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
Normalized Streamflow
3.0
2.5
Snow
Dominated
2.0
Transient Snow
1.5
Rain Dominated
1.0
0.5
0.0
10 11 12
1
2
3
4
Month
5
6
7
8
9
Typical hydrologic cycle of a western U.S. river basin
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)
Snow water content on April 1
Hydrologic prediction in the West – a long history
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
Snow
Land
Snow (SWE >= 5mm)
Clouds
No Data/No Decision/Saturated
Land (within Columbia River Basin)
Forecast System Schematic
local scale (1/8 degree)
weather inputs
soil moisture
snowpack
Hydrologic
model spin up
INITIAL
STATE
NCDC met.
station obs.
up to 2-4
months from
current
1-2 years back
LDAS/other
real-time
met. forcings
for spin-up
gap
streamflow, soil moisture,
snow water equivalent, runoff
Hydrologic forecast
simulation
ensemble forecasts
SNOTEL
SNOTEL
/ MODIS*
Update
Update
25th Day, Month 0
ESP traces (40)
CPC-based outlook (13)
NCEP GSM ensemble (20)
NSIPP-1 ensemble (9)
Month 6 - 12
* experimental, not yet in real-time product
Hydrologic forecasting – Ensemble approach
recently observed
meteorological data
Spin-up
hydrologic
state
ensemble of met. data
to generate forecast
ICs
Forecast
obs
Effects of the PDO and ENSO on Columbia River
Summer Streamflows
450000
Cool
Cool
Warm
Warm
350000
300000
250000
200000
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
150000
1900
Apr-Sept Flow (cfs)
400000
Naturalized Summer Streamflow at The Dalles
450000
WarmPDO/WarmENSO
400000
WarmPDO/ENSONeut
Flow (cfs)
350000
WarmPDO/CoolENSO
300000
CoolPDO/WarmENSO
250000
CoolPDO/ENSONeut
200000
CoolPDO/CoolENSO
150000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Probability of Exceedence
Forecast1
800000
Forecast2
700000
Forecast3
600000
Forecast4
Forecast5
500000
Forecast6
400000
Forecast7
300000
Forecast8
200000
Sim Min
100000
Sim Max
Observed Virgin Flow
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
nov
0
oct
Streamflow (cfs)
2001 cool PDO/ENSO neutral
Other sources of climate forecasts
CCA
NOAA
CAS
OCN
SMLR
CPC Official
Outlooks
Multiple Seasonal
Climate Forecast Data
Sources
CA
NCEP CFS
NASA
VIC
Hydrology
Model
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
Managed
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