Snowpack and seasonal low flows in the Sierra Nevada:
climate change and water availability in California
U53A-0704
Sarah E. Godsey* and James W. Kirchner, Dept. of Earth & Planetary Science, Univ. California-Berkeley, 94720. * godseys@eps.berkeley.edu
Introduction
Seasonal low flows help sustain aquatic ecosystems, and supply human needs during
mid-summer. In California, the gap in timing between water supply and demand is
bridged primarily by the Sierra Nevada snowpack, which slowly melts throughout the
spring and summer. However, most future climate scenarios suggest a decreased
snowpack in the Sierra. Previous studies have investigated changes in flow timing
and spring snowmelt flood events (Refs. 1-6). Here we explore how changes in the
Sierra Nevada snowpack will affect annual low flows.
Low flows are highly variable
and correlate with snowpack
4
Flow, Q (m^3/s)
2
1
0.6
0.4
890% Q at 227% SWE
600
500
400
300
200
300
200
SF Kern
100
300
100
300
0.1
0.06
0.04
190
160
130
100
70
40
10
01/01/1978
01/01/1980
01/01/1982
01/01/1984
01/01/1986
01/01/1988
01/01/1990
01/01/1992
01/01/1994
01/01/1996
01/01/1998
01/01/2000
01/01/2002
01/01/2004
-20
Date
Time series plots of the (a) daily 15-day running
median flow at Sagehen Creek in the Sierra
Nevada (Q, log-scale) and the (b) daily 15-day
running median snow water equivalent (SWE) at
the Independence Lake Snow Telemetry
(SNOTEL) site located on the divide between
Sagehen Creek and Independence Lake basins.
The minimum flow in Sagehen Creek varies from
year to year in response to the changing SWE.
Low Flows Respond
More-Than-Proportionally
to Snowpack
Upper Truckee
200
100
SF Mokelumne
200
100
The relationship between snowpack and
runoff was more than proportional (slope
minus 2 s.e. greater than 1) in five of the
nine catchments.
Pyramid Creek
300
200
100
Trout Creek
100
200
If snowpack decreases by 10%, low flows
in these catchments would decrease by
9-17%.
Merced River
100
Sagehen Creek
200
100
0
0
50
100
150
200
Maximum Snowpack, SWE
(% of normal)
250
Relative minimum runoff, Q vs. relative
maximum snowpack, SWE, for each study
catchment. The parallel thin dashed lines indicate
constant proportionality and the solid lines
indicate the best fit regression lines for each
catchment.
Name
Snow Pillow
Stream Gauge
Independence
Lake (IDP)
Ward Creek 3
(WC3)
Echo Peak 5
(EP5)
Heavenly Valley
(HVN)
Schneiders
(SCN)
Black Springs
(BLS)
Sagehen Creek near
Truckee
Ward Creek at Hwy
89 near Tahoe Pines
Upper Truckee River
at South Lake Tahoe
Trout Creek near
Tahoe Valley
Pyramid Creek at
Twin Bridges
SF Mokelumne near
West Point
Merced River at
Happy Isles Bridge
near Yosemite
Pitman Creek below
Tamarack Creek
SF Kern River near
Onyx
Ostrander Lake
(STR)
Tamarack Summit
(TMR)
Upper Tyndall
Creek (UTY)
This means that streams that are currently
perennial may run dry at a substantial
fraction of current "normal" snowpack.
Ward Creek
200
Site
Information
The regression line crosses the x-axis
(+/- 2 s.e., all errors propagated through
analysis) at a value greater than zero
percent of normal snowpack in five of
the nine catchments.
Pitman Creek
200
0.2
220
Snowpack, SWE (cm)
700
Minimum Runoff, Q (% of normal)
% We have identified all of the gauged catchments in the Sierra Nevada with
unimpaired streamflow records and with at least ten years of overlapping snowpack
and streamflow data. In each of these catchments, we have analyzed up to 33 years
of historical snow and streamflow records.
% We find that as the annual peak snowpack water content changes from year to
year in these catchments, the annual minimum, mean, and maximum flows in these
catchments all change proportionally, or more-than-proportionally. For every 10%
decrease in snowpack, there is a 9-17% decrease in annual minimum flow.
% Finally, in some catchments, annual low flows are significantly correlated not only
with that year's snowpack, but with the previous year's snowpack as well.
Streams Run Dry at a
Substantial Fraction
of Current Snowpack
900
800
Altitude (m)
Snow
Stream
Pillow
Gauge
2629
1966
27
Years of
overlapping
record
24
Drainage
Area (km2)
2100
1948
25
23
2427
1938
142
22
2738
1942
95
24
2722
1966
23
29
2022
607
195
24
2551
1250
469
15
2349
2184
59
22
3547
902
1373
33
Multiyear Memory Effect
Minimum Flow, Q (% of normal)
The Short Story
250
Previous Year Above Average
Previous Year Below Average
The annual minimum flow at Sagehen Creek
depends not only on this year's snowpack,
but also on the snowpack of the previous
year. When the previous year's snowpack is
above average, minimum Q is more sensitive
to the current year's maximum snowpack.
200
150
100
50
0
0
20 40 60 80 100 120 140 160 180
Maximum Snowpack, SWE (% of normal)
References
Cayan DR, Riddle LG, Aguado E. 1993. The Influence of Precipitation and Temperature on Seasonal Streamflow in CA. WRR 29: 1127-1140.
Dettinger MD, Cayan DR, Meyer M, Jeton AE. 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.
Lettenmaier DP, Gan TY. 1990. Hydrologic Sensitivities of the Sacramento-S. Joaquin River Basin, CA, to Global Warming. WRR 26: 69-86.
Risbey JS, Entekhabi D. 1996. Observed Sacramento Basin streamflow response to precipitation and temperature changes and its relevance
to climate impact studies. J. Hydrology 184: 209-223.
Stewart IT, Cayan DR, Dettinger MD. 2004. Changes in snowmelt runoff timing in western North America under a 'business as usual'
climate change scenario. Climatic Change 62: 217-232.
Acknowledgements
NSF funded this research.
Thanks to the USGS, NRCS and CA Snow Page for data.