In the Northern Cascade Mountains, Washington snowpack depth varies widely because of local topography and consequent microclimates. However, the average snowpack within a specific elevation band is determined by regional climate variations. Three changes are evident with respect to snowpack and they are robust occurring regardless of time period or stations analyzed. Published details in Northwest Science
Snowpack decline: Figure 1 at right shows the decline in April 1 winter snowpack in the North Cascades for various time intervals since 1936 at four stations and at six stations since 1946. Their is a negative slope for every time interval used, though the slope is highest for the period beginning in 1965 and lowest for the period beginning in 1980. The decline largely occurred in a step change in 1976-77.
Precipitation Increase The decline has occurred despite an increase in winter precipitation in the region, as noted in Figure 2 Diablo and Concrete. Again the precipitation is illustrated for various time periods, but increase for all. A rise in precipitation and a fall in snowpack can only be accounted for by greater melting or rainfall during the winter. The highest slope is for 1936 to the present, the lowest for 1950 to the present. The 1930's were a period of low snowpack, but due to limited precipitation.
Snowpack-Precipitation Ratio:
A key ratio that can be used
to identify the relationship between the snowpack and precipitation
is the ratio between winter precipitation (November-March) and April
1 SWE. There is minimal October accumulation, thus,
October precipitation is not included. An increasing ratio indicates
a greater percentage of precipitation is falling and remaining as
snow. A declining ratio indicates that greater percentages of
precipitation occur as rain instead of snow and/or that melt of
winter snowpack is increasing. The ratio of SWE to precipitation for various time periods (Figure 3 and Figure
4) illustrates that regardless of time period chosen or stations utilized the
ratio is declining. Figure 5 examines data from a higher elevation
location that is South Cascade Glacier versus local precipitation. The
declining swe-ppt ratio is apparent regardless of elevation in the North
Cascades. Winter balance and net balance for Easton Glacier
and South Cascade Glacier from 1990-2008 is in Figure 6 winter
balance is not declining since 1990 despite a significant
precipitation rise. Figure 7 illustrates the long term impact
of declining snowpack on North Cascade glacier mass balance.
The change in snowpack during the period
of warmer conditions since 1976 is not consistent. At low
elevations sites below 3000 feet, the decline is substantial note
Diablo and Concrete in figure 8. The decline in the ratio of snowpack to
precipitation is consistent regardless of altitude or time period
observed. This ratio is dependent only on temperature rise,
which reduces snowpack at all elevations for a given amount of
precipitation.
Figure 9 compares the measured
winter balance of Easton Glacier measured in the spring near the
time of maximum snowpack, with the snowpack swe from the five Snotel
sites and with maximum snowpack swe from Schreiber's Meadow 1000 m
below the glacier ELA. The maximum Shreibers Meadow snowpack
is a good predictor of winter balance for Easton Glacier.
The change in snowpack retained is due
to warmer winter temperatures in the North Cascades. Seen below.
Winter temperature has been consistently above normal at NOAA
stations in the North Cascades since the mid-1970's, but starting in
the mid-1990's winter temperatures increased even further.
Figure 1. April 1 SWE at USDA Snotel sites for five different time periods.
Seven stations are used for the 1944, 1950, 1965 and 1980 time series. Four
stations are used for the 1936 time series.
Figure 2. Mean total November-March precipitation at Diablo Dam and Concrete.
Figure 3. Ratio between April 1 SWE at seven SNOTEL stations and mean total November-March Precipitation at Diablo Dam and Concrete. For a specific level of precipitation the amount of April 1 SWE has declined for every period.
Three different types of data sets are utilized to identify the changes in snowpack across the North Cascades since 1946 with time and elevation:
1. Local climate stations at Diablo Dam and Concrete (NWS Cooperative stations) provide the most reliable records for precipitation, temperature and low elevation snowpack from 1935 to the present in the North Cascades.
2. April 1 snowpack at moderate elevations is assessed from seven long-term Snow Course and now SNOTEL stations; Rainy Pass, Lyman Lake, Stevens Pass, Miners Ridge, Stampede Pass and Park Creek and Fish Lake monitored by the USDA. These are the only North Cascades stations with continuous data for the period.
3. High elevation snowpack is assessed from winter mass balance measurements on the South Cascade Glacier from 1960-2005 and on Easton Glacier 1990-2007 monitored by the North Cascade Glacier Climate Project.
