RAINFED FLOOD RISKS IN A
WARMING WEST
Michael Dettinger & Dan Cayan, USGS
Scripps Inst Oceanography, La Jolla, CA
Jessica Lundquist, University of
Washington, Seattle, WA
16 May 2005 in Yosemite Valley
CEC’s California Climate Change Center
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
+1 std dev
MORE as snowfall
Mote, 2003
Observed: Earlier greenup
Observed:
Earlier
snowmelt runoff
Cayan et al., 2001
Stewart et al., 2005
Not many trends
in Annual-Flood
series that meet
even minimal
tests of
significance,
1947-2000.
But what flood-risk
changes should we be
expecting, and where?
What has happened to
Sierran floods recently?
Linear trend
10-yr avg
Have annual-peak flows
become more likely to be
precipitation-driven (rain- or
rain-on-snow fed) than
heat-driven (snowmelt-fed)
in recent decades?
+1 std dev
MORE as snowfall
Today:
•Quick look for historical flood-risk changes
• Brief consideration of a particular flood that we
observed reasonably well
• Implications for floods in a warmer world
What happens to snowlines & flood risk?
What happens to soil moisture & flood risk?
[What might happen to storms & flood risk?]
Basic Idea:
Rain on wet soil or snow --> Flood risk
Basic Idea:
Rain on wet soil or snow --> Flood risk
More frequent rainy (warm) storms --> More
risks of floods (seems likely)
Rain on bigger catchment areas --> Increased
risk of flood (very likely, example
of effectiveness of such changes)
Rain on wet soils --> Increased risk of flood
(likely, but let me explain why)
Larger storms --> Increased risk of flood
(but unknown likelihood)
But where and how much risk?
On 16 May 2005, in Yosemite Valley…
An inch or so of precipitation
combined with above
freezing temperatures…
…to generate major flood flows…
Highest stations
with runoff
derived from all
but the highest
elevations in the
Park.
Streamflows monitored
by Scripps/USGS Hydroclim
Network in Yosemite
So, what happened?
Temperatures measured
by Scripps/USGS Hydroclim
Network in Yosemite
high country
Snow-water Contents
What happened?
Temperatures measured
by Scripps/USGS Hydroclim
Network in Yosemite
Snow-water Contents
…but NOT the
prototypical
Rain-on-snow
MELTING event!
A brief aside about rain on snow:
HOW MUCH WARM RAIN DOES IT TAKE TO
MELT SNOW DIRECTLY?
• Each gram of liquid water contains about 4 J of heat per degree C
above freezing
• To melt a gram of ice requires about 333 J of heat
--> Ice melted ~ 1/80 gram per gram of rain per degree C above freezing
Instead, the warm temperatures increased the
area contributing rainfall-runoff by about 5 times
the area in a “normal” winter storm…
5 times the normal runoff-contributing
area*1 inch of precip --->
5 inches on ‘normal’ contributing area
How often and where might we expect such
events to take place, in a warming world?
If we assume a standard moist adiabatic lapse
rate during all historical storms in Yosemite…
Precipitation
(but, using all months and
min-daily temps in the figures
to follow)
Temperature
THEN, compare
Freezing
Altitudes to basin
hypsograph…
We can estimate the
snowline for historical
storms since 1916.
X -6.5C/km lapse rates
= freezing altitudes by storm
= rain-covered
Basin areas by
storm
Observations!
For such a large fraction of the basin above Yosemite Valley to
receive rain rather than snow was historically unusual….
…but it won't be unusual for long, if warming trends
continue.
Using1950-99 meteorologic records nationwide: How widespread is this vulnerability?
Estimating influence of a uniform +3ºC warming on
Rain-fed flood contributions
Notice highly nonlinear color bar
More risk
No change
Sierra Nevada especially
vulnerable
Changing flood risks
--> Sierra and Cascades are
likely rainfed flooding hotspots!
Meanwhile, back in
Yosemite, beneath
remaining
snowpacks, warming
brings a general
winter "drizzle" that
keeps snows & soils
wetter.
~ +100%
~ +40%
Dettinger et al 2004, Clim. Change
With all this
additional winter
melting going on,
the soils are more,
and more
frequently, wet
during the flood
seasons…adding
to the flood risks,
across the central
and northern
West.
Notice that reds are
order of magnitude
smaller than blues in
this color bar.
SO…
Until we have a much better grasp of what the frequencies of
various storm-precipitation totals will do under global
warming….
• Sensitivities of floods to trends in storm-time snowline
(Sierra/Cascades) & wetter winter soils (central/northern West) should be
immediate concern.
• Warming will increase scope & magnitude of raindriven flooding from the Sierra Nevada, despite
projected snowpack declines.
CEC’s California Climate Change Center
Lets look at how storm precipitation totals might change…
What is projected on the daily (storm) scale?
GFDL historical
simulation, 1961-2000
Focus on the warmest wettest
storms (above the green bar
here)
GFDL historical
simulation, 1961-2000
Observations
GFDL B1 scenario,
2061-2100
GFDL historical
simulation, 1961-2000
So, this scenario (with optimistic emissions in a sensitive climate
model) yields a potential doubling of # of occasions when warmstorm floods might occur…due mostly to elevated storm temperatures (notice: peak ppt
hasn't changed much).
THE storms to worry about…
e.g.,
• All 7 major floods of Russian River
since 1997 have been atmospheric
rivers (pineapple expresses)
• The 9 largest winter floods of
Carson River since 1950 have been
pineapple expresses
ONE MORE CONCLUSION(?)
At present….
• It is difficult--thus far--to judge how the numbers,
volumes or even temperatures of storms will change
• Our best bet for developing confidence about this will
depend on our ability to predict (or track) the future of
"atmospheric rivers"
• But, again, these are largely California/Cascades risks
CEC’s California Climate Change Center