bluuebunny/crossref_metadata_2025 huggingface dataset:
Subsurface water flow simulated for hillslopes with spatially dependent soil hydraulic characteristics
Authors: Sharma M. L. , Luxmoore R. J. , DeAngelis R. , Ward R. C. , Yeh G. T.
Published: February 2008

Water flow through hillslopes consisting of five soil layers, with varying spatial dependence in hydraulic characteristics in the lateral plane was simulated by solving Richards' equation in three dimensions under varying rainfall intensities and for two complexities of terrain. By concepts of similar media the variability in soil hydraulic characteristics was expressed by a single dimensionless parameter, the scaling factor α. The moments of log normally distributed α were set as: Mean = 1.0 and standard deviation = 1.0. Four cases of spatial dependence of α in the lateral plane were selected for simulation, using exponential variogram functions ranging in spatial structure from radom (no spatial dependence) to large dependence (large correlation lengths). The simulations showed that the rates of subsurface flow from the 30° hillslope, during and following rainfall, were significantly enhanced with an increase in spatial dependence. Subsurface drainage was also increased with increases in rainfall intensity and slope complexity. For hillslopes the relative effect of spatial dependence in soil hydraulic characteristics was smaller with 30° horizontal pitching than without pitching. Hillslopes with a random distribution of hydraulic characteristics provided greater opportunity for soil units with differing water capacities to interact than in cases with spatially correlated distributions. This greater interaction is associated with a greater lag in subsurface flow generation. These studies illustrate some of the expected effects of spatial dependence of soil hydraulic characteristics on the integrated hydrologic response of land areas.
Transport of reacting solutes subject to a moving dissolution boundary: Numerical methods and solutions
Authors: Willis Catherine , Rubin Jacob
Published: February 2008

A moving boundary problem which arises during transport with precipitation‐dissolution reactions is solved by three different numerical methods. Two of these methods (one explicit and one implicit) are based on an integral formulation of mass balance and lead to an approximation of a weak solution. These methods are compared to a front‐tracking scheme. Although the two approaches are conceptually different, the numerical solutions showed good agreement. As the ratio of dispersion to convection decreases, the methods based on the integral formulation become computationally more efficient. Specific reactions were modeled to examine the dependence of the system on the physical and chemical parameters. Although the water flow rate does not explicitly appear in the equation for the velocity of the moving boundary, the speed of the boundary depends more on the flux rate than on the dispersion coefficient. The discontinuity in the gradient of the solute concentration profile at the boundary increases with convection and with the initial concentration of the mineral. Our implicit method is extended to allow participation of the solutes in complexation reactions as well as the precipitation‐dissolution reaction. This extension is easily made and does not change the basic method.
A generalized kinematic catchment model
Authors: Field W. G. , Williams B. J.
Published: February 2008

The model requires the division of the catchment into a (branched) network of subcatchments or reservoirs. Rainfall on these elements of the model is distributed to a surface or a groundwater storage which, in turn, at differing rates, provides inflow into the channel network. The flows in the channels are simulated by a kinematic wave equation which is solved using an implicit finite difference scheme. Typical parameter estimates for catchments of varying sizes are presented. The model is considered to have particular potential for flood forecasting as well as being a useful adjunct to design.
Convective transport within stable river sediments
Authors: Savant S. Anne , Reible Danny D. , Thibodeaux Louis J.
Published: February 2008

Local pressure variations of the order of 100–1000 N/m2 can be observed between the upstream and downstream faces of the typically triangular‐shaped dunelike sediment structures that form at the sediment‐water interface of rivers. Laboratory experiments were conducted examining the influence of this localized pressure variation on contaminant transport processes within the sediment. Numerical modeling of the in‐bed flow via boundary element methods was also undertaken in order to predict convective transport under typical field conditions. The laboratory experiments and numerical simulation of the in‐bed flow in several rivers verified that the pressure distribution observed on the sediment surface and the resulting interstitial fluid convection can control transport of chemically inert, nonsorbed contaminants in stable sediments. In‐bed Peclet numbers were of the order of 100–1000, indicating the negligible influence of diffusion under the conditions examined.
Variable density flow and solute transport simulation of regional aquifers containing a narrow freshwater‐saltwater transition zone
Authors: Voss Clifford I. , Souza William R.
Published: February 2008

