摘要:

A concise representation of hysteretic soil hydraulic properties is given based on a combination of M. T. van Genuchten's (1980) parametricK‐θ‐hmodel and P. S. Scott et al.'s (1983) empirical hysteresis model modified to account for air entrapment. The resulting model yields compact closed‐form expressions for the hysteretic water retention curveθ(h) and soil water capacity C(h), as well as for the hydraulic conductivity functionK(h). Depending on the degree of simplification involved, the model entails a total of 6–9 parameters which can be calibrated from direct measurements ofθ(h) and saturated conductivity or by an inverse solution approach from transient flow experiments. Comparison of model‐predicted and measuredK‐θ‐hrelations for eight soils revealed one case in which model predictions were very poor. Model accuracy was judged to be acceptably good in the other cases. Mualern's modified (Y. Mualem, 1984) dependent domain model was found to be more accurate for soils with very narrow pore size distributions. Use of a simplified version of the proposed model with two parameters eliminated provided overall accuracy very similar to that of the more complex model. Numerical simulations of flow during transient infiltration and drainage using the proposed model and a variant of Y. Mualem's (1984) modified dependent domain model did not differ greatly and agreed reasonably well with experimental water content distributions, even when scanning curves were not described very accurately. By contrast, simulations without consideration of hysteresis produced highly unacceptable results. It is concluded that the proposed model provides a convenient and simple means of incorporating hysteretic effects into numerical flow models to provide significant improvement in prediction accuracy with little additional effort and with minimal

ISSN:0043-1397    

DOI:10.1029/WR023i001p00105

年代:1987

数据来源: WILEY

摘要:

The moving‐bed flume, where the cross‐sectional mean velocity is zero, has proven to be useful for visualization of coherent structures in the flow, especially streamwise vortices with a size scale equal to the depth. The temporal mean of these streamwise vortices is turbulence driven secondary motion. Hydrogen bubbles illuminated by a plane of laser light are used to visualize and measure these vortices. Flow visualization at the zero mean velocity point provides a unique view of the streamwise vortices without the interference of the mean flow veloc

ISSN:0043-1397    

DOI:10.1029/WR023i001p00115

年代:1987

数据来源: WILEY

摘要:

A method has been developed to establish steady state flow of water in an unsaturated soil sample spinning in a centrifuge. Theoretical analysis predicts moisture conditions in the sample that depend strongly on soil type and certain operating parameters. For Oakley sand, measurements of flux, water content, and matric potential during and after centrifugation verify that steady state flow can be achieved. Experiments have confirmed the theoretical prediction of a nearly uniform moisture distribution for this medium and have demonstrated that the flow can be effectively one‐dimensional. The method was used for steady state measurements of hydraulic conductivityKfor relatively dry soil, giving values as low as 7.6 × 10−11m/s with data obtained in a few hours. Darcy's law was tested by measuringKfor different centrifugal driving forces but with the same water content. For the sand at a bulk density of 1.82 Mg/m3and 27% saturation, results were consistent with Darcy's law forKequal to 5.22 × 10−10m/s and forces ranging from 216 to 1650 times normal

ISSN:0043-1397    

DOI:10.1029/WR023i001p00124

年代:1987

数据来源: WILEY

摘要:

Snow accumulation was compared between forested and clear‐cut plots in the transient snow zone of the western Cascade Range of Oregon, and measured snowmelt in both plots was compared to melt predicted by energy balance analyses. The absence of forest vegetation affected both snow accumulation and amount of energy available for melt during rainfall. Because intercepted snow melted in the forest canopy and reached the ground as meltwater, water equivalents in the clear‐cut plot were commonly 2–3 times greater than those in the forested plot. During the largest rain‐on‐snow event of the study, measured water outflow (rain plus snowmelt) in the clear‐cut plot was 21% greater than in the forested plot. Estimates made from microclimatological data show that during the common period of melt, total energy available in the clear‐cut plot was 40% greater than that in the forested plot. Because of greater wind speed in the clear‐cut plot, combined sensible and latent heat transfers in the clear‐cut plot were nearly triple those of

ISSN:0043-1397    

DOI:10.1029/WR023i001p00135

年代:1987

数据来源: WILEY

摘要:

Availability of extensive field data permitted realistic calibration and validation of a hydrodynamic model of tidal circulation and salt transport for Suisun Bay, California. Suisun Bay is a partially mixed embayment of northern San Francisco Bay located just seaward of the Sacramento‐San Joaquin Delta. The model employs a variant of an alternating direction implicit finite‐difference method to solve the hydrodynamic equations and an Eulerian‐Lagrangian method to solve the salt transport equation. An upwind formulation of the advective acceleration terms of the momentum equations was employed to avoid oscillations in the tidally averaged velocity field produced by central spatial differencing of these terms. Simulation results of tidal circulation and salt transport demonstrate that tides and the complex bathymetry determine the patterns of tidal velocities and that net changes in the salinity distribution over a few tidal cycles are small despite large changes during each tidal cycle. Computations of tidally averaged circulation suggest that baroclinic and wind effects are important influences on tidally averaged circulation during low freshwater‐inflow conditions. Exclusion of baroclinic effects would lead to overestimation of freshwater inflow by several hundred m3/s for a fixed set of model boundary conditions. Likewise, exclusion of wind would cause an underestimation of flux rates between shoals and channels by

ISSN:0043-1397    

DOI:10.1029/WR023i001p00143

年代:1987

数据来源: WILEY

摘要:

