UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

California Water Service Group

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

130788102

(CUSIP Number)

12/31/2015

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

x Rule 13d-1(b)

¨ Rule 13d-1(c)

¨ Rule 13d-1(d)

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Item 1(a) Name of issuer:

California Water Service Grp

Item 1(b) Address of issuer’s principal executive offices:

1720 N. First Street

San Jose

CA, 95112

United States

2(a) Name of person filing:

Pictet Asset Management SA

2(b) Address or principal business office or, if none, residence:

60 Route des Acacias

1211 Geneva 73

Switzerland

2(c) Citizenship:

Switzerland

2(d) Title of class of securities:

Common Stock

2(e) CUSIP No.:

130788102

Item 3. If this statement is filed pursuant to §§240.13d–1(b) or 240.13d–2(b) or (c), check whether the person filing is a:

(a) ¨ Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);

(b) ¨ Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);

(c) ¨ Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);

(d) ¨ Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C 80a–8);

(e) x An investment adviser in accordance with §240.13d–1(b)(1)(ii)(E);

(f) ¨ An employee benefit plan or endowment fund in accordance with §240.13d–1(b)(1)(ii)(F);

(g) ¨ A parent holding company or control person in accordance with §240.13d–1(b)(1)(ii)(G);

(h) ¨ A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i) ¨ A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a–3);

(j) ¨ A non-U.S. institution in accordance with §240.13d–1(b)(1)(ii)(J);

(k) ¨ Group, in accordance with §240.13d–1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with §240.13d–1(b)(1)(ii)(J), please specify the type of institution:             

Item 4. Ownership

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a) Amount beneficially owned: 2,765,877

(b) Percent of class: 5.77%

(c) Number of shares as to which the person has:

(i) Sole power to vote or to direct the vote : 0

(ii) Shared power to vote or to direct the vote : 2,765,877

(iii) Sole power to dispose or to direct the disposition of : 0

(iv) Shared power to dispose or to direct the disposition of: 2,765,877

Instruction. For computations regarding securities which represent a right to acquire an underlying security see §240.13d–3(d)(1).

Item 5. Ownership of 5 Percent or Less of a Class. If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than 5 percent of the class of securities, check the following ¨.

Instruction. Dissolution of a group requires a response to this item.

Item 6. Ownership of More than 5 Percent on Behalf of Another Person. If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required.

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person. If a parent holding company or control person has filed this schedule pursuant to Rule 13d–1(b)(1)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company or control person has filed this schedule pursuant to Rule 13d–1(c) or Rule 13d–1(d), attach an exhibit stating the identification of the relevant subsidiary.

Item 8. Identification and Classification of Members of the Group

If a group has filed this schedule pursuant to §240.13d–1(b)(1)(ii)(J), so indicate under Item 3(j) and attach an exhibit stating the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to Rule 13d–1(c) or Rule 13d–1(d), attach an exhibit stating the identity of each member of the group.

Item 9. Notice of Dissolution of Group. Notice of dissolution of a group may be furnished as an exhibit stating the date of the dissolution and that all further filings with respect to transactions in the security reported on will be filed, if required, by members of the group, in their individual capacity. See Item 5.

Item 10. Certifications

(a) The following certification shall be included if the statement is filed pursuant to §240.13d–1(b):

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

(b) The following certification shall be included if the statement is filed pursuant to §240.13d–1(b)(1)(ii)(J), or if the statement is filed pursuant to §240.13d–1(b)(1)(ii)(K) and a member of the group is a non-U.S. institution eligible to file pursuant to §240.13d–1(b)(1)(ii)(J):

By signing below I certify that, to the best of my knowledge and belief, the foreign regulatory scheme applicable to [insert particular category of institutional investor] is substantially comparable to the regulatory scheme applicable to the functionally equivalent U.S. institution(s). I also undertake to furnish to the Commission staff, upon request, information that would otherwise be disclosed in a Schedule 13D.

(c) The following certification shall be included if the statement is filed pursuant to §240.13d–1(c):

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.