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<br />EM 1110-2-1406
<br />5 Jan 60
<br />
<br />3-05. CONVECTIO\.CONDENSATION MELT. The Rimplified general equation 14 (par. 2-10)
<br />for convertion.condl'Ilsation melt may be further simplified for evaluating snowmelt during rain,
<br />on the assumption of saturated air, In this case, Ta equals Ta, and equation 14 becomes
<br />
<br />M,,=O,0084v(Ta-82)
<br />
<br />(15)
<br />
<br />For convection-condensation melt on basins) it is necrssary t.o introduce a. basin constant, k, whi,~h
<br />r('presents the mean exposure of the basin or segment thereof to wind, considering topographic and
<br />forest effects, For unforestcd plains, k would be 1, but for forested areits, the value may be as low
<br />u'i D.:;, depending upon the density of forest stands, Tite fullowing equation may be used to estimate
<br />convection-condensation snowmelt for open or partly forested arcas:
<br />
<br />M,,= (k) (0.0084v) (Ta-82)
<br />
<br />(16)
<br />
<br />where M" is daily eonvection-cond(,nsation snowmelt in inches, k is the basin convection-conden,a-
<br />tion constant as defined above, v is the mean wind speed itt the 50-foot level in miles per hour, and
<br />1 a is the mean temperature of saturated air at the lO-foot level in OF, For heavily forested areas in
<br />which the variable wind factor has been exeluded, the following equation may be used:
<br />
<br />M,,=O,045(Ta-82)
<br />
<br />(17)
<br />
<br />3-06. RAIN MELT. Snowmelt by the transfer of heat from rain is relatively small and may be
<br />expressed simply in terms of rainfall and free air temperature, as expressed by the following equation:
<br />
<br />Mp=0.007 P,(Ta-82)
<br />
<br />(18)
<br />
<br />where }.tp is the daily snowmelt from rain, P, is the daily rainfall in inches, and Ta is the air tem-
<br />perature in 0 F.
<br />
<br />3-07. GROUND MELT. Snowmelt from ground heat may be taken as at 0.02 inch per day,
<br />
<br />3-08. GENERAL E(:!UATION FOR TOTAL BASIN MELT DURING RAIN. Components of
<br />melt may now be combincd to form a general equation for total basin melt during rain. Since
<br />total melt, }.t, is expressed by the relationship
<br />
<br />Al=.M,,+M,,+M,,+M,+Mp
<br />
<br />(19)
<br />
<br />the terms may be combined for enyironmental conditions as follows:
<br />
<br />
<br />a, for open or partly forested basin areas
<br />
<br />M~ (0.029+0,0084kv+0.007 P,) (Ta-32) +0.09
<br />
<br />(20)
<br />
<br />b. for heavily forested areas,
<br />
<br />M= (0,074+0.007 P,) (Ta-32) +0.05
<br />
<br />(21)
<br />
<br />where 1\1 is total daily snowmelt in inches per day, Ta is the mean temperature of satW'ated air at
<br />the lo-foot level in C .F., v is the mean wind speed at the 50-foot level in miles per bour, Pr is the
<br />rate of precipitation in inches per day, and k is the basin constant as defined in paragraph 3-05,
<br />It is emphasized that total basin melt can be computed from the above equations only by use of
<br />values of wind and temperature which are representative of average conditions over the snow-
<br />covered area of the basin or by integration of melts computed from representative zonal averag'~s
<br />of temperature and wind. Where measurements of air temperature and wind are at significantly
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