摘要:

The existence of several problems associated with the short‐term operation of multipurpose reservoirs has been demonstrated in the past. A principal difficulty arises from uncertainties inherent in the prediction of future stream flows which are required to effect the requisite reservoir operation. It has been shown that the performance of the reservoir operation, as determined by an optimization model, is a function of the operating horizon. This results from the existence of a relationship between the operating horizon and both the reliability of the inflow forecasts and the reservoir operation. With an increase in the length of the operating horizon, the reliability of the forecasts will necessarily decrease. However, the reservoir operation model yields an improved operation on the assumption that the forecasts are accurate. An improved methodology is presented which considers the operating horizon as a decision variable which can change in real time. The optimal value of the operating horizon is selected based on the trade‐off between more reliable inflow forecast for shorter horizons and better reservoir operation associated with the use of longer operating horizons. The benefits of the new methodology are illustrated through an application of the technique in simulating the operation of Green Reservoir in Kentu

ISSN:0043-1397    

DOI:10.1029/WR025i001p00001

年代:1989

数据来源: WILEY

摘要:

Open‐loop and feedback equilibria are compared using a common property aquifer model. Open‐loop solutions correspond to assuming that participants commit themselves in the initial period to pumping rates in all future periods. Feedback solutions represent a more realistic assumption as farm operators adopt pumping strategies that depend on the reserve stock of water. Comparing the two equilibrium concepts reveals two sources of dynamic inefficiency in the common property aquifer: a pumping cost externality and a “strategic externality” that arises from the competition among users to capture the groundwater reserves. The qualitative results show that the open‐loop solution captures only the pumping cost externality. The feedback solution captures both externalities and exacerbates the over‐ exploitation of the commons compared to the open‐loop solution. Moreover, the dynamic inefficiency resulting from both externalities increases in the number of independent landowners with access t

ISSN:0043-1397    

DOI:10.1029/WR025i001p00009

年代:1989

数据来源: WILEY

摘要:

This paper introduces the general theory of water exclusion from, or entry into, subterranean holes from steady uniform downward unsaturated seepage. Buried holes serve as obstacles to the flow and so increase water pressure at parts of the hole surface. When downward seepage is fast enough and/or the hole is large enough, water pressure increases to the point where a seepage surface forms and water enters the hole. Contrary to the conventional picture drawn from capillary statics, hydrodynamics shows that the larger the hole the more vulnerable it is to water entry. Cavity shape is important also. Applications include optimal design of configurations of tunnels and underground repositories (e.g., for nuclear wastes) against entry of seepage water. The theory also embraces the disturbance of seepage flows by buried impermeable obstacles such as stones and structures. The quasi‐linear exclusion problem for circular cylindrical cavities is solved. Both exact solutions and simple asymptotic results are found, and graphs and tables presented. The moisture field about the cavity exhibits upstream and downstream stagnation points and retarded regions: “roof‐drip lobes” in which the moisture content and downward flow velocity are augmented by water essentially deflected from the cavity roof, and the “dry shadow” region of reduced moisture content and flow velocity, essentially sheltered by the cavity. A practical consequence is that we can establish, for any given combination of downward seepage velocity, cavity radius, saturated hydraulic conductivityK1, and sorptive number α, whether or not seepage water will enter a cylind

ISSN:0043-1397    

DOI:10.1029/WR025i001p00016

年代:1989

数据来源: WILEY

摘要:

The quasi‐linear problem of water exclusion from, or entry into, spherical cavities from steady uniform downward unsaturated seepage is solved. Both exact solutions and simple asymptotic results are found. These are qualitatively similar to those given previously for circular cylindrical cavities, exhibiting such features as the dry shadow and the roof‐drip lobes. A major practical result of the analysis is the function ∂max(s), the dependence of the maximum potential (at the roof apex) on the dimensionless quantitys= ½ αl, with α the sorptive number andlcavity radius. ∂max(s) is almost indistinguishable from ∂max(½s) for circular‐cylindrical cavities. This implies that physically the most relevant configuration parameter is total curvature at the apex of the cavity surface. Our results enable us to establish, for given values of downward seepage velocity, cavity radius, saturated hydraulic conductivityK1and of α, whether or not seepage water will enter a

