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Sensitivity of Snow-Dominated Hydrologic Regimes to Global Warming Dennis P. Lettenmaier1, Jennifer C. Adam1, Tim P. Barnett2 1. Dept. of Civil and Environmental Engineering, University of Washington 2. Climate Research Division, Scripps Inst. of Oceanography European Geosciences Union General Assembly 2006 Thursday, April 6 Vienna, Austria 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 oC/km. Typical hydrographs of snow, transient (rain and snow) and rain dominated watersheds in northwestern U.S. Map of Snowmelt-Dominated Regions Legend {Snowfall÷Runoff ≥ 50%} – {Basins with large storage} Basins with ≥ 50% Runoff Derived from SnowmeltDominated Regions Population • includes approximately one-sixth of the global population Gross Domestic Product • includes roughly one-quarter of global GDP Mechanisms for shift in seasonal hydrographs in a warming climate Warming Earlier Onset of Snowmelt Less Storage of Water in Snow pack (snow rain) Earlier Peak Runoff Reduction in Peak Runoff Reduced Surface Water Availability During Summer/Autumn (seasons of peak demand) 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) Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April 1) March Relative Trend (% per year) Figure courtesy of Alan Hamlet, U. Washington June 3. Hydrologic implications of climate change globally Global Climate Change Selected Basins 90 -150 -120 -90 -60 -30 0 30 1 60 60 0 120 150 5 4 2 30 90 7 9 6 8 90 60 30 0 3 -30 -30 -60 -60 -90 -150 -120 -90 1 2 3 4 5 -60 -30 MacKenzie Mississippi Amazon Severnaya Dvina Yenisei 0 30 60 90 6 7 8 9 120 150 -90 Amur Yellow Xi Mekong from Nijssen et al, Climatic Change, 2001 Runoff Sensitivity Season in which change was experienced Change in Runoff as a result of change in Precipitation Mackenzie Yenisei Severnaya Dvina Amur Mississippi Yellow Mekong Xi Amazon SON JJA MAM DJF SON JJA MAM DJF SON JJA MAM DJF DJF MAM JJA SON DJF MAM JJA SON DJF MAM JJA SON Season in which change was imposed -5% -10% +5% +10% from Nijssen et al, Climatic Change, 2001 Runoff Sensitivity Season in which change was experienced Change in Runoff as a result of change in Temperature Mackenzie Yenisei Severnaya Dvina Amur Mississippi Yellow Mekong Xi Amazon SON JJA MAM DJF SON JJA MAM DJF SON JJA MAM DJF DJF MAM JJA SON DJF MAM JJA SON DJF MAM JJA SON Season in which change was imposed -5% -10% +5% +10% from Nijssen et al, Climatic Change, 2001 4. Western U.S. impact studies Diminishing Sierra Snowpack % Remaining, Relative to 1961-1990 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 1961-1990 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 • • • • 28-41 days (4-6 weeks) shorter for B1 scenario 39-44 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 Minimum ski conditions never attained •49-106 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. 5. Water resources implications in the western U.S.: The Accelerated Climate Prediction Initiative (see Climatic Change special issue, Jan-Feb. 2004, for details) BAU 3-run average historical (1950-99) control (2000-2048) PCM Business-as-Usual scenarios Columbia River Basin (Basin Averages) Percent of Control Run Climate 2040-2069 140 120 PCM Control Climate and Current Operations 100 PCM Projected Climate and Current Operations PCM Projected Climate with Adaptive Management 80 60 Firm Hydropower Annual Flow Deficit at McNary BAU 3-run average historical (1950-99) control (2000-2048) PCM Business-as-Usual scenarios California (Basin Average) PCM Business-as-Usual Scenarios Snowpack Changes California April 1 SWE Central Valley Water Year Type Occurrence Percent Given WY Type 0.6 hist (1906-2000) 0.5 2020s 2050s 2090s 0.4 0.3 0.2 0.1 0.0 Critically Dry Dry Below Normal Water Year Type Above Normal Wet Current Climate vs. Projected Climate Storage Decreases • Sacramento Range: 5 - 10 % Mean: 8 % • San Joaquin Range: 7 - 14 % Mean: 11 % Current Climate vs. Projected Climate Hydropower Losses Central Valley Hydropower Production 1400000 • Central Valley Range: 3 - 18 % Mean: 9 % • Sacramento System Range: 3 – 19 % Mean: 9% • San Joaquin System Range: 16 – 63 % Mean: 28% Ctrl mean 2000-2019 2020-2039 2040-2059 2060-2079 2080-2098 1200000 Megawatt-Hours 1000000 800000 600000 400000 200000 ct O ov N D ec Ja n b Fe M ar pr A M ay n Ju l Ju ug A p Se PCM Projected Colorado R. Temperature Timeseries Average Annual ctrl. avg. hist. avg. Period 1 2010-2039 Period 2 2040-2069 2098 Period 3 2070- PCM Projected Colorado R. Precipitation Timeseries Average Annual hist. avg. ctrl. avg. Period 1 2010-2039 Period 2 2040-2069 2098 Period 3 2070- Annual Average Hydrograph Simulated Historic (1950-1999) Period 1 (2010-2039) Control (static 1995 climate) Period 2 (2040-2069) Period 3 (2070-2098) Total Basin Storage Figure 8 70 Minimum 60 Average Maximum Storage, BCM 50 40 30 20 10 0 Historical Control Period 1 Period 2 Period 3 Annual Releases to the Lower Basin Figure 9 14 1.2 Average Annual Release to Lower Basin (BCM/YR) Probability release to Lower Basin meets or exceeds target (probability) 12 1 target release 10 8 0.6 6 0.4 4 0.2 2 0 0 Historical Control Period 1 Period 2 Period 3 Probability BCM / YR. 0.8 Annual Releases to Mexico Figure 10 1.2 Average Annual Release to Mexico (BCM/YR) 3 Probability release to Mexico meets or exceeds target (probability) BCM / YR. 2.5 2 target release 1.5 1 0.8 0.6 0.4 1 0.2 0.5 0 0 Historical Control Period 1 Period 2 Period 3 Probability 3.5 Figure 12 Annual Hydropower Production 18000 Minimum 16000 Average Energy, GW - hr 14000 Maximum 12000 10000 8000 6000 4000 2000 0 Historical Control Period 1 Period 2 Period 3 Summary of ACPI results • Columbia and California reservoir systems primarily provide within-year storage (total storage/mean flow ~ 0.3 – 0.5), whereas Colorado is an over-year system (~4) • Climate sensitivities in Columbia basin and California 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); temperature changes are much less important. 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 Rhine River (Middelkoop et al. 2001) Discharge, m3/s Aare River at Brugg H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: 105-128, 2001. (Image: Ultrecht Univ., Netherlands) Discharge, m3/s Rhine River at Rheinfelden Rhine River (Middelkoop et al. 2001) Discharge, m3/s Rhine River at Rees Some Implications: • reduction of water availability during season of peak demand • increase in number of low-flow days (affects ship transport) H. Middelkoop et al., Impact of climate change on hydrological regimes and water resources management in the Rhine Basin, Clim. Change, 49: 105-128, 2001. (Image: Ultrecht Univ., Netherlands) • decrease in level of flood protection • decrease in annual hydropower production (some sub-basins) Canadian Prairies (de Loë et al. 2001) • agriculture sensitive to drought (irrigation derived primarily from surface waters) • predictions include: decrease in snow-pack, earlier peak runoff, and lower summer soil moistures R. de Loë et al., Adaptation options for the near term: climate change and the Canadian water sector, Global Env. Change, 11, 231-245, 2001. • 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)