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Water: Stormwater

Welcome to the Rainfall Erosivity Factor Calculator!

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EPA’s stormwater regulations allow NPDES permitting authorities to waive NPDES permitting requirements for stormwater discharges from small construction sites (disturbing less than five acres) if the value of the rainfall erosivity factor (“R” in the Revised Universal Soil Loss Equation or RUSLE) is less than 5 during the period of construction activity. Small construction sites that qualify for the waiver generally take place during a relatively short time in arid or semi-arid areas.

EPA has updated its Rainfall Erosivity Factor Calculator to correct known problems and to use updated data from the Natural Resources Conservation Service’s (NRCS) Revised Universal Soil Loss Equation, Version 2 (RUSLE2) database. To use this tool to estimate the rainfall erosivity factor for your site, you will need your project’s location (either latitude/longitude or address) and the estimated start and end dates of construction. The period of construction activity begins at initial earth disturbance and ends with final stabilization.

If your small construction project is located in an area where EPA is the permitting authority and your R factor is less than 5, you qualify for a waiver from NPDES stormwater permitting and you may submit a waiver certification using the electronic Notice of Intent (eNOI) system. Several states that are authorized to implement the NPDES permitting program also accept low erosivity waivers. Check with your state NPDES permitting authority for more information.

If your small construction project does not qualify for a waiver, then NPDES stormwater permit coverage is required.

For more information, please see the Construction Rainfall Erosivity Waiver Fact Sheet (12 pp, 2.5MB, About PDF) and Appendix C (4 pp, 87K, About PDF) of the 2012 CGP. For questions or comments, e-mail cgp@epa.gov.

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