bluuebunny/crossref_metadata_2025 huggingface dataset:
Landslide triggering by rain infiltration
Authors: Iverson Richard M.
Published: February 2004

Landsliding in response to rainfall involves physical processes that operate on disparate timescales. Relationships between these timescales guide development of a mathematical model that uses reduced forms of Richards equation to evaluate effects of rainfall infiltration on landslide occurrence, timing, depth, and acceleration in diverse situations. The longest pertinent timescale is A/D0, where D0 is the maximum hydraulic diffusivity of the soil and A is the catchment area that potentially affects groundwater pressures at a prospective landslide slip surface location with areal coordinates x, y and depth H. Times greater than A/D0 are necessary for establishment of steady background water pressures that develop at (x, y, H) in response to rainfall averaged over periods that commonly range from days to many decades. These steady groundwater pressures influence the propensity for landsliding at (x, y, H), but they do not trigger slope failure. Failure results from rainfall over a typically shorter timescale H2/D0 associated with transient pore pressure transmission during and following storms. Commonly, this timescale ranges from minutes to months. The shortest timescale affecting landslide responses to rainfall is
, where g is the magnitude of gravitational acceleration. Postfailure landslide motion occurs on this timescale, which indicates that the thinnest landslides accelerate most quickly if all other factors are constant. Effects of hydrologic processes on landslide processes across these diverse timescales are encapsulated by a response function,
, which depends only on normalized time, t*. Use of R(t*) in conjunction with topographic data, rainfall intensity and duration information, an infinite‐slope failure criterion, and Newton's second law predicts the timing, depth, and acceleration of rainfall‐triggered landslides. Data from contrasting landslides that exhibit rapid, shallow motion and slow, deep‐seated motion corroborate these predictions.
Saturated area formation on nonconvergent hillslope topography with shallow soils: A numerical investigation
Authors: Ogden Fred L. , Watts Brent A.
Published: February 2004

Prediction of saturated area formation is important for hydrologic modeling of watersheds with shallow, highly pervious soils. This simulation study examines the relative importance of hillslope properties and rainfall rate on the evolution of saturated source areas during wetting. The study focuses on homogeneous, nonconvergent hillslope topography with constant slope. The two‐dimensional, variably saturated groundwater model VS2D [Healy, 1990; Lappala et al. 1993] is used to simulate saturated area formation under the action of steady uniform rainfall. The study methodology systematically varies four hillslope properties, depth to impervious layer, slope length, slope angle, and average saturated hydraulic conductivity, and the rainfall rate. Results indicate that the fraction of the hillslope length that is saturated at equilibrium is a function of a parameter Φ, which is defined as the rainfall rate multiplied by the slope length, divided by the slope angle, soil thickness, and saturated hydraulic conductivity. The temporal evolution of saturated area is analyzed in terms of the equilibrium time. Nonlinearity in the unsaturated zone and differences in hillslope properties result in a nonunique time to equilibrium relation. The time to equilibrium is maximum when factors that tend to cause surface saturation are in approximate balance with those that tend to dissipate surface saturation. The temporal evolution of surface saturation during wetting follows a wide variety of trajectories. Equivalent hillslope soil water storage ratios on slopes with different properties can result in a wide range of surface saturation conditions. Hillslopes with shallower soils and smaller slope angles are most susceptible to saturated area formation. However, sudden changes in surface saturation are possible on steep hillslopes when 1 < Φ < 6.
Simulation of snowmelt in a subarctic spruce woodland: 1. Tree model
Authors: Woo Ming‐ko , Giesbrecht Mark A.
Published: February 2004

A model is presented for the simulation of snowmelt under a subarctic spruce tree, making use of the physical processes associated with melt, the geometry of the canopy, and empirical functions and coefficients obtained from the field. This model allows detailed study of the effect of a tree on the micropattern of snow ablation. Simulated results compare well with the daily snow depths measured along transects that extend in two directions outward from the tree trunk. Radiation represents the principal component of the total melt energy, and the tree canopy enhances the longwave radiation balance for the snow surface. Strong asymmetry in melt rates among different azimuths within and beyond the tree canopy is simulated and can be explained by the changing intensities of the snowmelt processes due to the presence of a tree.
Alternative principal components regression procedures for dendrohydrologic reconstructions
Authors: Hidalgo Hugo G. , Piechota Thomas C. , Dracup John A.
Published: February 2004

