ISSN:0043-1397    

DOI:10.1029/92WR02799

年代:1993

数据来源: WILEY

摘要:

A coupled formulation of withdrawal and sampling designs for ground water supply models is presented. The withdrawal design is described as a discrete time optimal control problem and solved using a closed loop stochastic control (CLSC) method. Two main features of the CLSC method are the anticipation of the future observation program and decomposition of the objective function into the deterministic and stochastic part. The former characteristic indicates the necessity for coupling the withdrawal and sampling designs, while the later feature allows a decision maker to estimate the uncertainty in the objective function if certain withdrawal rates are applied. The sampling network is sequentially developed, with the design criterion defined as a sensitivity of the objective function stochastic part to the uncertainty in the hydraulic head distribution multiplied with the variance of the hydraulic head. The concept of minimizing the stochastic part of the objective function with respect to the hydraulic head uncertainty provides a convenient way to couple the withdrawal design objectives with the monitoring network development weighted with the magnitude of the prediction error in the hydraulic head distribution. The Bayesian concept of measurement conditioning is employed to sequentially adjust the withdrawal rates and sampling network development by accounting for the information conveyed in field observations. Between the sampling sessions the uncertainty in the hydraulic head prediction is evaluated using the first‐ and second‐moment analysis applied to the discretized ground water flow model. The uncertainty in the hydraulic head prediction is assumed to come from the natural uncertainty in the hydraulic conductivity and uncertainty in the boundary condition values and other external fluxes (e.g., leakages and recharge). A hypothetical example is included to demonstrate the application procedure and to illustrate the main features of the proposed coupled formulat

ISSN:0043-1397    

DOI:10.1029/92WR02088

年代:1993

数据来源: WILEY

摘要:

A new method is developed to efficiently compute exact Scheffé‐type confidence intervals for output (or other function of parameters)g(β) derived from a groundwater flow model. The method is general in that parameter uncertainty can be specified by any statistical distribution having a log probability density function (log pdf) that can be expanded in a Taylor series. However, for this study parameter uncertainty is specified by a statistical multivariate beta distribution that incorporates hydrogeologic information in the form of the investigator's best estimates of parameters and a grouping of random variables representing possible parameter values so that each group is defined by maximum and minimum bounds and an ordering according to increasing value. The new method forms the confidence intervals from maximum and minimum limits ofg(β) on a contour of a linear combination of (1) the quadratic form for the parameters used by Cooley and Vecchia (1987) and (2) the log pdf for the multivariate beta distribution. Three example problems are used to compare characteristics of the confidence intervals for hydraulic head obtained using different weights for the linear combination. Different weights generally produced similar confidence intervals, whereas the method of Cooley and Vecchia (1987) often produced much larger confidence inter

ISSN:0043-1397    

DOI:10.1029/92WR01863

年代:1993

数据来源: WILEY

摘要:

Calibration data (observed values corresponding to model‐computed values of dependent variables) are incorporated into a general method of computing exact Scheffé‐type confidence intervals analogous to the confidence intervals developed in part 1 (Cooley, this issue) for a function of parameters derived from a groundwater flow model. Parameter uncertainty is specified by a distribution of parameters conditioned on the calibration data. This distribution was obtained as a posterior distribution by applying Bayes' theorem to the hydrogeologically derived prior distribution of parameters from part 1 and a distribution of differences between the calibration data and corresponding model‐computed dependent variables. Tests show that the new confidence intervals can be much smaller than the intervals of part 1 because the prior parameter variance‐covariance structure is altered so that combinations of parameters that give poor model fit to the data are unlikely. The confidence intervals of part 1 and the new confidence intervals can be effectively employed in a sequential method of model construction whereby new information is used to reduce confidence interval widths at ea

ISSN:0043-1397    

DOI:10.1029/92WR01864

年代:1993

数据来源: WILEY

摘要:

A new numerical model for the fate and transport of nonconservative metals in an estuarine environment is introduced. ELAmet solves the depth‐averaged advection/dispersion/transformation equation on a two‐dimensional grid using a finite element formulation in an Eulerian‐Lagrangian framework. The model incorporates aqueous speciation and adsorption/desorption. Adsorption to any number of solid types can take place through linear (partitioning) reactions or nonlinear (complexation) reactions. The model accommodates chemical equilibria and kinetics simultaneously. Rate constants can span any range of time scales, as the computational time step for solving the chemical transformation equations has been decoupled from that dictated by the circulation. Verification of the model is included for a one‐dimensional channel and up to 15 chemical species. Preliminary applications are included which illustrate the concept of diagnostic modeling for a synthetic estuarine system in which the effects of source location, chemical kinetics, and sediment deposition on the mixing plot are e

ISSN:0043-1397    

DOI:10.1029/92WR02126

年代:1993

数据来源: WILEY

摘要:

