Gravity is free, but it is often weak. A standard rain barrel elevated 1 foot off the ground generates less than 0.5 PSI of water pressure. This is enough to slowly fill a watering can or run a very short drip line, but if you want to run a garden hose, a lawn sprinkler, or an extensive automated irrigation system, you need an electric water pump.
Types of Irrigation Pumps
When selecting a pump for a rainwater tank, you generally have two main choices:
1. External (Surface) Pumps
These pumps sit on the ground next to the tank. A suction hose runs from the pump into the bottom bulk-head fitting of the tank. They are highly accessible for maintenance and generally cheaper than submersible options.
Drawbacks: They can be noisy. Because they are rarely self-priming, they must be positioned below the tank's water line to maintain a gravity prime, or they require a foot valve to keep the suction line full of water. You must also build a small cover to protect them from rain and direct sun.
2. Submersible Pumps
These pumps are dropped directly into the the rainwater tank and sit on the bottom (or hang suspended). They push the water up and out of the tank.
Drawbacks: They are more expensive and harder to service, as you must physically pull them out of the tank through the top hatch. However, they are completely silent to the outside world, inherently protected from the weather, and never lose their prime.
Understanding Pump Sizing: Flow and Head
To choose the right pump, you must look at two key specifications:
- Flow Rate (GPM): Gallons Per Minute. How much water the pump can move at once. A standard garden hose running an oscillating sprinkler requires about 5 to 8 GPM.
- Max Head / PSI: "Head" is how high the pump can push the water vertically. PSI (Pounds per Square Inch) is the pressure. To run pop-up sprinklers across a lawn, you typically need a pump that can deliver at least 40 to 50 PSI at your desired GPM.
The Importance of a Pump Controller
You cannot simply plug a pump into a wall outlet whenever you want water. You need an automated system. A Pump Controller (or pressure switch) acts as the brain. When it senses a drop in pressure (because you opened a garden hose valve or an irrigation timer opened a zone), it instantly turns the pump on. When you close the valve, the controller senses the pressure build up and shuts the pump off.
Most modern controllers also feature "run-dry protection." If your rainwater tank runs completely empty, the controller will shut down the pump to prevent the motor from burning itself out.
Filtering Before Pumping
Always ensure water passing through an electric pump has been filtered first. Whether using a floating intake or an inline strainer, keep leaves and heavy sediment out of the pump impellers to dramatically extend the life of your equipment.