Western Washington Hydrology Model

All versions of the WWHM2012 model are available in our WWHM archive. *Note because of a change with the database only the most recent versions of the model are currently available. 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. 

Can I still use WWHM3?

Ecology continues to support the use of WWHM3 Check with your jurisdiction to ensure that you are using an accepted model. See our WWHM3 User Manual for more information.

This program is not compatible with Windows 7, 64 bit version)

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

Based on a recent model calibration effort, the soil groups and precipitation gages for Clark County in WWHM have changed. The new soil groups and precipitation data for Clark County can be accessed within WWHM2012 when “Clark 2012SG” is selected in the “Map Information” screen.  Please see Clark County Stormwater code and manual for instructions and other modeling requirements. 

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.