Alan F. Hamlet, Philip W. Mote, Nate Mantua, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington Implications of Climate Change for Streamflow and Water Quality in the Western U.S.

Example of a flawed water planning study: The Colorado River Compact of 1922 The Colorado River Compact of 1922 divided the use of waters of the Colorado River System between the Upper and Lower Colorado River Basin. It apportioned **in perpetuity** to the Upper and Lower Basin, respectively, the beneficial consumptive use of 7.5 million acre feet (maf) of water per annum. It also provided that the Upper Basin will not cause the flow of the river at Lee Ferry to be depleted below an aggregate of 7.5 maf for any period of ten consecutive years. The Mexican Treaty of 1944 allotted to Mexico a guaranteed annual quantity of 1.5 maf. **These amounts, when combined, exceed the river's long-term average annual flow**.

Despite a general awareness of these issues in the water planning community, there is growing evidence that future climate variability will not look like the past and that current planning activities, which frequently use a limited observed streamflow record to represent climate variability, are in danger of repeating the same kind of mistakes made more than 80 years ago in forging the Colorado River Compact. Long-term planning and specific agreements influenced by this planning (such as long-term transboundary agreements) should be informed by the best and most complete climate information available, but frequently they are not. What’s the Problem?

Global Climate Change Scenarios and Hydrologic Impacts for the PNW

Pacific Northwest °C °C °C °C Observed 20th century variability +1.7°C +0.7°C +3.2°C

Pacific Northwest % -1 to +3% -1 to +9% -2 to +21% Observed 20th century variability +1% +2% +6%

The warmer locations are most sensitive to warming +2.3C, +6.8% winter precip 2060s

Simulated Changes in Natural Runoff Timing in the Naches River Basin Associated with 2 C Warming Impacts: Increased winter flow Earlier and reduced peak flows Reduced summer flow volume Reduced late summer low flow

April 1 SWE (mm) 20 th Century Climate“2040s” (+1.7 C)“2060s” ( C) -3.6%-11.5% Changes in Simulated April 1 Snowpack for the Canadian and U.S. portions of the Columbia River basin (% change relative to current climate) -21.4%-34.8%

Effects of Basin Winter Temperatures Northern Location (colder winter temperatures) Southern Location (warmer winter temperatures)

Mapping of Sensitive Areas in the PNW by Fraction of Precipitation Stored as Peak Snowpack HUC 4 Scale Watersheds in the PNW

Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): Trends in April 1 SWE

Water Resources Implications Reductions in natural storage in mountain watersheds Potential increases in water demand and evaporation Increasing drought and altered flood risks Increasing competition over water resources Tradeoffs between traditional water resources objectives such as water supply and hydropower production and environmental services related to instream flow Need for changes in flood control evacuation and refill schedules Disruption of existing water allocation agreements Disruption of transboundary agreements

Flood Control vs. Refill Full : Current Climate

Flood Control vs. Refill Streamflow timing shifts can reduce the reliability of reservoir refill Full : Current Climate o C : o C No adaption

Flood Control vs. Refill Streamflow timing shifts can reduce the reliability of reservoir refill Full : Current Climate : o C plus adaption o C : o C No adaption

Water Quality Implications Increasing water temperature Altered sediment transport processes Altered chemical processes (dissolved gas, BOD) Altered biological processes (algae, periphyton, macrophytes) Need for increased use of managed storage to maintain equivalent dilution flows in summer.

Temperature thresholds for coldwater fish in freshwater +1.7 °C +2.3 °C Warming temperatures will increasingly stress coldwater fish in the warmest parts of our region –A monthly average air temperature of 68ºF (20ºC) has been used as an upper limit for resident cold water fish habitat, and is known to stress Pacific salmon during periods of freshwater migration, spawning, and rearing

Conclusions Climate change will result in significant hydrologic changes in the Western U.S. including reduced natural storage as mountain snowpack, increased flow in winter, and reduced flow in summer. Changes in extremes (droughts and floods) are likely to occur. Impacts will not be equally distributed, and areas near freezing in mid winter will be the most sensitive to warming related losses of snowpack and streamflow timing shifts. A number of impact pathways related to water resources management and water quality are likely to be activated by these changes. There is a wide-spread need to incorporate expected changes in climate into long range planning.