Rainwater Harvesting

Learn the best methods of collecting, storing and using rainwater.


Cisterns can be attractive sign of a commitment to water conservation. The Lady Bird Johnson National Wildflower Research Center in Austin collects rainwater in a series of cisterns. Rainwater from the central rooftop of nearly 17,000 square feet feeds two 20,000-gallon cisterns and the 5,000-gallon tower cistern. Several other cisterns ass to the total capacity of 65,000 gallons. With an average rainfall of 30 inches annually, these systems can collect more than 300,000 gallons of rainwater every year.

© Barbara Harper

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Gardening with Less Water (Storey Publishing, 2015), by David A. Bainbridge, offers simple, inexpensive, low-tech techniques for watering your garden much more efficiently — using up to 90 percent less water for the same results.

You can purchase this book from the GRIT store: Gardening with Less Water.

Rainwater Harvesting

Rainwater harvesting is a great complement to the super-efficient irrigation systems described in this book and can help provide critically needed water for growing crops, landscaping, and other uses around the home, farm, school, or office. Even in very arid areas, gardens and orchards can benefit from captured rainwater used in the most efficient, demand responsive ways possible.

Ancient civilizations developed very sophisticated rainwater harvesting systems. The Nabatean people of the Negev Desert were the masters of this practice. They developed finely tuned, site-adapted catchment, runoff, dam, and channel systems that harnessed rains and flash floods to irrigate crops and fill the cisterns needed to support farms and a city in the harsh desert. Long ignored, these practices have been rediscovered as rainwater harvesting becomes a key component of more sustainable living, gardening, and farming.

Rainwater harvesting requires a catchment area, a collection system, and storage. The most common catchments are roofs, patios, and walkways, but other impermeable surfaces such as roads and even compacted soil can also work. Keep in mind, however, that road runoff is often contaminated with antifreeze, oil, and other materials. This can be fine for landscape plants but not so good for the garden. Water is usually harvested from relatively clean impervious surfaces where an inch of rain will deliver more than a half gallon per square foot of surface. Ten inches of rain on a 1,000-square-foot roof will provide about 6,000 gallons of water. An intense thunderstorm can deliver 1,000 gallons in less than an hour.

Many cities and states now encourage rainwater harvesting in development regulations or offer incentives for people or firms installing rainwater harvesting systems. Texas was the first state in the US to embrace rainwater harvesting, but Hawaii, North Carolina, and other states are now catching up. North Carolina and the Florida Keys have rainwater harvesting rebate programs.

The city of Tucson, Arizona, has been a leader in rainwater regulations and now requires some commercial facilities to meet 50 percent of their landscape demand

using harvested rainwater, prepare a site water harvesting plan and water budget, meter outdoor water use, and use irrigation controls that respond to soil moisture conditions at the site. Parts of Australia have relied on rainwater harvesting for decades and many building codes now require catchment and storage systems, with tanks up to 800 gallons required along the Gold Coast.

Capturing Rainwater at Your Home

Roof catchment systems are the simplest and most common forms of rainwater harvesting systems.


The catchment area you need can be determined by estimating the water you need for the crops you want to grow. For a home with 1,000 square feet of impervious tile or metal (preferably baked enamel or approved epoxy coated — not aluminum), the rainfall collection over the year may be 70–90 percent of annual rainfall.

Gutters and Downspouts

Gutters and downspouts are needed to collect the water. To take advantage of high rainfall rates, roof gutters must be big enough and sloped for good drainage. The rule of thumb for gutter slope is 1/16-inch per foot to the downspouts.

Downspouts must be large enough to drain the gutters rapidly. A minimum of 1 square inch of downspout opening for every 100 square feet of roof is suggested. Gutter screens can help keep debris out of the water. Additional filtering can be done at the gutter-to-downspout transition with a sloped screen. Rain barrels should have a screen as well to keep debris and insects out. For the simplest system, downspouts can be run through a first flush and roughing filter into an aboveground tank or rain barrel.


First flush filtering lets the rain clear the roof of dirt, bird droppings, and other debris before actually being collected for use. It is most often used in arid climates where there may be long periods of time between rainfalls. One of the easiest first flush filters runs water into a chamber that must fill up before clean water can flow into the cistern or tank. In some a floating ball rises up and seals the dirty water in the chamber, letting the water from the now clean surface run into the collection tank. The first flush chamber usually has a slow drip or outlet to a planted area so that it will be empty again by the next time it rains.

