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Minimum Depth to Groundwater for the Coastal San Francisco Bay Area

Citation

Plane, Ellen; Hill, Kristina (2017), Minimum Depth to Groundwater for the Coastal San Francisco Bay Area, v5, Dataset, https://doi.org/10.6078/D1W01Q

Abstract

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.

Methods

Well data California State Water Control Board: GAMA GeoTracker. http://geotracker.waterboar... Ground elevation data U.S. Geological Survey, The National Map, 2016, 3DEP products and services: The National Map, 3D Elevation Program Web page. 1/3 arcsecond DEM. http://nationalmap.gov/3DEP... 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#s... GIS FILE PROPERTIES: File format: ESRI ArcGRID Cellsize: 30.54 Linear unit: Feet Z unit: Feet Projected Coordinate System: NAD_1983_2011_StatePlane_California_III_FIPS_0403_Ft_US 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, we eliminated it from the analysis. If the minimum depth to water was a negative value (artesian wells), we changed the negative to zero, indicating a groundwater level at the ground surface. We joined coordinates from the latitude/longitude table to the well table for each well ID. If a well’s reported coordinates were outside the reported county, we excluded it from the analysis. For wells within one mile of the SFEI Bay/wetlands file, we calculated maximum water table elevation by extracting a ground elevation from the USGS digital elevation model (DEM) at each well point, then subtracting the minimum depth to water value from this ground elevation. To create a water table surface, we interpolated 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). Root Mean Square Error (RMSE) for the multiquadric radial basis function method was 4.57 feet, and Mean Error (ME) was -0.046 feet. We subtracted the interpolated water table surface from the ground surface DEM to produce a depth to water map. Results are shown within one kilometer of the bay, a distance used in other studies of sea level rise-induced coastal aquifer forcing (see Rotzoll and Fletcher, 2012). Wells are not evenly distributed across the bay shorefront; we excluded areas greater than 1km 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, especially in areas where there were no well sample points at the base of a slope or in a valley. In the provided data files, we have changed these negative values to zero for clarity.

Usage Notes

Note: results shown only for areas within 1km of well points in our 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.

References