The use of any waters developed by the drainage system of any drainage improvement district shall be subject to the control of the drainage improvement district and such district shall have the right to dispose of and contract for the use of such waters for irrigation or other uses, as hereinafter provided: PROVIDED, That the waters developed by any existing drainage system, and the waters developed by any drainage system hereafter constructed which shall remain undisposed of for three years after the completion of the improvement and the levy of the assessment to pay the cost thereof, shall not be subject to disposal by such district where such waters shall have been appropriated by any person at a point below the outlet of the drainage system of such district. The term “waters developed” as used in this chapter shall not be held to include surface wastewaters from irrigation.