ABSTRACT
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In the North Cascade Range, Washington snowpack accumulation and the
resultant ablation provides critical summer water resources.
Utilizing SWE data from 10 USDA Snotel sites and 13 glaciers in the North
Cascades the variation in maximum SWE accumulation with location are analyzed.
There is strong correlation between precipitation as measured at Diablo
Dam and maximum SWE accumulation at each Snotel site, with the strongest
correlation being the highest elevation sites.
Cross-correlation of maximum SWE between sites is only strong for sites
in the same elevation range. Maximum
SWE at the glacier locations is 299% and 212% of that at the low elevation and
high elevation Snotel sites.
The
regional nature of accumulation-Diablo Dam precipitation relationships indicates
that neither is particularly dependent on microclimates.
It further suggests that once ground truth data is available for a
limited duration for a site, maximum SWE could be determined from climate data.
INTRODUCTION
The spatial and temporal variation of snowpack accumulation, snowpack
ablation and consequent alpine runoff is crucial to determining regional summer
water resources in the North Cascades Range, Washington.
Glaciers alone provide 750 million m3 of runoff each summer
(Fountain and Tangborn, 1985). What
is the spatial and temporal variability of accumulation and ablation across the
North Cascades? Rasmussen and
Tangborn (1976) noted a poor relationship between observed annual precipitation
and annual runoff. They also noted,
in plotting mean annual runoff versus basin mean altitude in 36 basins in the
North Cascades, that there was a poor relation between runoff and basin
altitude.
These
observations demonstrate that precipitation variation is complex and significant
in the region and that extrapolations cannot be made from a standard measurement
site to a secondary location based simply on elevation change.
That further extrapolations could not be made from site to site with the
available datq. This paper examines
whether maximum winter season snowpack, and snowpack and glacier ablation, can
be determined for secondary locations from standard locations once baseline data
exists for the secondary sites.
DATA
SETS
The
following data sets are used (Table 1 and Figure 1): 1) Annual glacier mass
balance measurements from thirteen North Cascade glaciers (NCGCP on 8; USGS on 1
and NPS on 4). 2) 3) Daily snow water equivalent and temperature data from 10
USDA Snotel sites, and three snow course sites.
The
Natural Resources Conservation Service (NRCS) of the US Department of
Agriculture (USDA) operates an extensive automated Snotel system to collect
snowpack and climatic data in the western United States.
Snotel sites have at a minimum a pressure sensing snow pillow, storage
precipitation gage and air temperature sensor.
The snow pillows are envelopes of stainless steel or synthetic rubber
containing an antifreeze solution. As
snow accumulates on the pillow it exerts a pressure that is measured and
converted to a reading of snow water equivalent and telemetered to two NRCS
master stations. Each site measures
snow water equivalent (SWE) maximum, minimum and average daily temperature
(Figure 1).
From
1000-1900 m the USDA Snotel network provides an excellent network of snowpack
and temperature data recorders in the North Cascades, but no sites are found on
or adjacent to the highest accumulation areas, which are glaciers.
The necessity of using Snotel sites and glaciers to adequately identify
snowpack water resources in the North Cascades is emphasized by the difference
in mean maximum winter accumulation in SWE from 1.17 m at the ten USDA Snotel
sites, ranging in altitude from 1000-1900 m, and 2.93m at nine glacier locations
ranging from 1650-2200 m. By July
15 the nine glacier locations still average 1.3 m SWE, while the Snotel sites
have no snowpack remaining. Thus,
Snotel sites provide a good indicator of late spring and early summer runoff and
glaciers a better measure of mid and late summer runoff.
Figure 2. Shows the similar annual pattern of SWE at USDA North
Cascade snowpack sites. |
Mass
balance measurements have been made using the same methods at the same time of
the year on nine North Cascade glaciers by NCGCP (Pelto, 1996; and Pelto and
Riedel, 2001). The USGS has
maintained a mass balance record and weather records at South Cascade Glacier
since 1958 (Krimmel,1993-1999). The
North Cascades National Park Service began measuring mass balance on four
glacier in 1993 (Pelto and Riedel, 2001). Each program monitors ablation during
specific time periods using stakes emplaced in the glacier surface. Revisiting each site through the ablation season and
measuring the emergence of each stake identifies the ablation rate.
