Frequency of heavy rainstorms on areas from 10 to 10 000 km2, defined using dense rain gauge networks

Operation in Illinois of eight dense rain gauge networks of varying sizes, with each operating for 5-22 yr, provided data used to assess the temporal frequency of very heavy rain events on small-to moderate-sized areas. Initial testing reveals that incidences of heavy rainstorms with amounts equating to 25-yr values for 6-12 h, and those equating to 100-yr values for 1 day, increase rapidly as area increases and that values for the average storm incidence for 5-yr periods fit a logarithmic distribution. Results indicate an area of 300 km(2) will experience one 25-yr 6-12-h storm in an average 5-yr period, whereas a 10 000 km(2) area would experience 10 such storms in an average 5-yr period. Assessment of incidences of 100-yr storms or greater with a 1-day duration also found that the eight network values fit a logarithmic distribution of the area and storm frequency. In a 5-yr period, one such severe 100-yr rainstorm will occur in an area of 1500 km(2), and three such storms are average for a 10 000 km(2) area. The results serve as a meaningful basis for more extensive research, and the values derived have relevance to storm transposition as part of hydrologic design and for storm water management for urban and rural basins. The results should be applicable for estimating storm occurrences in areas with similar climate throughout the central United States.