Cal. Pub. Cont. Code § 20355.1

The purchase of all supplies, equipment, and materials when the expenditure required exceeds forty thousand dollars ($40,000), and the construction of facilities and works when the expenditure required exceeds ten thousand dollars ($10,000), shall be by contract let to the lowest responsible bidder. Notice requesting bids shall be published at least once in a newspaper of general circulation. This publication shall be made at least 10 days before the bids are received. The district may reject any and all bids and readvertise in its discretion.