All assessments made by an irrigation district for the payment of the bonds or improvement warrants to be refunded must inure to the benefit of the holders of the refunding bonds and the proceeds of the assessments must be utilized for the purpose of paying the interest and principal of the refunding bonds, and the board of directors of the irrigation district shall levy an assessment against the lands of the district, as provided by law for levying assessments, sufficient to pay the interest on the refunding bonds and to create a sinking fund to retire the refunding bonds at maturity.