Leaking sprinkler system area top of valve?
If you turn off the water to your sprinkler system you can them remove those two screws and take off the top of the pressure vacuum breaker.  Inside you should see an O-ring that makes a nice seal.  Check that the O-ring is in good condition and then also look for things like spider webs or wasp nests that are preventing a good seal.  

If water is flowing into the sprinkler system the O-ring of the pressure vacuum breaker will prevent the water from leaking out.  If water flows the reverse direction, the pressure vacuum breaker will open and prevent the water from going back into your house (and you drinking it).  So if something is blocking the O-ring from making a good seal, then you could end up with a leak.