Rain Events & Stormwater Compliance
Last reviewed: August 8, 2026
Most construction stormwater permits require an inspection after a storm that produces a certain amount of rainfall within a set window. This guide explains what makes a storm a "qualifying rain event," how the federal baseline works, and how several state permits set different thresholds and timing.
EnviroReport tracks rainfall and flags qualifying storms automatically.
When rainfall at your site crosses your permit's threshold, EnviroReport schedules the post-storm inspection and notifies your team. Up to 5 sites free, no credit card required.
What Is a "Qualifying" Rain Event?
Construction stormwater permits typically allow two ways to satisfy routine inspection requirements: a fixed calendar schedule (e.g., every 7 days), or a longer calendar schedule (e.g., every 14 days) paired with an additional inspection whenever a storm of a certain size occurs. A "qualifying" or "triggering" rain event is simply a storm that meets or exceeds the rainfall amount your permit sets as that trigger.
When a qualifying rain event happens, it creates a separate inspection obligation — on top of, or in place of, your routine schedule — with its own deadline, usually measured in hours from the end of the storm. This is distinct from a routine inspection under either the SWPPP or stormwater inspection schedules.
1.The EPA Construction General Permit
Under the EPA Construction General Permit, operators on the 14-day routine schedule must conduct an inspection within 24 hours of the end of any storm that produces 0.25 inches or more of rainfall. Sites discharging to impaired waters must inspect every 7 days regardless of rainfall.
- Threshold
- 0.25 inches of rainfall, measured over a 24-hour period.
- Deadline
- Within 24 hours of the end of the storm — the clock starts when precipitation stops, not when it begins.
- Applies To
- Only operators who chose the 14-day routine schedule. Operators on the 7-day schedule already inspect frequently enough that a separate rain trigger isn't layered on top.
2.How to Measure Rainfall
How you measure rainfall matters as much as the threshold itself, since it determines whether a given storm triggers an inspection at all.
- On-Site Rain Gauge
- The most reliable and defensible method. A simple tipping-bucket or manual gauge installed at the site gives you a site-specific, permit-ready reading.
- Nearby Weather Station
- Acceptable under most permits when a site-specific gauge isn't practical, provided the station is reasonably close to the site. Document the station used and the distance from the site in your inspection report.
- Multi-Day Storms
- Some permits total rainfall across a continuous storm rather than resetting at each 24-hour mark. If a storm spans several days, check whether your permit measures the whole event or each 24-hour period separately.
- Document the Source
- Always record where your rainfall figure came from — gauge or station — in the inspection report. This is the first thing regulators check if a post-storm inspection timeline is disputed.
Not sure if a storm is coming that will trigger an inspection?
Our free Precipitation Forecast Lookup shows a 72-hour rainfall outlook for any site. No login required.
3.State-Specific Examples
Many states that run their own construction stormwater permit program (rather than relying on the federal EPA CGP) set their own rainfall threshold and timing. The examples below illustrate how much these can vary — they are not a substitute for reading your permit, and permit language changes over time.
| Program | Rainfall Threshold | Inspection Deadline |
|---|---|---|
| Federal EPA CGP | 0.25 in. / 24 hrs | Within 24 hours of storm end |
| California CGP (2022) | Forecast-based "Qualifying Precipitation Event" (≥0.5 in. forecast with ≥50% chance); post-event inspection required if the on-site gauge records 0.5 in. or more | Within 96 hours of the event ending (Risk Level 2/3) |
| Texas TXR150000 | 0.5 in. / 24 hrs | Within 24 hours of storm end |
| Georgia GAR100001 | 0.5 in. / 24 hrs | Within 24 hours of storm end |
| Florida CGP (draft) | 0.5 in. / 24 hrs | Within 24 hours of storm end, or 7 calendar days since the last inspection — whichever comes first |
| Arizona AZG2025-001 | 0.5 in. / 24 hrs (14-day schedule only; 0.25 in. just requires a rain log entry) | Within 24 hours of each occurrence of 0.5 in. in a 24-hr period — not just once at storm end |
Arizona's trigger (CGP AZG2025-001, Part 4.2.2.c) is not measured from the end of the storm, but rather from the point during a storm that produces 0.5 inches or more of rain in a 24-hour period (including several smaller bursts that add up to 0.5 inches). An inspection is due within 24 hours of each time the rolling 24-hour total reaches 0.5 inches, for as long as the storm continues. Treat the figures above as a starting point and verify current requirements directly with each program before acting on them. Program links for every state are in our State Permit Overview.
4.What to Do When a Threshold Is Met
Once a storm meets your permit's threshold, treat it the same way you would a routine inspection, with a focus on storm-related damage:
- Walk BMPs, perimeter controls, and storm drain inlets for displacement or failure
- Check discharge points for sediment, turbidity, sheen, or other signs of pollution
- Note the rainfall amount, the data source, and the time the storm ended in your report
- Open a corrective action for any BMP that failed or needs maintenance
See our stormwater inspection guide for the full inspection and documentation checklist.
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Create Your Free Account See All FeaturesDisclaimer: This article is for general informational purposes only and does not constitute legal, regulatory, or professional engineering advice. Requirements vary by state, county, and jurisdiction. Always consult a licensed professional for your specific situation.