1. The Importance of Realistic Spatial Forcing in Understanding Hydroclimate Change-- Evaluation of Streamflow Changes in the Colorado River Basin Hydrology Conference 2010 Tapash Das Scripps Institution of Oceanography Dan Cayan Scripps Institution of Oceanography, USGS Julie Vano University of Washington Dennis Lettenmaier University of Washington David W. Pierce Scripps Institution of Oceanography

2. 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)

3. 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

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. Research Objectives Understand runoff sensitivity to warming, and drying in the Western United State T o investigate how streamflow and other hydrologic measures vary due to changes in the resolution of the temperature and precipitation input to VIC

6. c) Spatial distribution of VIC simulated variables a) Spatial distribution of observed gridded forcing Roughly 75% of runoff in Colorado River Basin is generated from about 25% of the area, mostly at high elevations b) Elevations, m Colorado River Basin

7. River Basins: GREEN: Green River, Utah GRNJC: Gunnison River near Grand Junction, Colorado BLUFF: San Juan River near Bluff, Utah GREEN GRNJC BLUFF 108720 km2 59472 km2 20880 km2 GREEN BLUFF GRNJC GREEN There is a substantial decrease in runoff and streamflow production using basin averaged model forcing compared to the baseline simulation 1) r1: p, t observed gridded 2) r2: t averaged 3) r3: p averaged 4) r4: p, t averaged Impact of model forcing resolution to VIC simulations: River Basins in Colorado

8. Runoff climatologies at Green River Substantial changes in runoff volume, timing in baseline VIC run and VIC run using averaged model forcings Impact of model forcing resolution to VIC simulations: Green River, Utah There is higher actual evapotranspiration (AET) in the simulations using coarse model forcing. The coarse resolution simulations produced increased AET in early of the winter months Changes in soil moisture volume and seasonal patterns also occur in the coarse resolution runs in comparison to the baseline run

9. VIC simulated April 1 Snow Water Equivalent (SWE) as driven by climate model meteorology from NCAR CCSM3 from two cases a) downscaled meteorology (black color) coarsend meteorology (red color) Impact of model forcing resolution to VIC simulations: Results from Climate model data at Green River, Utah Substantial changes in April 1 SWE in downscaled climate model data driven VIC run and VIC run using averaged p, t climate model forcings Fine scale forcing driven p,t averaged forcing driven

10. Sensitivity of runoff to Climate Warming GREEN GRNJC BLUFF NORSN River Basins: GREEN: Green River, Utah GRNJC: Gunnison River near Grand Junction, Colorado BLUFF: San Juan River near Bluff, Utah NORSN: Northern Sierra Nevada, California Sensitivity of streamflow to warming varies across different River Basins Overall sensitivity approximately 6% decline in streamflow per 1°C warming at Colorado River Basins. Californian Northern Sierra shows 1% decline per 1°C warming

11. Hydrologic characteristics: Region by Region In Californian Northern 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

12. 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 warming: Non-linear effects Changes in annual runoff as a function of the imposed year-round temperature change at Green River, Utah. Dashed lines show the tangent line at the origin, illustrating what the response would be if there were no non- linear effects

13. BLUFF LESFY Streamflow changes to climate warming using multi land surface models River Basins: BLUFF: San Juan River near Bluff, Utah LESFY: Colorado River at Lees Ferry, Arizona Colorado River at Lees Ferry San Juan River near Bluff Annual runoff change ranges between apprx. -3% to -10% per 1°C warming

14. Summary  Sensitivities of Runoff to warming: Colorado > Northern Sierra  The region with the highest evaporative demand has the greatest sensitivity.  Sensitivity of runoff due to temperature varies greatly across the South 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