CA Civ Proc Code Section 1240.640

(a)

Where property has been appropriated to public use by any person other than the state, the use thereof by the state for the same use or any other public use is presumed to be a more necessary use than the use to which such property has already been appropriated.

(b)

Where property has been appropriated to public use by the state, the use thereof by the state is presumed to be a more necessary use than any use to which such property might be put by any other person.

(c)

The presumptions established by this section are presumptions affecting the burden of proof.