What is a well yield test?

What the number on the log is

When a driller completes a well, they measure how much water it produces during a test and record that figure in gallons per minute. That value goes onto the well log and into the state database, where it becomes the permanent record for the site.

The critical detail is duration. Many of these are short tests. In the Montana records we have loaded, a substantial share ran two hours or less, using methods including air, bailer, and pump tests. A two-hour test establishes that the well produced at that rate for two hours. It does not establish that the well will sustain that rate through a dry August with neighbours pumping.

This is why our reports display the actual test durations and methods behind the yield figures they show, rather than presenting the numbers stripped of that context.

Why sustained capacity is a different question

Long-term well capacity depends on the aquifer's ability to transmit and store water, on seasonal recharge, and on how much other users are drawing from the same source. None of that is captured by a short completion test.

A well can test at 25 GPM and later struggle in late summer. Conversely, a modest recorded yield may be perfectly adequate with appropriate storage. What matters for a household is not peak instantaneous rate but whether daily demand can be met reliably across the year.

Yields that matter in practice

A few thresholds are worth knowing when reading nearby yield data:

• Under 5 GPM — low. Usually workable for a single household with storage, but tight, and some lenders and appraisers will ask questions.
• 5 to 10 GPM — modest. Generally fine for domestic use; limited headroom for irrigation or livestock.
• 10 to 20 GPM — comfortable for a household with some outdoor use.
• Above 20 GPM — good domestic supply with real margin.
• 35 GPM — not a hydrology threshold but a legal one. It is Montana's exempt-well flow limit outside a stream depletion zone.

What to ask for on a property with an existing well

If a property already has a well, the recorded completion test is the starting point, not the conclusion. A longer-duration pumping test performed for you gives a far better picture of sustained capacity, and it can be arranged as part of due diligence.

For a property with no well, nearby yield records are the best available signal — which is exactly what our reports compile, along with the share of nearby wells falling below each threshold above, so you can see the distribution rather than a single reassuring average.