Earlier quoted context omitted.
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 ne…
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…
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.