First to determine: Is it a mole or gopher? Moles leave tunnels at the surface in their search for insects. A pain, but not too much crop damage. Gophers, however have deeper tunnels and leave good sized mounds of dirt they have excavated from their tunnels. Gophers do eat crop roots and can cause quite a bit of damage. Killing/trapping a mole is MUCH easier than trying to eradicate gophers!! My best luck with gophers is to open the tunnel to light and wait with a 22 pistol till they come to recover the hole. Plus you get the satisfaction of personally blasting them
Moles, however are much easier to kill with the standard spiked mole traps. Just find a tunnel, flatten it with your foot and watch it for a day or two, if it's been pushed back up, you have an active tunnel and can set the trap. I have had good luck with this method. I am also sure there will be other posts with additional suggestions.