Hello Sultan Salahuddin, ...

Hello Sultan Salahuddin,

You are asking some excellent and pertinent questions here!  There are many reasons why dams and other water infrastructure are inadequate to prevent water shortages. A primary reason, as you suggest, is that an insufficient period of record for river flows is being considered, or that drought periods within a historical record were not given sufficient attention. Additionally, there are few existing dams in the world that were designed with adequate attention to climate change forecasts -- we are now learning that climate change is leading to lessened river flows and increased consumptive demands in many regions.  But most importantly, if consumptive uses of water within the source watershed area above the dam are not adequately controlled or restrained, upstream water users can deplete the river flows to the point where not enough water ends up flowing into the dam/reservoir.  

These issues are very well illustrated by the Colorado River in the Western US. Even though the big dams on the river were designed to store four full years of the average river flows, the consumptive use of water upstream of these dams has severely depleted the inflow to the reservoirs and the water levels in the reservoirs such as Lake Mead and Lake Powell are now plummeting rapidly.

There have been many good papers written about the Colorado River situation, but if you're interested in a more global and general treatment of these issues, you might find my book "Chasing Water" to be of interest.