Cal. Public Contract Code § 21221

In all work of improvement or repair of any of the works or property of the district and in the furnishing of materials or supplies therefor, when the expenditures exceed four thousand dollars ($4,000), the work shall be done by contract, and shall be let to the lowest responsible bidder, after notice by publication in the district pursuant to Section 6066 of the Government Code and by posting the notice for at least five days at or near the door of the meeting place of the board of trustees prior to the date set for opening bids; the notices shall distinctly state the work contemplated or the materials or supplies required therefor; provided the board of trustees may reject any bid presented and readvertise and post in their discretion, and provided further, that the board may declare and determine that in its opinion the work in question can be performed more economically by day labor or the materials or supplies can be furnished at a lower price in the open market, and they may proceed to have the work done or the materials purchased without further observance of the provisions of this section.