Short-term desalination of Pulicat lagoon (Southeast India) due to the 2015 extreme flood event: insights from Land-Ocean Interactions in Coastal Zone (LOICZ) models

Introduction: We investigated the magnitude and duration of desalination of Pulicat—a coastal lagoon ecosystem connected to the Bay of Bengal on the South-eastern coast of India—during the 2015 South India flood event which was a period of high-magnitude precipitation and riverine flooding. Methods: We estimated freshwater runoff into the lagoon using flow accumulation models for a period of 55 days (November 1 to December 25, 2015) using daily gridded precipitation data from the Global Precipitation Measurement and a digital elevation model. Using the estimates of freshwater runoff, direct precipitation and observed salinities, we simulated water and salinity fluxes of the lagoon using the Land-Ocean Interactions in the Coastal Zone model. Further, we also used Monte Carlo simulation to estimate the uncertainty in system salinity, the residual salinity at the boundary and the freshwater residence times in the lagoon. Results: We estimated that a high volume (~ 760 × 10⁶ m³) of relatively low salinity waters (residual salinity = 23.47 psu) had been exported from the lagoon to the Bay of Bengal during the period which is likely to have caused a strong dip in the daily salinity profile of the coastal sea. We contend that the lagoon experienced ~ 40% desalination due to the 2015 event with a freshwater residence time of 18.5 days. Conclusions: The study highlighted the short-term, high-magnitude desalination undergone by Pulicat lagoon during the 2015 South India floods. Considering the high residual and exchange volumes obtained from the study, we conclude that Pulicat could be a major exporter of relatively low salinity waters to the Bay of Bengal during monsoons.