html xmlns="" xml:lang="en" lang="en"> From the archives: An interlude.

Wednesday, January 31, 2007

An interlude.

Originally posted 2/5/7, 12:03am. Moved so the regulation posts could be continuous.

The cognoscenti call center pivot irrigation systems “circular moves”, in contrast to “linear moves”, in which an overhead sprinkler pipe spans the width of the field and rolls straight the length of the field. I was talking to this guy Vince one time, and when I mentioned circular moves, he narrowed his eyes at me and asked me if the word “moves” was indeed a noun in that sentence. I had to admit that it was. Vince was also the only person to give me a good explanation for the song lyrics “And I miss yoooouuuuuu, like the deserts miss the rain.” I’d never thought much about the song, until my sister mentioned that she hated it. “Deserts don’t miss the rain,” she said. “Deserts hate the rain. When deserts get rain, they stop being deserts.” When I told Vince these objections, he suggested that the deserts and rain were missing each other in the sense of passing each other without intersecting paths. He did hand gestures to illustrate.

You don’t see a lot of circular moves in California because circular moves have a fixed delivery rate, and that rate is too fast for our soil intake rates. Circular moves have to cover the whole circle before the soil dries out at the starting point. Every point on the circle will need some amount of water to meet evapotranspiration, say 4-5 inches in the summer. The circular move can put that amount of water out as it passes overhead, but on California soils a lot of that will run off. Some will infiltrate, but not enough to support a crop. If you have a center pivot irrigation system and see your crop wilting, the counterintuitive solution is to slow your circular move. It’ll hurt, because you’ll want to rush your sprinklers over the whole field. But that means that every spot will get not enough water. Slow your sprinkler to match the soil intake rate, and put enough down on most of the circle. Abandon some quadrant; you can rush your sprinklers over that to get back to the portion you can save.

Circular moves are rumored to be wicked hard to design. You want to save weight, so you shrink the pipes as they move outward. It is hard to size the sprinkler nozzles so that sprinklers close to the inside deliver the same amount of water as sprinklers close to the outside, which are traveling at a different speed and are on a different size pipe with different pressures. A4 asked whether the water pressure itself drives the circular move. They used to, but if you have to pressurize the water, you are better off just having motors on the pivot and peripheral wheel, ‘cause of the friction losses down the length of the arm. When I was doing irrigation system evaluations, I did one on an olive orchard right below Shasta Dam. He said he has pressure regulating valves on his main, ‘cause his water gets delivered to him with 90 feet of head. But that is exceptional.



Anonymous Mitch said...

I'm confused. If a circular move has a fixed delivery rate, what does it mean to slow it down? If the arm is moving slower, is it still delivering just as many gallons per minute? Seems like a waste of water.

In other words, if the soil can absorb water at a rate of x volume per unit time per unit area, and if your circular move delivers water at a fixed rate of y volume per unit time, but can cover a variable area in that time, then if your problem is that y is too high it seems like the solution is to speed up the circular move and make more passes.

2:38 AM  
Anonymous Mitch said...

Also, I've got both circular and linear moves, if you know what I mean.

Multiple passes are not a problem.

2:44 AM  
Anonymous Anonymous said...

Center pivot systems certainly make an impressive sight for airline passengers :)

Iron Rails & Iron Weights

7:24 AM  
Blogger Mark said...

My first thought when I saw center pivot irrigation was "why are they OK with not using (4-pi)/4 of their land?" Thinking about it a little bit, it seemed that the mechanism must be much simpler that way.

But now I'm totally confused. Why does anyone use central pivot? Other than create cool overhead views like this:

8:55 AM  
Blogger Megan said...

Hmmmmm. The circular move can't make passes fast enough. I'm sure of that.

I think I must have mentally switched (without warning you guys) to a case where the delivery rate is not exceeding the soil intake rate. Where it does exceed the soil intake rate, Mitch is right. Slowing down to apply more at one point is just wasteful. This is why you don't see a lot of them in California. Matching the delivery rate to the soil intake rate would mean that you couldn't get back to the start fast enough. Going faster wastes the runoff.

