What does the location of the reservoir matter? If you're taking a river and pumping it up on top of somewhere (a mesa in your example) aren't you going to cause the river to basically run dry everywhere downstream of where your pump is?
There's no river at the bottom, either. I thought the "desert" part would make that clear, but I guess it had to be said.
Water in this system is part of the capital cost (charging it up when you start), then a minor maintenance cost (replacing evaporation). Otherwise, unlike primary hydro, there is no large constant flow out of the system, and no need to be on a river.
The water lost to evaporation is at least an order of magnitude less than water evaporated from a nuclear plant of the same levelized power.
I think the confusion is a couple of comments up you said "There's a plan to put a reservoir for one on top of a mesa in the desert in Arizona" which is really confusing since of course they're planning to put a pair of reservoirs for one in the desert in Arizona. I guess only one is on top of the mesa though.
