Development of a Coastal Drought Index Using Salinity Data

Abstract

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. It influences community composition in freshwater and saltwater ecosystems, determines fisheries spawning habitat, and controls freshwater availability for municipal and industrial water intakes. These dynamics may be affected by coastal drought through changes in Vibrio bacteria impacts on shellfish harvesting and occurrence of wound infection, fish kills, harmful algal blooms, hypoxia, and beach closures.Many definitions of drought have been proposed, with most describing a decline in precipitation having negative impacts on water supply and agriculture. Four general types of drought are recognized: hydrological, agricultural, meteorological, and socio-economic. Indices have been developed for these drought types incorporating data such as rainfall, streamflow, soil moisture, groundwater levels, and snow pack. However, these drought indices were developed for upland areas and may not be appropriate indices for characterizing drought in coastal areas. Because of the uniqueness of drought impacts on coastal ecosystems, a need exists to develop a coastal drought index. The availability of many real-time and historical salinity datasets provides an opportunity to develop a salinity-based coastal drought index.The challenge of characterizing salinity dynamics in response to drought is excluding responses attributable to occasional saltwater intrusion events. We applied various statistical and numerical techniques to evaluate the most appropriate approach to develop salinity drought indices. An approach similar to the Standardized Precipitation Index was modified and applied to salinity data obtained from sites in South Carolina and Georgia, USA. Coastal drought indices characterizing 1-, 3-, 6-, 9-, and 12 month drought conditions were developed. Evaluation of the coastal drought index 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. The development of the various drought characteristic intervals (1-, 3-, 6-, 9-, and 12 month) allow for the coastal drought index to be correlated with environmental response variables that occur on different time intervals.

Saltwater Intrusion in the Upper Tagus Estuary during Droughts

“…Since tides and the freshwater discharge are the main drivers of this dynamics, during low river discharge periods the saltwater can propagate further upstream. Salinity is, thus, one of the major stressors associated with droughts in estuaries [10] and the estuarine response to changes in salinity can occur at different time scales [11]. Changes in the salinity dynamics can affect several resources and economic sectors, such as public health, the fishing industry and the aquifers found near estuaries [11].…”

Spatial gradients in the characteristics of soil-carbon fractions are associated with abiotic features but not microbial communities

“…Relatively fast changes of organic C chemistry compared to relatively slow changes in microbial composition may underlie the lack of association between assembly processes and C chemistry (Bramucci et al, 2013). Supporting this interpretation, a recent study evaluating microbial community composition and C biogeochemistry of soils in a mesohaline marsh following saltwater intrusion reported immediate changes in C mineralization rates with delayed shifts in microbial community composition (Dang et al, 2019). Similarly, a 17-year dryland soil transplant experiment showed large shifts in microbial activity with no change in community composition (Bond-Lamberty et al, 2016).…”

Modeling Soil Porewater Salinity Response to Drought in Tidal Freshwater Forested Wetlands

“…Incoming tidal river salinity is determined by the position of the freshwater-saltwater interface, which is affected by rising sea level, tides, tidal cycles, precipitation, streamflow, winds, and tropical storms (Conrads & Darby, 2017). The location of the freshwater-saltwater interface is balanced between upstream freshwater river streamflow and downstream tidal forcing (Conrads et al, 2013;Conrads & Darby, 2017). Salinity intrusions are often concurrent with spring-tide water levels along tidal rivers (Conrads et al, 2013).…”

“…TFFW are especially vulnerable to saltwater intrusion from alterations in climate-induced rainfall variability, sea level rise (SLR), river discharge, and heightened frequency and extent of storm surge (Baldwin, 2007;Kaplan et al, 2010;Neubauer et al, 2013;Pierfelice et al, 2015;Thomas et al, 2015). TFFW are also sensitive to coastal droughts through declines in direct precipitation, perennially lower riverine base flows, and human altered incoming tidal level that affect tidal range and salinity along many rivers (Conrads & Darby, 2017). For example, the combination of dredging (deepening) the navigation channel and the tidal gate along the Savannah River and its tributaries in the southeastern United States have affected flow paths and resulted in increased river salinity upstream, which led to shifts in vegetation structure (Jones et al, 2017;Krauss, Doyle, & Howard, 2009).…”

“…Saltwater intrusion is caused by drivers at the short-term (e.g., episodic events such as storm surge of hours to days), medium-term (e.g., few days of drought), and long-term (e.g., months and years of drought and decades of SLR) (Conrads & Darby, 2017;Krauss et al, 2007;Neubauer et al, 2013;Stagg et al, 2017;Wetzel & Kitchens, 2007). It is this chronic saltwater exposure that leads to persistent changes in structure and function of plant and microbial communities and influences rates and pathways of biogeochemical C and nutrient dynamics (Conrads et al, 2013;Neubauer et al, 2013).…”