I belong to a "rain guage" site. Recently this came around and I've been intrigued. I just came in from watching the snow melt. We were down to 15" this morning.
Any takers on this Mystery? I've already submitted a letter in response. BTW, if anyone is located in the area covered by our site, please be encouraged to join in!
http://www.cocorahs.org/*_*
From the site originator, Nolan Doesken:
The Mystery
Some of us are trying to measure SWE. What, you might ask, is SWE? SWE
simply means "Snow Water Equivalent" and it is the depth [amount] of water in the
snow that remains on the ground. Here in the Rockies, SWE is measured
religiously by the Natural Resources Conservation Service and their many
collaborators. SWE in the mountains is equivalent to liquid gold. It is
our water supply for the coming year -- held in a frozen reservoir of
white until next spring's snowmelt begins. But even at lower elevations
and in areas where snowcover is intermitent, SWE (Snow Water Equivalent)
is interesting and fun to study.
WE measure SWE by taking a core sample with our rain gauge outer cylinder
and melting and measuring the water content. I've done this each morning
since the snow fell last week. Since then, the skies have been mostly
clear and we've had no additional precipitation. The snow depth has
steadily decreased and the snow is now icy and granular. BUT THE WATER
CONTENT OF THE CORE SAMPLE HAS INCREASED!!. Originally I had about 0.56"
in my core samples, but now it's climbed to 0.60" Curiously, several of
you have written to me wondering what you were doing wrong. Your core
sample water contents were also going up. Where is this extra moisture
coming from?
It turns out that for those of us who have done measurements of SWE for a
long time, we've seen this before. In the cold of midwinter (not so much
in spring or fall), there is a tendency for the snow water content to go
up gradually until the snow reaches a point where it is between 25% and
40% water (i.e. 2" of old snow could have 0.50" or more of water content
before it finally melts). But we STILL DON'T KNOW for sure where the
extra water is coming from.
I've heard several theories. Perhaps there is frost deposition on the
surface at night. Maybe water vapor from the warmer soil below is moving
upward and then condensing on the colder snow crystals. Maybe as nearby
snow begins to melt and bare spots appear, melt water is being wicked
horizontally into the snow crystal structure rather than soaking into the
ground below. Or maybe it's even simpler. Maybe as the snow structure
changes, our coring techniques and catch efficiency changes. Maybe the
fresh snow was still stuck down in the grass when we took those first
cores but now we're doing a better job of getting all the snow into our
sample. I have may ideas, but I really don't know the answer.
For you snow hounds in cold places, I challenge you to discover this
mystery on your own by taking core samples each day of the old snow
remaining on the ground. Plot out your data and see what you come up
with. Then, if you experience increasing SWE let me know and provide me
your theory for why this is happening. Perhaps, together, we can figure
this out.
A great website for snow and water
If you like tracking mountain snowpack and water content, try this
website. This is the favorite of many water managers here in our part of
the country.
http://www.wcc.nrcs.usda.gov/snow/*_*
~samwik
