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is applied to concurrent ordinates. The instantaneous peak flows should be averaged, regardless of differences in lag time and plotted at the average lag time. The average unit hydrograph can then be sketched to confOITl) to the shape of the graphs, passing through the computed average peak, and having a Volume of 1 unit of runoff. By inspection of the heaviest rainfall and snowmeit excesses that caUSed the peak flows of the flood hydrograph, unit hydrograph ordinates around the peak are first estimated. The remainder of unit hydrograph is then estimated so that the volume is equal to 1 unit Of runoff. In estimating the overall shape of the unit hydrograph, consideration is given to the rate of rise and recession 01 the flood hydro graph and the estimated lag time from the heaviest excess period to the peak of the flood hydrograph. This first-trial unit hydrograph is then applied to the eX<::ess estimates, and the resuitinQ computed hydrograph is checked against the observed flood hydrograph. The unit hYdrograph is then adjusted as needed, and the process is repeated until the computed hydrograph approximates the observed hydrograph to the desired accuracy. The computation of a hydrograph rElquires not only developing a unit hydro graph but estimating rainfall and snowmeit losses as well, and ordinarily the analysis involves beth procEldures at the same time. This is normally required since it is not always obvious whether the unit hydrograph or the loss nUes should be adjusted to obtain a better fit between the computed and observed hydrographs. The reconstitution of historical f100q hydrographs involves the estimation of base flow, loss rates, and unit hydrographs; and it becomes clear that many combinations of these three items can reconstitute the same flood hydrograph equally as well. Consequently, a considerable amount of enllineering judgment will be requireq to establish reasonable estimates for each of these items. Some subjectivity can be eliminated by techniques such as the base flow separation. However, there is no way to completely eliminate all of the subjectivity involved. cCmsistency in approach can be gained through the use 01 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 enllineer, however, to evaluate the computed coefficients for reasonableness. 3. S-CUFlVE HYDROGRAPHS. 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, <::ontinuous 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-clJrve Hydrograph. The S-curve hYdrograph, as defined abeve, should not be confused with mass curves of runoff that simply represent accumulative volumes. Colorado Flood Hydrology Manual 7-38 a:w=r