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minutes/3 points prior to peak = 23,3 minutes, rounded down to 20 minutes). Note that the 24-hour storm would now be subdivided into 72 twenty-minute intervals to define the rainfall distribution fOl" the basin, compared with the 6 four-hour intervals where only the discharge hydrograph at the outlet was needed, Only one time interval can be specified for a given HEC-l simulation, This means that the smaller intervals must be carried throughout the hydro~lraph construction, routing, and combining operations within the HEC- 1 simulation. This means that the smaller intervals must be carried throughout the hydrograph construction, routing, and combing operations within the HEC-l model, and this greater number of intervals will require more effort and computer time, Extreme cases occasionally occur, such as a large drainage area with a few of its subareas very small in size. Although the storm duration may be 2 days or more to reflect travel time to the outlet, a time interval of 5 minutes may be required to accurately capture the peak discharge from the smallest subareas. It may be more economical to construct a separate HEC-l model (most likely with a short-duration storm) for each small subarea requiring a 5-minute subdivision, thereby allowing the model of the large area to use multi-hour time increments for the longer storm duration. 3.3. DATA EXTRACTION FROM NWS PUBLICATIONS. The methodology for hypothetical storm development for the Western states will be described separately from that for the remainder of the United States. Procedures for developing hypothetical storms in Hawaii and Alaska are similar to procedures for the other states, 3.3.1. EASTI:RN AND CENTRAL UNITED STATES. Once the storm duration and computation time interval have been established, the rainfall depths for key durations and each desired retum period are taken from the appropriate NWS publications. For a 6-hour storm duration and 15-mlnute increment, for example, the 2-year hypothetical storm data for the area of interest would be obtained from TP-40 and HYDRO-35. TP-40 gives isopluvial maps of the 2-, 3- and 6-hour duration 2-year-return-period total rainfall. The 30- minute and 1-hour maps in TP-40 have been superseded by the procedures given in HYDR0-35. By determining the location of the study area on each map, one can select the 2-year rainfall depth for each of the three durations. Since durations of one hour and less are also needed, one must use the HYDRO-35 maps of 15- and 6o-minute rainfall depth and repeat the extraction of the desired 2-year rainfall depths. A value of the 30- minute rainfall depth is also obtained by applying the equation given in HYDRO-35. At this point, all available rainfall-depth information for a 6-hour-duration l5-minute-interval storm has been ex1racted from the NWS publications, 3.3.2. WESTI:RN UNITED STATES. Procedures for extracting basic rainfall data from NOAA Atlas 2 for the Western states are significantly different from procedures for the rest of the United States, Maps are available for only the 6- and 24-hour-duration for various return periods. To determine a 6-hour duration l5-minute-intervaI2-year-retum- period storm, one would use the appropriate volume for the state in which the study area lies and ex1ractthe 6-hour and 24-hour rainfall depth at the location of the study area from the two maps. Although the total storm duration is only 6 hours, the 24-hour depth is usually needed to solve for the l-hour-duration depths using the appropriate ratio and equation. Equations for the 2- and 3-hour-duration depths can be solved once the 1- and 6-hour depths are known. Durations of less than one hour are determined by multiplying Colorado Flood Hydrology Manual DRAFT 7.4