HILL, P.J.

Certified for Partial Publication.

Messing Adam & Jasmine, Gary M. Messing, Jason H. Jasmine, San Francisco, and Lina Balciunas Cockrell for Defendants and Appellants.

Daniel C. Cederborg, County Counsel, Stockton, Catherine E. Basham, Deputy County Counsel, Fresno, for Plaintiff and Respondent.

OPINION

HILL, P.J.

Two sheriff's deputies, through their employee organization, filed a grievance challenging their involuntary reassignment from their specialty assignments to patrol assignments. They asserted the reassignments violated both the Memorandum of Understanding (MOU) between the County of Fresno (the county) and the employee organization, and an established past practice that deputies would not be involuntarily reassigned in the absence of disciplinary issues, documented performance issues, layoffs, or pending disability retirement. The administrative hearing of the grievance resulted in a decision in favor of the deputies. The county filed a petition for a writ of mandate to reverse the decision, and the trial court granted the petition. The employee organization and the deputies appeal. We conclude the arbitrator who heard the matter abused his discretion because his findings were not supported by substantial evidence. Accordingly, we affirm the judgment of the trial court.

FACTUAL AND PROCEDURAL BACKGROUND

Appellants Greg Isaac and Kirby Alstrom are employed by the Fresno County Sheriff's Office (the department) as deputy sheriffs. For several years prior to December 2014, they held specialty assignments as detectives, with Isaac in the Vice/Intelligence Unit (vice intelligence) and Alstrom in the Agricultural Crimes Task Force (ag task force).

A specialty assignment is an assignment to a detective unit, a non-patrol function. Specialty assignments include positions in units such as the U.S. Marshals Fugitive Task Force, the Multi-Agency Gang Enforcement Consortium (MAGEC), the ag task force, and vice intelligence. The department fills vacancies in specialty units by sending out flyers to the entire department, which announce an opening and specify the minimum qualifications, taking applications, and conducting examinations. Any deputy who has passed the patrol training program and is a working deputy in the courts or patrol unit is eligible to apply.

In April, Isaac's sergeant told him he was being reassigned from vice intelligence to patrol because of his tenure with the unit. In September, Alstrom's sergeant told him he would be returning to patrol in December. Alstrom was told he was being reassigned because some detectives had been in the specialty unit for a long time, and patrol deputies were complaining of the lack of opportunities to move into specialty assignments. Each deputy who works in patrol is assigned to a slot that designates the area, shift, and days off the deputy will have. Each year, the department has an opportunity to reconfigure its patrol shifts in the different areas it covers. It allows deputies to choose their patrol shift assignments based on seniority. These annual seniority shift sign-ups generally occur every October or November, and the shift changes go into effect every December. Isaac and Alstrom both participated in the annual seniority shift sign-ups for patrol in October and moved into their patrol assignments in December.

Both deputies testified they did not believe they could be reassigned from their specialty assignments to patrol involuntarily, absent documented performance problems or grounds for discipline. Neither deputy had disciplinary nor documented performance problems; both had received positive performance evaluations. They did not consent to reassignment and, through the deputies' bargaining representative, appellant Fresno Deputy Sheriff's Association (the association), they filed a grievance in protest.

The parties apparently agreed to have the grievance heard by an arbitrator, in lieu of the grievance committee provided for in the grievance procedure in the MOU. The issues presented to the arbitrator, as framed by the parties, were: "Did the [department] violate the [MOU] between the [c]ounty and the [association] when [Isaac and Alstrom] were involuntarily transferred from their special[ty] assignments? And if so, what is the remedy?" The arbitrator sustained the grievance, concluding that, in violation of the department's obligation to negotiate terms and conditions of employment, the department unilaterally changed its past practice of only involuntarily transferring deputies from their specialty assignments on the basis of disciplinary issues, documented performance issues, layoffs, or disability retirement. The decision ordered the department to cease and desist from involuntarily transferring deputies from their specialty assignments on other grounds. It also gave Isaac and Alstrom the opportunity to return to their specialty assignments, if they chose to do so.

