I was actually looking for the est. loss due to UV-A (sunlight). It is my understanding that narrow-band UV-C (like that used for sanitation) does not have a substantive effect on chlorine.
Sorry I missed that. First, let me finish off the UV lamp discussion. The low-pressure UV lamp produces primarily two lines at 185 nm and 254 nm and is primarily for disinfection and not so much for oxidation while the medium-pressure UV lamp produces many spectral lines that are much broader in spectrum that are both for disinfection and for more oxidation (i.e. for dichloramine and some for nitrogen trichloride). The UV absorption peak for monochloramine is 246 nm while for DNA damage it is 264 nm. See
this link for more info, but it looks like there is a decline in Free Chlorine (FC) even at disinfection doses, but it seems to be fairly small at perhaps 10% per pass (turnover), and this is without CYA in the water. So with CYA, the loss may be quite low.
So I was wrong in assuming much higher dosing for UV. If low-pressure UV is used, then one can have fairly low UV doses for disinfection and this minimizes the amount of loss of FC. On the other hand, this sort of UV is for disinfection and will only reduce monochloramine but not the bulk of what chlorine has to deal with which is urea.
Now as far as sunlight is concerned, the half-life of hypochlorous acid in shallow water with no CYA is 2 hours and 10 minutes while with hypochlorite ion it is 20 minutes. Near a pH of 7.5 where the mix is close to 50/50, this is a half-life of 35 minutes. However, lower depths are shielded by the chlorine itself (i.e. the breakdown in the shallow water absorbs photons that don't make it to lower depths) so that near a pH of 7.5 at 4.5 feet in depth the half-life is around 1 hour. Basically, you can figure that without CYA in the water, most pools in direct noontime sun would lose about half the FC every hour. So over a day with equivalent of 6 hours of peak sun (14 hours of daylight total) this is a 98% loss of chlorine so with other fixed non-sunlight losses, it essentially wipes out the chlorine in less than a day. If you're really interested, I can send you a spreadsheet on this with the derivation using spectral curves (PM me your E-mail address).
With CYA in the water, the half-life is significantly extended, but this is where things get pretty dicey since there isn't really good data that is consistent with what we've seen with users on this forum (i.e. the industry data seems to be wrong). For full-day sun on a pool in the summer, the daily FC loss at 30 ppm CYA can be up to 60%. At 50-55 ppm CYA, this can be up to 30-35% while at 80 ppm CYA it gets to around 20% and at 100 ppm CYA it's around 15%. These are very rough estimates and will vary depending on number of hours of sunlight, time of year, latitude, pH, etc. [EDIT]
Mark's experiments showed a greater difference where there was a 50% loss at 45 ppm CYA and a 15% loss at 80 ppm CYA. Every situation is a bit different so these are just rough guidelines. [END-EDIT]