Variable density flow and solute transport simulation of aquifer systems containing narrow transition zones between freshwater and saltwater requires particular attention to certain aspects of the numerical method and its application to be successful. Typically, only cases involving wide transition zones have been simulated with variable density transport models, possibly because of inaccuracies in the modeling approaches used. The major components of a successful approach are threefold. First, functionally consistent approximation of terms involved in fluid velocity calculations is necessary. In the case of Galerkin finite element methodology, a significant modification is required to the standard approach in order to achieve consistency. Second, the simulator must be verified in a particular series of tests. The usual tests using Henry's problem for verification of density‐dependent transport simulators are inadequate to check for consistency of the velocity approximations and for the accuracy of simulating flow driven by bouyancy forces. Third, adequately fine spatial discretization is required when applying the simulator for spatial stability of the numerical transport solution and to allow accurate representation of narrow transition zones and the effects of low transverse dispersivity. The effectiveness of this approach is demonstrated through simulation of the flow of fresh and saline groundwater in the layered basalt aquifer of southern Oahu, Hawaii. The transition zone in this regional flow system is narrow except near the discharge area where it is broadly dispersed. Simulation of this common situation with an inconsistent approximation gives grossly incorrect results, while simulation with a consistent model provides a robust tool for analysis of system hydrology.
Simulation of solute transport using a continuous time Markov process: 2. Application to transient field conditions
Authors: Knighton R. E. , Wagenet R. J.
Published: February 2008

A technique for modeling the movement of solute in the unsaturated zone using a continuous time Markov process is presented and field‐tested under a transient water flow regime. Three treatments, a fallow and two cropped, were used to quantify the effect of crop uptake on water and solute movement and to test model performance under field conditions. Field measured NO3− and Br − concentrations were simulated quite well using three scenarios of water flux. Average water fluxes calculated from 40 observations within a treatment and used to estimate water flow resulted in the best simulations,while the average water flux plus one standard deviation (SD) represented a net downward movement of solute, and simulations using average flux minus one SD represented a net upward movement of solute. Under fallow conditions, solute which was unaccounted for by mass balance of measured data was simulated as leached under extreme flux conditions. Simulations of the cropped treatments were improved substantially in inclusion of either a continuous time crop uptake function or a constant crop uptake coefficient. Model predictions were found to be much more sensitive to crop uptake than to the flux of soil water.
Drainage from layered field soils: Fixed gradient models
Authors: Sisson James B.
Published: February 2008

Fixed gradient models result when the gradient term in the soil moisture equation is assumed to vary only with depth (remains invariant in time). The fixed gradient assumption results in a first‐order partial differential equation that is transformable to a mathematical form identical to that for a uniform soil. When the transformation was applied to field data, all water content data were found to plot along a single curve. Assuming a fixed gradient and an exponential form for K(Θ) resulted in a fitted curve with an r2 = 0.847 (d.f. = 405) when data from three sites and seven depths were used. Assuming a power function for K(Θ) resulted in a similar r2. Prior to applying the transform, hydraulic conductivity required 42, 42, and 63 parameters to fit data obtained at the 21 spatial points sampled, assuming a Davidson, Watson or Brooks and Corey function, respectively. With the transform 23, 23, and 24 parameters were required for the three K(Θ) functions, respectively.
Soil heat and water flow with a partial surface mulch
Authors: Chung Sang‐Ok , Horton Robert
Published: February 2008

A computer model using the alternating direction implicit (ADI) finite difference method to study two‐dimensional coupled soil heat and water flow with a partial surface mulch cover is developed. A new, simplified computational procedure, which has only tridiagonal matrix problems, for the ADI method is introduced. The model uses a soil surface energy balance equation to determine soil surface boundary conditions for both heat and water flow. The inputs required for the computer simulations are weather data, soil thermal and hydraulic properties, and mulch data. Numerical experiments are performed to examine the effects of soil type, mulch width, and weather conditions on soil heat and water movement. For continuous evaporation and drainage, 10‐day simulations were performed for each combination of clay, loam, and sand soil and fractions of mulch cover of 0, 0.5, 0.8, and 1.0 of the row interval width. For repetitive evaporation and infiltration, 15‐day simulations were performed. The mulch cover greatly reduces evaporation loss and the amplitude of daily soil temperature, water content, and pressure head variations. Large spatial variations in temperature and soil water content are predicted near the interface of mulch and bare soil surface. The soil hydraulic properties have important roles in controlling soil surface water content. The present model reasonably describes the soil thermal and hydrologic environments and thus can be applied successfully in soil science and groundwater hydrology and can be extended to related disciplines.
The annual carbon dioxide cycle in a montane soil: Observations, modeling, and implications for weathering
Authors: Solomon D. Kip , Cerling Thure E.
Published: February 2008

