bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of clastic wedge provenance in the Appalachian foreland using U/Pb ages of detrital zircons
Authors: Gray Mary Beth , Zeitler Peter K.
Published: February 2004

We used the sensitive high‐resolution ion microprobe (SHRIMP) to determine the U/Pb ages of zircons within clasts from quartz‐pebble conglomerates in two Appalachian clastic wedges in order to further constrain the provenance and tectonic architecture of the central Appalachian basin at key times in its development. The ages of 64 zircons from samples of quartz pebbles in the Lower Silurian Shawangunk Formation and the Middle Pennsylvanian Pottsville Formation were determined using the SHRIMP. Zircons from Shawangunk quartz pebbles are no younger than 900 Ma, and most yield ages of 950–1200 Ma, coincident with the Grenville orogeny. Because multicycle sedimentary sources for the Shawangunk pebbles are not obvious, we conclude that a Grenville terrane, apparently unaffected by significant late Precambrian or early Paleozoic metamorphism, was uplifted, unroofed, and incised during the Taconic orogeny. Zircons from quartz clasts in the Pottsville Formation range in age from 380 Ma to 2100 Ma and also contain a major Grenville component. Some clasts contain zircons in the age range 380–640 Ma, including a significant grouping between about 400 and 450 Ma. These zircons may have been derived from internal terranes affected by the waning stages of the Taconic orogeny. Clasts containing zircons as much as 2.1 Ga are problematic. They may have been derived from the distant interior craton (e.g., Superior Province) or were perhaps derived from the adjacent West African craton during the Pennsylvanian.
Rotation of horizontal stresses in the Australian North West continental shelf due to the collision of the Indo‐Australian and Eurasian Plates
Authors: Hillis Richard R. , Mildren Scott D. , Pigram Chris J. , Willoughby Don R.
Published: February 2004

There is a 40° rotation of regional maximum horizontal stress (σhmax) orientation between the western end of the Australian North West Continental Shelf (Carnarvon Basin) and its eastern end (Bonaparte Basin). A total of 625 borehole breakouts covering a cumulative length of 7.7 km in 42 wells in the Carnarvon Basin indicates a σhmax orientation of 090°–100°N. A total of 616 borehole breakouts over 6.8 km in 46 wells in the Bonaparte Basin indicates a σhmax orientation of 055°N–060°N. Together with extant data from the World Stress Map, these results indicate that regional σhmax orientation is 050°–060°N from New Guinea westward through the Bonaparte Basin to the Canning Basin (central North West Shelf). Between the Canning Basin and the Carnarvon Basin, σhmax rotates to 090°–100°N. The parallelism of σhmax orientation in the Bonaparte Basin to the Australia/Banda Arc collisional zone indicates that this collision is not generating significant net push. Rather, the 050°–060°N σhmax orientation of much of the northern Australian margin is probably controlled by the more mature New Guinea orogen to which it is approximately orthogonal. The observed rotation of σhmax can be explained solely by the focusing of the forces balancing ridge push along collisional segments of the northeastern boundary of the Indo‐Australian Plate (such as the New Guinea orogen). Although not required to account for the observed stress rotation, a slab pull force from oceanic Indo‐Australian Plate being subducted beneath the Sunda Arc cannot be dismissed.
The role of extension in the Miocene denudation of the Nevado‐Filábride Complex, Betic Cordillera (SE Spain)
Authors: Johnson Christopher , Harbury Neil , Hurford Anthony J.
Published: February 2004

The Internal Zone of the Betic Cordillera, SE Spain, consists of a nappe stack of three complexes, the deepest of which is the Nevado‐Filábride Complex. The zone is separated from the overlying Alpujárride Complex by a crustal scale shear zone that has variously been interpreted as a thrust or an extensional detachment. A suite of 74 new apatite and zircon fission track results have been obtained from the Nevado‐Filábride Complex and these have been used to define regional cooling patterns for the complex. Rapid cooling (105°C–200°C Ma−1) is spatially related to the tectonic contact with the overlying Alpujárride Complex‐Cooling to near‐surface temperatures occurred first in the east (Sierra de los Filabres) during the mid‐Serravallian (12±1 Ma) and was completed by the early Tortonian (9–8 Ma) in the west (Sierra Nevada). There is no correlation between fission track age and sample elevation. These results are consistent with tectonic unroofing of this complex, a finding that favors extension as the mechanism by which the two complexes were brought into contact. Extension spans the middle and earliest upper Miocene (12–8 Ma) in the study area and therefore lasted much longer than previously documented. A hypothesis is advanced which links oblique convergence between the Iberian plate and the Betic Internal Zones, resulting in crustal contraction at depth, with orogen parallel extension in the middle and upper crust.
Numerical Evaluation of Flowmeter Test Interpretation Methodologies
Authors: Ruud N. C. , Kabala Z. J.
Published: February 2004

