bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Heterogeneous Adsorption Behavior on Ion Transport
Authors: Parker J. C. , Jardine P. M.
Published: February 2008

A nonlinear, nonequilibrium, two‐site adsorption model is employed to describe ion transport in soil subject to binary monovalent‐divalent cation exchange. The model assumes that adsorption sites can be divided into two fractions each exhibiting different equilibrium and kinetic adsorption behavior. Equilibrium isotherms for binary exchange at constant ionic strength are described by the Vanselow equation with selectivity coefficients, kν, for each site taken as functions of the solid phase equivalent fractions of exchanging ions. To evaluate the effects of concentration‐dependent selectivity on ion transport, a numerical solution of the convection‐dispersion equation incorporating the nonlinear absorption model was used to simulate K‐Ca exchange for a sandy soil using measured concentration‐dependent kν reported in a previous study for equilibrium‐ and kinetic‐controlled adsorption sites. Breakthrough curves are simulated for various solution ionic strengths and kinetic site rate coefficients using (1) measured kν functions, (2) single‐valued kν obtained by different procedures, and (3) an otherwise analogous model with a linearized isotherm. Errors which may be incurred by employing the various simplified models for the direct problem solution, as well as implications of parameter estimation analyses for the inverse problem, are discussed.
Periodic Gamma Autoregressive Processes for Operational Hydrology
Authors: Fernandez Bonifacio , Salas Jose D.
Published: February 2008

A number of models have been suggested for hydrologic time series in general and streamflow series in particular. Most of them are normal autoregressive (AR) of order 1 with either constant or periodic parameters. Since generally hydrologic time series are nonnormal (skewed), transformations have been suggested to make the series approximately normal. A new class of univariate models is proposed herein which incorporates skewed and correlation properties within the model structure without the necessity of transformations. Such models assume a gamma marginal distribution and a constant or periodic autoregressive structure. The models may be additive gamma, multiplicative gamma, or a mixed model which incorporates properties of both additive and multiplicative models. The gamma models were tested and compared in relation to (transformed) normal AR models by computer simulation studies based on five weekly streamflow series with samples varying from 35 to 40 years of record. The results show that the new class of gamma models compares favorably with respect to the normal models in reproducing the basic statistics usually analyzed for streamflow simulation. It is expected that the proposed gamma models will be of interest to other researchers for further developments and applications to hydrologic and geophysical time series.
Groundwater Seepage Vectors and the Potential for Hillslope Failure and Debris Flow Mobilization
Authors: Iverson Richard M. , Major Jon J.
Published: February 2008

Insight for understanding the effect of groundwater flow on the potential for hillslope failure and liquefaction is provided by a novel limit‐equilibrium analysis of infinite slopes with steady, uniform Darcian seepage of arbitrary magnitude and direction. Normalization of the limit‐equilibrium solution shows that three dimensionless parameters govern completely the Coulomb failure potential of saturated, cohesionless, infinite homogeneous hillslopes: (1) the ratio of seepage force magnitude to gravitational body force magnitude; (2) the angle θ − Φ, where θ is the surface slope angle and Φ is the angle of internal friction of the soil; and (3) the angle λ + Φ, where λ is the angle of the seepage vector measured with respect to an outward‐directed surface‐normal vector. An additional dimensionless parameter affects the solution if soil cohesion is included in the analysis. Representation of the normalized solution as a single family of curves shows that minimum slope stability universally occurs when the seepage direction is given by λ = 90° − Φ. It also shows that for some upward seepage conditions, slope stability is limited by static liquefaction rather than by Coulomb failure. Close association between these liquefaction conditions and certain Coulomb failure conditions indicates that slope failure in such instances could be responsible for nearly spontaneous mobilization of destructive flowing soil masses on hillslopes.
Solute transport parallel to an interface separating two different porous materials
Authors: Van Duijn C. J. , van der Zee S. E. A. T. M.
Published: February 2008

The transport of solute is studied for a flow domain consisting of two regions that are separated by a sharp interface parallel to the direction of water flow. The two regions have different flow velocities, linear adsorption coefficients, and porosities. An approximate analytical solution is given for the depletion of solute in the most permeable region caused by transfer of solute into the less permeable region. To take into account the boundary conditions at the interface, an approximate expression is derived for the concentration at the interface. In the derivations the longitudinal dispersion coefficient is assumed to be zero, and the transversal dispersion coefficient is taken finite. For comparison to numerical results, an expression for the concentration averaged over the height in the permeable region, at given distance and time, is presented. The agreement of numerically obtained breakthrough curves and interface concentrations with the analytical results is shown. Because of the zero longitudinal dispersion coefficient in the analytical approach, differences occur at initial breakthrough. Agreement between numerical and analytical results is good after initial breakthrough provided the assumption of two infinitely thick regions is valid. Lower bound constraints for the thickness of the two regions are given. The assumption made often of infinite transversal dispersion leads to significant differences compared to numerical results in the case of a large retardation factor R2 in the region with the smallest transport velocity.
Green and Ampt analyses for rising water table and intermittent surface flux conditions
Authors: Watson K. K. , Awadalla S. A.
Published: February 2008

