摘要:

A stochastic model is described that allows transfer of information from general circulation models to precipitation gauge locations using a weather state classification scheme. The weather states, which are based on present and previous day's sea level pressure, are related stochastically to gauge daily precipitation and temperature. Weather states are defined to give maximal separation of test station precipitation distributions using the Classification and Regression Trees procedure. Precipitation amounts are resampled from historical observations, conditional on the weather state and the previous day's rain state. Daily temperature maxima and minima are simulated by conditioning on the present and previous day's rain state, with the residual modeled as a first‐order autoregressive process. The model parameters are estimated using 9 years (1965–1973) of concurrent U.S. National Meteorological Center (NMC) gridded observations and four precipitation stations in the Columbia River Basin. The model is illustrated using both historical NMC gridded observations of sea level pressure and lower atmosphere temperature, as well as the same variables from the General Fluid Dynamics Laboratory general circulation model for present climate and CO2doubling. A 40‐year sequence of simulated precipitation and temperature is used to estimate seasonal streamflows and flood frequency distributions under present and doubled CO2cli

ISSN:0043-1397    

DOI:10.1029/93WR01066

年代:1993

数据来源: WILEY

摘要:

To evaluate the hydrologic and biogeochemical response of freshwater watersheds to climatic variability properly, a mathematical model with detailed parameterization in describing the hydrologic and thermal processes in a watershed is needed. For this purpose, the Enhanced Trickle Down model was modified to predict the hydrologic and thermal responses of freshwater watersheds to various climate change scenarios. Modifications of the model included the incorporation of an energy transfer submodel, an improved hydraulic conductivity scheme, and the coupling with a point source snowmelt model. The results of calibration and verification of the model using 8 years of field data collected at the Agricultural Research Service, W‐3 watershed, located near Danville, Vermont, are presente

ISSN:0043-1397    

DOI:10.1029/93WR01491

年代:1993

数据来源: WILEY

摘要:

This paper describes laboratory and field experiments which were conducted to study the dynamics of trichloroethylene (TCE) as it volatilized from contaminated groundwater and diffused in the presence of infiltrating water through the unsaturated soil zone to the land surface. The field experiments were conducted at the Picatinny Arsenal, which is part of the United States Geological Survey Toxic Substances Hydrology Program. In both laboratory and field settings the gas and water phase concentrations of TCE were not in equilibrium during infiltration. Gas‐water mass transfer rate constants were calibrated to the experimental data using a model in which the water phase was treated as two phases: a mobile water phase and an immobile water phase. The mass transfer limitations of a volatile organic compound between the gas and liquid phases were described explicitly in the model. In the laboratory experiment the porous medium was nonsorbing, and water infiltration rates ranged from 0.076 to 0.28 cm h−1. In the field experiment the water infiltration rate was 0.34 cm h−1, and sorption onto the soil matrix was significant. The laboratory‐calibrated gas‐water mass transfer rate constant is 3.3×10−4h−1for an infiltration rate of 0.076 cm h−1and 1.4×10−3h−1for an infiltration rate of 0.28 cm h−1. The overall mass transfer rate coefficients, incorporating the contribution of mass transfer between mobile and immobile water phases and the variation of interfacial area with moisture content, range from 3×10−4h−1to 1×10−2h−1. A power law model relates the gas‐water mass transfer rate constant to the infiltration rate and the fraction of the water phase which is mobile. It was found that the results from the laboratory experiments could not be extrapolated to the field. In order to simulate the field experiment the very slow desorption of TCE from the soil matrix was incorporated into the mathematical model. When desorption from the soil matrix was added to the model, the calibrated gas‐water mass transfer rate constant is 2 orders of magnitude lower than that predicted using the power law model develop

ISSN:0043-1397    

DOI:10.1029/93WR01414

年代:1993

数据来源: WILEY

摘要:

Kinetically controlled volatilization and dissolution of nonaqueous phase liquids (NAPLs) may play an important role in the transport of volatile compounds in the unsaturated (vadose) zone. In this study, some one‐ and two‐dimensional steady state transport problems are solved analytically. The one‐dimensional case is pertinent to pollution by a relatively long, mainly horizontally spread leak of NAPL. The two‐dimensional case corresponds to situations in which the pollution spreads primarily vertically, originating at the ground surface and migrating to the top of the capillary fringe, and in which the solution domain may be represented by a cross‐sectional model. Solutions of the steady state transport problems are used to investigate effects of several parameters, characterizing the advective‐dispersive and purely diffusive transport regimes, on the NAPL concentration distribution for the one‐ and two‐dimensional cases, respectively. Results of this analysis indicate that the mass exchange between NAPL and other phases may not reach equilibrium, even for relatively large mass transfer rate coefficients and small water infiltration rates, if this zone has relatively small vertical or horizontal extent and is located close to the fully open ground surface. Analysis of local volatilization and dissolution fluxes shows that, under equilibrium conditions, the main losses of the organic phase take place at the upper part o

ISSN:0043-1397    

DOI:10.1029/93WR01530

年代:1993

数据来源: WILEY

摘要:

Many researchers have used freezing as an effective, short‐term, water sample preservation method for subsequent nutrient analysis. In this study, filtered samples held at −16±2°C for 4–8 years were reanalyzed for orthophosphate, nitrate plus nitrite, and ammonia. Orthophosphate and ammonia concentrations decreased by 0.2 μg P/L and 5 μg N/L, respectively, at mean concentrations of 69.4 μg P/L and 246 μg N/L. Nitrate plus nitrite increased by 1.1 μg N/L at a mean concentration of 139.1 μg N/L. An anaerobic well sample proved to be unsuitable for freezing because it lost significant amounts of orthophosphate during the freezing process. None of the differences observed over long periods of frozen storage were more than twice the estimated standard deviation of the analytical methods used in the study. The small changes observed demonstrate the effectiveness of frozen storage as a means of nutrient preservation in water samples that are unaffected by the freezing p

ISSN:0043-1397    

DOI:10.1029/93WR01684

年代:1993

数据来源: WILEY

摘要:

This study has two objectives: (1) to compare the attitudes of the water‐using public, water officials, and elected officials toward the risk of water supply shortage; and (2) to develop a methodology for incorporating water users' valuation of reliability in system design. Using contingent valuation techniques, we have measured the benefits and costs of different reliability levels in terms of water users' willingness to pay (WTP) for increases in reliability and in terms of their willingness to accept (WTA) compensation in the form of lower water bills for lower levels of reliability. Three cities in northern Colorado with diverse baseline levels of water supply reliability (Aurora, Boulder, and Longmont) are the study sites. Contrary to our hypothesis that water managers are unjustifiably risk averse, we find that water managers' preferences are consistent with customer WTP (WTA) values associated with the risk of water shortages and the system costs associated with reliability. Water managers in Boulder (high reliability) were willing to consider reductions in the level of system reliability while water managers in Aurora and Longmont (low reliability) favored the status quo or increased reliability. While these attitudes were sometimes contrary to a majority of customers' expressed interests in change, they were shown to be justified by comparison of supply system costs (savings) with aggregate WTP for additional reliability (WTA for less reliability

ISSN:0043-1397    

DOI:10.1029/93WR01040

年代:1993

数据来源: WILEY

摘要:

Groundwater quantity management problems with fixed charges have been formulated in the past as mixed integer and linear programming problems. In this paper a new methodology is presented where the fixed charges are incorporated into the objective function in an exponential form and the problem is solved as a concave minimization problem. The principal difficulty in the minimization of a concave function over a linear or nonlinear set of constraints is that the local minima which are determined by the classical minimization algorithms may not be global. In an effort to circumvent this problem the outer approximation method is introduced. This method is applicable to the global minimization of a concave function over a compact set of constraints. In the present work the outer approximation is applied to concave minimization problems over a convex compact set of constraints. Two applications of the method to groundwater management problems are presented herein, and the results are compared with an existing solution obtained using a different optimization approach.