We evaluate systematic errors inherent in two flowmeter test interpretation methodologies. A physically consistent two‐dimensional groundwater flow model is used to numerically simulate the single flowmeter test and the double flowmeter test in 25 different two‐layer confined aquifers of a priori known horizontal hydraulic conductivities, Ki; specific storativities, Ssi, and hydraulic diffusivities, vi=Ki/ssi, in each layer (i=1, 2). Values selected for the layer hydraulic parameters span those encountered in natural sandy formations with the ratios of the corresponding parameters in the two layers falling in the following ranges: 1 ≤ K1/K2 ≤ 100, 0.0001 ≤ Ss1/Ss2 ≤ 100, and 1 ≤ V1/v2 ≤ 10,000. We find that the size of the hydraulic diffusivity contrast rather than the hydraulic conductivity ratio is the dominating factor for the parameter estimation accuracy in the two flowmeter methodologies. For the single flowmeter test methodology the ratio of the estimate over the corresponding true value falls within the range
, whereas for the double flowmeter test it falls within
. The double flowmeter test also provides estimates of layer specific storativity, Ssi. These are accurate only for the special case of equal layer hydraulic diffusivities. The test yields order‐of‐magnitude estimates of Ssi for layer hydraulic diffusivities differing from each other by up to an order of magnitude. For larger differences the estimation errors are much larger. Hydraulic parameters of a given layer are better estimated when the flowmeter is placed either exactly at the interlayer boundaries or, ideally, at two different points within each layer away from the boundaries. The results from simulated flowmeter tests in a five‐layer system are consistent with those for the two‐layer aquifers. This implies that the presented results most likely apply to flowmeter tests in arbitrary multilayer aquifers. Existence of significant errors in aquifer parameters estimated from synthetic flowmeter data demonstrate that the Theis [1935] model, which is assumed to be valid in each layer by the two considered interpretation methodologies, does not fully capture the flow dynamics in layered aquifers. This is illustrated by numerically calculated examples of simultaneously nonuniform and transient well face flux distributions in a layered aquifer. A model more sophisticated than that of Theis [1935] needs to be found.
Analysis of Feedback Mechanisms in Land‐Atmosphere Interaction
Authors: Brubaker Kaye L. , Entekhabi Dara
Published: February 2004

The initiation of a hydrologic drought may depend on large‐scale or teleconnective causes; however, local positive feedbacks in the land‐atmosphere system are believed to contribute to the observed persistence and intensification of droughts. In this study a basic linearization technique is combined with a nonlinear stochastic model of land‐atmosphere interaction to analyze and, more importantly, quantify feedback mechanisms that arise in the coupled water and energy balances at the land surface. The model describes land‐atmosphere interaction by four coupled stochastic ordinary differential equations in soil moisture, soil temperature, mixed‐layer humidity, and mixed‐layer potential temperature. The solution is a physically consistent joint probability distribution. The steady and perturbation‐induced parts of the model equations are decomposed into the dependence of each component physical process upon each model state. Because of the negative correlation between soil moisture and soil temperature, the physical mechanisms that serve to restore each state individually (largely soil moisture control of evaporation and temperature dependence of saturation specific humidity) act as significant anomaly‐reinforcing mechanisms for the other state.
Heterogeneity in Sedimentary Deposits: A Review of Structure‐Imitating, Process‐Imitating, and Descriptive Approaches
Authors: Koltermann Christine E. , Gorelick Steven M.
Published: February 2004

Numerical models that solve governing equations for subsurface fluid flow and transport are commonly applied to analyze quantitatively the effects of heterogeneity. These models require maps of spatially variable hydraulic properties. Because complete three‐dimensional information about hydraulic properties is never obtainable, numerous methods have been developed to interpolate between data values and use geologic, hydrogeologic, and geophysical information to create images of aquifer properties. Image creation approaches fall into three general categories: structure‐imitating, process‐imitating, and descriptive. Structure‐imitating methods rely on one or more of the following to constrain the geometry of spatial patterns in geologic media: correlated random fields, probabilistic rules, and deterministic constraints developed from facies relations. Structure‐imitating methods include spatial statistical algorithms and geologically based sedimentation pattern‐matching approaches. Process‐imitating models include aquifer model calibration methods and geologic process models. Aquifer model calibration methods use governing equations for subsurface fluid flow and transport to relate hydraulic properties to heads and solute information through history and steady state data matching. Geologic process models combine fundamental laws of conservation of mass and momentum with sediment transport equations to simulate spatial patterns in grain size distributions. At the sedimentary basin scale, multiprocess models include thermomechanical mechanisms of basin subsidence. Descriptive methods couple geologic observations with facies relations to divide an aquifer into zones of characteristic hydraulic properties. All approaches are capable of reproducing heterogeneity over a range of scales and considering some types of geologic information. Some approaches are strictly spatial while some are linked to the time evolution of sedimentation. Some approaches can be conditioned on measurements. Recent advances aimed at infusing geologic information into images of the subsurface include extracting more information from sedimentological facies models, incorporating qualitative geologic information into random field generators and simulating depositional processes. Classes of research missing from the literature include multiprocess models that incorporate diagenesis and three‐dimensional surface water flow, hybrid methods that combine features of existing approaches, and approaches that can make use of all available geologic, geophysical, and hydrologic data.
An Isotopic Study (2H and 18O) of Water Movements in Clayey Soils Under a Semiarid Climate
Authors: Mathieu Renaud , Bariac Thierry
Published: February 2004

