Water is simply not an issue in my area, and I pray I never have to eat those words. That said, we do not take it for granted. Annual precipitation is 34" per year. We do have a catchment system for rainwater, and can easily add more.
Storage: My short term goal is to have 150 gallons on hand at all times. This would be about a one month supply.
Processing: We have the means to both filter, boil and store all of the water we can collect.
Surface: There is a creek within 100 yards of my property with easy public access. There is a lake about 1/2 mile away, but that is on private property. There is a river about 1 mile away with public access.
Wild Card: Supposedly my house was built over a spring. I have no idea how far down I would have to dig to find it, and I am certainly not going to tear up my basement to look. We do have a sump pump, and it runs occasionally when the weather is rainy. If the power went out, and the sump pump didn't run I might have more water than I could have ever imagined. I hope I never find out.
For those that live in really dry climates you might want to look at how the Israeli's live in a desert. They have done remarkable things managing limited resources.
https://www.jewishvirtuallibrary.org/israel-s-chronic-water-problem