bluuebunny/crossref_metadata_2025 huggingface dataset:
The Variability of Alternative Decisions in a Water Resources Planning Problem
Authors: Harrington Joseph J. , Gidley James S.
Published: February 2008

Optimization models do not perfectly represent the real world. Therefore the mathematically optimal solution is not necessarily the “best” decision to implement. Water resources planning models tend to have many solutions that are nearly optimal in terms of objective function value, but which are significantly different decisions. Methods for generating nearly optimal solutions to linear programming models have been devised. The use of such methods is illustrated with an example problem involving the sizing of reservoirs and hydroelectric power plants and demands for municipal water supply and irrigation. The results show that the interactions among the decision variables and the economics of the system determine the range of choice among nearly optimal decisions. The examination of these interactions yields insight into the basic structure of the decision problem.
Longwave Radiation in Mountainous Areas and Its Influence on the Energy Balance of Alpine Snowfields
Authors: Olyphant Greg A.
Published: February 2008

Field studies in the Colorado Front Range provide data for calibrating and testing a model that computes incoming longwave radiation in alpine terrain. The model estimates longwave irradiance on the basis of empirical terrain and atmospheric radiance distributions. A statistical analysis of estimated versus observed longwave irradiance indicates no difference between the regression line and a line of one‐to‐one correspondence. Simulations of longwave irradiance to cirque floors within the study area reveal subtle variations that are due to differences in ground slope and the configuration of surrounding ridges. The simulated variations in longwave irradiance are not adequately represented by the commonly employed istoropic approximation. Surrounding rockwalls enhance the energy balance of alpine snowfields by reducing their net longwave loss circa 50% below that of an unobstructed ridge top. The added energy is equivalent to roughly 0.5 m of melt when integrated over an entire snowmelt season.
Review of Parameter Identification Procedures in Groundwater Hydrology: The Inverse Problem
Authors: Yeh William W‐G.
Published: February 2008

The purpose of this survey is to review parameter identification procedures in groundwater hydrology and to examine computational techniques which have been developed to solve the inverse problem. Parameter identification methods are classified under the error criterion used in the formulation of the inverse problem. The problem of ill‐posedness in connection with the inverse problem is addressed. Typical inverse solution techniques are highlighted. The review also includes the evaluation of methods used for computing the sensitivity matrix. Statistics which can be used to estimate the parameter uncertainty are outlined. Attempts have been made to compare and contrast representative inverse procedures, and direction for future research is suggested.
Solute Dispersion in Multidimensional Periodic Saturated Porous Media
Authors: Gupta Vijay K. , Bhattacharya R. N.
Published: February 2008

Suppose in a convection‐dispersion equation, governing solute movement in a saturated porous medium of infinite extent, the convection velocity components are periodic functions of spatial coordinates. Then it follows from a general mathematical result that the solute concentration can be asymptotically approximated by a Gaussian density. Two theoretical examples, with and without a constant vertical velocity, are given to illustrate an application of this mathematical result to solute dispersion in a parallel‐bedded, three‐dimensional aquifer of infinite extent. Analytical expressions are obtained for dispersion coefficients in the asymptotic Gaussian approximations in the two examples. The dispersion coefficients functionally depend on a velocity scaling parameter U0 and a spatial scaling parameter (period) a via their product aU0, and this functional dependence on aU0 is different in the two examples. The expressions for dispersion coefficients are formally contrasted and compared with those obtained previously by other authors in a parallel‐bedded infinite aquifer in which the convection velocity is a random field. Some physical implications of dependence of dispersion coefficients on a spatial scaling parameter (i.e., the scale effect) are discussed in the context of modeling in an aquifer with evolving heterogeneities. This interpretation of “scale effect” in dispersion coefficients is contrasted with the preasymptotic interpretation in the current literature.
Estimation of Aquifer Parameters Under Transient and Steady State Conditions: 2. Uniqueness, Stability, and Solution Algorithms
Authors: Carrera Jesus , Neuman Shlomo P.
Published: February 2008

Paper 2 of this three‐part series starts with a discussion of the question, Under what conditions is the aquifer inverse problem well‐posed? After defining the terms uniqueness, identifiability, and stability, theoretical considerations and synthetic examples are used to demonstrate that ill‐posedness can be mitigated by including prior information about the parameters in the estimation criterion to be minimized. At the same time, the inclusion of such information is shown to be insufficient to guarantee uniqueness and stability in all cases. Several test problems in the recent literature, which have resulted in pessimistic conclusions about the solvability of the aquifer inverse problem, are shown to be ill‐posed; a question is thus raised about the validity of these conclusions in the general case. Various conjugate gradient algorithms, coupled with the adjoint state finite element method for computing the gradient of the estimation criterion, and with Newton's method for determining the optimum step size downgradient, are compared. A marked improvement in the rate at which these algorithms converge is shown to be achieved by switching from one method to another when the former slows down or fails to converge.
Porosities and Diffusivities of Some Nonsorbing Species in Crystalline Rocks
Authors: Skagius Kristina , Neretnieks Ivars
Published: February 2008

