1. 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

2. 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

3. Hydrologic Characteristics of PNW Rivers

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

8. 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)

9. 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

10. Typical SNOTEL Site

12. 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)

15. 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
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

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

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

18. Naturalized Summer Streamflow at The Dalles

19. 2001 cool PDO/ENSO neutral

20. 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

21. 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

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

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

24. 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