The directors of such irrigation districts may designate a statewide association dedicated to the promotion of irrigated agriculture as a coordinating agency in the execution of the duties imposed by this chapter, and pay dues or assessments, or both, to the association from district expense funds, and the several districts may levy assessments against the lands therein for this purpose. Such dues and assessments shall be paid only on vouchers approved by the board of directors of the contributing district in the manner provided for the approval of district vouchers generally. The total of such voucher claims for any district in any calendar year shall not exceed two percent of the total amount or its equivalent of the expense fund levy of the district for that year.