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Abstract
IntroductionWe 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.MethodsWe 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.ResultsWe 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.ConclusionsThe 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.
Highlights
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
Based on aquifer characteristics and management in the region, we considered the contribution from groundwater flow (VG) and the contribution of the salinity of groundwater (SG) during the period as insignificant compared to the surface flow which was apparently higher during the floods
In the years leading to the major desalination event in 2015, the marine dominance of the lagoon had been attributed to failure of monsoons and the insignificance of the riverine inputs which play an important role in a b imparting the brackishness (e.g. Coulthard 2008)
Summary
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. Shifts in the salinity regimes in coastal lagoon systems from brackish water to freshwater or marine salinities are known to be seasonally progressive; such shifts can occur at variable rates and spatio-temporal scales that could go unnoticed. Salinity and nutrient budgets of the Bay of Bengal (BoB) show that the system alternates as a net source or sink in tandem with the regional climate variations (e.g. Mukhopadhyay et al 2002). These changes to the fluxes of salt and nutrients are further constrained by the divisory contributions of large coastal systems at the boundary (e.g. Bonthu et al 2016 and Prasad et al 2016) which are seldom accounted for. While extensive monsoonal dilution of lagoons is relatively commonplace, detailed understanding of the impacts of high-magnitude flooding remains quite critical in deciding whether such events would impact salinity and its regime shifts (e.g. Jeppesen et al 2007)
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