Friday, August 3, 2012

Snowpack and Snowmelt

Above is the graph of the snow water equivalent (SWE) for the snowpack and snowmelt at the Paradise NRCS SNOTEL site, about 2 miles southeast of Paradise.

The SWE is the depth of snow on the ground each day converted to water, or the snow to water ratio. This ratio varies with the location and during the snow season, but normally for Mt. Rainier and many locations in the western side of the Cascade Mountains the ratio is about 2 1/2 inches of snow per inch of water.

This compares with general ratios of dry snow found in the Rockies of about 4-6 to 1 and something in between in the Sierra Mountains. The SWE is not the amount of snowfall, that's the snow in inches which falls on the ground. The SWE is the snowpack once the snow has fallen and added to the existing snow on the ground.

The snow, once on the ground, becomes part of the snowpack and melts during the season from rain on snow or ablation, or snow evaporation from exposure. These processes change the ratio during the season and requires routine calibration of the snow to water ratio to ensure the calculation of the total water in the snowpack is accurate to detrmine the quantity of water in the snowmelt.

This is done with field work at the data collection sites where the snow to water ratio is deterined to calibration the real-time data and at specific locations called snow courses which are a line of points in the mountains where the snow to water ratio is determined and then averaged to determine an area snow to water ratio. These data are used to determine the total available water in the snow over larger areas, such as the Cascade Mountains.

As you can see, this year the snowmelt ended July 29th, a month earlier than last year, and about two week later than normal. The "normal" curve on the graph is inaccurate from the calcuation of the longterm average. The more accurate averate end of the snowmelt is around July 14th, plus or minus a day or two.

The NRCS is aware of this and is in the process of updating their database and calculations for the snowmelt. The current calculation does not correctly account for anomalous years which skews the data to the few rare long snowmelts, like last year. The new calculations and graph were supposed to be on-line this year but doing the work for all of their sites takes time.

Anyway, this means for the most part, the snow is gone except at the higher elevations, about 6,000 feet or in pockets below 6,000 feet depending on the location, exposure (direction of slope face), and other factors. So the color in the NP now is green with just snow in the usual places.

No comments: