The natural,loss of CYA is minimal, maybe 1-2 ppm per month. Rain and evaporation will have no effect on the value unless you pump water (with CYA in solution) out of the pool.
Now, with that being said changes in water level will cause the CYA to change, but most probably below the values we can see with our test. As an example, your pool,has a CYA of 50 and you have 2" of water evaporate. Technically because there is the same amount of CYA (CYA does not evaporate, it stays behind) in less water the CYA may go up, let's say to 52. But, as soon as you add the 2" of water back with your hose the CYA goes back to the original. 50 you had.
Rain has the opposite effect, you start with 50 and get 2" of rain. You now have the original amount of CYA in a larger body of water, so the tested CYA level will go down slightly, let's say to 48. Now as the water evaporates the CYA will return to 50. But, if you pump 2" of water out while the level is high you now have a new baseline of 48.
