MUNICIPAL WATER SUPPLY PLANNING FOR DROUGHT

It is the mission of municipal and other domestic water providers to provide high quality water to their customers at a reasonable cost. Included in this mission is the presumption that an adequate amount for use will be provided on a reliable basis. Common criteria have not been adopted by municipal water providers when evaluating the reliability of the raw water supply that will be developed.

Definitions and selection of the critical or planning drought to be used for reliability evaluation vary widely. Drought planning typically includes the analysis of how the supplies and demands compare during a series of critically dry (or drought) years. The firm or safe annual yield of the water supply system is used for water demand planning. Firm yield is usually defined as the maximum constant base demand that can be met with the available supply during a representative hydrologic period. The selection of a representative hydrologic period has come under increased scrutiny as the result of tree-ring and climate change research.

Historic gage records may be insufficient for reliability-based decision making as they fail to provide the decision maker with any meaningful information about the reliability of water supplies from a risk-based perspective. Rational decisions regarding increasing or decreasing the reliability of a system cannot be made. There is a need for statistical information about the performance and yield of water supply systems under varying hydrologic conditions of known (or assumed) probabilities. Appropriate analytical techniques should be employed for estimating the yield of existing water supply systems as a function of their statistical reliability (risk analysis.)

In addition to the selection of a representative hydrologic period, considerations in developing additional system reliability and drought protections, such as the high cost for extreme occurrence, the allocation of costs to new or existing customers and the recovery methods for these costs are discussed. Structural and non-structural options potential actions to address the issues with improving water supply system reliability are compared and evaluated. Considerations in developing additional system reliability and drought protections, including implications of recent climate change research are discussed.