Is a Gravity-Feed System Viable with a Smaller Cistern?

Cistern Size Considerations

When determining a suitable cistern size for a gravity-feed system, it’s essential to consider the available water depth and the pressure drop required for the system to function efficiently. A general rule of thumb is to maintain a water depth of at least 6-8 feet to ensure sufficient head pressure for most residential applications. For example, a 1,000-gallon cistern with 6 feet of water depth can provide approximately 4.3 psi (pounds per square inch) of pressure, which is suitable for most residential irrigation systems.

Pipe Sizing and Head Pressure

Proper pipe sizing is crucial to maintain the desired head pressure and prevent excessive pressure drops. A common approach is to use the “pressure drop method,” where the pipe diameter is calculated based on the desired pressure drop and the water flow rate. For instance, a 1" schedule 40 PVC pipe can handle a flow rate of approximately 10-12 GPM (gallons per minute) with a pressure drop of 4.3 psi. By selecting the correct pipe size, you can ensure a stable and efficient gravity-feed system, even with a smaller cistern.