Alan F. Hamlet, Philip W. Mote, Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington Effects of Climate Change and Climate Variability on Water Supply 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 water supply planning and specific agreements influenced by this planning (e.g. long-term water allocation agreements) should be informed by the best and most complete climate information available, but frequently they are not. What’s the Problem?

DJF Temp (°C) NDJFM Precip (mm) PNW CACRB GB Cool Season Climate of the Western U.S.

Natural Climate InfluenceHuman Climate Influence All Climate Influences Natural AND human influences explain the observations of global warming best.

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%

Hydrologic Changes Associated with Warming

For areas that accumulate snowpack in winter, the warmest locations are most sensitive to warming +2.3C, +4.5% winter precip

April 1 SWE (mm) 20 th Century Climate“2020s” (+1.7 C)“2040s” ( 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%

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

Observed Changes in the 20 th Century

A Time Series of Temporally Smoothed, Regionally Averaged Met Data for the West

Linear Trends in Cool and Warm Season Climate for and (% per century for precip, degrees C per century for temperature)

TMAX Global T as a Predictive Variable for TMAX Trends Over the West R 2 = 0.62

R 2 = 0.81 TMIN Global T as a Predictive Variable for TMIN Trends Over the West

Global Climate Models Reproduce These Patterns of Variability at the Global Scale

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

The recession of the Illecillewaet Glacier at Rogers Pass between 1902 and Photographs courtesy of the Whyte Museum of the Canadian Rockies & Dr. Henry Vaux Wide-Spread Glacial Retreat has Accompanied 20 th Century Warming. Loss of glacial mass may increase summer flow in the short term and decrease summer flow in the long term.

Simulated Change in Date of 90% Melt Snowpack is melting earlier Hamlet A.F.,Mote P.W, Clark M.P., Lettenmaier D.P., 2005: Effects of temperature and precipitation variability on snowpack trends in the western U.S., J. of Climate, 18 (21):

As the West warms, spring flows rise and summer flows drop Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8):

Summer Water Availability is Declining 55 years

Precipitation

Differences in cool and warm season precipitation trends suggest different mechanisms (large-scale advective storms vs. smaller scale convective storms) and differing sensitivity to regional warming. Trends in warm season precipitation in the CRB are very different than the other regions and may function more like cool season precipitation (e.g. related to circulation rather than locally generated storms)

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

Sample Size = 270 years Super ensemble CDFs of PNW cool season precipitation for four 30 year time slices from nine GCM simulations

Regionally Averaged Cool Season Precipitation Anomalies PRECIP

Sacramento and Upper Colorado Annual Flow Reconstructions Concurrent periods of low flow are indicated by pink bands Meko, D.M. and C.A. Woodhouse, 2005: Tree-ring footprint of joint hydrologic drought in Sacramento and Upper Colorado river basins, western USA. Journal of Hydrology, 308 (1-4):

Trends in Annual Streamflow at The Dalles from are strongly downward.

Remarks Regarding Precipitation Trends in cool season precipitation and the summer monsoon in the southwest U.S. are ambiguous and do not seem to be related to regional expressions of global warming. Warm season precipitation in the PNW, CA, and GB, however, seems to be steadily increasing with warming. Unambiguous changes in cool season precipitation variability have occurred starting in about 1975 in the West, coincident with (but not necessarily related to) rapid greenhouse-forced warming. Are trends in warm season precipitation and changes in cool season precipitation variability linked to warming?

Water Supply Impacts

Reductions in Supply Increases in Demand Conflicts with Other Water Resources Objectives Water Supply Impact Pathways Combined Impacts Climate Change Increasing Population

In Managed Systems the Storage to Flow Ratios are Important High Storage to Flow Ratio (e.g. the Colorado River basin) = Low sensitivity to streamflow timing shifts, high sensitivity to systematic changes in precipitation and multi-year drought Low Storage to Flow Ratio (e.g. the Seattle Water Supply System) = High sensitivity to streamflow timing shifts and changes in single year droughts.

Transient SWE simulation from HadCM3 (A2) GCM run (with running 10 year average smoothing) Simulated from observed climate shows a declining trend of ~3KAF per decade ( ) HadCM3 simulated declines ~4KAF per decade Figure courtesy of Matt Wiley and Richard Palmer at CEE, UW

Master's Thesis: Wiley, M.W. (2004). "Analysis Techniques to Incorporate Climate Change Information into Seattle’s Long Range Water Supply Planning," University of Washington In sensitive areas, systematic reductions in summer water availability will decrease the yield of water supply systems.

Climate Change May Exacerbate Other Impacts climate change and growth growth climate change Exceedance Probability of Reduced Storage Value from Current Climate and 2000 Demands

Source: Payne, J.T., A.W. Wood, A.F. Hamlet, R.N. Palmer and D.P. Lettenmaier, 2004, Mitigating the effects of climate change on the water resources of the Columbia River basin, Climatic Change, 62 (1-3): Climate change adaptation may involve complex tradeoffs between competing system objectives

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

Adaptation

Evidence of Autonomous Adaptation Increasing water buyouts and water market transactions from irrigation to municipal water supply in response to the recent severe droughts in the Southwest. Population increases are also driving permanent transfers [cost of new municipal supply ~$2500 per acre-ft, subsidized irrigation ~$50 per acre-ft] Increases in conjunctive use and shifts from increasingly regulated surface sources to more lightly regulated ground water sources as supplies tighten (e.g. Idaho) Changes in irrigation technology in response to shortage (e.g. Idaho)

Management Alternatives Supply Side Increase conventional storage Build new storage projects Enlarge existing storage projects Off-stream surface water storage projects Ground water storage and recharge Advanced waste water treatment systems (Yelm, Squim, WA) Reverse osmosis in coastal areas Improve forecasting systems and management Re-allocation of existing supplies Demand Side Conservation and plumbing codes Pricing of water to reflect scarcity and demand Market based transfers and water banks

Anticipate changes. Accept that the future climate will be substantially different than the past. Use scenario based planning to evaluate alternatives rather than the historic record. Expect surprises and design for flexibility and robustness in the face of uncertain changes (particularly when sensitivity to precipitation changes dominates). Plan for the long haul. Where possible, make adaptive responses “self tending” to avoid repetitive costs of policy intervention as impacts increase over time. Approaches to Adaptation