Rainwater Harvesting

Harvesting rainwater is an old idea that is gaining popularity in Texas, where our growing population and frequent drought make it important to use our water resources efficiently.

One of four cisterns at the Denton Co Government Center

Collecting and using rainwater in the landscape helps reduce demands on surface and ground water supplies and can save money by reducing water bills.  Rainwater is good for plants because it is free of salts and other minerals that harm root growth.

Rainwater often ends up as storm runoff, carrying with it fertilizer, grass clippings, toxic chemicals, sediment and bacteria into our lakes and streams. Rainwater harvesting helps decrease storm runoff and associated pollution and provides a healthy source of water for the landscape.

How much rain can you collect?

One inch of rain will provide about 0.6 gallons of water per square foot of catchment surface. For example, if a house is 2000 square feet, with 4 downspouts that drain equal areas of the roof, each “catchment surface” will be 1/4 of the roof, or 500 sq. ft. There is no need to take the slope of the roof into account in your calculations. So a rain barrel at one of the downspouts could collect

500 sq. ft. x 0.6 gal./sq. ft. = 300 gallons per inch of rain.

The 30-year normal precipitation (1981-2010) for the city of Denton, from the National Weather Service, is shown below:

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual
Normal Precip.(in.) 2.06 2.81 3.23 3.25 5.11 3.59 2.39 2.14 3.09 4.96 2.90 2.56 38.09


How much water do you need?

You can start small with an inexpensive rain barrel that provides water for your container plants, wildlife or a small garden.   Add a raingarden to conserve water that would normally run off and add color and texture to your landscape.

The amount of water needed for a larger landscaped area can be calculated from the monthly average evapotranspiration (ET) for your city, the plant water use coefficient (which depends on the types of plants in the landscape), and the square footage of the landscaped area.   Sample calculations and worksheets can be found in the Texas AgriLife Extension Rainwater Harvesting publication, found on the Texas AgriLife Extension Rainwater Harvesting page.   The Texas AgriLife Extension also provides a Storage and Supplemental Use Calculator.

Mulching to minimize runoff and evaporation and using native and adapted plants will help reduce water needs.   Grouping plants with the similar water requirements together helps simplify maintenance of the landscape. See the Texas Urban Landscape Guide:  water wise principles page for more suggestions for water-wise landscaping.

Components of a Rain Harvesting System

A rainwater harvesting system consists of a catchment surface (usually the roof of a house or other structure, but it can also be a sloping lawn or paved area), a distribution system (gutters and downspouts or channels to route the water from the catchment area), and a collection or storage area (from a simple raingarden, where rain is collected and used immediately, to rain barrels, tanks or cisterns, where water is stored for future use).

Raingardens:  Collect the Rain Right in the Garden


With a raingarden, the rain is collected right where it is needed—in the garden. To construct a raingarden, a shallow depression is created in a location where rainwater normally drains or where runoff can be directed.   If needed, a berm is constructed on the lower side of the depression to retain the water that flows into it.   Drought tolerant plants are planted, and their water needs are provided by the rain that collects and slowly infiltrates into the soil over a few hours.  Raingardens help minimize runoff, and the soil and plants in the garden help remove pollutants.  A raingarden isn’t a bog or a pond. Water collected in a raingarden should seep slowly into the soil—usually in fewer than 24 hours—to minimize the risk of breeding mosquitoes.

See the Texas AgriLife Extension Raingardens page for more information.   The Texas AgriLife Extension publication “Rainwater Harvesting: Raingardens”, available from the raingardens page,  contains diagrams and instructions for designing and building a rain garden.  A list of plants that are suitable for a raingarden is also available on the raingardens page.  The article “Raingardens for North Texas Landscapes” describes one local Master Gardener’s experience with constructing raingardens.

Rain Barrels and Cisterns:  Save the Rain and Use It Later

What equipment do you need?

Trash can rain barrel

With a few tools and easy-to-obtain materials, you can make your own rain barrels.   Here are two methods for making do-it-yourself rain barrels from a plastic trash can and from a food-grade plastic storage barrel.  There are also many online retailers that sell ready-made rain barrels, or check with local hardware stores or nurseries.   Most of these barrels hold 30-60 gallons.  If you’d like to collect more, you can connect additional barrels to the first one, or consider purchasing a large cistern and install it yourself or have it commercially installed.

Note: Texas Senate Bill 2 exempts rainwater harvesting equipment and supplies from sales tax, and you may be eligible for other tax incentives.  See the Texas Manual on Rainwater Harvesting for more information.

What can you do with the water you collect?

You can use the rain you’ve collected to water your landscape and container plants or to fill a pond.   Connect a drip irrigation system to a rain barrel or cistern to water plants or supply water for wildlife.   Elevate the rain barrels so that they are higher than the area you want to water to help increase water pressure.   A pump may be required if gravity doesn’t provide enough force.   Water for indoor use and human consumption must be treated before use. Barrel, watering can and drip

The Texas AgriLife Extension Rainwater Harvesting In-Home Use page contains additional information about collecting water for indoor use.

Want to know more?

Here are some good rainwater harvesting resources:

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