The commissioners may provide the funds necessary for conducting drainage work by levying any tax or forced contribution which is authorized by the constitution and laws of this state.  If the tax to be levied is an acreage tax levied by the vote of the electors of the gravity drainage district for gravity drainage, then the tax shall by resolution of the governing authority of the district, be levied for the full term for which it was voted.  If the tax levied be an ad valorem tax assessed for the purpose of paying the principal and interest of the bonded indebtedness issued upon the vote of the property taxpayers, then the ad valorem tax shall be levied annually.