Water harvesting
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Issue: April 24, 2004

Water harvesting

Question:

Are there any unique techniques for harvesting water for semi-xeric plants other than creating surface channels in the ground? Can deeper levels of the planting bed be modified to receive and retain harvested water in some manner?

Gary H.

South of Socorro, NM

Answer:

Water harvesting (collecting rainwater), especially water that runs off roofs and paved areas, is a good idea in the arid Southwest. It is often important to move the water from the area where it is collected to where it is needed in the landscape. There are many methods of harvesting water and directing it to its point of use. Some methods are passive, requiring no storage or input of energy, and others require a cistern and pump. The best system depends on the terrain, soil conditions, and location of constructed elements in the landscape.

If the water will be used downhill from the source of collected water (roof, paved area, or just downhill over soil), a passive-type system works well. The water may be directed over the surface through the channels you mentioned or through buried pipes or French drains. As long as there is sufficient soil to dig a trench to bury the pipe or gravel of a French drain, you can make sub-surface channels. There are many different ways to create a French drain, from the use of special perforated pipe that collects water to plastic-lined (or unlined) trenches filled with gravel and covered with soil. French drains are most often used where precipitation and poor drainage cause problems. Their purpose is to drain water from low areas near building foundations; however, the French drains are useful in arid regions for water harvesting. It is important that the trench or piping slope downward from the point of water collection to the discharge area. A slope of one-fourth-inch per foot or greater is recommended.

At the location where the water is discharged into the landscape, it may be allowed to flow over the surface and infiltrate the soil from the top down. It may also be more quickly and deeply moved into the soil by creation of a drywell of gravel or by burying a straw bale in the ground near the planting bed. The point of discharge and the drywell may be covered with soil so that it is not visible. This will also reduce evaporative water loss and minimize muddy areas in the landscape.

Remember that the roots of landscape plants are nearer the surface than most people realize. The majority of absorptive roots of even trees and shrubs are in the top 12 inches of soil. Some absorption of water and minerals occurs more deeply, but water in soil below three feet may be unavailable to the plants.

There is limited space here to describe the many technologies for harvesting rainwater for landscape use. More information is available in books that discuss permaculture. Water harvesting is included among the concepts of permaculture (permanent culture).

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Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.