摘要:

Discharge, piezometric, and other field observations from a 3‐year monitoring program indicate that runoff from a low‐gradient source area in Marin County, California, is controlled by the interaction of throughflow, macropore flow, and saturation overland flow. Throughflow response integrates multiple storm events and exhibits both seasonal trends and rapid response to midwinter storms upon saturation of highly conductive near‐surface soils. After saturation of the deeper colluvium along the hollow axis, macropore flow responds rapidly to individual storm events and locally provides a ceiling to piezometric response. Saturation overland flow occurs along the axis of the unchanneled valley only during large storms when both soil matrix and macropore transmissivity are exceeded. During large, runoff‐producing storms, saturation overland flow extends continuously over most of the unchanneled valley axis. During smaller runoff‐producing events, however, ground surface saturation may be discontinuous, reflecting either variations in the conductivity of the underlying soil/bedrock or a variable soil thickness along the hollow axis. Results of this study document a sequence of flow path activation in which the temporal distribution of rainfall events within a season determines both the mechanism and magnitude of runoff g

ISSN:0043-1397    

DOI:10.1029/94WR02270

年代:1995

数据来源: WILEY

摘要:

This paper presents a theoretical analysis of the experimental evidence reported in papers 1 and 2 (Lu et al., this issue (a, b)). The analysis of force is conducted on spherical particles serving as an idealized porous medium. Four close packing conditions were studied and two models were developed to describe liquid movement in glass bead porous media. According to the analysis of forces acting on the contact point of gas‐liquid interface, the direction as well as the magnitude of the total surface tensile force changes in contrast to a constant total surface tensile force acting in a capillary tube. It is shown that the velocity of the liquid plays an important role during capillary rise into porous media. Equations for the height and velocity of capillary rise into initially dry porous media are given for four different geometries of close packing. The models and equations present an improved explanation of the Haines' “jump” phenomenon and the instability observed in experiments of capillary rise in porous

ISSN:0043-1397    

DOI:10.1029/94WR00999

年代:1995

数据来源: WILEY

摘要:

A simulation model developed to explore patterns of fracture enlargement within incipient limestone karst aquifers has been extended to turbulent flow. In contrast to the highly selective passage enlargement that occurs early in cave network development under laminar flow, the transition to turbulent flow results in more general passage enlargement, leading to maze networks when initial fractures are large and hydraulic gradients are high. These results support previously published hypotheses for the development of maze patterns, including formation within structural settings that have created initially large fractures or within flow systems periodically inundated by flooding. Maze development is also favored under turbulent flow when passages are entirely water filled, and where the groundwater flow system is long‐lived. By contrast, branched patterns are most common when passages become free‐surface subterranean streams, because depression of the piezometric surface along main passages, downcutting along main passages, and possible infilling with sediment of side passages limit the sharing of discharge among interconnected fractures or bedding planes that promote maze developm

ISSN:0043-1397    

DOI:10.1029/94WR01964

年代:1995

数据来源: WILEY

摘要:

A method is developed to analyze steady horizontal flow to a well pumped from a confined aquifer composed of two homogeneous zones with contrasting transmissivities. Zone 1 is laterally unbounded and encloses zone 2, which is elliptical in shape and is several orders of magnitude more transmissive than zone 1. The solution for head is obtained by the boundary integral equation method. Nonlinear least squares regression is used to estimate the model parameters, which include the transmissivity of zone 1, and the location, size, and orientation of zone 2. The method is applied to a hypothetical aquifer where zone 2 is a long and narrow zone of vertical fractures. Synthetic data are generated from three different well patterns, representing different areal coverage and proximity to the fracture zone. When zone 1 of the hypothetical aquifer is homogeneous, the method correctly estimates all model parameters. When zone 1 is a randomly heterogeneous transmissivity field, some parameter estimates, especially the length of zone 2, become highly uncertain. To reduce uncertainty, the pumped well should be close to the fracture zone, and surrounding observation wells should cover an area similar in dimension to the length of the fracture zone. Some prior knowledge of the fracture zone, such as that gained from a surface geophysical survey, would greatly aid in designing the well test.

