bluuebunny/crossref_metadata_2025 huggingface dataset:
The Las Cruces Trench Site: Characterization, Experimental Results, and One‐Dimensional Flow Predictions
Authors: Wierenga P. J. , Hills R. G. , Hudson D. B.
Published: February 2004

A comprehensive field trench study was conducted in a semiarid area of southern New Mexico to provide data to test deterministic and stochastic models of vadose zone flow and transport. A 4 m by 9 m area was irrigated with water containing a tracer using a carefully controlled drip irrigation system. The area was heavily instrumented with tensiometers and neutron probe access tubes to monitor water movement and with suction tubes to monitor solute transport. Approximately 600 disturbed and 600 core samples of soil were taken to support deterministic and stochastic characterization of the soil water hydraulic parameters. The core sample‐based saturated hydraulic conductivities ranged from 1.4 to 6731 cm/d with a mean of 533 cm/d and a standard deviation of 647 cm/d, indicating significant spatial variability. However, visual observation of the wetting front on the trench wall shows no indication of preferential flow or water flow through visible root channels and cracks. The tensiometer readings and the neutron probe measurements also suggest that the wetting front moves in a fairly homogeneous fashion despite the significant spatial variability of the saturated hydraulic conductivity. In addition to the description of the experiment and the presentation of the experimental results, predictions of simple one‐dimensional uniform and layered soil deterministic models for infiltration are presented and compared to field observations. These models are presented here to provide a base case against which more sophisticated deterministic and stochastic models can be compared in the future. The results indicate that the simple models give adequate predictions of the overall movement of the wetting front through the soil during infiltration. However, the models give poor predictions of point values for water content due to the spatial variability of the soil. Comparisons between the one‐dimensional infiltration model predictions and field observations show that the use of the layered soil model rather than the uniform soil model does not consistently improve the accuracy of the predictions for this particular field application. This result illustrates that increasing the spatial resolution of the deterministic characterization of the site in the vertical direction does not always improve the model predictions. Uncertainties due to horizontal spatial variability and due to other difficulties associated with experimental characterization appear to be more significant.
The Effect of Quadratic Gradient Terms on the Borehole Solution in Poroelastic Media
Authors: Wang Yarlong , Dusseault Maurice B.
Published: February 2004

When compressible fluid continuity in a fluid‐saturated compressible porous medium under transient conditions is considered, we cannot obtain a standard linear diffusion equation in terms of pressure unless we ignore the quadratic terms in the pressure gradient expression, for example [∂p/∂r]2 for cylindrical plane strain coordinates. They are assumed to be so small that their contribution can be ignored in pressure analysis. Thereby, a nonlinear equation can be avoided. During hydraulic fracturing, rapid drawdown, or slug testing, the pressure difference can reach a high value “instantly.” An extremely steep pressure gradient is generated, and it may not be appropriate to neglect quadratic terms. In this paper, an analytical solution for pore pressure coupling with the deformation in a porous medium is developed by taking the quadratic term into account. By Laplace transformation, we obtain a solution for a nonlinear diffusion equation by setting up a fluid continuity equation according to the mass conservation law rather than from energy principles in terms of volume. Deviations from existing solutions are identified in cases of high pressure gradients, and these deviations are related to the compressibility of the pore and injected fluids. It would seem that the new solution gives a more correct early time response in these cases. To calculate the effective stresses and pore pressure, we need to carefully define hydraulic diflfusivity. We have related the coefficients of Biot and Geertsma to those of mass conservation, which are commonly used in hydrology.
Effects of Rainfall, Vegetation, and Microtopography on Infiltration and Runoff
Authors: Dunne Thomas , Zhang Weihua , Aubry Brian F.
Published: February 2004

Apparent, or effective, infiltration rates on grassland hillslopes vary with rainfall intensity and flow depth because of the interaction between rainfall, runoff, and vegetated microtopography. The higher parts of the microtopography are occupied by greater densities of macropores and therefore have much greater hydraulic conductivities than the intervening microdepressions. On short hillslopes and plots the apparent infiltration rate is simply the spatial average of the saturated and unsaturated conductivities of this surface. The proportion of the surface which is saturated and the value to which the unsaturated conductivity is raised depends on the rainfall intensity. On longer hillslopes the downslope increase in flow depth in microtopographic depressions progressively inundates more permeable, vegetated mounds so that the hydraulic conductivity of a greater proportion of the surface is raised to its saturated value. For this reason the apparent infiltration rate increases downslope, even in the absence of spatial trends in any of the surface characteristics that affect infiltration. Apparent, or effective, infiltration rate depends on hillslope length. Consequently, steady state discharge does not increase linearly with distance downslope. These two fundamental relationships between infiltration, rainfall intensity, and runoff are analyzed on the basis of sprinkling‐infiltrometer measurements and a mathematical model.
Unmeasured Residuals in Sediment Budgets: A Cautionary Note
Authors: Kondolf G. Mathias , Matthews W. V. Graham
Published: February 2004

