Here in Southern California, we are in the midst of a series of storms that are expected to drop about 6-12 inches of rain over the course of 4-5 days. I'm using this as an opportunity to do a partial drain of my pool, which has very high calcium hardness values (over 1000 ppm). I figure: free water (with very low mineral hardness), and cloudy, humid air make for ideal conditions to protect the plaster.
Now I know that in general, it is not a good idea to completely drain a pool during the winter because of the risk of floating the pool shell. However, we live in a pseudo-desert (Temecula valley) area, and we've had less than 3" of rain in the last 8 months. The pool has a hydrostatic valve in the main drain which is supposed to prevent pressure from building up under the shell.
Given that, approximately how much water could I safely drain? The pool depth is 3.5'-5.5'-4.5' (sport pool). FWIW, when the pool was dug about 3 years ago (also in January), the ground was bone dry, so it appears that the normal water table is well below the bottom of the pool shell.
I've already drained about 10" of water, and I'd like to drain about 24-30", which I estimate is about half the volume of the pool.
Thanks in advance for any advice.