Defining drought in California:
Precipitation variability over
annual to millennial time scales
Dr. Jim Sickman
Department of Environmental Sciences and
Center for Conservation Biology
University of California, Riverside
International Drought Symposium
March 24-26, 2010
Riverside, California
Today’s Talk
California’s water supply:
Importance of snowpack
Definition of drought
Indices of precipitation
variability:
Spatial and Temporal
Refined definition of
drought for California
Spatial variability: 20 fold
Spatial variability: 2 fold
California Snowpack:
Present and Future
By 2100
+ 1.4 – 5.8
ºC degrees
Result
~ 1/3 loss
of
snowpack
Knowles and Cayan, 2004
Climate Change - Predicted
Changes in Inflows to Delta

higher winter peaks

drier summer
Change in inflows to Delta by 2060 (Knowles and Cayan, 2004)
On-going shift in
runoff timing toward
winter, extending
low-flow periods –
1/3 loss of snowpack
by 2050
Increase in intensity
and frequency of
winter runoff events
Hetch
Hetchy
Shasta
Dam Reservoir
Oroville
Dam
Pardee
Reservoir
CVP
Mokelumne River
Aqueduct
California
Aqueduct
Hetch
Hetchy
System
SWP
Groundwater,
Local
East
US
CASan
Bureau
Department
BayFrancisco
Municipal
of Reclamation
ofPublic
Water
Utility
Utilities
District
Resources
Resources
&
Conservation
Mokelumne
Central
State
Commission
Water
Valley
River
Project
Project
Aqueduct
1929
1940
1960
Hetch
- -1st
Burns
Hetchy
water
Porter
delivered
System
Act (Contra
Costa
1973
1913
Canal)
- 1st
- Raker
waterAct
to So.Cal.



American Heritage Dictionary: “A long
period of abnormally low rainfall”
Merriam-Webster Dictionary: “A period of
dryness especially when prolonged”
Concise Oxford Dictionary: “The
continuous absence of rain”
Lodgepole (Central Sierra Nevada) 1968-2009
Monthly Precipitation (cm)
30
25
20
15
10
?----------Drought--------?
5
0
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug Sep
Donner Summit Snow Course 1911-2009
200
Minimum =
15 cm (1963)
Maximum = 221 cm (1983)
150
100
50
0
19
11
19
18
19
25
19
32
19
39
19
46
19
53
19
60
19
67
19
74
19
81
19
88
19
95
20
02
20
09
Snow Water Equivalence (cm)
250
Water Year
Donner Summit 1911-2009
50%
45%
Mean SWE = 96 cm
STDEV = 41 cm
2009
Probability of SWE
40%
35%
30%
25%
20%
15%
10%
5%
0%
25
50
100
150
Snow Water Equivalence (cm)
200
83
Crested Butte Colorado Snow Course 1936-2009
Snow Water Equivalence (cm)
80
Minimum = 11 cm (1981)
70
Maximum = 68 cm (1952)
60
50
40
30
20
10
0
1936 1943 1950 1957 1964 1971 1978 1985 1992 1999 2006
Water Year
Crested Butte Colorado 1936-2009
60%
Normal
Probability of SWE
50%
40%
Mean SWE = 37 cm
STDEV = 12 cm
30%
20%
0.5x
10%
1.5x
0.25x
2x
0%
9
18
37
55
Snow Water Equivalence (cm)
73
http://beetlesinthebush.files.wordpress.com
F. Biondi, J.A. Kleppe and S. Strachan UNR

USA Today

Submerged, standing trees in
Lake Tahoe date to periods of
known severe drought
Older ages of some trees point
to undocumented droughts
100,000 yr
variability
10,000 yr
variability
Millennialscale
variability
Century-scale
variability
Decadal-scale
variability



Climate variability in California is extreme in
relation to more mesic zones
In the context of century to millennial scale
records of precipitation, our society overuses the
term drought
Overuse of the term gives society a false
impression of California’s hydrology and obscures
the fact that water shortages are driven principally
by human demand and are a near-permanent
condition
Q
U
E
S
T
I
O
N
S
?
[email protected]