We applied the natural variations of stable isotopes in water to the study of infiltration in clayey soils and to the study of groundwater recharge in the small drainage basin of Barogo in West Africa. The comparison between the variations in isotopic contents of soil water and rainwater in 1988 and 1989 led us to characterize two infiltration processes: (1) an infiltration through the soil matrix of rainwater that mixed with mobile water which was held in the superficial part of the soil and was enriched in heavy isotopes by evaporation and (2) a fast and deeper infiltration by preferential flow along macropores that bypassed the superficial part of the soil. The study of the variations in groundwater isotopic content led us again to separate the recharge process into two components: (1) a slow infiltration through the soil matrix and the weathered basement (distributed recharge) and (2) a fast and direct recharge through conducting fissured zones (localized recharge). A schematic and local representation of groundwater recharge in the Barogo basin allowed us to estimate that direct recharge represented around 70% of the total recharge in 1989.
The Economic Value of Water in Recreation: Evidence from the California Drought
Authors: Ward Frank A. , Roach Brian A. , Henderson Jim E.
Published: February 2004

A significant barrier to economically efficient management of most reservoir systems is lack of reliable information about how recreational values change with reservoir levels. This paper presents evidence on marginal values of water for recreation at Corps of Engineer reservoirs in the Sacramento, California, District. Data on visitors were collected by origin and destination before and during the early part of the 1985–1991 California drought. Because lake levels varied widely during the sample period, water's effect on visits was isolated from price and other effects. An estimated regional travel cost model containing water level as a visit predictor provided information to compute marginal values of water in recreation. For the range of the lake levels seen, annual recreational values per acre‐foot (1234 m3) of water vary from $6 at Pine Flat Reservoir to more than $600 at Success Lake. These findings are limited to use values of visitors who travel to the reservoirs and do not reflect passive use values to people who value the reservoirs but never visit them. Analysts could apply similar methods to other river basins in which a public agency controls the management of multiple water uses.
A Reassessment of the Groundwater Inverse Problem
Authors: McLaughlin Dennis , Townley Lloyd R.
Published: February 2004

This paper presents a functional formulation of the groundwater flow inverse problem that is sufficiently general to accommodate most commonly used inverse algorithms. Unknown hydrogeological properties are assumed to be spatial functions that can be represented in terms of a (possibly infinite) basis function expansion with random coefficients. The unknown parameter function is related to the measurements used for estimation by a “forward operator” which describes the measurement process. In the particular case considered here, the parameter of interest is the large‐scale log hydraulic conductivity, the measurements are point values of log conductivity and piezometric head, and the forward operator is derived from an upscaled groundwater flow equation. The inverse algorithm seeks the “most probable” or maximum a posteriori estimate of the unknown parameter function. When the measurement errors and parameter function are Gaussian and independent, the maximum a posteriori estimate may be obtained by minimizing a least squares performance index which can be partitioned into goodness‐of‐fit and prior terms. When the parameter is a stationary random function the prior portion of the performance index is equivalent to a regularization term which imposes a smoothness constraint on the estimate. This constraint tends to make the problem well‐posed by limiting the range of admissible solutions. The Gaussian maximum a posteriori problem may be solved with variational methods, using functional generalizations of Gauss‐Newton or gradient‐based search techniques. Several popular groundwater inverse algorithms are either special cases of, or variants on, the functional maximum a posteriori algorithm. These algorithms differ primarily with respect to the way they describe spatial variability and the type of search technique they use (linear versus nonlinear). The accuracy of estimates produced by both linear and nonlinear inverse algorithms may be measured in terms of a Bayesian extension of the Cramer‐Rao lower bound on the estimation error covariance. This bound suggests how parameter identifiability can be improved by modifying the problem structure and adding new measurements.
Identification of Hydraulic Conductivity Structure in Sand and Gravel Aquifers: Cape Cod Data Set
Authors: Eggleston J. R. , Rojstaczer S. A. , Peirce J. J.
Published: February 2004

