alt Hacker News> It seems like we're mixing up terms here and miscommunicating. I thought we were talking about actually replacing the water here (as in, down the drain, fresh water from the tap)
We are. I changed it while you were writing.
Everything I'm seeing suggests typical loss to backwash of the filters for a short course pool is in the 5k - 10k liters of water a day range, possibly less if you invest in expensive additional filters. 5k a day is ~1.8 million liters a year.
To get below a full replacement every year for a small short-course pool, you need to lose less than 1370 liters a day, or ~1.4 m^3
You're shifting things from normally needing to fully drain and fill the pool multiple times a year from specific contamination events to now talking about routine backwashing events, which I agree do happen as a part of normal routine maintenance (a part of the "partial drain" and "good filtering" I was talking about before).
I agree you'd probably use something like 5-10k liters of water for a backflush for filters that size, but that wouldn't normally be a daily thing. The amounts needed here would be highly variable depending on a wide range of factors.
I still stand by if you're having to actually fully drain and refill your pool multiple times a year (as in, an actual drain and fill, not slow partial replacement from weekly/every other week filter flushes) you're either very unlucky or you don't know what you're doing maintaining the pool.