(a) In addition to all other disclosures required by this chapter, a party shall disclose to the other parties the identity of any expert witness it may use at trial to present evidence.
§ 843.Disclosure of Expert Witnesses
Title 10. Actions In Particular Cases · Chapter 7. Actions Relating to Groundwater Rights · Article 10. Expert Witnesses · Enacted 2015 · no amendments on record · Last verified July 28, 2026
In one sentenceSection 843 requires a party in a comprehensive adjudication to disclose each expert witness it may call at trial, generally with a signed written report covering the expert's opinions and qualifications, on a schedule tied to each phase of the case, with a duty to supplement.
Full Text of § 843
(b) Unless otherwise stipulated by the parties or ordered by the court, the disclosure made pursuant to subdivision (a) shall be accompanied by a written report prepared and signed by the expert witness if the witness is retained or specially employed by the party offering the expert witness to testify as an expert in the action, or if the expert witness's duties as the party's employee regularly involves giving expert testimony. The report shall include all of the following:
(1) A complete statement of all opinions the witness will express and the basis and reasons for those opinions.
(2) The facts or data considered by the witness in forming his or her opinions.
(3) Any exhibits the witness will use to summarize or support his or her opinions.
(4) The witness's qualifications, including a list of all publications authored by the witness in the previous 10 years.
(5) A list of all other cases in which the witness testified as an expert at trial or by deposition in the last five years.
(6) A statement of the compensation to be paid for the witness's work and testimony in the comprehensive adjudication.
(c) If subdivision (b) does not apply to an expert witness because of a stipulation by the parties or an order of the court, the witness's disclosure shall include both of the following:
(1) The subject matter on which the witness is expected to present evidence.
(2) A summary of the witness's opinions, and the facts or data considered by the witness in forming his or her opinions.
(d) Unless otherwise stipulated by the parties, a party shall make the disclosures of any expert witness it intends to present at trial, except for an expert witness presented solely for purposes of impeachment or rebuttal, at the times and in the sequence ordered by the court. If there is no stipulation or court order, the disclosures of an expert witness shall be made as follows:
(1) At least 30 days after the court's entry of an order establishing the scope of the relevant phase of the comprehensive adjudication.
(2) Except for a supplemental expert witness described in paragraph (3), at least 60 days before the date set for trial of the relevant phase of the comprehensive adjudication.
(3) For a supplemental expert witness who will express an opinion on a subject to be covered by another expert witness designated by an adverse party that was not among the subjects covered by an expert witness initially disclosed by the party offering the supplemental expert witness, no more than 20 days after the initial expert witness disclosure date.
(e) The court may modify the disclosure requirements of subdivisions (b) to (d), inclusive, for expert witnesses presented solely for purposes of impeachment or rebuttal. In modifying the disclosure requirements, the court shall adopt disclosure requirements that expedite the court's consideration of the issues presented and shall ensure that expert testimony presented solely for purposes of impeachment or rebuttal is strictly limited to the scope of the testimony that it intends to impeach or rebut.
(1) A party whose expert witness has made a disclosure pursuant to this section shall promptly supplement or correct the expert witness's disclosure in either of the following instances:
(A) In a timely manner if the party learns that in some material respect the disclosure is incomplete or incorrect, if the additional or corrective information has not otherwise been made known to the other parties during the disclosure or discovery process.
(B) As ordered by the court.
(2) A party's duty to supplement or correct its expert witness's disclosure includes the information included in the report and the information given during the expert witness's deposition. Unless otherwise stipulated by the parties or ordered by the court, any supplementation or correction shall occur at least 14 days before trial of the applicable phase of the comprehensive adjudication.
(3) The court may authorize a supplemental deposition of an expert witness based on a supplemental disclosure made pursuant to this subdivision. The court shall appropriately condition the authorization of a supplemental deposition of an expert witness to ensure the expeditious completion of the applicable phase of the comprehensive adjudication. The court may require the party whose expert makes the supplemental disclosure to pay some or all of the costs associated with the supplemental deposition.
(g) To the greatest extent possible, the parties shall serve expert witness disclosures electronically through an electronic service system, an electronic document repository, email, or another method of electronic transmission. If it is not possible for the party to serve his or her expert witness disclosures electronically, he or she shall serve the expert witness disclosures in an electronic format saved on a portable storage media device such as a compact disc or flash drive.
Plain-English Summary
Given the technical, hydrogeological questions comprehensive adjudications often turn on, this section builds a structured expert-disclosure process on top of the ordinary initial disclosures required by § 842. Subdivision (a) requires disclosure of any expert witness a party may use at trial. Subdivision (b) generally requires that disclosure to come with a written, signed report covering the expert's opinions and their basis, the facts or data relied on, supporting exhibits, the expert's qualifications and recent publications, other cases where the expert testified in the last five years, and the expert's compensation — unless the parties stipulate or the court orders a lighter disclosure under subdivision (c), which applies to experts not independently retained for the case.
Subdivision (d) sets the default timing absent a stipulation or court order: disclosures at least thirty days after the court sets the scope of the relevant phase, and at least sixty days before trial of that phase, with a narrower window for a supplemental expert responding to a new subject an adverse party's expert raised. Subdivision (e) lets the court adjust these requirements for experts offered solely for impeachment or rebuttal, while making sure that testimony stays limited to what it's rebutting or impeaching.
Subdivision (f) requires prompt supplementation of an incomplete or incorrect disclosure, including information from a deposition, generally due at least fourteen days before trial of the applicable phase, and lets the court authorize a supplemental deposition and allocate its cost. Subdivision (g) again pushes toward electronic service to the greatest extent possible.
Frequently Asked Questions
What must accompany a typical expert witness disclosure under this section?
A written report signed by the expert covering the opinions and their basis, underlying facts and data, exhibits, qualifications, recent publications, prior testimony history, and compensation.
When is expert disclosure due, absent a stipulation or court order?
At least thirty days after the court sets the scope of the relevant phase, and at least sixty days before trial of that phase.
Are experts used only for impeachment or rebuttal treated differently?
Yes. Section 843(e) lets the court modify the disclosure requirements for those experts, while limiting their testimony strictly to what they're rebutting or impeaching.
What if new information affects a disclosed expert's opinions?
The party must promptly supplement or correct the disclosure, generally no later than fourteen days before trial of the applicable phase.
Amendment History
Added by Stats 2015 ch 672 (AB 1390),s 1, eff. 1/1/2016.
Source & verification. Section text is reproduced verbatim from
the Deering's California Codes Annotated / vLex. Enacted by the California Legislature.
Last verified July 28, 2026.
· Official source
Also known as: expert witness disclosure groundwater adjudication california