You will find out that groundwater is disappearing in many parts of the globe. And you will find that water is a big percentage of all foods and is used in all manufacturing. Once the groundwater has been used up, we will have no choice but to get along with rainfall.
How would rational people prepare for the coming water shortage?
Rational people would reduce fertility to the point where the population would match the rainfall. What are we doing as a species? We are reproducing as fast as we can. And our numbers are still exploding currently.
It takes 60 years for a fertility reduction to show up in population demographics. Who is going to kill whom when we run out of water?