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 five 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 does not qualify for a waiver, then NPDES stormwater permit coverage is required.

Visit Rainfall Erosivity Factor Calculator

For more information, please see the Construction Rainfall Erosivity Waiver Fact Sheet (PDF)(12 pp, 2.5MB, About PDF) and Appendix C (PDF)(4 pp, 87K) of the 2012 Construction General Permit (CGP).