What 1 inch per hour actually means
Written 30 Jul 2026. “One inch per hour” is a rate with a hidden biography. It means: from a presoaked hole, the water level fell one inch during one hour of waiting, with the hole refilled to a standard depth. It does not mean one inch of rain disappears per hour — rain falls on a surface; the test measures a wetted hole wall. Same units, different object.
Written 2026-07-30 · re-checked 2026-09-10
The three inflations
- The dry hole. A hole dug in dry soil drinks fast for the first hour — that is the soil quenching its own thirst, not its true rate. The perk test job presoaks twice for exactly this reason. An unpresoaked number can read two to three times too fast.
- The shallow hole. A hole that ends in topsoil tests topsoil. The water you plan to manage sits deeper. Test at the depth that matters.
- The short watch. A 5-minute check multiplied out inherits every slosh and bubble. Thirty minutes is the desk's minimum watch; an hour is better on slow soil.
Why the unit matters to the verdict
The 0.5 in/hr gate on the perk page only means something against an honest rate. A vendor's “your soil drains fine, over 1 inch an hour” measured on a dry hole in August is a 0.3 in/hr clay in November. The unit is the honesty contract; the presoak is the fine print.
Where to go next
- Run a percolation test — the method that makes the number honest.
- Percolation rate — what the bands permit.
- Size a swale — where the honest number goes to work.
Sources
- Minnesota Stormwater Manual, infiltration measurement — stormwater.pca.state.mn.us
- USDA NRCS, soil permeability — nrcs.usda.gov
- USGS Water Science School, infiltration — usgs.gov