F ig u r e 2 . Tr en d s in w in t er S WE (left ) a n d P R CP (r ig h t). N o t e d o m in a n ce o f S WE
tr en d s in ca u sin g S WE /P t r en d s, w ith slig h t excep tio n s in CO , N M (cf. F ig ur e 1 ).
Figure 2 shows the corresponding individual trends in SWE and P. The dominance of trends in SWE
deposition in generating the SWE/P trends is evident. SWE trends generally concur with the direction of
trends in the ratio SWE/P, while trends in P are in the “wrong” direction, with the slight exception of CO and
NM, where higher P has led to higher SWE, with the ratio SWE/P still generally decreasing.
F ig ur e 1 . Le f t : Win ter (N o v-M a r ) S WE /P tr end s: R ed sig n ifie s a n eg a t ive tr en d ,
b lu e p o sit ive; sym b o l size is p r o p o r t io n a l t o ch a n g e in sta n d a r d d evia tio n s a s
in d ica t ed [b ig g e st tr en d is -2.2 *sig m a o ver 5 3 yr s]; cir cle s in d ica t e h ig h t r en d
sign ifica n ce (p < 0 .05 ), squ a r e s in d ica te lo w e r tr en d sig n ifica n ce (p > 0 .0 5 ).
R ig h t : M ea n w in ter S WE /P .
Results
a. Winter Trends
SWE/P trended negatively for water years 1949-2001 over most of the West (Figure 1a). Of all 222 stations,
156 trend negative versus 65 trending positive (64 vs. 15 at p<0.05). Downward trends are most pronounced at
lower-elevation sites in WA, OR, and CA, though significant negative trends occur throughout the West.
Steps 1-2 were also applied to the pair TMAX and TMIN. Daily snow water equivalent (SWE) of deposited
snow (not to be confused with the SWE of the accumulated snowpack) was determined by assuming that any
precipitation measured on a day when snowfall was also measured was entirely in solid form; for no measured
snowfall, any measured P was assumed liquid. Time series of winter (Nov-Mar) and monthly totals of SWE
and P were calculated, the corresponding ratios SWE/P were calculated, and a Kendall's-tau trend analysis
was performed on all time series.
1) Any year for which at least 10 days between 11/1 and 3/31 have missing precipitation or snowfall data
was considered incomplete and excluded from the analysis.
2) Any station which was missing >50% of precipitation or snowfall data in any given 10-year period was
discarded.
3) Any station whose mean total annual SWE (see below) was less than 50mm was discarded.
Data and Methods
NCDC Summary of the Day historical data comprising daily snowfall depth (S), precipitation (P), maximum
(TMAX) and minimum (TMIN) surface air temperature for the 11 westernmost states of the conterminous
U.S. were assembled for the 53-year period 10/1/1948-9/30/2001. For precipitation and snowfall, there were
1,653 stations active during some or all of this period; for temperature, the number of stations was 1,517.
Emulating the approach of Huntington et al. (2004), the data pair of precipitation and snowfall were processed
according to the following sequential steps:
Introduction
Previous studies have shown that over the past several decades, runoff from snow-dominated basins has
arrived progressively earlier due to large-scale warming trends. To better understand the nature of this shift
and the climatic factors behind it, it is useful to distinguish reduced initial snowfall and earlier snowmelt as
separate causes of early runoff. Warming trends have historically been (and are projected to continue as)
marked by strong seasonal patterns (Knowles and Cayan, 2002). The temporally separated processes of snow
deposition and melt are therefore subject to different warming trends, and trends in these processes follow
accordingly. This study isolates the effect of historical warming on snow deposition throughout the western
U.S.
F ig u r e 3. M o nt h ly t r en d s in S WE /P . N o te do m in a n ce o f Ja n ua r y (to p r ig h t) a n d
esp ecia lly M a r ch (low er m id d le). [r ed = n ega t ive t r en d ]
Mar
Dec
Apr
Jan
Feb
Nov
Apr
Jan
Dettinger, M.D. and D.R. Cayan, 1995. Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California.
Journal of Climate, 8:3 p. 606-623.
Hamlet A.F., Mote P.W, Clark M.P., Lettenmaier D.P., 2004, Effects of temperature and precipitation variability on snowpack trends in the
western U.S., J. of Climate (in review).
Thomas G. Huntington, Glenn A. Hodgkins, Barry D. Keim and Robert W. Dudley. 2004: Changes in the Proportion of Precipitation
Occurring as Snow in New England (1949–2000). Journal of Climate: Vol. 17, No. 13, pp. 2626–2636.
Knowles, N., and D. Cayan, 2002. Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco
estuary, Geophysical Research Letters, 29:18.
Mote P.W., Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS (in press).
Stewart, I.T., D.R. Cayan, and M.D. Dettinger, 2004. Changes towards earlier streamflow timing across western North America. Journal of
Climate (in review).
References
Thanks to Sasha Gershunov and Emelia Bainto of Scripps Institution of Oceanography for providing some of the data used in this
study.
This work was funded by the USGS through the National Research Council Associateship Program.
Acknowledgements
F ig u r e 4 . M o n th ly tr en d s in w et-d a y TM AX, N o v-Ap r .
Mar
Dec
Several studies have linked decreasing snowpack and earlier runoff in the West with increasing temperatures
(Dettinger and Cayan 1995, Hamlet et al. 2004, Stewart et al., 2004, Mote et al., 2005). This trend has also affected
the amount of snow deposited. The patterns of monthly SWE/P trends show a striking resemblance to trends in wetday TMAX below. In particular, January showed significant warming along the coast, while in March, broad
warming occurred across the West.
