Snowpack Depth and Density Changes
during Rain on Snow Events at Mount Hood, Oregon
Jon Lea1 and Jolyne Lea2
The U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS)
operates a near-real-time remote snow-measuring system called SNOTEL in the Western United
States. The SNOTEL system has been in full operation since 1980 and currently has 693 stations
across the West. Each station collects snow water equivalent, precipitation, and temperature. Addi-
tional sensors can be installed at the remote sites to measure other types of data that are of interest
to cooperators. One of the additional sensors is an ultrasonic snow-depth sensor, which was field
tested at several SNOTEL sites beginning in the fall of 1996. This study focuses on the results of the
first year of operation of the snow-depth sensors installed at three stations on the slopes of Mount
Hood in the Cascade Mountains located east of Portland, Oregon. These sensors were installed to
add to the snowpack information already collected at each site. This additional information has
improved our knowledge on how the snowpack reacts during melt events.
Rain-on-snow events can occur at any time during the winter, but are more prevalent during
November through February on the west slopes of the Cascades. During rain-on-snow events in the
1997 water year, snow depth, snow water equivalent, precipitation, and temperature readings were
taken on an hourly basis. Data indicate that the snowpack reacted differently in each location. The
amount of reaction to the rain-on-snow was very much dependent on the depth of the snowpack,
and was most volatile during early season snowpack accumulation when large and rapid changes in
density were observed. During rain-on-snow events, the density of the snowpack usually increased
to 45 to 50 percent, but did not always produce a loss of snow water equivalent at the site. Con-
versely, in some rain-on-snow events, the snowpack with a lower density of 35 to 38 percent den-
sity did produce snowmelt runoff from the site. While the relationship of streamflow runoff to
snowpack density is a complex issue, the additional snow depth data is a valuable tool for snow-
pack analysis and early warning of a major runoff from mountain snowpacks. These real-time data
are easily accessible and analyzed by cooperators and can be used in preparation for rain-on-snow
induced flooding in the Pacific Northwest and elsewhere.
1 USDA NRCS Oregon Snow Surveys, 101 SW Main Street, Suite 1300, Portland, Oregon 97204, USA
22 USDA NRCS National Water and Climate Center, 101 SW Main Street, Suite 1600, Portland, Oregon
97204, USA
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