In studies that develop sediment budgets, one or more terms (constituting up to 94% of the total budget) have been obtained by subtraction. These unmeasured residual terms incorporate not only the sediment budget components attributed to them, but also the net sum of all errors in measured components. Thus, budgets may appear to balance only because errors are hidden in the residual term. The nature of sediment measurements is such that some error is virtually certain, and in sediment budget studies where all terms have been quantified, imbalances range as high as 104% of total basin yield. Despite these potential problems, the process of constructing the budget is still informative (even when terms are obtained as residuals), because it requires sediment sources and transport pathways to be identified and quantified. However, terms obtained by subtraction should be clearly identified as such, and where possible, error analysis of all terms should be included.
Analytical Solutions to Predict the Long‐Term Surface Energy Balance Components and Temperatures of a Bare Soil
Authors: Novak Michael D.
Published: February 2004

A periodic analytical theory previously tested only against diurnal variations is applied to a 66‐day springtime period at Agassiz, British Columbia. Comparison is made with daily average values of the surface energy balance components, soil temperatures, and near‐surface atmospheric potential temperatures and water vapor densities measured throughout this period. It is shown that the inability of the periodic theory to predict the measured warming trend at the 0.5 m depth in the soil can be overcome by applying the theory to the whole of 1978, i.e., by imbedding the period of interest in a longer period. A new analytical theory that accounts for aperiodic transient effects is also described and compared with measured soil temperatures. This theory can also account for the observed soil wanning trend but is sensitive to small errors in the prescribed soil surface heat flux density. Additional analytical calculations show that advection of heat within the soil associated with evaporation and rainfall could have significantly influenced the long‐term temperature trends observed at Agassiz. The poorer success with the analytical approach when applied to long‐term rather than diurnal variations suggests that to predict the former accurately a numerical model is required.
An Assessment of the Equable Change Principle in at‐a‐Station Hydraulic Geometry
Authors: Miller Theodore K.
Published: February 2004

The at‐a‐station hydraulic geometry of stream channels that are in quasi‐equilibrium has been associated with the minimum variance concept. In this study, minimum variance is viewed as a particular approach to implementing a more fundamental idea, that of equable change in at‐a‐station hydraulic geometry in response to change in discharge. An alternative approach to characterizing equable change is formulated in terms of variables transformed to z scores. The hypothesis of equable change is tested by imposing a restriction of equal exponents on a system of hydraulic geometry equations formulated in terms of these variables. This is equivalent to testing the hypothesis that the correlations between hydraulic geometry variables and discharge are all equal. The hydraulic geometry of 35 of 61 sites analyzed is consistent with the equable change principle, implying that these channels are in a quasi‐equilibrium state. Constraints on width adjustment or velocity adjustment appear to operate in a large number of the remaining sites. Channels that are consistent with the equable change principle are compared across a number of characteristics with channels that manifest width and velocity constraints, in order to gain insight into the nature of these constraints.
Systematic Comparison of ILWAS, MAGIC, and ETD Watershed Acidification Models: 2. Monte Carlo Analysis Under Regional Variability
Authors: Rose K. A. , Brenkert A. L. , Cook R. B. , Gardner R. H. , Hettelingh J. P.
Published: February 2004

A combination of input mapping (Rose et al., this issue) and Monte Carlo simulation was used to quantitatively compare the predictions of the Integrated Lake Watershed Acidification Study (ILWAS), Model of Acidification of Groundwater in Catchments (MAGIC), and Enhanced Trickle Down (ETD) watershed acidification models. Monte Carlo simulation was used to impose regional variability on selected model inputs of two dissimilar watersheds resembling those found in the northeastern United States for four different SO42− deposition scenarios. Model predictions were viewed in aggregate (distributions, medians) corresponding to population‐level predictions, and on an iteration‐by‐iteration basis corresponding to watershed‐specific predictions. For each of these, predicted values of acid‐neutralizing capacity (ANC) were treated as relative scale (changes in ANC in response to changes in deposition) and absolute scale (ANC concentrations, number of acidic lakes) predictions. Aggregate and iteration‐specific predictions viewed on a relative scale were similar for three models under small to moderate changes in SO42− deposition. Aggregate and iteration‐specific predictions differed among models when viewed on an absolute scale. Absolute scale predictions were most similar under conditions of high variability in deposition. Proper interpretation of multiple model forecasts requires objective quantification of the effects of model differences on predictions.
Systematic Comparison of ILWAS, MAGIC, and ETD Watershed Acidification Models: 1. Mapping Among Model Inputs and Deterministic Results
Authors: Rose K. A. , Cook R. B. , Brenkert A. L. , Gardner R. H. , Hettelingh J. P.
Published: February 2004