Equations developed for a Green and Ampt type analysis for moving water table conditions in an initially saturated homogeneous profile of porous material and used previously in a falling water table study are now applied to a falling‐rising water table sequence in a sand. The ability of the developed equations to be used in a system involving a directional change in water table movement relates to the inclusion in the equations of a term describing the initial saturated depth applicable to each new stage of the analysis. The predictive capability of the Green and Ampt approach for such a system is tested by comparing the results with those obtained from a computer‐based numerical analysis. The relevant subroutine in the computer program required certain restructuring to accommodate the rising water table component. An interpolative hysteresis model is used to describe the accompanying wetting process with the material in question wetting up along primary wetting scanning curves. In comparison with the complexity of the real system during rewetting, the simplicity of the Green and Ampt approach is very marked. Notwithstanding this, it is found that reasonable correspondence occurs, although care is necessary in specifying an appropriate value, applicable to the wetting process, for the characteristic pressure head at the sharp front. The Green and Ampt approach is also extended to include a falling water table system where an applied surface flux of constant magnitude is discontinued during the gravity drainage process. The analysis can be readily adapted to such intermittent inflow and, again, comparisons with a numerical simulation indicate that satisfactory accuracy is achievable.
The “physics” of soil water physics
Authors: Sposito Garrison
Published: February 2008

Some aspects of the underlying conceptualizations (as opposed to experimental methodologies or mathematical algorithms) in soil water physics are reviewed. Contemporary issues relating to the symmetry properties of the Richards Equation, the status of the energy picture of soil water, and the theoretical description of coupled heat and water flows in soil are raised to exemplify as yet unresolved problems in the “physics” of soil water physics. Four basic questions which formalize these problems are posed as suggestions for future research.
Development and evaluation of closed‐form expressions for hysteretic soil hydraulic properties
Authors: Kool J. B. , Parker J. C.
Published: February 2008

A concise representation of hysteretic soil hydraulic properties is given based on a combination of M. T. van Genuchten's (1980) parametric K‐θ‐h model and P. S. Scott et al.'s (1983) empirical hysteresis model modified to account for air entrapment. The resulting model yields compact closed‐form expressions for the hysteretic water retention curve θ(h) and soil water capacity C(h), as well as for the hydraulic conductivity function K(h). Depending on the degree of simplification involved, the model entails a total of 6–9 parameters which can be calibrated from direct measurements of θ(h) and saturated conductivity or by an inverse solution approach from transient flow experiments. Comparison of model‐predicted and measured K‐θ‐h relations for eight soils revealed one case in which model predictions were very poor. Model accuracy was judged to be acceptably good in the other cases. Mualern's modified (Y. Mualem, 1984) dependent domain model was found to be more accurate for soils with very narrow pore size distributions. Use of a simplified version of the proposed model with two parameters eliminated provided overall accuracy very similar to that of the more complex model. Numerical simulations of flow during transient infiltration and drainage using the proposed model and a variant of Y. Mualem's (1984) modified dependent domain model did not differ greatly and agreed reasonably well with experimental water content distributions, even when scanning curves were not described very accurately. By contrast, simulations without consideration of hysteresis produced highly unacceptable results. It is concluded that the proposed model provides a convenient and simple means of incorporating hysteretic effects into numerical flow models to provide significant improvement in prediction accuracy with little additional effort and with minimal data requirements.
Some dynamic aspects of river geometry
Authors: Yu Bofu , Wolman M. Gordon
Published: February 2008

Natural alluvial river channels, in contrast to most regime or equilibrium canals, are characterized by variable streamflows. This paper relates channel geometry to measures of streamflow variability as distinct from mean discharge or flow magnitude. A relationship between mean channel width and the mean and coefficient of variation of channel‐forming discharges is derived. For the given mean discharge it is shown that the more variable the flow, the narrower the Channel is in the mean. To a great extent, this relationship is confirmed by data on alluvial channels in the Missouri River basin. Variability of water discharges proves to be an important factor influencing natural river geometry. This initial approach to the understanding of the dynamics of river geometry leads to the construction of a simple simulation model in which a series of channel forming discharges is generated and corresponding changing channel geometry is calculated. Results from the simulation indicate (1) the channel has, in effect, a kind of truncated memory; i.e., the impact of an existing channel exists so long as the prevailing discharge does not exceed the present channel capacity; (2) the effect of memory or the impact of the existing channel on the mean channel geometry is opposite to that of flow variability (i.e., the memory tends to keep the channel geometry larger in the mean); (3) the simulated channel width, as expected, increases abruptly at high discharges, declining over time along a die‐away curve until the advent of a recorded high flow.
A boundary element‐finite element procedure for porous and fractured media flow
Authors: Elsworth Derek
Published: February 2008

