Irrigation

Center Pivot Irrigation

Center Pivot

Drip Irrigation

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Go to Clay Robinson, Ph.D., CPSSc

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The next innovation in irrigation put irrigation pipes on wheels. These systems used high pressure (75 to 90 psi or more) and impact sprinkler heads. These systems improved both application and distribution efficiency, providing uniform water applications and very little runoff. These systems also require less labor because the system could be moved rather than individual pieces of pipe. The earliest systems were lateral move systems, using a hose to supply water from a riser at one end of the line. A motor mounted in the center of the line is operated by the irrigator to move the line when the desired amount of water is applied.

The water is supplied to the center of the field in pivot systems. The irrigation line is the length of the radius of the circle, and goes around the field, pivoting at the center. If you have flown over the Western Great Plains in the summer, you may have wondered what the regular circles of various diameters were. Now you know. Center pivot systems in the Great Plains are typically quarter-mile or half-mile systems. Quarter-mile systems irrigate 120 acres while the larger systems irrigate about 500 acres. The corners of the field are not irrigated by the system, and many farmers use the corners in dryland crop production. Since the tower closest to the pivot irrigates the smallest land area, and the last tower irrigates the greatest, the nozzle size of the sprinkler heads is adjusted to obtain a uniform water application.

High-pressure pivot systems can have irrigation efficiencies up to 90%. One of the main efficiency losses is evaporation of water before it gets to the ground. The pictures at left show the impact of wind on the spray. Hot, dry winds are common in the Great Plains, and can evaporate a lot of water.

Low-pressure systems were developed to decrease the amount of water lost to wind drift, and to decrease the energy costs associated with pumping enough water to maintain the high pressures required for the impact heads. The low-pressure systems run on 15-30 psi, just enough pressure to get the water to the end of the line. The pipes get progressively smaller from the pivot to the end of the line to help maintain pressure. These low-pressure systems have application efficiencies up to 95%, with the added benefit of lower energy consumption, and so lower pumping costs.

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Many of these low-pressure drops have adjustable heads or pads, allowing them to be used for applying insecticide inside a crop canopy when spraying upward. The flat fan shown in the picture at right is used to wet the whole soil surface just after planting when germinating a crop to get a good plant stand. There is also a bubbler mode which applies water closer to the surface just above the row. Finally, there are drag hoses which allow the water to be applied on the soil surface directly in the row. These drops are usually spaced at twice the row width, watering alternate furrows. To optimize the advantages of these low-pressure irrigation systems, crops must be planted in a circle.