The five USDA-SNOTEL stations have provided April snow water equivalent measurements since 1943, with four reporting data since 1936. These are the only North Cascade alpine snowpack records that exist for the entire interval. The snowpack maximum for most of the SNOTEL sites is reached near April 1 (Pelto, 1993; Mote, 2004). The April 1 date is also the date for which longer term snow course measurements were completed prior to the SNOTEL continuous measurement program started in 1980 (Pelto, 2008).
At
the ten Snotel sites from November 1-February 15 snowpack development is rapid
reaching 68-80% of the maximum (SWE). The
average maximum SWE for sites above 1500 m is May 5, and for sites below 1500 m
is April 10 (Table 2). The actual
maximum accumulation varies with elevation ranging from 0.8 m to 1.6 m, with a
mean of 0.98 m for the six sites below 1500 m and a mean of 1.38 m above 1500 m
(Figure 2). The maximum glacier
snowpack SWE is distinctly larger with an average accumulation of 2.93 m.
The
mean and maximum SWE depth is variable from site to site, however, the annual
pattern of development and relative amount is consistent in response to specific
annual climate conditions for each elevation band.
MAXIMUM
SNOWPACK ACCUMULATION:
Comparison
of annual maximum SWE on glaciers yields cross correlations of 0.82-0.99,
indicating the strong regional control of accumulation.
The mean correlation of glaciers to low elevation sites range from 0.37-0.82 for
individual glaciers, and for Lyman Lake the best Snotel site from 0.72-0.95.
Ablation The onset of ablation is
also seen in the spring ablation rates at the Snotel sites (Table 3).
The early ablation season, is marked by freezing levels that more
frequently result in snowfall at Lyman Lake and rainfall at the lower sites.
May ablation rates between Snotel sites are poorly correlated as are the
daily ablation rates, correlation coefficients ranging from 0.43-0.76.
Average ablation after June 1 is a limited data set in many seasons due
to the disappearance of the snowpack at the lower elevation stations.
However, at any melt station with snowpack enduring past June 15 the
daily ablation rate have been correlated for the entire June-July period when
snowpack was present. Ablation
rates after June 1 are highly correlated ranging from 0.73-0.92.
Average June ablation in SWE is 3.8 cm/day at Lyman Lake, 3.7 cm/day at
Rainy Pass, 3.8 cm/day Miners Ridge, 3.7 cm/day at Columbia Glacier, and 3.6
cm/day at South Cascade Glacier. The
lack of variation between various glacier and Snotel sites contrasts with the
sharp variation in accumulation between sites.
Thus, within the elevation zone from 1400-2000 m across the North
Cascades ablation after June 1 has a low degree of variability.
Early season ablation rate varies widely with elevation, but after June 1
ablation rates fall within a narrow range.
Accumulation is widely variable and can only be estimated if baseline
data is available, ablation rates are similar in the summer season and can be
extrapolated from primary to secondary sites without substantial baseline data.
The most important ramification is that if the distribution and depth of
the snowpack is known on June 1, than summer water resources can be estimated
for a wide range of basins from a limited number of primary ablation measurement
sites.
Figure 4. Ratio between April 1 SWE at SNOTEL stations and mean total November-March Precipitation at Diablo Dam and Concrete. The four station series is Lyman Lake, Miners Ridge, Fish Lake and Stevens Pass. For five station series Rainy Pass is added and for the six station Harts Pass.
Figure 5. Ratio between observed SWE on South Cascade Glacier (USGS) at the start of the melt season and Diablo Dam and Concrete precipitation.
Figure 6. Winter balance and net balance of Easton Glacier and South Cascade Glacier since 1990.
Figure 7. Cumulative mean glacier mass balance in meters of water equivalent on five North Cascade glaciers in the Skagit River basin.
Figure 8 Winter snowfall at Diablo and Concrete since
1976. This indicates that low elevation snowpack has declined.
Figure 9. Winter balance of Easton Glacier measured in the spring near the time of maximum snowpack, April 1 snowpack swe from the five Snotel sites, and with maximum snowpack swe from Schrieber's Meadow 1000 m below the glacier ELA. The maximum Shriebers Meadow snowpack is a good predictor of winter balance for Easton Glacier.