We used electrical resistance tomography (ERT) to map the subsurface distribution of a steam flood as a function of time as part of a prototype environmental restoration process performed by the Dynamic Underground Stripping Project. We evaluated the capability of ERT to monitor changes in the soil resistivity during the steam injection process using a dipole‐dipole measurement technique to measure the bulk electrical resistivity distribution in the soil mass. The injected steam caused changes in the soil's resistivity because the steam displaced some of the native pore water, increased the pore water and soil temperatures and changed the ionic content of the pore water. We could detect the effects of steam invasion by mapping changes in the soil resistivity as a function of space and time. The ERT tomographs are compared with induction well logs, formation temperature logs and lithologic logs. These comparisons suggest that the ERT tomographs mapped the formation regions invaded by the steam flood. The data also suggest that steam invasion was limited in vertical extent to a gravel horizon at depth of approximately 43 m. The tomographs show that with time, the steam invasion zone extended laterally to all areas monitored by the ERT techniqu

ISSN:0043-1397    

DOI:10.1029/92WR01608

年代:1993

数据来源: WILEY

摘要:

A numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography. At St. Kevin Gulch, an effluent stream in the Rocky Mountains of Colorado, subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well‐defined flow paths through the alluvium. Stream water‐filled flow paths in the alluvium (referred to as substream flow paths) returned to the stream a short distance downstream (1 to 10 m). Recharge to the substream flow paths occurred where stream water slope increased, at the transition from pools (<1%) to steeper channel units (5–20%). Return of substream flow paths to the stream occurred where stream water slope decreased, at the transition from steeper channel units to pools. A net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments. Along our study reach at St. Kevin Gulch the net inflow of water from subsurface to stream (1.6 mL s−1m−1) underestimated the gross inflow (2.7 mL s−1m−1) by 40%. The influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water‐sediment contact times; these effects could have important consequences for solute transport, retention, and transformatio

ISSN:0043-1397    

DOI:10.1029/92WR01960

年代:1993

数据来源: WILEY

摘要:

A new single‐borehole measurement technique for confined aquifers, the dipole flow test, yields the vertical distributions of the horizontal hydraulic conductivity, the vertical hydraulic conductivity, and the specific storativity when applied to different borehole intervals. The test utilizes straddle packers to isolate two chambers in the borehole, pressure transducers to monitor drawdown in them, and a small pump to create a dipole flow pattern in the aquifer by pumping water at a constant rate from the aquifer into one chamber, transferring it within the well to the next chamber, and finally injecting it back to the aquifer. A mathematical model describing the drawdown in each chamber is derived for the transient as well as the steady state cases. The aquifer parameters may be estimated from data produced by the dipole flow test alone or in conjunction with conventional pumping tests. The dipole flow regime reaches a steady state relatively quickly, especially in well permeable aquifers. A robust computational methodology for estimating the aquifer parameters, suitable for automatization, is based on the Newton‐Raphson algorithm applied to a system of up to three nonlinear equations, each describing the well drawdown at a different judiciously chosen time. Due to the relatively small drawdown it invokes, the dipole flow test may be applicable to unconfined aquifers as w

ISSN:0043-1397    

DOI:10.1029/92WR01820

年代:1993

数据来源: WILEY

摘要:

The coefficients of diffusion for water vapor in snow and other porous media are fundamental parameters of great importance. While they are generally thought to be greater than the coefficient for air, there is much uncertainty about what value to use. Attempts to determine the diffusion coefficient in snow experimentally have led to contradictory results, but Yosida's experiments seem to give reasonable results. Yosida's idea of hand‐to‐hand diffusion is quantified here in a particle‐to‐particle model which shows that the diffusion coefficient depends on the mean pore size and that the coefficient in snow is four to seven times greater than

ISSN:0043-1397    

DOI:10.1029/92WR02301

年代:1993

数据来源: WILEY

摘要:

Reacting, nonlinearly adsorbing solute transport in chemically heterogeneous soils is studied. Assuming adsorption is adequately described with the Freundlich equation, random variation of the adsorption coefficient is assumed to describe the heterogeneity. In a homogeneous case, traveling wave fronts develop, characterized by a constant velocity and a constant front shape. Using the method of moments, an analytical expression is derived to describe the constant variance of the traveling wave front. Deviations from the analytical variance and velocity, both calculated with an average adsorption coefficient, show that column scale heterogeneity has significant effects on front spreading and front movement. Expected values of front velocity and variance are computed as averages of values of 600 randomly generated columns. The nonlinear process causes small deviations from the case with average parameters. The ensemble average concentration front, representing an average front for the flow domain, shows that three mechanisms are responsible for the front spreading. At early displacement times the front spreading is caused by the thickness of the individual traveling waves. Subsequently, the effect of the internal variation of the adsorption coefficient (column scale heterogeneity) increases, whereas at large displacement times the front spreading is dominated by the different retardation coefficients of the different columns. The latter effect causes the variance to increase in proportion tot2. An analytical approximation is derived for the ensemble average front, ignoring column scale heterogeneity.

ISSN:0043-1397    

DOI:10.1029/92WR01859

年代:1993

数据来源: WILEY