Cisterns have been used for water storage for thousands of years and continue to be used today. A cistern is a large water storage container that is often underground. Many of you will remember Masada where the Roman Legion had the Jews besieged. This mountain top fortress was able to hold out for as log as they did, in part, because of the large cisterns where they stored rain water. In fact without cisterns this would have been nothing other than another uninhabited mountain.
These water storage tanks can range up to thousands of gallons, or liters if you prefer. The size of your cistern should be determined by your water usage and the water source. If your water source is seasonal then a large enough capacity to get you through the dry spell would be real nice. A cistern can be above ground, below ground or partially buried. This storage is something of a midpoint in you water system sitting as it does, between the collection and distribution systems.
I grew up in a community where thousands of homes collected their water off the roof and stored it in a cistern. I have seen, used, and built many different cisterns. The first one I actually put together was an inexpensive above ground pool. We made a level spot near the eve of the roof and ran the down spouts from the gutter into the pool. While today I might question whether the plastic liner was appropriate for potable water, back then the question never came up. We drank from that pool for years and it didn't affect me… affect me… affect me. Actually because of the price and ease of installation this type of cistern became fairly popular around the community for a couple decades. While they will last for a few years the plastic eventually deteriorates in the sun or the thin metal sides rot out so this is not a permanent solution. In a SHTF scenario your down spouts could be run to your in ground pool to collect what ever rain you do get and replenish what you have consumed. If this becomes part of your plan you might want to secure and store adequate downspout and/or pipe.
Another popular way to build a cistern is with a ten foot length of culvert. The suppliers would nest these starting with an eight foot culvert inside a larger and larger culvert till the largest was about twelve feet across. This greatly reduced shipping cost. Since the freight company cubes something like this you are essentially paying freight for only the largest culvert. The culverts need to be manufactured in such a manner as to have water tight seams. Delivered laying on its side it could be transported on the road with little problem. When placed on a low trailer the twelve foot height would fit under the power lines and the ten foot width was legal. The process is to dig a flat spot larger than the culvert to a depth that the top of the culvert will be lower than the eve of the house. You then make a form for your concrete and place reinforcing inside the form. Pour and level your concrete. Tip the culvert into the wet concrete and vibrate it to create a seal. The culvert should set so that it is four to six inches into the concrete. After a week or so the concrete has cured enough to start filling your tank.
The tank off an old water truck was a quick answer in that it only required a flat spot. I would expect an old milk truck tank to work as well. A local mill had been serviced by a four foot diameter wooden water line. We wound up with a twenty foot section and built ends in it.
A friend of mine built a tank out of plywood and put a plastic liner in it. He started with eight sheets of plywood. Standing up two on each side he attached 2X6's every foot from the bottom past midway up then spaced them further apart. The 2X6's were laid on their side, run past the plywood and bolted to the intersecting 2X6's. This is a relatively inexpensive tank but be aware that eight feet of head generates quite a bit of pressure at the bottom so do quality work. Stringers tying the bottom sides together are essential as well as the top.
The newer systems often choose the plastic tanks made for that purpose. The largest of these are cylindrical. A buddy of mine had room to place two, five thousand gallon tanks behind his house. There was a small ledge on the hillside next to these that allowed him to place another two thousand gallon tank. With twelve thousand gallons available they can go quite a while without rain.
My personal favorite is to build the cistern as part of a concrete foundation. This requires a foundation of at least four feet tall to get adequate volume. A full basement would be even better. If this is the way you go I strongly suggest that you design the house so that no sewer lines run above the water tank. This leaves your entire water system accessible inside the house and protected against freezing.
One of the problems with outside water storage is the possibility of freezing. I had an eighteen hundred gallon plastic tank freeze solid one winter with no apparent damage. It was not in current use and had been filled without my knowledge so I did not know to empty it. This tank had also been sprayed with four inches of insulation so it took over a month for it to thaw completely in the spring. Insulating a tank can help as can putting it in a shed. Two or three wraps of PEX pipe around the outside near the bottom before you spray the tank works well if you have a boiler. Your outside water storage could then be another zone off the boiler. My outside tank has seen -40°F with no problem. Okay, maybe a few problems but I worked them out.
If you do not have really severe winters a heat tape on a Hula Hoop will keep your tank from splitting. Just a heat tape on the water line will leave an open passage that allows the water to escape out the top if the ice expands reducing pressure on the tank walls. You still lose that volume of water that turns to ice. At least until it warms up. We had a particularly long stretch of cold weather this year and a neighbor of mine ran the water from his water heater back into his tank to melt some of the ice and reclaim some of the lost volume. You can also put a purpose made electric heater in your tank. If the bottom of the tank is buried below the frost line freezing problems are greatly reduced. These are some of the heat sources at your disposal if you opt for outside storage.
You might also want to consider PEX for your water line especially outside or any other place that is likely to freeze. PEX has a memory and will return to its original shape after it thaws. Copper will stretch until it ruptures, usual between the first and third freeze. Not only is it expensive to replace water lines but the time required is a factor as well.
If you collect rain off your roof the roofing material is an important part of the system. Metal roofing is the best as it sheds water faster and does not retain as much as other materials. Three tab works but it holds a surprising amount of water and in a light misty rain it takes a bit before it starts dripping, where a metal roof might shed some water in a fog or when a frost thaws. Some three tab shingles are also built with chemicals that I am uncomfortable with but most of the roofs that I have seen collect drinking water are of this type. Cedar roofs are of particular concern. Cedar is toxic so special care must be taken with a cedar roof. I lived in an area with heavy rain. Those people who wanted to collect from their cedar roof waited for over a year with a new roof to allow the rain to flush most of the oil from the surface of the wood. This community is in the middle of a rain forest with thousands of homes collecting rain water.
While I have run into people who look at me like I have a third eye, when I discuss drinking rain water, I consider rain water generally safe. What I like to call God distilled water (rain) is generally free of contamination with some rare exceptions. Were I down wind of a frisky volcano or a forest fire I might redirect my down spouts for a while. City water can become contaminated as well. How many times have you heard news reports where the community has been told to boil their water. I worked with a man who was replacing his copper water lines because his wife was having a reaction to the copper. As long as reasonable care is taken with the construction, material selection, and maintenance rain collection and a cistern is a viable option in many climates.
I have seen cisterns filled by wells and wind mills. If you had a hill above your house you could also place your cistern at a useful height to provide water pressure for your home. If you have a stream on the property you could use a hydraulic ram pump/water hammer pump (clacker) to fill your cistern. This system could give you water and suitable water pressure with no electricity.
If you decide to haul your water in a large tank in the back of your truck or on a trailer make sure the tank is full. If your vehicle won't haul the weight of a full tank get a smaller tank or larger truck. Most tanks are built without baffles and when you get the weight of the water slamming back and forth you can have all sorts of problems so it is best to travel with a full tank.
We used bleach about once a year to kill what ever might be growing in the cistern. The chlorine smell for the next two or three days was a bit much, but it worked. I preferred in the summer when we ran low and a truckload of city water was purchased. This was already chlorinated so the tank was sterilized but with far less odor.
While a gravity collection system is preferred I have put smaller collection containers (50 to 200 gallons) under the down spouts and then used a sump pump to fill the larger tank. This method is most often incorporated when adding an out building to the collection system or when the tank can't easily be placed below the roof line. I've seen the power go out and pumps get old but somehow gravity keeps working so that is my preferred method whenever possible.