From tree-rings to GCMs: A new methodology for water resources planning and reservoir management at the Salt River Project in Arizona

The Salt River Project (SRP) delivers nearly 1 million acre-feet of water annually, mainly from the 13,000 square mile watershed of the Salt and Verde rivers, to a 375 square mile service area in central Arizona. SRP was created in the early 1900s (as an early federal reclamation project) to assure an adequate water supply for its shareholders in the Salt River Valley.

The straight forward job of storing inflows and meeting demand from a single reservoir system soon became more complex. As the population of the Salt River Valley swelled (and became the Phoenix metropolitan area), additional reservoirs were added to the system, alternative supplies of water were developed, and hydro-power generation became a financial consideration in reservoir operations. The primary operational objective continues to be the conjunctive management of multiple sources of surface and ground water to ensure an adequate year-to-year carryover supply of water for SRP's shareholders.

This objective has traditionally been accomplished by managing the reservoir system as each time the reservoirs fill to capacity, an extended drought comparable to the worst drought in recorded history (7 years) is assumed to begin. Over the past 20 years, several subtle yet significant events have taken place which raise concerns regarding SRP's traditional method of water planning and management.

Changes in demand patterns as land is converted from mainly agricultural use to urban use, an ongoing drought rivaling the historical drought of record, tree-ring studies suggesting even more severe droughts have occurred in pre-historic times and the specter of a changing climate due to global warming all suggest that a business-as-usual approach to water management and planning is no longer be appropriate.

In response, the SRP Water Resource Operations group developed a new planning guide and operations procedures that allow better tracking of current water supplies and improve decision making in the context of uncertainty about future short-term weather and longer-term climate outcomes.