The effects of investigator‐dependent configuration and calibration procedures on model predictions are difficult to evaluate when sufficient data for model testing are not available. We derived a set of rules and algorithms (referred to as input mapping) to provide consistent inputs for the Integrated Lake Watershed Acidification Study (ILWAS), Model of Acidification of Groundwater in Catchments (MAGIC), and Enhanced Trickle Down (ETD) watershed acidification models without calibration. Model predictions of lake chemistry based on input mapping were similar for two dissimilar northeast U.S. watersheds, and were within the variability obtained with independent calibration of the three models and the interannual variability observed in two studies of natural watersheds. In a companion paper (Rose et al., this issue), Monte Carlo analysis is used, in conjunction with input mapping, to compare model predictions under varying inputs.
Fluid Flow Patterns Around a Well Bore or an Underground Drift With Complex Skin Effects
Authors: Bidaux Pascal , Tsang Chin‐Fu
Published: February 2004

A semianalytic solution is given for steady state flow around a well bore or a drift with a complex skin. The hydraulic conductivity of the skin may vary continuously as a function of the radial distance from the well (or drift) and may also be radially anisotropic. Such configurations can be found around damaged or acidized well bores or around a drift near which the stress redistribution induces changes in hydraulic conductivity. Purely radial flow, regional flow around an open or cemented hole without pumping or injection, and combined regional and radial flow are considered. Variations of hydraulic potential and Darcy velocity in various radial directions are studied for several cases and are found to be strongly affected by a complex skin. It is shown that the convergence of the streamlines toward the borehole, which is used in applications involving the point dilution method of measuring regional flow velocity, is significantly enhanced by a complex negative skin. Negative skin values may also reduce the size of the capture zone of a withdrawal well. Head distribution and tracer transport in combined radial and regional flow around a drift have also been modeled. It is demonstrated that tracer breakthrough curves from an experiment of tracer transport from an injection zone in the rock to the drift can be significantly affected by the fluid flow pattern around the drift, so the commonly used radial pattern may lead to erroneous results for tracer dispersivity.
Weighted Sum Transferable Discharge Permit Programs for Control of Multiple Pollutants
Authors: Lence Barbara J.
Published: February 2004

Transferable discharge permit (TDP) programs for controlling several pollutants may manage such pollutants as several individual commodities or as a single weighted sum of the various pollutants. This paper shows that the weighted sum permit program may be appropriate for managing pollutants that have an additive or a noninteractive effect on environmental quality. However, under this approach, administrators do not have direct control over the amount of each pollutant that is discharged, and environmental quality may be jeopardized unless the selected weighting factors induce a market equilibrium that satisfies environmental quality standards for the region. Estimates of the cost‐effective weighting factors that would tend to satisfy environmental quality standards under such programs are developed here. These estimates require complete water quality information and, in the case of noninteractive pollutants, treatment costs. An approach is described for estimating the cost‐effective weighting factors for noninteractive pollutants in cases where treatment cost information is unknown. This approach is demonstrated for a weighted sum TDP program for biochemical oxygen demand (BOD) and phosphorus. The results of this demonstration suggest that for weighted sum permits for these pollutants it is possible to determine a set of weighting factors that achieve adequate environmental protection, cost efficiency, and certainty of system outcome.
Visualization of residual organic liquid trapped in aquifers
Authors: Conrad Stephen H. , Wilson John L. , Mason William R. , Peplinski William J.
Published: February 2004

Organic liquids that are essentially immiscible with water migrate through the subsurface under the influence of capillary, viscous, and buoyancy forces. These liquids originate from the improper disposal of hazardous wastes, and the spills and leaks of petroleum hydrocarbons and solvents. The flow visualization experiments described in this study examined the migration of organic liquids through the saturated zone of aquifers, with a primary focus on the behavior of the residual organic liquid saturation, referring to that portion of the organic liquid that is trapped by capillary forces. Etched glass micromodels were used to visually observe dynamic multiphase displacement processes in pore networks. The resulting fluid distributions were photographed. Pore and blob casts were produced by a technique in which an organic liquid was solidified in place within a sand column at the conclusion of a displacement. The columns were sectioned and examined under optical and scanning electron microscopes. Photomicrographs of these sections show the morphology of the organic phase and its location within the sand matrix. The photographs from both experimental techniques reveal that in the saturated zone large amounts of residual organic liquid are trapped as isolated blobs of microscopic size. The size, shape, and spatial distribution of these blobs of residual organic liquid affect the dissolution of organic liquid into the water phase and the biotransformation of organic components. These processes are of concern for the prediction of pollution migration and the design of aquifer remediation schemes.
Chlorine 36 Dating of Very Old Groundwater: 3. Further Studies in the Great Artesian Basin, Australia
Authors: Torgersen T. , Habermehl M. A. , Phillips F. M. , Elmore David , Kubik Peter , Jones Geoffrey B. , Hemmick T. , Gove H. E.
Published: February 2004