Streamflow reconstruction using tree ring information (dendrohydrology) has traditionally used principal components analysis (PCA) and stepwise regression to form a transfer function. However, PCA has several procedural choices that may result in very different reconstructions. This study assesses the different procedures in PCA‐based regression and suggests alternative procedures for selection of variables and principal components. Cross‐validation statistics are presented as an alternative for independently testing and identifying the optimal model. The objective is to use these statistics as a measure of the model's performance to find a conceptually acceptable model with a low prediction error and the fewest number of variables. The results show that a parsimonious model with a low mean square error can be obtained by using strict rules for principal component selection and cross‐validation statistics. Additionally, the procedure suggested in this study results in a model that is physically consistent with the relationship between the predictand and the predictor. The alternative PCA‐based regression models presented here are applied to the reconstruction of the Upper Colorado River Basin streamflow and compared with results of a previous reconstruction using traditional procedures. The streamflow reconstruction proposed in this study shows more intense drought periods, which may influence the future allocation of water supply in the Colorado River Basin.
Assimilation of active microwave observation data for soil moisture profile estimation
Authors: Hoeben Rudi , Troch Peter A.
Published: February 2004

This paper discusses the potential of retrieving information about the soil moisture profile from measurements of the surface soil moisture content through active microwave observations of the Earth. Recently, Mancini et al. [1999] have shown through laboratory experiments that the volumetric moisture content of the first few centimeters of a bare soil can be determined within 5% vol accuracy by means of C and L band active microwave observations and inverse modeling. Here we use active microwave observations of the surface soil moisture content in a data assimilation framework to show that this allows the retrieval of the root zone soil moisture profile. The data assimilation procedure developed is based on the Kalman filter technique. Kalman filtering allows reconstruction of the state vector of a system when this system is represented by a dynamic model and when at least part of the state variables are observed regularly. The dynamic model of the system used here is based on the one‐dimensional Richards equation. The observation equation is based on the Integral Equation Model [Fung et al., 1992; Fung, 1994] and is used to link the radar observations to surface soil moisture content. It is shown that even in the presence of model and observation noise and infrequent observations, accurate retrieval of the entire moisture profile is possible for a bare soil.
Preferential states of seasonal soil moisture: The impact of climate fluctuations
Authors: D'Odorico P. , Ridolfi L. , Porporato A. , Rodriguez‐Iturbe I.
Published: February 2004

The impact of climate fluctuations on the dynamics of soil moisture is studied through a stochastic model of soil water balance. The analysis focuses on the changes of soil water content induced by the interannual variability of rainfall observed at the decade‐to‐century timescale. Extensive data analyses have been performed to characterize the statistical properties of such a variability. Particular attention is paid to the year‐to‐year variability of the average value of soil moisture during the growing season because of its relevance to the mechanisms affecting the physiology of plants and the dynamics of ecosystems. It is found that the probability distribution of the average seasonal soil moisture may be either unimodal or bimodal depending on the different combinations of climate, soil, and vegetation parameters. The possible occurrence of a double mode has both hydrologic and ecologic implications that are analyzed here.
Duration and frequency of water stress in vegetation: An analytical model
Authors: Ridolfi L. , D'Odorico P. , Porporato A. , Rodriguez‐Iturbe I.
Published: February 2004

In the study of the evolutionary dynamics of soil moisture at a site, it is particularly important to define some characteristic properties of the temporal structure of the periods in which the soil water content is below certain levels indicative of water stress conditions in vegetation. The analysis of such properties provides an approach to establish some hydrologic basis for the understanding and modeling of ecosystems functioning in water‐limited environments. This paper deals with a stochastic point process model of soil water balance. Expressions for both the mean number and the mean duration of time intervals during which the soil moisture is below a given threshold are analytically derived as a function of climate, soil, and vegetation. The seasonal mean value of water deficit is also analytically obtained. These properties are used to characterize the state of water stress in plants and to study its dependence on the interrelated dynamics. Estimates are included for the probability distributions of the frequency and duration of the stress and soil water deficit, for different hypotheses on climate, soil, and vegetation. Both the hydrologic and the ecologic implications of the results are briefly outlined.
Risk assessment of consuming agricultural products irrigated with reclaimed wastewater: An exposure model
Authors: van Ginneken Meike , Oron Gideon
Published: February 2004

This study assesses health risks to consumers due to the use of agricultural products irrigated with reclaimed wastewater. The analysis is based on a definition of an exposure model which takes into account several parameters: (1) the quality of the applied wastewater, (2) the irrigation method, (3) the elapsed times between irrigation, harvest, and product consumption, and (4) the consumers' habits. The exposure model is used for numerical simulation of human consumers' risks using the Monte Carlo simulation method. The results of the numerical simulation show large deviations, probably caused by uncertainty (impreciseness in quality of input data) and variability due to diversity among populations. There is a 10–orders of magnitude difference in the risk of infection between the different exposure scenarios with the same water quality. This variation indicates the need for setting risk‐based criteria for wastewater reclamation rather than single water quality guidelines. Extra data are required to decrease uncertainty in the risk assessment. Future research needs to include definition of acceptable risk criteria, more accurate dose‐response modeling, information regarding pathogen survival in treated wastewater, additional data related to the passage of pathogens into and in the plants during irrigation, and information regarding the behavior patterns of the community of human consumers.
Hydrologic uncertainty processor for probabilistic river stage forecasting
Authors: Krzysztofowicz Roman , Kelly Karen S.
Published: February 2004

