This is pumped hydro to store energy, it's a bit different...

This is pumped hydro to store energy, it's a bit different because the same water is being used.
As long as the two reservoirs are properly sealed and evaporation is reduced to a minimum it's not a bad system. Batteries would be more efficient but cost more and with less lifetime, currently.
The water could be sealed by cheap floats or better still, floating solar panels.