Harvesting rainwater to use for growing vegetables makes a great deal of sense. Unfortunately, the most common method of rainwater harvesting isn’t the most effective. Typically, gardeners invest in a rain barrel — which holds only 50 or 60 gallons of water — and then dole out the captured water to plants as needed, hopefully emptying the barrel before the next storm.
But 50 gallons is only a small fraction of the water you could be harvesting each time it rains. During a 1-inch shower, more than 900 gallons of water flow off the roof of a 30-by-50-foot house or barn. Instead of catching just a little bit of it in a rain barrel, why not capture it all? You can do just that with a simple setup that diverts rain from your downspouts directly to your garden. We’ll tell you more about how to do this in a minute, but first, we’ll explain why we think it’s such a good idea.
How Soil Stores Water
Even many experienced gardeners have trouble comprehending just how much water soil can hold. Except in areas with consistently high rainfall, your garden soil’s moisture level will seldom be at “field capacity.” That’s the term scientists use to describe the maximum amount of water a soil can hold. When it rains or when we irrigate, gravity pulls the water down into the soil. After a heavy rain, some of the water may move all the way down to the water table or the bedrock, but a large amount of it is held by capillary forces that cause water to coat each soil particle and partially fill the spaces between particles. (An example of capillary action is the way a paper towel absorbs liquid.) That capillary water is what your crops use as they grow.
Each soil’s field capacity varies depending on how much sand or clay is in it. One cubic inch of coarse sand may contain 125,000 particles, while the same amount of the finest silt could contain 15.6 trillion particles! Soil particles have an astonishing amount of surface area. One cubic inch of an ordinary soil (with a mix of sand, silt and clay particles) could have a surface area of 25 square feet.
What those numbers mean is that many soils can hold 2 to 3 inches of water in each foot of soil depth, and garden soils that contain lots of organic matter can hold even more. Crop roots can reach down 4 feet — sometimes even 8 feet deep — to tap this capillary water. To be sure crops get the water they need, gardeners would ideally want to keep their soil moisture near field capacity to a depth of at least 4 feet. During peak growth, crop transpiration together with surface evaporation can draw as much as a half-inch of water per day. The more water you’ve stored in your soil, the less you will need to provide supplemental irrigation.
To understand how soil moisture levels vary in your area, check out the soil moisture maps from the National Weather Service. These maps will tell you whether soil moisture levels in your region are above or below normal at any particular time.
Your Improved Rainwater-Harvesting System
To store as much rainfall as possible in your garden soil, you can set up a rainwater irrigation system that diverts your roof runoff water directly onto your garden beds (or lawn, if you prefer).
This rainwater-harvesting system relies on gravity to carry rainwater from your downspouts out into your garden or lawn. For it to work, the area you choose to irrigate must be level with or below your downspout. If your garden is not near a downspout, you may need to modify your setup, perhaps attaching a larger pipe to the downspout to carry the water out to a manifold and a set of perforated hoses in the garden.
Step 1: Go Shopping. Study the illustrations in the Image Gallery, and then head to a hardware store and buy one or more standard plastic trash cans. Get rectangular ones if you can — they will fit up against the wall behind your downspout a little better than round ones. (If you already have a rain barrel, you can just use it.) In the lawn and garden department, look for a hose manifold that will let you attach several hoses to it (see illustration in the Image Gallery). Next, you’ll need one manifold per trash can. Go to the plumbing department and find a clerk who can locate the bulkhead fitting you’ll need in order to attach the hose manifolds to your trash cans. Printing this article and taking it along to show the clerk what you’re planning to do may be helpful.
Step 2: Install the Bulkhead Fitting and Manifold. Cut a hole near the bottom of the trash can, attach the bulkhead fitting to the can, and then screw on the manifold.
Step 3: Cut a Hole in the Trash Can Lid; Install the Can Under a Downspout Near Your Garden. If there will be leaves in the water coming off the roof, you may want to cut the downspout off above the trash can and install a screen over the entrance hole into the trash can. The screen will allow water from the downspout to enter but will prevent leaves from being washed into the can and clogging the manifold.
A small screen secured over the bulkhead entrance in the bottom of the can is also a good idea to keep debris from clogging the hoses. To prevent mosquitoes from breeding between rainstorms, drill a few small holes in the bottom of the can so water can drain away completely after each storm.
Step 4: Install Perforated Hoses. Decide which areas you want to direct the rainwater to, then round up some old garden hoses. If you don’t have any, ask around — many of us can seldom bring ourselves to throw out hoses even after they’ve aged and begun to leak. Cut them to the lengths you need, and then cap the ends or fold the ends back and secure with wire. Drill holes in the hoses every foot or so, and then attach them to the manifold. You could also buy no-pressure soaker hoses designed to work with gravity flow from Mr. Drip. (Commercial low-pressure soaker hoses will not work well with this setup.) If you use irrigation ditches between your crop rows, you can skip drilling holes in the hoses and just lay the hose ends in the ditches.
Each time it rains, roof water will flow into the trash can and out through the manifold to wherever you’ve directed the hoses. Check the system during a heavy downpour to confirm that the hoses are distributing the water where you want it. You may need to add more holes or possibly tape some holes closed.
Rainfall patterns vary greatly from region to region, but even if most of your rain comes in fall or winter, this system will let you store it right in your garden beds. (Be sure the manifold is fully drained during cold weather to prevent damage from freezing.) If you have periods during which your garden gets too much rain and the soil reaches field capacity, you can simply redirect your hoses away from the garden for as long as you need to.
There’s a bonus to using this DIY rainwater-harvesting system: If the rain doesn’t come often enough and you need to irrigate using your household water supply, you can just aim your main hose into the trash can and turn on the water, and your network of hoses will distribute the water wherever you want it.
Cheryl Long is the editor in chief of MOTHER EARTH NEWS magazine, and a leading advocate for more sustainable lifestyles. She leads a team of editors which produces high quality content that has resulted in MOTHER EARTH NEWS being rated as one North America’s favorite magazines. Long lives on an 8-acre homestead near Topeka, Kan., powered in part by solar panels, where she manages a large organic garden and a small flock of heritage chickens. Prior to taking the helm at MOTHER EARTH NEWS, she was an editor at Organic Gardening magazine for 10 years. Connect with her on Google+.