The hydrologic uncertainty processor (HUP) is a component of the Bayesian forecasting system that produces a short‐term probabilistic river stage forecast based on a probabilistic quantitative precipitation forecast (PQPF). The task of the HUP is to quantify the hydrologic uncertainty under the hypothesis that there is no precipitation uncertainty. The hydrologic uncertainty is the aggregate of all uncertainties arising from sources other than those quantified by the PQPF; these sources include the hydrologic model (model and parameter uncertainties), inputs estimated deterministically (measurement, estimation, and prediction uncertainties), and inputs not forecasted (e.g., precipitation beyond the period covered by the PQPF). Bayesian theory for the HUP is presented, and a meta‐Gaussian model is developed. This parametric model allows for (1) any form of marginal distributions of river stages, (2) a nonlinear and heteroscedastic dependence structure between the model river stage and the actual river stage, and (3) an analytic solution of the Bayesian revision process. Estimation and validation of the model are described using data from the operational forecast system of the National Weather Service for a 1430‐km2 headwater basin.
A compartmentalized solute transport model for redox zones in contaminated aquifers: 2. Field‐scale simulations
Authors: Abrams Robert H. , Loague Keith
Published: February 2004

This paper, the second of two parts [see Abrams and Loague, this issue], reports the field‐scale application of COMPTRAN (compartmentalized solute transport model) for simulating the development of redox zones. COMPTRAN is fully developed and described in the companion paper. Redox zones, which are often delineated by the relative concentrations of dissolved oxygen, have been observed around the globe. The distribution of other redox‐sensitive species is affected by redox zonation. At the U.S. Geological Survey's Cape Cod research site, an anoxic zone containing high concentrations of dissolved iron has been observed. Field data were abstracted from the Cape Cod site for the one‐dimensional and two‐dimensional COMPTRAN simulations reported in this paper. The purpose of the concept‐development simulations was to demonstrate that the compartmentalized approach reported by Abrams et al. [1998] can be linked with a solute transport model to simulate field‐scale phenomena. The results presented in this paper show that COMPTRAN successfully simulated the development of redox zones at the field scale, including trends in pH and alkalinity. Thermodynamic constraints were used to prevent lower‐energy redox reactions from occurring under infeasible geochemical conditions without imposing equilibrium among all redox species. Empirical methods of reaction inhibition were not needed for the simulations conducted for this study. COMPTRAN can be extended easily to include additional compartments and reactions and is capable of handling complex velocity fields in more than one dimension.
Hydraulic tomography: Development of a new aquifer test method
Authors: Yeh T.‐C. Jim , Liu Shuyun
Published: February 2004

Hydraulic tomography (i.e., a sequential aquifer test) has recently been proposed as a method for characterizing aquifer heterogeneity. During a hydraulic tomography experiment, water is sequentially pumped from or injected into an aquifer at different vertical portions or intervals of the aquifer. During each pumping or injection, hydraulic head responses of the aquifer at other intervals are monitored, yielding a set of head/discharge (or recharge) data. By sequentially pumping (or injecting) water at one interval and monitoring the steady state head responses at others, many head/discharge (recharge) data sets are obtained. In this study a sequential inverse approach is developed to interpret results of hydraulic tomography. The approach uses an iterative geostatistical inverse method to yield the effective hydraulic conductivity of an aquifer, conditioned on each set of head/discharge data. To efficiently include all the head/discharge data sets, a sequential conditioning method is employed. It uses the estimated hydraulic conductivity field and covariances, conditioned on the previous head/discharge data set, as prior information for next estimations using a new set of pumping data. This inverse approach was first applied to hypothetical, two‐dimensional, heterogeneous aquifers to investigate the optimal sampling scheme for the hydraulic tomography, i.e., the design of well spacing, pumping, and monitoring locations. The effects of measurement errors and uncertainties in statistical parameters required by the inverse model were also investigated. Finally, the robustness of this inverse approach was demonstrated through its application to a hypothetical, three‐dimensional, heterogeneous aquifer.
Nonlocal nonreactive transport in heterogeneous porous media with interregional mass diffusion
Authors: Huang Hai , Hu Bill X.
Published: February 2004

