Don't forget that you don't need any water in the bottom of the pit. Just enough for storage to allow for one cycle of generation/recirculation, be that a day or a week depending on what system you design to be relied upon for the recirc, eg solar or wind or the generated power from the system + losses, but on the off cycle, ie most likely during the night.
Even if you allowed for 200metres of water in the pit that would still be a fair fall/drop. You can calculate the volume by starting with a cone to make it easy, then extrapolate from the known dimensions of the pit, its a fair bit of water.
A potential issue that I can see is that it might be noisy, due to the megaphone shape of the pit. That and the fact that the noise would off the storage.