The Great Artesian Basin (GAB), Australia, contains several groundwater types within the Lower Cretaceous‐Jurassic confined aquifer system with distinctly different chemistry. The groundwater flow pattern of the GAB has been analyzed with over 40 36Cl measurements from this study and that of Bentley et al. (1986a). The results indicate general agreement between the hydrologie model ages (Darcy's law) and the radioisotopic model ages suggesting that flow conditions over ∼1 m.y. have remained largely unchanged. The results confirm that the chemically distinct western GAB operates as a separate flow system with its own recharge area and its own geochemical evolution. The results indicate an area of groundwater with relatively young 36Cl ages in the central portion of the basin which cannot be explained by flow or mixing along the presently known flow lines. It is hypothesized that an additional source of young water originated from the basin margin region to the north and northwest between the Simpson Desert and Mount Isa. This additional recharge area, active more strongly over the last 500 kyr than at present, is delineated by 36Cl ages which integrate a history and evolution over the age of the groundwater.
Rotational dispersion in porous media due to fluctuating flows
Authors: Webster I. T. , Taylor J. H.
Published: February 2004

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.
On the integration of risk aversion and average‐performance optimization in reservoir control
Authors: Nardini Andrea , Piccardi Carlo , Soncini‐Sessa Rodolfo
Published: February 2004

The real‐time operation of a reservoir is a matter of trade‐off between the two criteria of risk aversion (to avoid dramatic failures) and average‐performance optimization (to yield the best long‐term average performance). A methodology taking into account both criteria is presented m this paper to derive “off‐line” infinite‐horizon control policies for a single multipurpose reservoir, where the management goals are water supply and flood control. According to this methodology, the reservoir control policy is derived in two steps: First, a (min‐max) risk aversion problem is formulated, whose solution is not unique, but rather a whole set of policies, all equivalent from the point of view of the risk‐aversion objectives. Second, a stochastic average‐performance optimization problem is solved, to select from the set previously obtained the best policy from the point of view of the average‐performance objectives. The methodology has several interesting features: the rnin‐max (or “guaranteed performance”) approach, which is particularly suited whenever “weak” users are affected by the consequences of the decision‐making process; the flexible definition of a “risk aversion degree,” by the selection of those inflow sequences which are particularly feared; and the two‐objective analysis which provides the manager with a whole set of alternatives among which he (she) will select the one that yields the desired trade‐off between the management goals.
Stable sulfur isotopes of sulfate in precipitation and stream solutions in a northern hardwood watershed
Authors: Stam A. C. , Mitchell M. J. , Krouse H. R. , Kahl J. S.
Published: February 2004

Stable S isotopes of SO42‐ in precipitation, throughfall, and stream water solutions in a northern hardwood watershed (Bear Brook Watershed, Maine) were examined to determine sources of stream SO42‐ and to identify watershed processes that may affect atmospherically deposited SO42‐ prior to reaching the streams. Similarity among temporal patterns in δ34S of precipitation, throughfall, stream SO42‐ (range: +3.4 to +9.4‰), and
of stream SO42‐ (5–10%) indicate that the δ34S values of stream SO42‐ are controlled principally by atmospheric deposition. Lack of correlation between δ34S of stream SO42‐ and stream
, elevational position, and stream discharge supports the interpretation that fractionation of stable S isotopes within the watershed appears to have, at most, a minor influence on δ34S of stream SO42‐.
An advection‐aridity evaporation model
Authors: Parlange Marc B. , Katul Gabriel G.
Published: February 2004

Actual evaporation is calculated by means of an advection‐aridity complementary model which requires, as input, the meteorological data used in classical combination models. The main advantage of the model is that it does not require site‐specific calibration. The advection‐aridity formulation presented here includes the effect of atmospheric stability, which can be important to describe the diurnal evaporation variation, and a theoretical model of the scalar roughness height. A parameterization for advection over the surface of interest is incorporated which allows the model to be used for a wide range of natural conditions. The model was found to give good agreement with evaporation measurements obtained with a large sensitive lysimeter over a bare soil field.
Friction angle measurements on a naturally formed gravel streambed: Implications for critical boundary shear stress
Authors: Buffington John M. , Dietrich William E. , Kirchner James W.
Published: February 2004