ISSN:0043-1397    

DOI:10.1029/93WR01388

年代:1993

数据来源: WILEY

摘要:

This paper describes the outline of an approximate mathematical technique capable of including stochastic components in environmental models based on ordinary differential equations. The technique is based on the decomposition of the solution into additive components, with the first component being the solution of a simplified deterministic equation and each of the other components being found in terms of those preceding it. Moments of the solution process can then be calculated by performing appropriate averaging. The method provides natural statistical separability, that is, no truncations in the derivation stage or closure approximations are necessary. The random quantities involved can be either random variables or stochastic processes. No unrealistic assumptions, like white noise behavior or small randomness, are necessary. Solutions, which are in the form of infinite but convergent series, can be easily implemented into computer programs, providing convenient support in management and decision analysis. A demonstration application of a simple biochemical oxygen demand model for river waters illustrates the usefulness of the technique in environmental modeling.

ISSN:0043-1397    

DOI:10.1029/93WR01167

年代:1993

数据来源: WILEY

摘要:

Important features of multiphase flow in porous media that distinguish it from single‐phase flow are the presence of interfaces between the fluid phases and of common lines where three phases come in contact. Despite this fact, mathematical descriptions of these flows have been lacking in rigor, consisting primarily of heuristic extensions of Darcy's law that include a hysteretic relation between capillary pressure and saturation and a relative permeability coefficient. As a result, the standard capillary pressure concept appears to have physically unrealistic properties. The present paper employs microscopic mass and momentum balance equations for phases and interfaces to develop an understanding of capillary pressure at the microscale. Next, the standard theories and approaches that define capillary pressure at the macroscale are described and their shortcomings are discussed. Finally, an approach is presented whereby capillary pressure is shown to be an intrinsic property of the system under study. In particular, the presence of interfaces and their distribution within a multiphase system are shown to be essential to describing the state of the system. A thermodynamic approach to the definition of capillary pressure provides a theoretically sound alternative to the definition of capillary pressure as a simple hysteretic function of saturatio

ISSN:0043-1397    

DOI:10.1029/93WR01495

年代:1993

数据来源: WILEY

摘要:

In situ sensing technology, used in a series of natural‐gradient tracer tests at the Chalk River Laboratories in Ontario, leads to the introduction of a conceptually new approach to the study of groundwater motion in porous media. As opposed to the conventional approach, based on the consideration of a fictitious fluid continuum with fluid properties distributed over both voids and solids, in the new approach the actual groundwater motion in the void space of a porous medium is considered and described at the local scale by the statistical characterization of the propagation of gamma‐radiation energy associated with the moving water as a tracer. The essential feature of the new approach is that the mean free path of a gamma‐energy photon instead of the porosity is used as a scaling factor in transferring information associated with pore‐scale fluid motion to the local scale. This scaling factor is employed for reintroducing the familiar particle model of fluid motion but at the local scale. It is shown that when the local‐scale dispersion is neglected, the evolution of local‐scale fluid particles making up the tracer plume can be described by the advection equation; its equation of characteristics describes trajectories of local‐scale particles. A simple analytical solution to the advection equation is then used to produce three‐dimensional images of the spatial distribution of local‐scale particles observed in the Twin Lake test. It is also shown that the spatial averaging procedure with regard to the weighting function for a spherical averaging volume of one mean free path radius may be used to introduce the three‐dimensional field of local‐scale concentration. The averaging procedure is then used to illustrate that the concept of the three‐dimensional field of plume‐scale concentration does not make physical sense and only the one‐dimensional plume‐scale concentration field may be i

ISSN:0043-1397    

DOI:10.1029/93WR01274

年代:1993

数据来源: WILEY