cyanuric level dropped?

I was able to use the fantastic advice learned here last summer to turn my slimy, green pond into a sparkling, clean swimming pool. I gradually got the CYA level up to 95 by Sept. to stabilize chlorine use. I only tested FC & pH over the winter. All was well until last weekend when we got such heavy rain that it flooded my pool with dirt & debris. It took a few days to get the water clear again. I decided to do a complete test with these results: FC=8, pH=7.4, TA=70, CH=310, CYA=33. What happened to my CYA? I thought it could only be reduced by water replacement?

Generally that's true, dilution is the main way to lower CYA. However, extreme algae and contamination of the water can also break it down. Not sure why yours has gone down so much other than a possible testing error.

My cya falls to almost 0 over every winter in NE fl. With cold temps I don't do too much but maintain cl and pH levels, the cl stays high with little work, and the pool is relatively clear all winter, we dont' get enough rain to dilute it out that much . I usually end the swimming season with a cya level of about 50-60. I'm not sure where it goes, but I assume it's getting consumed either chemically or biologically. I usually wait until all the oak pollen is done and the oak pollen pods are done before adjusting. I have to use a lot of liquid cl to get the stain from them out of the water and don't want cya levels making it necessary for more cl than need. After things are done falling from trees, I jack the cya back to normal and things stay pretty good all summer and fall. I many need to add smaller amounts of cya over the summer but not start from 0 like in the early spring. I just consider it a fact of life with an outdoor pool and no screen enclosure.

CYA is typically destroyed over the winter by bacterial bio-degradation. If you maintain a slime and algae free pool over the winter, this will not happen. If you do get slime, you will probably lose your CYA.

Otherwise, the only way you lose CYA is through water leakage or splashout. Evaporation does NOT cause you to lose CYA.

Keastman, you should NOT be losing your CYA *if* you maintain chlorine 100% of the time, with no slime. However, even a week of low/no chlorine can be enough to cost you your CYA.

Once your CYA is gone, you need to find out whether you have a little problem (no CYA => need to add some) or a big problem (no CYA *and* high ammonia => need to add HUMONGOUS amounts of chlorine to clean up!).

If your pool is clear, the easiest test is simply to add 2x a normal dose of granular chlorine or of bleach (whatever is normal for your pool) and see if you get the expected FREE chlorine rise. If you do not get the expected rise, OR if you have a slimy pool, you can use the chlorine demand test:
http://www.poolforum.com/pf2/showthread.php?16973
to find out just how bad the problem is.

If you do have a problem, be SURE to use borax to raise your pH above 7.6, BEFORE you start adding lots of chlorine. Continue to use borax to keep the pH above 7.6, until the problem is resolved.

The issue that you need