SCD is characterized by a predominantly obstructive pattern of lung function that increases in prevalence over time. There was no apparent causal relationship between the pattern of somatic growth and the pattern of lung function.

We estimated the economic benefits resulting from controlling soybean aphid infestation by using a multi-regional competitive dynamic equilibrium model. Results indicate that the reduction of soybean production resulting from a soybean aphid infestation is largely absorbed by reducing soybean exports, due to the higher price elasticity of export demand compared to domestic demand. Producer benefits resulting from controlling soybean aphids would increase by between $949 million and $1.623 billion in ten years under various scenarios. Results also suggest that it is economically more efficient to control soybean aphids when the rate of intrinsic growth is relatively lower, the supply price elasticity of soybean acreage is relatively more elastic, and insecticide treatment costs per acre are lower. However, if the discovery of the gene Rag-1 (TF04048) leads to new cultivars that withstand the soybean aphid, our estimates will overestimate the actual damages. Even so, our analysis demonstrates that it is critical to control soybean aphids early in their infestation cycle to avoid a rapid increase in damages.

Water development and allocation to competing uses without well-defined water quality rights contribute to water use externalities. Federal legislation to address the salinity externalities in the Colorado River Basin comprises a set of arbitrary quality standards and millions of dollars in federal projects. This study specifies economic criteria to empirically determine first- and second-best quality standards and to indicate opportunities for efficiency gains in existing policy. A basin-wide, nonlinear programming model optimizes river water quality, resource allocation, production levels, and total expenditures for control. Revealed are the economic tradeoffs between water uses, regions, and control strategies. Copyright 1996, Oxford University Press.

Dominant users of Lake Okeechobee water resources are agricultural producers and recreational anglers These uses will be directly affected, should the lake become infested with zebra mussels. We employ a probabilistic bioeconomic simulation model to estimate the potential impact of zebra mussels on consumptive water uses, recreational angling, and wetland ecosystem services under alternative public management scenarios. Without public management, the expected net economic impact from zebra mussels is –$244.1 million over 20 years. Public investment in prevention and eradication will yield a net expected gain of +$188.7 million, a superior strategy to either prevention or eradication alone.

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Using a bio-economic model of zebra mussels (Dreissena polymorpha), we examine the expected economic value of prevention, control and eradication alternatives for the freshwater mussel in Lake Okeechobee (Florida, USA). We include two emerging technologies for zebra mussel (ZM) control: (1) a natural pesticide called Zequanox, and (2) hot wash stations at boat ramps. We employ water management district data, user data collected via a phone survey, and mitigation expenditures from infested locations elsewhere to estimate the potential damage from the introduction of zebra mussels in Florida. Methods used include static cost transfer estimation, econometric cost estimation, and stochastic-dynamic simulation. We use our bio-economic model to compare costs and risks with and without the emerging technologies. We also consider the impact of technology adoption rates by anglers, management policy efficacy, and opportunity costs associated with ZM control. Results indicate that, without investment in prevention, there is a very high probability that Florida waterways will be infested with zebra mussels by year 2025, and expected environmental damages and management costs are high. Slow response due to poor detection methods or insufficient control efforts will lead to a moderate probability of a significant infestation. Rapid reaction and enhanced prevention efforts are expected to greatly reduce the probability of ZM infesting Lake Okeechobee by 2025, and to generate much higher expected net benefits.

This research combines mass transport theory with stochastic methods to derive a stochastic, variable flow, surface water quality model for water quality policy analysis. Steady-state flow assumptions and a series of differential equations reduce the information and number of equations required to completely specify the model. The newly developed hydrology model is applied to river salinity in the Colorado Basin to evaluate the effectiveness of alternative salt load reduction strategies in water quality management. Model results are used to illustrate the influence of salt load reduction, stochastic river flows, and water conservation on river water quality. INTRODUCTION This paper describes a stochastic, variable flow river water quality model for water quality policy analysis. The model comprises a series of differential equations that approximate a large-scale hydrology simulation model but is compact and can be executed simultaneously within an economic decision-making framework. Use of the model is illustrated with an application to salinity in the Colorado River Basin. Large-scale models such as those by Udis et al. [1973] and the Colorado River Simulation System [U.S. Department of the Interior, 1981] model the hydrological flows of the Colorado River Basin using extensive databases and numerous equations. These large models can simulate a wide range ofhydrologic scenarios. For economic policy analysis, however, large-scale hydrology models can be unwieldy. Thus in an earlier work, Gardner [1983] assumed constant river flows and used a linear equation to model the hydrologic link between two regions in the Colorado River Basin. Constant flow assumptions, however, fail to account for the effect of upstream changes in water use and the implications of natural fluctuations in river volume (from snowmelt and precipitation) on downstream salinity. Several studies have looked at using upstream water to dilute downstream river salinity. Scherer [1977] developed a hypothetical stream model that incorporated upstream water uses and allowed for dilution as a mitigation alternative. In Scherer's [ 1977] model, however, streamflows were deterministic. Although Colorado River flows are largely controlled by a system of dams, lakes, and reservoirs, seasonal and annual flows can vary widely. As a result, river quality falls during low-flow