We present a Eulerian stochastic analysis for nonreactive transport in a heterogeneous, structured medium. A first‐order mass diffusion model (or mobile and immobile model) is applied to describe interregional mass diffusion between advection and nonadvection (mobile and immobile) regions. Spatial variabilities in the media motivate us to treat the interregional mass diffusion coefficient a and hydraulic conductivity K as spatial random variables. The two random variables are assumed to correlate with each other. The analytical solution for mean concentration is given explicitly in Fourier and Laplace transforms and is numerically inverted to real space via fast Fourier transform. Various factors that affect mass diffusion and transport processes are investigated by plotting spatial moments up to third and mean concentration contours. It is shown from the calculation results that interregional mass diffusion will significantly increase plume dispersion in both longitudinal and transverse directions and make the plume negatively skewed and give the breakthrough curve a long tail. In comparison with the case with a deterministic a, randomness of the parameter will increase the plume dispersion and make the plume more skewed, especially for the case where a is negatively correlated with K.
Importance of poroelastic coupling in dynamically active aquifers of the Po River Basin, Italy
Authors: Gambolati Giuseppe , Teatini Pietro , Baú Domenico , Ferronato Massimiliano
Published: February 2004

Uncoupling between the flow field and the stress field in pumped aquifers is the basis of the classical groundwater hydrology. Recently, some authors have disputed the assumption of uncoupling with regard to both fluid dynamics and porous medium deformation. The issue is very important as it could undermine the traditional approach to simulate subsurface flow, analyze pumping tests, and predict land subsidence caused by fluid withdrawal. The present paper addresses the problem of coupling versus uncoupling in the Po river plain, a normally consolidated and normally pressurized basin which has experienced in the last 50 years a pronounced pore pressure drawdown because of water and gas removal and where a large hydromechanical database is available from the ground surface down to 4000 m depth. A numerical study is performed which shows that the matrix which relates flow to stress is very similar to the capacity matrix of the uncoupled flow equation. A comparison of results obtained with the finite element integration of the coupled and uncoupled models indicates that pore pressure is rather insensitive to coupling anywhere within the pumped formation while in the adjacent aquitard‐aquifer units, coupling induces a slight overpressure which quickly dissipates in time with a small initial influence on medium deformation, and specifically on land subsidence. As a major consequence the uncoupled solutions to the fluid dynamic and the structural problems appear to be fully warranted on any timescale of practical interest in a typical normally consolidated and pressurized basin.
Degassing of deep groundwater in fractured rock around boreholes and drifts
Authors: Jarsjö Jerker , Destouni Georgia
Published: February 2004

Deep groundwater contains dissolved gases that may come out of solution if the water pressure is lowered. Water pressures are often decreased down to atmospheric pressure when water is withdrawn from deep boreholes and drifts in the bedrock, for example, in investigations regarding nuclear waste disposal. Groundwater degassing may then contribute to the development of a local, unsaturated zone around the borehole or drift, which may affect inflow and, as a consequence, the outcome of hydraulic and tracer tests in fractured rock. Laboratory experiments with gas‐saturated water in rock fracture replicas have demonstrated that degassing causes considerable hydraulic conductivity reduction under certain conditions. Degassing has also been hypothesized to be the cause of observed flow reductions in the field; however, supporting field experiments have so far been lacking. We report results that constitute the first field support of the development of an unsaturated zone and, as a consequence, hydraulic conductivity reduction due to groundwater degassing around a borehole. The borehole tests were conducted approximately 450 m below the groundwater table at the Äspö Hard Rock Laboratory in southeastern Sweden. No hydraulic conductivity reductions were observed at gas contents of about 1%, whereas a 50% reduction in hydraulic conductivity was observed at a gas content of 13%. Formal hypothesis testing, based on all available field and laboratory degassing tests, supports degassing as the actual cause of the observed hydraulic conductivity reduction at the higher gas content. We also show analytically that hydraulic conductivity reductions due to degassing may occur at much lower gas contents around drifts than around boreholes.
Estimating fluvial sediment transport: The rating parameters
Authors: Syvitski James P. , Morehead Mark D. , Bahr David B. , Mulder Thierry
Published: February 2004

Correlations between suspended sediment load rating parameters, river basin morphology, and climate provide information about the physical controls on the sediment load in rivers and are used to create predictive equations for the sediment rating parameters. Long‐term time‐averaged values of discharge, suspended load, flow duration, flow peakedness, and temporally averaged values of precipitation, temperature, and range in temperature were coupled with the drainage area and basin relief to establish statistical relationships with the sediment rating parameters for 59 gauging stations. Rating parameters (a and b) are defined by a power law relating daily discharge values of a river (Q) and its sediment concentration Cs, where Cs = aQb. The rating coefficient a (the mathematical concentration at Q = 1 m3/s) is inversely proportional to the long‐term mean discharge and is secondarily related to the average air temperature and the basin's topographic relief. The rating exponent b (the log‐log slope of the power law) correlates most strongly with the average air temperature and basin relief and has lesser correlations with the long‐term load of the river (which is related to basin relief and drainage area). The rating equation describes the long‐term character of the suspended sediment load in a river. Each river undergoes higher‐frequency variability (decadal, interannual, and storm event) around this characteristic response, controlled by weather patterns and channel recovery from extreme precipitation events.
Behavior of indicator variograms and transition probabilities in relation to the variance in lengths of hydrofacies
Authors: Ritzi Robert W.
Published: February 2004

