This study was designed to update the Illinois State Water Survey (ISWS) Bulletin 70,evaluating rainfall frequency relations in Illinois using current precipitation datasets. The studyprimarily used the National Oceanic and Atmospheric Administration (NOAA) daily precipitationdata from 1948 to 2017 to perform regional frequency analysis (RFA) using the L-momentsapproach. Additional information on precipitation relationships for less than 24 hours wereobtained from NOAA hourly precipitation data from 1948 to 2014 and Cook CountyPrecipitation Network (CCPN) data from 1989 to 2016. Precipitation frequency relations weredeveloped for storm durations from 1 to 240 hours and for recurrence intervals from 2 to 500years. The results are presented for the same 10 geographic sections as in Bulletin 70 (Figure 1) to maintain the continuity of hydrologic studies and compatibility with regulations.
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