Mapping the 95th Percentile Daily Rainfall in the Contiguous U.S.

The U.S. Environmental Protection Agency (USEPA) published technical guidance to help federal agencies in implementing Section 438 of the Energy Independence and Security Act of 2007, which requires federal development or redevelopment projects to preserve or restore predevelopment hydrology of the site. The technical guidance recommends retaining stormwater runoff from rainfall events equal to or less than the 95th percentile rainfall event by using a variety of stormwater management practices often referred to as Green Infrastructure or Low Impact Development practices. The 95th storm rainfall depths for a few selected cities were reported in the technical guidance. In this paper, the 95th percentile daily rainfall depths were computed following the USEPA guidelines from weather stations at 206 U.S. cities. An isohyetal map for the contiguous U.S. and supporting table with rainfall depths from the 206 weather stations were generated for 95th, 90th, and 85th percentiles, because the 90th or 85th percentile rainfall depths are extensively used in many stormwater management manuals in the U.S. to determine water quality volume. When compared with NOAA's 1-year 24-hour rainfall depths derived from TP-40 rainfall frequency atlas, the 95th percentile rainfall depths were smaller, and a correlation was developed between these two rainfall depths.