Instruction Manual for Watering North American Farm Crops
In the heart of North American agriculture, center-pivot irrigation systems have become a recognizable sight, characterised by large, circular or semi-circular crop patterns. These systems, manufactured by leading companies such as Lindsay Corporation and Valley Irrigation, are designed to efficiently water crops over vast areas.
The central component of these systems is the pivot tower, a sturdy structure typically made of galvanized steel, positioned over a riser pipe that connects to the water supply. The irrigation boom is formed from individual sections of pipe, called spans, each about 180 feet long and usually made from galvanized steel or aluminum. Each span is connected to the center pivot using flexible elastomeric couplings, and the outer end of each span is attached to a wheeled tower.
Most center-pivot machines are electrically driven, with a motor mounted on each span's tower, and only the outermost span rotates continuously. However, some manufacturers have adopted hydrostatic drives to combat copper theft and offer mechanical simplicity and safety. Hydrostatic drives use hydraulic motors on each wheel and a powerful electric- or diesel-driven hydraulic pump at the pivot.
While hydrostatic pivots offer advantages, they do pose an environmental hazard due to potential hydraulic leaks. To mitigate this risk, these systems generally use a non-toxic hydraulic fluid specifically engineered for pivots.
In contrast, wheel line irrigation, which is less common, involves lengths of pipe being connected to large spoked wheels and manually moved across the field. This method, while requiring more manual labour, does not pose the same environmental risks as hydrostatic pivots.
Hose reel irrigation, another method, covers a circle or semi-circle a hundred feet or more in diameter and retracts the hose back onto the reel, evenly irrigating a large rectangular swath of the field. This method, too, does not pose the same environmental risks as hydrostatic pivots.
End guns are often used to extend coverage into the corners of square fields, making better use of space that otherwise would go fallow. For rectangular fields, center-pivot booms can be set up with drive wheels at both ends.
Sprinkler heads (drops) are attached to the span by elbows that exit at the top of the pipe and act as siphon breakers. Different sprinkler heads are typically used along the length of the boom, with lower flow rate heads used near the center pivot and closer spacing near the pivot to limit water delivery.
Center-pivot irrigation systems require a water supply at the center of the field, able to supply approximately 1,000 gallons per minute. Hose reel irrigation uses a large reel of flexible polyethylene pipe towed into the field and connected to a tower-mounted gun.
In conclusion, center-pivot irrigation systems, with their efficient design and ability to cover large areas, play a crucial role in North American agriculture. While hydrostatic drives offer advantages, the environmental risks they pose must be carefully managed. Other methods such as wheel line irrigation and hose reel irrigation, while less common, also have their place in the irrigation landscape.
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