We report the first measurements of friction angles for a naturally formed gravel streambed. For a given test grain size placed on a bed surface, friction angles varied from 10° to over 100°; friction angle distributions can be expressed as a function of test grain size, median bed grain size, and bed sorting parameter. Friction angles decrease with increasing grain size relative to the median bed grain size, and are a systematic function of sorting, with lower friction angles associated with poorer sorting. The probability distributions of critical shear stress for different grain sizes on a given bed surface, as calculated from our friction angle data, show a common origin, but otherwise diverge with larger grains having narrower and lower ranges of critical shear stresses. The potential mobility of a grain, as defined by its probability distribution of critical shear stress, may be overestimated for larger grains in this analysis, because our calculations do not take into account the effects of grain burial and altered near‐bed flow fields.
A deterministic gamma‐type geomorphologic instantaneous unit hydrograph based on path types
Authors: Jin Chang‐Xing
Published: February 2004

A geomorphological instantaneous unit hydrograph is obtained by dividing the whole catchment into subcatchments, according to the different “path types” which water follows to the outlet of catchment. Each subcatchment produces, at its own outlet, an impulse response which is described by a two‐parameter (n, K) gamma function. In traveling to the outlet of whole catchment, each impulse response undergoes a further pure translation with delay time τ, thus producing at the outlet of the catchment a partial impulse response described by three parameters (n, K, and τ). Summation of these partial impulse responses with area weights W forms the impulse response of whole catchment. The parameters K and τ are estimated from catchment characteristics and the flow velocity of each flood event. Area weights W can be calculated from maps after defining the subcatchments. The parameter n is estimated by optimization. The proposed model is tested in two catchments in China and shows encouraging results.
Space‐time model for daily rainfall using atmospheric circulation patterns
Authors: Bardossy Andras , Plate Erich J.
Published: February 2004

A multidimensional stochastic model is developed for the space‐time distribution of daily precipitation. The rainfall is linked to the atmospheric circulation patterns using conditional distributions and conditional spatial covariance functions. The model is a transformed conditional multivariate autoregressive AR(I) model, with parameters depending on the atmospheric circulation pattern. The model reproduces both the local rainfall occurrence probabilities and the distribution of the rainfall amounts at given locations, and the spatial dependence described with the help of cross‐covariances of the transformed series. Parameter estimation methods based on the moments of the observed data are developed. A simulation procedure for the model is also presented. Its link to atmospheric circulation patterns makes it suitable for local precipitation simulation under stationary and nonstationary arrivals of atmospheric circulation patterns such as climate change. The model is applied using the classification scheme of the German Weather Service which is available for the time period 1881–1990. Precipitation data measured at 44 different stations for the time period 1977–1990 in the catchment of the river Ruhr (Germany) are used to demonstrate the model.
Benefit transfer of outdoor recreation demand studies, 1968–1988
Authors: Walsh Richard G. , Johnson Donn M. , McKean John R.
Published: February 2004

The accumulation of studies on outdoor recreation demand creates an opportunity to apply the growing science of reviewing research for purposes of benefit transfer. The process involves developing an understanding of the variables that explain the observed difference in estimates. This paper illustrates how the results of previous studies could be adjusted to develop some tentative estimates of nonmarket values for future policy analysis. Also, the evaluation of some potentially important variables should help improve statistical analysis and the allocation of resources to new studies. The challenge is to build each subsequent work on the knowledge gained from previous ones. In this experimental phase, there is a need to examine additional variables that might conceivably be more important than those considered in the past.
The effect of heterogeneity on the drainage capillary pressure‐saturation relation
Authors: Ferrand Lin A. , Celia Michael A.
Published: February 2004

Proper mathematical description of multiphase fluid displacement in porous media requires specification of appropriate constitutive relations, including the relationship between capillary pressure and saturation. This pressure‐saturation relationship may be measured at the laboratory core scale, but extension to larger scales is difficult in heterogeneous materials. To date, stochastic and volume averaging methods have been used to estimate the “effective” pressure‐saturation relation in heterogeneous media. A third method, based on network percolation models, may be developed and used to define effective relationships. This last method is appealing in that pore scale physics are directly incorporated into the model, assumptions are clear, and numerical tests are most similar to those performed in the laboratory. Numerical results based on network models indicate that heterogeneities have a significant impact on the effective relationship. In addition, these results indicate that application of any sort of linear averaging of the individual small‐scale relationships to define an effective relationship may provide incorrect results.
Optimum operation of a multiple reservoir system including salinity effects
Authors: Dandy Graeme , Crawley Philip
Published: February 2004

