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<br />- <br /> <br />M, = O.007*P*(T,-32) <br /> <br />(11) <br /> <br />Where: Mp = Daily snowmelt from rain <br />P = Daily rainfall <br />T. = Saturated air temperature <br />This assumes that the temperature of the rain is the same as the surrounding air, While this <br />equation is for daily values, it can be subdivided into smaller time intervals (Le, 5 minutes), <br /> <br />Table 14 supplies a table of the quantity of snowmelt for a given rainfall depth at a given <br />temperature, This table was used in conjunction with the hourly temperature values to provide an <br />estimate of the snowmelt caused by the rain, Table 15 contains the results of the snowmelt caused <br />by rain during the duration of the storm and calculates anew rainfall hyetograph, which is then <br />used in the HEC-I model. <br /> <br />3, Results <br /> <br />The results ofthis analysis are provided in Tables 12 and 13 for Spar Gulch and Vallejo Gulch, <br />respectively, The return period associated with these values is in respect to the rainfall amount <br />and distribution only, In reality, the occurrence of a major rain event while snow was still on the <br />ground would involve a multiple probability analysis, which is beyond the scope of this study, The <br />actual return frequency of these flows occurring would be much less than those shown, <br /> <br />Some of the reasons that the Rain on Snow event is so much greater than the Rain Only flow may <br />be: <br /> <br />a. The Rain Only event includes an abstraction amount of 0,2 inches while the Rain on <br />Snow event has no abstraction, <br /> <br />b, The Rain Only event has a greater final infiltration rate, <br /> <br />c. The Rain on Snow event would carry more runoff due to the melted snow caused by the <br />air temperature and the rain, <br /> <br />d, The Rain Only event used the Green-Ampt infiltration method and the Rain on Snow <br />analysis used the uniform loss rate, <br /> <br />22 <br />