SNOWMELT - THE PROCESS EXPLAINED
1. Snowpack accumulation
a. sun goes south - life begins to be fun
b. less solar input, more oblique angle, less effective
c. temperature drops, things get cold
d. rainfall turns to snow
1. first snows melt due to stored warmth in the soil
2. later snowfall starts to accumulate at higher elevations
3. this gradually descends to valley floors
e. snowpacks continue to accumulate, getting deeper and deeper
f. snowpacks are continually be ‘redistributed’ wind and sun
1. some areas get more, some get less through redistribution
g. sun comes back from vacation, beginning the end of good times
h. snowpack begins to suffer from warmth and compaction
I. snowmelt begins! - things get hot, life ends!
2. Physical Properties and characteristics
a. color - obviously white
1. highly reflective 80 to 95% reflective of incoming short wave
2. becomes 50% or so during melt due to detritus
b. crystalline structure
1. always changing, becoming more dense and compact with time
c. highly insulative: 12” of snow is R6 insulation
1. temperature at surface not reflected at deeper layers, hot or cold!
d. internal temperature typically near 20 to 30 degrees
e. temperature at ground surface of deep packs - near 30 to 35 degrees
f. temperature at ground of shallow packs reflects ambient temperature
g. temperature near pack surface - near ambient up to 32 degrees
h. tremendous thermal energy required to execute phase change
1. 80 ca/gm ice to water, 640 ca/gm ice to vapor
I. snowpacks readily pass water thru it’s structure without retaining much
3. Snowmelt - the process
a. Pack absorbs energy bringing it to isothermal
1. proceeds incrementally with respect to elevtion, aspect, vegetation
and other microclimatic conditions.
b. Isothermal means: equal temperature throughout at 32 degrees
c. When pack is isothermal,
1. density is 40 to 50%
2. needs only energy for phase change - 80 ca/cm
3. up to this point, energy is put into portions of the pack to get to
isothermal conditions with very little melt and then only on the
periphery of the pack; Analagous to a pot of water at 32 degrees
with ice cubes in it on a burner - temp of water remains 32 until
the ice has melted, then increases.
thus the pack can absorb massive amounts of energy and still produce
very little melt and later on in the snowmelt season, comparatively little
energy results in massive melt. Remember we are dealing with huge
amounts of water. Because of the albedo, 50 to 90% of the energy is
reflected back.
d. Energy exchange:
1. soil surface - little but more or less continuous, tends to keeps soil in a
static saturated state
2. within the pack - typically few energy sources: potentially water
percolating thru with minor amounts of energy
3. Surface - most energy exchange occurs here thru radiation, conduction
and convection - convection being capable of highest transferral rate
a. convection keeps air flow high and prevents high relative
humidity from forming at the snow surface, decreasing energy
transfer
b. radiation is next most effective in the mountain west in terms
of getting energy into the pack - but because this process is
continuous, it accounts for the most snowmelt! Observation:
look at snowmelt patterns - south slopes melt off first indicating
that radiation has a critical role. If convection were the dominant
player, aspect would not be significant. Convection can put a lot
of energy into the pack over a short period of time, but it is usually
not as steady as incoming shortwave radiation.
c. conduction: via physical objects can be very significant for
short time frame phenomenom such as rain on snow or
condensation. - Rain on snow typically doesnt melt much snow
but because of saturated conditions, high initial streamflow and
the some additional snowmelt on top of a significant rainfall
event, streamflow can easily become a flood.
d. Example of rain on snow: rain 10 degrees C, wet bulb temp
you need 8 inches of rain to produce 1 inch of snowmelt!
e. CONDENSATION: 640 ca/cm of energy to the snowpack
this is 8 times as much as average temp rainfall so if you get
rainfall combined with significant condensation from fog,
clouds etc directly on the snowpack you can melt a bunch.
Note the relative contributions from various energy sources. Shortwave is steady day in
and day out, rain melt has very little impact and convection/condensation in the short
term can outweigh everything.
*** quick definition of FLOOD: strictly a hydrologic definition - e.g. flows in the
top 10% or 20% of all recorded flows. NOT whether some field will be inundated
(agricultural flooding) or if campground that gets water every year will get it this year ,
or enything classified as the “usual flooding”.
f. RAIN ON SNOW flooding occurs primarily due to:
1. existing saturated conditions yielding a pavement type
hydrologic runoff situation such that nearly all precip turns
to streamflow
2. high existing streamflow conditions that with even minor
additional inputs to the system push streamflow into the
red zone
3. last and least is the additional snowmelt caused by rain
which is negligible compared to direct input by rain.
4. More about the Process (it’s not an event)
a. begins at low elevations and proceeds to highest
b. typically begins in march and proceeds through july
c. is typically incremental in nature although it can occur from all
elevations simultaneously such as 83 - this takes relatively rare
climatic conditions: i.e. isothermal conditions across all but
the highest elevations.
d. Frozen ground not generally a problem here in Utah due to the
deep snowpacks - ground is not generally frozen except at the
lower elevations where there isn’t much snow to begin with
e. rain on snow, not typically a huge problem but can occur. Is
generally misunderstood regarding the root cause of the flood
event with most people assuming that there is an instantaneous
and catastrophic meltdown of the entire pack. NOT - most of the
pack remains there for the next storm to dump on.
f. south facing slopes and windy ridges melt/blow off first
g. vegetation affects how snow accumulates and melts
h. best indicator of snowmelt flood potential is how much snow
is in the modal elevation band of the watershed
1. low elevation snowpack typically generates nuisance
type flooding because there typically just isnt enough
snow accumulated here to produce much water even
though it represents a large part of any watershed and has
athe potential to melt quickly.
2. mid elevation snowpacks generally encompass the modal
watershed band. High snowpack in this area gives a clue to
snowmelt flood potential. Snowpack in this area can melt
quickly, typically has the greatest area and hence the most total
snow and very importantly, is proximal to the stream: e.g. it
can get melted snow to the channel immediately, shorting the
hydrograph timing and with less chance of infiltration or
evaporation.
3. High elevation snowpack generally takes too long to melt,
is far from the channel and more often melts in late june and
july, providing late streamflow as opposed to peak flows.
4. flooding will most likely occur as a result of a combination
of low and mid elevation snowmelt, or as a combination of all
three zones. There is a smaller chance of flooding if there is one
of the zones missing, eslpecially the low and mid elevations. High
elevation snowpack can produce flood flows, but it must be fairly
large (snowpack) and the timeframe fairly short as with ashley creek
in 95.
I. Flooding in general:
1. exacerbating features
a. high snowpack at low and mid elevations
b. Extremely high snowpack at high elevations
c. isothermal conditions at all elevations
d. extreme thermal inputs to the snowpack at isothermal
e. direct precip input to already high streamflow with
some additional snowmelt
f. snowmelt keeps the gound under and around it
saturated, raises the potential for fast runoff
g. in this area, frozen ground is not generally widespread
or a critical element in flooding when it occurs, it is typically
not deep or extensive nor does it last long