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Coastal Salinity Index (CSI)

Associated Agencies

U.S. Geological Survey

The Coastal Salinity Index (CSI) is a long-term monitoring tool that characterizes relative changes in coastal salinity regimes for salinity gages with long periods of record. The CSI was developed to characterize coastal drought, monitor changing salinity conditions, and improve our understanding of the effects of changing salinities on fresh and saltwater ecosystems, fish habitat, and freshwater availability, providing economic, human health, and environmental benefits.

  • The CSI uses an approach similar to the Standardized Precipitation Index (SPI) to show the probability of recording a given amount of salinity. A value of zero indicates historical mean salinity, and positive and negative values represent increasingly fresh and saline conditions, respectively. 
  • The CSI uses the same classification scheme as the U.S. Drought Monitor for high saline (or drought) conditions and the inverse for wet conditions.
  • The CSI can be computed for multiple time intervals from 1 to 24 months to characterize short- and long-term conditions.
  • The CSI does not depict hourly to daily salinity fluctuations, but the response to monthly (and longer) precipitation and streamflow conditions.

The Coastal Salinity Index project has been supported by the National Integrated Drought Information System (NIDIS), the National Oceanic and Atmospheric Administration (NOAA), the Carolinas Integrated Sciences and Assessments program (a NOAA RISA), and the U.S. Geological Survey.

Interactive Map: Coastal Salinity Index (CSI)

Coastal Salinity Conditions

How To

Click on any station icon on the map to access the following information:

  • The station identification number, station name, and a link to the National Water Information System station current conditions page data.
  • The current CSI 12-month interval value.
  • The CSI stacked graph that allows the user to evaluate short- (1-month) to long-term (24-month) conditions in one graph. Learn how to read the stacked graph.
  • A link to all the CSI graphs for that particular gauge.

You can also download CSV files with data for specific stations or all stations.


Coastal Salinity Index Tool: View the CSI on the U.S. Geological Survey website


About the CSI: Learn about the motivations and methodology behind this tool

Conrads, P.A. and L.S. Darby, 2017, Development of a Coastal Drought Index Using Salinity Data. Bull. Amer. Meteor. Soc., 98, 753–766,

McKee, T. B., Doesken, N.J., and Kleist, J., 1993, The relationship of drought frequency and duration to time scales. Preprints, Eighth Conference on Applied Climatology, Anaheim, CA, American Meteorological Society, p. 179–184.