This study evaluates commonly used geostatistical methods to assess reproduction of hydraulic conductivity (K) structure and sensitivity under limiting amounts of data. Extensive conductivity measurements from the Cape Cod sand and gravel aquifer are used to evaluate two geostatistical estimation methods, conditional mean as an estimate and ordinary kriging, and two stochastic simulation methods, simulated annealing and sequential Gaussian simulation. Our results indicate that for relatively homogeneous sand and gravel aquifers such as the Cape Cod aquifer, neither estimation methods nor stochastic simulation methods give highly accurate point predictions of hydraulic conductivity despite the high density of collected data. Although the stochastic simulation methods yielded higher errors than the estimation methods, the stochastic simulation methods yielded better reproduction of the measured ln (K) distribution and better reproduction of local contrasts in ln (K). The inability of kriging to reproduce high ln (K) values, as reaffirmed by this study, provides a strong instigation for choosing stochastic simulation methods to generate conductivity fields when performing fine‐scale contaminant transport modeling. Results also indicate that estimation error is relatively insensitive to the number of hydraulic conductivity measurements so long as more than a threshold number of data are used to condition the realizations. This threshold occurs for the Cape Cod site when there are approximately three conductivity measurements per integral volume. The lack of improvement with additional data suggests that although fine‐scale hydraulic conductivity structure is evident in the variogram, it is not accurately reproduced by geostatistical estimation methods. If the Cape Cod aquifer spatial conductivity characteristics are indicative of other sand and gravel deposits, then the results on predictive error versus data collection obtained here have significant practical consequences for site characterization. Heavily sampled sand and gravel aquifers, such as Cape Cod and Borden, may have large amounts of redundant data, while in more common real world settings, our results suggest that denser data collection will likely improve understanding of permeability structure.
Analysis of nonlinear effects on tracer migration in heterogeneous aquifers using Lagrangian travel time statistics
Authors: Cvetkovic V. , Cheng H. , Wen X. ‐H.
Published: February 2004

Advective transport in a heterogeneous two‐dimensional aquifer is simulated and quantified by the statistical moments of tracer travel time and transverse displacement. These moments depend on the statistics of the Lagrangian velocity given as a function of space rather than time. A comparison is made with first‐order results that appear to be robust for log transmissivity variance at least up to 1. Lagrangian statistics for travel time clearly expose a few essential features of nonlinear transport: nonstationary distribution and the increasing integral scale of the Lagrangian velocity. These reflect the tendency of streamlines to concentrate into relatively few flow paths (“preferential flow”) for increasing log transmissivity variance. Two simple empirical relationships are identified that in combination with first‐order results for the flow capture the main features of nonlinear transport. The proposed methodology can readily be generalized for analyzing advective transport under more complex flow conditions and for establishing relatively simple analytical models for estimating solute mass flux in heterogeneous aquifers.
Evaluation of Water Conservation Programs in Thorny Nonexperimental Settings
Authors: Bamezai Anil
Published: February 2004

Nonexperimental program evaluations are performed under numerous constraints that require a judicious blend of theory, attention to data quality, and judgment. This paper describes the conceptual and measurement issues that arise while evaluating nonresidential water conservation programs implemented during a drought in the absence of a true control group. Sensitivity analysis techniques are presented to assess the impact of the drought on comparisons of preprogram and postprogram water consumption.
Using partial probability weighted moments to fit the extreme value distributions to censored samples
Authors: Wang Q. J.
Published: February 2004

The estimation of floods of large return periods from lower bound censored samples may often be advantageous because interpolation and extrapolation are made by exploring the trend of larger floods in each of the records. The method of partial probability weighted moments (partial PWMs) is an useful technique for fitting distributions to censored samples. In this paper, partial PWMs are redefined along with a brief discussion on the definition of ordinary PWMs. The expression for partial PWMs is derived for the extreme value type I or Gumbel distribution. Combined with those for the extreme value type II and III distributions, an unified expression for partial PWMs is presented for the generalized extreme value (GEV) distribution. The equations for solving the distribution parameters are provided. Monte Carlo simulation shows that lower bound censoring at a moderate level does not unduly reduce the efficiency of high‐quantile estimation even if the samples have come from a true GEV distribution.
Field determination of sulphide oxidation rates in mine tailings
Authors: Elberling Bo , Nicholson Ronald V.
Published: February 2004

A field study was conducted to assess the rates of oxygen consumption in the vadose zone of sulphide mine tailings. Oxygen uptake rates were measured directly at the ground surface and were compared to rates of sulphate production in the tailings profile. A nonreactive tracer gas was also used to assess effective diffusion coefficients in the shallow subsurface. The 26 measurement sites were located within a 0.1 km2 area in tailings with a water table depth ranging from 0.6 m to ponding at the surface. Ambient oxygen consumption rates in the 8‐year‐old tailings were compared to measurements through 0.2‐m‐thick covers of fine sand and 0.2 m layers of fresh tailings replacing oxidized tailings at ground surface. The oxygen flux across the tailings surface ranged from 0.1 to 250 mol O2 m−2 yr−1 for ambient oxidation conditions. Fresh tailings at the surface produced rates that were as much as twice the ambient rates. The 0.2 m fine sand cover lowered rates by a factor of 100 below ambient rates. Sulphate production rates agreed well with oxygen consumption measurements. Independent diffusion flux measurements with a nonreactive gas exhibited an excellent correlation with oxygen uptake fluxes, indicating the dominance of oxygen diffusion in the control of oxidation rates in the tailings. Spatial trends in oxidation rates were consistent with depth to the water table, and temporal changes by as much as a factor of 2 were attributed to a 0.16 m variation in the depth to the water table over a 2‐week period. The results of this study support the use of simple Fickian models to evaluate oxygen diffusion and oxidation rates in sulphide tailings.
Self‐Affinity in Braided Rivers
Authors: Sapozhnikov Victor , Foufoula‐Georgiou Efi
Published: February 2004