A coupled boundary element‐finite element procedure is presented for linear and nonlinear fluid flow simulation in porous and fractured aquifers. Quadratic variation of both element geometry and fundamental singularity is used in the constitutively linear direct boundary element formulation. Compatible 3‐to 9‐noded Lagrangian finite elements are used to represent the plane flow domain for mixed linear and nonlinear flows, alike. Nodes on the external contour of the boundary element domain are only retained if flux boundary conditions are not prescribed, thus resulting in reduced matrix dimension. The geometric conductance of the linear boundary element region is evaluated only once. The resulting system matrices remain sparse, positive definite, and may be arranged for symmetry. Nonlinearity, in this context, is restricted to turbulent flow at high Reynolds numbers, although other nonlinearities may be easily accommodated using a similar procedure. A Missbach relationship is implemented to represent turbulent flow in rock fractures. Turbulent effects are confined to the finite element domain, and the resulting nonlinear equations are solved by direct iteration. Validation studies are completed against analytical solutions to linear and nonlinear flow problems. Excellent agreement is obtained with relatively sparing nodal coverage.
Balancing public trust resources of Mono Lake and Los Angeles' Water Right: An economic approach
Authors: Loomis John B.
Published: February 2008

The contingent valuation method (CVM) is used to quantify the Public Trust values of Mono Lake at alternative lake levels. The dichotomous choice approach to contingent valuation is employed using a logit model. The economic benefit to California residents of preserving Mono Lake is estimated to be $1.5 billion. Purchase of replacement water and power would cost $26.2 million annually. On efficiency grounds, reallocation of water for maintenance of Public Trust values at Mono Lake is warranted. The CVM appears to be a useful methodology to evaluate the balancing and feasibility tests of the expanded Public Trust doctrine suggested by the California Supreme Court.
Anthropogenic anoxification (“eutrophication”) of the water table region of a deep phreatic aquifer
Authors: Ronen Daniel , Magaritz Mordeckai , Almon Ehud , Amiel Abraham J.
Published: February 2008

Depletion of dissolved oxygen was detected at the water table region (upper 2.5 m) of a 30‐m‐deep, sandy, phreatic aquifer containing high oxygen concentrations (7.5 mg L−1) in bulk groundwater. Samples spaced at 3‐cm intervals disclosed very sharp oxygen gradients of up to 1.9 mg O2 L−1 cm−1 between two consecutive samples. High concentrations of labile organic matter arrive at the water table region after a transport period of more than 15 years through the unsaturated zone. Part of the organic matter oxidizes at the water table region, as evidenced by the decrease in the dissolved oxygen content. The concentration of the remaining dissolved organic matter can be as high as 8 mg C L−1, decreasing with depth. The dissolved organic carbon derives from a large pool of water soluble organic matter found in the unsaturated zone. The average oxygen and organic carbon fluxes at the unsaturated‐saturated interface are 8.5 × 10−2 mg O2 and 3.1 × 10 −2 mg Corg cm −2 yr −1, respectively. Anaerobiosis may result either from the high organic load (140 kg Corg ha−1 yr−1) provided by irrigation with sewage effluent since the 1960s or from the retarded influx of organic matter mobilized at the onset of the intensive agricultural development of the area in the 1930s. The high oxygen content in deep pumping wells and the positive correlation found between the depth of the reduced zone and the organic loads from sewage effluents supports the first model.
Groundwater flow to a well in a layered porous medium: 1. Steady flow
Authors: Maas C.
Published: February 2008

Analytical solutions of steady multiple‐aquifer problems usually assume the flow of groundwater to be essentially horizontal in aquifers and vertical in aquitards. These assumptions yield formulae that are economical to evaluate. The simplifications, however, ask for a theoretical check. This paper is dealing with the derivation of an analytical formula that may serve as such. It considers steady flow of groundwater to a partially screened well in a layered porous medium. Both horizontal and vertical flow components are taken into account.
Are rain rate processes self‐similar?
Authors: Kedem Benjamin , Chiu Long S.
Published: February 2008

We clarify the implication, with respect to self‐similarity, of the facts that the probability of rain tends to increase with the size of the area of observation and that the distribution of rain rate contains an atom at zero. These properties observed in precipitation processes lead to subtle difficulties which are not compatible with self‐similarity.
Stochastic water quality optimization using imbedded chance constraints
Authors: Ellis J. Hugh
Published: February 2008