The MOU provided that the decision on the grievance would be reviewable by administrative writ of mandate, pursuant to Code of Civil Procedure section 1094.5. The county petitioned for a writ to reverse the arbitrator's decision. After considering the administrative record of the hearing and the arguments of counsel, both written and oral, the trial court granted the petition. It ordered issuance of a writ of mandate vacating the administrative decision and denying the grievance filed by the association on behalf of Isaac and Alstrom. The association, Isaac, and Alstrom appeal the judgment. DISCUSSION

I. Standard of Review

When the trial court reviews the decision of an administrative body pursuant to section 1094.5, the inquiry "shall extend to the questions whether the respondent has proceeded without, or in excess of, jurisdiction; whether there was a fair trial; and whether there was any prejudicial abuse of discretion. Abuse of discretion is established if the respondent has not proceeded in the manner required by law, the order or decision is not supported by the findings, or the findings are not supported by the evidence." ( § 1094.5, subd. (b).)

The appellate court applies substantial evidence review. ( SP Star Enterprises, Inc. v. City of Los Angeles (2009) 173 Cal.App.4th 459, 469, 93 Cal.Rptr.3d 152.) If a fundamental vested right was involved, and the trial court therefore exercised independent judgment, the appellate court reviews the decision of the trial court. ( Ibid. ) If the trial court "applied substantial evidence review because no fundamental vested right was involved, then the appellate court's function is identical to that of the trial court. It reviews the administrative record to determine whether the agency's findings were supported by substantial evidence, resolving all conflicts in the evidence and drawing all inferences in support of them." ( Ibid. ) "If the administrative findings are supported by substantial evidence, the next question is one of law—whether those findings support the agency's legal conclusions or its ultimate determination." ( JKH Enterprises, Inc. v. Department of Industrial Relations (2006) 142 Cal.App.4th 1046, 1058–1059, 48 Cal.Rptr.3d 563.) Here, the parties agree the case does not affect a fundamental, vested right, so we review the decision of the arbitrator and apply the substantial evidence standard.

"Because [memorandums of understanding] are binding agreements between local agencies and designated employee representatives, when the meaning of a[ memorandum of understanding] is in dispute we apply de novo review, exercising our independent judgment. [Citation.] ‘It is a judicial function to interpret a contract or written document unless the interpretation turns upon the credibility of extrinsic evidence.’ " ( National City Police Officers' Assn. v. City of National City (2001) 87 Cal.App.4th 1274, 1278, 105 Cal.Rptr.2d 237.) II. Prior Proceedings

A. Administrative proceedings

On or about September 23, the association, on behalf of Isaac, Alstrom, and "all those similarly situated" (capitalization omitted), filed a grievance alleging the deputies were adversely affected by the misapplication of (1) the MOU, and (2) a clearly established lawful past practice. The association alleged there was no written agreement regarding rotation in and out of specialty assignments. However, they asserted there was a long-standing past practice of allowing deputies to remain in their specialty assignments until they chose to leave, promoted out of the assignment, or retired. The association asserted the MOU did not allow the department to involuntarily reassign deputies out of their specialty assignments, absent documented performance issues, such as a disciplinary transfer. The department, however, believed it could transfer a deputy from a specialty assignment based on being in the assignment too long; Isaac and Alstrom were the subjects of this type of transfer. The association alleged the involuntary transfers were a change from past practice that violated article 53 of the MOU, the waiver clause.

The department denied the grievance, rejecting the contention it misapplied the MOU. It asserted it had the right, under article 38 of the MOU, the management rights clause, to involuntarily move deputies and sergeants from one assignment to another, based on the department's needs, and it had done so in the past. It asserted: "The ability to move employees from one assignment to another is critical to giving employees the necessary job knowledge and experience to promote the growth and development of our employees and the [department]." It provided a list of some who had been involuntarily reassigned.

