摘要:

Remedial action decisions for groundwater contamination are made under conditions of uncertainty regarding both human exposure levels and chemical potency. The value of groundwater monitoring in reducing exposure uncertainty can be estimated within a decision‐analytic framework. The value of information from a particular monitoring strategy will depend on the expected accuracy of the strategy in estimating exposure, prior uncertainty about exposure and potency, the costs and efficiencies of the alternative remedial actions, and values assumed for the discount rate and the cost per unit risk. The proposed framework for assessing the value of hydrogeologic information at a groundwater contamination site consists of three steps: (1) determine a probability distribution for average lifetime exposure; (2) estimate the value of perfect exposure information in reducing the expected opportunity losses of decision making; and (3) compare hydrogeologic monitoring costs with the expected value of the sample information provide

ISSN:0043-1397    

DOI:10.1029/WR025i007p01451

年代:1989

数据来源: WILEY

摘要:

Most crop production in the western United States is dependent on water from irrigation, often using substantial amounts of energy to obtain and apply such water. The sensitivity of crop production to rising energy costs is an issue of concern to producers, input suppliers, and others. In this paper the effects of increasing electricity prices on electricity consumption and other production inputs are assessed using a dual cost estimation procedure. The empirical focus is on selected regions of the western United States where electricity is the primary source of irrigation energy. Demand for irrigation electricity varies across regions but is found to be price elastic.

ISSN:0043-1397    

DOI:10.1029/WR025i007p01461

年代:1989

数据来源: WILEY

摘要:

A theoretical analysis of the linear programming (LP) gradient method for optimal design of water distribution networks is presented. The method was first proposed by A. Alperovits and U. Shamir (1977) and has received much attention in the last 10 years. It consists of two stages that are solved in alteration: (1) a LP problem is solved for a given feasible flow distribution and (2) a search is conducted in the space of flow variables, based on the gradient of the objective function (GOF). In this paper a matrix formulation is given for both stages using well‐known graph theory matrices. It is proven that the mathematical expression of the GOF is independent of the choice of the sets of loops and paths along which the head constraints are formulated. This is contrary to the claim made by I. C. Goulter et al. (1986). The original GOF expression is shown to have been an approximation of the steepest direction, but still gives good results. Finally, the search procedure is improved by using the projected gradient metho

ISSN:0043-1397    

DOI:10.1029/WR025i007p01469

年代:1989

数据来源: WILEY

摘要:

Injection of liquid waste into a highly transmissive, saltwater‐bearing, fractured dolomite underlying the city of St. Petersburg, Florida, provided an opportunity to study density‐dependent flow associated with two miscible and density‐different liquids. The injection zone was 98 m thick with a radial hydraulic conductivity of 762 m/d and a vertical hydraulic conductivity of 152 m/d. Mean chloride concentrations of the injectant during two tests of 91 and 366 days duration were 180 and 170 mg/L, respectively, whereas chloride concentration of native salt water ranged from 19,000 to 20,000 mg/L. During the 366‐day test, chloride concentration in water from a well open to the upper part of the injection zone 223 m from the injection well approximately stabilized at about 4000 mg/L. Relatively constant chloride concentrations in water from this observation well at a level significantly greater than the injectant concentration suggested the hypothesis that circular convection with saltwater flow added chloride ions to the injection zone flow sampled at the observation well. In order to assess the acceptability of the circular convection hypothesis, information was required about the velocity field during injection. Mass transport model simulations were used to provide this information, after determining that the fractured injection zone could be treated as an equivalent porous medium with a single porosity. The mass transport model was calibrated using the 91‐day test data from two observation wells 223 m from the injection well. The model was then run without parameter changes to simulate the 366‐day test. Mass fractions of injectant computed for four observation wells during the 366‐day test compared favorably with observed mass fractions. Observed mass fractions were calculated as a function of chloride concentration and density. Comparisons between model‐computed mass fraction and velocity fields in a radial section showed circular convection, with salt water flowing toward the injection well in the lower part of the injection zone. The salt water then mixed with the injectant, and the mixture flowed away from the injection well in the upper part of the injection zone. On the basis of the model results and the assumed reasonableness of treating the injection zone as an equivalent porous medium with a single porosity, the hypothesis of circular convection with saltwater flow during subsurface injection of liquid waste into a highly transmissive saltwater‐bearing fractured dolomite was

ISSN:0043-1397    

DOI:10.1029/WR025i007p01481

年代:1989

数据来源: WILEY

摘要:

A numerical solution is developed for the analysis of groundwater flow problems using semianalytic time integration procedures and the boundary element method. The secondary reduction method, developed earlier, is an integral part of this computation process. Compared to finite difference procedures, which are commonly used for time integration in boundary element applications, the semianalytic computation yields more efficient and accurate results. The proposed method is tested here for two‐dimensional confined flow applications. Numerical results obtained are verified using analytical procedures that are available in the literature and are compared with other boundary element solution

ISSN:0043-1397    

DOI:10.1029/WR025i007p01495

年代:1989

数据来源: WILEY

摘要:

Estimates of snow water equivalent derived from measurements of the attenuation of terrestrial gamma radiation are systematically biased downward if variability in the snow water equivalent exists in the snowpack. The degree of underestimation is a function of both the shape and the variance of the distribution. The effects of these two factors on the estimate of snow water equivalent are examined for three assumed distributions (discrete, normal, and uniform) over a wide range of variances. The downward bias that results in each of the examined distributions can be corrected if the variance in the snow water equivalent is known. Examples of the application of the methodology are provided for flight lines located in the plains of western Canada and in the mountainous region of Colorado. The methodology presented can aid National Weather Service hydrologists in improving river and flood forecasts, water supply forecasts, and spring flood outlooks for large areas of the United States and Canada.

