On your garden variety potted plant, say a small ficus tree or whatever,
how much of the water it is given gets evaporated from the top of the
soil?

If substantial, what are good ways to prevent this?

I travel a lot so I'm trying to come up with ways to reduce watering
needs. I've heard of people putting rocks on top of the soil to reduce
evaporation. And I was thinking that I could fit a plastic picnic plate
to the top of the pot. But then there would be a dark and damp place
between this and the soil. Might mold grow here?

What's the best way to reduce evaporation losses?

Thanks for you help.