Many studies have been carried out regarding the development of optimum operating policies for multiple reservoir systems but few of these have considered water quality as an objective. This study considers operations of the headworks system for Adelaide, Australia. The system is rather unusual because of its high salt concentrations which impose large economic costs on the consumers. In addition, a significant fraction of the supply is pumped from a distant river. An existing linear programming model for operation of the Adelaide system is modified to identify policies which minimize total system costs (including salt damage costs). These changed procedures can reduce the average salinity of the supplied water significantly but at some increase in annual pumping costs. The benefit‐cost ratio of the changes is estimated to be 3.0 for the entire system.
Tritium and helium: 3 as groundwater age tracers in the Borden Aquifer
Authors: Solomon D. K. , Poreda R. J. , Schiff S. L. , Cherry J. A.
Published: February 2004

Vertical profiles of 3H and He isotope ratios have been measured in groundwater from the well‐characterized Borden aquifer, Ontario. The sum of 3H and tritiogenic 3He (3He*) is used as an equivalent nondecaying tracer, while the ratio of 3He* to 3H is used to compute groundwater ages. The mid‐1960s 3H peak is not always apparent using the 3H data because both dispersion and radioactive decay have significantly reduced the magnitude of the 3H bomb peak. However, the sum of 3H and 3He* clearly defines the bomb peak at several locations. The accuracy of the 3H/3He dating method depends on the ability of the saturated zone to retain 3He* against diffusive loss at the water table and on the amount of dispersive mixing that occurs within the saturated zone of shallow unconfined aquifers. Helium 3 confinement is strong while dispersive mixing is weak in the Borden aquifer, resulting in an excellent delineation of groundwater travel times. Computed 3H/3He age profiles are compared with travel times predicted using a previously calibrated flow model. Although the 3H/3He age profiles are vertically offset from the modeled travel times, the travel time and 3H/3He age gradients compare exceptionally well. Recharge rates have been computed using the 3H/3He age gradients and vary from 62 cm yr−1 beneath the Borden landfill to 14 cm yr−1 north of the landfill. The 3H/3He‐computed recharge agrees well with the recharge function used in previous flow modeling.
Gravity‐driven groundwater flow and slope failure potential: 2. Effects of slope morphology, material properties, and hydraulic heterogeneity
Authors: Reid Mark E. , Iverson Richard M.
Published: February 2004

Hillslope morphology, material properties, and hydraulic heterogeneities influence the role of groundwater flow in provoking slope instability. We evaluate these influences quantitatively by employing the elastic effective stress model and Coulomb failure potential concept described in our companion paper (Iverson and Reid, this issue). Sensitivity analyses show that of four dimensionless quantities that control model results (i.e., Poisson's ratio, porosity, topographic profile, and hydraulic conductivity contrast), slope profiles and hydraulic conductivity contrasts have the most pronounced and diverse effects on groundwater seepage forces, effective stresses, and slope failure potentials. Gravity‐driven groundwater flow strongly influences the shape of equilibrium hillslopes, which we define as those with uniform near‐surface failure potentials. For homogeneous slopes with no groundwater flow, equilibrium hillslope profiles are straight; but with gravity‐driven flow, equilibrium profiles are concave or convex‐concave, and the largest failure potentials exist near the bases of convex slopes. In heterogeneous slopes, relatively slight hydraulic conductivity contrasts of less than 1 order of magnitude markedly affect the seepage force field and slope failure potential. Maximum effects occur if conductivity contrasts are of four orders of magnitude or more, and large hydraulic gradients commonly result in particularly large failure potentials just upslope from where low‐conductivity layers intersect the ground surface.
Prediction uncertainty in solute flux through heterogeneous soil
Authors: Destouni Georgia
Published: February 2004

The Lagrangian framework presented by Dagan et al. (1992a) is used to analyze the uncertainty in predictions of the field scale mass flux of solute through the unsaturated zone. Transport of both nonreactive and degradable solutes is investigated for input sources that are located at the soil surface of fields with spatially variable hydraulic conductivity at saturation. The variances of the solute flux and accumulated mass, which quantify the corresponding prediction uncertainties, are illustrated at an arbitrary depth below the soil surface for different sizes and shapes of the input domain, and for different flow and degradation conditions. The greatest solute flux variances arise when the expected breakthrough curve has a steep slope. The coefficient of variation for the solute flux is minimum at the peak arrival time of the expected breakthrough curve; this minimum value is relatively insensitive to the assumed distribution for solute travel time and to the loss rate coefficient for degradante solute. The prediction uncertainty decreases with increasing size of the input domain and is smaller for a planar source than for a linear one. The relative uncertainty in the total leached mass of degradable solute increases with increasing loss rate coefficient.
Energy dissipation, runoff production, and the three‐dimensional structure of river basins
Authors: Rodríguez‐Iturbe Ignacio , Rinaldo Andrea , Rigon Riccardo , Bras Rafael L. , Marani Alessandro , Ijjász‐Vásquez Ede
Published: February 2004

