This dataset shows estimated values for minimum depth to groundwater in the coastal Bay Area. The estimation is based on an interpolation that uses ground elevation data and minimum depth to water values measured at monitoring wells in the nine Bay Area counties over the past 20 years.

NOTE: The most recently uploaded data file (August 24, 2018) has the highest accuracy in the interpolation. Please use this file rather than the older versions.

A full description of the methods is available in the article where this dataset is published: Plane, E.; Hill, K.; May, C. A Rapid Assessment Method to Identify Potential Groundwater Flooding Hotspots as Sea Levels Rise in Coastal Cities. Water 2019, 11, 2228. https://www.mdpi.com/2073-4441/11/11/2228

Well data:

California State Water Control Board: GAMA GeoTracker. http://geotracker.waterboards.ca.gov/data_download_by_county.

Ground elevation data:

U.S. Geological Survey Coastal and Marine Geology Program https://topotools.cr.usgs.gov/coned/sanfrancisco.php. 2m Digital Elevation Model (DEM).

SF Bay extent (includes open water and tidal wetlands):

San Francisco Estuary Institute (SFEI): Bay Area Aquatic Resource Inventory (BAARI). http://www.sfei.org/baari#sthash.AZdz9gDM.yexzxDn5.dpbs.

Tidal datums:

Produced by AECOM for FEMA and the San Francisco Bay Conservation and Development Commission (Mak et al., 2016).

GIS FILE PROPERTIES:

File format: TIFF.

Cellsize: 2m.

Linear unit: Meters.

Z unit: Meters.

Projected Coordinate System: NAD_1983_2011_StatePlane_California_III_FIPS_0403

Geographic Coordinate System: GCS_NAD_1983_2011.

METHODOLOGY:

We downloaded well depth to water and well location files from the CA State Water Board GeoTracker database for each of the nine Bay Area counties. We concatenated “Global ID” and “Field Point Name” to create a unique ID for each well in the database, then calculated the minimum depth to water value for each well for the years 1996-2016 (any measurements before 1996 were excluded). If a well had no recorded measurements for depth to water, or if the minimum depth to water was a negative value (artesian wells), we eliminated the well from the dataset.

For wells within one mile (1.6km) of the SFEI Bay/tidal wetlands edge file, we calculated maximum water table elevation by extracting a ground elevation from the 2m USGS digital elevation model (DEM) at each well point, then subtracting the minimum depth to water value from this ground elevation.

We also included tidal datum data to smooth the resulting surface toward the Bay, adding 0.3 meters to the mean tide line elevation because the freshwater aquifer usually lies above the mean tide line.

To create a water table surface, we interpolated between the well and tidal datum elevation points using a multiquadric radial basis function. Using cross-validation, we were able to determine that this method produced the lowest error values relative to other deterministic interpolation methods (inverse distance weighting, global polynomial interpolation, and other radial basis functions). The root-mean-square error for the multiquadric radial basis interpolation was 1.167 m and the mean error was -0.010 m.

We subtracted the interpolated water table surface from the ground surface DEM to produce a minimum depth to water map. Results are shown within one kilometer of the Bay.

Wells are not evenly distributed across the bay shorefront; we excluded areas greater than 1 km from the nearest well due to the uncertainty introduced by the lack of sample points in these areas.

The interpolation and subtraction method we used produced some negative values for depth to water, indicating water above the ground surface. These areas most likely have very shallow groundwater, but there is considerable uncertainty associated with them. In the provided data file, we have changed these negative values to zero.

Note: results shown only for areas within 1km of well points in the dataset. If data is missing for a coastal location, this does not indicate anything about the water table at that location, but rather a lack of available data. Please reference the shapefile included in the NoDataZones folder for areas within 1km of the Bay but excluded from the output for this reason.