Sensitivity of Snow-Dominated Hydrologic Regimes to Global Warming Dennis P. Lettenmaier Ohio State University Department of Geological Sciences Department of Geography Thursday, April 13, 2006

Outline of this talk 1.Background 2.Observational evidence 3.Hydrologic implications of climate change globally 4.Western U.S. impact studies 5.Water resources implications in the western U.S.: The Accelerated Climate Prediction Initiative

1. Background “On a global scale, the largest changes in the hydrological cycle due to warming are predicted for the snow-dominated basins of mid- to higher latitudes …” (Barnett et al, 2005) Approximately one-sixth of the world’s population lives in river basins that are strongly affected by snowmelt, and for which reservoir storage is unable to substantially attenuate seasonal shifts in runoff. This region accounts for roughly one-quarter of the global gross domestic product. Reduction of snow affected area can roughly be estimated on the basis of movement of the snowline (lower boundary of transient rain-on-snow zone) by the psuedo-adiabatic lapse rate, or roughly 6 o C/km.

Typical hydrographs of snow, transient (rain and snow) and rain dominated watersheds in northwestern U.S.

Map of Snowmelt-Dominated Regions {Snowfall÷Runoff ≥ 50%} – {Basins with large storage} Basins with ≥ 50% Runoff Derived from Snowmelt- Dominated Regions Legend

Population includes approximately one-sixth of the global population

Gross Domestic Product includes roughly one-quarter of global GDP

Less Storage of Water in Snow pack (snow rain) Warming Earlier Onset of Snowmelt Earlier Peak Runoff Reduction in Peak Runoff Reduced Surface Water Availability During Summer/Autumn (seasons of peak demand) Mechanisms for shift in seasonal hydrographs in a warming climate

Mountainous Regions snowmelt dominated regions occupy regions pole-ward of 45° exceptions include mountainous areas (lower latitudes) and areas warmed by ocean waters (higher latitudes)

2. Observational evidence

As the West warms, winter flows rise and summer flows drop I.T. Stewart, D.R. Cayan, M.D. Dettinger, 2004, Changes toward earlier streamflow timing across western North America, J. Climate (in review) Figure courtesy of Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

March June Relative Trend (% per year) Trends in fraction of annual runoff (cells > 50 mm of SWE on April 1) Figure courtesy of Alan Hamlet, U. Washington

3. Hydrologic implications of climate change globally

Season in which change was imposed -5% -10% +5% +10% Runoff Sensitivity Change in Runoff as a result of change in Precipitation from Nijssen et al, Climatic Change, 2001

Runoff Sensitivity Change in Runoff as a result of change in Temperature from Nijssen et al, Climatic Change, 2001

4. Western U.S. impact studies

Diminishing Sierra Snowpack % Remaining, Relative to Total snow losses by the end of the century: 29–73% for the lower emissions scenario (3-7 MAF) 73–89% for higher emissions (7-9 MAF – 2 Lake Shastas) Dramatic losses under both scenarios Almost all snow gone by April 1 north of Yosemite under higher emissions Visual courtesy Ed Maurer

Future Spring Snowpack Remaining by Elevation as a % of levels Losses greatest below 3,000 m: 37–79% for B1 81–94% for A1fi. Below 1800 m (~6000 ft) >80% April 1 snow loss under all simulations Below 2600 m (8500 ft) >75% loss for 3 of 4 simulations, both of high emissions scenarios Visual courtesy Ed Maurer

Impacts on Ski Season Warmer temperatures result in: Less precipitation falling as snow in winter Earlier melt of accumulated snow These combine to shorten the ski season Photo: SwissRe Visual courtesy Ed Maurer

Length of Ski Season days (4-6 weeks) shorter for B1 scenario days (6 weeks) shorter for A1fi Retreat of season start: 5-14 days (losing end of November and early December) This is at midpoint year of 2035 – in our lifetimes. Visual courtesy Ed Maurer

Length of Ski Season days (7-15 week) shorter for B1 scenario 103 days shorter (15 week) to zero day ski season for A1fi Retreat of season start: at least 22 days This is at midpoint year of 2085 – in our childrens’ and grandchildrens’ lifetimes. Minimum ski conditions never attained Visual courtesy Ed Maurer

5. Water resources implications in the western U.S.: The Accelerated Climate Prediction Initiative (see Climatic Change special issue, Jan-Feb. 2004, for details)

Climate Scenarios Global climate simulations, next ~100 yrs Downscaling Delta Precip, Temp Hydrologic Model (VIC) Natural Streamflow Reservoir Model DamReleases, Regulated Streamflow Performance Measures Reliability of System Objectives

Reservoir Model Hydrology Model Coupled Land- Atmosphere-Ocean General Circulation Model

Accelerated Climate Prediction Initiative (ACPI) – NCAR/DOE Parallel Climate Model (PCM) grid over western U.S.

