Corporations formed under the laws of this state for the purpose of constructing and operating canals for irrigation by gravity have power to acquire all land needed by them for rights of way over the routes selected by them and for reservoirs, and in the event of failure to agree with the owners of the land on the price to be paid for it, they have power to expropriate this land, such power to be exercised in the manner now provided for by the general expropriation laws of the state.  This power of expropriation shall not be exercised within any township or section occupied by any canal in actual use, without the consent of the owners of such canal.