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can be eliminated by techniques such as the base flow separation. However, there is no way to completely eliminate all of the subjectivity involved. Consistency in approach can be gained through the use of computer programs such as the HEC-1 Flood Hydrograph Package, which will automatically derive "best fit" unit hydrograph and loss rate coefficients based on optimization techniques to minimize the differences between observed and computed flows. Care must still be exercised by the engineer, however, to evaluate the computed coefficients for reasonableness. 3. S-CURVE HVDROGRAPHS. According to the unit hydrograph concept, if the unit rate of rainfall excess over a drainage area should continue indefinitely with the same areal distribution and intensity characteristics, successive units of rainfall excess would contribute runoff at rates corresponding to the basic unit hydrograph. An accumulation of runoff ordinates corresponding to a particular time would give the total rate of runoff produced by the uniform, continuous rate of rainfall excess antecedent thereto. At a time equal to the rate of rainfall excess and would remain constant thereafter. The hydrograph generated in this fashion will be referred to herein as an S-curve Hydrograph. The S-curve hydrograph, as defined above, should not be confused with mass curves of runoff that simply represent accumulative volumes. An S-curve hydrograph may be computed by tabulating a series of identical unit hydrographs, arranged with origins spaced progressively one unit-rainfall duration apart of the time scale and accumulating the ordinates for specific times. With the unit hydrograph values known, the S-curve hydrograph would be computed by steps. During the first period of unit-rainfall duration, the unit hydrograph and S- curve values are identical. The S-curve values for the first period are added to unit hydrograph discharges for the second interval to obtain corresponding S-curve ordinates. The process is continued until the S-curve discharge rate is equal to the basic rate of rainfall excess. The operations involved are more apparent if the reverse procedure is considered, assuming that the S-curve hydrograph values were originally known. Discharge values represent the rates of runoff that would result from a uniform continuous rate of rainfall excess of 1 inch per unit-rainfall duration beginning at time zero. In accordance with the unit hydrograph principle, the ordinates of an S- curve hydrograph representing runoff from a uniform continuous rainfall excess rate of 1 inch per 12 hours may be multiplied by 2 in order to obtain values applicable to a rainfall excess rate of 1 inch per 6 hours. Accordingly, S-curve hydrographs developed from runoff data for unit storms of various durations may be adjusted to apply to any unit rainfall duration desired, within practical limits. 7-28 I