A chance‐constrained stochastic programming model is developed for water quality optimization. It determines the least cost allocation of waste treatment plant biochemical oxygen demand (BOD) removal efficiencies, subject to probabilistic restrictions on maximum allowable instream dissolved oxygen deficit. The new model extends well beyond traditional approaches that assume streamflow is the sole random variable. In addition to streamflow, other random variables in the model are initial in‐stream BOD level and dissolved oxygen (DO) deficit; waste outfall flow rates, BOD levels and DO deficits; deoxygenation k1, reaeration k2, and sedimentation‐scour rate k3 coefficients of the Streeter‐Phelps DO sag model; photosynthetic input‐benthic depletion rates Ai, and nonpoint source BOD input rate Pi for the Camp‐Dobbins extensions to the Streeter‐Phelps model. These random variables appear in more highly aggregated terms which in turn form part of the probabilistic constraints of the water quality optimization model. Stochastic simulation procedures for estimating the probability density functions and covariances of these aggregated terms are discussed. A new chance‐constrained programming variant, imbedded chance constraints, is presented along with an example application. In effect, this method imbeds a chance constraint within a chance constraint in a manner which is loosely associated with the distribution‐free method of chance‐constrained programming. It permits the selection of nonexpected value realizations of the mean and variance estimates employed in the deterministic equivalents of traditional chance‐constrained models. As well, it provides a convenient mechanism for generating constraint probability response surfaces. A joint chance‐constrained formulation is also presented which illustrates the possibility for prescription of an overall system reliability level, rather than reach‐by‐reach reliability assignment.
Measurements of cesium and strontium diffusion in biotite gneiss
Authors: Skagius Kristina , Neretnieks Ivars
Published: February 2008

A significant retardation of radionuclides transported by flowing water from an underground repository can be expected if the nuclides are able to diffuse into the water filled micropores in the rock. This diffusion into the pores will also increase the surface available to interactions between the nuclides in the groundwater and the rock material, such as sorption. To calculate the retardation, it is necessary to know the sorption properties and the diffusivities in the rock matrix for the radionuclides. Diffusion experiments with cesium and strontium in biotite gneiss samples have been performed. Both the transport of strontium and cesium through rock samples and the concentration profiles of cesium and strontium inside rock samples have been determined. The result shows that diffusion of cesium and strontium occurs in the rock material. A diffusion model has been used to evaluate the diffusivity. Both pore diffusion and surface diffusion had to be included in the model to give good agreement with the experimental data. If surface diffusion is not included in the model, the effective pore diffusivity that gives the best fit to the experimental data is found to be higher than expected from earlier measurements of iodide diffusion in the same type of rock material. This indicates that the diffusion of cesium and strontium (sorbing components) in rock material is caused by both pore diffusion and surface diffusion acting in parallel.
Hydrogeochemical cycling and chemical denudation in the Fort River Watershed, central Massachusetts: An appraisal of mass‐balance studies
Authors: Yuretich Richard F. , Batchelder Gail L.
Published: February 2008

The Fort River watershed in central Massachusetts receives precipitation with a composition similar to that in Hubbard Brook (New Hampshire), yet the average stream water chemistry is substantially different, showing higher pH and TDS. This is largely a function of bedrock and surficial geology, and chemical differences among small streams within the Fort River watershed are apparently controlled by the composition and thickness of the prevailing surficial cover. The surficial deposits determine groundwater and surface water flow paths, thereby affecting the resultant contact time with mineral matter and the chemistry of the runoff. Despite the rural setting, over 95% of the annual sodium and chloride in the streams comes from road salt; after correcting for this factor, cation denudation rates are about equal to those at Hubbard Brook. However, silica removal is occurring at a rate more than 30% greater in the Fort River. When climatic conditions in Hubbard Brook and Fort River are normalized, weathering rates appear consistently higher in the Fort River, reflecting differences in weathering processes (i.e., cation exchange and silicate breakdown) and hydrogeology. Because of uncertainties in mechanisms of cation removal from watersheds, the silica denudation rate may be a better index of weathering intensity.
Processes affecting the distribution of selenium in shallow groundwater of agricultural areas, western San Joaquin Valley, California
Authors: Deverel S. J. , Fujii Roger
Published: February 2008

A study was undertaken to evaluate the processes affecting the chemistry of shallow groundwater associated with agricultural drainage systems in the western San Joaquin Valley, California. The study was prompted by a need for an understanding of selenium mobility in areas having high selenium concentrations in shallow groundwater. Groundwater samples were collected along transects in three artificially drained fields where the age of the drainage system varied (15, 6, and 1.5 years). Selenium concentrations in the drain water also varied (430, 58, and 3700 μg/L, respectively). Isotopic enrichment and chemical composition of the groundwater samples indicate that saline‐ and selenium‐enriched water has evolved as a result of evaporation or transpiration of groundwater. This evaporated, isotopically enriched water is being displaced by more recent, less saline irrigation water percolating through the root zone. This displacement seems to be a process whereby sodium chloride and sodium sulfate water is being replaced by more dilute calcium sulfate and calcium bicarbonate water.
Capacity sharing of water reservoirs
Authors: Dudley Norman J. , Musgrave Warren F.
Published: February 2008