Monthly trends in TMAX and wet-day (P>0) TMIN (not shown) correspond closely to wet-day TMAX (below).
Trends in TMIN are notably different, with stronger warming across the west. Thus, dry-day TMIN warming trends
are particularly strong and would have an impact on the heat budget of the deposited snowpack. But it is the wetday trends that drive changes in the amount of snow that is actually deposited shown in Figure 1.
Feb
Nov
b. Monthly and Seasonal Changes
Plotting trends in SWE/P by month (Figure 3) reveals that the strongest declines occur in January along the
West Coast, and in March throughout the entire West.
Abstract. The western U.S. depends heavily on snowpack to help retain its wintertime freshwater
endowment into the drier spring and summer months. A well-documented shift towards earlier runoff
can be attributed to 1) more precipitation falling as rain instead of snow, and 2) earlier snowmelt. The
present study shows a region-wide trend toward a decreasing ratio of winter snow water equivalent
(SWE) to total precipitation since 1948. This trend is attributable to a shift toward more rainfall rather
than a decrease in overall precipitation. At the monthly scale, the trend is most pronounced in January
(coastally) and March (West-wide), corresponding to warming trends in those months for average wetday maximum temperatures. The trends exhibit a clear dependence on elevation, becoming less
negative with increasing altitude.
Trends in Snowfall versus Rainfall
for the Western United States
Michael D. Dettinger
mdettinger@ucsd.edu
Scripps Institution of Oceanography,
U.S. Geological Survey
Daniel R. Cayan
dcayan@ucsd.edu
Scripps Institution of Oceanography,
U.S. Geological Survey
+
a vg
+
(S W E / P )
+
SWE
+
fr e q
+
x
in t
x
+
a vg
+
PRCP
+
fr e q
x
x
in t
+
–
–
–
–
wet
TMIN
a ll
Ta b le 1 . Ra n k co r r ela tion s o f lo n g-ter m ch a n ge s in va r io u s fa ct or s w ith eleva tion . Th e sign s o f t h e co r r ela t io n s a r e
in d ica ted . A cir cled r ed sym b o l in d ica te s p < 0 .05 , a n d a n ' X' in dica tes p > 0 .2 0 . Resu lts a r e sh o w n for ch a n g e s in w in t er
a ver a g e s o f a ll q u a n t itie s, fo r ch a n g e s in n um b er o f w int er d a ys w it h m ea su r a b le S WE a n d P RC P (fr eq uen cy), a n d fo r
ch a ng e s in a ver a g e m a g n itu d e o f S WE (P RC P ) o n t h o se d a ys w it h m ea su r a b le S WE (P RC P ) (in ten sity).
a vg
+
–
a ll
x
–
wet
x
TMAX
Changes in SWE/P and in SWE become less negative with altitude. This appears to be the result not only of
colder average conditions with altitude, but also a general tendency toward less long-term warming at higher
altitudes, particularly in the Rockies In fact, the highest altitudes show evidence of cooling (not shown here) and a
long-term increase in SWE/P.
Nonetheless, most stations, both spatially and with regard to elevation, show a long-term trend of decreasing
SWE/P. In the case of both the spatial and the elevational distributions, SWE/P changes appear to be dominated by
SWE changes, which are caused by changes in SWE frequency, all ultimately a result of the broad warming evident
over the last half-century.
> -1 1 4 E
Lon
< -1 1 4 E
How do the underlying factors contribute to this relationship? Table 1 shows the results of a non-parametric
correlation (Kendall's tau) analysis for the various contributing factors in which each factor's rank correlation with
Z was calculated, along with its statistical significance.
F ig u r e 6 . Le f t : S t a t io n eleva tio n s. yello w : Z > 1 4 0 0 m , b lu e: Z < 1 40 0 m .
R ig h t :
M a g n itu d e o f S WE /P ch a n g e ver su s eleva tio n . To isola te r egio n a l effects fr o m
eleva t io n a l, b lu e sym b o ls cor r es p o nd to st a tio n s w est o f -1 1 4E , a n d r ed to
sta tio n s ea st . A lin ea r t r en d is evid en t in b o th sa m p le s.
c. Elevational Dependence
Most processes related to snowpack exhibit strong elevational dependence, as do the results presented above. A
difficulty with the present analysis is that there are clear regional biases in station elevation (Figure 6a), a fact
which must be taken into account when investigating elevational dependence. Figure 6b shows that the change in
SWE/P is clearly altitude-dependent, becoming less negative with height. In fact, some increase in SWE/P is
apparent at high altitudes. To ensure that this relationship is not in fact due to a regional pattern, the plot is colorcoded to indicate stations east and west of -114E (roughly , stations in the Rockies, and stations west of them).
Each set of data exhibits a statistically strong correlation with Z (p<0.05).
F ig u r e 5 . Le f t : M ea n sn o w sea son d ur a t io n in d a ys. R ig h t : Tr end in sn o w
sea so n du r a t io n . (b lu e = lo ng er )
Figure 5 shows means and trends in the duration of the snow season, defined as the time between the first
and last days of measurable snowfall (using the period Oct-Jun). The means show a significantly (2-3 months)
longer snow season in the Rockies than in the near-coastal regions or the extreme south. The trends show an
even longer snow season in the Rockies, and an even shorter season to the west. The trends in duration appear
to correspond generally to changes in SWE deposition (Figure 2). Trends in average annual deposition timing
were also calculated, but were generally weak.
Noah Knowles
USGS NRC Associate
nknowles@usgs.gov
http://tenaya.ucsd.edu/~knowles