I don’t think it is a given that, this time, there will be a post-drought that fills the current reservoirs. If so, if extra reservoirs were built years ago, they would have been useful exactly once.

On top of that, that ‘useful’ could have been very limited. For example, shortening a half year drought by a week would not be worth anything if it just meant crops would die a week later.

You have to do some serious modeling on expected rainfall, the costs of reservoirs and water usage to figure out whether additional reservoirs are worth the investment.