The diffusion of nonsorbing species in different rock materials and fissure coating materials has been studied on a laboratory scale. The nonsorbing species were iodide, Uranine, and Cr‐EDTA. The results show that the effective diffusivity of iodide in rock materials with fissure coating material is of the same magnitude as or higher than the effective diffusivity of iodide in rock materials without fissure coating material. The results also show that the variations in the rock material are too large to give one value of the diffusivity in a rock material from a certain area. The estimated effective diffusivity of iodide in rock materials without fissure coating material was found to be in the range 1 × 10−14 m2/s to 70 × 10−14m2/s.
Innovative Approaches to Water Allocation: The Potential for Water Markets
Authors: Howe Charles W. , Schurmeier Dennis R. , Shaw W. Douglas
Published: February 2008

Exchange through markets is one way of allocating water. In many settings this method possesses a number of desirable attributes to a greater degree than alternative allocative mechanisms: flexibility, security, predictability, fairness …. Markets have shortcomings, too, especially related to quantity and quality return flow effects, but these can be mitigated through changes in the administrative framework of the water rights system. It is shown that an economically efficient water allocation system must integrate quantity and quality management and would be neither a priority nor a proportional rights system. However, the nature of the water rights system may not be very important if short‐term rental markets are operative. The potential for intrastate and interstate water markets is discussed.
Continuous‐Time Versus Discrete‐Time Point Process Models for Rainfall Occurrence Series
Authors: Foufoula‐Georgiou Efi , Lettenmaier Dennis P.
Published: February 2008

Several authors have had apparent success in applying continuous‐time point process models to rainfall occurrence sequences. In this paper, it is shown that if rainfall occurrences are interpreted as the events of a point process (and not as a censored sample), the continuous‐time point process methodology and estimation procedures are not directly applicable since they fail to account for the time discreteness of the sample process. This is demonstrated analytically by studying the effects of discretization on selected statistical properties of a Poisson process, a Neyman‐Scott process, and a renewal Cox process with Markovian intensity. In general, the study of rainfall occurrences under the continuous‐time point process framework may result in misleading inferences regarding clustering (dispersion), and consequently incorrect interpretations of the underlying rainfall generating mechanisms. For example, daily rainfall occurrence structures underdispersed relative to the Poisson process are usually overdispersed relative to the Bernoulli process (the discrete‐time analogue of the Poisson). These findings are confirmed by the statistical analysis of six daily rainfall records representative of a range of U.S. climates, two of which are described in detail.
A Comparison of Several Methods for the Solution of the Inverse Problem in Two‐Dimensional Steady State Groundwater Flow Modeling
Authors: Kuiper Logan K.
Published: February 2008

Two geostatistical approaches for the estimation of hydraulic conductivity and hydraulic head from hydraulic conductivity and hydraulic head measurements are developed for two‐dimensional steady flow with sinks. For both approaches the field of the logarithm of hydraulic conductivity (log‐conductivity) is represented as a random field with mean θ1+θ2x+θ3y where x and y denote Cartesian coordinates, variance σ2, and covariance σ2, exp (−αR), σ2, exp (−αR2), or, σ2ΔRr where α, Δ, and r are constants and R is separation distance. The first approach uses linearization of the discretized flow equations to allow the construction of the joint covariance matrix of the hydraulic head and log‐conductivity measurements as functions of the parameters θ, θ2, θ3, σ2, and α or Δ. It then uses maximum likelihood estimation to obtain these parameters and also a parameter associated with log‐conductivity measurement error. Having found values for the parameters, it then uses kriging to form predictors for log‐conductivity and hydraulic head from measured values of hydraulic conductivity and hydraulic head. The second approach uses kriging to form a parameter‐dependent predictor for log‐conductivity from measured hydraulic conductivity, and then uses this predicted log‐conductivity placed into the discretized flow equations to compute hydraulic head. The parameters are determined by the minimization of the sum of the squares of the difference between the measured and computed hydraulic heads. A third approach simply allows the hydraulic conductivity field to be a step function with a different value for hydraulic conductivity assigned to each of several chosen regions in the two‐dimensional aquifer. The assigned hydraulic conductivities are determined by the minimization of the sum of the squares of the difference between the measured and computed hydraulic heads. The three approaches are tested for hydraulic head prediction accuracy on two generated test problems, one of which is statistically generated, and also on a field problem. The third approach, despite its simplicity, performs as well or better than the other approaches.
Compatibility of Continuous Rainfall Occurrence Models With Discrete Rainfall Observations
Authors: Foufoula‐Georgiou Efi , Guttorp Peter
Published: February 2008

