The district shall have authority at its option of turning any district revenues not probably required during the current year to the surplus fund by adopting a resolution to that effect and filing a copy of the same with the county treasurer in charge of such fund. For this purpose unrequired moneys may be transferred from other funds, except from either of the two bond funds.
Assessments, not exceeding twenty percent of the total levy for a given year, may be levied for the purpose of supplying moneys for the surplus fund.
The surplus fund may be used for any district purpose authorized by law, by resolution of the board of directors specifying said purpose, and the duration of such use.