PRMS Snow Module

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Detailed Description

Presents the USGS Precipitation Runoff Modeling System (PRMS) Snow simulation module.


Date Taken:

Length: 00:10:49

Location Taken: Lakewood, CO, US


Steve Markstrom: Hello.

Continuing on with the PRMS training video
series, talking about the snowcomp module


This satisfies the snow process and this is
the only module, so there's no choice.

This module develops the snow pack, simulates
the accumulation, depletion process and computes

the energy for melting the snow, and that's
what we're going to go through.

Properties of snow -- snow is a porous media.

Snow does change through time.

The albedo changes with time.

The density increases with time, and snow
does have the ability to store melted water,

free water.

In PRMS, we model the snow pack as a two-layered

Since PRMS runs on a daily time step, we divide
the day into two twelve-hour periods, basically

night and day, the warm period and the cold

The snow pack exchanges heat between the two
layers, and also, mass, basically in the form

of liquid water, moves down through the top
to the bottom.

The temperature of this surface layer is based
on the air temperature.

It's either the air temperature or zero, if
the air temperature is above zero.

Then the temperature in the pack, the majority
of the snow down here is actually computed

with a heat balance.

If you took this whole depth of snow and melted
it, that would be the pack water equivalent


What are the steps, the computational steps
for the snow pack on each HRU?

Basically the first thing that happens is
any new precipitation is added to the snow

pack, then the snow-covered area is computed
based on the pack water equivalent, albedo

is computed, the depth -- and that's the actual
depth -- the pack density is computed, the

energy for the night, the energy for the day,
then sublimation, and finally, any melted

ice is computed.

Precipitation falling on a snow pack - If
snow falls and a pack doesn't exist yet because

no snow has fallen or whatever snow there
was has melted, then a new pack is initiated,

and if rain falls, it's added as melted water
to the pack.

You can see there's a few parameters, freeh2o.

This example here, you can see we've got some
liquid water inside of our snow pack.

All right, depletion curves, these are pretty
important to PRMS.

Along the x-axis, we've got the fractional
amount of snow water equivalent, and then

on the y-axis we've got the snow-covered area.

If you really are concerned about this once
the pack moves into the melt phase, during

the melt season, or anytime the pack is melting,
basically you start out here and you move

in towards having less pack water equivalent
and less amount of snow-covered area as the

pack melts off.

Really, the idea here is to describe the driftiness
and the patchiness of the snow.

If you think about it, if an inch of snow
falls on a parking lot and the sun comes out,

it's all going to melt off in one day.

But if that same amount of snow gets plowed
up into a big snow bank, that could last weeks

or months, the same volume.

It's just this relationship between snow water
equivalent and snow covered area is different

and it really determines how things melt off.

So here's an example of a couple different
shapes of depletion curves.

Typically if you have a forested area, your
snow-covered area can go all the way up to

100 percent cover because the trees can keep
the snow from drifting.

In the case of Alpine, above Timberline, a
lot of times no matter how much snow you'll

get, you still have patchiness because the
wind will redistribute the snow into snow


Another important aspect of the snow module
in PRMS is this idea of albedo, which is basically

the reflectiveness of the snow.

The whiter and the more reflective the snow
is, the less solar radiation, short wave radiation,

the less effect that energy is going to have
on the snow pack because it's reflected away.

You see a real difference here.

This plot is showing that the reflectiveness
is much lower during the melt period than

it is when the snow is fresh, white and finer

Once the snow starts melting, it melts a lot
faster because it absorbs more energy.

You can see the idea, how PRMS is doing this.

Number of days since the last time it snowed,
basically, once you're in the melt phase or

if you're in the accumulation phase up here.

Here's some equations from the PRMS users'

You can see these bold, den init and den max,
those are parameters, and what we're trying

to do is compute the actual pack depth.

It's not the water equivalent, but that's
the actual depth of the snow.

This describes basically how the snow is settling
through time.

If you've got an aging snow pack, that's what
this delta pack depth is.

So PRMS is computing the actual depth of the

We're getting ready to look at some of the
energy equations in PRMS.

You can see all of the different terms we
take into account in the energy balance.

This is kind of the full energy equation.

I'm not going to go through all this.

But you can see there's all these different
sources of heat that can be putting heat into

the snow pack for our melting snow.

If these terms are negative, then it's going
to be heat moving out of the snow pack and

the snow pack actually getting colder.

In some form or another, you have to kind
of keep track of all this stuff.

When we're looking at the heat through conduction
-- that was one of the terms in the previous

equation -- we're using this formulation from

It's a temperature index.

Basically if you give it the temperature,
it'll tell you the heat being conducted into

or out of the snow pack according to this.

All this stuff is the thermal conductivity.

If we look at these terms, here's the basic
form of the energy balance when the pack is

in melt or getting ready to melt some snow.

This first term here is the short-wave radiation.

We've got this equation, which the DD or CC
solrad, this albedo term, depending on how

long it's been since it snowed, and then here's
our parameter here, rad transmission coefficient.

Solve this.

This gives you this heat short wave term.

The heat long-wave term is this down here,
you can see.

It's kind of a black-body type idea where
the air and the vegetation and the snow itself

is emitting long-wave radiation, and that's
where those different terms come from.

This is condensation and evaporation terms.

There's different forms depending on if you've
got a lot of moisture in the air, based on

the fact if it's precipitating or not, the
amount of heat that's in the precip.

Basically, if it's raining on a snow pack
we assume the rain is coming down at the temperature

of the air.

This final term is the conduction from the
snow pack to the ground.

In PRMS, we assume the snow pack and the ground
are at the same temperature, which is usually

a pretty good assumption.

If you're really interested in this kind of
stuff, I suggest you go read the users' manual

where these things are really spelled out
in a lot more detail.

Once you've got all your heat figured out,
that's this term here, t-cal, the number of

calories going into the snow pack, and here
you can see snow-covered area.

We're only melting snow where there is snow,
so that's what this equation is about here.

Sublimation - here's the snow evap equation
from the PRMS users' manual.

Here's a parameter, potet_sublim, the sublimation

These other terms are being computed either
by other modules or further up the chain in

this module.

That's the end.