But, where your plants are wilting because you aren't putting out enough water and the soil could be drinking more, go ahead and slow your center pivot, like we talked about.

9:02 AM  
Anonymous Mitch said...

Bonus question: if you're in an area where everyone uses center pivot irrigation systems of roughly the same radius, what's the optimal shape for land plots?

My gut says hexagons or parallelograms, but I'm not sure it even matters.

10:02 AM  
Anonymous Anonymous said...

Very interesting. So that explains why you see so many circular fields with a portion that is not irrigated. Never knew that.

The soil type issue is interesting, also. May that explain the different density of center pivot* systems north and south of highway 50 on this map?


*A4 is not one of the whatever the singular of cognoscenti is.

10:05 AM  
Blogger Megan said...

The soil type issue is interesting, also. May that explain the different density of center pivot* systems north and south of highway 50 on this map?

Could be the soil, but it could well also be a local fad, or a good salesperson for one type of system.

4 is not one of the whatever the singular of cognoscenti is.
I looked it up! Cognoscente.

10:10 AM  
Anonymous ptm said...

I still don't understand the constraints here. The rate that the water comes out is not variable?

10:21 AM  
Blogger Megan said...

Don't think so. I should look this up to be sure. We didn't study circular moves very closely, 'cause they aren't common here.

My impression is that circular moves deliver water at a constant rate and the amount of water you deliver is determined by the length of time the arm stays at one position. A full rotation takes weeks, I believe.

You know, I'm on shaky ground with this stuff, so if someone shows up to correct me, I would believe that person.

10:50 AM  
Anonymous Anonymous said...

Dang. I looked it up also (paper *and* interweb), and still didn't find the singular form.

Wouldn't the nozzles/sprinkler heads/thingies be designed for a fixed delivery rate?

I'm intrigued by the question of field shape. I've never seen center pivots arranged like a honeycomb, only like a carton of eggs. Land area must not be the binding constraint.


11:43 AM  
Blogger Megan said...

Wouldn't the nozzles/sprinkler heads/thingies be designed for a fixed delivery rate?

I think they have to be, and that you have to keep constant pressures through the overhead pipe to make the sprinklers work at design flow. That's why I think the only variable you can mess with the the speed the system moves at. Like I said, though... if someone knows better, I'm willing to hear it.

12:59 PM  
Anonymous Anonymous said...

Bonus question: if you're in an area where everyone uses center pivot irrigation systems of roughly the same radius, what's the optimal shape for land plots?

Answering this question the way it's asked, obviously the optimal size for land plots is circular, however I think what you're trying to ask is what is the optimal layout for multiple plots of land. Meaning, how do you align the various circles, in something other than a square plot, to maximize the amount of land used for growing and minimizing the amount of land outside of the radius of the irrigation system.

One could easily optimize this in terms of geometry, but there is more to consider when answering, for example, what is the surrounding landscape like. Obviously if you've got a mountain range on the eastern side of the field, you're limited on that side to the geometry of the mountain range.

A lot also depends on the size of the individual plots with respect to the size of the overall plot. The smaller your individual plots, the more closely you can pack them into a given area.

My guess, without any math or geometry to back it up, would be a triangle. It raises an interesting question, though, and perhaps as time allows, I'll try throwing something together to see if I can provide something more than a WAG.


1:40 PM  
Anonymous Tom said...

If the delivery rate from the sprinkler heads was a constant along the radius, an annulus of land at R/3 with will receive 2 times as much water per area as the same-width annulus at 2*R/3 in a given interval of time.

If you want a constant delivery over the field, you'll need sprinkler heads with delivery rate proportional to the distance from the center.

Of course, if you assume a small drop over the system (pressurized line, I guess), you could use a constant-sized sprinkler orifice and increase the density of sprinklers linearly with distance from the pivot. Or you could increase the diameter of the sprinkler orifice with the square root of the distance from the center. Of course, that'd increase the pressure drop per length, I guess.

I don't really know about sprinklers. They do sound interesting.

4:30 PM  

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