To comply with the requirements of setting aside and paying into the local improvement guaranty fund a proportion of the monthly gross revenues of the public utilities of a district, for which guaranteed local improvement bonds and/or warrants have been issued and are outstanding, the district shall bind and obligate itself so long as economically feasible to maintain and operate the utilities and establish, maintain and collect such rates for water and/or electric energy, as the case may be, as will produce gross revenues sufficient to maintain and operate the utilities, and make necessary provision for the guaranty fund. The district shall alter its rates for water and/or electric energy, as the case may be, from time to time and shall vary them in different portions of its territory to comply with such requirements.