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<br />Ie <br /> <br />e <br /> <br />e <br /> <br />EM 1110-2-1406 <br />5 Jan 60 <br /> <br />CHAPTER 3 <br />BASIN SNOWMELT DURING RAIN <br /> <br />3-01. BASIC CONSIDERATIONS. The snowmelt equations presented in the preceding section <br />have dealt with evaluation of snowmelt at a point. Evaluation of basin snowmelt during rain repre- <br />sents a special condition for which further simplifying assumptions can be made in tbe snowmelt <br />equation. During rainstorms, solar radiation nlClt is relatively small, and snowmelt resulting frDIu <br />longwavc radiation is easily evaluated frorn known theoretical considerations. Heat transfer by <br />convection and condensation represents the major source of energy for snowmelt, and the form of the <br />equatiol1 is dependent somewhat upon tbe type of area, Basil1 coefficients must be assumed wbich <br />express tbe effect of wind over the basin for the convection-condensation term. Evaluation of heat <br />transfer to the snowpack during rain involves the following basic considerations: <br />a. Shortwave radiation being relatively unimportant can be considered as a constant amount. <br />b. Longwave radiation exchange between forest or low clouds and the snowpack may be com- <br />puted as a linear function of air temperature, <br />c. Air is assumed to be saturated, so that an air temperature-wind speed expression may be <br />used in evaluating both convection and condensation melt, by assuming a linear relat.ionship <br />between vapor pressure and dewpoint. <br />d, On heavily forested basins, wind variation is so reduced b,meath the forest canopy that an <br />average wind condition may be assumed, thereby eliminating tbe wind variable. <br />e. Rain melt is simply expressed as a function of rainfall intensity and air temperature. <br />f. Ground melt is normally unimportant and may be treated as a constant amount. <br />g. Water lost by evapotranspiration is negligible. <br /> <br />3-02. CLASSIFICATION OF FOREST DENSITY. The form of the basin snowmelt equation <br />during rain (as well as for elear-weather periods) depends partly upon the forest cover. The follow- <br />ing limits of average basin forest density arc presented, for use as a guide in selecting snowmelt <br />equations: <br /> <br />Mea.n Ca.1WPII cover, <br />percent <br /> <br />a. Heavily forested_ _ _ _ _ _ __ _ __ __ __ ____ __ __ _ ___ _ __ ___ <br />b. Forested_ _ _ _ ____ _ __ _ _ __ __ _ __ __ _ _ _ _ _ _ __ ___ ___ _ _ _ _ ___ <br />c. Partly forested____ n _ _ _ _ _ _ _ _ _ _ _ __ ___ __ __ _ n _ __ ___n n <br />d. Open_______________________________________________ <br /> <br />>80 <br />60-80 <br />10-60 <br /><10 <br /> <br />3-03. SHORTWAVE RADIATION MELT. Snowmelt by shortwave radiation, !vI", is relatively <br />unimportant during periods of rain. Studies of incident radiation during these periods show that it <br />is reasonable to assume a constant average of 40 langleys per day for an open area and an average <br />albedo of the snow surface of 65 percent. The resulting net snowmelt is 0.07 inch per day, For <br />forested areas it may be less, depending on the areal extent and density of forest cover, <br /> <br />3-04. LONGWAVE RADIATION MELT. As explained in paragraph 2-06, daily longwave <br />radiation melt during periods of significant precipitation (with complete eloud cover) can be ade- <br />quately estimated by equation 9: <br /> <br />M,,=O,029(Ta-32) <br /> <br />(9) <br /> <br />During storm conditions, turbulent mixing in the lower layers of the atmosphere establishes an <br />equilibrium between air t.emperature measured at the normal instrument height and the tempera- <br />ture of tbe forest or low clouds, When the cloud base is less than 1,000 feet abo~e the ground, air <br />temperature lapse rate corrections Illay be ignored. <br /> <br />9 <br />