The 72-hour standing-water rule
Written 2 Sep 2026. The rule: a swale, ponded area, or dry well should drain within 72 hours of the storm's end. It appears in stormwater manuals and mosquito-control guidance alike, and the reason is arithmetic, not taste: a mosquito can go from egg to adult in still water in roughly a week, and several common species beat that. Three days buys a margin against a four-day clock.
Where the number comes from
Mosquito-control guidance from the CDC is blunt: still water is habitat, so empty it. Stormwater manuals turn that into a design number — drawdown limits of 24 to 72 hours are the working range across US manuals, with 72 the common outer edge for vegetated practices. The desk adopts 72 as its ceiling and treats 48 as the comfortable target for anything near a house.
What the rule is not
- It is not a performance goal for aesthetics. A swale that drains in 70 hours passes the rule and still spends three days as a puddle.
- It is not the same as “dry between storms.” Two storms 40 hours apart can leave a compliant swale wet for a week of calendar time.
- It is not universal law. Your local manual may set 24 or 48 hours. The rule on this desk is a ceiling, not your jurisdiction — read the code.
What it does to sizing
The rule turns “soak it eventually” into a budget. The sizing job uses it directly: bottom area must pass the volume through the perk rate within 72 hours. Skip the rule and the arithmetic has no deadline; a swale without a deadline is a pond with a promise.
Where to go next
- Swale — the object the rule governs.
- Size a swale — the rule as arithmetic.
- Diagnose standing water — the walk that starts it all.
Sources
- CDC, mosquitoes and standing water — cdc.gov/mosquitoes
- Minnesota Stormwater Manual, drawdown requirements — stormwater.pca.state.mn.us
- US EPA NPDES — epa.gov/npdes