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Eels are fish

Field Guide to North American Crop Irrigation

Human existence boils down to one brutal fact: however much food you have, it’s enough to last for the rest of your life. Finding your next meal has always been the central organizing fact of life, and whether that meal came from an unfortunate gazelle or the local supermarket is irrelevant. The clock starts ticking once you finish a meal, and if you can’t find the next one in time, you’ve got trouble.

Working around this problem is basically why humans invented agriculture. As tasty as they may be, gazelles don’t scale well to large populations, but it’s relatively easy to grow a lot of plants that are just as tasty and don’t try to run away when you go to cut them down. The problem is that growing a lot of plants requires a lot of water, often more than Mother Nature provides in the form of rain. And that’s where artificial irrigation comes into the picture.

We’ve been watering our crops with water diverted from rivers, lakes, and wells for almost as long as we’ve been doing agriculture, but it’s only within the last 100 years or so that we’ve reached a scale where massive pieces of infrastructure are needed to get the job done. Above-ground irrigation is a big business, both in terms of the investment farmers have to make in the equipment and the scale of the fields it turns from dry, dusty patches of dirt into verdant crops that feed the world. Here’s a look at the engineering behind some of the more prevalent methods of above-ground irrigation here in North America.

Crop Circles

Center-pivot irrigation machines are probably the most recognizable irrigation methods, both for their sheer size — center-pivot booms can be a half-mile long or more — and for the distinctive circular and semi-circular crop patterns they result in. Center-pivot irrigation has been around for a long time, and while it represents a significant capital cost for the farmer, both in terms of the above-ground machinery and the subsurface water supply infrastructure that needs to be installed, the return on investment time can be as low as five years, depending on the crop.

Pivot tower in an alfalfa field in Oregon. You can clearly see the control panel, riser pipe, swivel elbow, and the boom. The slip rings for electrical power distribution live inside the gray dome atop the swivel. Note the supporting arch in the pipe created by the trusses underneath. Source: Tequask, CC BY-SA 4.0.

Effective use of pivot irrigation starts with establishing a water supply to the pivot location. Generally, this will be at the center of a field, allowing the boom to trace out a circular path. However, semi-circular layouts with the water supply near the edge of the field or even in one corner of a square field are also common. The source must also be able to supply a sufficient amount of water; depending on the emitter heads selected, the boom can flow approximately 1,000 gallons per minute.

The pivot tower is next. It’s generally built on a sturdy concrete pad, although there are towable pivot machines where the center tower is on wheels. The tower needs to stand tall enough that the rotating boom clears the crop when it’s at its full height, which can be substantial for crops like corn. Like almost all parts of the machine, the tower is constructed of galvanized steel to resist corrosion and to provide a bit of anodic protection to the underlying metal.

The tower is positioned over a riser pipe that connects to the water supply and is topped by a swivel fitting to change the water flow from vertical to horizontal and to let the entire boom rotate around the tower. For electrically driven booms, a slip ring will also be used to transfer power and control signals from the fixed control panel on the tower along the length of the boom. The slip ring connector is located in a weather-tight enclosure mounted above the exact center of the riser pipe.

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