Hydrologic Implications of 20th Century Warming in the Western U.S. Alan F. Hamlet, Philip W. Mote, Martyn Clark , Dennis P. Lettenmaier JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington Western Water Assessment, CIRES, University of Colorado, Boulder

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

Regionally Averaged Cool Season Temperature Anomalies TMIN

Regionally Averaged Cool Season Precipitation Anomalies

Overview of Research Questions: How have variations in temperature and precipitation from the early 20th Century on (1916-2003) affected trends in hydrologic variables such as snowpack, volume and timing of runoff and baseflow, seasonal evaporation and soil moisture, and flood risk in the western U.S.? Is a consistent global warming signal apparent over the western U.S. in this period, and is it possible to make a clear distinction between “natural” variations such as decadal precipitation variability and more systematic effects associated with global warming signals? Are temperature and precipitation different in this regard? What role do regional climatic regimes and topographic variations play in defining the role of temperature and precipitation variability on hydrologic variations? What areas of the western U.S. are most sensitive to changes in temperature or precipitation changes and why?

Trends in Temperature and Precipitation

Daily Precipitation, Tmax, Tmin Long-Term Meteorological Driving Data for the West Result: Daily Precipitation, Tmax, Tmin 1915-2003

Trends in Cool Season (Oct-Mar) Precipitation and Temperature Tmax Tmin 1916- 2003 DJF Avg Temperature Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr) 1947- 2003 DJF Avg Temperature Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

Trends in warm season (Apr-Sept) Precipitation and Temperature Tmax Tmin DJF Avg Temperature 1916- 2003 Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr) DJF Avg Temperature 1947- 2003 Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

In temperature sensitive areas of the West, we should be able to see the effects of observed global warming in the historic snow and streamflow records. Using models we should be able to more fully analyze these changes, and explore the effects on other hydrologic variables which are not typically measured (evaporation and soil moisture).

Schematic of VIC Hydrologic Model and Energy Balance Snow Model PNW CA CRB GB 12 km 1/8th Deg. 12 km Snow Model

Trends in April 1 Snowpack

Trends in April 1 SWE 1950-1997 Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): 39-49

Overall Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Temperature Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Precipitation Related Trends in April 1 SWE from 1947-2003 DJF avg T (C) Trend %/yr Trend %/yr

Trends in SWE 1916- 1997 a) 10 % Accumulation b) Max Accumulation c) 90 % Melt Trends in SWE 1916- 1997 Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) DJF Temp (C) Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) FP DJF Temp (C) Change in Date Change in Date Change in Date DJF Temp (C) DJF Temp (C) DJF Temp (C) FT Change in Date Change in Date Change in Date

Trends in Runoff Timing

winter flows rise and summer flows drop As the West warms, winter 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): 1136-1155 Spring snowmelt timing has advanced by 10-40 days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

June March Trends in simulated fraction of annual runoff in each month from 1947-2003 (cells > 50 mm of SWE on April 1) March June Relative Trend (% per year)

Trends in March Runoff Trends in June Runoff DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr

Trends in Soil Moisture

Trends in Simulated Soil Moisture from 1947-2003 DJF Temp (°C) April 1 Trend %/yr July 1 DJF Temp (°C) Trend %/yr

Trends in April 1 SM Trends in July 1 SM DJF Temp (°C) DJF Temp (°C) Trend %/yr Trend %/yr

50% WY runoff, 80% max soil moisture recharge, and 50% WY ET Trends in the Dates of 50% WY runoff, 80% max soil moisture recharge, and 50% WY ET

Cumulative Trends in the Date of Hydrologic Events 50% WY Runoff 80% Max SM 50% WY ET Cumulative Trends in the Date of Hydrologic Events (1947-2003) BR BR BR DJF Temp (°C) FPR FPR FPR Effects of Temp alone DJF Temp (°C) FTR FTR FTR Effects of Precip alone DJF Temp (°C) Trend days/50 yr

Trends in the “Runoff Ratio” (runoff/precipitation)

Effects of Cool Season Precipitation Trends on Trends in the Runoff Ratio Trend Runoff Ratio Trend Oct-Mar PCP

Temperature Related Downward Trends in Annual Streamflow in the Columbia River at The Dalles Compared with the Effects of Precipitation Variability Black trace = constant precip Magenta trace = with precip variability

Changes in Flood Risk Associated with 20th Century Warming and Increased Precipiatation Variability

Detrended Temperature Driving Data for Flood Risk Experiments “Pivot 2003” Data Set Temperature Historic temperature trend in each calendar month “Pivot 1915” Data Set 1915 2003

Simulated Changes in the 20-year Flood Associated with 20th Century Warming DJF Avg Temp (C) X20 2003 / X20 1915 Fig 3 20 year flood A spatial scale DJF Avg Temp (C) X20 2003 / X20 1915 X20 2003 / X20 1915

20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime DJF Avg Temp (C) X20 ’73-’03 / X20 ’16-’03 X20 ’73-’03 / X20 ’16-’03

Summary Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West as a result of increasing temperatures. Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in summer soil moisture. Trends in the runoff ratio are predominantly linked to winter precipitation trends, which are not necessarily related to global warming Flood risks appear to be declining overall due to warming, but changes in precipitation variability since 1975 suggest increasing flood risks due to changes in precipitation variability. Because these effects are shown in many cases to be predominantly due to temperature changes, we expect that they will both continue and increase in intensity as global warming progresses in the 21st century.

Implications for Water Management The hydrologic changes shown in these studies will create problems for water management in the western U.S. by disrupting the existing balance between water resources objectives such as flood control, hydropower production, water supply, instream flow augmentation, and water quality. Implications for Ecosystems Synchronized temperature changes affecting very large spatial scales have important implications for the structure and integrity of ecosystems that are affected by water or air temperatures. Some examples of this kind of large scale ecological change are the recent bark beetle outbreaks which have devastated forests in Canada and Alaska, changes in fire ecology, and impacts to cold water fish species such as salmon.