You need a motion sensor (from a light perhaps) a solenoid controlled valve for your water supply and a fixed watering system.

Cat arrives, triggers motion sensor, Motion Sensor triggers solenoid valve and watering system switches,on. No more cat and your garden gets watered.

Same scenario applies to any intruder in your garden. (Yes officer we just had our wheelbarrow nicked, no we dont know who by but we do know he/she will be wet.)

Then of course there are land mines, but that's going too far.