The concept of a water use property right is developed which does not apply to water volumes as such but to a share of the capacity (not contents) of river storage reservoirs and their inflows. The shareholders can withdraw water from their share over time in accordance with their preferences for stability of water deliveries. The reservoir authority does not manage reservoir releases but keeps record of individual shareholder's withdrawals and net inflows to monitor the quantity of water in each shareholder's capacity share. A surplus of total reservoir contents over the sum of the contents of the individual shareholder's capacity shares will accrue over time. Two different criteria for its periodic distribution among shareholders are compared. A previous paper Dudley (this issue(b)) noted a loss of short‐run economic efficiency as reservoir and farm management decision making become separated. This is largely overcome by capacity sharing which allows each user to integrate the management of their portion of the reservoir and their farming operations. The nonattenuated nature of the capacity sharing water rights also promotes long‐run economic efficiency.
On the diffusion wave model for overland flow: 2. Steady state analysis
Authors: Govindaraju R. S. , Jones S. E. , Kavvas M. L.
Published: February 2008

Following our study in the first paper (Govindaraju et al., this issue) on steep slopes, we are now presenting some results on analysis of the steady state phase of the solution. It is physically obvious that a steady state is reached for constant rainfall, since the depth of overland flow at the outflow section increases until equilibrium is achieved and continuity is satisfied. Numerical and analytical steady state results for flux type boundary conditions are presented as new material in this paper. The upstream boundary condition is one of zero inflow. Both the zero‐depth gradient and the critical depth down‐stream boundary conditions are investigated here. For steep slope situations, the upstream boundary condition of zero depth adopted in paper 1 of these companion papers is found to be justified, A method of finding the complete solution (i.e., including the time‐dependent part) by superimposing the steady state profile and a transient component is discussed for the zero‐depth gradient lower end condition. The steady state results clearly show that the critical depth condition at the downstream boundary is a stringent requirement and therefore is likely to pose problems. This helps us to understand why some of the numerical techniques used by previous researchers failed for certain parameters.
Analyses of equations for free water evaporation estimates
Authors: Warnaka Karen , Pochop Larry
Published: February 2008

A comparison of six equations which estimate free water evaporation using climatological data show that the equations vary greatly in their ability to define the magnitude and variability of evaporation. The equations compared included the Kohler‐Nordenson‐Fox, Kohler‐Parmele, Linacre, Priestley‐Taylor, Stewart‐Rouse, and deBruin, which range from equations requiring only temperature data to those requiring temperature, radiation, wind, and humidity. Several years of monthly estimates for May through September were made at two locations in Wyoming using each of the six equations. Means and standard deviations of the monthly data were calculated and t‐tests and regressions comparing the data from the various equations and against pan data were performed.
Condensed disaggregation procedures and conservation corrections for stochastic hydrology
Authors: Grygier Jan C. , Stedinger Jery R.
Published: February 2008

Disaggregation models are perhaps the best available method for generating multiseasonal multsite streamflow sequences for use in water resource system simulations. They are the basis of two general packages: LAST and SPIGOT. Both packages use staged disaggregation procedures and condensed disaggregation models to avoid the excessive number of parameters required by the multisite monthly Valencia‐Schaake model. This paper examines the model size issue and what is gained and lost by the use of condensed models. In particular, when modeling the normal transforms of streamflow volumes, generated seasonal flows generally fail to sum to the specified annual total. A Monte Carlo study demonstrates that the discrepancy can be quite large and that alternative correction schemes perform quite differently.
Twin Lake Tracer Tests: Transverse dispersion
Authors: Moltyaner G. L. , Killey R. W. D.
Published: February 2008

Two natural‐gradient dispersion tests were performed at the Chalk River Nuclear Laboratories in Ontario in 1982 and 1983. During the field tests 131I was used to label the groundwater, and its activity was measured in situ, collecting continuous records of activity versus depth at selected times and of activity versus time at selected locations. Curves generated by a three‐dimensional analytical solution to the advection dispersion equation were fit to activity versus depth scans recorded at various locations in the test aquifer. Within the individual velocity regions the vertical dispersivities were 0.0006 and 0.0008 m, with the arithmetic average 0.0007 m as calculated from the 1982 data. The vertical dispersivity values calculated from 1983 data were 0.0014 and 0.0010 m for the high‐ and intermediate‐velocity regions, respectively, with an arithmetic average 0.0012 m. The increase in dispersivity was attributed to the error associated with the sampling and fitting procedures. The average vertical dispersivity of the tracer test aquifer was an order of magnitude less than the average longitudinal dispersivity (0.0010 m versus 0.010 m for the intermediate‐velocity region). Within individual velocity regions the vertical dispersivity of the porous medium was independent of the travel distance of the tracer and was proportional to the velocity.
Groundwater circulation in a closed desert basin: Topographic scaling and climatic forcing
Authors: Duffy Christopher J. , Al‐Hassan Sumani
Published: February 2008