The arbitrator heard the matter over the course of two days. In his decision, he identified the baseline issue as one of contract interpretation—interpreting the application of articles 38 and 53 of the MOU. He found "based on the four corners of the MOU the [a]ssociation has been persuasive in [its] contention that the [department] violated the terms of the MOU. This is due to the fact that the [d]epartment had stepped over the line in [its] interpretation of the MOU, when [it] determined that [Isaac and Alstrom] could be involuntarily transferred even though they did not want to leave the [specialty] assignments." The arbitrator concluded there was a longstanding past practice that involuntary transfers out of specialty assignments were limited to transfers based on discipline, documented performance issues, layoffs, or pending disability retirement. The arbitrator sustained the grievance, concluding: "based on the evidence the arbitrator's finding is that the [d]epartment violated Government Code Section 3505 and [a]rticle 53 of the MOU, when it unilaterally changed the past practice of involuntar[il]y transferring the deputies from their special[ty] assignments based on criteria other than that of disciplinary issues, documented performance issues, layoff[s,] or disability retirement."

B. Trial court proceedings

The county petitioned the trial court for an administrative writ of mandate overturning the decision of the arbitrator. It asserted the arbitrator abused his discretion, "because the evidence submitted does not support the finding that [the c]ounty misapplied any section of the MOU or acted contrary to any lawful past practice." Additionally, the arbitrator's decision constituted an error of law, because the arbitrator "failed to interpret [a]rticles 38 and 53 of the MOU in a manner that harmonized the two provisions."

The trial court granted the petition. It noted that, while the arbitrator identified the baseline issue as one of contractual interpretation, he "devoted almost no discussion to the relevant MOU provisions." The trial court analyzed articles 38 and 53 of the MOU. It concluded article 38 granted the department "the power to assign, reassign, and determine the procedures and standards for reassignment of employees." Article 38 did not limit the department's power to reassign employees to situations in which there was a performance, disciplinary, or similar justification for the reassignment. Article 53 precluded unilateral changes in matters included in the MOU during its term. It waived the parties' obligation to meet and confer over matters included in the MOU or within the scope of negotiation during the term of the MOU; it did not limit the county's rights as set out in the MOU.

Regarding the issue of past practice, the trial court noted the arbitrator did not identify any testimony or other evidence that supported his conclusion that the parties had a past practice of limiting involuntary transfers out of specialty assignments to situations in which the transfer was due to disciplinary reasons, documented performance issues, layoffs, or pending disability retirement. It reviewed the evidence in the administrative record and applied the test for determining when a past practice has been established by the evidence. A binding past practice must be unequivocal, clearly enunciated and acted upon, and readily ascertainable over a reasonable period of time as a fixed and established practice accepted by both parties. The trial court concluded the administrative record did not contain substantial evidence supporting the arbitrator's finding that there was an established past practice of making involuntary transfers only in the limited situations he described. Further, it decided the arbitrator exceeded his jurisdiction by concluding the department violated Government Code section 3505. The arbitrator did not discuss the statute in his decision; the issue was not raised in the grievance and was not properly before the arbitrator. C. Grievance

Article 49 of the MOU sets out the employee grievance procedure, pursuant to which the association and the deputies filed their grievance. It provides, in pertinent part:

"A grievance is a complaint relating to any phase of an employee's employment or working conditions which the employee believes has been adversely affected because of:

"A misapplication of a Memorandum of Understanding, Ordinance or Resolution of the Board of Supervisors, or of the written policies, administrative orders, or a clearly established lawful past practice of a department, relating to the employment of the individual ...."

The grievance in this case alleged a misapplication of both the MOU and a past practice of the department. We analyze both allegations.

