Storage holds water for later use. Later use can be between frequent rains, requiring less storage volume, or in the dry season, requiring more storage volume. Typical storage for built (active) systems includes plastic storage tanks (Figure 2-24), commercial rainwater tanks, 55-gallon drums (Figure 2-25), custom ferrocement (cement or lime, with sand, applied over a metal mesh such as fencing) tanks (Figure 2-26), and Intermediate Bulk Containers (IBCs) (Figure 2-27).
In all systems, it is important to think about weight, pest avoidance, and vacuum prevention. A vacuum can be introduced in a relatively sealed system by quickly evacuating the storage. Once a vacuum is introduced it can prevent the system from continuing to function. You can create an example vacuum by (1) filling a bottle with water, (2) inverting the bottle underwater, (3) raising the bottle while keeping the lip below the waterline, and (4) observing that the jar is still full of water even though it is mostly above the waterline (Figure 2-28). This water would evacuate if you drilled a small hole in the base of the bottle or if you raised the bottle above the waterline, allowing air in.
To prevent any vacuum, a breather hole (a hole above the water line that allows air to flow freely in and out of the system) should be introduced and is usually included in any conventional plastic tank. This breather hole can be double purposed with the overflow pipe if necessary (Figure 2-29). In both cases, any holes should be covered with mosquito mesh to prevent mosquitos (in applicable regions). The weight of the tank can be calculated using the formulae in Section 3.1. The weight can be critically important, as systems can fail if the storage becomes too heavy for the platform, or worse, the roof or hill on which it stands (Figure 2-30).