me too. i finally got to ask a professor at an agricultural college that question. all i can do is give you the non-definitive answer he gave me: "there are access roads between all those circles for planting, harvest, and, maintenance. straight roads are a helluva lot easier to build. land is cheap, gravel isn't."
> Circular irrigation sprayers; now those I have to wonder about versus
> a single over-farm arm that can run down a length.
> Also, if the water is valued as it should be, why aren't 'growing season'
> tent structures with build in irrigation used instead?
I suspect that the answers to those are maintenence and materials respectively.
If you have a circular arm you essentially just need an engine, a pivot, and a wheel at the end of the arm, whereas if you have an arm that runs linearly you need a lot more moving parts (some sort of mechanism to turn the rotational motion of the engine into a linear motion, some sort of mechanism to keep the two ends of the arm in sync, some sort of mechanism to make the arm travel in the opposite direction, etc.). All that adds up to a bigger initial cost and more maintenence.
For the tent on the other hand you would really need a lot of raw materials. If we just make the tent 1 mile (which is the size of the bigger circles) in diameter and 10 feet high, then you would need over 500 acres[1] of plastic/tarp/(whatever you're gonna cover the tents in) per circle. Add on to that the metal needed for the irrigation system and building costs, and I really start to doubt whether it would be cost effective.
If I'm going to build a giant greenhouse in Oklahoma, it's going to need to be weather resistant. A slack covering over a grid of 10-foot-tall supports won't work. If you consider the cost of structural supports will likely dwarf the tent covering, different architectures probably make more sense.
I would suggest a half-torus with major radius 0.25mi and minor radius 0.25 mi. The structural shell would be a tensegrity grid, with the covering affixed over it. That's 790 acres of covering over 502 acres of land. You're going to need a lot of steel cable, and a few truckloads of rigid 50' pipes. You will also need at least two types of pipe-to-cable junction.
Sprinklers under the apex ring (0.25mi above the ground) could easily irrigate the covered area from a fixed position, without rotating, and condensers above the apex ring could recycle water out of humid exhaust air and feed it right back into the sprinklers. There would be no moving parts at all, other than in the groundwater pump and sprinkler heads. You have one ring-shaped pipe (a gutter could work, if the sprinkler heads don't require pressure to operate) to supply the sprinklers that is 1.57mi long, and one pipe from ground to apex ring 0.39mi long.
But with center-pivot, you only need a well pump, 0.5mi of pipe, A-frame supports, and wheels. But then you lose water to evaporation and transpiration. So I think giant greenhouses could not appear until the aquifer dries up, and water costs skyrocket. Even then, I'm not sure it wouldn't be better to just put a bunch of mirrors and a collector tower up, and transmit the solar energy to a vertical farm that is closer to a cheaper, more reliable water source.
From what I've observed, each of the wheels on the center pivot has its own controller to keep the system in line. Some appear to have GPS controllers. The issued with having a large linear arm is re-connecting the water supply every time the arm moves.
A single large arm operating the way you describe would need synchronized drive wheels on both sides, adding cost and complexity over a single-drive anchored system. You'd also need a lot more slack in the hose that feeds it.
