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the SPS and PMS), one must estimate two storm parameters: total duration and time intelVal for each rainfall increment. Both parameters must reflect the type and size of the drainage areas being examined, the type of basin features one intends to analyze, and the location of these features. The total duration of the hypothetical storm is directly related to the time of concentration of the watershed (the traveltime from the upper portions of the watershed to the most downstream point of interest). For example, if the estimated traveltime is 14 hours (determined from actual records or by computation) from the watershed boundary to the lower limits of the study area, the storm duration must be at least 14 hours and preferably more. For most applications, the duration would be set to an even day (24 hours). Since a storm duration of less than 14 hours would not allow all portions of the drainage basin to contribute direct runoff to the outlet simultaneously during the course of the storm, the peak discharge at the basin outlet would not be reflective of the rainfall event if the storm duration were made less than 14 hours. Runoff from the lower portions of the basin from, say, a 6-hour event would have left the basin before the inflow from the upper portion reached the outlet. Therefore, a minimum storm duration should be selected at least equal to, and preferably well in excess of, the estimated travel time (time of concentration) at the downstream-most point of interest. This selected duration should be increased considerably if total volume of runoff as well as peak discharge is of importance in the study. Drainage basins having an unusually large amount of flood plain storage (wide flood plains and/or large areas of swamps) may require a storm of longer-duration to capture the attenuation effect of these large natural storage areas. ReselVoir studies require long-duration events for full assessment of the reselVoir flood storage needed. Therefore, a maximum storm duration of 10 days may be used even if the travel time to the reselVoir site is only 14 hours. Total storm duration is normally taken as some increment of a 24-hour day (3, 4, 6, or 12 hours), or a multiple of a day (1 to 10 days). 3.2. TIME INTERVAL Once the storm duration has been established, the time intelVal for subdivision of the total storm must be selected. The time interval must be small enough to accurately define the flood hydrograph (especially the peak); however, too small an Interval will result In excess computations by the indMdual or the computer. The time intelVal will generally be established by the fastest peaking subarea of the overall basin model for which the peak discharge is required, i.e., for later use in developing water surface profiles, to evaluate the effects of a flood control component, etc. The time intelVal must be small enough to define the rising limb and peak for the hydrograph for this subarea. It has been found from past experience that a time intelVal that gives at least 3 points on the rising limb of the hydrograph prior to the peak provides an estimation of the peak discharge that is accurate enough for most work. Extending the previous example of the selection of a storm duration of 24 hours basedon a traveltime of 14 hours to the outlet, one can now select a time intelVal of 4 hours (14 hours divided by 3 points prior to peak = 4.67 hours, rounded down to 4 hours). This would be an appropriate time intelVal if one were interested only in the peak discharge at the outlet. However, if there is a subarea upstream for which a peak discharge estimate was also necessary, then the time intelVal for the entire drainage basin would be based on the requirements for this subarea. For example, if the time of concentration for this subarea is 70 minutes, the intelVal required would be 20 minutes (70 Colorado Flood Hydrology Manual DRAFT 7.3