Three braided rivers of different scales and different hydrologic/geomorphologic characteristics (the Aichilik and Hulahula in Alaska and the Brahmaputra in Bangladesh) are analyzed for spatial scaling using a logarithmic correlation integral method developed earlier by the authors. It is shown that the rivers exhibit anisotropic scaling (self‐affinity) with fractal exponents vx = 0.72–0.74 and vy = 0.51‐0.52, the x axis being oriented along the river and the y axis in the perpendicular direction. The fact that despite large differences in scales (0.5–15 km in braid plain width), slopes (7 × 10−3 to 8 × 10−5), and types of bed material (gravel to sand), the analyzed braided rivers show similar spatial scaling deserves special attention. It might indicate the presence of universal features in the underlying mechanisms responsible for the formation of the spatial structure of braided rivers. Also, comparison of fractal characteristics of braided rivers with those of single‐channel rivers and river networks suggests that braided rivers form a class of fractal objects lying between the classes of single‐channel rivers and river networks.
Source of radiogenic helium 4 in shallow aquifers: Implications for dating young groundwater
Authors: Solomon D. K. , Hunt A. , Poreda R. J.
Published: February 2004

Radiogenic helium 4 (4Herad) has been used in numerous studies as a tracer of groundwater age in the range of 103–108 years. We have measured 4Herad along shallow groundwater flow paths at a variety of hydrogeologically distinct sites and postulate its use for dating groundwater as young as 101 years. Groundwater travel times and fluid velocities are particularly well documented at one site in northern Ontario because of detailed profiling of tritium, 3H/3He ratios, and chlorofluorocarbons (CFCs). Metamorphic rocks of the Canadian Shield (>1 Ga) that contain large quantities of 4He are the protolith of this unconsolidated aquifer and observed 4Herad values increase linearly with distance along a flow path and with increasing groundwater age. A solute transport model suggests that the aquifer solids are the source of 4Herad as vertical fluid velocities are too great to allow upward diffusion of 4Herad from the underlying shield rocks. The apparent rate of 4Herad release is 130 μcm3 m−3 yr−1 and is 300 times greater than can be supported by the in situ decay of U and Th series nuclides (i.e., the “steady state” approximation). Laboratory release experiments (conducted by sequentially heating the aquifer solids, measuring the amount of 4He released, and then extrapolating release rates to the in situ temperature) agree well with the field results and suggest that diffusion from aquifer solids is the source of 4Herad. The combined laboratory and field release data yield 4He diffusion coefficients that exhibit an Arrhenius temperature dependence that is similar to4Herad diffusion in quartz reported by other researchers. The 4Herad release rate at the Ontario site is extraordinarily similar to sites in Tennessee, Nebraska, and Germany in spite of major hydrogeologic differences. A model of 4He diffusion from spherical grains suggests that aquifer solids derived from old protoliths will release 4He at rates greater than supported by U/Th production for up to 50 million years in fine sands that have typical U/Th concentrations. Both observations and modeling suggest that 4He may be useful as a groundwater dating tool over a range of tens to hundreds of years. The latter is particularly important because no other groundwater dating techniques are accurate for waters ranging from 40 to about 500 years old.
Flow and nutrient export patterns for an agricultural hill‐land watershed
Authors: Pionke H. B. , Gburek W. J. , Sharpley A. N. , Schnabel R. R.
Published: February 2004

Nutrient patterns and export in streamflow were determined for a 7.4‐km2agricultural hill‐land watershed located in Pennsylvania. P export, whether associated with sediment or water phase, was dominated by storm periods. About 70% of water phase P was exported during the 10% of time defined as storm flow. P export during storms becomes less controlled by water phase P as the P fraction being exported is expanded from labile P (50–70%) to algal‐available P (20–40%) to total P (10–20%). Water phase P accounted for 25–50% of the algal‐available P exported in total flow. Nitrate export was mostly in nonstorm periods (60%) with the higher concentrations in elevated base flows. The elevated base flow appears to result from increased drainage from a shallow fracture layer that underlays cropland. Because storm periods dominate P export, control of the hydrologically active P source areas within the watershed provides the greatest opportunity for controlling export of algal‐available P.
Modeling the Transport of Water Stable Isotopes in Unsaturated Soils Under Natural Conditions: 2. Comparison With Field Experiments
Authors: Melayah A. , Bruckler L. , Bariac T.
Published: February 2004

