Hi!
Nice looking build!
I did a quick check from the values given in your excel and it seems that the greatest deviation between air and water is maximum ~6°C at any given point (i had to translate to Celsius to get a better grip...).
What kind of humidity are you looking at during this period? I was thinking that maybe the system is working kind of like a cooling tower at the moment? It would be interesting to correlate these values to the wet bulb temperature.
It might be possible to utilize this to keep the temperature even lower in the system, but unfortunately optimizing for cooling will probably give an increased evaporation loss in the system and also increase the rate of buildup of unwanted stuff in the water.

But for a hot climate it might be an idea to give some other properties to the return flow to maximize cooling?