Scenario planning for robust water supply infrastructure design

In the southwestern U.S. and numerous semiarid areas of the world, reclaimed water forms a key component of regional water sustainability planning. Integrated planning of potable water, wastewater and reclaimed/recycled water infrastructures is highly beneficial in regional water sustainability planning that can lead to economic efficiency and increased water system robustness and reliability. Scenario planning is a strategic planning method organizations use to make flexible long-term plans by providing a structured account of a range of possible future transformations. Scenario planning within this framework identifies key uncertainties with respect to future planning and assesses the potential impacts of these uncertainties. Scenario planning for integrated water systems can identify infrastructure elements that are needed over a range of future conditions related to community growth, access to water sources, and public acceptance/rejection of indirect or direct potable reuse of reclaimed/recycled wastewater. Elements common to multiple futures can then be constructed to reduce the likelihood of overbuilding or under-building due to limited knowledge of anticipated future conditions. The primary elements of scenario planning include (i) identifying a decision or focal issue, (ii) identifying forces that drive critical decisions, (iii) ranking key forces by their degree of importance and level of uncertainty, (iv) identifying the most critical uncertainties, (v) building a set of scenarios (futures), (vi) describing the scenarios, (vii) creating scenario paths, and (viii) finding shared elements amongst scenarios. All are briefly described as is the application of the first five steps of the scenario planning procedure within a largely undeveloped (green-field) area situated in southern Pima County and southeastern Tucson, AZ. Limitations of scenario planning and generalizations that can be derived from the case study are discussed.