Three principles of optimal energy expenditure are used to derive the most important structural characteristics observed in drainage networks: (1) the principle of minimum energy expenditure in any link of the network, (2) the principle of equal energy expenditure per unit area of channel anywhere in the network, and (3) the principle of minimum total energy expenditure in the network as a whole. Their joint application results in a unified picture of the most important empirical facts which have been observed in the dynamics of the network and its three‐dimensional structure. They also link the process of runoff production in the basin with the characteristics of the network.
Determination of the effective hydraulic conductivity for heterogeneous porous media using a numerical spectral approach: 1. Method
Authors: Dykaar Bruce B. , Kitanidis Peter K.
Published: February 2004

A numerical spectral method is described for solving the equations governing the effective hydraulic conductivity of a stationary heterogeneous porous medium. The numerical method is efficient in the sense that the solution can be achieved in order Nt ln (Nt) operations and order Nt storage, where Nt is the total number of sampling points used in an n‐dimensional flow domain. The numerical method is extensively tested and shown to be quite robust. In the test cases, very accurate results are achieved even for an extremely heterogeneous porous medium. The developed numerical algorithm is quite general and can be used to compute effective conductivities for cases where analytic results are unavailable. In the companion paper (Dykaar and Kitanidis, this issue) the method is used to find the effective conductivity of (1) multidimensional isotropic, stationary, lognormally distributed conductivity and (2) core samples of a shale and sandstone formation.
A revised model for water flow, sediment transport, bed topography and grain size sorting in natural river bends
Authors: Bridge John S.
Published: February 2004

A revised model for the interaction of water flow, bed topography, and the rate and mean grain size of bed load in river bends is presented and compared with a large range of observational data. The model represents a modification of the approaches used by F. Engelund, J. S. Bridge, G. Parker, S. Ikeda, and coworkers. Model predictions generally agree with data from the River South Esk, Muddy Creek, River Dommel, and Hooke's laboratory channel. This simple model apparently performs at least as well as more complicated flow models. To make further improvements it is necessary to more accurately specify the interaction between local bed shear stress, sediment transport, and the local bed configuration.
A linear conceptual subsurface storm flow model
Authors: Koussis Antonis D.
Published: February 2004

A zero‐dimensional (lumped), linear conceptual model of the subsurface storm flow from hillslopes has been developed by integrating, over the length of the hillslope, the linearized one‐dimensional (Dupuit‐Forchheimer approximation) discharge relationship for saturated flow over an inclined plane. The derived two‐term storage function accounts for gravity‐ and gradient‐driven flow. Coupled with the storage balance equation for the plane, the model allows direct, analytical computation of the outflow hydrograph at the foot of the hill in response to an infiltration step input. The model is the conceptual counterpart of the linear advection‐diffusion (LAD) flow equation derived from the extended Boussinesq equation through linearization. In contrast to the kinematic wave approximation, the outflows normalized by their equilibrium values are properly given as functions of the infiltration rate. The outflow hydrographs computed by the conceptual LAD model approximate nonlinear behavior closely over a wide range of conditions encountered in field applications.
Contribution of induced and remanent magnetization to long‐wavelength oceanic magnetic anomalies
Authors: Cohen Yves , Achache Jose
Published: February 2004

The Magsat mission has provided very precise measurements of Earth's magnetic field which have been used to derive accurate maps of the long‐wavelength magnetic anomaly field. Analyses of this field over localized areas have shown that induced magnetization is mainly responsible for the anomalies and that remanent magnetization is rarely detectable at Magsat altitude. A careful analysis of the data, taking into account the various effects due to non lithospheric sources, allows this field to be better described and, consequently, suggests two observations: (1) long‐wavelength oceanic anomalies display many similarities with the distribution of seafloor topography, suggesting a relationship between crustal thickening and magnetization enhancement; (2) age‐related pattern (i.e., ridge parallel) of anomalies is observed in two of the larger oceanic basins (Indian and North Atlantic oceans). Several forward models are constructed to investigate these correlations. In the first model, the effect of variations in crustal thickness was modeled assuming uniform susceptibility and that thickness varies proportionally to the seafloor topography. In the second model, the field due to remanent magnetization acquired during the Cretaceous Long Normal period was modeled and shown to contribute significantly to the Magsat field. In order to make the modeled and observed maps fully comparable, several effects must be taken into account: a global magnetization contrast exists between oceans and continents and induces widely distributed anomalies because of the truncated spherical harmonic analysis; the spectral content of the observed map in the north‐south direction is reduced because of the along‐track filtering due to the polar orbit of Magsat, When these effects are taken into account, the model combining the contribution of induced and remanent magnetization display remarkable similarity to observed data. This comparison constrains both induced and remanent magnetization of oceanic lithospheric rocks.
The effect of vertical soil heterogeneity on field scale solute flux
Authors: Destouni Georgia
Published: February 2004

