摘要:

The classical assumption that the lowest dissolved oxygen (DO) occurs at the highest temperature may not always hold. The DO saturation concentration decreases monotonically with increasing temperature, lowering the DO, but the reaeration coefficient increases monotonically with increasing temperature, tending to raise it. The decay coefficient monotonically increases with increasing temperature, lowering the DO for single discharges but not necessarily for multiple discharges. (Lower decay rates attending lower temperatures could result in low DO at the point where the impact from one discharge meets that of another.) This paper addresses the question of whether DO might under some circumstances worsen with decreasing temperature. Using a linear programing model it is shown that for a uniform stream at constant streamflow, the pattern of discharge that maximizes the derivative of critical dissolved oxygen with respect to temperature is an infinite uniformly distributed load. This suggests that streams receiving a large number of discharges may be more susceptible to DO increasing with decreasing temperature than streams receiving a small number of discharges. The corresponding maximum value of the derivative is shown to bedCS/dT+ (CS‐C*) ln Φ mg/L‐1°C−1, whereTis the temperature in °C,C*is the dissolved oxygen standard,CSis the DO saturation concentration (both in milligrams per liter), and Φ is the temperature adjustment factor for the reaeration coefficient. The maximum derivative value does not depend on the decay coefficient or its temperature adjustment factor. For the maximum reported value of Φ of 1.047, the maximum derivative value is 0.26273 ‐ 0.0028610T+ 0.00013371T2‐ 0.0000035718T3‐ 0.045929C*, for which the assumption that DO decreases monotonically with increasing temperature is valid forC*greater than about 6 mg/L. However, this assumption breaks down for values of Φ just above the range reported in the literature and values ofC*just below the normally chosen

ISSN:0043-1397    

DOI:10.1029/WR025i002p00145

年代:1989

数据来源: WILEY

摘要:

Does optimization systematically lead to solutions that appear better than they actually turn out to be when implemented? The answer can be yes if there are errors in estimating objective function coefficients. Even if such errors are unbiased, the calculated value of the objective function for the optimal solution will, in an expected value sense, overstate that solution's true performance. This presupposes that errors in the constraint set are relatively unimportant. The existence of such a bias is shown by proof; Monte Carlo simulations of two realistic water resources optimization problems show its significance for water planners. The most important implication is that the estimated net benefits of model solutions may be exaggerated compared to existing water systems, whose performance is generally known with more accuracy.

ISSN:0043-1397    

DOI:10.1029/WR025i002p00152

年代:1989

数据来源: WILEY

摘要:

The sediment economics model is used to determine whether optimal responses of farmers to constraints on cumulative sediment loads from cropland are similar to or different from optimal responses to constraints on episodic loads. Stream impacts are not directly addressed. Control costs for episodic loads are found to be consistently higher than control costs for the same proportionate reductions in annual average loads. A shift of policy emphasis from reducing cumulative sediment loads to reducing impacts from storm events may entail different analytical procedures, different management responses, and greater costs.

ISSN:0043-1397    

DOI:10.1029/WR025i002p00161

年代:1989

数据来源: WILEY

摘要:

Multireservoir, multiperiod linear programing models are typically computationally very expensive. Some of these models can be formulated as network linear programs (NLPs), for which computer codes about 100 times faster than general linear programing codes are available. A NLP formulation is presented for determining water assignments in a multireservoir system over some time horizon. It provides for demand zone shortfalls due to drought or transfer limitations, instream flow requirements, which can be violated during droughts, and seasonal reservoir target volumes. It also allows the trade‐off between reliability and demand shortfall severity to be explored. A case study illustrates computational performance. A NLP was formulated for a three‐reservoir, two‐demand zone water supply system modeled over 324 periods. It was solved in 26 CPU seconds on a VAX 8550 computer. Finally, it is shown that a NLP can be formulated to find the required minimum capacity of one reservoir in a multireservoir system so that system demand is just meet over some planning p

ISSN:0043-1397    

DOI:10.1029/WR025i002p00169

年代:1989

数据来源: WILEY

摘要:

Inaccuracies in parameter values, parameterization, stresses, and boundary conditions of analytical solutions and numerical models of groundwater flow produce errors in simulated hydraulic heads. These errors can be quantified in terms of approximate, simultaneous, nonlinear confidence intervals presented in the literature. Approximate confidence intervals can be applied in both error and sensitivity analysis and can be used prior to calibration or when calibration was accomplished by trial and error. In this paper, the method is expanded for use in numerical problems, and the accuracy of the approximate intervals is evaluated using Monte Carlo runs. Four test cases are used to compare results. In two numerical test cases, the approximate, simultaneous confidence intervals are generally accurate and are calculated efficiently enough to indicate that the method is useful for real‐world problems. In one of the two analytical test cases considered, the approximate intervals are too wide to be of practical interest. Improving the method used to approximate the uncertainty of the model inputs could make the confidence intervals useful in more problem

