Using Forecasts in Water Supply Management: History and Applications

Traditionally, municipal water supply planning and operations have focused on assuring reliable sources of water to meet all but the most extreme demands under the drought of record. Systems have been designed to deliver peak daily demands by evaluating distribution limitations and to meet long-term demands by developing sufficient storage or other reliable sources. As demand increased or new, record hydrologic conditions were experienced, new facilities were constructed to both meet the peak demands and seasonal (or over seasonal) requirements. Today, changes in water supply related to climate change, limitations on acceptable new reservoir sites, depletion and restriction of groundwater sources, rising development costs, and growing public concern for environmental impacts have refocused the emphasis from new sources to better management of existing supplies.

Recent improvements to climatological streamflow forecasts have greatly enhanced the ability of water utility and resource managers to plan for and respond to changes in hydrologic conditions, both near term and long term. Improvements to ensemble forecasts, as demonstrated by the National Weather Service’s release of the Hydrologic Ensemble Forecast System (HEFS) and other ensemble products, have facilitated a major shift in water supply planning and operations, from a largely deterministic process to a probabilistic, risk-based approach. This shift has coincided with a growing recognition of the need to get more benefit out of existing infrastructure and water resources; and the need to explicitly consider non-stationarity in water supply planning.

In this presentation, we will provide a historical overview on how water supply planning has evolved to make better use of weather and climate information in both planning and operations contexts. We will discuss the use of system models and other analytical tools to help water supply managers and operators make risk-based decisions. Finally, we will discuss ongoing needs and provide recommendations for continued collaboration between climate scientists and water resources engineers.