Profiles of CO2concentrations in soil and snow, soil respiration, soil and snow temperatures, and shallow groundwater chemistry were monitored from March 1984 to July 1985 in a montane region near Brighton, Utah. Significant seasonal variations in the concentrations of CO2in soil and snow occurred, and two principal rise‐decline cycles were observed. During the first cycle the concentration of soil CO2at 35 cm rose from 4200 ppmv in July to a maximum of 12,400 ppmv in August and then declined to 4300 ppmv by October. This cycle is attributed to the changing production rate of soil CO2during the growing season. During the second cycle the concentration of CO2at 35 cm began to rise in November, reached a maximum of 7200 ppmv in early spring, and quickly declined to 3200 ppmv by late spring shortly after the snow cover had melted. This cycle is attributed to deterioration in the exchange of CO2between the soil and atmosphere due to a deep snowpack. A model based on Pick's second law of diffusion was developed to account for the temporal and spatial distribution of soil CO2. The model predicts that soil CO2at 35 cm is increased by as much as 15 times due to the deep snowpack. The elevated concentration of soil CO2, abundance of water, and above‐freezing soil temperatures imply that significant soil weathering occurs during the winter in montane regions.
Simulation of the effects of forest cover, and its removal, on subsurface water
Authors: Hillman G. R. , Verschuren J. P.
Published: February 2008

A distributed, physically based mathematical model was developed to simulate the effects of forestry operations on both soil water and streamflow. It combines Galerkin's finite‐element method with an implicit backward finite‐difference scheme to solve the Darcy‐Richards equation of porous media flow, and uses sink concepts to simulate water use by trees. The validity of the model was checked by simulation of one‐dimensional vertical infiltration into a soil column. Five forest configurations, imposed on a hillslope, were simulated: fully treed, upper slope clear‐cut, lower slope clear‐cut, patch‐cut, and entire slope clear‐cut. The results, displayed as total potential and soil water fields, were qualitatively compatible with accepted hydrological concepts. A field study was simulated and the results compared with field measurements. Explanations are offered for discrepancies between the simulation results and the field measurements. The model's primary usefulness rests in its capability to simulate soil water movement and redistribution, on a distributed basis, in response to the processes of infiltration, evaporation, and transpiration.
Contaminated groundwater remediation design using simulation, optimization, and sensitivity theory: 1. Model development
Authors: Ahlfeld David P. , Mulvey John M. , Pinder George F. , Wood Eric F.
Published: February 2008

The problem of designing contaminated groundwater remediation systems using hydraulic control is addressed. Two nonlinear optimization formulations are proposed which model the design process for the location and pump rates of injection and extraction wells in an aquifer cleanup system. The formulations are designed to find a pumping system which (1) removes the most contaminant over a fixed time period and (2) reduces contaminant concentration to specified levels by the end of a fixed time period at least cost. The formulations employ a two‐dimensional Galerkin finite element simulation model of steady state groundwater flow and transient convective‐dispersive transport. To make the optimization problems computationally tractable sensitivity theory is used to derive a general relationship for computing the derivatives of an arbitrary function of the simulation outputs with respect to model inputs. This relationship is then applied to the convective‐dispersive transport equation.
Colors of glacier water
Authors: Aas Eyvind , Bogen Jim
Published: February 2008

The optical characteristics of sediment suspensions in the glacierfed lake Veitastrondsvatn are examined to explain observed color variations. The color depends on the wavelength where the ratio between the backward scattering coefficient and the absorption coefficient of the suspension has its maximum, and this usually coincides with the wavelength where the absorption has its minimum. It is found that the absorption coefficient of the small particles of clay and silt size decreases with increasing wavelength, but that the absorption coefficient of the suspension obtains a minimum in the green part of the spectrum, due to the strong absorption of red light by water. The suspension of such particles in the surface layer of the lake will then appear green. The coefficients of larger particles of sand size are almost independent of wavelength, with no significant maximum or minimum, and river waters close to the glacier fronts, which are dominated by these particles, will appear milky.
Analytical solution incorporating nonlinear radial flow in confined aquifers
Authors: Şen Zekâi
Published: February 2008

Type curve expressions are derived for the nonlinear flow in confined aquifers on the basis of Forchheimer (1901) flow law which presents a combination of the laminar and turbulent flows of the groundwater. Solutions of the relevant initial boundary value problem are achieved by the application of the Boltzmann transformation that permits the coupling of the continuity equation and Forchheimer's law. The validity of the obtained results are checked against the well‐known Theis and Jacob equations which are only applicable to linear (Darcian) flow. Necessary type curves matching of all times and a straight line method for large times are proposed to evaluate the aquifer parameters with aquifer tests conducted in nonlinear flow conditions. An example application of the proposed methodology is given for aquifer test results in Nova Scotia, Canada.
Stable width and depth of straight gravel rivers with heterogeneous bed materials
Authors: Ikeda Syunsuke , Parker Gary , Kimura Yoshitaka
Published: February 2008

Mathematical model for defining stable width and depth of straight gravel rivers is presented, in which it is revealed that the major agency for maintaining stable channel is lateral diffusion of longitudinal fluid momentum due to turbulence. The primary quantities which determine the stable channel geometry are found to be flow discharge, longitudinal free surface slope, the median size of bed material, and gradation of the bed material. Increasing gradation is found to increase the depth and decrease the width. Introduction of one loose and the other rigid bank in laboratory experiments provides good examples to test the model, in which four kinds of sand were used to test the gradation effect. These laboratory data support the mathematical model presented herein. The theory also performs well when it is applied to natural gravel rivers.
Water resources assessment: A spatial equilibrium approach
Authors: Marin Carlos M. , Smith Mark Griffin
Published: February 2008

