When estimating how much rainwater is needed to store, it should always be kept in mind that a drought or dry-period could happen at anytime, any year. That's going to factor into how much extra water to keep on hand. It's a good idea to look up the 100-year records and study them. It's also important to find the months that get the least/most amount of rain, recurring dry-spells, drought patterns and average rainfall numbers. Some records can be found, here!
These numbers can help calculate how much water storage is needed for general use, reserve and emergency purposes. These are also important to determine how large rainwater collection efforts need to be. The goal here is to never run out of water, even during the driest of dry spells. When it comes to water, it's better to be over prepared than under prepared.
It's also better to over-estimate general water use. It's important to store more than will ever be needed. Keep in mind, when prepping for SHTF, friends or extended family members may also take a toll on water supplies, as well.
Speaking of SHTF, backup, backup, backup. In the event of drought, a crack in a tank or any other SHTF scenario, a backup source is essential. It's crucial to determine how much to put away for the absolute worst-case scenario. Even with a backup source, it's important to secure another reliable source of water, just in case.
To collect and contain all of this water, a rainwater collection system will need to be designed. This could be a 55-gallon drum underneath a gutter, or a large structure with it's own collection/storage system. It all depends on the need. There are several kinds of delivery system, including gravity-fed and pumped. A gravity-fed system doesn't usually require electricity. It uses elevation and gravity to build up water pressure. A pump will require some type of power, but there are solar pumps available.
A rainwater collection system can have several components. The first component might be a collection system(like a roof and gutter.) A filtration system may be the next part(like a downspout diverter/filter or charcoal filter,) but there's some debate on filtration for rainwater tanks. Some people choose to use a sediment tank, but again, there's some debate on this.
After collecting the rainwater, it will need to be contained in a clean area, preferably out of the sun. One of the more common ways is by using several of these plastic tanks. Again, this all depends on how much water is needed to be stored. Plastic tanks can be ordered in many different sizes, from 500 gallons to 20,0000 gallons. Some people use different types of tanks, but it's important to be sure that they're safe for potable(drinking) water.
Another method is an underground cistern. These can be expensive to install and hard to maintain, so many homesteaders go for an above-ground system. These are easier to clean, empty if needed and manage. They are also much easier to repair or replace, if need be.
With all of the different options of rainwater collection, it's important to research the best methods for the area. A desert homestead may have different needs than a coastal homestead. It all depends on the use and area!
Use this information at your own risk. I'm not an expert. It's your responsibility to do your own research and make an informed, correct decision. This post should not be taken as advice of any kind. This post should not replace proper research or professional advice.
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I am not affiliated with plastic mart at this time. I wanted to provide a link to demonstrate prices of rainwater tanks.