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Streamflow/runoff sensitivity to warming and drying in the Colorado (Western US) River Basin Tapash Das, Dan Cayan, David Pierce, Mike Dettinger Reconciling Projections of Future Colorado River Stream Flow Collaborators: Robert Webb, Bradley Udall Martin Hoerling, Jonathan Overpeck Holly Hartman, Dennis Lettenmaier, Julie Vano, Dan Cayan, Tapash Das Levi Brekke, Kevin Werner Past Studies Table from Colorado Water Conservation Board (CWCB) report “ Colorado Climate Change: A Synthesis to Support Water Resource Management and Adaptation.” Oct 2008 (available online at: http://cwcb.state.co.us/NR/rdonlyres/8118BBDB-4E54-4189-A354-3885EEF778A8/0/CCSection5.pdf) Project Objectives 1) Reconcile discrepancies in projected Colorado River flow changes. 2) Assess the basins sensitive in runoff to changes in temperature, in precipitation, or in both. 3) Identify the underlying mechanisms for these sensitivities (e.g. soil moisture, ET). 4) Provide meaningful information for water managers and policymakers that incorporate uncertainties in future climate change projections. DATA, Model, Methods Data, Model Hydrologic model: Methods Compute change Variable Infiltration Capacity (VIC) Liang et al. (1994), JGR Historic streamflow/runoff forcing: Observed Gridded meteorology Responses to prescribed changes in T, P imposed on historic climate Q ref+1 - Qref Temp = sensitivity Qref deg C Wood and Lettenmaier (2006), B Am Meteorol Soc Hamlet and Lettenmaier (2005), J. Hydromet. Q ref-1% - Qref Precip = elasticity Qref % Colorado River Flows: highly sensitive to warming GN BL LE IM MA CA Catchments: GN: Green River at Greendale MA: Yampa River near Maybell CA: Colorado River near Cameo GA: Gunnison River near Grand Junction CI: Colorado River near Cisco BL: San Juan River near Bluff LE : Colorado River at Lees Ferry IM: Colorado River below Imperial Dam GA CI -6% Sensitivity of streamflow to warming varies across different catchments Overall sensitivity approximately 6% decline in streamflow per 1°C warming at Colorado at Lees Ferry Climatologies of monthly streamflow: Colorado at Lees Ferry Baseline Warming Temp increases: streamflow decreases annually, mainly because decreases flow in spring/summer Streamflow changes to climate change: Colorado at Lees Ferry The total decline in streamflow increases each time the temperature increases, but the total reductions decrease as the temperature increases more and more Colorado River Flows: Sensitive to Drying GN MA BL LE IM CA GA CI -2% Sensitivity of streamflow to drying varies across different catchments Catchments: GN: Green River at Greendale MA: Yampa River near Maybell CA: Colorado River near Cameo GA: Gunnison River near Grand Junction CI: Colorado River near Cisco BL: San Juan River near Bluff LE : Colorado River at Lees Ferry IM: Colorado River below Imperial Dam Overall sensitivity approximately 2% decline in streamflow per 1% reduction of precipitation at Colorado at Lees Ferry Impact of model forcing resolution to VIC simulations: Colorado at Lees Ferry baseline Annual mean streamflow (MAF) (WY1916 through 2000) P,T basin average Naturalized flow VIC simulated flow (baseline) VIC simulated flow (p, t basin wide) VIC simulated flow (t basin wide) VIC simulated flow (p basin wide) Runoff climatologies at Colorado at Lees Ferry Substantial changes in runoff volume, timing in baseline VIC run and VIC run using average model forcings : 15.1 : 15.7 : 8.1 : 11.8 : 7.8 Research Objectives: Sensitivity of Runoff to Climate Change Over The Western U.S. Understand runoff sensitivity to warming, and drying in the Western United States Compare major river basins in the West, and understand the underlying differences Apply sensitivity method to IPCC AR4 climate change ensemble projection Study domain: Western US (+ part of British Columbia) Columbia River Snow dominated oC Northern Sierra Rain-Snow fed Colorado River Snow dominated Southern Sierra Snow dominated Hydrographs drawn from VIC simulated streamflow forced by observed meteorology, 1915-2002 Sensitivity of runoff to Warming varies greatly across the West %oC-1 Major Basins: Upper Colorado shows the highest sensitivity to warming, followed by Columbia, and least sensitivity in the Californian Nevada Hydrologic characteristics: Region by Region Runoff efficiency (R/P) Evaporative demand (AET/P) In Columbia and Sierra Nevada, about 50% of total precipitation is lost via ET However, in Colorado, about 80% of total precipitation is lost via ET Actual ET and runoff are computed using VIC as driven by observed meteorology, 1915-2002 Sensitivity of runoff to Drying varies across the West %%-1 Major Basins: Upper Colorado 2.2% reduction, Northern Sierra 1.4% for 1% reduction of precipitation Does snowpack produce higher Sensitivity? Snowdominated region Other region Snow-dominated region has higher sensitivity in Colorado, Columbia, and Southern Sierra, but NOT in Northern Sierra Region considered as snow-dominated if VIC simulated mean April 1 Snow Water Equivalent (SWE) in the period 1915-2002 is larger than 50 mm (VIC simulation with varying snowline) Sensitivity of Streamflow to Warming: Month by month Warming in the warm season yield larger reduction (as compared to reduction in cool season warming) for Columbia, Colorado and Southern Sierra Runoff fraction: Region by Region In Columbia, Colorado and Southern Sierra Nevada >70% of total runoff produced in warm season Precipitation fraction: Region by Region In Columbia and Sierra Nevada >65% of total precipitation falls in cold season Summary Sensitivity of runoff, especially due to temperature, varies greatly across the West The region with the highest evaporative demand has the greatest sensitivity. Greatest sensitivities appear in zones of snow and snow-rain transition Sensitivities of Runoff to warming: Colorado > Columbia > Southern Sierra > Northern Sierra Warming in the warm season yield larger reduction (as compared to cool season) for Columbia, Colorado and Southern Sierra Topography and hydrologic structure is crucial: resolving snow vs. rain and other processes in the Colorado River Basin is crucial. Distributing precipitation over the basin produces less runoff, but lesser effect when temperature is smoothed out