Vertical heterogeneity is incorporated in a stochastic transport model, in which the soil is viewed as consisting of an ensemble of essentially vertical, independent stream tubes. The saturated hydraulic conductivity in each stream tube is defined as the vertical average of the corresponding point random process. The statistics of the vertically averaged hydraulic conductivity and the resulting statistics of solute travel time are depth dependent. The evolution of the field scale breakthrough curve with depth is analyzed in terms of the first two moments of solute travel time to an arbitrary depth below the soil surface. The arrival time of the solute center of mass, quantified by the expected travel time, grows linearly with depth also for vertically heterogeneous soil. The temporal solute spreading around its center, quantified by the travel time variance, may exhibit a compression‐expansion that is qualitatively analogous to the solute plume behavior in the studies of Butters and Jury (1989), Russo (1991), and Ellsworth and Jury (1991). The relative solute spreading, quantified by the travel time coefficient of variation, decreases with increasing depth.
Impact of fracture coatings on fracture/matrix flow interactions in unsaturated, porous media
Authors: Thoma Steven G. , Gallegos David P. , Smith Douglas M.
Published: February 2004

Groundwater flow in unsaturated, fractured rock is often assumed to be dominated by the porous matrix component. This is frequently the result of reasoning that water flowing in the fractures is rapidly imbibed into the rock matrix by capillary suction forces with negligible resistance to uptake at the fracture/matrix interface. However, the existence of a low‐permeability mineralized layer or coating at this interface may substantially reduce matrix imbibition and could consequently result in fracture‐dominated flow. To test this idea, four tuff samples containing natural fractures were obtained from tuff formations in southern Nevada. By performing imbibition experiments into the matrix rock, across a mineralized fracture face and then across a fresh uncoated fracture face, water uptake as a function of time and coating was measured. A model combining Darcy's law and the Washburn equation has been used to describe the imbibition behavior. Before testing the tuff samples, the experimental technique and imbibition model were first tested using ordered sphere packings with known pore size and structure. From the tuff imbibition data, the ratio of the permeability through the mineralized face to the permeability through the uncoated face was found to be 0.3 (relatively little effect of the mineralized layer) for two samples, 0.01 for one sample, and 10−7 (uptake strongly restricted by the mineralized layer) for one sample. Using a simple fracture flow model and the imbibition model mentioned above to simulate matrix uptake from the fracture, numerical simulations indicate that the existence of fracture coatings could significantly decrease the travel time and increase the travel depth of water flowing through fractures.
Application of a coupled adjoint sensitivity and kriging approach to calibrate a groundwater flow model
Authors: LaVenue A. Marsh , Pickens John F.
Published: February 2004

A new technique is employed to calibrate a regional‐scale groundwater flow model to an extensive data base of undisturbed (i.e., assumed to be steady state) and transient heads. The methodology presented in this study is similar in concept to one presented by de Marsily et al. (1984) in that an adjoint sensitivity technique is coupled with a kriging algorithm to calibrate a flow model. The notable difference of the methodology presented in this paper is that it directly identifies the regions where modification of the model's kriged transmissivity or boundary pressure values will directly improve the overall fit between measured and model‐calculated heads at selected wells. At the locations identified as most sensitive to transmissivity changes, synthetic transmissivity values, referred to as pilot points, are added to the transmissivity data base and used as input for kriging the transmissivity field. An application of the methodology to data originating from approximately 10 years of regional hydrogeologic site characterization efforts that have been conducted in the Culebra dolomite at the Waste Isolation Pilot Plant in southeastern New Mexico is presented.
Contaminant transport in layered porous media: 1. General solution
Authors: Tang Yi , Aral Mustafa M.
Published: February 2004

Analysis of transport of contaminants in a layered aquifer system still poses significant challenges, although considerable progress has been made in the literature toward the understanding of this problem. In this study, an analytical approach is chosen as the mode of analysis, and a closed form solution is developed for a quasi‐three‐dimensional advective‐diffusion equation defined for a layered aquifer. The layered aquifer is idealized as an aquifer‐aquitard system in which the solution domain is considered to be finite and two dimensional in the aquifer region and infinite and one dimensional in the adjacent aquitards. The analytical solution outlined emphasizes the advection of contaminant in the longitudinal direction in the aquifer and also in the transverse vertical direction in the aquiferaquitard system. The analytical solution also includes the two‐dimensional diffusion of the contaminant in the aquifer, one‐dimensional diffusion in the aquitards, and first‐order reaction and retardation terms in both aquifer and aquitards.