Climate-Change Scenarios for Water Planning Studies: Pilot Applications in the Pacific Northwest

NOVEMBER 2003 AMERICAN METEOROLOGICAL SOCIETY | espite uncertainties in predictions of the magnitude and timing of anthropogenic climate change, many Pacific Northwest (PNW) water managers now recognize climate change as a significant issue that should be addressed in water resources planning. This represents a significant change over the last several years. These managers are requesting detailed information about potential climate impacts on their systems, in a form suitable for inclusion in planning. Academic climate-change assessments, which have been available since the mid-1980s, often produce information that is inconsistent with the specific periods of the historic streamflow record and/or the internally developed water management models used by individual water management agencies in formal water planning studies. While water managers may find an academic assessment of a system’s sensitivity to climate change (produced by coupling academically developed climate, hydrologic, and water management models) to be perfectly believable in the abstract, they are unlikely to consider such an assessment directly relevant to the decisions they make about the system they manage. This is one reason that water-planning studies have rarely addressed the implications of climate change in any more than a very cursory manner. We therefore believe that water management agencies would be more apt to incorporate climate change in their planning in the short term if they had access to scenarios of future streamflow that could easily and inexpensively be used within the confines of their existing planning frameworks. A few water management agencies in the PNW have taken the lead in incorporating climate change in their planning efforts. The Portland (Oregon) Water Bureau, for example, recently completed a study in partnership with the University of Washington’s Climate Impacts Group (CIG) that evaluated several long-range planning alternatives under four different scenarios of future climate. Seattle Public Utilities (which manages Seattle’s water supply) has begun a similar planning process. This paper describes how we are working to expand the availability of climate-change streamflow scenarios that would allow more planners to assess the impacts of climate change on PNW water systems. In the pilot project described here, streamflow scenarios are being generated to support two large-scale PNW water-planning studies.