The circulation of groundwater and solutes in a closed basin in western Utah (Pilot Valley) is studied to gain insight into the climatic processes driving flow in this desert environment. The study area is located within the Great Basin portion of the Basin and Range Physiographic Province, a region which is hydrologically closed with no outlet except to the atmosphere. The distinctive topography of steep mountains adjacent to flat playas has a major influence on the climatic and hydrologic patterns of the region. A scaling relationship is found for the distribution of elevation in Pilot Valley, suggesting that the mountain slopes are self‐similar. This relationship is used to approximate the spatial distribution of orographic precipitation and recharge for groundwater model input. On the playa, or discharge zone, high evaporation rates produce concentrated brine solutions which lead to large horizontal and vertical density gradients in underlying groundwater. Numerical experiments demonstrate the existence of a free convection cell and recirculation of solutes under the playa and towards the basin margin. It is shown that the motion of the salt nose which forms is a result of a balance between the buoyancy velocity of the evaporating brines on the playa and recharge from the mountain range. A Rayleigh number for solutes is defined for basin scale hydroclimatic parameters. The simulated flow pattern is consistent with field observations in Pilot Valley and elsewhere, in that both fresh and brackish springs emerge along the margin of the playa. The concept of recirculation may also account for the apparent “loss” of salts in closed lakes as a subsurface storage mechanism.
Stochastic analysis of boundaries effects on head spatial variability in heterogeneous aquifers: 1. Constant head boundary
Authors: Rubin Yoram , Dagan Gedeon
Published: February 2008

The spatial variability of log transmissivity Y and head H in aquifers assumes a major role in determining the solutions of the direct flow problem, as well as of the inverse problem. Investigation of the joint Y, H variability has been cast in the past in numerical and analytic frames. Analytic solutions for the Y, H statistical moments in two dimensions assumed the flow to occur in an unbounded domain. In this study, exact analytical expressions for the Y, H moments are derived, which account for the presence of a given head boundary in a half plane. This result is achieved by a first‐order approximation and for an exponential Y covariance. The result offers the opportunity to examine the effects of boundaries on the semivariogram of H and on the Y, H cross covariance. The assumption of unbounded domain is examined and shown to be accurate at distances larger than two Y integral scales from the boundary. By using conditional probabilities, a general and simple method to estimate the moments close to the boundary is presented. It requires the knowledge of the moments of Y and H in the unbounded domain only. The method is compared with the exact analytic solution, showing excellent results.
State‐space dynamic hydrological modeling of soil‐crop‐climate interactions
Authors: Protopapas Angelos L. , Bras Rafael L.
Published: February 2008

The basic biological and hydrological processes of plant growth, such as CO2 assimilation, transpiration, growth, maintenance, and water uptake are discussed and quantified mathematically. A model is constructed in state‐space form to simulate the seasonal growth of a crop under variable climatic forcing. The model takes into account the soil moisture and salinity profiles in a soil column. Salinity and moisture profiles are obtained by numerically solving the relevant mass transport equations. The morphology of the root system, which controls sink terms in the equation for transient unsaturated‐saturated water transport, is explicitly determined by distributing the produced root biomass over depth so that the water uptake rate is maximized. The salinity concentration profile is used for predicting osmotic effects on the behavior of roots. A case study for maize is presented. It is shown that the crop response depends drastically on the climatic forcing, the soil salinity, and the irrigation practice. Necessary conditions for optimal growth are investigated. The temporal variation of many crop and soil variables is discussed. Finally, the ability of the model to produce the crop production function is demonstrated.
Estimation of descriptive statistics for multiply censored water quality data
Authors: Helsel Dennis R. , Cohn Timothy A.
Published: February 2008

This paper extends the work of Gilliom and Helsel (1986) on procedures for estimating descriptive statistics of water quality data that contain “less than” observations. Previously, procedures were evaluated when only one detection limit was present. Here we investigate the performance of estimators for data that have multiple detection limits. Probability plotting and maximum likelihood methods perform substantially better than simple substitution procedures now commonly in use. Therefore simple substitution procedures (e.g., substitution of the detection limit) should be avoided. Probability plotting methods are more robust than maximum likelihood methods to misspecification of the parent distribution and their use should be encouraged in the typical situation where the parent distribution is unknown. When utilized correctly, less than values frequently contain nearly as much information for estimating population moments and quantiles as would the same observations had the detection limit been below them.
A critical evaluation of recent developments in hydrogeochemical transport models of reactive multichemical components
Authors: Yeh G. T. , Tripathi V. S.
Published: February 2008

