Swimming pool H2O evaporation.
If we add heat gain from sun (2540 Btu/ft^2/day or 105.8 Btu/h-ft^2 in Dagget), (460+88.9)-Tw + 105.8 = 100(Pw-0.3376), ie 688.5 - Tw = 100Pw, with Tw in Rankine degrees. Pw = e^(17.963-9621/Tw), so ln(100Pw) = 4.605+17.863-9621/Tw = ln(688.5-Tw), ie ...