There are plenty of hills much more than 100m high. People will favor using the best places for it. Efficiency of pumped hydro is always better than 70%, round

There are plenty of hills much more than 100m high. People will favor using the best places for it. Efficiency of pumped hydro is always better than 70%, round-trip, and commonly better than 85%. There is no reason to put all your storage in one place, and great reasons not to. A site moving 0.0001 km³ is useful for utility-scale storage. Of course, pumped hydro will be only one storage method among many. Birds mostly use the top of the water. Probably, the lower pond is easiest to tailor for multiple use, but "floatovoltaics" are probably better in the upper one.