摘要:

A model which includes the transport and retardation mechanisms of advective flow, axial dispersion, liquid‐phase mass transfer, diffusion into immobile liquid, and local adsorption equilibrium was developed to describe the migration of nondegradable, organic chemicals through a column of saturated, aggregated soil. A range of simplifying assumptions were explored to assess the relative importance of the various mechanisms. Solutions to the model were either adapted from the literature or derived from mass balances and mass transfer principles. The most general form of the model required the development of numerical solutions which employed orthogonal collocation. Soil column breakthrough predictions in terms of relative concentration as a function of total column pore volumes fed can be characterized by seven independent dimensionless parameters: the Peclet number, the Stanton number, a pore diffusion modulus, a surface diffusion modulus, an adsorbed solute distribution ratio, an immobile fluid solute distribution ratio, and the Freundlich parameter 1/n. For a strongly adsorbed chemical in long soil columns, a fifteen fold decrease of the Peclet number, a fivefold decrease of the Stanton number, or a onefold decrease in either the pore diffusion modulus or the surface diffusion modulus have an equivalent effect on the spreading of the breakthrough curve. The breakthrough curve tends to sharpen for favorably adsorbed chemical species (1/n1.0). The movement of chemical is retarded as the solute distribution ratios increase. A sensitivity analysis of model parameters, which were derived from literature correlations, column geometry, soil adsorption isotherms, and breakthrough curves, showed that adsorption capacity, adsorption intensity, and aggregate geometry have the greatest effect on chemical retardation and spreading, while liquid‐phase mass transfer has little eff

ISSN:0043-1397    

DOI:10.1029/WR022i003p00271

年代:1986

数据来源: WILEY

摘要:

Measured breakthrough and elution curves for trichloroethene, bromoform, and chloride in columns of a sandy loam soil were compared to various models describing one‐dimensional chemical transport through saturated soil columns. The local equilibrium model and the segregated flow model approximate the retardation of organic chemicals but do not account for the amount of spreading seen in the breakthrough and elution data. A dispersed flow, local equilibrium model (DFLEM) could simulate the breakthrough of the organic chemicals and tracer but only if the axial dispersion coefficient were adjusted to match the breakthrough data. Existing correlations for axial dispersion based on soil and fluid properties could not predict the apparent dispersion seen in the miscible displacement experiments. A dispersed flow, pore and surface diffusion model (DFPSDM) could also simulate the chemical break‐through if an aggregate radius were adjusted to fit the data. Neither the adjusted radii nor the apparent dispersivities could be related to the hydraulic characteristics of the column or to soil properties. Both models were able to reasonably predict the elution of chemical from the column when either an aggregate radius or an apparent dispersivity were estimated from the breakthrough data. Neither the DFLEM nor the DFPSDM were able to predict the increased asymmetry or the leftward shift of the breakthrough data when the average pore water velocity was increased from 12.0 to 36.6 cm/h. While the DFPSDM appears to be more phenomenologically correct, this work suggests that an additional kinetic mechanism should be included in the mo

ISSN:0043-1397    

DOI:10.1029/WR022i003p00285

年代:1986

数据来源: WILEY

摘要:

Each year the National Weather Service and the Soil Conservation Service issue a monthly sequence of five (or six) categorical forecasts of the seasonal snowmelt runoff volume. To describe uncertainties in these forecasts for the purposes of optimal decision making, a stochastic model is formulated. It is a discrete‐time, finite, continuous‐space, nonstationary Markov process. Posterior densities of the actual runoff conditional upon a forecast, and transition densities of forecasts are obtained from a Bayesian information processor. Parametric densities are derived for the process with a normal prior density of the runoff and a linear model of the forecast error. The structure of the model and the estimation procedure are motivated by analyses of forecast records from five stations in the Snake River basin, from the period 1971–1983. The advantages of supplementing the current forecasting scheme with a Bayesian analysis are disc

ISSN:0043-1397    

DOI:10.1029/WR022i003p00296

年代:1986

数据来源: WILEY

摘要:

This paper synthesizes the state‐of‐the‐art in modeling of the decision criteria for seasonal water supply planning under uncertainty, with particular focus on irrigation supply. We review, compare, and contrast three types of criterion functions: a utility function, a benefit function, and an opportunity loss function. Economic and preferential conditions imbedded in each function are identified. Theoretical relationships between these functions are established and illustrated using real data. The result of this synthesis is a methodological framework that unifies the three types of functions and offers analysts a systematic approach to choosing an appropriate decision crit

ISSN:0043-1397    

DOI:10.1029/WR022i003p00303

年代:1986

数据来源: WILEY

摘要:

