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Phase II Small Municipal Separate Storm Sewer System (MS4) Program

Announcements

  • Public Notice for Comments on the Draft Small MS4 Permit and Dates for Workshops and Hearings (Notice | Aviso) (posted July 2, 2026)
  • Draft Small MS4 Permit for Public Comments
    The Draft Phase II Small MS4 permit is available for review and public comments. The links below provide the complete draft permit and the individual attachment.
  • Informal Draft
    • Response to Comments on the Informal Draft Small MS4 Permit (coming soon)
    • The Informal Draft Small MS4 Permit is available for historical context. See the Small MS4 Reissuance Page for details.

Current 2013 Phase II Small MS4 Permit

The State Water Board adopted the Phase II Small MS4 General Permit Water Quality Order (WQO) 2013-0001-DWQ on February 5, 2013. Since then, six amendments have been adopted. The links below provide the 2013 adopted permit and the six amendments.

The most recent version of the Order, which incorporates all amendments, is available here: Phase II Small MS4 Permit

The links below provide the 2013 adopted permit and the six amendments.

Order WQ 2013-0001-DWQ (adopted 2/5/2013)

Resources

  • Phase II LID Sizing Tool

    A web-based tool to assist in selecting and sizing Low Impact Development (LID) Best Management Practices (BMPs) for Regulated Projects (that create and/or replace 5,000 square feet or more of impervious surface) as defined by the Phase II Small MS4 Permit (Order No. 2013-0001-DWQ [CAS000004]). This tool was developed and is maintained by the Office of Water Programs at Sacramento State University.

  • Small Project Site Design Measure Post-Construction Calculator

    NOTE: This calculator is only to be used for projects that create and/or replace between 2,500 square feet and 5,000 square feet of impervious surface (or detached single family homes that create and/or replace over 2,500 square feet of impervious surface). For projects that create and/or replace 5,000 square feet or more of impervious surface (Regulated Projects), see the Phase II LID Sizing Tool above.

    This calculator has been developed in response to the fact that the Post-construction Calculator developed in SMARTS and referenced in the Small MS4 Permit (Order No. 2013-0001-DWQ [CAS000004]) is only available to projects that file an NOI under the General Construction Storm Water Permit (CGP). Therefore, we have developed this spreadsheet calculator for those projects that will not be required to enroll under under the CGP through SMARTS or are otherwise not required to access the SMARTS calculator, but will nonetheless be required to quantify runoff reductions resulting from implementation of Site Design Measures as specified in Section E.12.b.(ii) and F.5.g.1.(ii) of the Small MS4 Permit.

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