IT IS YOUR LANDSCAPE. ENJOY IT.
Many times properties have PVB / Backflow for the irrigation system and sometimes for other things. They might look like the ones in the photos below. They may have a copper colored top instead of a black one like these. Often times we see them located in homes front or the side yards. Many times turning the PVB off will stop water from leaking in an irrigation system.
Never force knobs and make sure it is only for the irrigation.