美国环境政策的基本经济原则可以在外部性理论中找到。这一理论关注的是产生副作用的经济活动，而生产者却忽略了这些副作用。关于外部性的文献非常丰富——实际上有数百本书和论文讨论外部问题。将外部性成本内部化的尝试为美国的许多监管实践提供了理由。本文探讨了围绕水资源保护的外部性问题。虽然大多数关于外部性的讨论和例子都强调对环境的负面影响，但本文着眼于实践节约的积极外部性，例如应用节水项目的效益-成本分析，比较需求侧管理和供应增加。

The basic economic principles of environmental policy in the US are found in the theory of externalities. This theory focuses on economic activities that create side-effects ignored by the producer. The literature on externalities is huge--literally hundreds of books and papers address external issues. Attempts to internalize the cost of externalities provides the justification for much of the regulatory practices in the US. This paper examines the externality issues surrounding water conservation. While most discussions and examples of externalities emphasize negative impacts on the environment, this paper looks at the positive externalities of practicing conservation such as applying benefit-cost analysis of water conservation programs and comparing demand side management with supply augmentation.