III. Violation of the Express Terms of the MOU

Under the Meyers-Milias-Brown Act (MMBA; Gov. Code, §§ 3500 – 3511 ), public employees have the right to be represented by employee organizations in all matters of employer-employee relations. ( Gov. Code, §§ 3502, 3503.) "The MMBA applies to local government employees in California. [Citation.] ‘The MMBA has two stated purposes: (1) to promote full communication between public employers and employees, and (2) to improve personnel management and employer-employee relations. [Citation.] To effect these goals the act gives local government employees the right to organize collectively and to be represented by employee organizations [citation], and obligates employers to bargain with employee representatives about matters that fall within the "scope of representation" [citations].’ [Citation.] The duty to meet and confer in good faith is limited to matters within the ‘scope of representation ....’ " ( Claremont Police Officers Assn. v. City of Claremont (2006) 39 Cal.4th 623, 630, 47 Cal.Rptr.3d 69, 139 P.3d 532, fn. omitted.) The scope of representation includes wages, hours, and other terms and conditions of employment. ( Gov. Code, § 3504.)

The public agency employer must meet and confer in good faith with the employee organization on matters within the scope of representation prior to arriving at a determination of policy or a course of action. ( Gov. Code, § 3505.) If the public agency and the employee organization reach a tentative agreement, and the governing body of the public agency adopts the tentative agreement, "the parties shall jointly prepare a written" MOU. ( Gov. Code, § 3505.1.) Under the MMBA, an MOU adopted by the governing body of a public agency becomes a binding and enforceable contract that neither party may change unilaterally. ( City of Los Angeles v. Superior Court (2013) 56 Cal.4th 1086, 1092–1093, 158 Cal.Rptr.3d 1, 302 P.3d 194.)

MOU's are binding contracts and are interpreted in accordance with the general rules of contract interpretation. ( National City Police Officers' Assn. v. City of National City, supra , 87 Cal.App.4th at pp. 1278–1279, 105 Cal.Rptr.2d 237.) The goal of contract interpretation is to effectuate the mutual intent of the parties as it existed at the time of contracting insofar as it is ascertainable and lawful. ( Id. at p. 1279, 105 Cal.Rptr.2d 237.) " ‘Such intent is to be inferred, if possible, solely from the written provisions of the contract. [Citation.] The "clear and explicit" meaning of these provisions, interpreted in their "ordinary and popular sense," unless "used by the parties in a technical sense or a special meaning is given to them by usage" [citation], controls judicial interpretation. [Citation.] Thus, if the meaning a layperson would ascribe to contract language is not ambiguous, we apply that meaning.’ " ( Hervey v. Mercury Casualty Co. (2010) 185 Cal.App.4th 954, 961, 110 Cal.Rptr.3d 890.) No extrinsic evidence was offered regarding the meaning the parties intended be given to articles 38 and 53 at the time of contracting. Consequently, we determine the meaning of these provisions de novo.

The arbitrator did not interpret the language of the MOU. Although he concluded, "based on the four corners of the MOU," that the department violated the MOU by involuntarily transferring Isaac and Alstrom, when they did not fall within any of the categories he deemed permissible and they did not want to leave their specialty assignments, he did not identify any provision of the MOU prohibiting involuntary transfers. The trial court correctly interpreted the language of the MOU, concluding article 38 conferred on the county the right to reassign its employees and article 53 did not limit that right.

Article 38 of the MOU, the management rights clause, provided, in part:

"A. All [c]ounty rights, powers, functions, and authorities except as expressly abridged by this MOU shall remain vested in the [c]ounty whether or not they have been exercised in the past. [¶] ... [¶]

"H. The rights, powers, and authorities of the [c]ounty include, but are not limited to, the sole and exclusive right to: [¶] ... [¶]

"4. select, train, direct, assign, demote, promote, layoff, dismiss its employees; [¶] ... [¶]

"7. relieve its employees from duty or reassign employees because of lack of work or for other reasons the [c]ounty considers legitimate; [¶] ... [¶]

"9. determine and change the method, means, personnel, and standards by which [c]ounty operations are to be conducted; [¶] ... [¶]

"13. make rules and regulations pertaining to employees consistent with this MOU; ..."