Many hydrogeochemical models have appeared in recent years for simulating subsurface solute transport. The hydrological transport of solutes can be described by a set of linear partial differential equations, and the chemical equilibria are described by a set of nonlinear algebraic equations. Three approaches are currently used to formulate the problem: (1) the mixed differential and algebraic equation (DAE) approach, (2) the direct substitution approach (DSA), and (3) the sequential iteration approach (SIA). An extremely important consideration in any approach is the choice of primary dependent variables (PDVs). Six types of PDVs have been employed in the existing models: (1) concentrations of all species, (2) concentrations of all component species and precipitated species, (3) total analytical concentrations of aqueous components, (4) total dissolved concentrations of aqueous components, (5) concentrations of aqueous component species, and (6) hybrid concentrations. Because of many possible combinations of PDVs and approaches, many hydrogeochemical transport models for multicomponent systems have been developed. This paper critically evaluates and discusses these models. The discussion and evaluation are conducted in terms of (1) how severe can the constraints be that a model imposes on computer resources, (2) which equilibrium geochemical processes can a model include, and (3) how easily can a model be modified to deal with mixed kinetic and equilibrium reactions. The use of SIA models leads to the fewest constraints on computer resources in terms of central processing unit (CPU) memory and CPU time; both DAE and DSA models require excessive CPU memory and CPU time for realistic two‐ and three‐dimensional problems. Only those models that use the first three types of PDVs can treat the full complement of equilibrium reactions simultaneously. DAE and SIA models can be modified with reasonable ease to handle mixed chemical kinetics and equilibria. DSA models require strenuous efforts to modify for treating mixed chemical kinetics and equilibria. Therefore SIA models using the third type of PDVs are recommended for their practicality and flexibility. DSA and DAE models should remain research tools for one‐dimensional investigations.
Regional scale transport in a Karst Aquifer: 2. Linear systems and time moment analysis
Authors: Dreiss Shirley J.
Published: February 2008

Travel time distributions of water or tracers in conduit‐type karst aquifers can be found from linear systems analysis of either tracer test data or naturally occurring fluctuations in springflow chemistry. I use the chemical fluctuations at Maramec Spring, Missouri described in paper 1 (Dreiss, this issue) and results from a previous tracer test to derive a set of kernal functions that represent regional scale transport in the karst conduit network. A single kernel is sufficient to simulate the storm‐derived component of Maramec springflow, suggesting that rapid transport in the conduit network is well‐approximated by temporal stationarity. Time moment analysis of the kernels leads to several conclusions. The kernel for the tracer test exhibits a larger mean residence time and much smaller variance than the kernels derived from nonpoint source recharge. Thus the tracer travel distance appears to be longer than the mean travel distance of rapid recharge and much of the variance of nonpoint source kernels apparently results from the distribution of flow path lengths to the spring. By assuming an effective transport model and comparing the moments of the empirical tracer test kernel to the moments of the impulse response of the model, I compute an effective velocity between the tracer input point and Maramec Spring of 1.3 km/day and an effective dispersivity of 0.29 km. Because the time moments of the kernels and the effective transport parameters can be computed from readily measured springflow properties, they may prove to be a convenient means for studying and comparing regional scale transport in karst aquifers.
Stochastic analysis of nonstationary subsurface solute transport: 1. Unconditional moments
Authors: Graham Wendy , McLaughlin Dennis
Published: February 2008

This paper applies stochastic methods to the analysis and prediction of solute transport in heterogeneous saturated porous media. Partial differential equations for three unconditional ensemble moments (the concentration mean, concentration covariance, and velocity concentration cross covariance) are derived by applying perturbation techniques to the governing transport equation for a conservative solute. Concentration uncertainty is assumed to be the result of unmodeled small‐scale fluctuations in a steady state velocity field. The moment expressions, which describe how each moment evolves over time and space, resemble the classic deterministic advection‐dispersion equation and can be solved using similar methods. A solution procedure based on a Galerkin finite element algorithm is illustrated with a hypothetical two‐dimensional example. For this example the required steady state velocity statistics are obtained from an infinite domain spectral solution of the stochastic groundwater flow equation. The perturbation solution is shown to reproduce the statistics obtained from a Monte Carlo simulation quite well for a natural log conductivity standard deviation of 0.5 and moderately well for a natural log conductivity standard deviation of 1.0. The computational effort required for a perturbation solution is significantly less than that required for a Monte Carlo solution of acceptable accuracy. Sensitivity analyses conducted with the perturbation approach provide qualitative confirmation of a number of results obtained by other investigators for more restrictive special cases.
Estimation of spatial covariance structures by adjoint state maximum likelihood cross validation: 2. Synthetic experiments
Authors: Samper F. Javier , Neuman Shlomo P.
Published: February 2008

