Don't get caught short

Irrigation system design is critical Never get caught short when designing irrigation systems. Always keep water supply in mind when figuring system size and components, Robert Evans, North Carolina State University agricultural engineer, said at the recent Southeast Vegetable and Fruit/AgTech 2000 Expo in Greensboro, N.C.

First, figure it takes one-fourth inch of water daily to replace moisture lost by the crop, he says. That equals 6,789 gallons per acre. If the system has a 24-hour pumping cycle, that means it needs a 4.7 gallon per minute per acre capacity.

A 12-hour cycle requires 9.4 gallons per minute per acre. And a four-hour cycle, which may be more realistic on many farms, calls for 28 gallons per minute per acre.

Most irrigation systems are only 70 to 95 percent efficient, Evans says. He usually figures on 80 percent efficiency. That would mean a 24-hour pumping cycle could require as much as seven gallons per minute per acre.

Carefully consider water sources, as well. "Groundwater is usually better quality for irrigation than surface water, at least here in North Carolina, Virginia and South Carolina," Evans says.

If surface water is the only option, as it often is in the Piedmont and westward in the Carolinas, accessibility becomes the issue, Evans says.

"With irrigation from a stream and if you're using a centrifugal pump, if you lift water more than about 20 feet, you're looking at potential problems. And if there's much variability of the water level in the stream, that's a challenge. Smaller streams can go dry. You can put a dam across them but regulations limit our ability to do that," he says.

Floating pumping plants, sometimes used in streams, may not provide good answers, either. "They can be damaged or destroyed in floods. You may come back after a flood to find the floating pump somewhere downstream," he says.

"And wet wells are becoming more difficult to get in, due to regulations."

Embankment ponds may be better for most farm irrigation systems. They also have to be carefully designed, however.

"If you get most runoff in winter and spring, you've got to be able to store that water," he says.

The formula Evans uses to figure pond storage volume is .4 x depth x surface area. That would mean a two acre pond five feet deep at the stand pipe would hold the equivalent of four acre feet. Evans says that's enough water to irrigate six acres.

Dug ponds, on the other hand, don't help much with irrigation, Evans says. "Dug ponds on the farm are good to look at but not much good for irrigation. The option is to use wells to recharge them," he says.

An increasing number of vegetable farmers are using municipal water for irrigation needs, he says. "That's kind of expensive but if there's no other water source, farmers are choosing to do that.

Figure the economics carefully, with any system. A four-inch well costs $8 to $10 per foot while a 10-inch well costs $40 or more per foot, Evans says. Pond excavation usually costs $4 to $5 per cubic yard, he says.

Whatever you do, check with local and state laws regarding irrigation. "There are lots of regulations governing water use now," Evans says.

Even when rain does fall, all of it usually isn't available for crop use. "From 70 percent to 90 percent of rainfall is effective precipitation, available to the root zone. For designing irrigation systems, it's typical to use an 80 percent figure," says Garry Grabow, North Carolina State University assistant professor of biological and agricultural engineering.

If you're banking on rainfall, with some crops the odds are against you. "With corn, less than 10 percent of the time can you approach full yield with rainfall alone. If you want full yield, you've got to go with irrigation," Grabow says.

"When you're designing a system, don't count on rainfall for your peak design. The crop irrigation requirement is more important for considering the total water supply than for designing the system."

Carl Camp, USDA-ARS agricultural engineer at Florence, S.C., says sub-surface drip irrigation systems are looking good in tests. He's researched drip systems since 1984 and since 1991 has focused on sub-surface drip irrigation mostly in cotton rotated with peanuts and soybeans, followed by wheat.

His tests show no yield difference between sub-surface drip systems with tubes buried 12 inches deep under every 38-inch row, or every 76 inches under alternate furrows. A similar two-year test with cowpea, green beans, melons and squash showed little yield difference, as well.

Since 1995 Camp's cotton tests have been no-tilled. "If you have a soil subject to compaction with heavy equipment, keep in mind that shallow compaction means you can't get water to the roots some parts of the season.," he says.

Using Gossym/Comax as a scheduling aid, the sub-surface drip system increased yield two of four years, compared to non-irrigated cotton.

Camp now tests sub-surface drip in more than 30 crops, including tomatoes, lettuce and potatoes as well as row crops like cotton and corn.

"There's less disease in some crops because the soil surface remains dry. We're getting multiple year use of the system which reduces annual cost. And we're getting precise placement of water and fertilizer. We know exactly where it's going and we can put out whatever frequency we want. You can inject nutrients and even some pesticides. Sub-surface drip certainly uses less fertilizer and optimizes crop production and water supply," he says.

Subsurface drip also means odors are easier to control. In addition, there's no foliage contact and reduced human exposure, Camp says.

Subsurface drip may fit well in odd-shaped fields where center pivots are impractical. "But the poorer the water quality, the more complex the filtration system will have to be. Filtration is the first step in assuring a long life of the sub-surface drip system," Camp says.

Emitter plugging, often caused by root intrusion, is an irritating, difficult-to-fix problem when it develops with sub-surface drip systems. "The best solution is prevention," he says.

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