Recent Advances of a Box Model to Represent the Estuarine Dynamics: Time-Variable Estuary Length and Eddy Diffusivity

This study starts from a two-layer 1D Estuary Box Model (EBM), the so called Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC) EBM (Verri et al., 2020, ), devised to offer a proper representation of the net river release in a coupled modeling framework with hydrology models and ocean models reaching the mesoscale. Two key advances are proposed in the present study: (I) the estuary length and (II) the horizontal eddy diffusivity coefficient are no longer considered as static parameters to calibrate but two non-dimensional equations have been introduced in order to make them time-variable. The Goro branch of the Po river delta is considered as case study of salt-wedge estuary flowing into a microtidal sea. Regarding the estuary length, a non-dimensional equation has been tested and validated. This equation provides a time dependent estuary box volume which is more realistic choice. This time-variable estuary length represents the length of the salt wedge intrusion. Regarding the horizontal eddy diffusivity, the static eddy coefficient adopted in the previous version of the model is a coarse assumption. Thus a non-dimensional equation for this coefficient has been tested and validated. It makes the eddy diffusivity dependent on the velocity shear, the salinity gradient and the estuary geometry. The proposed dynamic formula is found to enhance the model capability to reproduce the salinity at the estuary mouth. Overall the high statistical performance in terms of RMSE and correlation coefficient, the short CPU time and the minimal calibration encourage to use the CMCC EBM in coupled mode with both mesoscale-resolving ocean and hydrology models to produce operational forecasts and climate scenarios.

Automated summary

This study improves the CMCC EBM model by introducing non-dimensional equations to make estuary length and horizontal eddy diffusivity coefficient time-variable, leading to enhanced model capability to reproduce salinity at the estuary mouth.