Dams without enough sustainable water supply

i am interested to know what percentage (just roughly) of medium large dams in the world are without enough water supply to be sustainable and what is the reason? Is it poor hydrological calculations due to droughts and climate change or  or anyone have examples in the your country?

Calculations for inflow to dam site based on previous 20-30 years of data is how reliable?

i am asking this question because there are so many dams here which are without water for a long time after construction. So can i guest this poor hydrological calculations or is this phenomenon happening all over the world?

Any supporting documents or data that you can suggest much appreciated.