In places such as San Diego, where it may not rain for nine months, a manual valve can be used to divert all the water from the season’s first rainfall to flush away accumulated dust and deposits. More elaborate flush or continuous filtering systems are also available. Water for use in porous irrigation systems should also be cleared by settling and/or with sand or mesh filters on its way into or out of the cistern.


To get the water to a storage tank, you can run pipes under sidewalks or decks and then back up to the storage tank. Water will be left in the lower parts of the pipe and can be used for irrigation with a spigot and valve. The water will have to be drained before winter and freeze risk.

How much storage do you need? It depends on the rainfall pattern and anticipated use. If you are using rainwater collection as a supplemental water supply, your storage can be as simple as a rain barrel. But while a barrel of rainwater is nice to have, it won’t go very far. For a garden, you might want to harvest a few hundred gallons or even a few thousand or tens of thousands of gallons. If you are using rainwater to fill specific irrigation needs, the storage volume needed can be estimated by setting up a water budget for the year, with monthly or weekly water use and average rainfall plotted in a table or chart. Think of the water supply like your bank account — you make withdrawals and rain makes the deposits. The chart on the following pages shows a simple example for the desert where I live.

In an area where all the rain that falls must be collected, then assume tank size based on annual rainfall.

Tanks must be fairly big to provide the water needed for a full garden season. Large concrete tanks can be placed under patios or decks. A 20-by-10-foot tank just 3 feet high will hold more than 4,000 gallons. In an area with rainfall spread out over the season or in a bimodal pattern, the storage capacity size can be reduced. If it rains almost every month, the storage can be much smaller. San Diego has an annual average rainfall of only 10 inches, with high year-to-year variability, and it falls mostly in just a couple of months in the winter; therefore, capacity needs to be large. Some of the early homes here included 30,000 gallons of storage.

Tanks or cisterns must be screened or managed to prevent insect problems and to keep out animals, children, and snakes. Access hatches should be locked. Water barrels and rainwater tanks can be purchased at your local hardware or garden center and are usually the most expensive element of a rainwater harvesting system. A 1,500-gallon polyethylene tank can cost as much as $1,000 plus transport. The lowest-cost tank is probably homemade with ferrocement, but this takes some practice. The simplest installation is an opaque plastic tank (it can be challenging to get paint to stick to plastic, so opaque is best for limiting algae growth). It is easiest to have it sitting on the ground but it may be placed on an elevated platform to provide more water pressure. If additional pressure is required, water can be pumped up to a tank on a hill (this can be done with a simple photovoltaic system and pump without batteries) or pressurized with a pump, pressure tank, pressure switch, and check valve (familiar to well owners), or an on-demand pump.

Narrow, stacking, and vertical tanks designed for rainwater harvesting are increasingly available at home building centers, and some will fit very nicely in side yards. Recycled food-grade plastic drums will also work. These can be put in a series so they fill up sequentially. (A series of tanks can also be used as insurance against a leak, valve failure, or errant rifle shot.) The tanks will last longer and the water will be cooler if they are kept in a tank barn or shed. A plastered straw-bale shed will keep the tank water at a very uniform temperature and minimizes risk of freezing pipes and valves.

A rainwater harvesting system is a living system, and to keep it operating efficiently it needs to be cared for properly. The gutters and roughing filters will need to be cleaned, and silt may need to be removed from the cistern or tanks periodically.

Going Big

Cisterns can be an attractive sign of a commitment to water conservation. The Lady Bird Johnson National Wildflower Research Center in Austin collects rainwater in a series of cisterns. Rainwater from the central rooftop of nearly 17,000 square feet feeds two 20,000-gallon cisterns and the 5,000-gallon tower cistern. Several other cisterns add to the total capacity of 65,000 gallons. With an average rainfall of 30 inches annually, these systems can collect more than 300,000 gallons of rainwater every year.

Excerpted from Gardening with Less Water (c) David A. Bainbridge. Used with permission of Storey Publishing. Buy this book from our store: Gardening with Less Water.