Why can’t the US build infrastructure that automatically siphons water from flood zones to the west during flood seasons?
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Basically for two reasons relating to how siphons work.
First is the target area is most likely higher in elevation than the source area. A siphon needs to move the liquid to a lower point overall in order to create a flow. That’s why when you siphon gas out of a car, the gas can needs to be sitting on the ground. That puts the gas can below the car’s gas tank.
The other is that (even if you found a flood that was higher elevation than some other desert place that needed water) siphoning is limited to about 30 feet of water depth between the openings of the tube and the highest point the tube.
Why about 30 feet? Because a 30 foot column of water an inch in cross section weighs 14 pounds. In other words, 30 feet of water exerts a pressure of 14 pounds per square inch (psi) and that’s enough to counter the air pressure around it.
That’s because the thing that “pulls” water into the tube is actually the air pressure *pushing* the water. A siphon works like a straw, except instead of creating “suction” with your cheeks, the suction comes from the water leaving the other end of the tube.
Well that “suction” that you get from a vacuum is actually pushing happening everywhere other than the vacuum. And that pressure is not unlimited, which means if you can create a “suction” scenario which involves as much pressure as the surrounding atmosphere, the suction fails.
I’m explaining this really badly.
Here’s a mental experiment: you take glass test tube, you dunk it underwater so all air is gone, then you invert the closed end of the test tube above the water level. What happens? As long as you keep the opening under water, there’s no way for air to get in and the tube stays full of water.
What happens if you use something bigger than a test tube? Some people put entire fish tanks jutting up over the surface of a pond and it lets fish swim *up* into the fish tank to look around.
The water stays up in the fish tank, again because there’s no way for air to get in.
So does that mean you can keep lifting more and more water above the surface of the pond, assuming you never let the opening to your glass tank come into contact with the air?
No. What happens eventually is the total water you’re trying to lift out becomes too heavy to be lifted that way, and you get a bubble at the top of the fish tank anyway. Except it’s not an air bubble; it’s a vacuum bubble. It’s a bubble full of empty space, not full of air.
There’s no way for air to get in, but vacuum doesn’t need to get in. Vacuum can be “created” anywhere just by pulling the matter out of a region of space.
You know what? I don’t think I’m explaining this well at all. TL;DR is the second reason you can’t siphon long distances is that if you’ve got a rise of more then about 30 feet, you get a vacuum bubble in your tube and it breaks the flow.
The why of this has to do with stuff you learn in fluid mechanics 101, and it’s a really great thing to understand but it takes a little concerted effort to memorize and internalize the rules about how pressure works in fluid.
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