Rain gauge requirements: how rainfall is measured for SWPPP inspections
Updated
Every rain-triggered inspection starts with a measurement: did this site get 0.25″ (or 0.5″, or 1.0″; triggers vary by state) in 24 hours? The permit doesn't just set the threshold; most also say how you're allowed to measure it, and several states are strict enough that the gauge itself is an inspectable item.
Get the measurement method wrong and the rest of the program collapses quietly: a storm your method missed is an inspection you didn't know you owed.
The two accepted methods
- On-site rain gauge: a physical gauge at the project, read and logged by site personnel. It's the legally strongest record because it measures the actual site: thunderstorms are hyperlocal, and the airport five miles away can read 0.1″ while your site takes an inch.
- Representative weather station: a nearby station (NWS/airport or commercial weather data) documented in the SWPPP as the site's reference. Accepted by the federal CGP and many states; Pennsylvania and Maryland, for example, allow an on-site gauge or a representative station.
Whichever method the SWPPP names is the one you're held to. Switching between sources storm by storm (gauge when it's convenient, airport when the gauge reads high) is the kind of pattern auditors are trained to spot.
States where the gauge is mandatory
| State | Measurement rule |
|---|---|
| North Carolina | On-site rain gauge required with daily readings; a cumulative reading is acceptable over unattended weekends. The 1.0″ trigger is measured by that gauge |
| Georgia | Daily rain gauge readings required except on non-working weekends and holidays; turbidity sampling is also tied to 0.5″ events |
| Arkansas | On-site rain gauge required for the 0.25″ trigger |
| South Dakota | On-site rain gauge required |
| North Dakota | On-site rain gauge or the nearest NWS gauge; inspections only during normal working hours |
| California | The post-storm inspection duty keys off the site rain gauge reading (0.5″ or more) under the forecast-based QPE system |
| Pennsylvania / Maryland | On-site rain gauge or documented representative weather station |
Permits change, so verify the measurement rule on your state's page against the current permit text.
The rain log: the record behind the record
The gauge reading only matters if it's written down. Most permits require rainfall records to be kept with the SWPPP: dates, amounts, and who read the gauge. That rain log is the first cross-check in an audit: the reviewer lines up your rainfall record against your inspection reports and looks for storms above the trigger with no inspection within the deadline. A complete inspection binder with a gappy rain log fails that check just as badly as missing inspections.
South Carolina makes the log explicit: its 2026 permit requires a Rain Log recording events of 0.5″ or more even though its post-storm check is only recommended.
Practical gauge discipline
- Mount it right: open area, away from buildings and trees, at the height the manufacturer specifies — a gauge under a roof drip line reads fiction.
- Read it daily where required (and empty it): cumulative multi-day readings are only acceptable where the permit says so, like North Carolina's unattended-weekend allowance.
- Name the backup source in the SWPPP: gauges freeze, clog, and vanish. A documented fallback station keeps the record continuous.
- Log zeros too: a log that only has entries on rainy days can't prove the dry days were dry.
Doing it without the daily walk to the gauge
RainCheck runs hourly precipitation monitoring at each site's exact coordinates and builds the rain log automatically, flagging any event that crosses your state's trigger and starting the inspection clock. Got a gauge on site? Log the reading in two taps and the gauge becomes the official record, and the monitoring becomes your backstop instead of your burden. Either way, the log and the inspections stay in one place, already lined up the way an auditor will read them.
Common questions
Do I need a rain gauge on my construction site?
It depends on the state. North Carolina, Georgia, Arkansas, and South Dakota require an on-site gauge (Georgia and North Carolina with daily readings); the federal CGP and states like Pennsylvania and Maryland accept a documented representative weather station instead. Check your state permit.
Can I use weather station data instead of a rain gauge?
Under the federal CGP and many state permits, yes, if the station is representative of the site and documented in the SWPPP. Some states mandate an on-site gauge regardless. Whichever source the SWPPP names is the one you're held to.
How often does a rain gauge need to be read?
Daily readings are the norm where gauges are required: Georgia requires daily readings except non-working weekends and holidays, and North Carolina requires daily readings with a cumulative reading allowed over unattended weekends. The reading should be logged with date, amount, and reader.
What is a rain log?
The running record of rainfall at the site (dates, amounts, and source) kept with the SWPPP. Auditors cross-check it against inspection reports to find triggering storms with no inspection, so it needs entries for dry days too, not just storms.