I posted that on mobile, but I'd like to clarify that the circular white fitting on the top is what is leaking. It can be unscrewed, which exposes a valve. It's threading is not common hose threading, it is a tighter thread. I'd like to find anything which can be attached here to block flow so I can leave the faucet on and not soak the ground.

I am not sure but it looks like an anti siphon valve, It sometimes they do leak a little.

If it is an anti siphon valve, it may be required by the water company to comply with city, state, or federal regulations.

Yes, looks like a vacuum breaker but I haven't seen ones that look so odd like that.

Thanks! That was what I needed. Searching on anti-siphon valve I also found vacuum breaker valves, and a drop in replacement from Home Depot for $2.50. It looks just like the part I photographed, but mine was missing gaskets and a cosmetic / protective covering cap.