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Increasing threat of coastal groundwater hazards from sea-level rise in California

Nature Climate Change volume 10, pages 946–952 (2020)Cite this article

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Abstract

Projected sea-level rise will raise coastal water tables, resulting in groundwater hazards that threaten shallow infrastructure and coastal ecosystem resilience. Here we model a range of sea-level rise scenarios to assess the responses of water tables across the diverse topography and climates of the California coast. With 1 m of sea-level rise, areas flooded from below are predicted to expand ~50–130 m inland, and low-lying coastal communities such as those around San Francisco Bay are most at risk. Coastal topography is a controlling factor; long-term rising water tables will intercept low-elevation drainage features, allowing for groundwater discharge that damps the extent of shoaling in ~70% (68.9–82.2%) of California’s coastal water tables. Ignoring these topography-limited responses increases flooded-area forecasts by ~20% and substantially underestimates saltwater intrusion. All scenarios estimate that areas with shallow coastal water tables will shrink as they are inundated by overland flooding or are topographically limited from rising inland.

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Fig. 1: California’s loss of shallow water tables with sea-level rise.
Fig. 2: Distribution of flux-controlled and topography-limited groundwater conditions along coastal California for higher sea levels.
Fig. 3: Saline groundwater wedge footprint in shallow coastal California groundwater.

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Data availability

Derived model outputs that were merged across overlapping model boundaries and compiled to county boundaries are available to download at https://doi.org/10.5066/P9H5PBXP. The available data include georeferenced rasters of hydraulic head (that is, water table elevation) and water table depth and georeferenced shapefiles of the water table depth categories. The saline groundwater wedge footprint shapefiles are available to download at https://doi.org/10.4211/hs.1c95059edcf041a0959e0b4a1f05478c. The other MODFLOW input, output and derived datasets are available upon request. All other input datasets are available from the original sources.

Code availability

The relevant portions of the pre- and post-processing functions and scripts used to develop the figures and datasets in this study are available at https://doi.org/10.5281/zenodo.3897502. All other codes are available upon request at the discretion of the authors.

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Acknowledgements

This project was funded by the California Safe Drinking Water, Water Quality and Supply, Flood Control, River and Coastal Protection Bond Act of 2006 (Proposition 84), the Ocean Protection Council and the USGS Coastal and Marine Hazards and Resources Program. NODC_WOA94 salinity data were provided by the NOAA/OAR/ESRL PSD from www.esrl.noaa.gov/psd/. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the US government.

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Authors and Affiliations

  1. Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY, USA

    K. M. Befus

  2. Department of Geosciences, University of Arkansas, Fayetteville, AR, USA

    K. M. Befus

  3. Pacific Coastal and Marine Science Center, United States Geological Survey, Santa Cruz, CA, USA

    P. L. Barnard, D. J. Hoover & J. A. Finzi Hart

  4. Water Mission Area, United States Geological Survey, Menlo Park, CA, USA

    C. I. Voss

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Contributions

All authors participated in conceiving the study, developing the analyses and writing the paper. K.M.B. performed the modelling and analyses with input from all authors.

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Correspondence to K. M. Befus.

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The authors declare no competing interests.

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Peer review information Nature Climate Change thanks Chunhui Lu, Christine May and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Difference in model water table response behavior.

Conceptual cross-section showing how the flux-controlled model can overpredict heads compared to the water tables that include the hydraulic conditions created by surface drains.

Extended Data Fig. 2 Distribution of flux-controlled (≤5%) and topography-limited (>5%) groundwater conditions along coastal California for higher sea levels.

The overprediction of the water table rise by the flux-controlled response was calculated for all K and tidal datum scenarios to 1 km inland with Methods Eq. 1.

Extended Data Fig. 3 Distribution of emergent groundwater, flux-controlled, and topography-limited conditions with increasing sea levels and varying the distance inland used in the analysis for the LMSL tidal datum scenarios.

The MHHW distributions showed very similar distributions and were visually indistinguishable from the LMSL distributions in this figure. Note the irregular spacing on the vertical axes.

Extended Data Fig. 4 Profile-based comparison with current analysis.

Spatial comparison between the overprediction calculated in this study (Eq. 1; LMSL + 1 m, K = 1 m/d, MODFLOW forecast) and the delineation of flux-controlled (that is, recharge-limited) and topography-limited profiles from the “base case” of Michael et al.13 for 1 m of sea-level rise.

Extended Data Fig. 5

Graphical definition of the saline groundwater wedge footprint and saltwater intrusion.

Extended Data Fig. 6

Growth of the saline groundwater wedge footprint across coastal California regions for the flux-controlled and MODFLOW model predictions.

Supplementary information

Supplementary Information

Supplementary Figs. 1–6, Tables 1–11 and Discussion 1.

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Befus, K.M., Barnard, P.L., Hoover, D.J. et al. Increasing threat of coastal groundwater hazards from sea-level rise in California. Nat. Clim. Chang. 10, 946–952 (2020). https://doi.org/10.1038/s41558-020-0874-1

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