A method of completely describing and analyzing the stochastic process of streamflow droughts has been recommended. All important components of streamflow droughts such as deficit, duration, time of occurrence, number of streamflow droughts in a given time interval [0,t], the largest streamflow drought deficit, and the largest streamflow drought duration in a given time interval [0,t] are taken into consideration. A streamflow drought is related here to streamflow deficit. Following the theory of the supremum of a random number of random variables a stochastic model is presented for interpretation and analysis of the largest streamflow drought deficit below a given reference discharge and the largest streamflow drought duration concerning a time interval [0,t], at a given location of a river. The method is based on the assumption that streamflow droughts are independent, identically distributed random variables and that their occurrence is subject to the Poisson probability law. This paper is actually a continuation of the previous E. Zelenhasić (1970, 1979, 1983) and P. Todorović (1970) works on the extremes in hydrology. Application of the method is made on the 58‐year record of the Sava River at Sr. Mitrovica and on the 52‐year record of Tisa River at Senta, Yugoslavia, and good agreement is found between the theoretical and empirical distribution functions for all analyzed drought components for both rivers. Only one complete example, the Sava River at Sr. Mitrovica, is given in the paper. The proposed method deals with hydrograph recessions of daily or instantaneous discharges in the region of low flows, and not with mean annual flows which were used by other investig

ISSN:0043-1397    

DOI:10.1029/WR023i001p00156

年代:1987

数据来源: WILEY

摘要:

The engineer is often required to estimate dispersion and first‐order reaction coefficients from data obtained in natural streams. One way to accomplish this is by use of routing methods, in which the coefficients are determined such that the convective‐dispersion (transport) equation most nearly describes the observed concentrations. In the past, analytic solutions to the transport equation have generally been used with the routing method (especially when determining dispersion coefficients) because numerical solutions often contained too much numerical dispersion for the results to be realistic. It is shown that a Lagrangian model can be used to determine realistic estimates of dispersion and reaction coefficients by use of the routing method. The numerical routing procedure is tested using data obtained analytically, under steady flow in a large and small river, in a river with unsteady flow and in a river with steady but highly nonuniform flow conditions. The numerical routing procedure based on a Lagrangian solution scheme performed well in all cases while offering the flexibility that only a numerical solution scheme can of

ISSN:0043-1397    

DOI:10.1029/WR023i001p00169

年代:1987

数据来源: WILEY

摘要:

The spatial and temporal structure of rainfall from storm events is investigated using point process techniques. Cells are assumed to be distributed in space either independently according to a Poisson process, or with clustering according to a Neyman‐Scott scheme. Cells are born randomly through the storm and their rain is spread in time and space according to functions which may include random parameters. Two processes are studied: the rainfall intensity process which in reality is never measured and the cumulative rainfall process through the life of the storm. The mean, variance, and covariance structure are obtained for both processes under the different assumed model

ISSN:0043-1397    

DOI:10.1029/WR023i001p00181

年代:1987

数据来源: WILEY

摘要:

A mathematical model is developed to describe the fate of hydrocarbon constituents of petroleum products introduced to soils as an immiscible liquid from sources such as leaking underground storage tanks and ruptured pipelines. The problem is one of multiphase transport (oil (immiscible), air, and water phases) of a reactive contaminant with constituents such as benzene, toluene, and xylene found in refined petroleum products like gasoline. In the unsaturated zone, transport of each constituent can occur as a solute in the water phase, vapor in the air phase, and as an unaltered constituent in the oil phase. Additionally, the model allows for adsorption. Molecular transformations, microbially mediated or abiotic, are incorporated as sink terms in the conservation of mass equations. An equilibrium approximation, applicable to any immiscible organic contaminant is applied to partition constituent mass between the air, oil, water, and adsorbed phases for points in the region where the oil phase exists. Outside the oil plume the equilibrium approximation takes on a simpler form to partition constituent mass between the air, water, and adsorbed phases only. Microbial degradation of petroleum products is first discussed in a general model, then the conservation of mass equation for oxygen is incorporated into the analysis which takes advantage of the key role played by oxygen in the metabolism of hydrocarbon utilizing microbes in soil environments. Approximations to two subproblems, oil plume establishment in the unsaturated zone, and solute and vapor transport subsequent to immiscible plume establishment are then developed from the general model.

ISSN:0043-1397    

DOI:10.1029/WR023i001p00191

年代:1987

数据来源: WILEY

摘要:

In this paper we develop a numerical solution to equations developed in part 1 (M. Y. Corapcioglu and A. L. Baehr, this issue) to predict the fate of an immiscible organic contaminant such as gasoline in the unsaturated zone subsequent to plume establishment. This solution, obtained by using a finite difference scheme and a method of forward projection to evaluate nonlinear coefficients, provides estimates of the flux of solubilized hydrocarbon constituents to groundwater from the portion of a spill which remains trapped in a soil after routine remedial efforts to recover the product have ceased. The procedure was used to solve the one‐dimensional (vertical) form of the system of nonlinear partial differential equations defining the transport for each constituent of the product. Additionally, a homogeneous, isothermal soil with constant water content was assumed. An equilibrium assumption partitions the constituents between air, water, adsorbed, and immiscible phases. Free oxygen transport in the soil was also simulated to provide an upper bound estimate of aerobic biodgradation rates. Results are presented for a hypothetical gasoline consisting of eight groups of hydrocarbon constituents. Rates at which hydrocarbon mass is removed from the soil, entering either the atmosphere or groundwater, or is biodegraded are presented. A significant sensitivity to model parameters, particularly the parameters characterizing diffusive vapor transport, was discovered. We conclude that hydrocarbon solute composition in groundwater beneath a gasoline contaminated soil would be heavily weighted toward aromatic constituents like benzene, toluene, and xylen

ISSN:0043-1397    

DOI:10.1029/WR023i001p00201

年代:1987

数据来源: WILEY