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

Sushban Shrestha¹, Xing Fang², and Junqi Li³

¹Hydro-Engineering Solutions, LLC, 2124 Moore’s Mill Road, Suite 120, Auburn, AL 36831-2889, Email: sshrestha@hydro-engineering.net

² Professor, Department of Civil Engineering, Auburn University, Auburn, AL 36849-5337, E-mail: xing.fang@auburn.edu

³Professor, Key Laboratory of Urban Stormwater System and Water Environment, (Beijing University of Civil Engineering and Architecture)，Ministry of Education, 100044, Beijing, P. R. China, E-mail: jqili6711@vip.163.com

Abstract

The U.S. Environmental Protection Agency (USEPA) published a 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 pre-development hydrology of the site. The technical guidance recommends retaining stormwater runoff from rainfall events equal to or less than the 95^th percentile rainfall event by using variety of stormwater management practices often referred to as Green Infrastructure or Low Impact Development practices. The 95^th storm rainfall depths for a few selected cities were reported in the technical guidance. In this paper, the 95^th 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 95^th, 90^th, and 85^th percentiles, because the 90^th or 85^th 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 95^th percentile rainfall depths were smaller, and a correlation was developed between these two rainfall depths.

Table 1a.  The 85^th, 90^th, and 95^th Percentile Rainfall Depths (in.) Derived Using Daily Rainfall Data for 206 Weather Stations or Cities in the Contiguous USA (SI units).

Tabke 1b. The 85^th, 90^th, and 95^th Percentile Rainfall Depths (in.) Derived Using Daily Rainfall Data for 206 Weather Stations or Cities in the Contiguous USA (Engineering units).

Note: The last column "90% Cumulative Rainfall Depth" in Table 1 is “the runoff from 90% of average annual rainfall” for BMPs in urban watersheds (Clar et al. 2004) - see more discussion in the paper.

Declaration: These data in Table 1 are provided for free as part of the paper.  It has been carefully verified. Neither I nor any university in which I am in the employment accepts responsibility or liability for its use by third parties.

Contact information for further questions: Dr. Xing Fang (Auburn University), (334) 844-8778 or xing.fang@auburn.edu

The 95th percentile daily rainfall in the contiguous USA.

The 90th percentile daily rainfall in the contiguous USA.

The 85th percentile daily rainfall in the contiguous USA.