Editing Center-pivot irrigation (section)

==Overview== 
{{multiple image
 | align = right
 | total_width = 335
 | image1 = Irrigation Sprinkler.jpg
 | image2 = Circular irrigation sprinklers 02.jpg
 | footer = Center-pivot irrigation systems on [[Corn Belt|corn fields]] in [[Wisconsin]]
}}

Center pivot irrigation is a form of overhead [[irrigation sprinkler|sprinkler]] irrigation consisting of several segments of pipe (usually [[galvanized steel]] or [[aluminum]]) with sprinklers positioned along their length, joined together and supported by [[truss]]es, and mounted on wheeled towers.<ref name="FencePost"/> The machine moves in a circular pattern and is fed with water from the pivot point at the center of the circle.<ref name="CAFNR">{{cite web | url=http://cafnrnews.com/2008/07/growing-rice-where-it-has-never-been-grown-before/ | title=Growing Rice Where it has Never Grown Before: A Missouri research program may help better feed an increasingly hungry world | publisher=College of Agriculture, Food and Natural Resources, University of Missouri | date=July 3, 2008 | access-date=June 6, 2012| url-status=usurped |archive-url=https://web.archive.org/web/20130917225624/http://www.cafnrnews.com/2008/07/growing-rice-where-it-has-never-been-grown-before/|archive-date=17 September 2013}}</ref>

For a center pivot to be used, the terrain needs to be reasonably flat; but one major advantage of center pivots over alternative systems that use gravity flow is the ability to function in undulating country. This advantage has resulted in increased irrigated acreage and water use in some areas. The system is used in parts of the [[Agriculture in the United States|United States]], [[Irrigation in Australia|Australia]], [[New Zealand]], [[Brazil]],<ref name="CAFNR"/> and in desert areas such as the [[Sahara]] and the Middle East.

Center pivots are typically less than {{convert|500|m|ft|sp=us|sigfig=2}} in length (circle radius), with the most common size being the standard {{convert|400|m|mi|frac=4|adj=on|sp=us}} machine, which covers about {{convert|125|acre|ha|round=5|order=flip}} of land.<ref name="CAFNR"/>

{{multiple image
| header= Center pivot irrigation [[nozzle]]s
| align = left
| image1 = Nelson A3000 Accelerator.png
| width1 = 150
| caption1 = Rotator style pivot applicator sprinkler
| image2 = Nelson Big Gun.png
| width2 = 150
| caption2 = End Gun style pivot applicator sprinkler
}}
Originally, most center pivots were water-powered. These were replaced by [[hydraulics|hydraulic]] systems and electric motor-driven systems, usually driven by a motor mounted at each tower.

The outside set of wheels sets the pace for the rotation. The inner sets of wheels are mounted at hubs between two segments and use angle sensors (microswitches) to detect when the bend at the joint exceeds a certain threshold.  When the angle is too large, the wheels rotate to keep the segments aligned. Typical periods for a full rotation include three days;<ref name="CAFNR"/> while other sources have the outer edge of the structure moving at {{cvt|2|to|3|m|ft}} per minute, which would equate to 14 to 21 hours for one full rotation of a {{convert|400|m|mi|frac=4|adj=mid|-radius|sp=us}} system.<ref> {{cite web |title=Pressurized Irrigation Techniques: The center pivot irrigation systems |url=http://www.fao.org/3/a1336e/a1336e10a.pdf |website=UN [[Food and Agriculture Organization]] |access-date=26 April 2020 |date=2007}}</ref>

To achieve uniform application, center pivots require a variable emitter flow rate across the radius of the machine. Since the outer-most spans (or towers) travel farther in a given time period than the innermost spans, [[nozzle]] sizes are smallest at the inner spans and increase with distance from the pivot point. Aerial views show fields of circles created by tracings of quarter-mile or half-mile (400 or 800&nbsp;m) radial irrigation pipes, which consume up to several thousands of gallons per minute."<ref name=NYT20May2013 />

[[File:Irkhaya Farms center-pivot irrigation.jpg|thumb|Center pivot irrigation at Irkhaya Farms in [[Al Rayyan]], Qatar]]
Most center pivot systems now have drops hanging from a U-shaped pipe called a ''gooseneck'' attached at the top of the pipe{{clarify|What is attached - the drops or the 'gooseneck'?|date=July 2012}} with sprinkler heads that are positioned a few feet (at most) above the crop, thus limiting [[evaporation|evaporative]] losses and wind drift. There are many different nozzle configurations available including static plate, moving plate and part circle. Pressure regulators are typically installed upstream of each nozzle to ensure each is operating at the correct design pressure.

Drops can also be used with drag hoses or bubblers that deposit the water directly on the ground between crops. This type of system is known as LEPA (Low Energy Precision Application) and is often associated with the construction of small dams along the furrow length (termed furrow diking/dyking). Crops may be planted either in straight rows or in circles to conform to the travel of the irrigation system.