Inferences about the underlying rainfall generating mechanism made through a model fitted to the sampled rainfall series at a particular time scale are meaningful only if they are invariant under the time scale of measurement at which the fitting of the model is made. Following the work of I. Rodriguez‐Iturbe et al. (1984), this assertion is tested by examining the compatibility of the Neyman‐Scott (N‐S) model for the continuous underlying rainfall with sampled realizations taken over intervals ranging from one hour to one day. The results suggest that the N‐S model is inconsistent at different time scales, and therefore mathematical descriptions of rainfall using this model are inevitably limited to the time scale of measurement at which a description of rainfall is sought. These findings are quite general, in that they apply to any model with rainfall deposited instantaneously at times described by a N‐S process, regardless of how the amounts associated with the events are distributed, and of the dependence structure of the amounts.
Simulation of Leaching of A Gypsiferous‐Sodic Desert Soil
Authors: Russo David
Published: February 2008

Leaching a gypsiferous‐sodic soil under different soil conditions and water qualities was investigated using the transport model of E. Bresler (1973) combined with the salinity model of C. W. Robbins et al. (1980b). Incorporating field‐measured values of the soil hydraulic properties and the dispersivity coefficient, profiles of the soil solution concentration of Cl, SO4, Ca and Na, and soil solution electrical conductivity (EC) obtained under intermittent leaching were simulated for a set of representative cases typical to gypsiferous‐sodic soils. For a given soil depth and amount of applied water the simulated soil solution concentration of chloride was affected only by the initial soil salinity and the salinity of the applied water. Simulated soil solution concentrations of calcium, sodium, and sulfate were also affected by the initial soil exchangeable sodium percentage (ESP), initial gypsum content, and the cation exchange capacity (CEC) of the soil. Sensitivity analysis of the model results suggest that soil CEC, initial soil salinity and ESP, and gypsum content have a significant impact on the soil salinity profiles during leaching. Possible applications of the combined model results for salinity control were demonstrated. Expressing the soil system response in terms of the soil solution EC, rather than the specific ion concentrations, the combinations of water quantity and quality (salinity) which yield a given soil solution EC at a given soil depth for different initial conditions and soil CEC were calculated. For a given salinity of the applied water, the amount of water required to maintain a given EC at a given soil depth increased as the initial soil salinity, soil ESP, gypsum content, and CEC rose. The calculated isosoil salinity lines can be used for economic evaluation of the least cost combination of the applied water quantity and quality which will ensure that a critical salinity at a given soil depth is not exceeded.
Streamline routing through fracture junctions
Authors: Hull Laurence C. , Koslow Karen N.
Published: February 2008

A series of laboratory tests was conducted to determine routing criteria for streamlines through fracture junctions. These tests showed that two criteria are all that is necessary to route streamlines through any two‐dimensional junction under laminar flow conditions. These criteria are (1) that streamlines do not cross and (2) that flow along adjacent streamlines must be in the same direction. Using these two criteria, a unique distribution of streamlines can be determined for both continuous and discontinuous fracture junctions.
Snow distribution patterns in clearings and adjacent forest
Authors: Golding Douglas L. , Swanson Robert H.
Published: February 2008

Snow accumulation patterns were determined for clearings and adjacent forest at Marmot Creek experimental watershed and James River, Alberta. At maximum accumulation snow water equivalent (SWE) was greater in clearings than in forest whether clearings were large, as in 8‐ to 13‐ha blocks where SWE averaged 20% more than in the forest, or small as in the ¼ to 6‐H (height) diameter circular clearings where SWE was 13–45% greater than in the forest. SWE was 42 to 52% less in north than in south sectors of 2–6 H clearings. These differences increased with clearing size and time since beginning of accumulation period and are caused by snow ablation (melt and evaporation), a function of direct solar radiation reaching the snowpack. In such situations the snow that has accumulated on the ground cannot be considered a measure of the snow that has actually fallen there. For water balances and hydrologic modeling, snow measurements in partially cleared watersheds must be adjusted for temporal and spatial factors specific to the watershed.
A natural gradient experiment on solute transport in a sand aquifer: 1. Approach and overview of plume movement
Authors: Mackay D. M. , Freyberg D. L. , Roberts P. V. , Cherry J. A.
Published: February 2008

