Western Washington Hydrology Model

The purpose of the Western Washington Hydrology Model (WWHM2012) is to design stormwater-control facilities so they can best mitigate the effects of increased runoff (peak discharge, duration, and volume). This model can also inform facility developers and managers on the effects likely to result from proposed land-use changes that impact nearby natural streams, wetlands, and other water courses.

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We released WWHM2012

WWHM2012 is an updated version of Western Washington Hydrology Model that includes several new elements related to Low Impact Development (LID). The last generation of the Western Washington Hydrology Model (WWHM3) could not model LID features/facilities. See a list of the changes between Model 3 to 2012.

Latest version of the model

To install the current version of the WWHM2012, download and install first the "WWHM Program" file, then the "Map Data" file below. Both files are .zip files.

Download the model first: WWHM2012 Version 4.3.2 (May 13, 2025)
Then the Map Data (July 2, 2024)

The Model will be updated as we receive comments and make improvements.

Archive of all 2012 versions

All versions of the WWHM2012 program are available in our WWHM archive. *Note because of a change with the database only the most recent versions of the model are currently availible. You will need to download these versions as .zip files and change the file exension to .msi.*

If you need an older version of the model, not currently downloadable, please contact Foroozan Labib.

WWHM2012 Version 4.3.2 (Dated: 05-13-2025) fixes a bug as described below:

An issue with autopond, autovault and autotank achieving a passing vault when the autosizing is complete, however, when the model is manually run the facility does not pass. This issue has been resolved.

WWHM2012 Version 4.3.1 (Dated: 06-26-2024)

Version 4.3.1 Includes a major update to the Wetland Fluctuation Method 2.

Criteria 2 - Mean Monthly Total Discharge Volumes from the Site

Total volume of water into a wetland on a monthly basis should not be more than:

20% higher or lower than the pre-project volumes for the months of October, November, and December.
15% higher or lower than the pre-project volumes for all remaining months (January through September).

Calculate the average of the monthly total discharge volumes from the site for each calendar month over the period of precipitation record in the approved model for pre- and post-project scenarios.

The guidance for implementing Method 2 and assessing the criteria above in the respective model is provided in section I-C.5 Wetland Hydroperiod Protection Data Collection, Evaluation Procedures, and Strategies.

Summer Months Allowable Exception for Criteria 2: The exception for summer months (July, August, and September) may be an option for projects that are not able to meet the monthly basis criteria above. To utilize this exception, additional information about the wetland is necessary. It is important to note that this information may not be available, as it goes beyond what is required to complete the standard method 2 analysis. If that is the case, then the exception will not be an option for the project being analyzed.

Western Washington Hydrology Model users manual

WWHM2012 manual (Feb. 2016)
New item WWHM Versioning Guidance (Oct. 2015)

WWHM 2012 trainings available (for a fee)

For information on training classes on Western Washington Hydrology Model WWHM2012 please contact Dan Morris at the AGC Education Foundation at 206-284-4500.

You may also go to the AGC Education Foundation website and navigate to find and register for WWHM2012 training classes. Class schedules are based on the number of people interested in training.

More information about the 2012 version of the model

Model version WWHM2012 provides:

A uniform technology and methodology for the 19 counties of Western Washington
A more accurate methodology than single-event design storms
An easy-to-use software package

Model version WWHM2012 is based on:

Continuous simulation hydrology (HSPF)
Washington's long-term recorded precipitation data
Measured pan-evaporation data
Historic vegetation (for pre-developed conditions)
Regional HSPF parameters

Parameter values can be modified to simulate local conditions and meet the needs of each county or region.

Join the Western Washington Hydrology Model email list for news, including new version releases.

Can I still use WWHM3?
Instructions for projects in Clark County only

Ecology continues to support the use of WWHM3 for those who choose to continue to use it. It is up to the individual jurisdiction to determine which model they will use. Check with your jurisdiction to ensure that you are using an accepted model.

WWHM3 User Manual.

Follow the steps below to install WWHM3 (This program is not compatible with Windows 7, 64 bit version)

Download “WWHM3.msi” file. (January 23, 2013)

Please note: When downloading this file, save it on the desktop or in a folder on your PC other than <C:\Program Files\WWHM3> (This is the default folder for the installation step).
Download “allmaps_ww.msi” file. (January 23, 2013)

Please note: When downloading this file, save it on the desktop or in a folder on your PC other than <C:\Program Files\WWHM3> (This is the default folder for the installation step.)

Western Washington County maps is to be downloaded and installed for use with WWHM3.msi.
Double click each file to open it. This starts the installation process. Follow prompts to complete installation.

Please note: When installing, let the programs install in the default subfolder <C:\Program Files\WWHM3>.

Based on a recent model calibration effort, the soil groups and precipitation gages for Clark County in WWHM3 have changed. The new soil groups and precipitation data for Clark County can be accessed within WWHM3 when “Clark 2010” is selected in the “Map Information” screen.

WWHM users can still access the old (default) soil groups and precipitation-gage data for the county by selecting “Clark” in the “Map Information” screen. However, users must check with Clark County for instructions on which county map is acceptable for their project in the county.

For instructions on soil groups and other modeling requirements in Clark County, use the following link to the Clark County website:

Clark County Stormwater code and manual

Contact information

Foroozan Labib
Water Quality Engineer
foroozan.labib@ecy.wa.gov
360-407-6439