RE: Some thoughts on recurrence intervals

Stacey Good (StaceyAG@classic.msn.com)
Wed, 1 Oct 97 02:53:54 UT

I was with you until you said water runs over the surface of a snowpack
efficiently. Unless it is solid ice, a snowpack is highly permeable and
rainwater does not run off the surface. A mountain snowpack will soak up the
water and energy from rainfall untill the snowpack is homothermous at zero
degrees C, then it is "ripe" and ready to melt from the bottom up. Any
melting of the surface of the snowpack from solar radiation and warm air soaks
in just like the rainfall and doesn't contribute to runoff until the snowpack
is ripe. Once the snowpack is ripe and melt begins, the ground beneath will
be saturated. That is when the rain-on-snow effect has its impact. Not only
does virtually all of the rain pass through the snowpack and run off because
there is no infiltration to the ground, it imparts energy to the snowpack and
results in additional runoff from melt.

It's kind of like saving for retirement.

----------
From: owner-gcboaters@songbird.com on behalf of Benjamin Harding
Sent: Monday, September 29, 1997 10:48 AM
To: gcboaters@songbird.com
Subject: Re: Some thoughts on recurrence intervals

On 29 Sep 97 at 14:00, Earl Perry wrote:

> Warm rain on isothermic snow banks is a partial explanation
> for the Christmas floods on the Eel in '64. This condition
> has the potential to put a hydrograph right over all previous
> scales;

(Are you all bored with all this hydrology? Should we stop
now?)

Well this rain on snow thing is overrated. It's all about
energy.

The energy required to turn one gram of ice (at 0 degrees C,
just ready to melt) to water is 80 calories. The energy given
up by a gram of water as it cools one degree C is 1 calorie.
Thus, lets just say that we had 20-degree C rain (room
temperature, very warm rain) falling on snow. As it cooled
to 0 degrees C, each gram of rain would give up 20 calories to
melt snow, enough to melt 1/4 gram. Thus, the rain would
supplement itself by 25%. Not a sufficient explanation for
extreme events. (It is a sufficient explanation of why its so
damned hard to defrost a freezer.)

(The heat released by a gram of water vapor as it condenses is
540 calories. A real bomb. That's the powerhouse that drives
storms.)

I think the real impact of snow is twofold. First, water
tends to run efficiently on its surface or channels within it
and thus get to streams _quickly_ (increasing peak flows) and
without too much infiltration. Second, the soil below
isothermal snow should be saturated and unable to take up much
additional water.

Disclaimer--I'm an engineer, not a hydrologyist.

Ben


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