Rainwater collection

Why do you think it's a short term solution during wet months? Collected water can be stored and used during dry seasons.

Well, it isn't necessary for rainwater to be purified for most uses. You don't need purified water to flush the toilet or water the garden, or wash your floors or the laundry, or even shower unless you are going to get the water in your mouth. Rainwater doesn't need to be purified to wash your dishes- it's not like well water that can be contaminated by nearby septic systems or agricultural chemicals.

If the rainwater catchment, pipes and storage are kept clean, rainwater is pretty safe stuff anyway. For years, in the summer, I have put my empty garafons under the rainspout that drains off my roof and balcony and drunk that water, with zero ill effects. I wait until it's been pounding rain for an hour, so any dirt on the roof and balcony are well washed away.

I started doing it years ago one summer when we had a big flood that washed out our road and broke the water mains. Not only was there no water coming through the pipes, the road was completely impassable for 3 weeks- you couldn't drive in and out from our area, so I couldn't get garafons. I caught water in pails for everything else.

I would only be concerned about rainwater if I lived in a heavily industrial area, acid rain and such.

Yes, I realize that, but it doesn't make much sense to me. Rainy season is when ground water levels rise and wells fill up- it's not the time of year when there are water shortages.

I've been thinking of doing much the same. Where I live here in Sayulita, the town has been super low on water since the beginning of April. They are only sending dribbles to my area and I've had to buy 2 pipas of water and be super conservative. The water trucks are going back and forth all day all over town.
If I dug up the area just inside my gate, under where I park, and buried a 10,000 litre tank that got filled in the rainy season, I could get through the times of no water in the late spring, early summer.

It sounds wonderful and it was our intention to do this when we built our house in San Pancho, Nayarit. The very real problem is that the rainfall here (and there) all falls in a very short time frame and requires a HUGE storage facility. A storage facility that is most likely far greater in size than your house itself.
1)

A one-inch rain will collect 600 gallons from a 1,000 square foot roof

https://waterwise.arizona.edu/ways-save-water/harvest-rain#:~:text=Think%20there%20isn't%20enough,lot%20will%20receive%202%2C800%20gallons!
2)

Volume 1 US liquid gallon = 3.78541 liter

3)1 Cubic Meter = 1000 Liters This means that there are 1000 liters in one cubic meter. To determine the number of liters in a cubic meter, we multiply the value in cubic meter by the conversion factor. How many liters in a cubic …
calculatorology.com
4) The last two years here, there has been an average of 40 inches of rain in the season. https://www.ajijicweather.com/wxrainhistory.php

Assume you only have 1,000 square feet of roof surface that can be harvested. On that surface will fall 600 gallons x 40 inches = 24,000 gallons of rain or 24,000 x 3.78 liters= 90,720 liters of rain. 1,000 liters of water in a cubic meter means that you have to build a storage facility of 10 meters width by 10 meters length by 9 meters deep. No matter what size of storage you actually provide for rainwater, make sure you have an overflow that can divert the water away from your house because you will overflow the space you have provided.

That presumes that you would want to capture and store all of the rainwater that falls on your roof.
The average water usage per day for the average American is 250 litres. (and I'm not nearly as wasteful with water as the average American) That means 90,720 liters would last for 363 days, an entire year.

I don't need to store enough water for a year. I'd just need storage for when the city wells run dry or their pump breaks or some idiot construction project breaks a municipal water pipe. Once whatever size rainwater tank I used was full, I would disconnect the pipes. 10,000 litres of water would last me for 2 months, at least.

I hear ya. I never draw from my cistern unless there's a problem...municipal or other. The street water comes in with enough pressure to take it up to the tinaco on the roof of my one storey house. There have been a few times where the tinaco became empty and I have had to draw from the 6,000 liter cistern and that is achieved with a simple 1/2 HP by-pass pump. Flip a lever, turn the pump on and up she goes to a 750 liter tinaco.
Sometimes, the filter that I have on the street water becomes clogged earlier than normal and that will also prevent it from getting to the tinaco. That involves changing the filter before drawing from the cistern. Either way, it's a good and simple system.
I also use very little water.