Issue: July 2005
Harvest Rainwater to Irrigate Gardens and Landscapes
In parched New Mexico, water harvesting techniques can help gardeners get more splash from every rainfall.
Most rainfall is lost to runoff. By harvesting rainwater, gardeners can reduce the amount of tap water needed to irrigate gardens and landscapes.
Rainwater also helps improve plant health because it's salt free, unlike most tap water. By supplementing ground water with rainwater, salts move below the root zone, improving plant growth.
And, by catching and storing rainwater, gardeners can reduce soil erosion because less water will gush across gardens and landscapes during storms.
Harvest rainwater either directly in the soil or in containers for future use. Compost and mulches can capture rainfall. When mixed with garden soil, compost acts like a sponge to absorb moisture in the root zone. Using organic mulches, landscape fabrics and perforated or woven plastic mulches allows rainwater to reach plants while reducing evaporation.
Water catchment areas can guide water to the soil. Catchment areas should have hard, smooth surfaces that water wonít penetrate, such as concrete driveways and metal or shingled roofs. Sloping a driveway or sidewalk toward the landscape will help distribute the captured rainwater to the growing area. Gutters and downspouts can help guide water from a roof to a flowerbed or lawn. Swales in a landscape will help keep water from running off site.
Use rain barrels to store water for future use. Most rain barrels are made of fiberglass, polyethylene, wood, metal or concrete. Locate barrels under downspouts to capture rainwater. Keep them near growing sites but at a higher elevation to allow gravity to carry water to the site.
Use nontranslucent barrels to block light. That prevents algae that can clog water distribution lines from the barrel to the growing site. Cover the barrel top with plastic door screen to filter out debris and keep mosquitoes from breeding in the tank.
Rain barrels generally have two water distribution outlets. The main outlet is usually located a few inches up from the bottom of the barrel. This allows any fine debris in the water to settle on the bottom of the tank to avoid plugging water distribution lines. The upper outlet is located near the top of the barrel and acts as an overflow valve when barrels get too full. Barrels should always sit on a surface that allows excess water from the barrel to drain away from the house to the landscape or growing site.
Distribution of water from the barrel to the growing site can vary from a simple garden hose to a drip irrigation system. Drip systems will require an extra filter to eliminate any fine silt that may plug the emitters in the drip line.
The amount of water that can be harvested from a catchment area varies with the areaís size and the texture and slope of the surface. When figuring square footage of a catchment area, keep the tape measure level to avoid over estimates caused by laying the measure directly on a sloping driveway or a pitched roof. When measuring a roof, measure in a straight line from one corner of the roof to the other corners.
The amount of water harvested depends on duration and intensity of rain. Use a rain gauge to measure captured water. Assuming optimum conditions in the catchment area, an inch of rainfall could produce approximately 0.62 gallons of water per square foot of catchment surface. Potentially, a house with a 1,500 square-foot flat roof could collect 930 gallons with an inch of rainfall. However, rough roof surfaces can reduce water harvested by 5 to 10 percent.back to top
For more gardening information, visit New Mexico State University's Cooperative Extension Service publications world wide web site at http://www.cahe.nmsu.edu/pubs/_h.
George W. Dickerson, Ph.D., is is a horticulturist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.