The purpose of this work is to test a mechanistic model for predicting depth profiles of stable isotope values in unsaturated soil under natural climatic conditions. An experiment was conducted on a (20 × 50 m) bare field (27% clay, 62% silt, and 11% sand) at Montfavet (France) for a 50‐day period. The field was irrigated with a sprinkler (approximately 7 mm h−1) at the beginning of the experiment (July 9), after which evaporation operated under natural conditions (July 10). Three to five gravimetric water content profiles, down to 0.20 m, were measured daily. The field was equipped with tensiometers at 0.10‐m intervals from 0.15 to 1.55 m depth at three sites. Temperature probes were installed at intervals down to 1.00 m depth at one site, and soil‐temperature measurements were recorded every 15 min. Soil‐water was sampled for isotopic analysis about every 5 days. A micrometeorological mast was set up in the center of the field to measure wind speed, air temperature, and vapor pressure and to estimate the sensible heat flux (eddy correlation method). Unsaturated hydraulic conductivity and aerodynamic roughness length were estimated before running a full comparison with field data. Good agreement was achieved between observed and calculated hourly fluxes for net radiation, soil heat conduction, sensible heat flux, and actual soil evaporation. The model described well the variations of both moisture content and soil temperature. The isotopic enrichment in the top soil layers was qualitatively well described, although systematic differences appeared in the shape of the calculated and measured isotope profiles. The model results were highly sensitive to small changes in convective transport in the liquid phase and to the initial isotope profile. More intensive soil sampling would be useful for validating the model.
Modeling the Transport of Water Stable Isotopes in Unsaturated Soils Under Natural Conditions: 1. Theory
Authors: Melayah A. , Bruckler L. , Bariac T.
Published: February 2004

A physical model is proposed for predicting the transport of the stable isotopes of water (H2 18O and HD16O) in soils under natural climatic conditions. The transport equation for stable isotopes in the unsaturated zone was coupled with a mechanistic model for water and heat exchange across the soil‐atmosphere boundary. Diffusive and convective isotope fluxes were each considered in liquid and vapor phases of the soil. Turbulent exchanges of isotopes between the soil surface and the atmosphere were modeled by use of similarity theory in the same way as water vapor. Boundary conditions at the soil surface are based on an energy balance and turbulent fluxes of heat, water, and isotopes. The resulting model provides a framework for understanding the behavior of stable isotopes in soils of variable hydraulic properties and under various climatic conditions.
Finite element methods for modeling water flow in variably saturated porous media: Numerical oscillation and mass‐distributed schemes
Authors: Pan Lehua , Warrick A. W. , Wierenga Peter J.
Published: February 2004

The finite element method is an attractive numerical method for modeling water flow in variably saturated porous media due to its flexibility in dealing with complicated geometries. It is well known that the conventional mass‐distributed finite element method suffers from numerical oscillations at the wetting front, especially for very dry initial conditions. Routinely, mass‐lumped procedures are used to eliminate them. This paper proposes a physical interpretation of the finite element method applied to the water flow problem. With the finite element method, mass conservation is applied at the element level. The water storage and the flux within each element are split into several components in the function space, each of which corresponds to one component of the boundary flux of the element. However, even though physical laws are correctly applied at the element level, it is shown that the traditional mass‐distributed scheme can still generate an incorrect neighboring node response due to the highly nonlinear properties of water flow in unsaturated soil and cause numerical oscillation. We propose two new mass‐distributed schemes which are free of the numerical oscillation, and reduce the smearing near the wetting front, at slightly increased CPU time.
Coupled estimation of flow and solute transport parameters
Authors: Medina A. , Carrera J.
Published: February 2004

A methodology for solving the coupled linear flow and transport inverse problem is presented. It allows the estimation of aquifer parameters (transmissivity, storativity, recharge, boundary heads and flows, leakage, dispersivity, molecular diffusion, porosity, retardation, linear decay, boundary concentrations), using their prior estimates as well as head and concentration measurements as basic information. Transient flow and transport equations are solved using the finite element method in space and a weighted finite difference scheme in time. Flow and transport domains can be one‐, two‐, or quasi three‐dimensional, applying linear one‐dimensional elements, linear two‐dimensional triangles, or bilinear two‐dimensional rectangles mixed at will. Time regimes can be transient or steady state in any equation (flow or transport). Parameters are estimated on the basis of maximum likelihood theory, which allows us to obtain information about parameters' uncertainty. Minimization of the negative log likelihood is performed using Marquardt's method. The applicability of the method is shown by means of two real examples. They illustrate that the proposed approach can successfully address model selection, parameter uncertainty, and nontrivial identifiability problems. Estimation of time‐dependent sources and the role of prior estimates are also addressed in the examples. While the formulation of the problem leaves room for both stochastic and deterministic treatment of hydraulic parameters, the latter has been adopted in both examples. Limitations of the approach are also apparent.
Improved Glass Micromodel Methods for Studies of Flow and Transport in Fractured Porous Media
Authors: Wan Jiamin , Tokunaga Tetsu K. , Tsang Chin‐Fu , Bodvarsson Gudmundur S.
Published: February 2004

