Reconciling Projections of Colorado River Stream Flow Over the Next Century

Transcription

1 Reconciling Projections of Colorado River Stream Flow Over the Next Century Julie Vano Dept of Civil and Environmental Engineering, Univ of Washington Dennis Lettenmaier Dept of Civil and Environmental Engineering, Univ of Washington Collaborators: Brad Udall, Robert Webb (WWA) Dan Cayan, Tapash Das (CAP) Jonathan Overpeck,, Holly Hartman (CLIMAS) Martin Hoerling, Kevin Werner (NOAA) Levi Brekke (Reclamation) March 26, 2010 Hydrology in the 21st Century: Links to the past, and a vision for the future Steve Burges Retirement Symposium, Seattle, WA Climate science in the public interest

2 Current Climate Trends Observed April 1 snow water equivalents, March Average Min Temp on Days with Precipitation ( ) Trends in Snow vs. Rain in Winter ( ) and many more Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1): Knowles, N., Dettinger, M.D., and D.R. Cayan, 2006, Trends in Snowfall verse Rainfall in the Western United States, Journal of Climate 19:

3 Intergovernmental Panel on Climate Change (IPCC) 2007 What are tends for Colorado River basin? AND, how does this relate to streamflow? Consensus Forecasts of Temperature and Precipitation Changes from IPCC AR4 GCMs

4 Natural Flow at Lee Ferry, AZ Flow measurements 30 for the Colorado River Basin Compact Annual Flow (BCM) allocated 16.5 MAF Currently used 13.2 MAF Annual Flow 10 Year Average Running Average Figure adapted from Christensen and Lettenmaier, 2007

5 Motivation

6 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:

7 Research Objectives 1) Why are there such large discrepancies in projected Colorado River flow changes? Climate model projections, hydrological model sensitivities? 2) How sensitive is runoff to changes in temperature, changes in precipitation, or changes in both temperature and precipitation? 3) What are the underlying mechanisms for these changes (e.g. soil moisture, ET)? In the context of hydrologic sensitivities to (global) climate change, does the land surface hydrology matter, or does it just passively respond to changes in atmospheric circulation? 4) What are meaningful measures for water managers and policymakers that incorporate uncertainties in future climate change projections?

8 Methodology Climate Forcings Land-surface Hydrologic Models Measures Historic Delta changes GCM scenarios Regional climate models Variable Infiltration Capacity (VIC) Noah LSM Sacramento (SAC) SAC operational Catchment LSM Community Land Model 2-Layer Soil Water Balance possibly others temp sensitivity precip elasticity = = with time, spatially Q ref Q ref Q ref 0.1 deg C Q ref-1% - Q ref Q ref 1%

9 Historic climate forcings Within VIC, temp sensitivities and precip elasticities similar regardless of historic dataset or time period of long-term avgs Naturalized flow Wood and Lettemaier PRISM Maurer et al.

10 Delta method climate forcings Applied uniform perturbations in temperature and/or precipitation at every timestep in historic record Temp increases: streamflow decreases annually, primarily because decreases flow in spring/summer Common across models? Where are these changes occurring? Specific landsurface characteristics? Thresholds? Base 2 ºC 1 ºC 3ºC

11 Delta method climate forcings VIC Noah 2.7 SAC SAC NWS At Lee s Ferry, flows differ between models, but models appear to have similar patterns in temp sensitivity Base 2 ºC 1 ºC 3ºC

12 Temp Sensitivity percent change in flow per C temp increase VIC Noah 2.7 SAC SAC NWS A tmin & tmax tmax tmin & tmax tmax tmin & tmax tmax tmin & tmax historic deg deg deg Precip Elasticities percent change in flow per percent increase in precip VIC Noah 2.7 SAC SAC NWS -30% B -20% B -10% historic % Photo courtesy of xxx

13 Temperature Sensitivity temp sensitivity = Q 0.1 deg C - Q historic Q historic 0.1 deg C temp sensitivity = Q 3.1 deg C - Q 3 deg C Q 3 deg C 0.1 deg C Preliminary results: most sensitive appear to be areas along marginal snow zone. Photo courtesy of xxx

14 Spatial Patterns of Sensitivity by Sub-basin -8% -7.0% -7% -6.2% -6.3% -6% -5.7% -6.1% -5.8% -5% -4% Historic sensitivity Station Key: GA: Gunnison River near Grand Junction CI: Colorado River near Cisco GR: Green River at Green River LI: Snake River near Lilly, CO LE or LF: Colorado River at Lees Ferry IM: Colorado River below Imperial Dam % streamflow change per deg C -3% -2% -1% 0% -8% -7% -6% -5% -4% -3% -2% -1% IM LF GA LI GR CI base tmin1.0tmax1.0 tmin2.0tmax2.0 tmin3.0tmax3.0 0% IM LF GA LI GR CI Green River by Greendale is most sensitive, Snake River undergoes largest change in sensitivities (from historic to 3 deg C)

15 Concluding thoughts Temperature sensitivities decrease with increasing temperature, although overall magnitude is larger Precipitation elasticities increase with decreasing precipitation, finding consistencies between models Sensitivities appear to be at marginal snow zone Trends more notable at sub-basin level, spatial patterns matter

16 Future work Further resolve areas and seasons producing the most runoff, and their role in modulating (or exacerbating) regional scale sensitivities Explore underlying mechanisms, i.e. differences in how temperature change applied Compare additional models and changes in combined temp and precip changes Explore further how improved understandings of land-surface interactions and model uncertainty can aid decision-making

17 Acknowledgements Ted Bohn Ben Liveh Shrad Shukla Qiuhong Tang Francisco Munoz-Arriola Tazebe Kiros Beyene Tapash Das And, others of the Land Surface Hydrology Group, both past and present Reconciling Colorado Flow Projections Questions?? Ideas??

18 Advanced Hydrology 2009, aspiring to be like Steve Thank you Steve!