The first step is to study the lay of the land and determine which one of all the gopher mounds in the tunnel system is at the highest elevation. Secondly, open the gopher hole at the highest mound so the entire tunnel system will be flooded. Be sure that you have enough water to flood the entire tunnel system in about two minutes. This is important as the water needs to get to the gophers with enough volume to make them feel that there is no other escape than to come straight to the surface. When the volume of water is insufficient the gophers have enough time to throw up a dirt plug or dig an escape route above the water. They will escape any way they can, including letting the water inundate them while sticking their nose out to breathe as they for the water to recede. If the land adjacent to their tunnel is at a higher elevation, they may quickly construct a new tunnel above the water level without surfacing. I have observed gophers come out of the ground on the run, which required chasing them. I am sure that many times gophers have surfaced and escaped without my seeing them.
I don't recommend trying to flood gopher tunnels as a means of gopher control. Flooding will water all the plant roots making them an improved food source. Flooding will soften the soil making it easier for the gophers to dig tunnels. Also flooding is inefficient and may cause the gophers to move or simply to enlarge their tunnel systems. My recommendation is trap the gophers and be done with your problem.
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