When using indicator geostatistics to represent the distribution of hydrofacies or lithofacies, the range and curvature of the autotransition probabilities or the autovariogram are related to the variation in facies lengths. As the coefficients of variation for the length of each facies increase toward unity, the effective range increases while the indicator‐correlation structure evolves from a periodic linear structure to a somewhat periodic spherical structure to an aperiodic exponential structure. Multimodal distributions of facies lengths can give rise to autotransition probabilities or autovariograms that appear to have nested structures. With the understanding of these relationships it is possible to choose a model form and model parameters for the autotransition probability or autovariogram based upon the facies proportions and the modality, mean, and variance in length of the facies. This is illustrated with data from a case study.
Regional depressurization and its impact on the sustainability of freshwater resources in an extensive midcontinent variable‐density aquifer
Authors: Lahm Terry D. , Bair E. Scott
Published: February 2004

Regional depressurization due to groundwater withdrawal from a large confined midcontinent aquifer may cause significant changes in the regional flow field and solute distributions through time. This study addresses the effects of depressurization in the Silurian‐Devonian carbonate aquifer in the western flank of the Appalachian Basin in Ohio, where salinity‐derived variable‐density flow effects are significant. This complex hydrogeologic environment is examined using an interpretative, transient, numerical flow and transport model to examine the hydrodynamics involved in the salinization process, to simulate changes in the velocity field and solute distributions over the past 100‐year period, and to forecast the effects of three different depressurizing scenarios over the next 100‐year period. The results indicate that large‐scale depressurization can lead to significant changes in the regional flow patterns, resulting in changes in solute distributions and salinization of a portion of the aquifer. Depressurization creates vertical flow gradients within the carbonate aquifer that transport brine from underlying and overlying shale units into the updip freshwater portion of the aquifer. Model results show the development of flow reversals in the downdip portions of the aquifer that facilitate transport of brine from the deeper portions of the carbonate aquifer. These processes result in significant water quality degradation in the updip portion of the aquifer, which is extensively used for municipal, industrial, and domestic water supplies. Our results indicate the need to monitor water levels, pumping rates, and water quality so that future management decisions regarding the sustainability of the resource are based on complete and accurate field data.
Modeling of subgrid effects in coarse‐scale simulations of transport in heterogeneous porous media
Authors: Efendiev Y. , Durlofsky L. J. , Lee S. H.
Published: February 2004

A methodology for incorporating subgrid effects in coarse‐scale numerical models of flow in heterogeneous porous media is presented. The method proceeds by upscaling the deterministic fine‐grid permeability description and then solving the pressure equation over the coarse grid to obtain coarse‐scale velocities. The coarse‐grid saturation equation is formed through a volume average of the fine‐scale equations and includes terms involving both the average and fluctuating components of the velocity field. The terms involving the fluctuating components are subgrid effects that appear as length‐ and time‐dependent dispersivities. A simplified model for the coarse‐scale dispersivity, in terms of these subgrid velocity fluctuations, is proposed, and a numerical scheme based on this model is implemented. Results using the new method are presented for a variety of two‐dimensional heterogeneous systems characterized by moderate permeability correlation length in the dominant (horizontal) flow direction and small correlation length in the vertical direction. The new method is shown to provide much more accurate results than comparable coarse‐grid models that do not contain the subgrid treatment. Extensions of the overall methodology to handle more general systems are discussed.
Stream temperature variation in regulated rivers: Evidence for a spatial pattern in daily minimum and maximum magnitudes
Authors: Lowney Cynthia L.
Published: February 2004

Elevated water temperatures have become an important concern in watersheds where aquatic species such as salmonids are present. To maintain cold water downstream of large reservoirs, releases sometimes are taken from beneath the thermocline where water temperature exhibits little diurnal variation. This unusual upstream boundary condition, that is, steady flow and temperature, causes a characteristic pattern of “nodes” of minimum diurnal variation and “antinodes” of maximum diurnal variation to form at regular intervals downstream of the regulating reservoir. Within this paper, an analytical model is derived in order to illustrate the phenomenon of nodes and antinodes. Field data as well as analytical and numerical model simulations illustrate the formation of nodes and antinodes over a 250 km reach of the Sacramento River, located in the Central Valley of California.
An experimental exploration of the transport and capture of abiotic colloids in a single fracture
Authors: Knapp R. B. , Chiarappa M. L. , Durham W. B.
Published: February 2004

