Me too.I'm just trying to understand the system you're designing.
It seems like we are arriving at a high-flow shut-off. So if a head or two is severed by the landscapers, the system when activated will shut down "quickly" when it senses that the flow is too great. The trip point would be set by prior calibration.
The flow could be sensed by ∆P across a flow restriction, venturi tube or whatever, or by some actual flowmeter such as a spinning magnet like those in water softeners.
A low-pressure shut-off is an alternative. It's really the same as measuring ∆P as above, except the second pressure is just atmospheric. If adequate pressure is not reached within some time of turning on the water, the system could shut itself down.
Either method might struggle with air in the system. As #12 noted also, detecting a lower pressure would be tough if all the normal ∆P is across the heads, and the supply pressure doesn't care. Most of the systems in my neighborhood would suffer this problem I think - plenty of pressure in the system whether a head is missing or not.