ISSN:0043-1397    

DOI:10.1029/WR025i007p01505

年代:1989

数据来源: WILEY

摘要:

An assessment of the importance of input uncertainty in long‐term predictions of lake acidification is presented. Mathematical models that simulate the behavior of hydrologic catchments under acid deposition require input data of precipitation, temperature, and deposition chemistry. In long‐term studies of lake acidification it is necessary to hypothesize a scenario of future input to the mathematical models. The present study indicates that uncertainty in precipitation amount and acid concentrations can be very important in the characterization of future lake alkalinity levels. It also suggests that the natural day‐to‐day variability present in precipitation and other weather variables significantly affects long‐term predictions of lake alkalinity in watersheds with short response time, even more than wet and dry weather

ISSN:0043-1397    

DOI:10.1029/WR025i007p01511

年代:1989

数据来源: WILEY

摘要:

This study involves the analysis of groundwater level time series for the purpose of obtaining details for a conceptual hydrogeologic model at a time when conventional hydraulic stress testing was not feasible due to regulatory considerations. The study area is located in south central Washington in the Pasco Basin which was a candidate site for underground disposal of high‐level radioactive nuclear wastes. Advantages of such passive methods of analysis may include relative simplicity, low cost, and avoidance of disturbances typically associated with stress testing of aquifers. Through this approach, natural and incidental man‐made groundwater level variations, most of which are quite small, are examined by statistical and analytical methods in conjunction with hydrogeologic models to draw inferences on the hydrogeology. Vertical connectivity of the hydrostratigraphic units is also examined by analyzing groundwater level time series of five units at three piezometer nests. It is concluded that a combination of statistical/analytical approaches used in a complementary fashion can provide useful information about the hydrogeology of a given area. A meaningful analysis requires that there is (1) a source of influence on the groundwater levels, (2) a response to that influence, (3) a sufficiently long data record, and (4) measurement and analytical techniques which allow the detection and identification of the influence and respo

ISSN:0043-1397    

DOI:10.1029/WR025i007p01519

年代:1989

数据来源: WILEY

摘要:

Asymptotic roof boundary layer solutions are obtained for the quasi‐linear seepage exclusion problem for elliptic‐cylindrical and spheroidal cavities of arbitrary aspect ratio h. The remarkably simple result for ϑmax, the maximum dimensionless potential at the cavity wall, is ϑmax(σ) ∼ 2σ +f0(h) + [f1(h)/σ] + ]f2(h)/σ2+ … where σ=½ αk−1(α is sorptive number;kis apical total curvature) and thefnare polynomials inh−2of degree (n+ 1). The results agree, as they should, with previous results for the circular cylinder and sphere (h=1) and the parabolic cylinder and paraboloid (h=∞). Connections noted in earlier studies between apical sharpness and the variation of potential along the cavity wall are generalized and made quantitative. The singular (h=0) strip and disc are of especial interest: they indicate behavior near the critical upstream stagnation point on cavities (and impermeable obstacles) with flat strip‐ and disc‐shaped roofs but otherwise (within some limits) arbitrary. For the strip, ϑmax(S1) ∼ 4S12+ 9 ‐ (36/S12) + ⋯, and for the disc, ϑmax(S1) ∼ 2S12+ 6 ‐ (27/S12) + ⋯, wheres1=½αl1(l1is semiwidth or radius). The central importance of ϑmaxis that seepage water enters, or is excluded from, the cavity, depending on whetherK0/K1(K0is downward steady seepage velocity far from the cavity;K1is saturated hydraulic con

ISSN:0043-1397    

DOI:10.1029/WR025i007p01531

年代:1989

数据来源: WILEY

摘要:

A comprehensive groundwater solute transport simulator is developed based on the modified method of characteristics (MMOC) combined with the Galerkin finite element method for the transport equation and the mixed finite element (MFE) method for the groundwater flow equation. The preconditioned conjugate gradient algorithm is used to solve the two large sparse algebraic system of equations arising from the MMOC and MFE discretizations. The MMOC takes time steps in the direction of flow, along the characteristics of the velocity field of the total fluid. The physical diffusion and dispersion terms are treated by a standard finite element scheme. The crucial aspect of the MMOC technique is that it looks backward in time, along an approximate flow path, instead of forward in time as in many method of characteristics or moving mesh techniques. The MFE procedure involves solving for both the hydraulic head and the specific discharge simultaneously. One order of convergence is gained by the MFE method, as compared with other standard finite element methods, and therefore more accurate velocity fields are simulated. The overall advantages of the MMOC‐MFE method include minimum numerical oscillation or grid orientation problems under steep concentration gradient simulations, and material balance errors are greatly reduced due to a very accurate velocity simulation by the MFE method. In addition, much larger time steps with Courant number well in excess of 1, as compared with the standard Galerkin finite element method, can be taken on a fixed spatial grid system without significant loss of accurac

ISSN:0043-1397    

DOI:10.1029/WR025i007p01541

年代:1989

数据来源: WILEY