Cal. Water Code § 9031

If the warrants of the drainage district, or any of its units or projects, are at the time of the offer for exchange, in the hands and custody of the county treasurer of the county in which the land of the reclamation district, or of the largest portion thereof, is situated, the county treasurer of the county shall make the exchange on the terms, and in the manner provided by this chapter, whenever application is made to him for such purpose.