The effect of microscale surface roughness in fracture walls is shown to accelerate colloid transport relative to a tracer and to increase colloid capture. These results are from experiments on duplicate laboratory experiments using synthetic, neutrally buoyant, abiotic, colloidal particles and two dense rhyolitic tuff cores. One core has a single, rough‐walled fracture with an average aperture of 450 μm. We have used a profilometer to measure the fracture topography and calculate aperture distributions and surface areas in the reconstructed core. The other core has a smooth, channelized slit that is a parallel‐plate analogue to the fractured core. Identical tracer and particle pulses were injected into each core. Particle capture by electrostatic attachment in the parallel‐plate core gives a retention capacity of 55×109 particles m−2. In the rough‐walled fracture core, average particle transport rates are a full fracture volume faster than average tracer transport rates. Reynolds flow simulation indicates the presence of highly channelized fluid flow that likely contributes to this accelerated particle transport. Capture in the fractured core was 90×109 particles m−2; retention capacity is unknown because particle concentrations in the effluent achieved a steady state value of only 85% of the injected concentration.
Moving packet model for variably saturated flow
Authors: Ewen John
Published: February 2004

The theory of moving packets is developed, and a practical moving packet model for Richards' equation is created, implemented using linked lists, and verified for vertical flow in variably saturated layered porous media. The model uses a Lagrangian approach. When simulating infiltration, for example, packets of water are injected at the ground surface, and an implicit finite difference approximation to Darcy's law is used to track the vertical movement of the boundaries which separate each packet from its neighbors above and below. In effect, the boundaries define a nonuniform self‐adapting grid which is carried along with the flow. The moving packet method was designed to be used as the basis for models of coupled flow, transport, and mixing. Its advantage is that transported properties and matter can be carried within the packets, simplifying the modeling of advection, dispersion, and mixing.
A dual‐domain mass transfer approach for modeling solute transport in heterogeneous aquifers: Application to the Macrodispersion Experiment (MADE) site
Authors: Feehley C. Erin , Zheng Chunmiao , Molz Fred J.
Published: February 2004

A large‐scale natural‐gradient tracer test in a highly heterogeneous aquifer at the Macrodispersion Experiment (MADE) site on the Columbus Air Force Base in Mississippi is simulated using three‐dimensional hydraulic conductivity distributions derived from borehole flowmeter test data. Two methods of hydraulic conduct(fBm), are used to construct the hydraulic conductivity distributions needed by the numerical model. Calculated and observed mass distributions are compared to evaluate the effectiveness of the dual‐domain mass transfer approach relative to the single‐domain advection‐dispersion approach. The results show that the classical Fickian advection‐dispersion model can reproduce reasonably well the observed tritium plume above a certain concentration limit but fails to reproduce the extensive spreading of the tracer at diluted concentrations as observed in the field. The alternative dual‐domain mass transfer model is able to represent the rapid, anomalous spreading significantly better while retaining high concentrations near the injection point. This study demonstrates that the dual‐domain mass transfer approach may offer a practical solution to modeling solute transport in highly heterogeneous aquifers where small‐scale preferential flow pathways cannot be fully and explicitly represented by the spatial discretization of the numerical model.
Pumping optimization in saltwater‐intruded coastal aquifers
Authors: Cheng A. H.‐D. , Halhal D. , Naji A. , Ouazar D.
Published: February 2004

This paper investigates the hydraulic issues of pumping well optimization in saltwater‐intruded coastal aquifers. The well field includes freshwater outflow, pumping wells, and a recharge canal. The objective is to maximize the total pumping rate subject to the constraint of no intrusion of the saltwater front into the wells. Analytical solutions are presented for one‐well, two‐well, and one‐well‐with‐recharge‐canal problems. For problems involving multiple pumping wells, a structured messy genetic algorithm is used to search for the optimal solution.
Multidimensional linearized nonsteady infiltration toward a shallow water table
Authors: Basha H. A.
Published: February 2004

The Green's function method is used to derive a general analytical model pertaining to multidimensional nonsteady infiltration toward a shallow water table with arbitrary initial conditions, boundary conditions, and root uptake forcing functions and for various simple source geometries. The general Green's function solution allows the derivation of particular analytical solutions pertaining to cases of surface and subsurface irrigation, evaporation, root uptake, and moisture redistribution. The model assumes that the hydraulic conductivity function is an exponential function of the water pressure and a linear function of the moisture content. One‐dimensional solutions are obtained for time‐dependent distributions of the flux at the land surface and of root uptake in the subsurface. Two‐ and three‐dimensional solutions are also derived for point and line sources in the domain, for a strip, a rectangle, and a disc source at the land surface, and for two‐ and three‐dimensional moisture redistribution and root uptake. Most of the solutions are a product of two terms; one term is a function of depth while the other term is a function of time and the lateral coordinates. Numerical results show the effect of the water table and soil type on the two‐dimensional moisture movement from a strip source.
Dating young groundwater with sulfur hexafluoride: Natural and anthropogenic sources of sulfur hexafluoride
Authors: Busenberg Eurybiades , Plummer L. Niel
Published: February 2004