This paper presents a spatial equilibrium approach for screening level water resources assessment. This approach complements traditional hydrologically based water resources assessment methods by providing a model capable of evaluating national‐regional strategies with respect to four principal objectives of water resources policy: supply‐demand balance, economic efficiency, cost recovery, and equity. The proposed approach has a number of advantages. First, it provides a simple yet generalizable representation of the spatial environment at a number of scales. Second, the assessment is freed from the constraint of requiring a highly detailed hydrologic description. Third, an objective‐constraint structure provides the basis for defining, organizing, and collecting the data necessary for the assessment. Finally, simulation is used in conjunction with optimization to evaluate alternative allocation scenarios. A case study in Cyprus demonstrates use of the model for policy analysis.
Dispersion in heterogeneous porous media: 2. Predictions for stratified and two‐dimensional spatially periodic systems
Authors: Plumb O. A. , Whitaker S.
Published: February 2008

The closure scheme which is presented in part 1 (Plumb and Whitaker, this issue) is solved for a stratified system and for a two‐dimensional spatially periodic system. By this we mean that the ω and η regions, which make up the heterogeneous media, are distributed in a spatially periodic manner. The solution to the closure problem yields the information necessary to determine the large‐scale dispersion tensor and the other coefficients that appear in the large‐scale dispersion equation. The theoretical results for the stratified system exhibit an increase in the longitudinal dispersion coefficient of several orders of magnitude over that which would be predicted using established correlations based on laboratory experiments with seemingly homogeneous porous media. For the cases studied, the coefficient of the mixed space‐time derivative appears to be significant and this coefficient increases for those conditions which yield increasing longitudinal dispersion coefficients. The results for the two‐dimensional spatially periodic system are in qualitative agreement with experimental observations, and we found that both the size of the heterogeneity and the difference in the hydraulic conductivity between the two regions had an important influence on the large‐scale dispersion tensor. As in the case of a stratified medium, the predicted values of the coefficient of the mixed space‐time derivative (the skewness vector) indicate that this term could be a significant factor in the prediction of mass transport in heterogeneous porous media.
Determination of electrical conductivity using time domain reflectometry: Soil and water experiments in coaxial lines
Authors: Topp G. C. , Yanuka M. , Zebchuk W. D. , Zegelin S.
Published: February 2008

The simultaneous measurement of water content and electrical conductivity of soils and KCl solutions was achieved using time domain reflectometry (TDR). Coaxial transmission lines varying in length from 90 to 300 mm contained either KCl solutions or soil of varied water and salt content. The water content of soil or dielectric constant of the water solutions was determined from the travel time. The measured dielectric constant of KCl solutions was unchanged from that of pure water (81) at those concentrations where there was sufficient reflected signal for measurement. Two analyses were used for determination of electrical conductivity, one based on signal attenuation after one “round‐trip” and the second based on a thin sample approximation for the signal reflection and attenuation. Reference measurements of conductivity were made on the same samples using low‐frequency conductance bridge measurements. These analyses of the TDR traces showed that for water solution both the thin sample analysis and the analysis after a signal had traversed one round‐trip yielded conductivity in agreement with bridge conductivity values. This indicated that the imaginary part of the complex dielectric constant was negligible. For soils the thin sample analysis was in general agreement with the bridge measurements. From the analysis of signal after one round‐trip in soils there was indication that the imaginary part of the dielectric constant should not be assumed negligible. Further investigation of the frequency dependence of the dielectric constant and attenuation will be required to identify the relative contributions of the real and imaginary parts of the dielectric constant to measurement by TDR. The effect of impedance‐matching transformers on conductivity measurements in the field has yet to be ascertained.
Use of map analysis to elucidate flooding in an Australian Riparian River Red Gum Forest
Authors: Bren L. J. , O'Neill I. C. , Gibbs N. L.
Published: February 2008

Red gum (Eucalyptus camaldulensis) forests occur on extensive floodplains along the river Murray in Australia. This type of forest is unusual because of its high quality in a semiarid area, the absence of woody species other than red gum, and its survival on a deep, intractable, swelling clay soil of depths exceeding 20 m. This soil probably acts as an aquiclude. The forests require flooding to thrive and regenerate. For many years there has been speculation that irrigation regulation of the river was reducing forest flooding. A grid cell analysis of flood maps of areas flooded over a period of 22 years showed that vegetation communities and forest site quality were statistically related to the flood frequencies of sites. The percentage of forest inundated was dependent on the peak daily flow during the period of inundation. A historical analysis of the estimated percentage of forest inundated showed a substantial influence of river regulation on both timing and extent of inundation. Estimates of historical floodings showed that the environment is one that changes rapidly from wetland to dry land. Although not without limitations, the analysis produced information not available from other sources.
Continuum models for contaminant transport in fractured porous formations
Authors: Berkowitz Brian , Bear Jacob , Braester Carol
Published: February 2008

