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unit hydrograph. For a storm with only one period of excess, the ordinates of the hydrograph are computed as follows: Q, = U. E , , (1 ) where: Q. = the hydrograph ordinate for any period, i , U, = the unit hydrograph ordinate for the same period, i , E = the rainfall plus snowmelt excess For complex storms in which excess occurs in more time than one period (the normal case), the resulting hydrograph can be determined by computing the individual hydrographs resulting from the various excess periods in their proper time sequence and adding the individual components. The individual components need not be tabulated separately, but accumulatively multiplied until all excess periods have been used, to obtain a total hydrograph ordinate. A general equation for the total hydrograph ordinate for any period, i, is: Q = U 1 . E. + U 2 . E _ + U 3 . E ;-2 + .., + Un' E_ I I 1-1 I-n+ (2) where: i = sequence number of excess Q. = the hydrograph ordinate for period, i , . U= the unit hydrograph ordinates in order from 1 through n E = the rainfall plus snowmelt excess in reverse order from i through i-n n = total number of unit hydrograph ordinates or i, whichever is smaller. 2. UNIT HYDROGRAPH DERIVATION. A theoretically simple method of deriving a unit hydrograph involves the analysis of runoff resulting from isolated precipitation that produces reasonably uniform excess rates for a period approximately equal to the desired unit duration. Base flow is separated from the direct runoff, and the individual ordinates of the direct runoff hydrograph are divided by the volume of direct runoff. The resulting ordinates form a unit hydrograph for the specified duration of unit excess. The occurrence of floods resulting from single bursts of uniform precipitation rates are rare and, 7-26