The human brain is 75% water - moderate dehydration can cause headaches and dizziness
Blood is 92% water.
Muscles are 75 % water
Bones are 22% water
Water is vital for body temperature regulation, therefore fluid replacement is vital. During summer when the temperature increases, your body requires even more fluid to maintain it's normal body temperature.
1-5% Thirst, vagueness. Discomfort, increased pulse, nausea
6-10% Dizziness, headache, labored breathing, walking difficulties
11-20% Delirium, twitching, inability to swallow. Severe lack of water, or extreme dehydration, when more than 1/2 to 2 liters of fluid are lost can be life threatening.
Water is heavy at 8.345 lbs per gallon. Figuring one gallon per person /per day/x4 would mean 50 million pounds of water..for 1.5 million people, not something that can be air dropped (at least in a usable form).
While many will go out and hoard bottled water, they do not realize that in most homes, you already have a 40 gallon reserve in the hot water heater, A wise, easy and inexpensive solution would be to require all tank hot water heaters to valve a simple valve to replace the drain spigot, shut off the incoming supply. and that would control a vent line to the top of the tank as well as controlling the flow through the spigot.An Interlock to shut off the burner/heater would be required as well. In addition, require the water heater be installed 6" above the floor to get a cup or pot under it.
This would allow (figuring a family of 4 in a home) more than double the requirements for 4 days.