This work examines some conditions under which contaminant transport in fractured porous rocks can be described by an equivalent porous medium (single continuum) model. For this purpose, a two‐dimensional mathematical and numerical model for flow and contaminant transport was developed. The model allows for contaminant transport by advection, diffusion, and dispersion in both fractures and porous blocks. Concentration distributions were calculated for different flow conditions and medium properties. The resulting S‐shaped breakthrough curves, characteristic of ordinary porous media, indicated the possibility of regarding the fractured porous medium as a single (equivalent) continuum. The results were compared to an existing analytical solution for contaminant movement in ordinary porous media. Analysis showed that within the range of considered parameter values, and except for the region close to the source, a single continuum model is sufficient for modelling the movement of contaminants. In such cases, application of the equivalent porous medium model is an actual field situation requires knowledge of the “equivalent” porosity and the equivalent coefficient of dispersion appearing in the governing transport equation. In practice, these coefficients must be determined by analysis of breakthrough curves obtained from field tests.
Landscape and rainfall indices for prediction of streamflow similarities in the Hunter Valley, Australia
Authors: Cook B. G. , Laut P. , Austin M. P. , Body D. N. , Faith D. P. , Goodspeed M. J. , Srikanthan R.
Published: February 2008

Multivariate analysis of grid cell landscape data and of daily rainfall and streamflow data is used to locate mesoscale catchments of the Hunter valley, Australia, in multidimensional spaces which define landscape, rainfall, and streamflow indices for each catchment. Multiple regression equations are developed and validated for predicting indices of streamflow from those of landscape and rainfall. This methodology provides a basis for using landscape and rainfall data to locate gauged and ungauged catchments in a common space in which proximity between catchments predicts similarity of streamflow.
Analytical solution for aquifer decontamination by pumping
Authors: Chen Chia‐Shyun , Woodside Greg D.
Published: February 2008

Rehabilitation of polluted aquifers is an important issue in groundwater study. The use of withdrawal wells to extract dissolved solutes from contaminated aquifers is a possible mechanical remedial technique. A mathematical model dealing with aquifer decontamination by pumping is developed. The pumping well with a constant flow rate is taken into account as a mathematical sink located at the center of the plume to be removed. This plume is assumed to have a circular geometry inside which the solute concentration is axial symmetric with respect to the well and is incorporated into the model as an initial condition that can be formulated in an analytic or a sectionally continuous function capable of representing a wide range of uniform or nonuniform profiles. It assumes advection and longitudinal mechanical dispersion to be the transport mechanisms on a radially converging groundwater flow field. The analytical solution detecting concentration variation inside the aquifer is determined in closed forms with the Green's function approach and the Laplace transform technique. Using the field data presented by Pickens and Grisak (1981), the analytical solution obtained very accurately reproduces the reported concentration history at the well during the withdrawal phase of the single‐well injection‐withdrawal tracer test. It is found that if the initial conditions are expressed in functions presenting noticeable concentration gradients at the plume boundary, adverse dispersion against the converging groundwater movement would cause spreading of solutes beyond the original extent of plume during pumping. If the initial conditions gradually decrease to zero concentration at the plume boundary where negligible concentration gradients exist, concentration distributions do not extend beyond the initial condition envelopes during the withdrawal process. Since the well is placed at the center of the plume where maximum concentration occurs, the analytical solution evaluated at the well bore can be employed to indicate the decontamination level achieved at various pumping times. Graphical relations are presented for approximating the pumping time required to lower the concentration at the well bore to 1% of the maximum initial concentration. In general, neglecting dispersion in estimating the total cleanup times yields shorter pumping times.
Drawdown interval analysis: A method of determining the parameters of shallow conduit flow carbonate aquifers from pumping tests
Authors: Thrailkill John
Published: February 2008

In shallow conduit flow (SCF) carbonate aquifers, the nature of the void space and groundwater flow differs from that of granular aquifers in that the voids are solution conduits, often of large size, and flow is generally non‐Darcy. These and other characteristics of SCF aquifers preclude the application of pumping test analysis methods based on the diffusion equation. The drawdown interval analysis (DIA) method, which is consistent with a conceptual model of the SCF aquifer, provides estimates of aquifer parameters using both direct calculation and simulation approaches. Application of the DIA method to two wells yielded parameter values which are consistent with the hydrogeologic setting of the wells.
Analytical simulation of water system capacity reliability: 1. Modified frequency‐duration analysis
Authors: Hobbs Benjamin F. , Beim Gina K.
Published: February 2008