Microscale experiments can provide mechanistic insights into larger‐scale flow and transport phenomena. Studies of the microscale mechanics involved in preferential flow in general, and unsaturated fast flow paths in particular, require the development of new experimental techniques. A new method for constructing glass micromodels has been developed which permits direct visualization and quantification of flow and transport phenomena in fractured porous media. In the fracture‐matrix micromodels a sequential etching procedure was developed in order to provide the necessary contrast of depths between matrix pores and fracture apertures. This high contrast in etching depths ensures that very different capillary properties are associated with micromodel “fractures” and “matrix” blocks. Improved techniques were also developed for reducing the pore sizes of the matrix to a natural fine‐grained sandstone pore scale. The improved micromodel pattern designs allow for previously unachievable control of boundary conditions. Various saturated and unsaturated fracture flow and transport processes can be visually and quantitatively studied with these micromodels. A method for directly measuring pore‐scale flow velocity distribution through tracing trajectories of suspended fluorescent microspheres was also developed. Examples of applications include measurements of velocity profiles in fractures, imbibition, fracture‐matrix transient flow, and matrix diffusion. In general, the improved micromodel method provides a unique tool for exploring some of the previously unrecognized flow and transport processes in fractured porous media. This research is directed at providing microscale explanations to some currently unresolved flow and transport issues important in predicting the larger‐scale flow processes.
Low‐flow analysis with a conditional Weibull Tail Model
Authors: Durrans S. Rocky
Published: February 2004

Estimates of low‐flow quantiles, such as the 7‐day, 10‐year low flow, which are usually obtained by statistical modeling of observed data series, are widely used in water quality management. This paper presents a conditional modeling approach to low‐flow analysis that employs only those data values which are less than or equal to a ceiling value. Modeling in this fashion has been motivated by the observation that annual low flows may derive from mixed processes and by the subjective nature of graphical methods, such as those employed by the U.S. Geological Survey, which are often employed in such cases. Results of Monte Carlo experiments demonstrate that the conditional modeling approach yields a low‐flow quantile estimator whose bias and RMSE are comparable to more conventional modeling approaches of fitting a classical textbook probability distribution on the basis of all observed data values, even when the underlying population is of a “well‐behaved” form. Since the complex forms of mixed low‐flow data distributions are not capable of being represented by classical textbook distributions and since the conditional modeling approach performs comparably to those models even when the data derive from well‐behaved probability distributions, these results imply that the conditional modeling approach is worthy of consideration for use by hydrologists. The conditional modeling approach also leads rather naturally to a scheme, much like that used in index flood methods, whereby a regional low‐flow estimator might be devised. An application of the conditional modeling approach to 48 low‐flow data series in Alabama is presented.
Estimation of Rates of Aerobic Hydrocarbon Biodegradation by Simulation of Gas Transport in the Unsaturated Zone
Authors: Lahvis Matthew A. , Baehr Arthur L.
Published: February 2004

The distribution of oxygen and carbon dioxide gases in the unsaturated zone provides a geochemical signature of aerobic hydrocarbon degradation at petroleum product spill sites. The fluxes of these gases are proportional to the rate of aerobic biodegradation and are quantified by calibrating a mathematical transport model to the oxygen and carbon dioxide gas concentration data. Reaction stoichiometry is assumed to convert the gas fluxes to a corresponding rate of hydrocarbon degradation. The method is applied at a gasoline spill site in Galloway Township, New Jersey, to determine the rate of aerobic degradation of hydrocarbons associated with passive and bioventing remediation field experiments. At the site, microbial degradation of hydrocarbons near the water table limits the migration of hydrocarbon solutes in groundwater and prevents hydrocarbon volatilization into the unsaturated zone. In the passive remediation experiment a site‐wide degradation rate estimate of 34,400 g yr−1 (11.7 gal. yr−1) of hydrocarbon was obtained by model calibration to carbon dioxide gas concentration data collected in December 1989. In the bioventing experiment, degradation rate estimates of 46.0 and 47.9 g m−2 yr−1(1.45 × 10−3 and 1.51 × 10−3 gal. ft.−2yr−1) of hydrocarbon were obtained by model calibration to oxygen and carbon dioxide gas concentration data, respectively. Method application was successful in quantifying the significance of a naturally occurring process that can effectively contribute to plume stabilization.
Parameter Estimation for Determining Hydraulic Properties of a Fine Sand From Transient Flow Measurements
Authors: Gribb Molly M.
Published: February 2004

A cone penetrometer method for measuring hydraulic conductivity of unsaturated soils at depth is under development. Successful advancement of this method hinges on using parameter estimation to obtain hydraulic parameter values from pore water pressure and flow rate data. A finite element model is employed to predict flow responses, and objective functions describe differences between “true” and simulated responses. Contour plots in parameter space show the relative sensitivity of objective functions to field‐saturated hydraulic conductivity, Kfs, field‐saturated moisture content,θfs, and the van Genuchten hydraulic parameters, α and n. Principal curvatures and directions in parameter space describe the nature of objective functions near “true” parameter values. An objective function based on flow rate and pore water pressures does not provide better parameter sensitivity than one based on pore water pressures alone. It appears possible to obtain estimates of Kfs and α but unlikely that the other parameters will be identifiable.
Modeling Steady State Conjunctive Groundwater and Surface Water Flow with Analytic Elements
Authors: Mitchell‐Bruker Sherry , Haitjema Hendrik M.
Published: February 2004

