SIERRA NEVADA SNOWPACK
MARCH 2016
Snowpack Resources in CA and NV
S nowpack in California and Nevada supplies water, ecosystem services, and recreation. Snowpack serves as a natural reservoir as well as a key source of surface water and groundwater. In California, the spring snowpack on average stores about 70% as much as the water stored in the State’s reservoirs, shown in figure 1. The snowpack plays a vital role in water management in accumulating water during the cool stormy season and then releasing water as snowmelt during the drier, warmer spring and summer period. In the Colorado River Basin, which supplies almost all of Southern Nevada and approximately 55% of Southern California water, runoff from snowmelt contributes about 70% of total water supplies. In addition, melted snowpack recharges ground water, often more effectively than run-off. For example, in the Spring Mountains, west of Las Vegas, approximately 50-90% of groundwater recharge comes from snow.
Snowpack varies considerably from year to year in response to precipitation delivered by North Pacific storms and temperature fluctuations. Snowpack is often reported as snow water equivalent (SWE), or the amount of water stored in the snow. To accumulate snow, temperatures must be cold enough to cause the precipitation to fall as snow and to prevent the snow from melting. In the past two years, 2014 and 2015, Sierra Nevada snowpack was disproportionately depleted because of record high temperatures (figure 2). Results from hydrologic model runs in which 2014 temperatures were replaced with temperatures from 1917-2013 suggests that temperatures caused the 2014 snowpack in California to be lower by 60% on average. The results ranged between 2014 snowpack decreasing by as much as 160% and increasing by 20%, with 92% of the scenarios showing that snowpack was lower in 2014. The low snowpack in the last two years provides a scenario of future water supply conditions under climate change.
Figure 1: The total water stored in the 12 major reservoirs defined by CA Department of Water Resources, and the other 161 reservoirs, and in the monthly snowpack. The solid red line is the average reservoir storage from 2000-2015 and the dashed red line is the average snowpack plus reservoir storage. Updated from Dettinger and Anderson, 2015.
STRONG EL NIÑO E VENTSStrong El Niño events tend to result in colder winter temperatures and increased precipitation throughout much of California and Nevada. This combination of cooler than average temperatures and increase precipitation typically leads to increased SWE during most strong El Niño events, with exceptions in 1965-66 and 1991-92.
CONTACTS: David Pierce – [email protected] Kalansky– [email protected]
— A N O A A R I S A —
Snow Water EquivalentTuolumne BasinMar 23 2014
SWE (meters)
Figure 3: SWE has traditionally been measured by manual snow samples and by fixed pillows. Recently these surface measure-ments have been supplemented by airborne lidar. Mapped here over the Tuolumne Basin. Image courtesy of NASA Airborne Snow Observatory.
Temp Precip
Figure 4: Temperature and precipitation Oct-Mar departure from average during strong El Nño events (1957/58, 1965/66,1972/73,1982/83,1991/92,1997/98). Courtesy of NOAA
2005 20
0620
07 2008
2009
2010
2011
2012
2013
2014
2015 20
160
102030405060
Calendar Years
Mill
ions
of A
cre-
Feet
Water Stored in California Reservoirs and Snowpack
161 other reservoirs
Total Snowpack
12 major reservoirs
2000-15Normals (snow & total reservoirs)
Milli
on o
f Acr
e Fe
et
Calendar Years200
6200
7200
8200
9201
0 2011
2012
2013
2014
2015
2016200
5
20
30
10
40
50
60
0
WATER STORED IN CALIFORNIA RESERVOIRS AND SNOWPACK
4848
50
52
54
56
0
50%
100%
150%
200%
1970 1980 1990 2000 2010
2014
2015
April
1st
Sno
wpa
ck (p
erce
nt o
f nor
mal
) Ave Winter Tem
p (˚F)
Figure 2: April 1st percent of normal SWE (blue bars and 2014 -2015 yellow bars ) and winter (DJF) temperatures (red line) for California. Data courtesy of the CA DWR and WRCC.