A large‐scale field experiment on natural gradient transport of solutes in groundwater has been conducted at a site in Borden, Ontario. Well‐defined initial conditions were achieved by the pulse injection of 12 m3 of a uniform solution containing known masses of two inorganic tracers (chloride and bromide) and five halogenated organic chemicals (bromoform, carbon tetrachloride, tetrachloroethylene, 1,2‐dichlorobenzene, and hexachloroethane). A dense, three‐dimensional array of over 5000 sampling points was installed throughout the zone traversed by the solutes. Over 19,900 samples have been collected over a 3‐year period. The tracers followed a linear horizontal trajectory at an approximately constant velocity, both of which compare well with expectations based on water table contours and estimates of hydraulic head gradient, porosity, and hydraulic conductivity. The vertical displacement over the duration of the experiment was small. Spreading was much more pronounced in the horizontal longitudinal than in the horizontal transverse direction; vertical spreading was very small. The organic solutes were retarded in mobility, as expected.
A natural gradient experiment on solute transport in a sand aquifer: 3. Retardation estimates and mass balances for organic solutes
Authors: Roberts Paul V. , Goltz Mark N. , Mackay Douglas M.
Published: February 2008

The long‐term behavior of five organic solutes during transport over a period of 2 years in ground water under natural gradient conditions was characterized quantitatively by means of moment estimates. Total mass was conserved for two of the organic compounds, carbon tetrachloride and tetrachloroethylene, while the total mass declined for three other compounds, bromoform, 1,2‐dichlorobenzene, and hexachloroethane. The declines in mass for the latter three compounds are interpreted as evidence of transformation of the compounds. Retardation factors for the organic solutes, relative to chloride, ranged from 1.5 to 9.0, being generally greater for the more strongly hydrophobic compounds. The retardation is attributed to sorption. The apparent retardation factor increased markedly for all compounds over the duration of the experiment, by as much as 150%. Results from temporal and spatial sampling were in good agreement when compared at the same scale of time and distance.
A generalized stochastic hydrometeorological model for flood and flash‐flood forecasting: 2. Case studies
Authors: Georgakakos Konstantine P.
Published: February 2008

The short‐ and long‐term performance of a stochastic‐dynamic hydrometeorological model in real‐time flood flow forecasting is examined with 6‐hourly data from a 2344‐km2 basin in Oklahoma. The model couples a local precipitation model with a soil and a channel model through (1) the conservation of mass law and (2) an extended Kalman filter. The input consists of surface meteorological data interpolated from observations at nearby National Weather Service stations. Hydrograph peak time and magnitude errors are quantified in an effort to judge performance from an operational hydrologist's point of view. Least squares criteria are also utilized. The effect of the use of forecasts of the input variables on model performance, at various forecast lead times, is indicated. Good overall performance of the model is observed with obvious hydrologic performance deterioration when no precipitation component is used.
Stochastic subsurface hydrology from theory to applications
Authors: Gelhar Lynn W.
Published: February 2008

Research on stochastic analysis of subsurface flow has developed rapidly in the last decade, but applications of this approach have been very limited. The purpose of this paper is to illustrate how currently available techniques and results can be used to answer important questions about the large‐scale behavior of naturally heterogeneous aquifers. Perturbation‐based spectral theory, which presumes local statistical homogeneity, provides generic theoretical results for the head variance, effective conductivity tensor, and macrodispersivity tensor in a field. These results emphasize the key role of the variance and spatial correlation scales of the log hydraulic conductivity field. Field information of variances and correlation scales of natural materials is summarized. The validity of some of the generic stochastic results is evaluated through comparisons with Monte Carlo simulations and field observations. A specific field application example is developed to illustrate how the stochastic results are used to estimate large‐scale parameters and determine the reliability of three‐dimensional numerical simulations. Using typical log conductivity covariance parameters, the effective hydraulic conductivity tensor, and the macrodispersivity tensor are estimated. The calculated head variance, based on the simulated mean hydraulic gradient, is used as a measure of the adequacy of the calculation of the steady state flow model. Discussion emphasizes limitations and extensions of this approach, and ongoing field evaluations of the results.
Use of systems analysis in water management
Authors: Rogers Peter P. , Fiering Myron B
Published: February 2008

Over the past 30 years systems analysis applied to the planning and operation of water resource systems has grown from a mathematical curiosity to a major specialty. We define systems analysis as that set of mathematical planning and design techniques which includes at least some formal optimization procedure. Based on the increasingly large number of systems‐oriented papers which appear in the civil engineering literature, it is not unreasonable to expect that the use of one or another optimization technique would have been undertaken in a significant number of completed projects and described in the literature; this turns out not to be the case. Moreover, U.S. federal agencies and major consultants do not appear to use these techniques in any but a handful of projects. We offer several explanations for this, including institutional resistance to use of the techniques, deficiencies in data bases, inadequacies in modeling, and the fundamental insensitivity of many systems (not merely the models thereof) to wide variations in design choices. We explore the differences between application in developed and developing countries.
An evaluation of data requirements for groundwater contaminant transport modeling
Authors: Chu Wen‐Sen , Strecker Eric W. , Lettenmaier Dennis P.
Published: February 2008

