Downscaling for Climate Change &
Water Resources
Mike Dettinger, Hugo Hidalgo & Dan Cayan
USGS @ SIO, La Jolla, CA
1. Quick review of western trends & projections
2. USGS downscaling needs
3. New downscaled scenarios
1. Review of recent trends & projections:
Mostly results from:
USGS Hydroclimatology
(Global Change Hydrology)
Program-funded studies
Funded by
Office of Surface Water:
Thanks to Harry Lins
Bill Kirby
Julie Kiang
& San Francisco Bay PES
efforts
2. USGS downscaling needs
Mostly lessons from:
Efforts contributing to the
2006 California Climate
Change Assessment
activities
Planning for 2008
California Climate
Change Assessment
http://meteora.ucsd.edu/cap/pdffiles/CA_climate_Scenarios.pdf
3. New downscaled scenarios: A product of
SF Bay PES & CASCaDE projects:
Computational
Assessments of
Scenarios of
Change in the
Delta
Ecosystem
1. Review of recent trends & projections:
The western states have been warming
in recent decades.
Geographically, here is the pattern of warming.
(Cayan et al., 2001)
Annual-mean warming, 1979-2005
(IPCC 4th WG1 Ch. 4)
Warming already has driven Observed: Less spring snowpack
TRENDS (1950-97) in
hydroclimatic trends.
April 1 snow-water content at
Observed: Less snow/more rain
western snow courses
Knowles et al.,
2006
-2.2 std devs
LESS as snowfall
Mote, 2003
+1 std dev
MORE as snowfall
Observed: Earlier greenup
Observed:
Earlier
snowmelt runoff
Cayan et al., 2001
Stewart et al., 2005
Greenhouse forcings
1900-2100
Under projected
greenhouse forcings,
all climate models
yield warmer futures
for California and
the West.
20 of 23 in this range
Dettinger, 2005
Under even the coolest
of these scenarios
(+2.5ºC), observed
hydrologic trends in
most Western streams
are projected to
continue in
21st Century.
With coming addition
of
increased flood risks
in Pacific coast states
low baseflows
warm streams
drier summer soils &
fuels
potentially large
changes in mountain
recharge
Stewart et al., 2004
Center-of-Timing of
Streamflow
No snow left
Most snow left
Knowles & Cayan, 2004
Knowles and Cayan 2002
Three Stages of Climate-Change Response
• Is there a problem?
• What should I do?
• What are my risks and options?
2. USGS downscaling needs
The downscaling problem
Downscaled
Global Climate Model
(actually, general circulation model)
Original GCM
values
USGS PES SF Bay (CASCADE) project
needs:
Downscaled series
needed for cascade of
hydrologic,
hydrodynamic,
geomorphologic,
geochemical &
ecological modelers:
Daily max & min temperatures
Daily precipitation
?
San Francisco
Monterey
Downscaling options:
• Broad-brush T and %P changes
Scenario options:
• Synthetic statistical
Describe the probability distributions of downscaled
variables conditional on statistical relations between
GCM and downscaled variables, and resample
accordingly
• Deterministic statistical *
Describe spatio-physical relations between GCM and
downscaled variables, and interpolate directly
• Dynamical
Simulate same physical processes as in GCM but at
higher resolution and (today) over smaller spatial
domains
Our downscaling wish list:
-- Resolution: Daily time resolution, ~ 10-km spatial
resolution, multiple climate variables
-- Accuracy: Reproduction of high-resolution historical records
-- Feasibility: Not too computationally burdensome (?)
-- Synchronicity: Downscaled weather synchronized with
climate model weather (just a Delta SLR thing?)