0%1970
2014
48
50
52
54
56
Average Winter Tem
p (˚F)
April
1st
SW
E (p
erce
nt o
f nor
mal
)
100%
2015
1980 1990 2000 2010
50%
150%
200%
0 -10 100Inches˚F NCEI NCEI
5.0 -5.0
CALIFORNIA APRIL 1ST SWE AND WINTER TEMPERATURES
0
40 km0.500.450.390.320.260.190.130.06
0 10 20 30
Snowpack and Climate Change
Historical observations, combined with climate models, indicate that climate change is already affecting spring snow accumulation in the western U.S. Historical snow measurements reveal that April 1st SWE values have declined in the northern Sierra, though less so in the southern Sierra. The difference is because the high elevation of the southern Sierra makes the region less susceptible to changes in temperature thus far. Observations across the West indicate that the proportion of precipitation as snow has declined also. Figure 5 shows the fraction of snow that falls between -2 and 0˚C; snow that falls in this temperature range is more likely to fall as rain as the climate warms. Besides water supply implications, snow instead of rain is an important deterrent for flooding, and has obvious impacts on recreation. Snowfall, snow accumulation, snowmelt and other aspects of snowpack are predicted to change at different rates in the future depending on their sensitivity to temperature as compared to precipitation. The table below shows the projected year when different variables related to snowpack are predicted to change beyond natural climate variability in different regions in the West.
n CNAP, the California Nevada Applications Program, is a NOAA RISA team conducting applied climate research that is inspired by and useful to decision makers in the region cnap.ucsd.edu.
n NIDIS, National Integrated Drought Information System, works with the federal, state, tribal and local partners to improve drought early warning, preparedness, and response to impacts, drought.gov.
n WRCC, the Western Regional Climate Center, develops products, provides services, and improves capabilities that enhance the delivery of climate information to the public in collaboration with NOAA partners, www.wrcc.dri.edu.
n SWCSC, the Southwest Climate Science Center, sponsored by the US Depart. of the Interior, provides scientific information, tools, and techniques to anticipate, monitor, and adapt to climate change, www.doi.gov/csc/southwest.
In mountain regions across the West as climate continues to warm over the 21st century, more precipitation will fall as rain rather than snow and the more snow will melt prior to April 1st. Projections of climate change in the Sierra Nevada find rain accounting for 50.6% (145 mm) more of precipitation and 62.3% (63mm) more of snow melt prior to April 1st than in historical norms (figure 6).
RCP 8.5 (Business as usual emission scenario)
RCP 4.5
(Emission reduction)
Sierra
Nevada Great Basin
Wasatch CO River
Basin
Sierra Nevada
Great Basin
Wasatch CO River
Basin
SWE 2055 2045 2055 2070 2075 2075
Precip Falling as Snow 2060 2055 2080 2090 2070 2100
Last day of snow season 2035 2028 2025 2045 2030 2033
For more information see Pierce and Cayan, 2013
Dettinger, M. D., et al., 2015, Ecological Applications, 25 (8), 2069-2093.Dettinger, M.D. and M. Anderson, 2015, San Francisco Estuary and Watershed Science, 13(2).Pierce, D. W. and D. R. Cayan, J. of Climate, 26,4184-4167.
0.28
0.20
0.15
0.10
0.05
0
Fraction Figure 5. Fraction of snow that falls between -2 and 0˚C. The snow that falls in this temperature range is most susceptible to fall as rain in a warming climate. From Dettinger et al. 2015
PROJECTED YEAR WHEN SNOW VARIABLE CHANGES BEYOND NATURAL CLIMATE VARIABILITY
2055 2055
2055
2045
2060 2080
20302035 2025
2070
20702090
2075 2075
2100
2045 2030 2035
(a) FRACTION OF ANNUAL PRECIPITATION FALLINGIN THE DAILY TEMPERATURE RANGE: -2C < Tavg < 0C
FRAC
TIO
N
0.00
0.05
0.10
0.15
0.20
0.28
0 1 2 3 4 5 6Warming, in degrees C
0
0.1
0.2
0.3
0.4
0.5
Frac
tion
of T
otal
Pre
cipi
tatio
nas
Sno
w v
s Rai
n
(b) Fraction of Western Precipitation Falling on Dayswith Tmin < 0C under Various Imposed Warmings
0 2 4 6 8Warming, in degrees C
0
0.2
0.4
0.6
0.8
1
Frac
tion
of W
est w
ith >
this
Prec
ip-F
orm
Cha
nge
10% increase in rain fraction20% change30% change40% change50% change
(c) Areas in the Western US subject to Rain-Snow Conversions of Various Percentages
Sierra Nevada
Tota
l pre
cip.
(mm
)
Rain+145 (50.6%)
Melts+63 (62.3%)
SWE−169 (−30.3%)Snow
2020 2060
020
060
010
00
Figure 6. Projected changes in snow variables in the Sierra Nevada throughout the 21st century relative to historical norms. From Pierce and Cayan, 2013
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