Sulfur hexafluoride (SF6) is primarily of anthropogenic origin but also occurs naturally. The troposphere concentration of SF6 has increased from a steady state value of 0.054±0.009 to more than 4 parts per trillion volume during the past 40 years. An analytical procedure was developed for measuring concentrations of SF6 to less than 0.01 fmol/L in water. Groundwater can be dated with SF6 if it is in equilibrium with atmospheric SF6 at the time of recharge and does not contain significant SF6 from other sources. The dating range of SF6 is currently 0 to 30 years. The tracer was successfully used to date shallow groundwater of the Atlantic Coastal Plain sand aquifers of the United States and springs issuing near the top of the Blue Ridge Mountains of Virginia. Significant concentrations of naturally occurring SF6 were found in some igneous, volcanic, and sedimentary rocks and in some hydrothermal fluids.
Gravity‐destabilized nonwetting phase invasion in macroheterogeneous porous media: Experimental observations of invasion dynamics and scale analysis
Authors: Glass Robert J. , Conrad Stephen H. , Peplinski William
Published: February 2004

We designed and conducted experiments in a heterogeneous sand pack where gravity‐destabilized nonwetting phase invasion (CO2 and trichloroethylene) could be recorded using high‐resolution light transmission methods. The heterogeneity structure was designed to be reminiscent of fluvial channel lag cut‐and‐fill architecture and to contain a series of capillary barriers. As invasion progressed, nonwetting phase structure developed a series of fingers and pools; behind the growing front we found nonwetting phase saturation to pulsate in certain regions when viscous forces were low. Through a scale analysis we derive a series of length scales that describe finger diameter, pool height and width, and regions where pulsation occurs within a heterogeneous porous medium. In all cases we find that the intrinsic pore‐scale nature of the invasion process and resulting structure must be incorporated into our analysis to explain experimental results. We propose a simple macroscale structural growth model that assembles length scales for substructures to delineate nonwetting phase migration from a source into a heterogeneous domain. For such a model applied at the field scale for dense nonaqueous phase liquid migration, we expect capillary and gravity forces within the complex subsurface lithology to play the primary roles with viscous forces forming a perturbation on the inviscid phase structure.
Stress partitioning in streams by large woody debris
Authors: Manga Michael , Kirchner James W.
Published: February 2004

Using simple theoretical models and field measurements from a spring‐dominated stream, we quantify how large woody debris affect channel hydraulics and morphology at both the local and reach‐averaged scales. Because spring‐dominated streams have nearly constant discharge, they provide a unique natural opportunity to study flow and transport processes near the channel‐forming flow. We first show that the drag on a floating log is identical to the theoretical value for widely separated cylinders at similar Reynolds numbers. We then use simple theoretical models to estimate the partitioning of flow shear stress between woody debris and streambeds. The inferred stress partitioning is consistent with an estimate based on a comparison of local and reach‐averaged measurements of the water surface slope. Our measurements show that even though large woody debris cover less than 2% of the streambed, they provide roughly half of the total flow resistance. As large woody debris are added to a stream, the total shear stress increases (because the water depth increases), but the shear stress borne by the bed decreases, as a growing fraction of the total shear stress is borne by the debris. Our analysis shows that simple theoretical models of stress partitioning may provide a convenient mathematical framework for assessing how changes in debris loading affect streams.
Boundary shear stress measurements in undular flows: Application to standing wave bed forms
Authors: Chanson H.
Published: February 2004

Waters flowing in natural streams and rivers have the ability to scour and to deposit materials, hence to change the bed topography. It is recognized that undular flows have great potential for sediment transport. In the present study, a fixed‐bed model was used to investigate the spatial variations of boundary shear stress under standing waves (i.e., undular flow). The results (Figure 8) highlight the nonuniformity of the boundary shear stress distributions. Minimum boundary shear stress is observed under the wave crests, and maximum shear stress is observed under the wave troughs. The experimental findings suggest the formation of three‐dimensional standing waves bed forms. Overall, the study highlights large variations of boundary shear stress in response to free‐surface undulations.
Modeling long‐term persistence in hydroclimatic time series using a hidden state Markov Model
Authors: Thyer Mark , Kuczera George
Published: February 2004

A hidden state Markov (HSM) model is developed as a new approach for generating hydroclimatic time series with long‐term persistence. The two‐state HSM model is motivated by the fact that the interaction of global climatic mechanisms produces alternating wet and dry regimes in Australian hydroclimatic time series. The HSM model provides an explicit mechanism to stochastically simulate these quasi‐cyclic wet and dry periods. This is conceptually sounder than the current stochastic models used for hydroclimatic time series simulation. Models such as the lag‐one autoregressive (AR(1))) model have no explicit mechanism for simulating the wet and dry regimes. In this study the HSM model was calibrated to four long‐term Australian hydroclimatic data sets. A Markov Chain Monte Carlo method known as the Gibbs sampler was used for model calibration. The results showed that the locations significantly influenced by tropical weather systems supported the assumptions of the HSM modeling framework and indicated a strong persistence structure. In contrast, the calibration of the AR(1) model to these data sets produced no statistically significant evidence of persistence.
A mechanistic study of nonlinear solute transport in a groundwater–surface water system Under steady state and transient hydraulic conditions
Authors: Boufadel Michel C.
Published: February 2004

