FLOOD07755

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Runoff from Snowmelt
• Date: 1/5/1960
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2-1406
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and B is the thermsI quality, For a melting mountain snowpack (thermal qnality assumed to be
0.97), tbe melt equation for shortwave radiation is

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M,,=O.00508I,(I-a)

(3:

In the middle latitud,'s during late spring, the maximum solar radiation for a clear day for a hori.
zontal surface is about 800 langlrys. With a minimum albedo of 40 percent, the rrsulting possible
shortwave radiation melt for an unforested area is in the order of 2.4 inches per day, We shull
see in the next paragrnph, however, that generally some of this energy is lost from the snowpaek
by longwave radiation,

No!e:
Tronsr."li$!Iion cu,-v~s ore For doily ins%llon
ornoun.fs. They represent mean condilions
during !h! sprinfl snow..m~U season. Acfual
fronsfT"SSton 0; msolahon may diFFer rrom
fho! indicofl1d due ;0 d;f'f'erenC'~s in the solor
o/fiflJd~ ~ heigh! ond 6pocing of' frees. and
dirrerences in types of' frees.

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F'aREST CANOPY DENSITY
(HORIZONTAL PROJECTION)

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Figure 4-. Transmission of solar radiation by forest canopy, based on daily insolation for coniferous forest during
spring snowmelt season.

2-06. LONGW A VE :RADIATION. Snow is very nearly a perfect blnck body with respect to
longwave (terrestial) radiation, absorbing all such radiation incident upon it and emitting tbe
maximum possible radiation in accordance with Stefan's law. This is expressed by th" ('quat ion

E=uT4

(4)

where E is the total energy of all wavelengths, u is Stefan's constant, * and T is the black body
temperature in degrees absolute. A rnelting sno\vpack has a surfuce temperature of 00 C., and,
according to Stefan's law, it loses energy at the rate of 0.-1.59 ly/min. Opposed to this loss is tbe
back radiation from the atmosphere Dr forest.. For ckar skies, the heat gain from back radiation
is gene.rally less than the heat loss, so thnt Hlt'r'- i" :1P[. IJ('ut loss from the snowpack by longwave
radiation. With cloudy skies or b12nl'l1:L :~-ll\.:::it canopy, however, the back radiation may be
greater or less than tbe loss from tb" sllowpack, depending principally upon the ambient air
temperature.

....=O.826Xlo--IO(ly!min)!(" K;-II,

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