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<br />EM 1110-2-1406
<br />5 Jan 60
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<br />4-02. SOLAR RADIATION. Observations of solar radiation are confined to about 70 pyrheli-
<br />ometer stations in the Vnited States. These are generally first-order U.S, Weather Bureau sta-
<br />tions, located at population centers and far removed from headwater drainage basins where
<br />determination of snowmelt is usually required. In engineering practice, therefore, it would usu-
<br />ally be necessary to obtain estimates of solar radiation indirectly, through use of observations of
<br />cloud cover, sunshine duratioll, diurnal tCluperaturc risc, or other related phenomena. Reference
<br />is made to figures 1 and 2, chapter 2, for guides of limiting amounts of solar radiation received at
<br />the earth's surface, Daily variation of solar rndiation can best be estimated indirectly through
<br />use of duration of sunshine obs('n~ations. Figure 5 is a nomograph derived by Hamon, 'Yeiss,
<br />and Wilson," which relatcs daily insolation to latitude, season, and duration of sunshine, Esti-
<br />mated values of incident solar radiation may be applied to the basic equation of snowmelt by
<br />shortwave radiatiou, equation 3 (par, 2-05), with the additional effect of the basin shortwave
<br />melt coefiicient, k'.
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<br />4-03. ALBEDO. There arc no rcgular observations of snow surface albedoes, but estimates may
<br />be made, based on relationships derived from laboratory observations. At best, such estimates
<br />must be considrn'd only as nppl'oximations, but limiting conditions are fuirly well known. Fig-
<br />ure (j shows a typical variation or snow. surface albedo .with time, for both the accumulation and
<br />Inclt seasons, which may be used for estimating purposes.
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<br />4~04. BASIN SHORTWAVE MELT COEFFICIENT, k'. 1!eusurements of solar radiation are
<br />expressed in terms or amounts on a horizontal surface. For basins whose exposure is predomin-
<br />antly north- or south-facing, a basin shortw!1Vc melt coefficicnt must be introduced in the melt
<br />equation. Hdercnce is made to figure 3 (par. 2-04) showing the effeet of u 250 slope at latit.ude
<br />46030' ~, on incident solar radiation. In general, averaged oyer a basin, the slope effect would
<br />not be as extreme as the particular example shO\vn in figure 3. The value of k' would be 1.0 for
<br />a basin which is essentially horizontal or whose north and south slopes arc areally balanced,
<br />The value of k' usually would fall within the limits of 0,9 and 1.1 during spring.
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<br />4~05. LONGW A VE RADIATION. The form of the longwave radiation term in the elear-weather
<br />snownwlt equation depC'nds upon the for('st and meteorological conditions. The basic form for
<br />overcast skies or forested basin is that of equation 9 (par. 2-06). 'Yith clear skies in open areas,
<br />equation 8 is applicable. For basins with partial forest cover, the reduction of the basin short-
<br />wave melt coefficient to account for net heat loss by long\vave radiation loss is warranted. By
<br />this assumption, the magnitude of the longwave loss varies inversely \..rith cloud cover, inasmuch
<br />us the incident solar radiation is primarily a function of eloud cover (in the middle latitudes
<br />during the spring snowmelt season). Errors introduced by this approximation are SIlHlll in C0111-
<br />parison with total daily snO\vmclt quantities, for areas whose forest cover exceeds 10 percent.
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<br />406. EFFECTIVE FOREST CANOPY COVER, F, For partly forested basins, it is necessary
<br />to estimate the effective forest canopy cover, F, which is applied to shortwave and longwave
<br />radiation snowmelt determinations. The coefficient, F, represents the average proportion of the
<br />basin shaded by the forest froIll direct solar radiation, expressed as a decimal fraction, and it
<br />is considered applicable to the long\vave radiation term. Determination of F must be based OIl
<br />a partly subjective estimate of the forest characteristics, considering density and spacing of for-
<br />est stands, latitudinal and diurnal effecls of the forest upon shading, and general knowledge
<br />gained from personal observation or photographic coverage of the area, In general, the value
<br />of F is somewhat greater than the theoretical cover takcn as t.he horizontal projection of the
<br />forest canopy, (See fig. 4, par. 2-04.)
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<br />4-07. CONVECTION-CONDENSATION MELT. Except for heavily forested areas, evaluation
<br />of convect.ion~l'ondensation mdt requires use of a basin convection-condensation melt coefficient,
<br />k, as defined in paragraph :1-05, When the forest cover is sufficiently dense (generally in
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