Factors in evaporation rate

[2poor2reef]

One again, I thought I understood how something worked in this hobby and it turns out I don't.

I have two systems. One has a 15g main and a 10g refugium attached via a common 10g sump. The other is a sumpless 10g. All tanks are open. The larger system has 250w mh lighting with a LOA 65w over the 'fuge. The 10g is lit with 65w LOA only. All lighting is pendant stype. I run a skimmer on the larger system and it turns 25X per hour. I run a tetratec powerhead on the sumpless 10g. Here's what I don't understand:

The larger system has over twice the total water column and almost three times the surface area of the smaller system. The larger system also has stronger water movement and lighting. The tanks are in the same room so airflow, temp and humidity are identical. I use two dosing pumps, same model, to replacem evap water in the two systems. The larger system evaps 60 m/l per hour and the smaller system 33 m/l per hour.

Why would the system with almost three times the exposed surface area experience less than twice the evap rate? Both dosers are medical grade and I have switched them with the same results. Also, the ratio holds regardless of season. Any thoughts?

 

[Reef Fever]

I'm sure you know this, so forgive me, but I notice that you don't mention temperature. Temperature is probably the biggest factor in evaporation. Hotter tanks empty MUCH quicker. Even if you are not running a heater, there may be big differences in temp from different pumps, etc-

Could this have anything to do with it?

 

[2poor2reef]

Thanks reef fever, I should have mentioned that I run both systems at 80 degrees. I have checked them with the same thermometer and they are consistent. That doesn't mean that they fluctuate in exactly the same way as the sensitivity of the heaters my be different and the time it takes each system to adapt to ambient room temp changes may differ slightly, but nothing major.