Simplified Method for Rainfall Intensities

Procedures are presented for estimating maximum rainfall amounts for 24 hours with a return period of 10 years from monthly and observation day measurements. Procedures are also presented for converting these values to other rainfall depth- duration amounts. The depth-duration amount varies with the fourth root of the time of duration and the sixth root of the number of years in the return period. Observation-day rainfall is converted to 24-hour values by multiplying by 1.13. Mean monthly rainfall and the maximum monthly values from a 30-year series are used to estimate 10-year return period, 24-hour duration amounts. The 10-year, 24-hour amounts are then used to estimate other depth-duration values. The relationships are good except in protected areas such as for the lee of mountains or in areas surrounded by mountains.