Issue: March 12, 2005

Passive water harvesting

Question:

What is "passive water harvesting"? I have been collecting rainwater from my roof for years in large barrels, so I understand "water harvesting", but I'm not sure what is meant by "passive" water harvesting.

Answer:

Passive water harvesting, like passive solar energy utilization, tends to be a system of water harvesting requiring little energy input and little effort on the part of the gardener after the system is constructed.

This method of water harvesting doesn't involve collection barrels and should have less need for screening and chemicals to prevent mosquitoes and other pests breeding in the harvested water. Water from rooftops and other hardscape areas of the landscape are directed into storage in the soil. This water is applied to trees needing deep irrigation and to areas needing leaching to remove salts that accumulated after application of poor quality water. There is no need for pumping or transporting the water from the storage site to the point of utilization. It is utilized at the storage site.

Water from the rooftop and other areas is directed by a system of swales (broad shallow ditches) to the area where it will be utilized. The soil is then distributed across that area and seeps into the soil. Water on the surface may evaporate, but the deeper water will be utilized by trees and other deeply rooted plants in the area.

The system of swales may be a shallow, almost unnoticeable, linear depression in a lawn area, or it may be a mock arroyo lined with plastic and filled with cobbles and boulders to become a distinctive element in the landscape. The arroyo may be landscaped with appropriate plants within this arroyo or plants may line its margin. The main purpose is to direct the water from near the house to a point of usage away from the foundation and among plants needing deeper irrigation. Passive water harvesting may not provide all the irrigation needed by trees in the landscape, but will reduce the need for purchased or pumped water, reducing costs for maintaining the trees. An interesting fact, long apparent to gardeners, is that rain water is better for plants than tap water. There is no reason to waste rainwater by allowing it to runoff. It is also a waste to allow rainwater to seep deeply into soil with shallow rooted plants. It is best used with the deeper rooted plants in the landscape.

Passive water harvesting does compete with "active water harvesting" in which water is collected in barrels to be distributed by pumping or gravity drainage at a later date. Gardeners should determine which system is most appropriate for their landscape. If the vegetable or flower garden is the place most in need of harvested rain water, then the active water harvesting system will be more appropriate. Other gardeners will find that passive water harvesting is most appropriate for them and provides an opportunity for creating an interesting element in the landscape. Either system allows gardeners to make best use of the (usually) sparse rainfall of the Southwestern U.S.

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Send gardening questions to Southwest Yard and Garden, Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu or at https://www.facebook.com/DesertBloomsNM/. Please include your county Extension Agent (aces.nmsu.edu/county) and your county of residence with your question!

For more gardening information, visit the NMSU Extension Horticulture page at desertblooms.nmsu.edu.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505-865-7340, ext. 113.