Coastal drought is unique in its effects on the salinity dynamics of creeks, rivers, and estuaries. The location of the freshwater-saltwater interface within coastal water bodies regulates the ecological and socioeconomic dynamics of coastal communities. Commonly-used drought indices characterize hydrological, agricultural, and meteorological conditions; however they do not incorporate salinity, a key stressor associated with coastal drought. Salinity is a critical response variable that integrates hydrologic and coastal dynamics including sea level, tides, winds, precipitation, streamflow, and tropical storms.

The motivation for a coastal drought index came from the National Integrated Drought Information System (NIDIS) Drought Early Warning System (DEWS) program for coastal North Carolina and South Carolina. The Coastal Carolinas DEWS is a collaborative federal, state, and local interagency effort to improve early warning capacity and resilience to drought. In the Carolinas, activities focus on improving the understanding of drought effects on coastal environmental resources and developing information to enhance drought monitoring and planning processes. In 2012, participants at the Coastal Carolinas DEWS Scoping Workshop recommended the development of an indicator that would be appropriate for assessing and characterizing drought conditions in coastal areas.

For more information on the need for an index of coastal drought see: Needs Assessment of Coastal Land Managers for Drought Onset Indicators in the Southeastern United States.

Conrads and Darby (2017) developed the CSI using an approach similar to the Standardized Precipitation Index (SPI), a readily available drought index that is widely used for monitoring meteorological droughts (McKee and others, 1993). The CSI substitutes total monthly precipitation with monthly mean salinity data to determine the probability of recording a given salinity value in a particular month. For the CSI, monthly mean salinity data are fit to a gamma distribution and then normalized (mean of zero and standard deviation of one). The index values are standard deviations from the normalized mean values. To align the CSI values with the same classification scheme as the U.S. Drought Monitor, the CSI values were multiplied by -1 so that negative numbers represent drought conditions.

An index value of zero indicates historical mean salinity; negative and positive values represent increasingly saline and fresh conditions, respectively. Typical SPI threshold values were adopted for the coastal drought classifications (Table 1). The thresholds characterize the historical range of measured salinity data at the site and provide an indication of the severity of the saline (or freshwater) conditions over the interval for which the CSI was computed. As with the SPI, the CSI can be computed for various time intervals, allowing the CSI to be used to evaluate and compare short- and long-term drought (saline) and wet (freshwater) conditions.

Reference:
Conrads, P.A., and Darby, L.S. (2017): Development of a Coastal Drought Index Using Salinity Data. Bulletin of the American Meteorological Society, 98, 753-766, https://doi.org/10.1175/BAMS-D-15-00171.1
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.

Stacked Graph
Table 1:
Coastal Salinity Index (CSI) Information
Coastal Salinity Classification Description Threshold Values Icon Cumulative Percentage
CD4 Exceptional salinity conditions -2.00 or less 2
CD3 Extreme salinity conditions -1.99 to -1.60 5
CD2 Severe salinity conditions -1.59 to -1.30 10
CD1 Moderate salinity conditions -1.29 to -0.80 20
CD0 Abnormal salinity conditions -0.79 to -0.50 30
Normal Normal salinity conditions -0.49 to 0.50 70
CW0 Abnormal freshwater conditions 0.51 to 0.80 80
CW1 Moderate freshwater conditions 0.81 to 1.30 90
CW2 Severe freshwater conditions 1.31 to 1.60 95
CW3 Extreme freshwater conditions 1.61 to 2.00 98
CW4 Exceptional freshwater conditions 2.01 or more 100
Not Available Missing Data
Applications

The Coastal Salinity Index (CSI) was developed to characterize coastal drought, monitor changing salinity conditions, and improve understanding of the effects of changing salinities on fresh and saltwater ecosystems, fish habitat, and freshwater availability for municipal and industrial use. The initial effort to develop the CSI used data from two USGS water quality stations, including the Waccamaw River at Hagley Landing (USGS Station 02110815), which is shown in the illustration below. This illustration depicts the CSI stacked plot for this station, as well as two drought monitor maps during a severe 2007-2008 drought. The results illustrate the unique contributions the CSI has to offer understanding coastal drought.

United States Drought Monitor maps, 2007-2008.

Caption: Comparison of Coastal Salinity Index with United States Drought Monitor maps, 2007-2008.

Table 2: Comparing the Coastal Salinity Index
with the U.S. Drought Monitor
Index Coastal Salinity Index U.S. Drought Monitor
Updated Daily Weekly
Timeframe 1- to 24-months Short- to long-term
Scale Coastal, station-specific values Regional- to local level data and information
Inputs Salinity Expert assessment of multiple data sources and drought indices

Preliminary evaluations of the CSI indicate that the index can be used for different estuary types, for regional comparison between estuaries, and as an index for drought and wet conditions. In the South Atlantic region, coastal watersheds vary from small tidal creeks, Coastal Plain rivers, to large rivers that originate in the Piedmont or Blue Ridge provinces of North Carolina, South Carolina, and Georgia. These different rivers and systems respond differently to precipitation, riverine flow, and tidal conditions, depending on the geologic setting. They also have different river and water management regimes. The various CSI intervals (1- to 24-months) allow the index to be correlated with environmental response variables that occur on different time intervals. As salinity is the primary environmental stressor during drought across various estuary types, researchers are currently using the CSI to improve understanding of the linkages between ecological indicators (for example, fisheries and habitat conditions) and varying salinity levels.