ISSN:0043-1397    

DOI:10.1029/WR025i002p00177

年代:1989

数据来源: WILEY

摘要:

The extended linear quadratic Gaussian (ELQG) control method (Georgakakos and Marks, 1987) is a stochastic control algorithm for the optimal operation of multiobjective reservoirs. Mathematically, this method optimizes a general functional of a stochastic system in state‐space form with upper and lower release constraints and probabilistic storage bounds. ELQG is a sequential algorithm which accounts for stochastic effects by preserving the first two statistical moments of the system's inputs and storages. In this paper, the method is first extended to handle nongaussian features which frequently characterize reservoir inputs. Second, ELQG's efficiency with respect to reliability storage constraints is discussed, and a new barrier function method is researched. These modifications are tested in case studies with the Savannah river syste

ISSN:0043-1397    

DOI:10.1029/WR025i002p00191

年代:1989

数据来源: WILEY

摘要:

The constitutive relations describing the fluid pressure response of a porous medium to changes in stress and temperature must reflect the microscopic processes that are operative over the time scale allowed for the deformation. Short‐duration deformations are readily described by undrained moduli, and intermediate duration deformations by drained moduli, both of which are formulated through linear elastic theory. Long‐term deformations that operate over geologic time are normally dominated by irreversible processes and result in considerably larger deformations, for the same applied stress conditions, than would be expected from their elastic counterparts. Model constitutive equations are developed for both elastic and irreversible processes and the magnitude and interpretation of the relevant material properties examined. Although the theory is presented in general terms, a sample calculation shows that for sandstone the inelastic deformation is one and one half orders of magnitude greater than the elastic deformation at the same applied stress. This difference in magnitude has a significant effect on the effective hydraulic diffusivity, various pore pressure coefficients, and the prospective fluid pressure development of the sedim

ISSN:0043-1397    

DOI:10.1029/WR025i002p00203

年代:1989

数据来源: WILEY

摘要:

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

ISSN:0043-1397    

DOI:10.1029/WR025i002p00215

年代:1989

数据来源: WILEY

摘要:

A field investigation was conducted to evaluate factors that regulate episodic variations of Al chemistry in an acid‐sensitive Massachusetts catchment. Stream water was collected from an upland subcatchment, a wetland subcatchment, and below the confluence of these two drainage areas in the West Wachusett Brook at 6 hour intervals during the first 2 weeks of November 1985. All major solutes and three Al fractions were analyzed. During the monitoring period, two different rainfall events of contrasting origin (i.e., a coastal storm and an inland storm) occurred. Rainfall volume, intensity, and duration were important factors regulating short‐term acidification of this stream. Ion exchange processes rather than mineral phase solubility appear to control stream water Al concentrations during high‐flow periods. The episodic acidification and subsequent release of Al in the upland stream were associated with a decrease in basic cations. In contrast, Al chemistry from the wetland drainage was regulated by the release of organic acids which served as mobile anions and were responsible for chelating and transporting Al. Chemical equilibrium calculations showed that waters draining the upland subcatchment were potentially toxic to fish during high‐flow periods, whereas the wetland stream exhibited minimal potential for Al t

ISSN:0043-1397    

DOI:10.1029/WR025i002p00233

年代:1989

数据来源: WILEY

摘要:

Analytical solutions of the general one‐ dimensional solute transport model for confined aquifers, applied to specific scenarios, are obtained. The mathematical formulation that is solved includes, in addition to the usual convection/dispersion formulation, (1) distributed first‐order loss, e.g., by metabolism, chemical reaction or irreversible dissolution; (2) linear equilibrium sorption rules; (3) a single, constantly emitting, source of finite length, which can be placed anywhere on the right half line and can be of any reasonable width; (4) mass conserving boundary conditions which include a highly tailorable time distribution of chemical concentration at the inlet boundary; and (5) a highly tailorable initial distribution on [0, ∞). The model constitutes an extension and generalization of the existing literature. Five transport and fate scenarios are presented as follows: (1) a compound initially distributed in stair step fashion, (2) a constant source, (3) a constant rate of entry at the inlet boundary, (4) an inlet boundary concentration that is decaying with time, and (5) conditions 1, 2, 3, and 4 specified at the same

ISSN:0043-1397    

DOI:10.1029/WR025i002p00241

年代:1989

数据来源: WILEY