The boards of commissioners of drainage districts are hereby authorized and empowered to file upon and appropriate in the manner provided by law, waters created or made available for irrigation purposes, by the construction of drainage works within such district, whenever the same can be applied to a beneficial use upon lands within the district, without impairing prior existing rights, which waters shall be equitably and ratably distributed in the manner provided by law to lands within the district which may beneficially use the same in the proportion that the assessment for drainage of each tract of said land bears to the whole assessment within the district: provided, that where lands within the district have an adequate water right and the water made available for irrigation by the construction of such works, may be beneficially used upon other lands within the district, the board of commissioners may supply such lands with water, upon their assuming to pay their proportionate share of the drainage assessments, and credit other lands within the district creating such water supply with their ratable proportion, of the actual cost of the delivery thereof.