To the payment of both principal and interest of the bonds and other negotiable evidences of debt to be issued under the provisions of this chapter, the entire revenues of the district from any and all sources and all real estate and railroads subject to taxation in the district are by this chapter pledged, in an amount not to exceed the amount of betterments assessed against said lands and railroads. The board of commissioners is hereby required to set aside annually from the first revenues collected from any source whatever a sufficient amount to secure and pay the interest on said bonds and evidences of indebtedness and a sinking fund for their ultimate retirement, if a sinking fund is provided for.