Water Resources Planning for an Uncertain Future Climate Center for Science of the Earth System and the Department of Civil Engineering University of Washington March, 2002 Alan F. Hamlet Dennis P. Lettenmaier

Overview Why include climate change information in water resources planning studies at all? What criteria can help identify the kinds of planning studies should include climate change information? What techniques can be used for including climate change information in hydrologic studies? Use of linked climate and hydrologic models Use of historic analogues Some issues in the Columbia basin and general considerations for generating alternate operating policies.

Why include climate change information in water resources planning studies at all? Most water systems cannot respond quickly to changes in climate. Significant physical or operational changes to water systems frequently requires 25 years or more to implement, the same time scale as expected changes in climate. Avoiding impacts will require innovative and flexible long-term planning. Many water systems in the PNW are vulnerable to loss of snowpack and shortages of water in summer, the most likely effect of climate change. Having a plan is not the same as implementing a plan. Long term planning always includes uncertain information, and including climate change information is prudent, does not cost a lot, and makes the planning process more robust to climate issues in general, even if contingency plans are never implemented. Addressing public and legislative concern. Being asked to redo a planning study because climate change was not considered is costly and embarrassing.

What criteria can help identify the kinds of planning studies that should include climate change information? Piggybacking: Planning is in progress or required for other reasons (climate change assessment adds relatively little cost to an existing planning process) Rare opportunity: The planning arena is unlikely to be revisited in the next several decades due to cost or other considerations Sensitivity: the water system in question is highly sensitive to reductions in snowpack and summer streamflow, or to other changes in streamflow timing. Durability: High costs and/or long economic life span associated with decisions addressed by a particular planning process Irreversibility: planning decisions made now that may permanently and irreversibly increase future vulnerability Inflexibility: The importance of long-term planning is elevated because planning at shorter time scales would be ineffective

What techniques can be used for including climate change information in hydrologic studies?

Climate Change Scenarios Long term planning for climate change may include a stronger emphasis on drought contingency planning, testing of preferred planning alternatives for robustness under various climate change scenarios, and increased flexibility and adaptation to streamflow uncertainty. Observed Streamflows Planning Study Altered Streamflows Climate Change Scenarios

Changes in Mean Temperature and Precipitation or Bias Corrected Output from GCMs ColSim Reservoir Model VIC Hydrology Model

Changes to Snow Extent and Naturalized Streamflow at The Dalles April 1 Snow Extent Estimated Range of Natural Flow With 2040’s Warming Current 20th Century Natural Flows ~2045

Changes to Mean Hydrographs Columbia Basin 2045

Historic Analogues: El Niño and drought years as a surrogate for climate change Water Year 2001

Water Year 1992

Years with Unusually High Winter Flows 1996 CC 3

Years with Unusually Early or Rapid Snowmelt 1998 CC 4

Some Water Management Issues in the Columbia Basin Affecting Potential Climate Change Impacts Conflicts between irrigation, hydropower, flood control, and instream flows for fish. Reductions in summer streamflow are likely to exacerbate these conflicts. The relatively fragmented 2001 drought response demonstrates that we are not well prepared for this eventuality in the PNW. Columbia River Treaty--~50% of the system storage is located in and controlled by Canada. For the future climate a larger proportion of the snowpack and late summer streamflow may also be in Canada. That water no longer flows across the border unimpeded. Lack of Drought Contingency Planning. The situation in the Klamath basin in 2001 is an example of what can happen if fundamental climate vulnerabilities are ignored until major impacts occur.

Implications for Flood Control in the Columbia Basin in a Warmer Climate Current flood control operations may provide insufficient flood protection in late fall and early winter (higher natural streamflows in winter due to warming) Flood evacuation quantity and timing may need to be adjusted to achieve appropriate flood protection in early spring AND acceptable reliability of reservoir refill and instream flow in spring and summer. (earlier snowmelt and reduced snowpack) Other changes in the system may significantly alter the current management framework in the basin. Flood control will need to be flexible enough to mesh with these changes. (E.g. move dominant hydropower production to summer.)

Qualitative Changes to Flood Evacuation Requirements to Adjust to Warmer Conditions Altered RC (Dry Year) Altered RC (Wet Year)

Conclusions The PNW’s vulnerability to reduced snowpack, and the time frame needed to make substantive changes in water systems places a strong emphasis on long-term planning as a means to successfully re-designing our water systems to be robust to the potential impacts of climate change. Piggy backing climate change adaptation studies with planning studies already proposed for other reasons is a good way to include climate change information without adding large costs. A number of straight-forward techniques are currently available for including climate change scenarios in planning studies, including integrated hydrologic modeling, and use of unusual historic water years as analogues for flows in a warmer PNW. The implications for flood control include the potential for increased risk of flooding in late fall and early winter (more run-off in winter), and changes in the timing and quantity of peak flows in spring (earlier and less spring and summer runoff) . Operational flexibility will be essential in coping with uncertain changes as they unfold.