3: Pressure

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Pressure is critical to moving water from where it is gathered or stored to where it will be used. In a rainwater harvesting system, the water must be able to flow from catchment through the filters and conveyance into the first-flush and storage or end use. This pressure can be provided by gravity from vertical height difference or by a pump.

As an idea of how much pressure you will need, typical US residential water pressure is between 40 to 80 psi (pounds per square inch), typical drip irrigation systems (and some micro-sprinklers) need between 15 to 25 psi, and some appropriate technology drip irrigation systems need only 4 to 10 psi.

In addition to flow from the catchment to the storage, the flow from storage to end use is critical. Using the existing topography and/or platforms can often yield enough pressure for end use. If necessary, a pump can be implemented to add sufficient pressure. While utilizing a pump increases the pressure, it also increases the upfront and operational costs.

Gravity acting on the vertical height of the water column is what produces the pressure, which is also referred to as the head. Make sure not to confuse volume with pressure (head). For instance, a 20-foot-tall water tower of 8000 gallons has the same water pressure as a 20-foot-tall pipe of 80 gallons. To illustrate this effect, picture (or build) a U-pipe with two different diameter legs and a valve in between them (Figure 3-1). Then fill the larger diameter pipe with water. Then see where the water level will be once you open the valve. Since water pressure is independent of volume, the levels should be equal to each other (also referred to as “water seeks its own level”).

Another experiment to convince yourself of water pressure being dependent on height and not volume is to feel the pressure at the bottom of a 10-foot-deep pool compared to 10 feet deep in a freshwater lake. Those two pressures should feel the same. Therefore, the head available in a system can be determined by measuring the available heights and applying basic physics or a standard conversion factor.

3.1: Mass and Weight
Before we calculate pressure, let’s calculate the mass of our storage. As stated before, weight is critically important as systems can fail if the storage becomes too heavy for the platform, or worse, too heavy for the roof or hill on which the storage sits. In addition, knowing how to calculate the weight of storage leads well into calculating pressure, which is force over area where the force is the weight of the water.
3.2: Calculating Pressure
Pressure is critical because it is what moves water from one point to another. A common constraint when designing a rainwater harvesting system is providing enough pressure for the end use. For systems with low roofs or challenging topography (e.g. the catchment area is in a depression), sometimes sufficient pressure can only be obtained with a pump.