Cal. Civ. Proc. Code § 841

(a) Except as otherwise provided in this section, the boundaries of the area subject to a comprehensive adjudication shall be consistent with the boundaries of a basin.
(b) If the department revises the boundaries of a basin pursuant to Section 10722.2, or subdivision (b) of Section 12924, of the Water Code after a comprehensive adjudication has been initiated, the court may revise the boundaries of the area subject to the comprehensive adjudication as the interests of justice and the objectives of this chapter require.
(c) Upon a showing that a revision of the basin boundaries would further a fair and effective determination of water rights, the court may direct any of the following to submit a request to the department pursuant to Section 10722.2 of the Water Code to revise the basin boundaries:
1. (1) A party to the comprehensive adjudication.
2. (2) The State Water Resources Control Board, if the court has made a reference pursuant to Part 3 (commencing with Section 2000) of Division 2 of the Water Code.
3. (3) A special master, if one has been appointed.
(d) A determination of the department on a submission made pursuant to subdivision (c) is subject to judicial review pursuant to Section 1085. Venue shall be in the court with jurisdiction over the comprehensive adjudication and the case shall be coordinated with the comprehensive adjudication.