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Hydrologic Forecasting: Effects of Climate Variability, Climate Change and Land Use Center for Science in the Earth System Climate Impacts Group and Department of Civil and Environmental Engineering University of Washington Feb, 2005 http://www.hydro.washington.edu/Lettenmaier/Presentations/2005/hamlet_AAAS_feb_2005.ppt Alan F. Hamlet Philip W. Mote Dennis P. Lettenmaier Natural Variability and Global Warming Pacific Decadal Oscillation El Niño Southern Oscillation A history of the PDO A history of ENSO warm warm cool 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Effects of the PDO and ENSO on Columbia River Summer Streamflows PDO 450000 Cool Cool Warm Warm 350000 300000 250000 200000 high high low low Red = warm ENSO Green = ENSO neutral Blue = cool ENSO 2000 1990 1980 1970 1960 1950 1940 1930 1920 1910 150000 1900 Apr-Sept Flow (cfs) 400000 The earth is warming -- abruptly Mostly human induced Mostly natural variability Hydrologic Climate Change Scenarios Water Management Model Downscaled Temperature and Precipitation from GCMs VIC Hydrology Model Time Scales: Monthly Weekly Daily Four Delta Method Climate Change Scenarios for the PNW Delta T, 2020s Delta T, 2040s 5 5 ~ + 1.7 C ~ + 2.5 C 4 hadCM2 3 hadCM3 2 PCM3 ECHAM4 1 Degrees C Degrees C 4 mean 0 hadCM2 3 hadCM3 2 PCM3 ECHAM4 1 mean 0 J F M A M J J A S O N D J -1 F M A Precipitation Fraction, 2020s J J A S O N D Precipitation Fraction, 2040s 1.75 1.75 1.5 1.5 hadCM2 hadCM3 1.25 PCM3 1 ECHAM4 Fraction Fraction M -1 hadCM2 hadCM3 1.25 PCM3 1 ECHAM4 mean 0.75 mean 0.75 0.5 0.5 J F M A M J J A S O N D J F M A M J J A S O N D Somewhat wetter winters and perhaps somewhat dryer summers The main impact: less snow VIC Simulations of April 1 Average Snow Water Equivalent for Composite Scenarios (average of four GCM scenarios) Current Climate 2020s Snow Water Equivalent (mm) 2040s 600 Area Average Water (depth in mm) Seasonal Water Balance Naches River 700 500 precipitation 400 swe runoff+baseflow soil storage 300 evapotranspiration 200 100 Current Climate sep aug jul jun may apr mar feb jan dec 600 500 precipitation 400 swe runoff+baseflow soil storage 300 evapotranspiration 200 100 sep aug jul jun may apr mar feb jan dec nov 0 oct 2040s Scenario (+ 2.5 C) 700 Area Average Water (depth in mm) More runoff in winter and early spring, less in summer nov oct 0 Basin Averaged Runoff (mm) Changes in Runoff in the Naches River Basin Associated with Systematic Warming 180.00 160.00 140.00 120.00 100.00 + 2.5° C current climate composite 2040 80.00 60.00 40.00 20.00 0.00 oct nov dec jan feb mar apr may jun jul aug sep Observed Climate Change: Hydrologic Impacts for the West Trends in April 1 SWE 1950-1997 Source: Mote et al. (2005), Declining Snowpack in the Western U.S., BAMS DJF AVG T (C) 1916-1997 Relative Trend in April 1 SWE (% per year) DJF AVG T (C) 1916-1997 Effects of Temp Relative Trend in April 1 SWE (% per year) DJF AVG T (C) 1916-1997 Effects of Precip Relative Trend in April 1 SWE (% per year) a) 10 % Accumulation b) Max Accumulation c) 90 % Melt Trends in SWE 19161997 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 Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April 1) March Relative Trend (% per year) June 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) Some Hydrologic Effects Associated with Changes in Land Use 1) Urban Development • Development or isolation of natural flood plains (dikes and levies) : Loss of ground water recharge area—reductions in summer streamflow and increased water temperatures • Increased impervious surface and soil removal: Increased runoff production in wet season, loss of groundwater recharge and storage, loss of water availability in dry season 2) Forest Disturbance • Loss of forest canopy due to logging, fire, insects, or disease: Modest increases in flooding, net increases in water availability (lower annual ET), potential for increased sediment loads until vegetation recovers. • Forest roads Modest increases in flooding. Modeling studies suggest that the effects are roughly additive when combined with the effects of clear cut logging. Typical Effects of Urbanization on a Small Watershed Des Moines Creek Source: Booth D.B., 2000, Forest Cover, Impervious-Surface Area, and the Mitigation of Urbanization Impacts in King County, WA http://depts.washington.edu/cwws/Research/Reports/forest.pdf Effects of Harvest Strategies on Magnitude of Flood Peaks Modeling studies (Storck 2000) and comparative analysis of observations in paired catchments (Bowling et al. 2000) show that large scale clearcutting results in increased flood peaks on the order of 10% for small basins in the transient snow zone of the Cascades. Sources: Storck, P., 2000, Trees, Snow and Flooding: An Investigation of Forest Canopy Effects on Snow Accumulation and Melt at the Plot and Watershed Scales in the Pacific Northwest, Water Resources Series Technical Report No. 161, Dept of CEE, University of Washington Bowling, L.C., P. Storck and D.P. Lettenmaier, 2000, Hydrologic effects of logging in Western Washington, United States, Water Resources Research, 36 (11), 3223-3240 Effects of Roads Networks on Peak Flows Bowling and Lettenmaier (1997) estimated that the 10-yr flood peak increased ~10% in two small transient snow basins due to road networks alone. Roads and logging together were estimated to increase the 10-yr flood peak on the order of 20% in the same two small transient snow basins. Bowling, L.C. and Lettenmaier, D.P., 1997, Evaluation of the Effects of Forest Roads on Streamflow in Hard and Ware Creeks, Washington, Water Resources Series Technical Report No. 155, Dept of CEE, University of Washington Conclusions •Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West as a result of increasing regional temperatures. These changes are not well explained by natural variability. •Precipitation changes remain uncertain and (unlike temperature) seem most clearly associated with decadal variability rather than long-term trends. •Hydrologic changes in the West include earlier and reduced peak snowpack, more runoff in winter, earlier snowmelt and peak flows, less runoff in summer, and reduced late summer low flows. Because these effects are shown 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, precipitation variability not withstanding. •Except in areas where permanent changes are made (e.g. urbanization), one can argue that changes in climate are likely to be more important hydrologic drivers overall than changes in land use. Climate affects entire river basins at once and is frequently much more persistent than vegetation disturbances (e.g. PDO vs. logging). Natural AND human influences explain the observations best. Temperature changes are much more certain than precipitation changes. Natural Climate Influence Human Climate Influence All Climate Influences Decadal Climate Variability Doesn’t Explain the Loss of SWE Due to Warming 1916-97 1947-97 1925-46 with 1977-95 Relative SWE Trends Due to Temperature Effects Alone (% per year) Decadal Climate Variability Does Explain Some of the Changes of SWE Due to Precipitation Changes 1916-97 1925-76 1947-97 Relative SWE Trends Due to Precipitation Effects Alone (% per year)