People trap time. First, private property, no trespassing sign. Someone on your property when you aren't around isn't just a security risk, it's a liability issue. The sign acts in theory, as informed consent if they don't heed it. That's not on legal guarantee though, just what I've been told by random persons. If someone fell down on your property for instance and you weren't with them on your property, I'm not sure how that would work.
I would set a sprinkler at about 4ft high (pvc stem) with a motion detector. You can get solar ones. Heck, I'd probably do four, one each corner of the yard and set the sensors to catch anything 4ft tall or taller and turn them on when you're away from home. If I were really going to get creative I'd find a way to rig dye inside the PVC stem and see if you can paint your perp.
If I were going to go this far, I might try to find or borrow a cheap security cam to capture the excitement.