After deciphering Mark Shipiro's MIME-laden comment
contradicting my comment, it seems the Irrascible Professor
may have more reliable information than my ignorant guess
as to the usefulness of such a back-pumping arrangement.
Maybe the system would be useful *if* cheap slack generating
capacity from nuclear and *other* hard-to-quickly-throttle
sources is used to store hydro energy for peak demand times.
It sure doesn't make much sense to do it if one is only
trying to save the energy in the reservoir itself. In that
case the smart thing to do is to just throttle back the water
flow when demand is down.