If only a portion of field is irrigated outside assured irrigable command area the water rates shall be chargeable on the whole field unless such portion shall have been clearly demarcated previous to the admission of water by a ridge not less than six inches high; and if a portion of a field has been irrigated by Lift Irrigation and the rest of other portion by other means the whole field shall be assessed to water rates unless clearly distinguishable boundary exists between the two portions.
