The flood irrigation taxes shall be collected by the county treasurer and all moneys so collected shall be credited to the flood irrigation fund to which they belong, and the county treasurer shall be the custodian of such funds. Payment of all the expenses and costs of locating and constructing any such project shall be made upon approval by the board of flood irrigation. Warrants therefor shall be signed by the chairman and secretary of the board. All such warrants, after presentation to the county treasurer for payment, if not paid for want of funds, shall be registered by the county treasurer, and thereafter shall bear interest at a rate not exceeding five percent per annum.