In laying out such proposed work and drains and ditches, the commissioners shall have the right to take and use any ditches heretofore constructed in any part of said district by any landowner owning the same. If any such ditches existing prior to the organization of such district shall be of any value to the said district, the said commissioners shall have the power to allow the said landowner reasonable compensation for the value thereof, which shall be put down on the assessment roll as a credit for ditches already constructed.