Evaporation's Effect on Pool Chemistry

Yes—evaporation is the opposite of rain: it removes pure water and leaves everything dissolved in it behind, so cyanuric acid, salt, calcium hardness, and total dissolved solids all trend upward over time even though nothing was added.

Why it happens

When water evaporates, only H2O leaves the pool -- dissolved chemicals (CYA, salt, calcium, and other minerals) stay behind in a smaller volume of water, so their concentration rises. Topping off with fresh fill water restores the volume but not the original dilution, because that fill water carries its own baseline hardness and alkalinity on top of what's already concentrated. Warm climates, wind, low humidity, spa jets, and water features all increase evaporation rate, which is why pools in hot, dry, or windy conditions tend to accumulate CYA and salt fastest and need more frequent partial drains.

What to do

• Track CYA and salt periodically, not just chlorine and pH -- these are the readings evaporation concentrates most noticeably over a season.
• When CYA or salt climbs above target range with no chemical added, a partial drain and refill is the standard fix -- see the CYA levels chart for specific thresholds.
• A pool cover reduces evaporation significantly and slows this concentration effect, alongside the other seasonal benefits covered in winterization guidance.

Safe ranges / timing

There's no separate "evaporation-safe" range -- the goal is keeping CYA, salt, and calcium hardness within their normal target ranges regardless of why they drifted. Retest these less-frequently-checked parameters monthly during high-evaporation conditions (hot, dry, or windy stretches) rather than only when chlorine performance seems off.

Calculator

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