Continuous evaporation.
I've never pumped water out of the pool (never enough rain to do it).

In the Summer, I add a couple inches a week of 300ppm water.

By the time I've added 15,000 gallons to the pool, I've got double in it.

I guess after 4 years, I've added 45,000 gallons.

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SWCG
15,000
Pebbletech.