Project Description
Within the Upper Colorado River Basin, projected reductions in naturalized streamflow by the mid 21st century tied to climate change range significantly based on findings from recent scientific literature. Projected reductions in streamflow range from ~45% by Hoerling and Eischeid (2007), 10 to 25% by Milly et al (2005), ~18% by Christensen et al. (2004), and ~6% by Christensen and Lettenmaier (2007). In addition, the recent Seager et al (2007) analysis of future P-E (a proxy for runoff) suggests an ”imminent transition to a more arid climate in southwestern North America”. This wide range of future Colorado River streamflow projections makes it difficult for decision makers and water managers to prepare and plan for potential future reductions in streamflow resulting from climate change. The goal of this project is to reconcile future Colorado River streamflow projections by evaluating the various methodologies and models being used in projections and to understand why different modeling approaches produce varying flow reduction amounts. The second component of this project entails identifying a common subset of appropriate climate scenarios and models in development of future projections of streamflow; and using these scenarios and models to generate a consistent suite of Colorado River streamflow projections by the mid to late 21st century. Results will directly apply to the needs of water planners across the western U.S., and similar mid-latitude continental regions where water resources are heavily dependent on snowmelt runoff from mountainous headwater areas.

Project Materials
Project Proposal

Methodology
Individual water years since 1995 cover a wide range of seasonal evolutions of climate and resulting Colorado River stream flow due to shifts in the average and seasonality of temperature and precipitation, snow pack development and decline, and antecedent soil moisture conditions. The proposed intercomparison methodology will involve running the models used to estimate future Colorado River stream flows retrospectively to see how well they simulated observed changes in stream flow to known changes in temperature and precipitation for Water Years 1995 to 2007. To partition the sensitivity of the methodological approaches and models to changes in temperature and precipitation, 24 retrospective simulations will be made where the observed input values for temperature and then for precipitation (e.g., hourly, daily, monthly) for individual years are replaced with long term climatological averages for an agreed upon span of years (e.g., 1970-1999). Simulations focused on elevation sensitivity will examine the hydrologic implications of significant spatial heterogeneity of snowfall in the basin and the fact that the resulting runoff is derived mostly from relatively small, high altitude headwater drainage areas. Sensitivity experiments will be used to determine the extent that negative feedbacks in the hydrologic system can offset expected reductions in streamflow due to warmer temperatures (e.g., earlier runoff during warm winters is associated with reduced evapotranspiration in late spring and summer). We will use IPCC AR4 climate model projections for multiple emission (SRES) scenarios as input to simulate Colorado River flows and the hydrologic status of the basin for 2050-2060 and 2090-2100. Finally, we will work with stakeholders to assess the best ways to communicate the range of possible future hydrologic change in the Colorado River Basin.

Project Duration: 12 months

Meetings
San Diego, CA, September 26, 2007

Background Materials
Effects of Climate Change on the Hydrology and Water Resources of the Colorado River Basin. (2004). Niklas S. Christensen, Andrew W. Wood, Nathalie Voisin, et al. Climatic Change, vol 62, pp. 337 - 363.

A Multimodel Ensemble Approach to Assessment of Climate Change Impacts on the Hydrology and Water Resources of the Colorado River basin, (2006). Niklas S. Christensen and Daniel P Lettenmaier, Hydrology Earth Syst. Sci. Discuss.: 9 July 2006

Past Peak Water in the West, Martin Hoerling (NOAA) and Jon Eischeid (CIRES), Southwest Hydrology, January/February 2007

Global pattern of trends in streamflow and water availability in a changing climate, (2005) P.C.D. Milly, K.A. Dunne, and A.V. Vecchia, Nature 438, 347-350 (17 November 2005)

Model Projections of an Imminent Transition to a More Arid Climate in Southwestern North America, Richard Seager, Mingang Ting, Yochanan Kushnir, Jian Lu, Gabriel Vecchi, Huei-Ping Huang, Nili Harnik, Ants Leetmaa, Ngar-Cheung Lau, Cuihua Li, Jennifer Velez, Naomi Naik, Sciencexpress, 5 April 2007 

Colorado River Basin Climate: What Lies Ahead?, Brad Udall, 2007 Regional Water Symposium, Tucson, Arizona, August 31, 2007

Recent Research on the Effects of Climate Change on the Colorado River, Brad Udall, May 2007 Intermountain West Climate Summary

Appendix U: Climate Technical Work Group Report, Colorado River Interim Guidelines for Lower Basin Shortages and Coordinated Operations for Lakes Powell and Mead

IPCC AR4 Report
Chapter 11: Regional Climate Projections
WWA Guide to the IPCC

University NOAA Funded Agencies
Regional Integrated Sciences Assessment ( RISA) Programs:
California Applications Program (CAP)
Climate Impacts Group (CIG)
Climate Assessment for the Southwest (CLIMAS)
Western Water Assessment (WWA)

Federal Agencies
USDA/ NRCS
DOI/USBR/USGS
NOAA/GFDL
NOAA/ESRL

For more information, contact WWA Team Members:
Brad Udall, Director, WWA, NOAA-CIRES
Robert S. Webb, Chief, Climate Analysis Branch, NOAA, Physical Sciences Division
Martin Hoerling, Physical Sciences Division, NOAA