ISSN:0043-1397    

DOI:10.1029/94WR01965

年代:1995

数据来源: WILEY

摘要:

Recently, a unified Eulerian‐Lagrangian theory has been developed by one of us for nonreactive solute transport in space‐time nonstationary velocity fields. We describe a combined analytical‐numerical method of solution based on this theory for the special case of steady state flow in a mildly fluctuating, statistically homogeneous, lognormal hydraulic conductivity field. We take the unconditional mean velocity to be uniform but allow conditioning on measurements of log hydraulic conductivity (or transmissivity) and/or hydraulic head. This renders the velocity field nonstationary. We solve the conditional transport problem analytically at early time and express it in pseudo‐Fickian form at later time. The deterministic pseudo‐Fickian equations involve a conditional, space‐time dependent dispersion tensor which we evaluate numerically along mean “particle” trajectories. These equations lend themselves to accurate solution by standard Galerkin finite elements on a relatively coarse grid. The final step is an explicit numerical computation of lower bounds on conditional concentration prediction variance‐covariance (and coefficient of variation), travel time distribution, cumulative mass release across a “compliance surface,” the associated error, and plume spatial moments. Our method also allows quantification of the uncertainty in the original source location of any solute “particle” located anywhere in the field, at any time. This paper describes the methodology and presents some unconditional results. Conditioning and more advanced computations are presented

ISSN:0043-1397    

DOI:10.1029/94WR02234

年代:1995

数据来源: WILEY

摘要:

In paper 1 of this series we described an analytical‐numerical method to predict deterministically solute transport under uncertainty. The method is based on a unified Eulerian‐Lagrangian theory which allows conditioning of the predictions on hydraulic measurements. Conditioning on measured concentrations is also possible, as demonstrated by Neuman et al. (1993). In this paper we condition velocity on log transmissivity and/or hydraulic head data via cokriging. We then combine an early time analytical solution with a pseudo‐Fickian Galerkin finite element scheme for later time to obtain conditional predictions of concentration and lower bounds on its variance and coefficient of variation. The pseudo‐Fickian scheme involves a conditional dispersion tensor which depends on information and varies in space‐time. Hence the predicted plumes travel along curved trajectories and attain irregular, non‐Gaussian shapes. We also compute a measure of uncertainty for the original source location of a solute “particle” whose position at some later time is known from sampling. Spatial maps of this “particle origin covariance” provide vivid images of preferential flow paths and exclusion zones identified by the available data. We illustrate these concepts and results on instantaneous poi

ISSN:0043-1397    

DOI:10.1029/94WR02235

年代:1995

数据来源: WILEY

摘要:

In paper 1 of this series we described an analytical‐numerical approach to predict deterministically solute transport under uncertainty. The approach allows conditioning such predictions on hydraulic measurements and assessing the corresponding reduction in uncertainty. In paper 2 we examined the effects of log transmissivity and hydraulic head data on conditional predictions of concentration due to instantaneous point and nonpoint sources. In this paper we show how the same approach can be used directly to estimate mass flow rate across a “compliance surface,” cumulative mass release, and the probability distribution of travel times across this surface and the associated uncertainty. Contrary to some other methods in the literature, our approach requires neither a special theory for travel times nor a prior assumption about their probability distribution. We also show how one can compute explicitly the second spatial moment of the conditional mean plume about its center of mass, the conditional mean second spatial moment of the actual plume about its center of mass, and the conditional covariance of the plume center of mass. We illustrate these quantities and the effect of conditioning on some of them by considering instantaneous point, line, and area sources in a two‐dimensional, statistically homogeneous, isotropic, mildly varying log transmissivity field under uniform prior (unconditional) me

ISSN:0043-1397    

DOI:10.1029/94WR02236

年代:1995

数据来源: WILEY

摘要:

