If you have a hill, put the tank at an elevation on it that yields adequate pressure. (230 ft elevation produces 100 PSI, the maximum recommended pressure for home.) In places where there is no hill handy, you can:
make a water tower (to artificially increase the elevation)
use a small pressure tank (to pressurize water as it is needed—and have no water when the power goes out)
use a huge pressure tank (to store pressurized water at low elevation)
put a tank on your roof (and live with low pressure, like most people worldwide)
Stability of Soil and Slope
You don’t want your tank to sink into the ground, or slide down the hill. The load per unit of area from water tanks is actually quite low. A person walking can easily place much higher point loads on the soil. On the other hand, no one has feet as big as a water tank. It’s the aggregate load from a water tank—all that area being pushed on at the same time—that can push your building pad down into the gully.
However, undisturbed native soil is sufficiently strong to support even large tanks. In the case of a tank on a slope, where you don’t have a natural flat spot, put the tank on cut (newly exposed, undisturbed soil), rather than fill (freshly dumped, loose soil). For a really large tank or any tank on fill, it’s a good idea to consult with an engineer.
Aesthetics, Sacred Spots
Water tanks can be big, and although they can be beautiful, they are more often ugly. When locating a water tank, either:
put it where it doesn’t matter
conceal it well
make it beautiful
Ideally, you want your tank downstream from whatever hazards and weak links lie between you and your water source. Rivers that flood, gullies that wash out, landslides, falling trees and rolling boulders—it’s best if as few of these hazards as possible are between you and your tank.
Burying your water tank makes it less obtrusive, cooler, and more secure, as well as providing good protection from frost and sunlight. However, you probably won’t be able to install a gravity drain, so you’ll need a pump to get the water out and cleaning is more difficult. The design of buried storage is more structurally challenging, and inspection, repair, and replacement are more difficult. The buried tank will need to be protected from surface water leaking in, and protected from children and pets falling in and drowning.
Sizing Water Tanks
The size of your storage is one of the main factors that will determine under what circumstances you will find yourself short of water, and for how long. Will demand outstrip supply every morning? When there is a fire? A day after the well pump goes out? It will also do a lot to determine what your system costs.
Sizing your tank is a matter of figuring out what degree of water security you want, then finding the tank volume that makes the most of your water supply within your budget and other limiting factors. This is a good time to remember the reason(s) you want storage, as they will drive the calculation of tank size: