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.
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 975 imperial gallons (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.
Water Storage Options
The usual solution for storing collected water is to use a water butt 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 personalised barrels.
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 litres). 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 water butts you can fit an overflow pipe near the top to divert excess water to another IBC, giving you a series of tanks.
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!