparameter uncertainty analysis are adapted and used to evaluate the effects of the uncertainties associated with the definition of the modeled processes and the values of the parameters governing these processes. Density‐driven flow caused by the mixing of fresh and salt water in a coastal aquifer and radionuclide mass transport resulting from the nuclear test are the two major processes analyzed. The parametric uncertainty analysis is performed in two stages. In the first modeling stage, one parameter is varied at a time, and the effects of this variability on transport results are evaluated. The analysis indicates that the solution to the flow problem, and thus the transport results, are very sensitive to the values of recharge and hydraulic conductivity. However, fracture porosity does not affect density‐driven flow but dramatically influences the transport results in terms of arrival times of radionuclides from the test cavity to the seafloor. Matrix diffusion is also crucial in affecting arrival times and mass flux values. The second modeling stage utilizes multiple realizations of the flow field generated by considering random combinations of recharge, conductivity, macrodispersivity, and fracture porosity drawn at random from their respective distributions. For each one of these realizations, transport parameters are also drawn at random from their assigned distributions. Different correlation models are considered and compared with scenarios with no parameter correlation. The results indicate that incorporation of correlation between hydraulic conductivity and recharge significantly reduces the uncertainty in both travel time and transverse plume location. This results from the impact of these parameters on the transition zone, which dominates flow velocity variability in this coastal aquifer setting. Correlation between fracture porosity and matrix diffusion has the largest impact on the transport results, with a positive correlation providing the critical scenario of most rapid transport (thus less radioactive decay) and a negative correlation leading to longer travel times with significant decay.
Serial correlation in water quality variables: Estimation and implications for trend analysis
Authors: Darken Patrick F. , Zipper Carl E. , Holtzman Golde I. , Smith Eric P.
Published: October 2002

Serial correlation in water quality time series invalidates tests of significance, such as seasonal Kendall analysis, that assume independence. One approach to analyzing such data is to estimate the magnitude of the serial correlation and then use the degree of serial correlation to adjust the variance and make valid inferences. Four methods for estimating serial correlation lag parameters are evaluated. R‐estimation outperformed the alternative methods considered. In case studies of water quality time series, the nonparametric rank von Neumann's ratio test of randomness is used to test for the presence of serial correlation, and R‐estimation is used to estimate the severity of the serial correlation. Case study results demonstrate that serial correlation is common in monthly water quality data and does not disappear when bimonthly intervals are used. Adjustment for flow and seasonal effects generally reduced estimates of serial correlation. However, even after flow adjustment and deseasonalization, the assumption of independence was often unreasonable.
Mathematical analysis of heat pulse signals for soil water flux determination
Authors: Wang Quanjiu , Ochsner Tyson E. , Horton Robert
Published: October 2002

Soil water flux is an important parameter in studies of runoff, infiltration, groundwater recharge, and subsurface chemical transport. Heat pulse sensors have been proposed as promising tools for measuring soil water fluxes. To date, heat pulse methods have required cumbersome mathematical analyses to calculate soil water flux from the measured data. We present a new mathematical analysis showing that a simple linear relationship exists between soil water flux and the natural log of the ratio of the temperature increase downstream from the line heat source to the temperature increase upstream from the line heat source. The simplicity of this relationship makes heat pulse sensors an attractive option for measuring soil water fluxes. In theory, this method is valid for fluxes with magnitudes between 10−4 and 10−7 m s−1. The range of measurable fluxes is defined by temperature measurement resolution at the lower end and by the assumptions used in the analysis at the higher end.
An integral transform approach for a mixed boundary problem involving a flowing partially penetrating well with infinitesimal well skin
Authors: Chang Chien‐Chieh , Chen Chia‐Shyun
Published: October 2002

A flowing partially penetrating well with infinitesimal well skin is a mixed boundary because a Cauchy condition is prescribed along the screen length and a Neumann condition of no flux is stipulated over the remaining unscreened part. An analytical approach based on the integral transform technique is developed to determine the Laplace domain solution for such a mixed boundary problem in a confined aquifer of finite thickness. First, the mixed boundary is changed into a homogeneous Neumann boundary by substituting the Cauchy condition with a Neumann condition in terms of well bore flux that varies along the screen length and is time dependent. Despite the well bore flux being unknown a priori, the modified model containing this homogeneous Neumann boundary can be solved with the Laplace and the finite Fourier cosine transforms. To determine well bore flux, screen length is discretized into a finite number of segments, to which the Cauchy condition is reinstated. This reinstatement also restores the relation between the original model and the solutions obtained. For a given time, the numerical inversion of the Laplace domain solution yields the drawdown distributions, well bore flux, and the well discharge. This analytical approach provides an alternative for dealing with the mixed boundary problems, especially when aquifer thickness is assumed to be finite.
Woody debris and channel morphology in first‐ and second‐order forested channels in Washington's coast ranges
Authors: Jackson C. Rhett , Sturm Christopher A.
Published: October 2002

