D. Marks et al., THE SENSITIVITY OF SNOWMELT PROCESSES TO CLIMATE CONDITIONS AND FOREST COVER DURING RAIN-ON-SNOW - A CASE-STUDY OF THE 1996 PACIFIC-NORTHWEST FLOOD, Hydrological processes, 12(10-11), 1998, pp. 1569-1587
A warm, very wet Pacific storm caused significant flooding in the Paci
fic Northwest during February 1996. Rapid melting of the mountain snow
cover contributed to this flooding. An energy balance snowmelt model
is used to simulate snowmelt processes during this event in the Centra
l Cascade Mountains of Oregon. Data from paired open and forested expe
rimental sites at locations at and just below the Pacific Crest were u
sed to drive the model, The event was preceded by cold, stormy conditi
ons that developed a significant snow cover down to elevations as low
as 500 m in the Oregon Cascades. At the start of the storm, the depth
of the snow cover at the high site (1142 m) was 1.97 m with a snow wat
er equivalent (SWE) of 425 mm, while at the mid-site (968 m) the snow
cover was 1.14 m with a SWE of 264 mm. During the 5-6 day period of th
e storm the open high site received 349 mm of rain, lost 291 mm of SWE
and generated 640 mm of runoff, leaving only 0.22 m of snow on the gr
ound. The mid-site received 410 mm of rain, lost 264 mm of SWE to melt
and generated 674 mm of runoff, completely depleting the snow cover.
Simulations at adjacent forested sites showed significantly less snowm
elt during the event. The snow cover under the mature forest at the hi
gh site lost only 44 mm of SWE during the event, generating 396 mm of
runoff and leaving 0.69 m of snow. The model accurately simulated both
snow cover depth and SWE during the development of the snow cover pri
or to the storm, and the depletion of the snow cover during the event.
This analysis shows that because of the high temperature, humidity an
d relatively high winds in the open sites during the storm, 60-90% of
the energy for snowmelt came from sensible and latent heat exchanges.
Because the antecedent conditions extended the snow cover to very low
elevations in the basin, snowmelt generated by condensation during the
event made a significant contribution to the flood. Lower wind speeds
beneath the forest canopy during the storm reduced the magnitude of t
he turbulent exchanges at the snow surface, so the contribution of sno
wmelt to the runoff from forested areas was significantly less, This e
xperiment shows the sensitivity of snowmelt processes to both climate
and land cover, and illustrates how the forest canopy is coupled to th
e hydrological cycle in mountainous areas. (C) 1998 John Wiley & Sons,
Ltd.