It shall be the duty of the purchasing agent of Los Angeles County, and the ex officio purchasing agent of Los Angeles County Flood Control District, unless otherwise ordered by the board of supervisors, to purchase for the district all materials, supplies, equipment, and other personal property necessary to carry out the purposes of this act, except emergency purchases, and to engage independent contractors to perform sundry services for the district, where the aggregate cost of such work, exclusive of materials to be furnished by the district, does not exceed twenty-five thousand dollars ($25,000). The purchasing agent shall make all such purchases and contracts upon proper requisition therefor, signed by the chief engineer of the district.