Yes, Theo, I believe your answer is plausible. Your idea led me to do a bit more research and I found this somewhat related link...
http://van.physics.uiuc.edu/qa/listing.php?id=1440 It states that evaporation rates are dependent upon: temperature of water at air/water surface, humidity of air, area of air/water surface, temperature of air, water currents convecting heat, and air flow past the air water surface. The site above stated that there is a tendency to raise humidity at the air/water surface and thus reduce evaporation.
That made me wonder if bottom diffusers could actually slow down evaporation in a pond by reducing surface temperature? Or, would the additional currents created in the water counteract any reduced temperature benefit? Could active efforts to lower water temperature reduce necessary watershed size, or would the reduction be negligible? Actually, this is mostly wishful (some might say desperate) thinking from someone who just needs a good rain
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