All special assessments and taxes levied and other revenues pledged under the provisions of this chapter to pay the cost of an improvement constitute a fund for the payment of that cost, including all principal of and interest on bonds and other obligations issued by the district to finance the improvement, and may be diverted to no other purpose. The district treasurer shall hold all moneys received for any such fund as a special fund to be applied to payment for the improvement. Each fund must be designated by the name and number of the improvement district in or for which the special assessments, taxes, and revenues are collected. When all principal and interest on bonds and other obligations of the fund have been fully paid, all moneys remaining in a fund may be transferred into the general fund of the district. Any     deficiency in any fund created for the payment of district bonds payable in whole or in part out of collections of special assessment taxes must be the general obligation of the district.