[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.

[0xbadcafebee]

Food collection, consumption is usually way too high at the outset of such a disaster and production later is in no way assured. Look at WWI, The Depression, The Dust Bowl, WWII. Many countries were effectively starving. Both rainfall and ground water assume you aren't having a drought.

Urban centers are not set up for large-scale rainwater collection. There literally isn't enough open space for all the people in cities to leave a pot outside to collect rain water, when it is raining, and it won't be enough water anyway. Even if every person in a city had a receptacle large enough to collect all the water they needed, many of them simply won't be able to haul it up and down stairs. It's completely infeasible.

What would actually happen is the military and national guard would be mobilized. They would use pumps and water trucks to visit acquifers and wells, and distribute water by truck neighborhood by neighborhood. A continuous stream of trucks constantly resupplying cities. If things got really bad they would get water from streams and boil it before distributing, but that wouldn't last long as it would take too much fuel and time. And they would need to completely secure the water supplies, both as a security concern, and to stop all removal of water except for what was absolutely necessary. Water isn't just for drinking and sanitation, it's also needed for a wide variety of processes, businesses, etc. Water really is a huge problem.

There is no way that gasoline, diesel, LPG, etc production/distribution would remain stable. It would be severely hampered and there would be shortages everywhere. Even during previous "normal" wars, fuel was a huge issue.

I don't know where you got the idea that heating wouldn't be necessary in winter? If you mean "humanity would survive", sure, but also a huge chunk of the population in cold places would die from cold and malnutrition over the first and second winter. Most people do not have a -20F sleeping bag, snow boots, wool underwear, etc even in cold places, because they have heating.