Paper 2 of this three‐part series uses synthetic data to investigate the properties of the adjoint state maximum likelihood cross‐validation (ASMLCV) method presented in paper 1 (Samper and Neuman, this issue (a)). More than 40 synthetic experiments are performed to compare various conjugate gradient algorithms; investigate the manner in which computer time varies with ASMLCV parameters; study the effect of sample size and choice of kriging points on ASMLCV estimates ; evaluate the ability of various model structure identification criteria to help select the most appropriate semivariogram model among given alternatives; study the conditions required for parameter identifiability, uniqueness, and stability; quantify the statistics of cross‐validation errors; test hypotheses concerning the distribution and autocorrelation of these errors; and illustrate the computation of approximate quality indicators for ASMLCV parameter estimates.
A channel dynamics model for real‐time flood forecasting
Authors: Hoos Anne B. , Koussis Antonis D. , Beale Guy O.
Published: February 2008

A new channel dynamics scheme (alternative system predictor in real time (ASPIRE)), designed specifically for real‐time river flow forecasting, is introduced to reduce uncertainty in the forecast. ASPIRE is a storage routing model that limits the influence of catchment model forecast errors to the downstream station closest to the catchment. Comparisons with the Muskingum routing scheme in field tests suggest that the ASPIRE scheme can provide more accurate forecasts, probably because discharge observations are used to a maximum advantage and routing reaches (and model errors in each reach) are uncoupled. Using ASPIRE in conjunction with the Kalman filter did not improve forecast accuracy relative to a deterministic updating procedure. Theoretical analysis suggests that this is due to a large process noise to measurement noise ratio.
Influence of silvicultural practices on the hydrology of pine flatwoods in Florida
Authors: Riekerk Hans
Published: February 2008

Slash pine (Pinus elliottii Engelm.) flatwoods in diked watersheds were harvested and regenerated with high and low levels of disturbance. One watershed received machine harvesting, slash burning, windrowing, bedding, and machine planting, which are practices causing a high level of disturbance. A second watershed received manual harvesting, slash chopping, bedding, and machine planting, which are practices causing a low level of disturbance. Water regimes of the two watersheds were compared against that of the third, an undisturbed control. Water table levels were higher after harvest because of reduced evapotranspiration (Et) posttreatment. The differences in water table levels due to treatment were more pronounced at deeper soil depths, probably owing to a lower specific yield of the soil matrix with depth. Daily runoff during the first year increased somewhat in proportion to the area clear‐cut in each watershed. However, only the 15‐cm first‐year runoff increase from the high‐disturbance watershed was significant at 150% of predicted runoff. This increase dropped to 65% of predicted runoff 6 years later, suggesting a return to normal about 11 years after treatment. In contrast, an unexpectedly early decrease in runoff from the low‐disturbance watershed was apparent by the last year indicative of anomalous watershed behavior likely due to greater seepage through the perimeter dikes. A severe drought which occurred during the study caused a very slow recovery of the hydrologie regime; it took a full year of normal rainfall to restore runoff patterns. Moreover, the drought demonstrated the importance of pond storage in the attenuation of surface runoff. High‐disturbance site preparation resulted in many windrows oriented toward the drainages, less resistance to surface runoff, and some compaction of normally wet soils. The net effect was less infiltration and more storm flow. First‐year storm flow was increased on the high‐disturbance watershed, but flooding by large storms often overwhelmed this effect. Pine Et estimates were used in the annual water balances to calculate seepage. A seepage value of 3 cm/yr, or 2% of annual rainfall, showed good agreement with previously published information for pine flatwoods.
An efficient laplace transform solution for multiaquifer systems
Authors: Cheng Alexander H‐D , Ou Kwotsong
Published: February 2008

An efficient numerical algorithm is developed for groundwater flow in multiaquifer systems. The method is based on Herrera's formulation which eliminates the need of explicitly modeling aquitards. Laplace transform is applied to the governing integrodifferential equations, which removes not only the time dependence in the solution system, but also the integral expression. The final system is a set of nontransient, linear partial differential equations in two spatial dimensions. A regular finite difference method is used for its solution. The solution in the Laplace transform domain is then inverted to the time domain using either a slow or a quick scheme, depending on the requirement of speed and accuracy. Examples of a two‐aquifer one‐aquitard system involving radial and regional geometries are examimed.
Regional flood quantile estimation for a Weibull Model
Authors: Boes Duane C. , Heo Jun‐Haeng , Salas Jose D.
Published: February 2008

Estimation of annual flood quantiles at a given site, based on a regional Weibull model with independence in space and time, is considered. A common shape parameter over sites, motivated by an index flood assumption, was assumed. An exact simple formula for the Cramer‐Rao lower bound for the variance of unbiased estimators of the quantile is obtained, and the gain of regional flood frequency analysis over single‐site analysis can be quantified via this formula. The estimation techniques of the method of moments, the method of probability‐weighted moments, and the method of maximum likelihood are compared.