Cal. PCC § 20826

The city may contract with the federal government of the United States or any branch of it, or with any county, city and county, municipal corporation, district, or other public corporation, or with any person, firm, or corporation, for the joint acquisition or construction or use of any improvement to serve the district and such other area as may be designated in the contract, when, in the judgment of the governing body of the city, it is for the best interests of the district so to do. Any such contract may provide for the acquisition, construction, or maintenance of such improvement and for the payment by or for the parties to the contract of such proportionate part of the cost of such acquisition, construction, or maintenance as may be stated in the contract. The payments shall be made at such times and in such amounts as may be provided by said contract. Any such contract may provide for the joint use of any improvement upon such terms and conditions as may be agreed upon by the parties to the contract, and for the flowage, treatment, or disposal of sewage, industrial waste, surface water, or storm water from such area for each of the parties to the contract as may be described in the contract.

Any city which has acquired or constructed, or which proposes to acquire or construct, any improvement for the benefit of a district may contract with the federal government of the United States or any branch of it, or with any county, city and county, municipal corporation, district, or other public corporation, or with any person, firm, or corporation for the use of any such improvement by any such county, city and county, municipal corporation, district, or other public corporation, or for the flowage, treatment or disposal of sewage, industrial waste, surface water, or storm water from any area designated by such person, firm, or corporation so contracting, upon such terms and conditions as may be provided in the contract.