3. Frequency Distributions of Heavy Rainfall Events

The data used in developing the frequency relations consisted of both daily and hourly precipitation records from Illinois and from nearby stations in surrounding states. The daily data, mostly from nonrecording raingages, spanned the 83-year period from 1901 through 1983. They included data from the 61 precipitation-reporting stations discussed previously (figure 71, whose records had been carefully edited in previous Water Survey studies and were considered of acceptable accuracy for the frequency study. Records for most stations were complete for the 83-year period, but several did not begin until 1902 to 1908, one (Kankakee) did not start until 1917, and another (New Burnside) was terminated in 1957. These stations were used to ensure adequate coverage in certain areas. The daily data were supplemented by hourly data for 34 recording-gage stations in Illinois and 21 stations in surrounding states for the 36-year period, 1948-1983 (figure 8). These data were used primarily in developing frequency distributions for rain periods of less than 24-hour durations. The recording stations are not as uniformly distributed as the daily reporting stations, and the quality of the data obtained from some of them was questionable. The 61-station, 83-year sample from the daily reporting stations was the primary database used in develop-