When the source of supply for a benefited district is obtained wholly or partly through another benefited district or if districts are supplied with their water from a common source, the board of supervisors having jurisdiction of those benefited districts, shall, upon ten days’ written notice to the trustees, hold a hearing relative to the establishment of a single benefited water district with a boundary encompassing all the area within the subject districts. If the board finds the residents and property owners in the proposed district would be benefited, it may establish the single district by resolution. In the case of districts with outstanding warrants in excess of the anticipated revenues and cash balance within the district fund, an assessment shall be drawn up by the auditor for an amount approximately fifty-five percent of the total indebtedness of the district and the board of supervisors must approve by resolution the final assessment as made and cause bonds to be issued at approximately ten percent greater than the total indebtedness of the district in accordance with sections 357.20 and 357.21 except that the bonds shall be paid, approximately equally, from user charges and the assessment. In the case of districts with bonded indebtedness, a subarea of the new single district with a boundary identical to each indebted district shall be designated and taxed in accordance with sections 357.22 and 357.23. When all bonds have been retired, the subarea shall cease to exist. In the case of districts with a surplus cash balance, all funds and credits shall become the property of the single district and used by it to the same extent as if acquired under the provisions of section 357.26. Upon establishment of the single district by the board of supervisors, a resolution shall be passed either appointing three trustees or designating the board of supervisors as the trustees for the single district. The operation of the single district constitutes a county enterprise under section 331.461, subsection 2.