The problem addressed is the computation of the unavailability and expected unserved demand of a water supply system having random demand, finished water storage, and unreliable capacity components. Examples of such components include pumps, treatment plants, and aqueducts. Modified frequency‐duration analysis estimates these reliability statistics by, first, calculating how often demand exceeds available capacity and, second, comparing the amount of water in storage with how long such capacity deficits last. This approach builds upon frequency‐duration methods developed by the power industry for analyzing generation capacity deficits. Three versions of the frequency‐duration approach are presented. Two yield bounds to system unavailability and unserved demand and the third gives an estimate of their true values between those bounds.
Analytical simulation of water system capacity reliability: 2. A Markov Chain Approach and verification of the models
Authors: Beim Gina K. , Hobbs Benjamin F.
Published: February 2008

A reliability model for water supply systems is presented which represents demand changes, failures and repairs of capacity components, and streamflows as independent Markov chains. Finished water storage is simulated using an operating policy that depends on the system state. Reliability statistics, including system unavailability, expected unserved demand, and the frequency and duration of failures, are obtained from the transition probabilities and resulting steady state probabilities. Both the Markov model and another analytical simulation approach, the modified frequency‐duration method, are applied to an actual water supply system. Their results are compared to those of a more realistic Monte Carlo simulation in order to verify that analytical simulation yields useful estimates of reliability indices. A procedure for interfacing models of capacity reliability with simulation models of large reservoirs is proposed.
A numerical transport model for oxygen‐ and nitrate‐based respiration linked to substrate and nutrient availability in porous media
Authors: Widdowson Mark A. , Molz Fred J. , Benefield Larry D.
Published: February 2008

A model to simulate organic carbon biodegradation by facultative bacteria in saturated porous media using oxygen‐ and/or nitrate‐based respiration is developed. Basic assumptions incorporated into the model concept include a simulated particle‐bound microbial population comprised of heterotrophic, facultative bacteria in which metabolism is controlled by lack of either an organic carbon‐electron donor source (substrate), electron acceptor (O2 and/or NO3−), or mineral nutrient (NH4+), or all three simultaneously. A system of nine coupled nonlinear equations is developed that describe the processes of transport, degradation, and microbial growth and decay. The solution technique is highly resistant to numerical dispersion and oscillation when applied to the advection‐dispersion equation, even for large Peclet numbers (100). Microbial utilization of materials is assumed to occur by intrapore scale diffusion of materials across a diffusion boundary layer separating the particle‐bound microcolonies of bacteria from the pore fluid. Denitrifying enzyme inhibition is modeled as a function of the oxygen concentration associated with the biomass. Simulations of oxygen‐based, nitrate‐based, and multiple‐electron acceptor respiration are presented for a hypothetical experiment using kinetic parameter value estimates available from the literature.
Chlorine 36 and tritium from nuclear weapons fallout as tracers for long‐term liquid and vapor movement in desert soils
Authors: Phillips Fred M. , Mattick Julie L. , Duval Thomas A. , Elmore David , Kubik Peter W.
Published: February 2008

Chlorine 36 and 3H have been widely used as laboratory tracers for the advection and dispersion of anionic solutes and water, respectively. Advances in accelerator mass spectrometry have recently made possible the analysis of 36Cl in natural soil water samples. Field investigations analogous to the laboratory experiments can now be conducted. Pulses of 36Cl and 3H released into the atmosphere by nuclear weapons testing in the 1950s and 1960s serve as the tracers. We report 36Cl and 3H measurements on desert soil profiles from New Mexico. In both cases where the two tracers were measured in the same profile, the 3H bomb pulse had penetrated between 1 and 3 m, but much of the 36Cl was retained near the soil surface. In contrast, the laboratory experiments usually show more rapid movement of 36Cl than 3H due to the anion exclusion effect. We attribute the difference to the lower water content, fluctuating temperature gradients, and longer time scale of the field situation, which may enhance vapor movement of the tritiated water relative to solute advection of the 36Cl. The spreading of the 36Cl and 3H pulses may be modeled by dispersivities ranging from 5 to 8 cm, much larger than most corresponding laboratory values. These larger apparent dispersivities are probably more a result of vertical velocity fields varying with depth and time than of soil heterogeneity. These results illustrate how the bomb pulse 36Cl and 3H can be useful links between laboratory solute transport studies and the corresponding field applications.
The role of advection in the energy balance of late‐lying snowfields: Niwot Ridge, Front Range, Colorado
Authors: Olyphant Greg A. , Isard Scott A.
Published: February 2008

