Ever see the film, Jarhead? One of the military aspects that comes up is dehydration and the effect it has on performance of soldiers. You see if you look this up, the Israelis did big time studies on dehydration.

So besides all the medical studies, governments know they have to keep their citizens hydrated or severe effects occur on performance.

In soldiers it's very pronounced as they are extremely active.

In citizens, since the thirst response is not resulting in hydration, the body has difficulty with controling blood pressure. People start passing out when they stand up, they are confused, they have blurry vision, they are largely incompentent, they make poor decisions, etc.

It's a worse case scenario if there are water shortages as that's going to compound medical issues all around plus hygiene.

By the time 48 hours comes around, people get listless and stuporous. It can decline rapidly based on being very young or elderly where they lose consciousness.