Idaho Code § 42-905

Any person, association or corporation which may contract to deliver a certain quantity of water to any party or parties, shall deliver the same to such party or parties, together with a reasonable and necessary allowance for loss by evaporation and seepage, at some convenient point on the main ditch, canal or reservoir of said person, association or corporation, or on any branch or lateral thereof belonging to the owner or owners of such ditch, canal or reservoir.