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

Abstract

There is a growing concern about the adverse effects of saltwater intrusion via tidal rivers, streams, and creeks into tidal freshwater forested wetlands (TFFW) due to sea level rise (SLR) and intense and extended drought events. However, the magnitude and duration of porewater salinity in exceedance of plant salinity stress threshold (2 practical salinity units, psu) and the controlling factors remain unclear. In this study, we developed a TFFW soil porewater salinity model, in which the feedback mechanisms between soil salinity and evapotranspiration and hydraulic conductivity were incorporated. We selected sites (upper, middle, lower tidal freshwater forest sites, and oligohaline marsh site) along the coastal floodplains of two rivers, the Waccamaw River (SC, USA) and the Savannah River (GA and SC, USA), that represent landscape salinity gradients from tidal influence of the Atlantic Ocean. The model results agreed well with field measurements and revealed that with drought-induced saltwater intrusion, the mean annual soil porewater salinity and duration of elevated soil porewater salinity (>2 psu) increased significantly compared to the normal (nondrought) condition, posing a threat to the health and ecosystem services of TFFW even in the absence of SLR. Model results also showed more severe salinity stress under drought for the lower forest sites along the two rivers, where soil salinity values have already been at or in exceedance of the 2 psu threshold.