Two laboratory experiments were conducted to investigate the effects of tides and buoyancy on beach hydraulics in the presence of a seaward groundwater flow due to an elevated “regional” water table. In the first experiment, case 1, the difference in concentration between the salt water at sea and the water of the regional aquifer was small, 2.4 g L−1, such that it did not engender density gradients; the salt acts as a tracer in this case. In the second experiment, case 2, the difference was ∼32.0 g L−1, which creates a significant density gradient. This case corresponds to the presence of fresh groundwater in the subsurface of the coasts of the continental United States. The experiments were numerically simulated by the marine unsaturated (MARUN) model, a numerical model for density‐and‐viscosity‐dependent flows in two‐dimensional variably saturated media. The long‐term experimental and numerical results showed that the seawater plume entered the beach from the sea and occupied most of the intertidal zone. The maximum depth of the seawater plume was near the midsection of the intertidal zone, and it decreased near the low and high tide lines. When viewed in the context of case 2, these results indicate an inverted salinity distribution in beaches subjected to tides with salt water from sea overtopping the freshwater lens. For both cases, water from the regional aquifer moved seaward beneath the seawater in the intertidal zone and pinched out near the low tide mark. We also noted that beach hydraulics are highly two dimensional with water entering the beach at a near‐vertical angle and leaving it at a near‐horizontal angle, which casts doubts on analyses of beach hydraulics based on the Dupuit assumption. Findings from this work have direct implications within the practice of bioremediation of oil spills on beaches. We found that applying dissolved nutrients on the beach surface at low tide is superior to applying them in a trench landward of the beach. This is because the residence time of the nutrient plume in the bioremediation zone of the beach in the prior situation is longer than that in the latter.
Temporal behavior of a solute cloud in a heterogeneous porous medium: 1. Point‐like injection
Authors: Dentz M. , Kinzelbach H. , Attinger S. , Kinzelbach W.
Published: February 2004

We investigate the temporal behavior of transport coefficients in a model for transport of a solute through a spatially heterogeneous saturated aquifer. In the framework of a stochastic approach we derive explicit expressions for the temporal behavior of the center‐of‐mass velocity and the dispersion of the concentration distribution after a point‐like injection of solute at time t=0, using a second‐order perturbation expansion. The model takes into account local variations in the hydraulic conductivity (which, in turn, induce local fluctuations in the groundwater flow velocities) and in the chemical adsorption properties of the medium (which lead to a spatially varying local retardation factor). In the given perturbation theory approach the various heterogeneity‐induced contributions can be systematically traced back to fluctuations in these quantities and to cross correlations between them. We analyze two conceptually different definitions for the resulting dispersion coefficient: the “effective”dispersion coefficient which is derived from the average over the centered second moments of the spatial concentration distributions in every realization and the “ensemble” dispersion coefficient which follows from the second moment of the ensemble‐averaged concentration distribution. The first quantity characterizes the dispersion in a typical realization of the medium, whereas the second one describes the (formal) dispersion properties of the ensemble as a whole. We give explicit analytic expressions for both quantities as functions of time and show that for finite times their temporal behavior is remarkably different. The ensemble dispersion coefficient which is usually evaluated in the literature considerably overestimates the dispersion typically found in one given realization of the medium. From our explicit results we identify two relevant timescales separating regimes of qualitatively and quantitatively different temporal behavior: The shorter of the two scales is set by the advective transport of the solute cloud over one disorder correlation length, whereas the second, much larger one, is related to the dispersive spreading over the same distance. Only for times much larger than this second scale, do the effective and the ensemble dispersion coefficient become equivalent because of mixing caused by the local transversal dispersion. The formulae are applied to the Borden experiment data. It is concluded that the observed dispersion coefficient matches the effective dispersion coefficient at finite times proposed in this paper very well.
Incomplete self‐similarity and flow velocity in gravel bed channels
Authors: Ferro Vito , Pecoraro Roberto
Published: February 2004

Velocity measurements, previously carried out using both a miniature current flowmeter and an acoustic Doppler velocimeter, are used to verify the applicability of the incomplete self‐similarity theory to deduce the velocity profile in a gravel bed channel. Then, for the velocity profiles having the maximum value below the free surface and for the S‐shaped profiles, the power velocity distribution is corrected using a new divergence function. For each value of the depth sediment ratio the nondimensional friction factor parameter is calculated by integration of the measured velocity distributions in the different verticals of the cross section. Finally, a semilogarithmic flow resistance equation is empirically deduced.