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 capacity should it have?
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 plastics ones, so it is worth checking into.
Capacity is a very relative subject. Here is an idea
of how much water your family needs per day for domestic use only (not including irrigation for agriculture which could require 2,000 gallons per day or more). 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 running, a larger cistern that will hold 4-6 days supply of water is a good idea. This could be a slow running spring, a ram pump that is placed under less than ideal conditions and pumps slowly, or a gasoline powered water pump (even though this pumps rapidly, having a larger capacity cistern means you won’t have to pump as frequently).
This gives you plenty of “buffer” against any surges in water usage that you might have. If your water source is fast enough to replenish your cistern very quickly and is a very reliable source, than it might not be necessary to have as much capacity. Maybe 2-3 days of storage would be adequate. But the larger your cistern is, the more “buffer time” you have to take care of mechanical problems. Consider it a one-time insurance policy. And if you plan to irrigate your garden from this cistern, it might not hurt to have two large cisterns installed. From personal experience, we know that a family of three can live very comfortably on 1,200 gallons of water for around a week. That is not even with water conservation in mind.
BUT PLEASE BEAR IN MIND...Once again, this is not taking irrigation for agriculture into account. That can be quite a consumer of water. Depending on one's climate and needs, 2,000 gallons per day or more could be required for this most essential need. Remember, your ability to grow food could depend on this.
Another use for a cistern is to act as a reservoir for low yield water sources. When using a powered pump, your source might not be able to keep up with it. In that case, install a small or medium sized cistern just below the water source to collect a quantity of water. Now your pump will not run dry until it empties the cistern.
Cisterns range in size from 300 or so gal. to 1,700 or more gal. 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.