Groundwater flow and contaminant transport models have been widely used for planning and design purposes in the past decade. Two of the most significant limitations for application of these models are data availability and parameter estimation. By use of a parameter identification algorithm and synthesized data, it is possible to isolate the effects of data availability and data uncertainty. This approach was implemented using the U.S. Geological Survey's method of characteristics (USGS‐MOC) model for a hypothetical aquifer. A parameter identification scheme attached to the USGS‐MOC model was used to determine unknown transmissivities and dispersivities. The study results showed that the predictive ability of the USGS‐MOC model (and, by implication, similar models) is limited unless relatively extensive and good quality data are available. For the example tested, it was found that extending the length of the observation series was more effective in improving parameter estimates and resolution of the contaminant plume prediction than adding observation wells. Further, when the boundary conditions were known, the contaminant predictions were much more sensitive to accurate estimation of transmissivity than to the estimation of dispersivities. The numerical results also showed that after a relatively short simulation period (less than 4 years), predicted contaminant concentrations could be significantly in error. This suggests the importance of integrating uncertainty analysis into the prediction of long‐term contaminant transport.
Direct comparison of kinetic and local equilibrium formulations for solute transport affected by surface reactions
Authors: Bahr Jean M. , Rubin Jacob
Published: February 2008

Modeling transport of reacting solutes in porous media often requires a choice between models based on the local equilibrium assumption (LEA) and models involving reaction kinetics. Direct comparison of the mathematical formulations for these two types of transport models can aid in this choice. For cases of transport affected by surface reaction, such a comparison is made possible by a new derivation procedure. This procedure yields a kinetics‐based formulation that is the sum of the LEA formulation and one or more kinetically influenced terms. The dimensionless form of the new kinetics‐based formulation facilitates identification of critical parameter groupings which control the approach to transport behavior consistent with LEA model predictions. Results of numerical experiments demonstrate that criteria for LEA applicability can be expressed conveniently in terms of these parameter groupings. The derivation procedure is demonstrated for examples of surface reactions including first‐order reversible sorption, Langmuir‐type kinetics and binary, homovalent ion exchange.
Simulation of the Borden Plume using the alternating direction Galerkin Technique
Authors: Frind E. O. , Hokkanen G. E.
Published: February 2008

The evolution of the chloride plume at the Borden landfill is simulated and a parametric analysis is performed. The plume is characterized by sharp vertical concentration gradients, indicating a weak transverse vertical dispersion mechanism. To handle the high‐resolution grid that is necessary because of the large contrast in the dispersive parameters, the alternating direction Galerkin technique is used. Various types of numerical error that can arise in simulations of such plumes are discussed. The parametric analysis reveals that an approximately three order of magnitude contrast between longitudinal and vertical transverse dispersivities exists in the Borden plume. A variable source concentration function will reproduce the concentration peaks that were observed in the field. However, a growing source concentration function can also produce longitudinal spreading resembling that obtained with a larger longitudinal dispersivity, indicating possible nonuniqueness. The vertical transverse dispersion parameter, which is unaffected by nonuniqueness, is within 1 order of magnitude of molecular diffusion.
Exploring existence value
Authors: Madariaga Bruce , McConnell Kenneth E.
Published: February 2008

The notion that individuals value the preservation of water resources independent of their own use of these resources is discussed. Issues in defining this value, termed “existence value,” are explored. Economic models are employed to assess the role of existence value in benefit‐cost analysis. The motives underlying existence value are shown to matter to contingent valuation measurement of existence benefits. A stylized contingent valuation experiment is used to study nonusers’ attitudes regarding projects to improve water quality in the Chesapeake Bay. Survey results indicate that altruism is one of the motives underlying existence value and that goods other than environmental and natural resources may provide existence benefits.
Estimating the value of water quality improvements in a recreational demand framework
Authors: Bockstael Nancy E. , Hanemann W. Michael , Kling Catherine L.
Published: February 2008

With the advent of Executive Order 12291, policymakers involved in water quality regulation are increasingly interested in assessing the benefits of their programs. Several methods for valuing water quality improvements using recreational demand models have been developed by economists, most of which depend on observing recreationists visiting an array of sites with varying water quality and costs of access. In this paper, three general types of models are described: systems of demands, discrete choice models, and the hedonic travel cost approach; the latter two models are demonstrated using a common data set on water quality and swimming behavior in the Boston area. The models are contrasted and their relative usefulness in answering policy questions explored.
Optimal groundwater quality management under parameter uncertainty
Authors: Wagner Brian J. , Gorelick Steven M.
Published: February 2008

