Thursday, May 10, 2012

Dam

The only way much of the western United States can be agriculturally productive is through irrigation, which essentially means taking one of the streams coming from the mountains and directing it through a series of ditches onto various peoples' fields. The system today isn't too much different from that of years ago: the irrigation water follows a particular schedule, and when it's your turn for the water, no matter the weather or time of day, you trot off to a ditch somewhere and redirect the water your way. There are stories of people killing each other over irrigation disputes, and they're very plausible; without the irrigation water, your fields will die, and with them whatever hope you had of harvesting from them.

Modernity has brought a few new methods to irrigation, though. One nice system involves redirecting your irrigation water into a pond, from which it eventually runs into a pressurizer -- a really big pump, typically electric around here. This pumps the water through underground pipes into large sprinkler systems. Sometimes these sprinklers are on wheel lines,

and sometimes they are on large, self-propelled pivot systems.
. These pivoting systems lead to the circular fields seen in aerial photos.

Those are nice, but they cost many thousands of dollars to set up. Another option to block up the irrigation ditch just downstream of an outlet. Water backs up behind the dam and flows out onto the field. This type of irrigation is called "flood irrigation", meaning that the water simply floods the field, and to make it work you need the help of physics, because the first rule of irrigation is water won't go uphill unless you push it. So ideally your field slopes consistently one direction, and you start irrigating at the top. Periodically you move the dam lower in the ditch, to help water flow across more of the field.

These dams are tricky business. With the advent of concrete came the concrete-lined ditch, advantageous because it's a standard size, with standard sized metal dams and regularly spaced openings, also with their own little valves built-in. They don't wash out (in theory) and you can walk on them pretty well without getting stuck in the mud. They also cost $24 per foot to install, which is an awful lot of money.

So we use the ghetto method, pictured at the top of this post (though our ditches are larger than the one pictured). One pole -- local wisdom says a 2x4 won't cut it, but two might -- sits on the ditch bank. Fence pickets, 2x4s, or other miscellaneous lumber rest firmly in the mud, and lean downstream against this pole. In front of it all, another pole runs through a loop sewn in what is essentially a plastic tarp. With some skill and luck, and a good shovel, you can position the tarp to block the water without washing out. I've been doing this every couple hours all day long, and frankly, I'm pooped. Other farms with far larger holdings than ours will sometimes go days without sleep, because their irrigation schedule demands they rush from one ditch to another moving dams for days at a time. We get the water for less than a day, once every two weeks, and that's enough work for me. Lugging soaked, slimy lumber through the mud down to the next ditch outlet can really take it out of you.

But this water turn (our second), we managed to water quite a bit of the field that we missed last time, which might mean we don't lose half our crop to my incompetence. And we're probably going to have our field leveled before next season, so then the water should flow decently. I It's kinda exciting...

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