Cal. Sts. & High. Code § 27247

If after paying the cost of constructing or acquiring or both the works and interest on the bonds during the actual period of construction and for a period of six months thereafter, there is a surplus of the proceeds of the bonds issued by the district, including all premiums paid there shall be set aside out of such surplus in a reserve interest fund an amount equal to one year’s interest on the bonds, or, if the surplus shall be less than one year’s interest on the bonds, all of the surplus shall be set aside.