California Code, Public Contract Code - PCC § 21131

(a) All contracts for the construction of any unit of work, except as provided in this article, estimated to cost in excess of two thousand five hundred dollars ($2,500) shall be let to the lowest responsible bidder in the manner provided in this article.  The board shall advertise by three placements in a daily newspaper of general circulation or two insertions in a weekly newspaper of general circulation published in the district inviting sealed proposals for the construction of the work before any contract is made, and may let by contract separately any part of the work.  The board shall require the successful bidder to file with the board good and sufficient bonds to be approved by the board conditioned upon the faithful performance of the contract and upon the payment of the claims for labor and material in connection therewith, the bonds to contain the terms and conditions set forth in Title 3 (commencing with Section 9000) of Part 6 of Division 4 of the Civil Code, and to be subject to the provisions of that title.  The board may reject any bid, in which case the board may advertise for new bids.

(b) In the event no proposals are received pursuant to advertisement therefor, or where the work consists of emergency work, the board of directors, by unanimous vote of all members present, 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).

(c) The district may purchase in the open market without advertisement for bids, materials and supplies for use in any work either under contract or by force account;  provided, however, that materials and supplies for use in any new construction work or improvement, except work authorized and required under circumstances referred to in subdivision (b), may not be purchased if the cost exceeds two thousand five hundred dollars ($2,500), without advertising for bids and awarding the contract to the lowest responsible bidder.

(d) This section does not apply to a contract entered into with the United States or the State of California or other governmental agency under the authority of Section 3 of Chapter 2126 of the Statutes of 1959, or to a contract authorized by a vote of the electorate of the district.

(e) The district may rent or otherwise contract for equipment with or without an operator and use it on works of the district, if the contract is approved by the board.