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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.

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Presentation on theme: "Streamflow/runoff sensitivity to warming and drying in the Colorado (Western US) River Basin Tapash Das, Dan Cayan, David Pierce, Mike Dettinger."— Presentation transcript:

1 Streamflow/runoff sensitivity to warming and drying in the Colorado (Western US) River Basin Tapash Das, Dan Cayan, David Pierce, Mike Dettinger

2 Collaborators: Robert Webb, Bradley Udall Martin Hoerling, Jonathan Overpeck Holly Hartman, Dennis Lettenmaier, Julie Vano, Dan Cayan, Tapash Das Levi Brekke, Kevin Werner Reconciling Projections of Future Colorado River Stream Flow

3 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)

4 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. Project Objectives

5 Data, Model  Hydrologic model: Variable Infiltration Capacity (VIC) Liang et al. (1994), JGR  Historic forcing: Observed Gridded meteorology Wood and Lettenmaier (2006), B Am Meteorol Soc Hamlet and Lettenmaier (2005), J. Hydromet. Methods  Compute streamflow/runoff change  Responses to prescribed changes in T, P imposed on historic climate Temp sensitivity Q ref+1 - Q ref Q ref deg C = Precip elasticity Q ref-1% - Q ref Q ref % = DATA, Model, Methods

6 Colorado River Flows: highly sensitive to warming 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 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 -6% GN MA CA GA CI LE BL IM

7 Warming Baseline Temp increases: streamflow decreases annually, mainly because decreases flow in spring/summer Climatologies of monthly streamflow: Colorado at Lees Ferry

8 The total decline in streamflow increases each time the temperature increases, but the total reductions decrease as the temperature increases more and more Streamflow changes to climate change: Colorado at Lees Ferry

9 Colorado River Flows: Sensitive to Drying 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 Sensitivity of streamflow to drying varies across different catchments Overall sensitivity approximately 2% decline in streamflow per 1% reduction of precipitation at Colorado at Lees Ferry -2% GN MA CA GA CI LE BL IM

10 Runoff climatologies at Colorado at Lees Ferry Substantial changes in runoff volume, timing in baseline VIC run and VIC run using average model forcings Impact of model forcing resolution to VIC simulations: Colorado at Lees Ferry Annual mean streamflow (MAF) (WY1916 through 2000) Naturalized flow: 15.1 VIC simulated flow (baseline): 15.7 VIC simulated flow (p, t basin wide): 8.1 VIC simulated flow (t basin wide): 11.8 VIC simulated flow (p basin wide): 7.8 baseline P,T basin average

11 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

12 Study domain: Western US (+ part of British Columbia) oCoC Colorado River Snow dominated Northern Sierra Rain-Snow fed Columbia River Snow dominated Southern Sierra Snow dominated Hydrographs drawn from VIC simulated streamflow forced by observed meteorology, 1915-2002

13 % o C -1 Sensitivity of runoff to Warming varies greatly across the West Major Basins: Upper Colorado shows the highest sensitivity to warming, followed by Columbia, and least sensitivity in the Californian Nevada

14 Hydrologic characteristics: Region by Region In Columbia and Sierra Nevada, about 50% of total precipitation is lost via ET Runoff efficiency (R/P) Evaporative demand (AET/P) 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

15 Sensitivity of runoff to Drying varies across the West Major Basins: Upper Colorado 2.2% reduction, Northern Sierra 1.4% for 1% reduction of precipitation % -1

16 Does snowpack produce higher Sensitivity? 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) Other region Snow-dominated region has higher sensitivity in Colorado, Columbia, and Southern Sierra, but NOT in Northern Sierra Snow- dominated region

17 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

18 Runoff fraction: Region by Region In Columbia, Colorado and Southern Sierra Nevada >70% of total runoff produced in warm season

19 Precipitation fraction: Region by Region In Columbia and Sierra Nevada >65% of total precipitation falls in cold season

20 Summary  Sensitivities of Runoff to warming: Colorado > Columbia > Southern Sierra > Northern Sierra  The region with the highest evaporative demand has the greatest sensitivity. Greatest sensitivities appear in zones of snow and snow-rain transition  Warming in the warm season yield larger reduction (as compared to cool season) for Columbia, Colorado and Southern Sierra  Sensitivity of runoff, especially due to temperature, varies greatly across the West  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

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