-- Theoretical: Doesn't constrain future 'weather' to be same
as historical
-- Aesthetic: Climate (& trends) arise from entire weather field
rather than being imposed GCM-grid cell by grid cell
-- Practical: Ability to downscale to grids + stations at same
time, maintaining internal consistency
The Delta
Lower salinity inflows
• The Delta encompasses 2,177
km2 of agricultural land; Central
Valley, including the Delta, yields
$31B/yr (45% of the US produce)
3
Annual Export Volume (km )
• 6 km3/yr of water is pumped from
the Delta by State & Feds for
agriculture & SoCal urban uses
(22M people)
Seawater intrusion
Higher salinity
freshwater
inflows
Export to south
8
6
State
4
2
Federal
0
60
70
80
W ater Year
90
2000
LEVEE STABILIZATION
• 1800 km of levees provide the
dry land for 57 manmade islands
• Aging levees, sea-level rise,
storm surges & flows, oxidizing
island peats, failing ecosystems
& earthquakes threaten
uncontrolled breaches
• Levee breaches continue,
despite massive structural
controls in the 20th Century
Florsheim & Dettinger, 2005
Sources of extreme water levels from San Francisco to Sacramento
Non-tidal
Range (floods)
Tidal range
NORTH
Sacramento R
Sacramento
Bay
Delta
San Joaquin R
San Francisco
Monterey
From 2006 California Climate Change Assessment:
Projected
extreme high
sea-level stands
(black)
Extreme high
sea-level stands
coinciding with
low-pressure
systems (red)
Cayan et al, in press, Clim. Chg
A distant proxy for
measuring the
coincidences of high
seas + floods
PROBLEM: BCSD-based downscaling yields enhanced floods, but
downscaled floods not synchronized with GCM storm surges
Downscaled Projected Trends in December Precipitation
by Two Approaches
(GFDL CM2.1, A2 emissions, 21st Century)
BCSD
Bias correction & spatial downscaling, from Ed
Maurer, SCU
Ours
Our approach towards downscaling climate change:
• Downscale weather day by day
• Downscale enough (daily) weather and you get
downscaled climate
• Downscale enough climate and you get downscaled
climate change
Don't impose climate or climate change after the fact !
The constructed-analogs method
Hidalgo, H.G., Dettinger, M.D., and
Cayan, D.R., in review, Downscaling
using constructed analogues daily US
precipitation and temperatures: J.
Climate, 24 p.
Hidalgo, H.G., Dettinger, M.D., and
Cayan, D.R., in review, Downscaling
using constructed analogues daily US
precipitation and temperatures: J.
Climate, 24 p.
Hidalgo, H.G., Dettinger, M.D., and
Cayan, D.R., in review, Downscaling
using constructed analogues daily US
precipitation and temperatures: J.
Climate, 24 p.
Skill of downscaling as indicated by application of method to
historical OBSERVATIONS
Skill at monthly
average scale
Distributions of daily precip at selected sites
Application of
method to
historical
OBSERVATIONS
shows that even
extremes are
captured
accurately
Hidalgo, H.G.,
Dettinger, M.D., and
Cayan, D.R., in
review, Downscaling
using constructed
analogues daily US
precipitation and
temperatures: J.
Climate, 24 p.
Our constructed analogs offer:
-- Resolution
-- done
-- Accuracy
-- just illustrated
-- Computationally feasible
-- yes
-- Synchronized with GCM
-- yes
-- Doesn't constrain future 'weather' to be same as historical
-- done
-- Source of trends
-- everything fitted to the whole field at daily level with no
trends imposed externally
-- Downscaling fields & stations
-- direct extension
3. A downscaling offer:
Available by end of (calendar) year 2007:
-- Daily downscaled Tmin, Tmax & Precip fields for
1950-2100 across CONUS on 12-km grid will be
online and available from PES/CASCADE:
•
From GFDL CM2.1 climate model (one of the
warmest models) under heavy emissions (A2) and
moderate emissions (B1) scenarios
From NCAR PCM climate model (one of the
coolest models) under heavy emissions (A2) and
moderate emissions (B1) scenarios
Planned by beginning of summer 2008:
-- Same for between 4 and 6 more models
Caveats with this approach
-- If you don't like the PRISM temperature &
precipitation interpolations in your area, you won't
like the downscaled fields
-- If the GCM's weather (historical or otherwise) is
flakey, this downscaling won't fix it
-- CA needs daily GCM fields and an archive of
observed weather data (analogs)
-- U of WA & USBR are hurrying to
distribute another set of downscaled fields
(BCSD), which may become the de facto
default
What do our constructed-analogs
downscaled climate changes look like?
