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discharge hydrograph at the outlet was nee<:ted, Only one time interval can be specHied for a given HEC-1 simulation, This mean~ that the smaller intervals must be carried throughout the hydrograph construction, routing, and combining operations wtthin the HEC- 1 simulation. This means that the smaller intervals must be carried throughout the hydrograph construction, routing, and combing operations wtthin the HEC-1 model, and this greater number of intervals will require more effort and computer time, Extreme cases occasionally occur, such as a large drainage area wtth a few of tts 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-1 model (most likely wtth a short,c!uration 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 ...WS PUBLICATIONS. The methodology for hypothetical storm development for the Westem states will be described separately from that for the remainder of the Untted States, Procedures for developing hypothetical storms in Hawaii and Alaska are similar to procedures for the other states, 3,3.1. EASTERN 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 fr<lm the appropriate NWS publications, For a 6-hour storm duration and 15-minute increment, for example, the 2-year hypothetical storm data for the area of interest would be obtained from TP-40 and HYORO-35, TP-40 gives isopluvial maps of the 2-, 3- and 6-hour duration 2-year-retum-period total rainfall, The 30- minute and 1-hour maps in TP-40 have bllen superseded by the procedures given in HYDRO-35, By determining the location of the study area on each map, one can select the 2-year rainfall depth for each of the threll durations, Since durations of one hour and less are also needed, one must use the HVORO-35 maps of 15- and 60-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 lor a 6-hour,c!uration 15-minute-interval storm has been extracted from the NWS publicati<)ns, 3.3.2. WESTERN UNITED STATES. Procedures for extracting basic rainfall data from NOAA Atlas 2 for the Western states are signHicantly different from procedures for the rest of the Untted States, Maps are available for only the 6- and 24-hour-duration for various return periods. To determine a 6-hour duration 15-minute-interval 2-year-return- period storm, one would use the appropriatfl volume for the state in which the study area lies and extract the 6-hour and 24-hour rainfall depth at the location of the study area from the two maps, ARhough the total storm dlJration is only 6 hours, the 24-hour depth is usually needed to solve for the 1-hour,c!uration depths using the appropriate ratio and equation. Equations for the 2- and 3-hour,c!uration depths can be solved once the 1- and 6-hour depths are known, Durations of less than one hour are determined by multiplying the one hour-depth by various ratios given" in Atlas 2, For the example, the maps and equations in Atlas 2 would be used to extract the 15- and 30-minute, 1-, 2-, 3- and 6-hour rainfall depths, Colorado Flood Hydrology Manual DRAFI' 7.4