I will be building a small pond, primarily to store water. Here in Colorado, the intense sun and dry air are very efficient at evaporating water. I will be filling a large area of the pond with big rocks, underlaid with large diameter PVC pipes to prevent this. However, there will still be an area of exposed water.
If I covered the surface with water lilies, azolla, duckweed, water hyacinth, etc., would this slow down evaporation? Or would the plants respire more than would have evaporated in the first place? Of course, the plants could be useful; pure evaporation would maybe cool down the surrounding temps a little, but so would passing the water through plants. So maybe it doesn't matter so much. However, I would like to know.
Many water plants will transpire much more water than would be lost from an equivalent area of water. I've heard of a 6 fold increase in water loss with water hyacinth. Trees for shading may be a better option.
I would second Dales comment and add that looking at the ratio of surface area to volume would also help . A very small deep pond will loose less water than a larger area not so deep pond of the same volume of water
Living in Anjou , France,
For the many not for the few
On my pond, we have been trying to keep summer temperatures down as much as possible. I found that greenhouse shade cloth is pretty cheap for large areas and would allow rain through. I second the notion about going deep, however it is a GOOD IDEA to hire this out to a knowledgable pond builder as there is a limit to how deep you should go on a given terrain, otherwise your water will just go through the substrate into the ground.
Lining the pond is a real issue as well, you need to think about that. If water conservation is your first and foremost goal, I would consider a synthetic pond liner that is professionally installed. Luckily we had clay and lots of it, as well as tons of water, so we do not have those issues, just the sun heating the water up more than I would like.
Good luck, and it is far better to take your time and put a project off to do it right than rush into something like this and regret a decision later!
Ajila Ama Farm Western North Carolina
The Greenhouse of the Future ebook by Francis Gendron