Cal. Public Contract Code § 21020.2

All contracts for any improvement or unit of work, except as provided in this article, estimated to cost in excess of ten thousand dollars ($10,000), shall be let to the lowest responsible bidder in the manner provided in this article. The board of supervisors of the district shall advertise by five or more insertions in a daily newspaper of general circulation, or by two or more insertions in a weekly newspaper of general circulation, printed and published in the district, inviting sealed proposals for the construction or work of the proposed improvement or work and may let by contract separately, any part of the work or improvement. The board shall have the right to reject any bids not suitable to the best interests of the district. In the event all proposals are rejected, or no proposals are received pursuant to advertising, or the estimated cost of the work does not exceed ten thousand dollars ($10,000), or the work consists of channel protection, maintenance work, or emergency work, the board of supervisors may, without advertising for bids, have the work done by force account. In case of an emergency, if notice for bids to let contracts will not be given, the board shall comply with Chapter 2.5 (commencing with Section 22050). The district shall have power to purchase in the open market, without advertising for bids, materials and supplies for use in any work being done by it either under contract or by force account whenever the estimated costs of the materials or supplies do not exceed five thousand dollars ($5,000); and provided further, that in case of emergency work, the district shall have the power to purchase in the open market, without advertising for bids therefor, materials and supplies for use in any work, even though the cost thereof should exceed five thousand dollars ($5,000).