Jim Stewart wrote:
I've wondered about this for years. Looking out at
an irriagated field, I see concrete pipe, about 1-3
feet in diameter sticking up anywhere from 4 to 8
feet. What is it for?

Pressure relief and vacuum relief. If you suddenly shut off the the
water down stream the water hammer effect
can damage the pipe. The stand pipe is a pressure regulator of sorts.

Bill K7NOM