Observation of a turbid plume using MODIS imagery: the case of Douro estuary (Portugal)

Abstract

The Douro is one of the longest rivers in the Iberian Peninsula and represents the most important freshwater input into the Atlantic Ocean in the northwestern Portuguese coast. In order to fill the gaps on the comprehension of the spatial variability of the Douro estuarine plume, an innovative methodology was applied to observe and characterize this plume and its spatial and temporal variability. Using images of normalized water-leaving radiance at 555 nm from MODIS (Moderate Resolution Imaging Spectroradiometer) in combination with long-term concurrent wind, tidal and river discharge data (2003-2011), this work evaluates the effect of the main drivers (river discharge, wind and tide) on the plume features. Expected strong relationships between river discharge and turbid levels were detected. A circular pattern is well distinct off Douro estuary when the flow exceeds 500 m³ s^-1. Under upwelling favorable winds and high river discharge regime, an offshore expansion is observed. During southerly winds, there is a plume confinement to the coast. Easterlywinds produce a bulge formation with an offshore detachment from the coast while westerly winds tend to confine freshwater close to the shoreline and decrease the cross-shore advection. Under high river discharges, a remarkable tidal influence in the near-field regions is observed close to the coast and near the river mouth.