There are really three main questions to decide in regard to a water cistern:
- Should it be above-ground or underground?
- Should it be concrete or plastic?
- What size (capacity) should it be?
Above or Below?
First, unless you live in an area that absolutely will not have any deep freezes, it only makes sense to have an underground cistern. Also, UV rays will deteriorate plastic tanks that are above ground. And that is not to mention the risk of a tree falling on it, a tractor running into it, etc.
Plastic or Concrete?
A concrete cistern should last longer, but is more expensive and a lot more difficult to install. Underground, a plastic cistern should work just fine as long as you don’t drive anything heavy on top of it. Just be sure to buy an underground “water cistern” rather than a “septic tank.” The two are almost identical but there is one big difference – Water cisterns are constructed with food-grade plastic whereas septic tanks are made from non-food-grade plastic. Some may have health concerns regarding any plastic, so if that is the case and you don’t mind spending a little more for the best, then go with concrete. We have even heard of concrete cisterns costing less than plastic ones, so it is worth checking into.
Capacity is a very relative subject that depends on your water system and how much water you consume. Here are a couple of examples…
Example 1: Solar Direct Pump - f you have a solar-direct water pump (i.e. no batteries or backup power) in your well, it is only pumping during daylight and only at maximum speed when it's sunny. Because of that, you could have days (or perhaps even weeks) when little to no water is being pumped to your cistern. And in that case, you would need a very large amount of water storage, capable of supplying you with water for as many days as necessary to carry you through the cloudy times. This is especially true in locations where sunny weather is extremely dependable and is why I only recommend solar-direct pumps in incredibly sunny areas.
Example 2: Conventional Setup - On the other hand, if your water pump has access to power 24/7 and it just needs to keep up with your water usage, you could get by with a much smaller cistern (assuming the water source is producing enough water).
So your first step is to determine what your water usage will be during the hottest & driest time of year.
Here is a simple calculator for getting you started with a rough idea of your average daily water usage. After you know this, it will be necessary to know what type of water system will be used. For applications where the water source is slow (low yield), a larger cistern that can hold several days of water is a good idea. That extra capacity enables you to handle times of higher demand (like irrigation) while allowing the collection of water from the slow source 24/7 so you can maximize the output of the well or spring.
Once again, if your water source is fast enough to replenish your cistern very quickly and is a very reliable source, then it might not be necessary to have as much capacity. Perhaps even just a day or two of storage would be adequate. But the larger your cistern is, the more “buffer time” you have to take care of mechanical problems or absorb times of high use. Consider it a one-time insurance policy.
From personal experience, we know that our family of 4 can easily live on 200 gallons a day for domestic use. This fits in with the conventional estimate of 50 gallons per person per day.
BUT PLEASE BEAR IN MIND…Once again, this is not taking irrigation for agriculture into account. That can be quite a consumer of water. If you have a large orchard and garden to irrigate (as well as a lawn), you could use 2,000 gallons per day or more when it's hot and dry. True, there are many ways to reduce irrigation water usage so that it's far less than typical. We've done that successfully at times out of necessity, but it's better to plan on having more than enough.
Cisterns range in size from 300 or so gal. to 1,700 or more gallons and can even be tied together in multiples if needed. Food-grade plastic water cisterns can usually be ordered from farm/agricultural supply and major building supply stores. Be sure to specify above-ground or below-ground plastic cisterns when ordering, as they are engineered differently. Concrete cisterns usually can be purchased locally at concrete septic tank manufacturers and can be delivered to your building site.