Measured profiles of temperature, humidity, and wind above alpine tundra are used as upwind boundary conditions in a numerical model for simulating turbulent transfer over late‐laying snow. Resulting simulations indicate that (1) sensible heat flux comprises the major source of turbulent melt energy, although its relative importance vis‐a‐vis latent heat flux depends upon ambient weather conditions, and (2) advected heat contributes greatly to the energy balance of late‐lying alpine snow. Near the leading edge of a snowfield, advection may contribute more than 30 MJ m−2 d−1 of melt energy on a very windy day and more than 12 MJ m−2 d−1 on a relatively windless day. The totals decrease to circa 5 MJ m−2 d−1 and 2 MJ m−2 d−1 at a distance of 1000 m from the leading edge. These findings suggest that the sensible heat flux may exceed net radiation as the major source of energy available for melting late‐lying snow in windy alpine environments.
An improved methodology for short‐term operation of a single multipurpose reservoir
Authors: Simonovic Slobodan P. , Burn Donald H.
Published: February 2008

The existence of several problems associated with the short‐term operation of multipurpose reservoirs has been demonstrated in the past. A principal difficulty arises from uncertainties inherent in the prediction of future stream flows which are required to effect the requisite reservoir operation. It has been shown that the performance of the reservoir operation, as determined by an optimization model, is a function of the operating horizon. This results from the existence of a relationship between the operating horizon and both the reliability of the inflow forecasts and the reservoir operation. With an increase in the length of the operating horizon, the reliability of the forecasts will necessarily decrease. However, the reservoir operation model yields an improved operation on the assumption that the forecasts are accurate. An improved methodology is presented which considers the operating horizon as a decision variable which can change in real time. The optimal value of the operating horizon is selected based on the trade‐off between more reliable inflow forecast for shorter horizons and better reservoir operation associated with the use of longer operating horizons. The benefits of the new methodology are illustrated through an application of the technique in simulating the operation of Green Reservoir in Kentucky.
A generalized maintenance of variance extension procedure for extending correlated series
Authors: Grygier Jan C. , Stedinger Jery R. , Yin Hong‐Bing
Published: February 2008

Hirsch's (1982) maintenance of variance extension technique for filling in missing observations or producing a unique extended streamflow sequence with a specified mean and variance is generalized to reproduce also the correlation between the variate of interest and other variables. The technique is used to extend several seasonal streamflow forecast series so that the generated forecasts have the appropriate mean, variance, and forecasting power. A multivariate model is developed which can extend several series simultaneously.
Interpretation of oxygen and sulfur isotopes from dissolved sulfates in tills of southern Alberta, Canada
Authors: Hendry M. Jim , Krouse H. R. , Shakur M. A.
Published: February 2008

Tills of the Interior Plains Region of southern Alberta, Canada, consist of an upper sulfate‐rich weathered zone and a lower sulfate‐deficient nonweathered zone. The δ18O of aqueous sulfate from the nonweathered zone
was enriched over the δ18O of the weathered zone
; however, no statistical difference was observed in the δ34S of the aqueous sulfate from the two zones
. The δ18O and δ34S data from the aqueous sulfate provide independent support for an earlier finding that most sulfate in the weathered zone is derived from the oxidation of reduced sulfur. The )δ18O of aqueous sulfate from the nonweathered zone is different from that of the associated groundwaters
, whereas the δ18O of aqueous sulfate from the weathered zone is similar to that of the groundwater
. These data are not consistent with equilibrium isotopic exchange between the aqueous sulfate and the groundwater and suggest that the rate of equilibration is very slow because the groundwater and sulfate in the nonweathered zone have been in contact for up to 37,000 years. Isotopic balance calculations imply that most of the oxygen in the sulfate produced by the oxidation of reduced sulfur in the weathered zone is incorporated from the associated water molecules.
Western irrigation response to pumping costs: A water demand analysis using climatic regions
Authors: Ogg Clayton W. , Gollehon Noel R.
Published: February 2008

Inelastic groundwater demand estimates were obtained for field crops throughout western regions. Results suggest that water pricing policies to achieve water conservation and water allocation efficiency affect income distributions as well as the intended objectives. Findings support earlier inelastic demand estimates from studies using experimental data, although demand for groundwater varies somewhat among climatic regions.
A discrete kernel method of characteristics model of solute transport in water table aquifers
Authors: Illangasekare Tissa H. , Döll Petra
Published: February 2008

A simulation model of two‐dimensional solute transport in water table aquifers is developed in this paper. The linearity of the governing partial differential equation of groundwater flow is exploited by using the discrete kernel approach that has been successfully employed previously in groundwater flow modeling. With this approach, the flow velocities resulting from different pumping and injection schemes can be efficiently computed. The conventional implementation of the method of characteristics is improved using an influence area particle tracking scheme that avoids the oscillations and step jumps of breakthrough curves. The concept of a porous medium consisting of mobile and immobile zones is introduced into the solution of the two‐dimensional advection dispersion equation. Model results are compared with those generated with an analytical solution and with a conventional method of characteristics model.
Contaminant transport and biodegradation: 1. A numerical model for reactive transport in porous media
Authors: Celia Michael A. , Kindred J. Scott , Herrera Ismael
Published: February 2008

