How do I drain the supply line?

[Rsmith99]

My sprinkler system freezes every couple of years because the water does not drain from the supply line. The riser that comes out of the ground and goes into the back flow preventer always has water in it.

How can I drain the line, at least below the ground?

 

[Terry]

You will need to add a lower shutoff and a way to drain that lower portion. I'm thinking a box to hold that in and to give access, and then foam over that in the Winter. For a drain I sometimes use a sillcock.

I can see that the spin on ball valve has frozen and cracked.