摘要:

This paper explores the implications of endogenous risk for the economic value of preventing groundwater contamination. We consider the analytical implications of endogenous risk for five key building blocks frequently used to structure studies of groundwater valuation: the probability and the location of contamination, the exposed population, risk perceptions, and intertemporal issues.

ISSN:0043-1397    

DOI:10.1029/90WR02135

年代:1991

数据来源: WILEY

摘要:

Many geophysical properties can be described as spatial stochastic processes, including spatially correlated hydraulic conductivity fields. Use of regional data can potentially improve estimation of such processes. We consider the case in which observations at each of several sites are described by a general linear model, while the parameters of these models arise from a common regional distribution. Parametric empirical Bayes methods enable the determination of the parameters of the regional distribution via maximum likelihood. However, such methods have not been utilized for spatial stochastic processes. We develop the application of a simple iterative technique for maximum likelihood estimation of the regional parameters, and demonstrate its use with a common parameterization of the spatial covariance structure. Synthetic data tests show the potential for substantial reduction in estimation risk through use of such techniques.

ISSN:0043-1397    

DOI:10.1029/90WR01879

年代:1991

数据来源: WILEY

摘要:

The transient hydraulic behavior of highly compressible aquitards is investigated through numerical analysis and field studies. Variations in the hydraulic parameters of an aquitard during consolidation are accounted for by incorporating empirical relationships derived from standard consolidation tests into the one‐dimensional flow equation. The resulting equation is highly nonlinear. The hydraulic response of an aquitard with physical properties typical of the compressible clays of Mexico City is evaluated with both standard linear analysis and with a nonlinear approach where the hydraulic parameters are stress‐dependent. The results indicate that decreases in the hydraulic diffusivity of the aquitard due to consolidation lead to diminished leakage flux, slower groundwater velocities and solute transport rates, longer transient response periods and less land subsidence than would be predicted by classical linear theory. Evaluation of field data from a site near Mexico City indicates that significant interpretive errors may arise if the stress dependence of the hydraulic parameters is ignored in these types of syst

ISSN:0043-1397    

DOI:10.1029/90WR01700

年代:1991

数据来源: WILEY

摘要:

Local approximation of functions based on values and derivatives at the nodes of a discretized grid are often used in solving problems numerically for which analytical solutions do not exist. In gradient dynamic programming (Foufoula‐Georgiou and Kitanidis, 1988) the use of such functions for the approximation of the cost‐to‐go function alleviates the “curse of dimensionality” by reducing the number of discretization nodes per state while obtaining high‐accuracy solutions. Also, efficient Newton‐type schemes can be used for the stage‐wise optimization, since now the approximation functions have continuous first derivatives. Our interest is in the case where the cost‐to‐go function is convex. However, the interpolants may not always be convex, introducing numerical problems. In this paper we address the problem of interpolating nodal values and derivatives of a one‐dimensional convex function with a convex interpolant so that potential computational difficulties due to approximation‐induced nonconvexity are avoided, and an efficient convergence to global instead of local optimal controls is guaranteed at every si

ISSN:0043-1397    

DOI:10.1029/90WR02032

年代:1991

数据来源: WILEY

摘要:

A theoretical model is formulated to investigate the development of the amplitude of alternate bars in unsteady flows. The problem is tackled by means of a weakly nonlinear analysis developed in a neighborhood of the threshold conditions for bar formation. Bar response to unsteady flow is found to depend on a parameterthat is a measure of the ratio between the time scale of the basic flow and the time scale of bar growth. The present theory shows that ifisO(1), as often occurs in nature, flow unsteadiness affects the instantaneous growth rate and phase of bar perturbations and controls the final amplitude reached by the bed configuration. A procedure for determining the final amplitude for a given flood event is proposed. Flume experiments were performed to test the main theoretical results. The bed response to unsteady flow was measured for different values of the period of the flood. The observed temporal behavior of the bar amplitude proves to be strongly affected by the unsteady character of the flow forofO(1), as predicted by the theory.