A2
Dettinger, 2005
One day in the 21st Century
(GFDL A2)
Downscaled
Original GCM
values
Trends, December temperatures
Trends, June-August temperatures
Trends, December precipitation
Trends, June-August precipitation
A2
B1
A2 Tmin Trends
B1 Tmin Trends
A2 Precip Trends
B1 Precip Trends
TO SUMMARIZE re: clim.chg.…
Warming by +2 to +6ºC,
likely more so at higher altitudes
•
Uncertain precipitation changes,
maybe less over much of western
mountains, mostly small declines
•
Significant changes in
• rain-vs-snow storms *
• snowpack amounts *
• snowmelt timing *
* Already detected
• flood risk
& PES/CASCADE is
• streamflow timing *
• low flows
producing a new generation
• growing seasons *
of downscaled scenarios for
• recharge?
CONUS
• total flow (drought?)
just in response to temperatures
REFERENCES
Bales, R., Molotch, N., Painter, T., Dettinger, M., Rice, R., and Dozier, J., 2006, Mountain hydrology of the western United States:
Water Resources Research, 42, W08432, doi:10.1029/2005WR004387, 13 p.
Cayan, D.R., Kammerdiener, S., Dettinger, M.D., Caprio, J.M., and Peterson, D.H., 2001, Changes in the onset of spring in the
western United States: Bulletin, American Meteorological Society, 82, 399-415.
Dettinger, M.D., 2005, From climate-change spaghetti to climate-change distributions for 21st Century California: San Francisco
Estuary and Watershed Science, 3(1), http://repositories.cdlib.org/jmie/sfews/ vol3/iss1/art4.
Dettinger, M.D., and Cayan, D.R., 1995, Large-scale atmospheric forcing of recent trends toward early snowmelt in California:
Journal of Climate 8(3), 606-623.
Dettinger, M.D., Cayan, D.R., Meyer, M.K., and Jeton, A.E., 2004, Simulated hydrologic responses to climate variations and
change in the Merced, Carson, and American River basins, Sierra Nevada, California, 1900-2099: Climatic Change, 62,
283-317.
Hidalgo, H.G., Cayan, D.R., and Dettinger, M.D., in preparation, Variability of spring-summer drought in regions of high and low
evaporative efficiency: for submission to J. Hydrometeorology
Intergovernmental Panel on Climate Change, 2007, Climate change 2007, The physical science basis—Summary for
Policymakers: IPCC Secretariat, http://www.ipcc.ch, 21 p.
Knowles, N., and D. Cayan, 2002, Potential effects of global warming on the Sacramento/San Joaquin watershed and the San
Francisco estuary: Geophys. Res. Lett., 29: 18-21.
Knowles, N., and D. Cayan, 2004, Elevational Dependence of Projected Hydrologic Changes in the San Francisco Estuary and
Watershed. Climatic Change, 62, 319-336.
Knowles, N., Dettinger, M., and Cayan, D., 2006, Trends in snowfall versus rainfall for the Western United States: Journal of
Climate, 19(18), 4545-4559.
Mote, P.W., 2003, Trends in snow water equivalent in the Pacific Northwest and their climatic causes. Geophysical Research
Letters, 30, DOI 10.1029/2003GL0172588.
Stewart, I., Cayan, D., and Dettinger, M., 2005, Changes towards earlier streamflow timing across western North America: Journal
of Climate,18, 1136-1155.