Suppose a commitment of water supply for the forthcoming season must be made within a prior period during which a sequence of seasonal runoff volume forecasts is available. The decision maker's dilemma is that on the average, the accuracy of forecasts increases with time, but the benefit from the supply allocation decreases as the commitment is delayed. This planning problem is modeled as a Markovian stopping process with maximization of the expected utility of outcomes as the planning criterion. The optimal decision strategy, obtained via dynamic programming, prescribes the timing of the commitment and volume of the planned supply as a function of forecasts. The strategy takes explicitly into account uncertainties in the forecasts, value of water (economic or subjective), risk attitude of the decision maker, and utility reduction factor caused by delaying the commitment. An extensive numerical example illustrates the capabilities of the model as an aid to real‐time decision making and evaluation of the economic worth of forecast

ISSN:0043-1397    

DOI:10.1029/WR022i003p00313

年代:1986

数据来源: WILEY

摘要:

The identification of a proper water demand model when all the variables are noisy is one of the major problems in the planning process. So far, this problem has been overlooked, and generally, regression or time series models have been used. It has been suggested that the existing methods to include noisy variables are fraught with problems such as nonidentifiability and unboundedness. An alternative procedure is proposed to identify a water demand model from a set of noisy data using noisy realization theory. A mathematical programming‐based solution algorithm is developed to identify a first‐order lag dynamical model for this noisy problem. This algorithm gives bounds on both the model parameters and on the noise covariance matrix. The unboundedness problem is addressed by using prior information about the upper bound on the noise covariance matrix. The monthly municipal water demand of Tucson, Arizona, from 1974 to 1977 is modeled as an example. The model assumes 10–30% noise in the variables and then correspondingly gives a range for each model para

ISSN:0043-1397    

DOI:10.1029/WR022i003p00322

年代:1986

数据来源: WILEY

摘要:

Municipal wastewater warrants increased attention as a potential environmental pollution and a possible irrigation water source. A long‐run mathematical programming model aimed at regional optimization with regard to plant capacity, treatment level, allocation of the effluent to the participating farms, and cropping patterns at each farm is presented. The model is applied to an agricultural region in Israel which includes a town and several farms. The results indicate the necessity of a subsidy for regional cooperation and provide a regional setup of reusing municipal wastewater for irrigation. The optimal solution enables each farm to reallocate the freshwater quota among the different land sections, to cultivate new land areas, and to expand the irrigated crops by using effluent. The acceptability of the regional optimal solution by the town and the farms is discussed in another pape

ISSN:0043-1397    

DOI:10.1029/WR022i003p00331

年代:1986

数据来源: WILEY

摘要:

This paper is concerned with the allocation of cost and benefits from regional cooperation, with respect to reuse of municipal effluent for irrigation at the Ramla region of Israel. An efficient regional solution provides the maximal regional income which has to be redistributed among the town and several farms. Different allocations based on marginal cost pricing and schemes from cooperative game theory like the core, Shapley value, generalized Shapley value, and nucleolus are applied. The town and farm A have the main additional gains according to all allocation schemes presented. Advantages and disadvantages of these allocation schemes are examined in order to suggest a fair and acceptable allocation of the regional cooperative gains. Although no method has been preferred, the marginal cost pricing was found to be unacceptable by the participants. The conclusion is that the theory of cooperative games may provide guidelines while comparing the different solutions.

ISSN:0043-1397    

DOI:10.1029/WR022i003p00339

年代:1986

数据来源: WILEY

摘要:

In recognition of hydrologic uncertainty and seasonality, reservoir inflows are described as periodic Markov processes. The optimization of reservoir operations involves determination of the optimal release volumes in the successive time periods so that the expected total rewards resulting from the operations are maximized. A two‐stage optimization framework, which consists of a real time model followed by a steady state model, is proposed. The steady state model that describes the convergent nature of the prospective future operations is regarded as a periodic Markov decision process and is optimized with the generalized policy iteration procedure. This result is in turn used as an interim step for deriving the optimal immediate decisions for the current period in the real‐time model. Significant computational efficiency results from this framework and the respective optimization proced

ISSN:0043-1397    

DOI:10.1029/WR022i003p00345

年代:1986

数据来源: WILEY

摘要:

The optimality of parameter estimates is commonly justified on the basis of their sampling properties in parameter space (e.g., minimum mean square error, unbiasedness). However, these criteria may not be necessarily appropriate within the small sample size environment and the management rather than inferential focus of water resource planning decisions. Using two general cases, it is shown that in the presence of parameter uncertainty, “optimal” parameters, selected on the basis of their sampling properties in parameter space, can instead lead to nonoptimal decisions relative to planning objectives. In either case, alternative (informative and noninformative) Bayesian estimators are presented which, by incorporating parameter uncertainty, lead to uniform improvements in efficiency in action space over the traditional estimators. Incorporation of parameter uncertainty will not necessarily lead to uniform improvements, however. A counterexample is presented which shows that the use of noninformative priors, while optimal in the first case, can instead result in uniformly poorer decisions relative to the traditional estimators in the second case. The potential inefficiencies due to the use of noninformative measures of uncertainty and/or the misspecification of the prior in Bayesian estimators, provide a motivation for the consideration instead of empirical Bayes methods of estimat

ISSN:0043-1397    

DOI:10.1029/WR022i003p00353

年代:1986

数据来源: WILEY