It will be a normal taking by Eminent Domain with the land owners receiving just compensation. If the feds sent letters that included monetary offers, the lands have already been appraised.
It’s been awhile that I read up on this, but in certain areas the Fed has already bought the land and fenced it off in preparation for construction of a “wall” (more like a fence...) Like several years ago. The trouble for the landowners is the government pays for acres along the Rio Grande, but now the cattle can’t get to the water, which makes many more acres useless to the rancher - which the government didn’t compensate him for.
An actual “wall” build on our side of the river will obviously prevent the cattle from getting to water. I suppose (hope?) that the wall or double fence or whatever will allow the cattle did get to the water.
They should offer property tax breaks for the lifetime of the current owner. They sell or move off - lose the tax break. THAT will work.