ISSN:0043-1397    

DOI:10.1029/WR025i001p00029

年代:1989

数据来源: WILEY

摘要:

Chromatographic theory of competitive solute movement through porous, reactive soils with nonlinear adsorption isotherms is extended to conditions of unsteady flow in the unsaturated zone under gravity. By treating water content as an extra solute component, equations governing coupled movement of solute and water through reactive soils under gravity are shown to be a special case of those governing competitive solute movement, leading to a quantitative description of the coupled movement of the water and solute fronts through a soil column. When one solution displaces another at a different concentration and water potential in a reactive soil, the solute and water front splits up into two distinct subfronts, separated by a region in which both concentration and potential are constant. Each subfront is characterized by a region in which water content and concentration vary in a predetermined manner, according to certain functions (characteristics), one of which must remain constant over each subfront. Only in certain simple cases will solute concentration and water content vary independently of each other. The case when the hydraulic conductivity depends on solute concentration is explicitly considered.

ISSN:0043-1397    

DOI:10.1029/WR025i001p00038

年代:1989

数据来源: WILEY

摘要:

A two‐dimensional finite element model based on Galerkin's weighted residual approach and an upstream weighting technique was developed to predict simultaneous flow of water and oil in a three‐fluid phase system with gas assumed at constant pressure. Element matrices were computed using the influence coefficient method for both Picard and Newton‐Raphson nonlinear iteration schemes. A number of hypothetical simulations were performed in both one and two dimensions to evaluate the accuracy and efficiency of the various schemes with respect to handling of nonlinear soil properties, time marching, mass balance errors, soil nonhomogeneity, and effects of upstream weighting. Results indicate that the Picard scheme appears to be as effective as the Newton‐Raphson scheme while requiring substantially less computational effort if upstream weighting is employed. The Picard method without upstream weighting did not provide satisfactory convergence behavior. Results from problems involving extreme soil nonhomogeneity indicate that accurate solutions can be obtained with the Picard method with the proper use of upstream weighting. Saturation‐pressure derivative terms in the formulation can be evaluated using both analytical and chord‐slope methods. Results indicate that time averaging of these terms is critical for good mass balance results especially during redistribution. Overall mass balance is also sensitive to the fluid‐dependent scaling parameters of the constitutive

ISSN:0043-1397    

DOI:10.1029/WR025i001p00043

年代:1989

数据来源: WILEY

摘要:

An effective conductivity (resistance) for unsaturated vertical flow through horizontal layers of quasi‐linear material above a water table is determined by summing “saturated” resistances in series, subject to an exponential weighting with depth. These effective conductivities essentially determine steady flows, while calculation of intercepts involve weighting changes in water saturation with effective conductivities (resistances) and an exponential function of depth. Exact expressions are determined for the intercept associated with changes in surface values of saturation, and for instantaneous rises in the water table, in a Davidson model

ISSN:0043-1397    

DOI:10.1029/WR025i001p00055

年代:1989

数据来源: WILEY

摘要:

