[hollerith]

>Reserves of fuel, food, and clean water will not last nearly that long [i.e., years].

Untrue. A survey done by nuclear-war planners in the 1980s found that there is enough food stored on or near farms to feed half the US population for about 3 years. During peacetime, most of this food is fed to farm animals, but there is no reason it cannot be used to keep people alive instead.

This food, mostly grains and soybeans, must be milled to be nutritious to people, but the (diesel-powered) equipment to do the milling tends to be stored near the food, so the trucks that bring the food from the farms to the population centers can just bring the milling equipment, too (and maybe the equipment for toasting grains, which I understand is widely done to grains fed to farm animals).

Water is continuously falling out of the sky and can also be obtained from underground.

[lunar-whitey]

You cannot sustain a population center of any size on rainfall. For that, you need electricity to operate municipal pumps (which will likely be absent, as above - if not immediately after generation and distribution is destroyed, then a few weeks later when fuel for the generators is exhausted). Without that the bulk of the population is dead in under a week.

Repurposing feed grain reserves is interesting, but you need fuel for that as well (plus significant coordination, which seems unlikely in a scenario where many people with the required knowledge and authority may be dead and telecommunications infrastructure is destroyed).

[hollerith]

The average person in the US uses about 80 to 100 gallons of water per day. Of course, just to survive, he or she would need vastly less than that (mostly for cooking and drinking, a little for washing and rinsing wounds). A person can of course survive for years without bathing or showering. Indirectly, the average consumption rate is a lot more -- about 2000 gallons. Food production accounts for about 90% to 95% of that, but again during the months and years right after a massive nuclear attack, the survivors of the attack can obtain most of their protein and calories from food grown before the attack. So, you can sustain a "population center" on rainfall, ground water and surface water especially of the members of the "population center" can relocate if they find themselves in a place (Phoenix AZ?) where there is not enough water to go around.

Fuel is similar: the amount currently used by the average person is much higher than the amount needed (i.e., to transport food and other essentials) just to keep people alive until our industrial base can be reconstituted enough that survival becomes easy again, so we can expect to be able to survive for a few years on fuel that was produced before the attack. Most motor vehicles will probably survive the attack, for example, according to analyses made by US war planners during the cold war, and the fuel tanks of each of them will on average be about half full even if no warning of the attack reaches the general public. Home heating is not strictly necessary for survival except maybe on the coldest nights of the year, which is good because I doubt there is enough firewood in the continental US to keep the survivors of the attack warm every night for a few years.