By using analytic elements to model steady state, two‐dimensional, Dupuit‐Forchheimer groundwater flow and its contribution to surface water flow, average base flows and groundwater flows in a groundwater and surface water system can be represented. Changing boundary conditions are modeled by adding or removing streams from the groundwater flow domain according to the availability of water in the stream. This dynamic representation of boundary conditions allows for the anticipation of unknown effects due to future aquifer stresses or stream withdrawals. The model does not account for transient streamflow but significantly improves the realism of steady state surface water and groundwater interactions by simulating average base flows and limiting the infiltration rates of losing streams. When steady state groundwater flow is modeled without considering the limits of available streamflow, recharge rates or hydraulic conductivities that are estimated during model calibration can be substantially in error. The capability to model groundwater and surface water conjunctively is accomplished without substantial increases in model complexity or data requirements.
Problems With Estimating the Economic Impacts of Averting Climate Change: A Look at Water Resources
Authors: Shaw W. Douglass
Published: February 2004

Averting climate change may have substantial water‐resource‐related benefits to agricultural, industrial, recreational, and residential household users. Economic benefits are usually estimated assuming that individuals face no uncertainty in decision making, but the benefits from averting climate change will accrue primarily to individuals in the future. Predicting future benefits is usually done using ex ante economic methods, and propogating uncertainty or specifiying the appropriate distributions in estimating the magnitude of the benefits is a problem. After describing the key issues relating climate change and water resources, the paper explains why economic theory fails to demonstrate which method appears more reasonable than the other. Alternatively, we might attempt to estimate the benefits from averting climate change using a benefits analysis of similar events that have already occurred. Many suggestions for conducting a benefits transfer, including one in the area of water‐based recreation, are offered as ways to glean something about possible water‐resource‐related benefits from averting climate change.
Diurnal Variation of Surface Fluxes During Thorough Drying (or Severe Drought) of Natural Prairie
Authors: Brutsaert Wilfried , Chen Daoyi
Published: February 2004

Experimental data recorded over a natural tallgrass prairie during the later stages of drying in the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment‐1987 showed (1) that the total daily values of evaporation exhibited a kind of second stage of drying behavior with a t−½ dependency at the daily timescale and (2) that this day‐to‐day evolution was modulated by the available energy at the surface, that is, the hourly radiation input. This allowed a simple description of the phenomenon by combining a desorptive diffusion‐type parameterization for the total daily evaporation or for its dimensionless counterpart (such as Priestley and Taylor's α, the evaporative fraction, and a few others), with an assumption of self‐preservation in the surface energy budget during the daytime hours. The resulting formulation, which involves two timescales, a daily and an hourly, was able to reproduce daytime hourly flux values over a 2‐week period of intensive drying. The method can also be useful in the disaggregation of daily, or even weekly, evaporation into hourly values.
Typology of Rainfall Fields to Improve Rainfall Estimation in the Sahel by the Area Threshold Method
Authors: Amani Abou , Lebel Thierry , Rousselle Jean , Taupin Jean Denis
Published: February 2004

The stratification of rainfall fields to improve specific rainfall models is a subject that has received relatively little attention in the literature. It is shown here that objective classification techniques, based on the intensities and spatial distribution of the rainfall fields, can produce meaningful results in terms of the area threshold method (ATM) model and climatology. Four approaches for rainfall classification, using rain gauge data, are proposed in order to improve the average areal rainfall estimation in the Sahel by the ATM model. Two of them are based on the structural behavior of the rainy area (area where it rains above a given threshold) function against a threshold. Based on this function, a new parameter, called the under profile area (UPA), has been proposed for the classification of rainfall fields. The groups obtained from the method based on this parameter are characterized by different average spatial structures. A significant improvement on the ATM model is observed by considering classification based on the UPA parameter. An average reduction of 34% of the root‐mean‐square error is observed in a validation term. This improvement is a direct consequence of the fact that the optimal thresholds are different from one group to another, which is an important point when considering the impact of classification on the ATM model.
A Functional Relationship Between Capillary Pressure, Saturation, and Interfacial Area as Revealed by a Pore‐Scale Network Model
Authors: Reeves Paul C. , Celia Michael A.
Published: February 2004

The constitutive relationships required for the parameterization of multiphase flow and transport problems are of critical importance to hydrologic modeling. Recently, a hypothesis has been developed that predicts a functional relationship between capillary pressure, saturation, and interfacial area. A network model was developed to test this hypothesis. Microscale physical processes were simulated and volume averaging was used to derive the macroscopic measures of saturation and fluid‐fluid interfacial area per volume of porous media. Results indicate that a smooth, though complex, functional relationship exists at the continuum scale. These results have direct relevance to constitutive theory and the modeling of nonaqueous phase liquid dissolution processes.