The maximum snowpack depth and water equivalent is also determined at
specific locations at approximately the same time each year in early to mid-May
utilizing probes driven through the snowpack on the glacier.
|
|
|
Elevation
|
Lati-tude
|
Long-itude
|
Source
|
|
Lyman
Lake
|
1805
|
48
12
|
120
55
|
USDA
|
|
Rainy
Pass
|
1460
|
48
33
|
120
43
|
USDA
|
|
Thunder
Basin
|
1285
|
48
31
|
120
59
|
USDA
|
|
Stevens
Pass
|
1245
|
47
44
|
121
05
|
USDA
|
|
Stampede
Pass
|
1190
|
47
17
|
121
20
|
USDA
|
|
Wells
Creek
|
1280
|
48
51
|
121
47
|
USDA
|
|
Park
Creek Ridge
|
1405
|
48
27
|
120
55
|
USDA
|
|
Fish
Lake
|
1030
|
47
31
|
121
04
|
USDA
|
|
Miners
Ridge
|
1890
|
48
10
|
120
59
|
USDA
|
|
Columbia
Glacier
|
1450-1750
|
47
58
|
121
21
|
NCGCP
|
|
Daniels
Glacier
|
2000-2250
|
47
34
|
121
10
|
NCGCP
|
|
Ice
Worm Glacier
|
1900-2050
|
47
34
|
121
10
|
NCGCP
|
|
Lynch
Glacier
|
1950-2250
|
47
34
|
121
11
|
NCGCP
|
|
Rainbow
Glacier
|
1350-2250
|
48
48
|
121
40
|
NCGCP
|
|
Easton
Glacier
|
1700-2900
|
48
44
|
121
50
|
NCGCP
|
|
S.Cascade
Glacier
|
1645-2100
|
48
21
|
121
03
|
USGS
|
|
L.Curtis
Glacier
|
1650-1950
|
48
50
|
121
37
|
NCGCP
|
|
Neve
Glacier
|
1830-2150
|
48
39
|
121
08
|
NCGCP
|
|
Cache
Col Glacier
|
1880-2100
|
48
22
|
121
03
|
NCGCP
|
Table
1. Location of USDA snotel,
USGS and NCGCP glacier
measurements
sites.
|
|
Year
|
Lyman
|
Rainy
|
Thun-der
|
Stevens
|
Stampede
|
|
|
|
|
|
|
|
|
1984
|
1-Jun
|
20-May
|
|
15-Apr
|
1-May
|
|
1985
|
10-May
|
1-May
|
|
25-Apr
|
10-April
|
|
1986
|
20-May
|
20-May
|
|
5-Apr
|
1-April
|
|
1987
|
1-May
|
25-Apr
|
|
15-Apr
|
15-April
|
|
1988
|
10-May
|
25-Apr
|
|
15-Apr
|
15-April
|
|
1989
|
1-May
|
15-Apr
|
15-Apr
|
15-Apr
|
5-April
|
|
1990
|
10-May
|
15-Apr
|
25-Mar
|
25-Mar
|
1-April
|
|
1991
|
10-May
|
1-May
|
15-Apr
|
15-Apr
|
20-April
|
|
1992
|
1-May
|
1-May
|
25-Mar
|
15-Mar
|
15-Mar
|
|
1993
|
10-May
|
1-May
|
5-Apr
|
5-Apr
|
1-April
|
|
1994
|
1-May
|
25-Apr
|
15-Apr
|
5-Apr
|
15-April
|
|
1995
|
10-May
|
25-Apr
|
25-Apr
|
25-Apr
|
25-April
|
|
1996
|
20-May
|
10-May
|
5-Apr
|
5-Apr
|
15-April
|
|
1997
|
10-May
|
10-May
|
25-Apr
|
15-Apr
|
25-April
|
|
1998
|
1
-May
|
15
April
|
15
April
|
15
April
|
15-April
|
|
1999
|
20-May
|
15-April
|
15-April
|
15-April
|
15-April
|
|
2000
|
20-May
|
5-April
|
5-April
|
5-April
|
15-April
|
|
2001
|
1-May
|
15-April
|
15-April
|
15-April
|
15-April
|
|
Mean
|
10-May
|
1-May
|
10-Apr
|
10-Apr
|
15-April
|
Table
2.
The date of maximum SWE accumulation at five USDA Snotel sites in
Washington.
|
MAXIMUM
SNOWPACK ACCUMULATION
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
correlation of daily SWE for the 1989-1997 period at the four Snotel sites
between 1500 m and 1900 m ranges from 0.88-0.99. From 1000 to 1500m the daily SWE correlation
coefficient at eight Snotel sites is 0.87-0.99.
The correlation between the high elevation and low elevation Snotel sites
is 0.63-0.96. Each
elevation band is then a good predictor of SWE only at the sites in the same
elevation band. The correlation
between annual maximum snowpack and total winter precipitation at Diablo Dam is
highest for Snotel sites above 1500 m (0.75-0.81). For sites below 1500 m the
correlation drops to (0.63-0.75).
This is expected since a higher proportion of total precipitation falls
as snow at the higher sites.
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 between low elevation sites and glacier maximum SWE
ranges from 0.37-0.82 and for Lyman Lake the best Snotel site from 0.72-0.95.
The two best sites for prediction of glacier snowpack are Lyman Lake and
North Klawatti Glacier.
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.