A new numerical solution procedure is presented for simulation of reactive transport in porous media. The new procedure, which is referred to as an optimal test function (OTF) method, is formulated so that it systematically adapts to the changing character of the governing partial differential equation. Relative importance of diffusion, advection, and reaction are directly incorporated into the numerical approximation by judicious choice of the test, or weight, function that appears in the weak form of the equation. Specific algorithms are presented to solve a general class of nonlinear, multispecies transport equations. This includes a variety of models of subsurface contaminant transport with biodegradation.
A physically based model of heterogeneous hillslopes: 1. Runoff production
Authors: Binley Andrew , Elgy John , Beven Keith
Published: February 2008

A fully three‐dimensional model of variably saturated flow on a hillslope has been used to explore the effects of different random patterns of saturated hydraulic conductivity on a 150 m by 100 m hillslope. Both surface and subsurface runoff production are simulated. The model's simulations suggest that peak discharges and runoff volumes are generally increased by the presence of heterogeneity, increasing with increasing variance and spatial dependence of the underlying random field. Simulations using different realizations with the same random field parameters show that for the case of soils showing no dependence, differences in runoff production between realizations are small at this scale of hillslope. The differences increase with increasing spatial dependence, particularly for the case of low‐conductivity soils producing surface runoff.
Some further evidence on the derived demand for irrigation electricity: A dual cost function approach
Authors: Connor Jeffery D. , Glyer J. David , Adams Richard M.
Published: February 2008

Most crop production in the western United States is dependent on water from irrigation, often using substantial amounts of energy to obtain and apply such water. The sensitivity of crop production to rising energy costs is an issue of concern to producers, input suppliers, and others. In this paper the effects of increasing electricity prices on electricity consumption and other production inputs are assessed using a dual cost estimation procedure. The empirical focus is on selected regions of the western United States where electricity is the primary source of irrigation energy. Demand for irrigation electricity varies across regions but is found to be price elastic.
Field scale transport of bromide in an unsaturated soil: 1. Experimental methodology and results
Authors: Butters Greg L. , Jury William A. , Ernst Frederick F.
Published: February 2008

Modeling the field scale movement of chemicals in unsaturated soil is of intense interest to both the public and private sectors and has become an area of active theoretical research in a number of environmentally based disciplines. However, the experimental data needed to validate existing solute transport models and to inspire the development of more refined approaches is very limited. In this research study, the movement of a mobile tracer (Br−) was monitored as it moved through the unsaturated zone beneath the soil surface of a 0.64 ha loamy sand field. Under flux‐controlled, steady state water flow achieved by bidaily sprinkler irrigation, a narrow pulse of 58.9 mol/m−3 NaBr(aq) was applied uniformly to the field and subsequently leached downward while monitored by vacuum solution samplers replicated 16 times at each of 6 depths between 0.3 and 3.0 m and 6 times at the 4.5‐m depth. Six deep soil cores to a maximum of 25 m were taken to characterize the final field average bromide depth profile after the pulse had passed the 4.5‐m depth. Although the mass recovery of the area‐averaged pulse was near 100% at all depths, the coefficient of variation (CV) of mass recovery between samplers at a given depth was near 50%. Lateral variations in apparent vertical solute velocity or in solute transport volume were considerable, with CVs near 50% in the shallow monitoring depths. However, variations in transport volume with depth at a given site were also large, even though the solution samplers for different depths were displaced laterally by only 0.3–0.6 m at different sites. The mean vertical velocity of the area‐averaged solute pulse was significantly less than the ratio of the average net water flux to the average volumetric water content, until approximately 1.8 m. The difference between the two average velocities near the surface was large enough (nearly a factor of 2) to suggest that transient effects from the bidaily irrigations were influencing solute transport.
Influence of slope on subsurface drainage of hillsides
Authors: Fipps G. , Skaggs R. W.
Published: February 2008

Hillside drainage was investigated using numerical solutions to the two‐dimensional form of the Richards equation for combined unsaturated and saturated flow. The objectives of the study were to examine the influence of slope on drain flow rates and water table depths in the center regions of sloping hillsides. Solutions were obtained for slopes 135 m long, containing parallel drains spaced 30 m apart and located l m below the surface in 3‐ and 7‐m‐deep profiles. Drainage under steady infiltration and for transient conditions was simulated on slopes of up to 0.40 m/m using two soils. Small slopes (≤0.15 m/m) were found to have little effect on drain flow rates and water table depths in the center of the region, indicating that drainage theory developed for flat land can be used for interior parallel drains on these slopes.