The quick answer
Cyanuric acid does not evaporate out. Reducing a high level normally requires partial water replacement or qualified reverse-osmosis treatment.
Why CYA rises
Stabilized chlorine products add cyanuric acid. Evaporation leaves CYA behind, so topping off does not reduce concentration.
Why high CYA matters
As stabilizer rises, more free chlorine is needed for equivalent sanitation. A pool may show chlorine but still be under-protected.
Confirm before draining
Use a reliable test and consider the test’s limits. Do not plan major water replacement from one questionable strip.
Replace water carefully
Consider groundwater, surface type, structural risk, local rules and discharge location. Partial replacement is often safer than a full drain.
Prevent recurrence
Track which chlorine products contain stabilizer and test CYA periodically. Match the sanitation plan to the pool.