If the water falls below the skimmer, the pump can suck air and ruin a seal. Topping up is ordinary work in a dry valley summer.
Where the line should sit
Mid-skimmer is the working line for most residential pools. Auto-fill valves stick; do not trust them after a windy week. In winter, leaving the pool full protects the shell. In a heat wave, evaporation of a quarter-inch a day is not unusual. Add water slowly so chemistry does not crash, then retest on the next visit.
Evaporation is not a leak — until it is
A typical Southern California summer can take a quarter-inch to a half-inch a day off an uncovered pool, more with wind and a spa that sits uncovered. That is weather, not a broken pipe. The bucket test still belongs here: if the pool drops faster than the bucket over a quiet 48 hours, we look for a leak before we keep filling. Keep filling a hidden leak and you can wash soil out from under the deck.
Auto-fill valves
A stuck-open auto-fill over-dilutes chemistry and can overflow a spa into the yard. A stuck-closed valve lets the water fall below the skimmer. We check the float and the shutoff on the bi-weekly visit. If the valve chatters, the washer is often worn; replacing it is cheaper than a flooded equipment pad.