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Rainwater: Use It or Lose It

Posted Jun. 30, 2009

Several recent events have combined to pique my interest in promoting rainwater harvesting as an alternative source of water for landscape irrigation. The 2008 drought forced several area municipalities and rural water districts to place restrictions on the use of water for irrigating landscape plantings and home gardens. This should concern every gardener as demand for water is only projected to increase while ground water reserves are projected to decrease.

In light of the 2008 drought, watching thousands of gallons of rainwater pour off the roof of my house and escape the confines of my property this past spring brought home the need for and appropriateness of harvesting rainwater. As sheets of water flowed across my lawn and down the street, all I could think of was opportunity lost.

Harvesting rainwater for use in the home landscape saves money on your water bill while also reducing demand on municipal water supply and treatment. When water treatment facilities are forced to expand due to increased demand, the cost of expansion is passed on to the end user.

Often overlooked is the exceptional quality of rainwater. Rainwater is virtually salt-free and has a neutral pH. The same can't be said of many municipal water sources. The greater the salt content of water, the more energy plants must expend to use it. Salt-induced stress impedes growth and reduces fruit yield.

There are several factors you need to consider before investing in a rainwater harvest system. Depending on the size and complexity of the system, it could take several years for the system to pay for itself. Will you be able to install the system or will you need to hire a professional? Companies exist that specialize in the installation of rainwater harvest systems.

A common residential rainwater harvest system consists of a catchment (usually a roof), conveyance system (usually gutters), storage (usually an aboveground tank) and a distribution system (usually a drip irrigation system.) Storage containers can be made of polyethylene, fiberglass, wood, concrete or metal. Underground containers cost more to excavate, to maintain or to remove, and the need to pump water out of them adds to the cost.

To calculate the water harvest potential of your roof in gallons, measure the square footage underneath the drip-line and multiply by 0.6 for each inch of precipitation. For example, a house located in Ardmore (37 inches annual rainfall) having a 2,000 sq. ft. roof catchment area can be expected to harvest 44,400 gallons annually.

When sizing a storage tank(s), keep in mind that storage capacity doesn't equate to harvest volume. Because storage is continually being depleted, primarily during the summer months, tank capacity can be smaller than harvest volume. To calculate required storage capacity and any need for supplemental water, you must first calculate demand (plant water use). A garden can easily use 2 inches of water per week during the summer. This is equivalent to 1.2 gallons per square foot of garden space.

Do you feel guilty watching rainwater run off your property? Maybe it's time you consider installing a rainwater harvest system. For a detailed discussion on rainwater harvesting including information on calculating storage capacity based on demand, consult Texas Agri Life Extension Publication B-6153, Rainwater Harvesting, available online at rainwaterharvesting.tamu.edu/publications.html.