To date optimization models for groundwater quality management give no assurance that water quality standards will be met. This is in part because they ignore errors in hydraulic heads, flows, and solute concentrations due to flow and transport model parameter uncertainty. Here we explicitly incorporate parameter estimation and estimate uncertainties into a model for the optimal design of an aquifer remediation scheme. Parameter uncertainty is incorporated into the decision‐making process. The objective is to identify the best remediation strategies (well site selection and pumping‐recharge rates) so that water quality standards are met at a specified reliability level. The procedure couples three methods: (1) a finite element flow and transport simulation model combined with nonlinear least squares multiple regression for simultaneous flow and transport parameter estimation; (2) first‐order first‐ and second‐moment analysis to transfer the information about the effects of parameter uncertainty to the management model; and (3) nonlinear chance‐constrained stochastic optimization combined with flow and transport simulation for optimal decision making. This joint approach enables one to estimate unknown aquifer parameters, quantify the uncertainty of the parameter estimates, simulate flow and transport responses, and automatically account for parameter uncertainty in the decision‐making process through the simulation management model. Results show that remediation requirements can increase dramatically due to parameter uncertainty. Risk‐averse design solutions automatically provide insurance by “overdesigning” the strategy relative to the risk‐neutral case. The approach is fairly general and is applicable to a variety of groundwater management problems. The influence on design solutions of the reliability level and verification of the underlying statistical assumptions of the first‐order analysis are explored in a sensitivity study and 2000 Monte Carlo simulations, respectively.
A theoretical study of the estimation of the correlation scale in spatially variable fields: 2. Nonstationary fields
Authors: Russo David , Jury William A.
Published: February 2008

A nonstationary field may be viewed as a process composed of two components, a stochastic component characterized by a relatively small‐scale, high‐frequency variation (HFV), and a deterministic (drift) component characterized by a relatively large‐scale, low‐frequency variation (LFV). In this case, the estimation of the variogram or the covariance function and its associated correlation scale requires the prior estimation of the drift. In this study, the effects of the type of drift function, the size of the integral scale, and the variances associated with the two components on the estimation of the correlation scale were analyzed using a theoretically generated master population of points representing the nonstationary field. A set of nonstationary fields was generated by superimposing different drift functions with a given variance σL2 on stationary fields with a given integral scale and variance σH2. Results showed that the shape of the estimated variograms and their associated estimated correlation scales, the fractal dimensions, and the estimates of the covariance function provided by the parametric estimation procedures were affected by the type of the imposed drift function, by the integral scale of the stochastic component, and by the ratio between the variances of the two components. Both the maximum likelihood (ML) and the restricted maximum likelihood (RML) estimation procedures may fail to correctly identify and separate the two components when their respective variances are of the same order of magnitude and when the correlation scale or spatial extent of the stochastic component is small compared with the typical size of the field domain. In these situations, the fractal dimension analysis also may not indicate the dominance of the LFV over the HFV even when σL2 ≃ 2σH2. The model validation tests also may fail to indicate inconsistency between the estimates of the parameters of the structural model and the data, when σL2 ≃ σH2. In general, the ML and the RML estimation procedures overestimated the integral scale J and the variance C(O) of the underlying process, whereas the iterative generalized least squares and the ordinary least squares procedures coupled with the minimum mean‐square deviation (MSD) criterion for variogram model fitting underestimated the value of the integral scale. However, results of model validation tests suggested that these estimates were accurate and consistent with the data and almost indistinguishable in terms of the values of the Akaike Information Criterion. Consequently, different sets of Ĵ and Ĉ(0) values may adequately describe a given realization of the nonstationary field to the same level of precision. Careful examination of the estimated variograms over the entire set of lag classes, application of fractal dimension analysis, and a validation test of the fitted variogram models are necessary steps for drift detection. Quantification of the drift, however, may not be achieved in certain situations.
Nearest‐neighbor methods for nonparametric rainfall‐runoff forecasting
Authors: Karlsson M. , Yakowitz S.
Published: February 2008

This paper introduces a nonparametric regression method for time series to the stochastic hydrology literature. The technique, known as the nearest‐neighbor (NN) method, is motivated by the classical rainfall‐runoff forecasting problem. We have shown in a companion paper (Yakowitz, 1987) that in the time series setting it has attractive large‐sample properties. As summarized herein, these include asymptotic convergence to the optimal forecaster under many circumstances in which the forecaster is not a linear function of the data. The technique is applied to a sequence of about 3000 seasonalized rainfall‐runoff pairs from the Coshocton Watershed. We compare the NN forecasting performance on a split sample with those of autoregressive maximum and instantaneous unit hydrograph forecasters.
Estimating the upper tail of flood frequency distributions
Authors: Smith James A.
Published: February 2008