Thus, on the face of the MOU, the county, with the agreement of the association, reserved to itself all rights not expressly abridged by other provisions of the MOU; the MOU specifically granted the county the "sole and exclusive right" to assign its employees, and to reassign them for reasons the county considered legitimate.

Article 53 of the MOU, the waiver clause (also referred to as the zipper clause), provided:

"The parties acknowledge that, for the life of this MOU, each voluntarily and

unqualifiedly waives the right, and each agrees that the other shall not be obligated, to bargain collectively with respect to any subject or matter pertaining to or covered by this MOU, or any other matter within the scope of negotiations, notwithstanding any other provisions of law to the contrary. Furthermore, the parties agree that neither party shall lobby the board of the other party regarding making changes on matters within the scope of negotiations that could occur during the term of the MOU."

" ‘[T]he general purpose of a zipper clause is to "zip up" the collective bargaining agreement. It insulates both parties to the agreement from a demand by the other party to reopen negotiations with the intent of modifying or adding to the current contract terms or otherwise changing the status quo.’ It does not insulate the unions (or the City) from timely requests to begin bargaining about the next [memorandum of understanding]." ( City of Fresno v. People ex rel. Fresno Firefighters (1999) 71 Cal.App.4th 82, 98, 83 Cal.Rptr.2d 603.) Thus, once the parties have agreed on the provisions of the MOU, the zipper clause locks them in place for the duration of the agreement. Neither party can compel the other to renegotiate the MOU's provisions or to negotiate additional provisions until it is time to negotiate a new MOU. Because any changes to the agreement must be negotiated, and they cannot be negotiated during the term of the MOU, the MOU cannot be unilaterally changed during its term.

As the trial court observed, the management rights clause granted the county the right to assign and reassign deputies, without limiting the reassignment right to cases of discipline, poor performance, and the like. Consequently, the department did not violate the express written terms of the MOU by reassigning Isaac and Alstrom involuntarily, even in the absence of disciplinary issues, documented performance issues, layoffs, or pending disability retirement.

Appellants argue that the MOU reserved to the county its reassignment rights "except as expressly abridged by this MOU" (article 38, section A), and the zipper clause expressly abridged those rights. We reject this argument. The zipper clause did not contain any provisions limiting the county's reassignment rights. Further, the function of a zipper clause is not to abridge or alter the substance of any of the other provisions to which the parties agreed in the MOU. Once an agreement is reached on the other provisions, the zipper clause locks them in for the duration of the MOU's term. It prevents either party from unilaterally changing those provisions and from compelling renegotiation of the agreed upon provisions, unless the parties agree to reopen negotiations. It does not change the substance of the other provisions of the MOU. The zipper clause in the parties' MOU did not change or limit the scope or application of the rights actually agreed upon in article 38.

Consequently, we conclude the arbitrator incorrectly interpreted the MOU when he determined that, "based on the four corners of the MOU the [a]ssociation has been persuasive in [its] contention that the [department] violated the terms of the MOU ... when [it] determined that [Isaac and Alstrom] could be involuntarily transferred even though they did not want to leave the [specialty] assignments." Appellants did not establish a violation by the county of the express terms of the MOU.

IV. Violation of Past Practice

The second ground for appellants' grievance was their allegation that the parties had established a past practice that deputies would not be reassigned out of their specialty assignments except for specified reasons, and that the practice was unilaterally changed by the department when Isaac and Alstrom were reassigned to patrol. The arbitrator found in their favor on that claim. A public agency may not "[r]efuse or fail to meet and negotiate in good faith with a recognized employee organization." ( Gov. Code, § 3506.5, subd. (c).) "An employer's unilateral change in terms and conditions of employment within the scope of representation is, absent a valid defense, a per se refusal to negotiate ...." ( California State Employees' Assn. v. Public Employment Relations Bd. (1996) 51 Cal.App.4th 923, 934, 59 Cal.Rptr.2d 488.) An existing and acknowledged practice affecting conditions of employment has the same dignity as an existing agreement, when it is sufficiently widespread and of sufficient duration. ( Vernon Fire Fighters v. City of Vernon (1980) 107 Cal.App.3d 802, 817, 165 Cal.Rptr. 908.) Thus, an employer's unilateral change in a well-established practice may violate its duty to meet and negotiate in good faith with the employee organization prior to changing the terms and conditions of employment.

