Hi
We first need to know where you are for climate details. Like in the USA they have Zones and it helps selection. I'm here in Brisbane which is a lot more humid than say Melbourne and we have different trees.
So if we know where you are we'll have a better chance of narrowing down the selection.
We're on heavy clay here too.
Now here's some interesting things, the amount of water a tree uses is pretty much proportional to the volume of canopy as the water is used when the tree transpires (a process used in photosynthesis). So trees with a sparse canopy will use less water than trees with a big full dense canopy.
Take a look at Aussie natives, most are quite sparse in the canopy compared to say a birch or elm. Also a 20' to 25' tree is only small, fruiting trees will tend to use more water than ornamentals as they have fruit to make.
Great question.
