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. COLORADO STATEWIDE DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL for 2-, 3- and 6-hour storm durations can be developed using the following procedures from the UDFCD Drainage Criteria Manual, 2001. For drainage basins less than 10 square miles but greater than 90 acres, two-hour storm distribution rainfall values without area adjustments of the values shall be used with CUHP. For drainage basins between ten and twenty square miles, three-hour storm distribution rainfall values with the area-adjustment shall be used. For basins equal to and larger than 20 square miles, six-hour storm distribution values with the area-adjustment shall be used. Area adjustments of the rainfall values for drainage basins equal to or greater than 10 square miles are necessary to determine the average depth of precipitation over the entire drainage basin being analyzed. The 1-, 3-, and 6-hour point rainfall depths estimated using the NOAA Atlas 2 procedure described previously can be used to develop storm distributions for a given recurrence frequency. The estimated NOAA point precipitation values can be distributed to develop 2-, 3- or 6-hour temporal distribution values using a 5-minute time increment following the distribution procedures from the UDFCD Drainage Criteria Manual, 2001. . The 2-hour temporal distribution for a given recurrence frequency can be developed by multiplying the NOAA 1-hour rainfall depth by the incremental distribution percentages (0 to 120 minutes) given in Table CH5- T103. The 2- hour design storm distribution can be used without further modifications wi,th CUHP for drainage basins less than 10 square miles. The 3-hour storm distribution can be developed by adding incremental precipitation values for the period between 125 minutes and 180 minutes to the 2-hour distribution discussed above. The incremental precipitation values for the period between 125 minutes and 180 minutes can be determined by evenly distributing the difference between the NOAA 3-hour rainfall depth and the 2-hour total precipitation developed using Table CH5- T1 03. In a similar approach, the 6-hour distribution can be developed by evenly distributing the difference between the NOAA 3-hour and 6-hour rainfall depths over the period of 185 minutes to 360 minutes. The first three hours of the 6-hour distribution is same as the three-hour distribution discussed above. 1.5.2 DEPTH-AREA ADJUSTMENT . CHAPTER 5 HYDROLOGY SECTION 1.0 RAINFALL The NOAA precipitation depths are related to rainfall frequency at an isolated point. Storms, however, cause rainfall to occur over extensive areas simultaneously, with more intense rainfall typically occurring near the center of the storm. Rainfall depth-area adjustment is necessary to determine the average depth of precipitation over the entire drainage basin being analyzed. This is normally performed using depth-area reduction curves relating point precipitation reduction factor to drainage basin area and storm duration. The depth-area adjustment curves from NOAA Atlas 2, as modified by UDFCD, are shown on Figure CH5-F115. The 3- and 6-hour design storm distributions should be adjusted in accordance with the suggested depth-area adjustment procedures outlined in NOAA Atlas 2 before using them with CUHP. In order to assist engineers AUGUST 2002 RAINFALL CH5-107