Distinguishing Evaporation from an Actual Leak
Not all water loss is a leak. Evaporation, splash-out, and filter backwash account for measurable water loss in any pool. The standard field test for distinguishing evaporation from a structural or plumbing leak is the bucket test: a bucket filled with pool water is placed on a pool step with the water level matched to the pool surface, and both are measured after 24–48 hours without bather use. If the pool water level drops more than the bucket, the difference represents water loss beyond evaporation — a leak is present somewhere in the system.
In Greater Boston, evaporation rates are relatively modest compared to warmer, drier climates, but uncovered pools in exposed locations can still lose a quarter to a half inch per day on hot, low-humidity days. Pools that lose an inch or more per day under calm conditions almost certainly have an active leak.