The San Francisco Bay Regional Watershed Spreadsheet Model (RWSM) was developed within the Regional Monitoring Program for Water Quality in San Francisco Bay (RMP) to estimate average annual regional and sub-regional scale pollutant loads to the Bay from stormwater. The RWSM provides estimates of relative load contributions from individual watersheds around the Bay to help identify high-load-generating watersheds. The model was also envisioned to provide information to prioritize high-leverage watershed areas. The RWSM was last updated in 2017, and new precipitation and land use information are available to update the model.
 

This report summarizes updates to the RWSM based on the new meteorological and land use information. First, we adjusted the RWSM runoff coefficients to reflect the land use classes of the MTC 2020 database. Then, we ran several simulations, progressively decreasing runoff coefficients until the errors were within the order of magnitude of those of the 2017 model update. Finally, we estimated a mean annual surface runoff volume of 1.9 x 109 m3 draining to San Francisco Bay. This value is 22% higher than the estimated surface runoff value of the 2017 model update (1.55 x 109 m3).
 

We provide the following suggestions for the model’s future use, considering the RMP management questions regarding contaminants of emerging concern (CECs). In the future, the ArcGIS Pro toolbox of the hydrology module could be adapted to execute from the command line, facilitating the application of an automated calibration technique. Estimating surface runoff as a function of precipitation and evapotranspiration can provide the model with a way to close the water budget at a regional scale. However, changes to the regional water storage (i.e., groundwater) will need to be assessed. An alternative to the current structure of the RWSM hydrology module is to adopt the hydrological output from the Watershed Dynamic Model (WDM). The output from the WDM can be organized and aggregated into an annual time step to resemble the output from the RWSM hydrology module.