Critical Aspects of the Coastal Drought Index: Length of Salinity Data Record and Ecological Response Data

Coastal droughts have a different dynamic from upland droughts that are typically characterized by agricultural, hydrologic, meteorological, and(or) socio-economic impacts. The location of the freshwater-saltwater interface in surface-water bodies is an important factor in the ecological and socio-economic dynamics of coastal communities. Because of the uniqueness of drought impacts on coastal ecosystems, a coastal drought index (CDI) that uses existing real-time and historical salinity datasets for sites in South Carolina, Georgia, and Florida, USA was developed by using an approach similar to the Standardized Precipitation Index (SPI). CDIs characterizing the 1- to 24-month salinity conditions were developed and the evaluation of the CDI indicates that the index can be used for different estuary types (for example, brackish, olioghaline, or mesohaline estuaries), for regional comparison between estuaries, and as an index for wet conditions (high freshwater inflow) in addition to drought conditions.

Unlike the SPI where long-term precipitation datasets of 50 to 100 years are available for computing the index, there are a limited number of salinity data sets of greater than 10 or 15 years for computing the CDI. To evaluate the length of salinity record necessary to compute the CDI, a 29-year dataset was resampled in 5-, 10-, 15-, and 20-year interval datasets. Comparison of the CDI for the different periods of record show that range of salinity conditions in the 10-, 15-, and 20-year datasets were similar and results were a close approximation to the CDI computed using the full period of record. The CDI computed with the 5-year dataset, with a smaller range of salinity conditions, had the largest differences with the CDI computed with the 29-year dataset but did provide useful information on coastal drought and freshwater conditions.

It is essential that a CDI be correlated to coastal drought response parameters to show the importance of a unique coastal drought index. However, identifying potential coastal drought response datasets is challenging. Coastal drought is a relatively new concept and existing datasets may not have been collected or understood as “drought response” datasets. Some potential drought response datasets include tree growth and liter fall in tidal marshes, harmful algal blooms occurrence, Vibrio infection occurrence, shellfish harvesting data, and shark attacks. An ongoing National Integrated Drought Information System (NIDIS) drought early warning project in the Carolinas is developing ecological linkages to the CDI and evaluating the effectiveness of the CDI as a prediction tool for adaptation planning for future drought.