All falling water
that could be harnessed and put to use.
It's debatable if this is the most efficient way to harness the energy of falling water. Compressed air is fairly inefficient as an energy storage medium. Now, if you need cool dry compressed air, bingo!
The two big controlling factors about how much energy you could harvest, is how far the water falls, and how much water falls, per minute.
Got a big river or creek on your property?
I suspect it is fairly expensive to dig a big hole 100, or 200, or 300 feet deep and install some hydraulic equipment down there.
I also suspect that there are at least 10 different government agencies that would have to approve your installation, keeping in mind they will have NO IDEA what this is or how it works.
Talk about the Department of Making you Sad...
Not saying it won't work, it will. There are working examples out there.
But there are some obstacles to overcome too.