Challenges in western water management: What can science offer? Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington for presentation at AAAS Symposium: Achieving sustainable water supplies in the drought-plagued West San Francisco February 16, 2007

The Issues Strongly winter-dominant precipitation regime, and dominant role of snow (responsible for 70+ percent of runoff) create timing mismatch between streamflow and water demand Rivers heavily managed (“Making the desert bloom” mentality pre ~1980 resulted in essentially all reservoir sites being taken) Climate change is reducing natural storage afforded by snowpack Rapidly growing population places stress on water allocation (especially agriculture, which remains responsible for much of water use) Water rights based on appropriative doctrine encourages inefficient allocation of water

Hydrologic Characteristics of PNW Rivers

Typical hydrologic cycle of a western U.S. river basin

winter flows rise and summer flows drop As the West warms, winter flows rise and summer flows drop I.T. Stewart, D.R. Cayan, M.D. Dettinger, 2005, Changes toward earlier streamflow timing across western North America, J. Climate Figure courtesy of Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

Hydrologic prediction in the West – a long history Snow water content on April 1 should add my personal pics of - snow sampling snotel sites (and scan in curve method figure) SNOTEL network McLean, D.A., 1948 Western Snow Conf. April to August runoff

Typical SNOTEL Site

MODIS Snowcover March 3, 2000 MODIS Snowcover April 4, 2000 Land Snow (SWE >= 5mm) Clouds No Data/No Decision/Saturated Land (within Columbia River Basin)

Forecast System Schematic NCDC met. station obs. up to 2-4 months from current local scale (1/8 degree) weather inputs soil moisture snowpack Hydrologic model spin up SNOTEL Update streamflow, soil moisture, snow water equivalent, runoff 25th Day, Month 0 1-2 years back LDAS/other real-time met. forcings for spin-up gap Hydrologic forecast simulation Month 6 - 12 INITIAL STATE SNOTEL / MODIS* Update ensemble forecasts ESP traces (40) CPC-based outlook (13) NCEP GSM ensemble (20) NSIPP-1 ensemble (9) * experimental, not yet in real-time product

Hydrologic forecasting – Ensemble approach recently observed meteorological data ensemble of met. data to generate forecast Spin-up ICs Forecast obs hydrologic state

Effects of the PDO and ENSO on Columbia River Summer Streamflows Cool Cool Warm Warm

Naturalized Summer Streamflow at The Dalles

2001 cool PDO/ENSO neutral

Other sources of climate forecasts Multiple Seasonal Climate Forecast Data Sources CCA NOAA CAS OCN CPC Official Outlooks SMLR CA NCEP CFS VIC Hydrology Model NASA NSIPP/GMAO dynamical model ESP ENSO UW ENSO/PDO

Comparison with RFC regression forecast for Columbia River at the Dalles UW forecasts made on 25th of each month RFC forecasts made several times monthly: 1st, mid-month, late (UW ESP unconditional forecasts shown) UW RFC

What are the implications of improved forecasts to a managed water resources system? “Natural” system

Potential gains in economic value of water use due to perfect seasonal hydrologic forecast (relative to climatology)

Conclusions Improvements in technology – both observations, and modeling – clearly have a role in Western water management They are not, however, a panacea – while the potential for improvements is substantial, the “capturable” economic gains (relative to no forecasts) are mostly single digit percentages Other factors (especially economic reallocation of water) have a potentially far larger impact