Water systems (WSs) use a large amount of energy and emit greenhouse gas (GHG). Accounting emissions from all phases of a project (construction, operation, and decommissioning) is called a life cycle approach. Energy consumption and GHG emissions are two critical elements in the life cycle of WSs. This paper aims to present a methodology for quantifying the life cycle energy (LCE) and life cycle greenhouse gas (LCGHG) implications of WSs in Iran. The primary objective is to represent a methodology of the cost–benefit analysis and a typology of the different scenarios, which respond to various policies. Future economic scenarios are designed and used to describe subsequent impacts. Results indicate that energy consumption accounts for about 60 percent of GHG emissions in Iran’s water system. The results show that cost–benefit analysis is highly sensitive to the social costs of reducing GHG emissions. Neglecting the social costs leads to economically infeasible scenarios. The benefit cost ratio of scenarios considering social costs is more than two times greater than scenarios without this consideration. Therefore, if a cost–benefit analysis is performed without considering social costs, the financial resources will not be allocated efficiently and in a way that reduces problems of air pollution.

供水系统 (WS) 使用大量能源并排放温室气体 (GHG)。对项目所有阶段（建设、运营和退役）的排放进行核算称为生命周期方法。能源消耗和温室气体排放是 WS 生命周期中的两个关键要素。本文旨在提出一种量化伊朗 WS 的生命周期能量 (LCE) 和生命周期温室气体 (LCGHG) 影响的方法。主要目标是代表一种成本效益分析方法和不同情景的类型学，以响应各种政策。未来的经济情景被设计并用于描述后续影响。结果表明，能源消耗约占伊朗供水系统温室气体排放量的 60%。结果表明，成本效益分析对减少温室气体排放的社会成本高度敏感。忽视社会成本会导致经济上不可行的情况。考虑社会成本的情景的效益成本比是不考虑社会成本的情景的两倍以上。因此，如果在不考虑社会成本的情况下进行成本效益分析，财政资源将无法有效分配，也无法以减少空气污染问题的方式进行。