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<br />Since the melt season usually lasts 2 to 4 Months, and occurs <br />during the spring when normal temperatures are changing rapidly, a <br />curve of temperature vs. duration should be constructed in terms of <br />temperatures above (or below) normal for each day. Envelope curves or <br />frequency curves of temp~rature for various durations can be constructed <br />and applied to the pattern of normal daily temperatures. Curves of <br />other melt factors can be established in the same manner. Envelope <br />values for a long period of record or values exceeded once in about 50 <br />years should be satisfactory for standard project use. The time se- <br />quence Df temperatures must be consistent with time sequences of pre- <br />cipitation and other factors during the melt season. Critical se- <br />quences may be different for different applications, and it is ordi- <br />narily necessary to make several runoff computation trials before a <br />critical sequence can be selected for any particular application. <br />In the case of rainfloods, the temperatures and other factors must <br />be maximum consistent with rainfall. Usually maximum surface dew <br />points and pseudo-adiabatic lapse rates will determine maximum rain- <br />storm temperatures. <br /> <br />Section 3.06. Standard project base flow <br /> <br />Base flow is the amount of runoff that occurs during a flood but <br />is not computed directly from rainfall and snowmelt. It is flow that <br />would occur if the current storm did not occur plus any return flow <br />that is not directly computed from the current rain and snowmelt. Some <br />hydrologists prefer to include an added amount to account for an in- <br />crease in subsurface flow resulting from the current storm and any <br />remaining water stored in the channel from an antecedent event. <br />Base flow to be used in standard project flood computations is <br />ordinarily obtained by enveloping the largest base flow amounts that <br />have occurred in past floods and applying this to the standard project <br />flood in the manner in which it occurred in past floods. <br /> <br />3-10 <br />