A NONSTATIONARY BOX MODEL TO DETERMINE RESIDUAL FLUXES IN A PARTIALLYMIXED ESTUARY, BASED ON BOTH THERMOHALINE PROPERTIES - APPLICATION TOTHE 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 pr evious models. Firstly, for calculating the horizontal and vertical ve locities and also vertical diffusion coefficients, the present model t akes into account the distribution not only of salinity but also of te mperature. Evaluation of both river discharge and heat exchange betwee n water surface and atmosphere is required. Two sets of fluxes are com puted; one for the distribution of salinity, and another for that of t emperature. They are then weight-averaged according to the vertical gr adient of both properties. Secondly, and due to the high density of sa mpling (twice a week), it is possible to introduce the non-stationary term in the convection-diffusion equation; thus, time variations of bo th 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 p reviously were inapplicable. The strong coupling between estuarine cir culation and upwelling is shown, and an equation is proposed in order to quantify this relationship. On the hypothesis of linear variation o f thermohaline properties with time, fluxes at the intermediate time b etween two samplings were also calculated. Comparison of salinity betw een two stations at the mouth of the ria allows the computation of the correction due to Coriolis outward flux deflection, which was then ap plied to the whole ria. (C) 1997 Academic Press Limited.