While there are conceptual and theoretical reasons to believe small streams behave differently than larger streams, the lack of information on small streams has lead land managers to rely on data from larger streams to guide management decisions. In response to the need for descriptive information on habitat and channel morphology specific to small, non‐fish‐bearing streams in the Pacific Northwest, morphologies and wood frequencies in 42 first‐ and second‐order forested streams <4 m wide were surveyed. Frequencies and size distributions of woody debris were compared between small streams and larger fish‐bearing streams as well as between second‐growth and virgin timber streams. Statistical models were developed to explore dominant factors affecting channel morphology and habitat. Findings suggest geomorphological relationships, specifically the role of woody debris in habitat formation, documented for larger streams do not apply to headwater streams. Relatively small wood (diameters between 10 and 40 cm), inorganic material, and organic debris (diameters <10 cm) were major step‐forming agents while big woody debris pieces (>40 cm diameter) created <10% of steps. Streams in virgin and managed stands did not differ in relative importance of very large woody debris. Because of low fluvial power, pool habitat was rare. These streams featured mostly step‐riffle morphology, not step‐pool, indicating insufficient both flow for pool‐scour. Stream power and unit stream power were dominant channel shaping factors.
Power spectrum of sea level change over fifteen decades of frequency
Authors: Harrison C. G. A.
Published: October 2002

The power spectrum of relative sea level change has been estimated over more than 15 orders of magnitude in frequency, from a frequency of 1/(591 Ma) to a frequency of 1/(5 s). Although there are still regions of the spectrum where data sampling and duration do not allow the power to be calculated, most notably between periods of 100–1000 years, the general shape of the spectrum is that in which the power depends on the square of the reciprocal frequency, apart from periods between 1 and 100 years where the power spectrum falls off less steeply with increasing frequency. A spectrum in which power depends on reciprocal frequency squared is the same spectrum as that calculated from a random walk signal of finite length. There are some causes that have defined frequencies, such as those associated with tides and the Milankovitch cycles of the ice ages, but there is also a continuum of relative sea level change that requires other causes. The implications of this are discussed in the light of global change and heating of the lithosphere from the bottom.
Measurement of silicon stable isotope natural abundances via multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS)
Authors: De La Rocha Christina L.
Published: October 2002

Widespread utilization of silicon isotope ratio variations (δ29Si and δ30Si) as a geochemical and paleoceanographic tracer has been hampered by the hazardous nature of the analysis, which requires the use of a fluorinating gas. Multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) provides a safer means of silicon isotope analysis. High backgrounds, long rinsing and sample introduction times, and interferences are significant difficulties. It is possible to work around these obstacles, and the measurement of δ29Si may be done to a precision averaging ± 0.1‰. The δ30Si, which cannot be measured directly due to a NO interference at mass 30, may be extrapolated from the theoretically derived and empirically verified relationship, δ30Si = 1.93δ29Si. Biological materials (sponge spicules and diatoms) measured via MC‐ICP‐MS yielded δ30Si values in the ranges expected from the prior analysis of similar samples by the method of fluorination.
Two‐component auroras
Authors: Shue J.‐H. , Newell P. T. , Liou K. , Meng C.‐I. , Kamide Y. , Lepping R. P.
Published: October 2002

By examining Polar Ultraviolet Imager auroral images at the 160–180 nm N2 Lyman‐Birge‐Hopfield emission band, we find that the auroral dynamics associated with substorms shows a two‐component feature. An auroral feature, which is called the two‐cell aurora, is associated with the increased magnetospheric convection in response to a southward interplanetary magnetic field. A substorm expansion may then occur, adding an additional aurora in the midnight sector. As the substorm continuously increases its strength, the substorm aurora connects with the two‐cell aurora, forming a well‐defined crescent‐shape aurora. It is also found that the occurrence rate of the two‐cell aurora depends on universal time.
Annual and solar rotation periodicities in IMF components: Evidence for phase/frequency modulation
Authors: Takalo J. , Mursula K.
Published: October 2002

We study the annual periodicity and the solar rotation periodicity of the in‐ecliptic components of the interplanetary magnetic field (IMF). It is well known that the annual periodicity undergoes a phase reversal in 11 years, reflecting the 22‐year Hale cycle of the solar magnetic field. We construct a model where the annual periodicity of the IMF components is phase/frequency modulated by the Hale cycle, and show that this model can reproduce the observed properties of the annual periodicity. We find that the solar rotation periodicity depicts an analogous nodal structure and a phase reversal with a period of about 3.2 years, implying a new periodicity in the properties of the IMF. We demonstrate that, using the observed spectral width, the phase/frequency modulation model can satisfactorily explain this structure. We also note that taking into account the observed phase reversal in the solar rotation periodicity implies that the most persistent synodic solar rotation period in the in‐ecliptic IMF components is 27.6 days, i.e., somewhat longer than recently suggested.
On the feedback of stratospheric clouds on polar climate
Authors: Kirk‐Davidoff Daniel B. , Schrag Daniel P. , Anderson James G.
Published: October 2002

Past climates, such as the Eocene (55 ‐ 38 Ma), experienced dramatically warmer polar winters. Global climate models run with Eocene‐like boundary conditions have under‐predicted polar temperatures, a discrepancy which has stimulated a recent hypothesis that polar stratospheric clouds may have been important. We propose that such clouds form in response to higher CO2 via changes in stratospheric circulation and water content. We show that the absence of this mechanism from models of Eocene climate may be attributable to poor vertical resolution in the neighborhood of the tropical tropopause. This may cause the models to underestimate future greenhouse warming.