Save Water: Rainwater Harvesting Tutorial

, written by Benedict Vanheems gb flag

Rain falling on a shed roof

Around the globe more and more areas are coming under severe water stress, meaning higher bills for treated water. It makes sense to save as much treated water as we can, and harvesting rainwater for use in the garden is a great way to do it.

Ways to Collect Rainwater

At its most basic, water collection starts with any receptacle capable of catching and holding water. A more effective method is to use a tarpaulin or other waterproof surface slanted at an angle to feed rainwater into a collection vessel. Setups like this can harvest a surprising amount of water and it’s a handy way to make the most of downpours. It’s a useful solution for getting water to growing areas detached from any buildings, such as community gardens.

An easier method, of course, is to use roofs. Whether on a house, greenhouse, garage, shed or other outbuilding, roofs make fantastic, ready-to-go rainwater collectors. And the bigger the surface area, the better.

Work out just how much water you can harvest using this simple calculation

Calculate How Much Water You Can Collect From Roofs

You can work out the rainwater harvesting potential of your roof by multiplying the area of the roof that will be doing the collecting by your average annual rainfall. About a quarter of this total will be lost to evaporation and rain bouncing off the surface, so multiply this figure by 0.75 to account for this.

Let’s look at an example: The half of my greenhouse roof that sheds to a rain barrel comes to an area of around 60 square feet (5.5 square meters). We get about 32 inches (800mm) of rain a year, so this means that half of the roof could yield up to 900 US gallons (975 imperial gallons or 3.4 cubic meters) of water.

That represents roughly a third of typical water use for the average garden, so that’s not bad! Nevertheless, it’s worth collecting from as many roof surfaces as possible, especially those with larger surface areas like a house roof, to store for use during dry spells.

A rain barrel with a lid makes a good storage option

Water Storage Options

The usual solution for storing collected water is to use a rain barrel connected to a downpipe by a rainwater diverter and hose. These typically come in at around 20 to 60 gallons (100 to 250 liters), which may sound like a lot but it’s easy to get through that during a dry spell. By linking multiple barrels together using overflow pipes we can store more water for use in the drier months.

Raising the barrel up on blocks or bricks should make it easier to fit your watering can beneath. Cover barrels with a tight-fitting lid or some sort of mesh cover to keep mosquitoes and other bugs out. It will also prevent leaves and other debris from clogging up your barrel to ensure cleaner, clearer water.

Aesthetics aren’t always a priority, but there are plenty of decorative options, such as repurposed or replica whiskey barrels, terracotta-style barrels, metal drums or personalized barrels.

Maximize your water storing capacity using one or more IBCs

To increase your ability to store rainwater you could stack barrels, or use multiple slimline barrels that sit snugly in corners or against walls. Or go large – very large – by repurposing an IBC tank. IBCs usually come in at around 250 gallons (1000 liters). Costs for tanks like these vary dramatically so definitely shop around, and fit a sun cover or protective hood to extend its life and stop the water from turning green. Just as with rainwater barrels you can fit an overflow pipe near the top to divert excess water to another IBC, giving you a series of tanks.

Bear in mind that while rainwater harvesting is almost always legal, some regions, particularly drier states in the US, impose restrictions that can prove a bit of a headache. In Colorado, for example, you’re limited to a measly capacity of 110 gallons, while in Texas your collection system must be incorporated into the building’s design and written notice given to your municipality. Regulations are constantly changing, so check restrictions local to you before you design your harvesting setup.

Storing water means you can have it on hand for when your plants need it most

Using Rainwater

While rainwater’s best for plants, stored water poses a minor risk to human health. Contaminants like bird droppings, insect larvae, midges and bacteria may compromise the purity of the water.

By all means irrigate crops with your collected water – just exercise a little common sense. Aim water right at the base of plants so it doesn’t splash up onto the foliage. Maybe leave it a few days before harvesting salads if you can and, of course, thoroughly wash any produce that’s come into direct contact with the water.

Rain is beautiful stuff, it really is. What are your plans for harvesting rainwater? I’d love to know your setup and how much you hope to collect and save, so pop me a comment below with all the stats!

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Show Comments


"I think you might have left a number out of your equation. I got something like 1440 when I plugged in your numbers, and I didn't understand and I googled it. The site I found also included a .0623 conversion factor to convert to gallons, which does equate to almost 900 gallons when using your numbers. Am I correct, or am I missing something?"
Lela on Saturday 3 September 2022

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