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Calculating water pressure for your rain barrel

Collecting rainwater in barrels for outdoor use is an appreciated practice. 

As interest has grown in home gardening and locally grown food, so has the popularity and interest in rain barrels for garden watering. However, our attention is often focused on collecting water, its cleanliness, and installing the collection/distribution system when it should be on 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 makes a lot of practical sense. It’s affordable, easy to setup, and extremely valuable if you experience annual, long-term droughts. Before buying a rain barrel to connect to your garden’s watering system, let’s look at a few important tips regarding the Barrel, water pressure and rainwater advantages.

Collecting rainwater: What type of barrel

First, we recommend using a food safe barrel. Typically, these are made from dense polyethylene, which has been deemed safe by the FDA. Other plastic barrels can be found, however, not all are food safe rated. 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 to provide visual appeal and personal style.

If you decide not to use a sump pump in your rain barrel to connect to your garden hose, ensure the barrel you purchase/make has a garden hose connection point. 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 (This is important - we’ll explain in detail below).
  • The higher the volume, the longer you can sustain watering.
  • Rainwater adds up very quickly. A moderate shower can easily produce over 50 gallons of water if funneled from your roof into the barrel.
  • Larger barrels provide a greater reserve in times of 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 sump-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.  Try to locate your garden as close to your rain barrel as possible and use a shorter garden hose; the shorter the hose the less pressure loss from friction you’ll have (albeit this is minor).  For those that are relying on gravity though, it’s important to understand the math so you can generate pressure:

A water level of 2.31 feet above its exit point will produce 1 PSI (pounds per square inch). Meaning the level of the water inside of your barrel must be 2.31 feet above your garden’s irrigation system for just this minimal amount of pressure.
Typical household pressure ranges from 40-50 psi, which is what most of us are accustomed to coming out of our hose spigots. 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 level in the barrel drops from use, 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 garden areas.
That’s right. If you want a sufficient water pressure from a rain barrel, 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, rainwater 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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Monday, 11 December 2023

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