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Grid Cells and Modeled Groundwater Levels to Characterize Hydrologic Conditions for Public-supply Aquifers in California's Central Valley, 1990-2020

June 3, 2021

This data release documents grid cells and modeled groundwater levels for hydrologic characterization of aquifer areas used for public drinking-water supply in California’s Central Valley from 1990 to 2020. Observations of spring groundwater level (median depth to groundwater measured from January through April) were calculated for selected wells from the California Department of Water Resources (DWR) periodic groundwater level measurements dataset (DWR, 2021) that had data for less than 50% of years from 1990 to 2020. Spring groundwater levels for years without data were imputed by multiple linear regression on the top five most correlated records within the same Central Valley subregion (Sacramento Valley, San Joaquin Basin, or Tulare Basin) using established methods described by Evans and others (2020). This resulted in annual time series for spring groundwater level at 3,239 locations throughout the Central Valley from 1990 to 2020. Observed and imputed spring groundwater level data were used to interpolate spring groundwater levels throughout the Central Valley at a five-kilometer resolution for individual water years from 1990 to 2020 using ordinary kriging. Annual time series for spring groundwater levels and associated kriging error were extracted from kriging model output at the locations of 6031 public-supply wells (PSWs). Spring groundwater levels interpolated to the locations of PSWs are not intended to approximate measured groundwater head values for specific geologic strata, but rather to assess the general direction and magnitude of interannual groundwater level changes in areas used for public-drinking water supply. The area used for public-drinking water supply was discretized into 187 hexagonal grid cells for geostatistical characterization of the groundwater resource.

This data release contains five tables formatted as tab-delimited text files and one geospatial shapefile containing hexagonal grid cells. Table 1 contains the location (coordinates and associated subregion) and observed spring groundwater levels (as feet below ground surface) for 3,239 wells in the DWR dataset, with years with no data represented as NAs. Table 2 is identical to Table 1 but contains imputed groundwater levels in place of NAs. Table 3 contains kriging parameters (nugget, sill, range) for each water year from 1990 to 2020. Table 4 contains depth to groundwater (in feet) interpolated to the locations of 6031 PSWs in the Central Valley. Table 5 contains kriging errors (in feet) for the corresponding interpolated groundwater levels presented in table 4. Depth measurements are presented in feet for consistency with DWR source data.

These data support the following publication:

Levy, Z.F., Jurgens, B.C., Burow, K.R., Voss, S.A., Faulkner, K.E., Arroyo-Lopez, J.A., Fram, M.S., In-review, Critical aquifer overdraft accelerates groundwater-quality degradation in California's Central Valley during drought: Geophysical Research Letters.

References

California Department of Water Resources [DWR], 2021, Periodic Groundwater Level Measurements: https://data.ca.gov/en/dataset/periodic-groundwater-level-measurements. Accessed online: January, 2021.

Evans, S.W., Jones, N.L., Williams, G.P., Ames, D.P., and Nelson, E.J., 2020, Groundwater Level Mapping Tool: An open source web application for assessing groundwater sustainability: Environmental Modelling and Software, 131, 104782. https://doi.org/10.1016/j.envsoft.2020.104782.