I don't know about Nevada, but the State of Montana has an amazingly detailed management of water rights across the whole State. Scarey actually. Water rights could easily be an underlining issue here.
Adverse possession doesn't appear to apply in this case.
First, under common law, one of the elements required to obtain title by adverse possession is 'hostile' use. That means you can't obtain title by adverse possession when your use of the land is with the consent of the owner. Second, under common law, another required element is exclusive use of the land. Without fencing, and with another 52(?) ranchers in the area at one time, I don't see how Bundy can prove, beyond a reasonable doubt, that his family's use of the federal land was exclusive.
Third, under Nevada law, color of title is required to obtain title by adverse possession. Unless Bundy can show a purported instrument of conveyance of title to the land in question, he can't obtain title by adverse possession under Nevada law.
Fourth, under Nevada law, to obtain adverse possession of land, all federal, state, and local taxes on the land must have been paid since the alleged date of occupation, or the claimant must pay them.