Robustness of reservoir storage reallocation decisions to climate change

Although numerous assessments of the hydrologic vulnerability of climate change have been performed in the last five years, essentially all of these studies have been based on static future climatic conditions (often doubled CO₂ scenarios), and have assessed climate effects on water supply or availability, without regard to demand changes that might occur over the same period. Many of these studies have shown that existing water supply systems, especially those in the West that are dependent on spring snowmelt, are vulnerable to climate change. The implication of these studies is that water resources planning should account for the possibility of future climate change. An alternative view is that existing planning strategies already are flexible enough to accommodate climate change, due to inherent uncertainty about future streamflows and demand. This paper examines the need to incorporate information about possible future climate change with respect to one particular case, reservoir storage reallocation for the Green River, WA, which is operated for municipal water supply, flood protection, and instream flow maintenance. The key questions are: would a different decision be made if uncertain knowledge about future climate were incorporated in the reservoir reallocation decision?, and what benefits would be foregone (or costs avoided) by failure to incorporate climate change in reservoir reallocation decisions? These questions were addressed using an existing water resources management model for the Green River system.