It appears that when the previous owner had the sprinklers put in out front they replaced a length of the incoming pipe with PVC. This is a little sub-optimal for any house, but given that our inlet pressure is really high (like 130psi) its even worse. We have a pressure reducer before the pipes enter the house, but the union joint was before that and before the house shutoff as well. So we had to go to the street shutoff in the box in our yard.
So, I took a quick trip to Home Depot to pick up some PVC glue and curb key to shut off the water coming from the main. The only tricky part was deciding how far I can safely turn this valve. I expected a quarter turn or something, but looking more closely I realized the ring on the valve top was to lock off my water if I didn't pay my bill. So I dug deeper and found the corresponding ring that it would lock to, so I could tell what the fully off position would be. I ended up turning it about 270 degrees. The last thing I wanted to do was break that 40 year old valve too.
So, Cara, claiming considerable expertise in PVC gluing, glued the pipe back to the joint. This went smoothly after we realized that we wouldn't be able to really keep the joint dry because water continually welled up out of the pipe. So we stuffed it with a rag hoping to wick most of the water away from the drying glue. It has held so far.