A mathematical model amenable to analytical solution techniques is developed for the investigation of contaminant transport from an injection well into a leaky aquifer system, which comprises a pumped and an unpumped aquifer connected to each other by an aquitard. A steady state groundwater flow field is assumed, where the injected fluids move horizontally in the pumped aquifer and vertically in the aquitard. The unpumped aquifer is assumed to remain in hydrostatic condition due to its large transmissivity. Descriptions of groundwater velocities are based on appropriate leaky aquifer well hydraulics. The model assumes that contaminants are transmitted in the pumped aquifer by radial advection, and in the aquitard by vertical one‐dimensional advection and longitudinal dispersion. Two coupled linear differential equations are formulated to deal with this transport problem; they involve variable coefficients dependent on Bessel functions. The unpumped aquifer is incorporated into the model as a boundary condition. Semianalytical transient solutions for concentration distributions in the pumped aquifer and the aquitard can be obtained by numerically inverting the Laplace domain solutions of the model with theCrump[1976] method. However, the steady state solutions of the model are determined in closed forms. In general, the modeling approach neglecting longitudinal dispersion in the pumped aquifer and lateral dispersion in the aquitard does not introduce significant error to the modelling results, as shown by the good agreement between numerical solutions including these two transport mechanisms and the solutions of the proposed model. Within the context of the assumed hydrogeological conditions, the proposed model appears to be a valid approximation to the problem of interest. Type curves for concentration evaluated at the well bore and the top of the aquitard are given, which can be employed to estimate whether the unpumped aquifer would be polluted as the consequence of the injection. Neglecting the leakage effects in a leaky aquifer can cause significant error in studying the transport proble

ISSN:0043-1397    

DOI:10.1029/WR025i001p00061

年代:1989

数据来源: WILEY

摘要:

Experiments were performed to test the ability of multiphase flow theory to predict the flow behavior of an immiscible organic liquid entering an unsaturated soil medium. The infiltration and redistribution behavior of water followed by oil, and of oil followed by water, in soil columns was observed. Experiments involved both liquid entry at the top of columns with downward flow and entry at the base of columns with upward flow. Two oils (specific gravity, 0.80 and 0.86; viscosity ratio relative to water, 4.7 and 77) were used in three soils: a silt loam, a sandy loam, and a sand. The results from 17 different infiltration scenarios, each about 8 hours in duration, are compared with the predicted results obtained from a simplified explicit finite difference code for three‐phase flow in a one‐dimensional system. Code input required only the soil water release curve, the bulk density, the initial water content, the saturated water conductivity, the oil viscosity, and the interfacial tensions between oil and air and water and oil. The code made reasonably good predictions of water and oil redistribution in the silt loam and loamy sand, when oil followed water infiltration. It made less than satisfactory predictions for the sand, possibly because hysteresis was neglected. Infiltration time for small amounts of oil was often shorter than the code predicted for the silt loam and loamy sand. The code, which did not account for hysteresis or unstable liquid flow, also did a poor job of predicting oil and water contents when water followed oil into the soils. Visual evidence of unstable flow of the more viscous oil entering the water wet loamy sand is presented, and the implications for one‐ and two‐dimensional experiments are discussed. The experimental results are presented to facilitate the preliminary validation of other immiscible flo

ISSN:0043-1397    

DOI:10.1029/WR025i001p00073

年代:1989

数据来源: WILEY

摘要:

The understanding of the processes of dissolution, volatilization, and gas‐liquid partitioning in porous media is very limited. The few models which attempt to characterize the transport of volatile organics in variably saturated media all assume that mass transfer processes are at equilibrium. In addition, gas phase advection is neglected by assuming that gas phase pressures are uniformly atmospheric and that density gradients are negligible. In this study a model was developed to solve for water phase flow and transport and density dependent gas phase flow and transport. Simple expressions for dissolution, volatilization, and gas‐liquid partitioning, employing the concept of an overall mass transfer coefficient, were incorporated into the model. The transport of trichloroethylene in a variably saturated vertical cross section, under a variety of conditions, was simulated. Results of the simulations appeared qualitatively correct. The importance of gas phase processes in increasing subsurface contamination from volatile organics, and in dissipating residual amounts of these substances, was demonstrated. The lack of similar analytical and/or numerical models, or suitable experimental studies, excluded the possibility of validating, or verifying, the mo

ISSN:0043-1397    

DOI:10.1029/WR025i001p00081

年代:1989

数据来源: WILEY