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discharge hydrograph at the outlet was needed. Only one time Interval can be specified for a given HEC-1 simulation. This means that the smaller intervals must be carried throughout the hydrograph construction, routing, and cornbining operations within the HEC.. 1 simulation. This means that the smaller intervals must be carried throu(Jhout the hydrograph construction, routing. and combing opera'lions within the HEC-1 model. and this greater number of intervals will require more effort and computer time. Extreme ca.ses 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 tile outlet, a time interval of 5 minutes may be required to accurately capture the peak discharge from the smallest subareas. It may be more econ~mical to construct a separate HEC-1 model (most likely with a short-duration storrn) for each small subarea requiring a 5-minute subdivision, thereby allowing the mcdel of the lar(je area to use multi-Ilour time increments for the longer storm duration. 3.3. DATA EXTRACTION FROM NWS PUBLICATIONS. The methodology for hypothetical storm development tor the Western states will be (!escribed separately from that for the remainder of the United States. Procedures for developing hypothetical stonTIS in Hawaii and Alaska are sirnilar to procedures for tfJe other states. 3.3.1. EASTERN AND CENTRAL UNITED STATES. Once the storm duration and computation time interval have been established. Hie rainfall dE'pths for key durations and each desired return period are taken frorn the approprate 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 HYDRO-35. TP-40 gives isopluvial maps of the 2-, 3- and 6-hour duration 2-year-return-period total rainfall. Tile 30. minute and 1-hour maps in TP-40 have been superseded by the procedu res (Jive~ in HYDRO-35. By determining the location ot tile study area on eacll 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 HYDHO-35 maps of 1 cj- and 60-minute rainfall depth and repeat the extraction of the desired 2-year rainfall depths. A value of tile 30- minute rainfall depth is also obtained by applying the E!quatic n given in HYDRO-35. At this point, all available rainfall-depth information for a 6-hour-duration 15-minute-interval st~rm has been extracted from the NWS publications. 3.3.2. WESTERN UNITED STATES. F'rocedures for extracting basic rainfall (lata from NOAA Atlas 2 for the Western states are significantly different from procedure,. for the rest of the United States. Maps are available for only Hle 6- and 24-hour-duratlon for various return periods. To determine a 6-hour duration 15.minuteinterval 2-year-return- period storm, one would use the appropriate volume for the state in which the study area lies and extract the 6-hour and 24-hour rainfall depth at tile location of the study area from the two maps. Although the total storm duration is only Ei hours. the 24-hour depth is usually needed to solve for the 1-hour-duration depths using tile 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. Curations of less tllan one Ilour 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 b'3 used to extract ',he 15. and30-minute. 1-. 2-. 3- and I)-hour rainfall depths. Colorado Flood Hydrology Manual D.RAFT 7.4