Water Code § 37162

An action, proceeding, defense, answer, or cross-complaint based on the alleged invalidity or irregularity of any agreement of sale, deed, lease, or option executed by a district in connection with property deeded to it by its collector or based on the alleged ineffectiveness of the instrument to convey or affect the title to the property described in it may be commenced or interposed only within one year after the execution by the district of the instrument.