How can you find the average water temperature for an inground pool in Phoenix Arizona in October relying on direct sun only?

For Los Angeles I take the low temp for the night plus the high temp for the day add them then and divide by 2. For the summer I add 3 degrees and for the winter I subtract 3. That usually pegs it pretty close. Also for a dark bottom pool I add an additional 3 degrees during summer. Not scientific but it generally gets it pretty close. For instance today it was 83 during the day and 64 last night so 83+64/2=73.5 + 3=76.5 and 79.5 for darker bottom pools. Depth, color, and amount of sun that hits the pool all change things. Oh this is for inground pools.