A Non-stationary Box Model to Determine Residual Fluxes in a Partially Mixed Estuary, Based on Both Thermohaline Properties: Application to the Ria de Arousa (NW Spain)

An improved two-layered box model for calculating the residual fluxes in a Spanish ria is developed in the present paper. This kinematic bi-dimensional model has two characteristics that make it different to previous models. Firstly, for calculating the horizontal and vertical velocities and also vertical diffusion coefficients, the present model takes into account the distribution not only of salinity but also of temperature. Evaluation of both river discharge and heat exchange between water surface and atmosphere is required. Two sets of fluxes are computed; one for the distribution of salinity, and another for that of temperature. They are then weight-averaged according to the vertical gradient of both properties. Secondly, and due to the high density of sampling (twice a week), it is possible to introduce the non-stationary term in the convection–diffusion equation; thus, time variations of both conservative tracers are taken into account. Coherent solutions of equations are obtained in strong upwelling–downwelling and/or very low river volume situations where the estuary behaves virtually as a well-mixed estuary, and salt balance/non-stationary box models described previously were inapplicable. The strong coupling between estuarine circulation and upwelling is shown, and an equation is proposed in order to quantify this relationship. On the hypothesis of linear variation of thermohaline properties with time, fluxes at the intermediate time between two samplings were also calculated. Comparison of salinity between two stations at the mouth of the ria allows the computation of the correction due to Coriolis outward flux deflection, which was then applied to the whole ria.