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11-2 <br />Methods of Estimation <br />??eeional analysis <br />This method is one of the moat uaefUl for snowmelt floods. See Chapter <br />for details of the method. <br />Decree-day method. uneaeed watersheds <br />This method is widely used because of its adaptability to usual data <br />conditions. Similar methods going into more detail are available but <br />seldom applicable because of lack of required data. <br />The degree lay method uses the equation: <br />M=KD (1 <br />where M = the watershed snowmelt in inches per day. <br />K = a constant that varies with watershed and climatic coalition: <br />D = the number of degree-days for a given day. <br />A 3eeree~av is a day with an average temperature one degree above 32° F. <br />Maximum and mini+mn,+ temperatures, as found in "Climatological Data," are <br />averaged to get the daily average temperature. A day with an average of <br />40° F. gives eight degree-days; with an average of 51° F., nineteen <br />degree days. The general form of the method is given below. A working <br />arrangement of the data is shown on table 11-1. In most cases the table <br />can be condensed. The steps iu the method are: <br />1. Using precipitation stations or snow survey data, show either <br />(a) the total available eater equivalent at the beginning of <br />the melt season (table 11-1) or (b} the precipitation and the <br />water equivalent by days (table 11-2). The first proceiure is <br />used where there is generally only one melt period per year; the <br />second, where melt periods occur intermittently through the <br />winter end spring. Water equivalent is the depth of eater, is <br />inches, that results from melting a given depth of snow, and <br />it is dependent on both depth and density of snow. Snow <br />surveys give field determinations of water equivalents. Where <br />such surveys are not made, it is customary to use one-tenth of <br />the snow 3epth as the depth of water equivalent. <br />2. For temperature stations in tie uaterahe3, tabulate average <br />temperatures for the melt perlods. (Note: ma~"+++m and <br />minimum values as given in "Climatological Data" can be <br />averaged aentally to avoid tabulation of averages below <br />33° F.) <br />f`.~~•v wY`~ fA~^ <br />~'~~: <br />jf~vi~~ V.> . <br />