California Code, Government Code - GOV § 895.6

Unless the public entities that are parties to an agreement otherwise provide in the agreement, if a public entity is held liable upon any judgment for damages caused by a negligent or wrongful act or omission occurring in the performance of the agreement and pays in excess of its pro rata share in satisfaction of such judgment, such public entity is entitled to contribution from each of the other public entities that are parties to the agreement.  The pro rata share of each public entity is determined by dividing the total amount of the judgment by the number of public entities that are parties to the agreement.  The right of contribution is limited to the amount paid in satisfaction of the judgment in excess of the pro rata share of the public entity so paying.  No public entity may be compelled to make contribution beyond its own pro rata share of the entire judgment.