Bias Correction from NCDC observations from PCM historical runraw climate scenario bias-corrected climate scenario month m Note: future scenario temperature trend (relative to control run) removed before, and replaced after, bias-correction step.

Downscaling observed mean fields (1/8-1/4 degree) monthly PCM anomaly (T42) VIC-scale monthly simulation interpolated to VIC scale

5a. Hydrology and water management implications: Columbia River Basin

PCM Business-as-Usual scenarios Columbia River Basin (Basin Averages) control ( ) historical ( ) BAU 3-run average

PCM Business-As- Usual Mean Monthly Hydrographs Columbia River The Dalles, OR 1 month 12

CRB Operation Alternative 1 (early refill)

CRB Operation Alternative 2 (reduce flood storage by 20%) 15,000,000 20,000,000 25,000,000 30,000,000 35,000,000 40,000,000 45,000,000 50,000,000 55,000,000 ONDJFMAMJJAS End of Month Total System Storage (acre-feet) Max Storage Control Base Climate Change Change (Alt. 2) Dead Pool

5b. Hydrology and water management implications: Sacramento-San Joaquin River Basin

PCM Business-as-Usual scenarios California (Basin Average) control ( ) historical ( ) BAU 3-run average

PCM Business-as-Usual Scenarios Snowpack Changes California April 1 SWE

PCM Business-As- Usual Mean Monthly Hydrographs Shasta Reservoir Inflows 1 month 12

Storage Decreases Sacramento Range: % Mean: 8 % San Joaquin Range: % Mean: 11 % Current Climate vs. Projected Climate

Hydropower Losses Central Valley Range: % Mean: 9 % Sacramento System Range: 3 – 19 % Mean: 9% San Joaquin System Range: 16 – 63 % Mean: 28%

5c. Hydrology and water management implications: Colorado River basin

Timeseries Annual Average Period Period Period hist. avg. ctrl. avg. PCM Projected Colorado R. Temperature

hist. avg. ctrl. avg. PCM Projected Colorado R. Precipitation Timeseries Annual Average Period Period Period

Annual Average Hydrograph Simulated Historic ( )Period 1 ( ) Control (static 1995 climate)Period 2 ( ) Period 3 ( )

Natural Flow at Lee Ferry, AZ Currently used 16.3 BCM allocated 20.3 BCM

Total Basin Storage

Annual Releases to the Lower Basin target release

Annual Releases to Mexico target release

Annual Hydropower Production

Uncontrolled Spills

Deliveries to CAP & MWD

Summary – ACPI studies Columbia River reservoir system primarily provides within-year storage (total storage/mean flow ~ 0.3). California is intermediate (~ 0.3), Colorado is an over-year system (~4) Climate sensitivities in Columbia basin are dominated by seasonality shifts in streamflow, and may even be beneficial for hydropower. However, fish flow targets would be difficult to meet under altered climate, and mitigation by altered operation is essentially impossible. California system operation is dominated by water supply (mostly ag), reliability of which would be reduced significantly by a combination of seaonality shifts and reduced (annual) volumes. Partial mitigation by altered operations is possible, but complicated by flood issues. Colorado system is sensitive primarily to annual streamflow volumes. Low runoff ratio makes the system highly sensitive to modest changes in precipitation (in winter, esp, in headwaters). Sensitivity to altered operations is modest, and mitigation possibilities by increased storage are nil.

6. Other locations and mechanisms

Rhine River (Middelkoop et al. 2001) H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: , (Image: Ultrecht Univ., Netherlands) Rhine River at Rees Discharge, m 3 /s Some Implications: reduction of water availability during season of peak demand increase in number of low-flow days (affects ship transport) decrease in level of flood protection decrease in annual hydropower production (some sub-basins)

Rhine River (Middelkoop et al. 2001) Aare River at Brugg Rhine River at Rheinfelden H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: , (Image: Ultrecht Univ., Netherlands) Discharge, m 3 /s

Canadian Prairies (de Loë et al. 2001) R. de Loë et al., Adaptation options for the near term: climate change and the Canadian water sector, Global Env. Change, 11, , agriculture sensitive to drought (irrigation derived primarily from surface waters) predictions include: decrease in snow-pack, earlier peak runoff, and lower summer soil moistures implications: agriculture more at risk in a warming climate; and heightened competition with other water needs (aquatic habitat and down-stream requirements)

Glaciers… Recession of Grinnell Glacier, Glacier National Park (1911 and 2000)

Conclusions Impacts of climate change on the hydrology of snowmelt dominated rivers (of which mountainous watersheds are a particularly important subset) are among the most predictable impacts of climate change Transient snow domains are most “at risk”, but impacts will be felt in all ephemeral snow domains Changes over the last century are detectable, and have already impacted the reliability of water supply systems in the western U.S. Planning methods that incorporate ongoing and future climate change are urgently needed as operating agencies begin to recognize the problems and issues