ISSN:0043-1397    

DOI:10.1029/90WR01699

年代:1991

数据来源: WILEY

摘要:

Laboratory experiments were conducted to determine the flow and transport properties of a fractured porous tuff block measuring 20 × 20 × 50 cm. One porous ceramic plate was placed immediately above a fracture and two other plates were placed on either side of the fracture above the rock matrix. The plates control the pressure head applied to the upper surface of the fractured rock block. Laboratory results are simulated using the boundary integral method for a single saturated fracture with an assumed uniform transmissivity embedded within a porous tuff block. The simulation is used to identify the saturated hydraulic properties of the fracture. Matrix hydraulic conductivity is estimated as 50 × 10−9m s−1and the fracture transmissivity is estimated as 5.0 × 10−9m2s−1. Much of the flow which exits the fracture at the lower surface first passes through the rock matrix even though a direct contact with a porous plate is present. Travel times and breakthrough curves are calculated by integrating the inverse velocity along a streamline, and then summing over all streamlines. Observed breakthrough curves were used to estimate fracture dispersivities which ranged from 0.0207 to 8.01 m. Breakthrough curves deviated from simulation results due to significant channeling of fr

ISSN:0043-1397    

DOI:10.1029/90WR01902

年代:1991

数据来源: WILEY

摘要:

Fracture flow under conditions of partial fluid saturation is studied where a portion of the fracture is filled with water and the remaining portion is filled with air. A discrete air‐water interface is used to separate the saturated from the drained regions within the fracture. A laboratory flow experiment is presented which demonstrates the interface concept. The steady state air‐water interface position is observed by maintaining a circular water source at constant positive pressure head between two vertical glass plates. An analytic solution to the flow problem is compared to the observed interface position and to simulated results which assume that a constant capillary pressure head can be assigned along the interface. Simulations performed for a hypothetical vertical fracture indicate zones of positive and negative pressure head in the saturated zone as well as an air‐filled zone. For the hypothetical fracture examined, fluid travel times are not substantially changed as fracture saturation decreases. This result may not hold generally for all frac

ISSN:0043-1397    

DOI:10.1029/90WR01901

年代:1991

数据来源: WILEY

摘要:

Surface waters, shallow groundwaters, and precipitation samples were collected along three traverses, which begin at the California coast and are oriented in an east–west manner, roughly parallel to the atmospheric flow path of meteoric water. The hydrogen isotope compositions from short‐term precipitation records varied as much as 40‰ at one location and were not representative of the average annual isotopic composition of precipitation. The hydrogen isotope ratios of surface waters and shallow groundwaters distinguish segments with isotopic variations that correlate with measured vertical fluxes of meteoric water. One coastal segment appears to closely approximate an open system in which precipitation dominates over evapotranspiration. Most segments are described by rather regular variations in δD: 3–45‰ per 100 km, which represent different degrees of closure of the hydrologic system caused by variable partitioning of precipitation between runoff and evapotranspiration. All partially closed systems imply terrestrial recycling of water. A simple model estimates this amount to be about 20% across northern and central California. Three segments, representing the Great Basin, show virtually no geographic variation and appear to represent isotopically clos

ISSN:0043-1397    

DOI:10.1029/90WR01708

年代:1991

数据来源: WILEY

摘要:

Extending a data record back in time using historical or paleoflood data has the potential to provide a considerable amount of information on very large floods. Parametric estimation methods are readily applicable to flood frequency analysis when historical or paleoflood estimates are available. However, all parametric approaches need an assumption about the underlying parent distribution, which is never known exactly. In recent years nonparametric methods of estimating probability density functions have been developed. Each of these involves the use of a suitable smoothing function known as a kernel. A new nonparametric variable kernel estimation model is proposed. Results obtained from a limited amount of real data and from simulation experiments show that quantiles estimated by the nonparametric method are better than those estimated by some selected parametric models both in descriptive ability and in predictive ability. The uncertainty in the choice of the threshold value of perception of a historical flood is also discussed.

ISSN:0043-1397    

DOI:10.1029/90WR01972

年代:1991

数据来源: WILEY

摘要:

Dissolved compounds in the water phase are rejected during freezing of soils. This phenomenon occurs due to the inability of ice crystals to incorporate most alien molecules. The ice grows only by association with water molecules, and molecules of solutes are rejected into the unfrozen water. A mathematical model for salt rejection in freezing saline soils has been developed by employing the mass balance equations for water and solute in a saturated porous medium, and the conservation of energy equation. Additional relations introduced include a moisture retention curve as a function of ice‐water capillary pressure, the Clapeyron equation and expressions for the heat capacity and the effective thermal conductivity of the soil. The resulting set of seven equations in terms of solute concentration, soil temperature, pore water, ice, and capillary pressures, and degrees of water and ice saturation have been solved simultaneously using a Newton‐Raphson linearization, with implicit iterative treatment of nonlinearities. Results indicate the development of unfrozen high salt concentration regions trapped in a frozen zone at sufficiently high freezing rates. The numerical results are compared with experimental data obtained by some researchers. Favorable match is obtained between the theoretical and experimental results. Finally, extension of the model to situations where ice lensing and soil deformations are significant is discus

ISSN:0043-1397    

DOI:10.1029/90WR01785

年代:1991

数据来源: WILEY