In previous papers of this series we described an analytical‐numerical method to predict deterministically solute transport under uncertainty based on a new Eulerian‐Lagrangian theory. We examined the effect that conditioning on hydraulic data has on predicted velocities and concentrations due to instantaneous point and nonpoint sources and discussed the same effect on spatial plume moments, total mass flow rate across a “compliance surface,” cumulative mass release across this surface, and the corresponding travel time distribution. Our analysis to date assumed that the initial state of the plume (the source term or initial concentration) is known with certainty and that the groundwater velocity field is Gaussian. In reality, the initial state of the plume is almost never known with certainty, especially when this state is inferred by sampling a plume at some arbitrary timet0following introduction of the solute into the subsurface (as is the case at many contaminated sites). Likewise, Monte Carlo simulations have shown that in Gaussian log hydraulic conductivity or log transmissivity fields, of the kind often indicated by in situ hydraulic test data, the longitudinal velocity becomes rapidly lognormal as the variance of the heterogeneities increases. In this paper we show how to handle both of these complications by means of our analytical‐numerical method of computation. By revisiting our previous examples, we explore the effects that uncertainty in plume initial state and non‐Gaussian velocities may have on predicted plume concentrations and uncertainty due to instantaneous point and nonpoint sources with and without conditioning on measurements of log transmissivity or hydraulic head in two dimensions. One of our findings is that the more common Gaussian models are nonconservative (in a regulatory sense) when applied during early dimensionless times to plumes that are initially short in the direction o

ISSN:0043-1397    

DOI:10.1029/94WR02237

年代:1995

数据来源: WILEY

摘要:

A new parameterization method for parameter identification is presented. This method allows us to incorporate all well log data and any other geological information into the inverse solution procedure for three‐dimensional groundwater modeling. In the proposed method, unknown parameters, such as hydraulic conductivity and storage coefficient, are directly related to the geological materials. Existing well logs of an aquifer can provide information of local geological structures along the vertical direction. By using these data, as well as any other geological information available, the three‐dimensional structure of the aquifer can be estimated by means of the geostatistical method. Then, art inverse problem can be formulated, leaving fewer unknown parameters to be identified. The advantages of the proposed method are the following: (1) all existing geological and hydrogeological information available is used for parameter identification, (2) the identified parameters are independent of the complexity of the simulation model, (3( the ill‐posedness of the inverse solution is mitigated, and (4) the identified parameters are distributed and physically meaningful. In a hypothetical example, the three‐dimensional distribution of hydraulic conductivity is easily identified using the proposed method. The results indicate that the identified distributed parameter vector is very close to the “true” distribution, and the inverse solution is highly stable with respect to observation errors. The proposed methodology has enabled us to determine the three‐dimensional distribution of hydraulic conductivity and storativity in the Hemet basin, Riversid

ISSN:0043-1397    

DOI:10.1029/94WR02276

年代:1995

数据来源: WILEY

摘要:

Breakthrough curves (BTCs) for reactive solutes commonly show earlier appearance, greater spreading, and more tailing than can be represented by the classical advection‐dispersion equation. Various types of kinetic models have been developed as a means of representing this nonideal behavior; however, several inconsistencies between the predictions of these models and experimental results have been noted. This paper presents an alternative mechanism that can contribute to the nonideality of BTCs for reactive solutes. The proposed mechanism is based on pore scale variation in the retardation factor (R) resulting from the pore size distribution. The effect of the mechanism on reactive solute transport was investigated using a stochastically derived macrodispersivity equation. In the example presented, the calculated stochastic dispersivity, which accounts for the pore scale variation inR, was almost 1 order of magnitude greater than that determined for a nonreactive solute. The BTC predicted using the stochastically derived dispersivity showed much better agreement with a measured BTC for strontium than that predicted using the nonreactive dispersivity. The simulated curve with stochastically derived dispersivity exhibited the common characteristics of nonideality, including enhanced spreading, early breakthrough, and tailing. Although the analysis was based on a highly idealized model of a porous medium, it is concluded that pore size variation inRcan contribute to nonideality in reactive BTCs, even in a homogeneously packed column under conditions of equilibrium solute partitionin

ISSN:0043-1397    

DOI:10.1029/94WR02261

年代:1995

数据来源: WILEY