The rainfall distribution type (I, IA, II, or III) and quantity of precipitation are required inputs to our hydrology (rainfall-runoff, peak discharge) calculation and detention storage calculation.  For regions within the USA, use the figures below to determine rainfall distribution type and quantity of precipitation.  For regions outside the USA, determine which rainfall distribution type best matches your region; find the precipitation quantity from local precipitation maps.

All figures are from:
U.S. Soil Conservation Service.  Technical Release 55: Urban Hydrology for Small Watersheds.  USDA (U.S. Department of Agriculture).  June 1986.  On the web in .pdf  format at http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf .

Quick links on this page:
SCS 24-hr rainfall distribution type graph
Geographic boundaries within the USA for SCS rainfall distributions
24-hr duration rainfall maps of USA with return periods of:  2-yr   5-yr  10-yr  25-yr  50-yr  100-yr

Fig. B-1.  SCS 24-hour rainfall distributions (SCS, 1986):
(y-axis reads "Fraction of 24-hour rainfall" and x-axis reads "Time, hours")

Fig. B-2.  Approximate geographic boundaries for SCS rainfall distributions (SCS, 1986):

Fig. B-3.  2-Year Return Period, 24-hour Duration Precipitation, inches (SCS, 1986):

Fig. B-4.  Five Year Return Period, 24 hour Duration Precipitation, inches (SCS, 1986):

Fig. B-5.  Ten Year Return Period, 24 hour Duration Precipitation, inches (SCS, 1986):

Fig. B-6.  25-Year Return Period, 24 hour Duration Precipitation, inches (SCS, 1986):

Fig. B-7.  50-Year Return Period, 24 hour Duration Precipitation, inches (SCS, 1986):

Fig. B-8.  100-Year Return Period, 24 hour Duration Precipitation, inches (SCS, 1986):