To Harvest the Rain
By Al Cooke
Agricultural Extension Agent
Chatham County Center
North Carolina Cooperative Extension Service
Printable Version
Whenever rainfall is in short supply, we become more aware of the costs of water, the depths of wells, and the limits of ponds.
Then there arises an interest often expressed as “rain barrel.” Catch-22 is that when you need a barrel full of water is usually
not a good time to try to fill a barrel. And when it starts to rain again, then we have short memories.
This issue comes around periodically, however. And property owners eventually face some tough choices. Under guidelines for
“severe drought” or “extreme drought” they must ask themselves what are non-essential uses of water. Is your vegetable garden essential?
Is your lawn essential? Is that oak tree that shades your home essential? All have value. But essential is a subject for philosophical
discussion. Some would say all of the above are essential. Others would respond, as existentialists, that the only essential is death.
One of the challenges of life, both as individuals and as members of civilized communities, is managing water resources. That’s why
we have wells, municipal water districts, and reservoirs. Another, perhaps primitive, method is the cistern or rain barrel. Essentially,
a cistern is a water storage container connected to a collection/storage system. These systems are in routine use in many areas where
groundwater is limited or where terrain may raise the cost of moving water great distances.
How safe is rainwater and how can it be used?
Water seldom exists in a pure state and almost always comes with contaminants. Atmospheric contaminants may consist primarily
of dust but other considerations include fungi, bacteria, insect parts, and even radioactive materials. Rainwater is generally of good
quality but there is no guarantee. It may be sufficiently acid to be corrosive. And that brings us to a bigger weakness in the safety of
rainwater use: our catchment, conduction, and storage facilities.
How clean is your roof? Are there bird droppings? Do bats fly overhead? Do birds ever nest in the gutters? Have you ever cleaned
out a stopped up gutter and noticed the odiferous fragrance. We may get thirsty enough to drink that water. But let’s begin by assuming
we are not going to use our water collection strategies for human consumption. If you are considering using rainwater for consumption,
then you will certainly want to investigate not only filters but also sterility techniques that are beyond the scope of this article. But even
if you only plan to use the water to irrigate or wash cars, there will be some filtration issues. Particles washed from a roof can clog
sprinklers and scratch car finishes. So we will need some filtering. But let’s not get ahead of ourselves.
How much water can you catch?
A rule of thumb is that a roof of 1,000 square feet will collect approximately 600 gallons of water in a one-inch rainfall. That’s about
10 or 12 barrels. In the course of a year in our neighborhood, you might catch more than 25,000 gallons. How much of that do you
want to store at any one time? The answer to that question varies with how much water you use, how quickly you can replace it, and
how long you might go without rain? There’s a lot we don’t know.
You are probably in the best position to estimate how much water you might use. If you develop a rain catchment system and begin
using it for irrigation before you need it, then you can see how far it goes. You can compare your needs with your supply. You can
modify as you see fit. You will learn how well it works, if you need greater capacity, and if you need a pump for water distribution.
A large storage tank can be an unnecessary expense if it stores more than you need. If you find you have insufficient storage it is
possible to increase it by adding more storage and linking them together.
Designing a system
While we tend to think in terms of a rain barrel or cistern, that is only the storage device. There are other issues to consider including
catchment, conveyance, screening, storage, and distribution.
The catchment is usually a roof. Depending on elevation and layout, you may also be able to collect water that runs from patios or
decks. Rooftops may include the home as well as outbuildings. Rooftops are excellent for catching non-potable water but roofing
containing asbestos, asphalt, or lead flashing should not be used for drinking water.
Conveyance includes the gutters, downspouts, and any other piping used to move water from the roof to storage. Gutters are usually
made of metal for strength although plastic gutters may be suitable for small collection areas. Copper gutters have been implicated
in plant damage from copper toxicity. Gutters should be supported every three feet with a downspout every 50 feet. If there are
trees nearby, the gutters should be screened with hardware cloth or similar material. Likewise the inlet to downspouts should be
screened to prevent blockage with sticks and debris. Avoid sharp turns in downspouts or consider including cleanouts.
Screening is an important technique to keep debris out of the storage tank. In addition to rainfall, roofs catch dust, bird droppings,
dead insects, leaves, branches, and a wide range of other material. A variety of screening devices can be used. A fairly common
design runs the water through a screen then to a small sediment tank. The sediment tank allows solid material to settle to the bottom.
When water reaches an overflow outlet near the top, it then flows into the main storage tank. A trickle valve near the bottom keeps
the sediment tank emptied. This sediment tank and screen need to be cleaned periodically. The diagram below is from
Rainwater Harvesting Systems for Montana
If you use just an open barrel with a screen, it should also be screened to exclude mosquitoes. If you are planning to use the water
for agricultural production, it is always advisable to have the water analyzed for pH, nutrients, carbonates, and other contaminants
that could affect plant growth.
The storage tank may be wood, metal, plastic, concrete, concrete block, or other material and it may be stored above or below ground.
The material chosen may depend on availability and cost, but consideration should also be given to the end use of the water. Some
sources suggest that barrels should be only food grade quality and that you should know the history, i.e. did someone use that barrel
to mix herbicide? Some contaminants may be detrimental to plants and are not easily removed.
If water is to be stored through the winter, then consideration must be given for freezing. A plastic barrel may freeze and burst.
If that is the chosen container, emptying it (install a drain line) may be the best option for winter. Large containers may be buried if
the tank material is designed for underground use. Large tanks may also need a manhole, vent, and cleanout sump. An overflow pipe
is essential. Once that open barrel next to your house fills up and overflows, you don’t want the overflow running under the house.
Plan for where the overflow will go.
Distribution raises the question of how you move water, which is actually rather heavy. But people do dip buckets into cisterns and haul
water by hand. If you have a slope with your storage above your gardens, you can take advantage of gravity. Gravity pressure may not be
sufficient to run sprinklers. So you’ll have to use trickle techniques or add a pump. But a 1/3 to 1/2 horsepower pump combined with
a pressure tank gives you lots of options.
Because rainwater is often acidic, it can also be corrosive. That’s another reason not to introduce it into household plumbing where it
can dissolve lead or copper fittings. Besides health issues, the household piping could be damaged.
Outdoor water lines that are not buried should have drains at a low point to prevent freeze injury. Actually, to prevent freeze injury,
some intelligent being has to remember to open the drains.
We’ve come a long way from the barrel beneath the downspout, and what started out as a cheap way to save water looks a bit more
complicated. You can make this as simple as that barrel with a dip bucket or as complicated as an underground cistern with pump
capable of storing thousands of gallons of water. The question you must answer is one of appropriate technology. Do you need thousands
of gallons of storage capacity? Do you need it underground? Do you need a pump?
Above ground you can cover it with your artwork. You can paint it to blend in. You can hide it behind a fence or shrub. You can use it
to reduce your dependence on commercial sources or to stretch your well supply. You can hold it in reserve for emergencies.
There are a lot of contingencies, and I never intended to write a book on the subject. But I have compiled a lot of references.
There are pictures of how to put a hose bibb in a plastic barrel. How to link several barrels together. How to disinfect a cistern.
How to build in diverters. I’ve put a lot of this together with a
web page of links on rainwater, cisterns, and rain barrels.
Do your homework. Then get started.
This page created and maintained by
al_cooke@ncsu.edu
Created September 19, 2007
Last Modified September 19, 2007
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