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Update for an ancient watering strategy.

Update for an Ancient Watering Strategy

Water harvesting is simply capturing and storing precipitation that would have seeped into soil or run off into stream channels. One creates a sloping surface that's impervious to water and fashions dams at the bottom to funnel precipitation onto crops or into storage containers.

The world's first harvesting system for drinking water was probably near the Ur areas in present day Iraq. Archaeological investigations in that region suggest water was collected as long ago as 4500 BC. Traces of those systems, which once supplied water for caravans and pilgrimages, can still be seen today along desert roads from the Arabian Gulf to Mecca.

During the past 6,500 years, many water harvesting designs have been tried and abandoned, but only relatively recently have scientists been called on to help ensure their success.

"There are many separate elements that must be considered, including precipitation patterns, land topography, cost of alternative water sources, and availability of materials, equipment, and labor," says Gary W. Frasier. Many of the factors are interrelated and must be considered simultaneously. Depending on the site and locally available materials, the collection surface can be water-proofed by compacted dirt (like those first ones), melted wax, asphalt, concrete, or plastic films.

Frasier, a hydraulic engineer with the Agricultural Research Service, has designed, modified, refined, tested, and evaluated hundreds of water harvesting systems during the past 30 years.

Recently he evaluated some of the water harvesting systems he helped build on the Fort Apache Indian Reservation in central Arizona. He cooperated with Sherri L. Simper, a range conservationist with the U.S. Department of the Interior's Bureau of Indian Affairs. Two of the original systems built there more than 25 years ago are still supplying drinking water to livestock.

One system is a combination of fiberglass matting and asphalt and the other a cover of chlorinated polyethylene sheets glued together.

With the fiberglass and asphalt, the matting is spread out on a smoothed area and saturated with a water-based asphalt. The asphalt soaks through the fiberglass and bonds the fabric to the underlying soil. This forms a waterproof, semirigid membrane that stands up to small animals that wander across it. Fences keep larger animals off.

Minor fence maintenance, plant removal, and re-coats of asphalt at 3-to-year intervals have been the only remedial measures necessary to extend its life beyond original design expectations. This treatment is still being used on other sites in various places in the United States. The USDA's Forest Service and Department of the Interior's Bureau of Land Management are two major users.

"Unfortunately, the polyethylene sheeting used on the other catchment is no longer manufactured. A different type of sheeting may be on the market, but we don't yet know if it will survive the ravages of wind and sun," says Frasier.

With flexible sheet covering, the key is finding a material that has chemical and physical characteristics that allow the cemented-together sheets to conform to all surface irregularities and withstand winds.

Frasier has authored a comprehensive how-to book on the subject and continues to receive inquiries from potential users.

Most recently, Texas ranchers have solicited his help to provide water for deer and other wildlife. Farmers and ranchers often supplement their incomes by charging hunters for using their land. Fees they collect are based on amount of wildlife their lands support.
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Author:Senft, Dennis
Publication:Agricultural Research
Date:Jul 1, 1991
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