Can't speak for Hungary specifically, but the problem with flat terrain wrt hydro storage isn't just elevation difference - which can be offset with flowrate - it's the large surface area to storage and to depth ratios.

Large lake means more losses, mainly due to evaporation, but seepage is also to be expected. Now put this fact in the thread's context: water shortage/stress...

> Now put this fact in the thread's context: water shortage/stress…

But doesn't adding a large reservoir mitigate both? You're not pumping the entire reservoir up and down hill every day.