Procedures for estimating recurrence intervals of extreme floods are developed. Estimation procedures proposed in this paper differ from standard procedures in that only the largest 10–20% of flood peaks are explicitly used to estimate flood quantiles. Quantile estimation procedures are developed for both annual peak and seasonal flood frequency distributions. The underlying model of flood peaks is a marked point process
, where
represents time of occurrence of the jth flood during year i and the mark
represents magnitude of the flood peak. Results from extreme value theory are used to parameterize the upper tail of flood peak distributions. Quantile estimation procedures are applied to the 92‐year record of flood peaks from the Potomac River. Results suggest that Potomac flood peaks are bounded above. The estimated upper bound is only 20% larger than the flood of record.
On parameter estimation of temporal rainfall models
Authors: Obeysekera J. T. B. , Tabios G. Q. , Salas J. D.
Published: February 2008

Characteristics and moment estimators of temporal rainfall models such as Poisson rectangular pulse (PRP), Neyman‐Scott white noise (NSWN), and Neyman‐Scott rectangular pulse (NSRP) are investigated. It is shown that PRP and NSWN have a correlation structure like that of an autoregressive moving average (ARMA) (1,1) model whereas the NSRP has a dependence structure like that of an ARMA (2, 2). The admissible regions of lag‐1 and lag‐2 autocorrelations are derived to demonstrate that in general they are more restricted than their ARMA counterparts. An additional property denoted as variance ratio, which is intimately related to the scale of fluctuation of a process, is defined and used for model comparison. The bias and mean square error properties of the moment estimators are investigated with emphasis on the NSWN model, for which it is suggested that the temporal scale T, defined by βT = 1, provides the most efficient estimators of the parameters. Parameter β defines the arrival of rain bursts relative to the origin of storm systems. All three models are fitted to an extensive data set covering hourly precipitation data at 38 stations in northeastern Colorado. The correlation and variance ratio plots are used to select the appropriate model for each month. However, it is shown that the diurnal periodicity of storm occurrence is predominant during the summer months, which is a characteristic not built into any of the temporal models used here.
Effects of hysteresis with air entrapment on water flow in the unsaturated zone
Authors: Kaluarachchi J. J. , Parker J. C.
Published: February 2008

Effects of hysteresis with air entrapment on water content and head distributions, surface fluxes, and water balance components are investigated by finite element simulations of flow in one‐ and two‐dimensional spatial domains. Results of one‐dimensional analyses indicate that effects of hysteresis are markedly influenced by surface boundary conditions and to a lesser extent by initial conditions. Stipulation of flux‐controlled boundary conditions results in minimal hysteretic effects, while potential‐type boundary conditions produce substantial deviations between hysteretic and nonhysteretic simulations. The latter effects are attributable in large part to differences in surface conductivity associated with air entrapment. Predicted water content distributions for nonhysteretic simulations using main drying functions deviated much more from hysteretic model results than did predicted water content distributions for nonhysteretic simulations based on main wetting functions. For two‐dimensional problems, predicted infiltration, evaporation, and seepage fluxes and soil water storage changes were much higher for nonhysteretic simulations using main drying functions than for hysteretic simulations; predictions based on main wetting functions deviated less markedly from hysteretic simulations. Introduction of heterogeneity in the porous medium diminished effects of hysteresis on infiltration and seepage but accentuated differences in evaporation due to interactions between evaporation and seepage boundaries, suggesting that the influence of heterogeneity on hysteretic effects will be problem‐specific.
Variability of bed load measurement
Authors: Pitlick John
Published: February 2008

The sampling efficiencies of two slightly different Helley‐Smith‐type bed load samplers were compared under carefully controlled field conditions. The samplers were identical in all respects except for the thickness of the metal used in their construction. The test results indicate that for coarse‐grained sand bed load, the sampler with a wall thickness of 1.5 mm has a sediment trap efficiency that can be as much as two times that of the sampler with walls that are 6.3 mm thick. In a separate test, bed load transport was monitored at a single channel cross section during a 10‐hour period of steady discharge. At individual sampling stations, measurement values ranged from nearly 0 to 3 times the mean. Composite bed load transport rates for full traverses of the channel width varied by a factor of 2.
Bed load transport regime of a small forest stream
Authors: Sidle R. C.
Published: February 2008

