Linking ROMS with watershed models for simulating hydrodynamics and thermohaline dynamics in a coastal lagoon affected by extreme weather events

• Hydrodynamic model of Mar Menor linked to groundwater and watershed models. • Sea level, currents, salinity and water temperature reproduced with great accuracy. • 2019 flash flood led to strong stratification lasting more than 1 month. • Prevalence of buoyancy over shear instability maintained stratification. • Water Renewal Time inside the lagoon decreased 52 days due to the CoL. Reproduction of hydrodynamic and hydrologic processes in complex coastal lagoons requires the development and calibration of linked numerical model implementations, that can show accuracy even in extreme weather scenarios. To achieve that, robust datasets for a wide variety of parameters are needed to validate the model. This study aimed to develop and validate a hydrodynamic model linked to a groundwater and a watershed model, for the microtidal Mar Menor coastal lagoon located in the southeastern Spain. Special concern was given to flash flood events, which, although infrequent, are proved to trigger mass mortality of species inside the lagoon. To achieve that, a ROMS numerical implementation was developed and linked to atmospheric (HARMONIE-AROME), groundwater (SUTRA), and watershed (TETIS) models. The model results were compared with a robust dataset with hydrodynamic, salinity, and water temperature data. Special attention was given to the September 2019 Cut-off Low (CoL) flash flood event. The model demonstrated high accuracy in reproducing the lagoon’s dynamics under normal conditions, including the currents in the narrow inlets connecting the lagoon with the Mediterranean Sea. After the CoL event, an extraordinary hydrological scenario developed — characterized by strong vertical stratification that persisted for over a month — explained by the lack of sufficient shear instability to overcome buoyancy forces induced by density gradients, despite the occurrence of a two-layer opposite direction flow. Runoff associated with the CoL event also led to a nearly 20 % reduction in the lagoon’s Water Renewal Time.