Cal. Water Code - WAT § 2900. Section 2900.
In rendering its decree for the determination of rights to water, whether in suits referred to the board under the provisions of Chapter 1 of this part or in adjudications under the provisions of Chapter 3 of this part, the court may provide that the board or any party affected by the decree may, at any time within three years from entry thereof, apply to the court for a modification of the decree insofar only as the decree determines quantities of water, and after hearing the motion and any competent and admissible evidence offered in support of or against the motion the court may modify the decree by increasing or decreasing the quantities of water therein allowed.