Bed load transport in a small gravel‐bedded stream on Chichagof Island, Alaska, was measured for 33 autumn storm flows during 1980 through 1985 to determine temporal and spatial trends within a riffle‐pool‐riffle sequence. The transport of fine sediment was more frequent than coarse sediment. Scouring of coarse material in the reach appeared to be triggered only by high flows with Tr ≥ 5 years. Within a given storm season, both antecedent storm history and cumulative flow (above the threshold for bed load transport, 0.25 m3 s−1) influenced bed load transport; however, the effects of these seasonal factors changed from year to year, presumably in response to storage and release of sediment around large organic debris upstream. Hysteresis loops existed in bed load transport versus flow plots for many storms. Fine bed load material was more subject to such differential transport over the storm hydrograph than was coarse material. During the 6‐year period, both riffles scoured along most of the channel width while the middle portion of the pool filled.
Further comments on sensitivities, parameter estimation, and sampling design in one‐dimensional analysis of solute transport in porous media
Authors: Knopman Debra S. , Voss Clifford I.
Published: February 2008

Sensitivities of solute concentration to parameters associated with first‐order chemical decay, boundary conditions, initial conditions, and multilayer transport are examined in one‐dimensional analytical models of transient solute transport in porous media. A sensitivity is a change in solute concentration resulting from a change in a model parameter. Sensitivity analysis is important because minimum information required in regression on chemical data for the estimation of model parameters by regression is expressed in terms of sensitivities. Nonlinear regression models of solute transport were tested on sets of noiseless observations from known models that exceeded the minimum sensitivity information requirements. Results demonstrate that the regression models consistently converged to the correct parameters when the initial sets of parameter values substantially deviated from the correct parameters. On the basis of the sensitivity analysis, several statements may be made about design of sampling for parameter estimation for the models examined: (1) estimation of parameters associated with solute transport in the individual layers of a multilayer system is possible even when solute concentrations in the individual layers are mixed in an observation well; (2) when estimating parameters in a decaying upstream boundary condition, observations are best made late in the passage of the front near a time chosen by adding the inverse of an hypothesized value of the source decay parameter to the estimated mean travel time at a given downstream location; (3) estimation of a first‐order chemical decay parameter requires observations to be made late in the passage of the front, preferably near a location corresponding to a travel time of
times the half‐life of the solute; and (4) estimation of a parameter relating to spatial variability in an initial condition requires observations to be made early in time relative to passage of the solute front.
Numerical modeling of sorption kinetics of organic compounds to soil and sediment particles
Authors: Wu Shian‐Chee , Gschwend Philip M.
Published: February 2008

A numerical model is developed to simulate hydrophobic organic compound sorption kinetics, based on a retarded intraaggregate diffusion conceptualization of this solid‐water exchange process. This model was used to ascertain the sensitivity of the sorption process for various sorbates to nonsteady solution concentrations and to polydisperse soil or sediment aggregate particle size distributions. Common approaches to modeling sorption kinetics amount to simplifications of our model and appear justified only when (1) the concentration fluctuations occur on a time scale which matches the sorption timescale of interest and (2) the particle size distribution is relatively narrow. Finally, a means is provided to estimate the extent of approach of a sorbing system to equilibrium as a function of aggregate size, chemical diffusivity and hydrophobicity, and system solids concentration.
The value of stochastic streamflow models in overyear reservoir design applications
Authors: Vogel Richard M. , Stedinger Jery R.
Published: February 2008

The design of storage reservoirs using stochastic streamflow models and synthetic streamflow sequences has received considerable attention in the water resources literature. Fewer studies have addressed the sampling properties of estimates of the design capacity of a storage reservoir obtained using available historical records or using synthetic streamflow sequences generated with models whose parameters were estimated from such data. Our experiments document the bias and root‐mean‐square error of estimates of overyear required storage capacity distribution quantiles corresponding to fixed or to random demand levels. The results show that the use of stochastic streamflow models can lead to improvements in the precision of reservoir design capacity estimates. Estimates of the design capacity of a storage reservoir based upon relatively simple stochastic streamflow models have smaller root‐mean‐square errors than corresponding estimates based solely upon the historic record, even when the correct model form is not known a priori.
Impact of rainfall and soil hydraulic property information on runoff predictions at the hillslope scale
Authors: Loague Keith M.
Published: February 2008

The stochastic‐conceptual model designed by Freeze (1980) to investigate rainfall‐runoff processes on a hillslope was modified to examine the impact of hydrologic data in simulating rainfall‐runoff events. Analysis of almost 12,000 synthesized rainfall‐runoff events generally shows that the spatial description of soil hydraulic properties has a greater impact on the characterization of hillslope runoff than corresponding descriptions of rainfall. Estimates of peak storm flow and time to peak were found to be less accurate than estimates of storm flow depth for the same level of information illustrating that different levels of information may be required to make predictions of equal efficiency for different summary variables.