Pivot irrigation is a method of crop watering that utilizes a long, rotating pipeline to distribute water over a field. The dominant form, known as center pivot irrigation, anchors the water supply at a single fixed point. This self-propelled system moves continuously across the landscape, uniformly irrigating expansive, circular areas of crops. It is instrumental in modern, large-scale agriculture.
The Center Pivot Mechanism
The mechanism begins at the central pivot point, which is a stationary tower anchored at the water source. This hub supplies water through a riser pipe to the main pipeline, which extends across the field. The pipeline is divided into multiple segments, known as spans, supported above the ground by wheeled towers.
These towers provide the structural support and the means of propulsion for the entire assembly. Each tower is equipped with its own motive power, typically a small electric motor or hydraulic system, which drives the wheels. The system rotates around the central point, with the outermost tower setting the pace for the rotation. Because the outer end of the pipe must travel a greater distance than the inner sections in the same amount of time, the inner towers use angle sensors to continuously adjust their speed. These sensors ensure that each span remains correctly aligned with the span ahead of it, preventing structural stress and maintaining the system’s integrity as it slowly circles the field.
Water Application and Efficiency
The design of the center pivot system ensures uniform water application across the entire circular area. To compensate for the difference in distance traveled, the sprinkler nozzles are not uniform in size along the pipeline. Sprinklers closer to the fixed center point have the smallest nozzles, while those near the end of the line are significantly larger to deliver more water and match the faster speed and greater area covered by the outer spans.
Modern systems enhance water conservation by utilizing low-pressure sprayers on drop tubes, often called goosenecks, which hang down from the main pipe. These low-pressure methods, such as Low Elevation Spray Application (LESA) or Low Energy Precision Application (LEPA), position the water emitters close to the crop canopy or the ground. Placing the water delivery closer to the soil significantly reduces losses from wind drift and evaporation, a common issue with older, high-pressure overhead sprinklers.
Alternative Movement Systems
A variation of pivot irrigation is the lateral move system, or linear move system. Unlike the center pivot, the lateral system is not anchored at a fixed point and travels in a straight line across rectangular fields. This design allows for nearly complete coverage of square or rectangular plots, avoiding the unwatered corners characteristic of center pivot circles.
A key distinction is the water source, as lateral move systems must receive water continuously along their path. This is typically achieved by drawing water from an open ditch or canal that runs the length of the field, or by trailing a flexible supply hose connected to a mainline at the edge of the plot. All towers in a lateral move system travel at the same speed, meaning all sprinklers can have the same flow rate for uniform water delivery. While requiring more complex water supply infrastructure and guidance systems to maintain a straight path, the linear movement provides an efficient means of irrigating non-circular agricultural land.