"[U]nder standards established by PERB, to prevail on a complaint of illegal unilateral change, the union must establish: (1) the employer breached or altered the parties' written agreement, or own established past practice ; (2) such action was taken without giving the exclusive representative notice or an opportunity to bargain over the change; (3) the change is not merely an isolated breach of the contract, but amounts to a change of policy, i.e., the change has a generalized effect or continuing impact on bargaining unit members' terms and conditions of employment; and (4) the change in policy concerns a matter within the scope of representation." ( California State Employees' Assn. v. Public Employment Relations Bd., supra , 51 Cal.App.4th at p. 935, 59 Cal.Rptr.2d 488, italics added.) The issue argued by the parties in this appeal is whether substantial evidence supports the arbitrator's conclusion on the first element: that the department unilaterally altered the parties' established practice of not involuntarily reassigning deputies from their specialty assignments, unless the reassignment was the result of discipline, documented performance issues, layoffs, or pending disability retirement. This encompasses two subissues: (1) whether substantial evidence supports the implied finding that such an established practice existed; and (2) whether substantial evidence supports a finding that the department unilaterally changed the past practice when it reassigned Isaac and Alstrom.

A. Alleged trial court error

Appellants argue that the trial court failed to look for evidence supporting the administrative decision and substituted its judgment for that of the arbitrator. The argument is moot. In this appeal, our review standard is identical to that of the trial court. ( Sierra Club v. California Coastal Com. (1993) 12 Cal.App.4th 602, 610, 15 Cal.Rptr.2d 779.) We review the administrative decision to determine whether substantial evidence supports the findings, and whether the findings support the decision. ( § 1094.5, subd. (b).) "Possible misapplication of the review standard below has no bearing on the outcome here." ( Sierra Club , at p. 611, 15 Cal.Rptr.2d 779.)

B. Effect of the terms of the MOU

The county argues that, when the language of an MOU is clear and unambiguous, it is unnecessary and inappropriate to examine past practice (citing Marysville Joint Unified School District (1983) PERB Dec. No. 314 [7 PERC ¶ 14163] (Marysville )). It asserts the MOU was clear in setting out the county's right to assign and reassign deputies, so the arbitrator should not have considered past practice in interpreting the contract.

In Marysville , the collective bargaining agreement "entitled teachers to ‘one duty free lunch break of no less than 30 minutes each day.’ " (Marysville, supra , PERB Dec. No. 314 at p. 8 [7 PERC ¶ 14163, p. 8].) The consistent practice of the district had been to allow teachers a 55-minute duty-free lunch break. During a budget crisis, the district assigned teachers to supervise students during the lunch period, reducing their duty-free lunch break to 30 minutes. (Id. at pp. 3–4.) The teachers challenged this as a unilateral change in their hours. The PERB decision stated: "An employer violates its duty to negotiate in good faith when it unilaterally changes an established policy affecting a negotiable subject matter without affording the exclusive representative a reasonable opportunity to bargain. [Citations.] Established policy may be embodied in the terms of a collective agreement [citation] or, where a contract is silent or ambiguous as to a policy, it may be ascertained by examining past practice or bargaining history. [Citations.] However, where contractual language is clear and unambiguous, it is unnecessary to go beyond the plain language of the contract itself to ascertain its meaning." (Id. at pp. 8–9.) Because the provision for a duty-free lunch break of no less than 30 minutes was unambiguous, PERB concluded the district acted consistently with its contractual obligations. Further, "[t]he mere fact that an employer has not chosen to enforce its contractual rights in the past does not mean that, ipso facto, it is forever precluded from doing so. [Citation.] Accordingly, we find that the Association, by agreeing to a contractual provision which plainly permitted the District to grant teachers a lunch period of 30 minutes or longer at its discretion, waived its right to negotiate over the District's reduction of the lunch period to 30 minutes." (Id. at p. 10.)

