Wash. Rev. Code § 87.03.700
When an irrigation district desires to connect its system of drainage with that of a lower drainage district or districts, it shall make the lower district or districts a party to the proceedings to construct its system, and allege in its petition that the connection is needed to afford a proper outlet and that the outlet is sufficient for both districts. If the lower system or systems must be improved to support the additional burden, the petition shall be accompanied by plans and specifications therefor. The owners of all lands in the lower district or districts affected thereby and also persons having an interest therein shall be made parties to the action and assessment for damages shall be the same as is provided by law for the establishment of the drainage system in the irrigation district.
RCW 87.03.700