Designing Your Gravity Rain Barrel Pressure Pump

Learn here about everything you need for a rain barrel water pressure calculation. Designing a gravity rain barrel pressure pump doesn't have to be difficult to be functional.

Reader Contribution by Bryan Traficante and Gardeninminutes
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by AdobeStock/schulzie

Learn here about everything you need for a rain barrel water pressure calculation. Designing a gravity rain barrel pressure pump doesn’t have to be difficult to be functional.

One of the most overlooked aspects of using collected rainwater is the amount of water pressure it requires to successfully feed from the barrel to a garden’s irrigation system. We are often concerned mostly with collecting the water, its cleanliness, and installing the collection/distribution system, that it’s easy to overlook the water movement. It’s not entirely our fault; we are accustomed to faucets that provide pressurized water practically everywhere in the U.S.

Collecting rainwater to water your garden is a capital idea. It’s affordable, practical, and a critical practice for those who experience annual, long-term droughts. Before buying a rain barrel, let’s look at a few important tips.

Collecting Rainwater: What Type of Barrel

First, you need a rain barrel. To be safer and ensure the water isn’t polluted by the material, find a food safe barrel. These are made from dense polyethylene, which has been deemed safe by the FDA. Containers made of other materials, like wood or metal, can be used but are more prone (depending upon the material) to rot and rust. Often a rain barrel will have a polyethylene core and wood/metal exterior for visual appeal and personal style.

  • The barrel should also be on the larger side; at least 50 gallons. This is for multiple reasons:
  • The higher the water level in a rain barrel the incrementally higher the pressure (we’ll explain this more in detail).
  • The higher the volume, the longer you can sustain watering.
  • Rain water adds up quickly. If you funnel water into your barrel from your roof or gutters a moderate shower can easily produce over 50 gallons of water in a ‘roof’ sized’ space. Be ready to collect!
  • Save some for the hotter seasons and drought. When water is scarce, you’ll be thankful you have a barrel full of water for your yard and garden.

Water Pressure: Without a Pump, It’s All about Gravity

If you have a water pump (generally a 1/3HP+ submersible utility pump is sufficient), then your barrel placement in terms of height off the ground really doesn’t matter. For those that are relying on gravity though, it’s important to understand the math so you can generate pressure:

A water level 2.31 feet above its exit point will produce 1 PSI (pounds per square inch). This means the level of the water inside of your barrel has to be 2.31 feet above the irrigation system which it supplies to generate just 1 PSI.

Typical household pressure ranges from 40-50 PSI. Most moderate flow garden watering systems need at least 10 PSI.

A full barrel that is elevated a few feet above the watering system will send water to it, but the irrigation system will have very reduced flow and likely uneven watering. As the water in the barrel drops, the pressure decreases further reducing the flow and ‘even-ness’ of irrigation further.

For consistent, stable water pressure that meets your garden’s watering needs (e.g. to create 10 PSI), the water level of your barrel needs to be 23.1 feet above the garden and higher for additional pressure or larger irrigation systems.

That’s right. If you want a sufficient water pressure from a rain barrel (without using a pump), your barrel’s water level needs to be 23.1 feet above your garden. This distance may seem exaggerated, but the math doesn’t lie! Using a taller rain barrel will help make achieving this height easier and you can also get creative by placing your garden in a lower spot in your yard if your landscape allows for it.

The Advantages of Rainwater

During times of drought water restrictions are often put in place to help conserve. The first thing to go in times of regulation is often water for your yard and, thus, your garden. Using a rain barrel for water collection helps buffer these times of drought and restriction if you are wary and able to conserve a portion of the barrel’s water. As a rule of thumb, if you live in a drought prone area try to keep at least a few gallons of water as a reserve in your barrel. Evaporation is a natural distillation process, effectively making rainwater clean. However, upon landing on your roof or any prior medium to the rain barrel, rain water can collect organic matter. If planning to hold water for months at a time, ensure you keep the water free of debris and as closed off as possible to reduce evaporation.

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