In Oakland Unified School District (2005) PERB Decision No. 1770 [29 PERC ¶ 143], the school district, after negotiating a new collective bargaining agreement with the school employees' association, eliminated its police force and entered into a contract in which it agreed to pay the city to provide police protection. (Id. at p. 3) The school employees' association challenged the change as subcontracting of bargaining unit work to a third party, which the district failed to negotiate. The district claimed its action was authorized by the management rights clause in the collective bargaining agreement. In that clause, the district "retain[ed] the exclusive right to manage the school district including, but not limiting, its rights to determine the methods, means and personnel by which the District operations are to be conducted; and to determine the missions and functions of each of its departments, sites, facilities and operating units, ..." (Id. at p. 48.) A prior PERB decision had determined that a broadly based management rights clause would not be construed as a waiver of statutory bargaining rights, because any waiver of a right to bargain was required to be clear and unmistakable. (San Jacinto Unified School District (1994) PERB Dec. No. 1078, p. 18 [19 PERC ¶ 26036, p. 18].) The management rights clause in Oakland Unified School District did not specifically address either subcontracting or police services. PERB concluded the management clause before it was "a generally worded clause without reference to subcontracting or police services and therefore [did] not support the unilateral action by the District." (Oakland Unified School District , at p. 9.) In County of San Bernardino (Office of the Public Defender) (2015) PERB Decision No. 2423-M [39 PERC ¶ 165], PERB rejected the public defender's argument that past practice trumped the wording of the memorandum of understanding, stating: "[W]hen parties' past practice conflicts with the wording of their [collective bargaining agreement], each party ‘still maintains the right to adhere to and enforce the contractual language of the [collective bargaining agreement].’ " (Id. at p. 53.)

Thus, when the action of the employer that is being challenged is directly addressed in the MOU, the parties are bound by the terms of the written agreement even when the employer has not exercised its rights previously. When the employer's action is addressed in an ambiguous provision, evidence of a past practice may assist in interpreting the ambiguous provision. When the MOU does not specifically address the situation, a past practice that is not inconsistent with the terms of the MOU may be enforceable if it is sufficiently established.

In this case, as in Oakland Unified School District , the employer relies on a management rights provision in the MOU as authorizing the challenged change in working conditions. Unlike the general management rights provision in Oakland Unified School District , which did not mention subcontracting or police services (the changed working conditions), the management rights clause in this case (article 38) specifically granted the county the right to assign and reassign employees. Although the clause was not as specific to the challenged conduct as the provision for a 30-minute lunch period in Marysville , article 38 did expressly permit the county to assign and reassign employees for reasons it considered legitimate, and to determine the procedures and standards for reassignment. Involuntarily reassigning deputies, even in the absence of disciplinary issues, documented performance issues, layoffs, or pending disability retirement was consistent with article 38. The association, by agreeing to the management rights clause, agreed that the county had the sole and exclusive right to assign and reassign employees for reasons it considered legitimate, and to determine the procedures and standards for reassignment. The department's action in reassigning Isaac and Alstrom was consistent with these terms of the MOU. By granting the county this authority to reassign employees, the association made a clear and unmistakable waiver of its right to renegotiate the grounds on which the county was permitted to reassign its employees, and the procedures for doing so, during the term of the MOU.

We conclude the association failed to establish that the MOU was ambiguous or silent regarding reassignment of employees, and therefore there was no need to consider the past practices of the parties to determine whether the county violated its obligation to negotiate a change in practice.

C.-D.

DISPOSITION

The trial court judgment granting the petition for writ of mandate and vacating the arbitrator's decision is affirmed. The county is entitled to its costs on appeal.

WE CONCUR:

SMITH, J.

MEEHAN, J.