摘要:

When estimating demand models, such as those used in travel cost analysis, the procedures used to collect data must be considered. When the statistical assumptions underlying the chosen model are inconsistent with the sample design, biased results will be produced. In this paper, three types of bias related to the improper treatment of nonparticipants are reviewed: censoring, truncation, and endogenous stratification. Methods that control for these sources of bias are considered in the context of both continuous and count models.

ISSN:0043-1397    

DOI:10.1029/92WR00762

年代:1992

数据来源: WILEY

摘要:

In a previous paper it was established that a simple relationship exists between the peak reduction ratio and the relative storage ratio of unregulated detention reservoirs. The relationship, named the efficiency function of the reservoir, depends on the physical features of the reservoir and its outlet, and also on the shape of the inflow hydrograph. For a specified type of reservoir and a given inflow hydrograph, the efficiency function predicts the storage volume needed to produce a desired peak flow reduction. In this paper, it is demonstrated that it is possible to achieve the same peak flow reduction with a smaller reservoir if a parallel drain design is used. In this design procedure the inflow hydrograph is split into two parts. One part is routed through a detention reservoir and the second part is allowed to bypass the reservoir. The two parts are recombined below the reservoir to form the outlet hydrograph which is the sum of two hydrographs with staggered peaks. By a proper choice of the proportion of flow directed through the reservoir it is possible to achieve a desired peak flow reduction with a significant reduction in active storage.

ISSN:0043-1397    

DOI:10.1029/92WR00761

年代:1992

数据来源: WILEY

摘要:

Hydraulic conductivity (K) variability in a sand and gravel aquifer on Cape Cod, Massachusetts, was measured and subsequently used in stochastic transport theories to estimate macrodispersivities. Nearly 1500Kmeasurements were obtained by borehole flowmeter tests and permeameter analyses of cores. The geometric mean for the flowmeter tests (0.11 cm/s) is similar to that estimated from other field tests. The mean for the permeameter tests (0.035 cm/s) is significantly lower, possibly because of compaction of the cores. The variance for the flowmeter (0.24) is also greater than that for the permeameter (0.14). Geostatistical analyses applying negative exponential models with and without nuggets reveal similar spatial correlation structures for the two data sets. Estimated correlation scales range from 2.9 to 8 m in the horizontal and from 0.18 to 0.38 m in the vertical. Estimates of asymptotic longitudinal dispersivity (b.35–0.78 m) are similar in magnitude to that observed in the natural gradient tracer test (0.96 m) previously conducted at this sit

ISSN:0043-1397    

DOI:10.1029/92WR00668

年代:1992

数据来源: WILEY

摘要:

A global atmospheric general circulation model is used to examine seasonal river runoff for the world's largest rivers. The model's river runoff is not directly comparable to observed runoff at the mouth of a river basin because the model assumes that all runoff within a river's drainage basin reaches the river mouth instantaneously. It is useful, however, to examine the model's monthly runoff to determine the extent to which it does agree with runoff as understood by hydrologists, The runoff and precipitation for the river drainage basins are compared with observations. For most river basins the model produces too much precipitation and runoff. However, for the world's largest river, the Amazon, the mean annual runoff is less than half the observed. A common characteristic of high‐latitude rivers is a large runoff peak in the spring, primarily due to melting snow. The model reproduces these peaks but generally leads the observed runoff by one month. This arises because the model does not allow runoff to move between grid boxes but calculates only the total monthly runoff into the entire drainage basin. The runoff in dry river basins is poorly simulated by the model, in part because the precipitation is too large but also because the model's surface parameterizations do not accurately apportion the water lost between evapotranspiration and runoff. The present version of the atmospheric model contains two soil layers and specifies the grid box runoff at each time step as a linear function of the soil moisture storage and the precipitation. A more physically based model should improve the model's simulation of river runoff. A model's ability to simulate monthly river runoff is a good test of its hydrologic cycle and surface parameterizations in different climatic regions and provides a useful diagnostic for climate modeler

ISSN:0043-1397    

DOI:10.1029/92WR00917

年代:1992

数据来源: WILEY

摘要:

Four years of runoff measurement (29 events) in Kangussano, Mali, were analyzed on the basis of the partial area contribution concept. The study region is semiarid and the use of runoff water to satisfy crop consumption.is of high importance. A first‐order basin which included two flow channels draining an area of 1.14 106ha (1.14 km2) was used for this purpose. The objective of this study was to develop and test a conceptual model to predict runoff on natural catchments of about this size. The model assumes that a basin is composed of a large number of pixels (satellite picture elements). Each pixel covers an area of 900 m2. The hydraulic properties of a representative pixel are determined by runoff simulation experiments. The model calculates the number of runoff generating pixels at a given time and rain depth. The areal runoff is a product of these two factors. Analysis of satellite images from LANDSAT and SPOT indicated similarities of soil cover complexes between the study area and an experimental area in Upper Volta. Results of runoff simulation from this area were adapted for the study area and used for the calculations. A good agreement between predicted and measured contributing area was obtained when the number of runoff generating pixels was 80 pixels/ram effective rain. Predicted and measured hydrographs were also in good agreement. It was therefore concluded that the model can satisfactorily predict actual hydrographs from data generated by rainfall‐runoff simulators. Dividing the watershed to surface elements of pixel's size makes the model capable of linking remote sensing information with simulation data in order to predict areal run

ISSN:0043-1397    

DOI:10.1029/92WR00906

年代:1992

数据来源: WILEY

摘要:

Previous work has shown that a one‐dimensional water content‐based finite difference algorithm can offer improved CPU efficiency in modeling infiltration into very dry heterogeneous soil when compared to similar pressure‐based algorithms. The usefulness of the water content‐based method is limited, however, since it cannot be applied to saturated soils. In this paper we develop two new methods which retain the advantages of water content‐based methods while still permitting fully saturated conditions. In the first method we define a new variable for the transformed Richards equation which has the characteristics of water content when soil is unsaturated and of pressure when soil is at or near saturation. In addition to the transformed Richards' equation method, an improved pressure‐based method which uses flux updating is presented. Both methods are implemented in algorithms using a preconditioned conjugate gradient method equation solver. The performance of the algorithms is compared to that of an algorithm based on a mixed form of Richards' equation using modified Picard iteration. Two test cases are examined. The first test case examines two‐dimensional infiltration into very dry heterogeneous soil which remains unsaturated throughout the simulation period. The second test case examines a two‐dimensional developing perched water table. Results indicate that the new methods retain the advantages of the water content‐based method for fully unsaturated heterogeneous problems while allowing for fully saturated conditions. For the unsaturated problem the new algorithms were from 16 to 59 times faster than the algorithm based on the mixed form of Richards' equation using modified Picard iteration. For the saturated problem the new algorithms were from 6 to 25 times faster than the mixed base algorithm. Results also indicate that the performance of the mixed base algorithm could be significantly improved if a more effective adaptive time st

ISSN:0043-1397    

DOI:10.1029/92WR00802

年代:1992

数据来源: WILEY

摘要:

A suitable equation for describing overland flow and the values for parameters of hydraulic resistance are required in surface irrigation models. The approaches to describe the flows and to estimate hydraulic resistance parameters in the past varied from simple empirical studies to complex semiempirical and theoretical studies. The Manning equation and Manning n are widely used for shallow flows in surface irrigation although their suitability for such flows is questionable. In this article the various flow equations are reviewed for their suitability to surface irrigation flows. It is revealed that there are only a few equations which are relevant to these flows. A need for a dependable method of estimating the hydraulic resistance parameters in surface irrigation is highlighted.

ISSN:0043-1397    

DOI:10.1029/92WR00424

年代:1992

数据来源: WILEY

摘要:

A boundary integral formulation is developed for flow of two immiscible, incompressible phases in a porous medium. With the boundary integral formulation, large computational domains may be considered with little impact on accuracy or computational effort, as the boundary integral mesh is placed only along contours of phase saturation and the domain boundaries. Time stepping is performed by updating the position of the contours based on phase fluxes obtained with a total flux formulation. Capillary pressure gradients are treated by interpolating saturation between contours using a separate boundary integral step; accordingly, the formulation is not restricted to the special form of the constitutive relationships implied in the quasi‐linear approximation to unsaturated flow. Examples of one‐dimensional and radial two‐phase flow are simulated, with excellent agreement to the corresponding analytical solutions. Vertical imbibition of trichoroethylene into an initially pristine aquifer is simulated with both a finite element simulator and the boundary integral simulator, with good agreement between the two me

ISSN:0043-1397    

DOI:10.1029/92WR00542

年代:1992

数据来源: WILEY

摘要:

Increment cores were collected from trees growing at two sites where groundwater is contaminated by nickel. Proton‐induced X ray emission spectroscopy was used to determine the nickel concentrations in selected individual rings and in parts of individual rings. Ring nickel concentrations were interpreted on the basis of recent concentrations of nickel in aquifers, historical information about site use activities, and model simulations of groundwater flow. Nickel concentrations in rings increased during years of site use but not in trees outside the contaminated aquifers. Consequently, it was concluded that trees may preserve in their rings an annual record of nickel contamination in groundwater. Tulip trees and oaks contained higher concentrations of nickel than did sassafras, sweet gum, or black cherry. No evidence was found that nickel accumulates consistently within parts of individual rings or that nickel is translocated across ring boundarie

ISSN:0043-1397    

DOI:10.1029/92WR00731

年代:1992

数据来源: WILEY

摘要:

The dispersion of a solute plume resulting from unsteady flow in three‐dimensional, heterogeneous porous media was analyzed using stochastic continuum theory. Asymptotic stochastic solutions of the perturbed unsteady flow and solute transport equations were used to construct the macroscopic dispersive flux and evaluate the resulting macrodispersivity tensor in terms of a three‐dimensional, statistically anisotropic input covariance describing the hydraulic conductivity and an input covariance describing the temporal variability of the mean hydraulic gradient. The flow equation was approximated by neglecting the influence of internal storage due to medium and fluid compressibility (specific storage,SS≈ 0). The predictive expression for the macrodispersivity tensor is the sum of two components: a spatial variability component identical to previous steady state theory and an unsteady component. Two special cases of unsteady flow were examined: variation only in the magnitude and variation only in the direction of the hydraulic gradient. Gradient magnitude variation produces a slightly larger longitudinal macrodispersivity than does the steady flow case, whereas gradient direction variation produces a significantly larger transverse macrodispersivity. Longitudinal and horizontal transverse macrodispersivities predicted with the unsteady stochastic theory were shown to be of a magnitude similar to observed values from the Borden, Cape Cod, and Columbus tracer experi

ISSN:0043-1397    

DOI:10.1029/92WR00750

年代:1992

数据来源: WILEY