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Study of the water quality of Alqueva reservoir in the south of Portugal using MERIS data

Miguel Potes(1) and Maria João Costa(1)

(1) Centro Geofísica de Évora (CGE), Rua Romão Ramalho, 59, 7000-671 Évora, Portugal

Abstract

M. Potes1, M. J. Costa1,2

1 Évora Geophysics Centre (CGE), University of Évora, Rua Romão Ramalho 59, 7000-671 Évora, Portugal 2Department of Physics, University of Évora, Rua Romão Ramalho 59, 7000-671 Évora, Portugal

The control and monitoring of water quality in artificial lakes is crucial, since these constitute essential renewable water resources for domestic, agricultural, and industrial purposes, amongst many others. Alqueva is the largest artificial lake in Europe in terms of surface area. It is located in the south of Portugal - Alentejo, a region that seriously faces the problem of droughts, and therefore is a good example of the importance of water quality control in artificial lakes. This control is done through laboratory analyses of water samples taken in several places of the lake. However, these are point measurements only available once a month, which motivated a study to develop a method for global monitoring of water quality parameters, through the combination of satellite and ground-based data. The study of surface water properties from satellite remote sensing techniques requires the correction for the effects of the atmosphere. The present study is only concerned with clear sky days. Major gas absorption bands are avoided, and therefore, the atmospheric correction depends essentially on the type and amount of aerosols present in the atmosphere. Aerosol measurements are continuously obtained at the observatory of the Évora Geophysics Centre (CGE) from the inversion of spectral radiation measurements taken by a Sun-sky photometer connected to the AErosol RObotic NETwork (AERONET). Due to the small distance between Évora site and Alqueva area (about 40 km), a significant variation is not expected, especially with respect to aerosol type, therefore the atmospheric correction over Alqueva is accomplished using the aerosol characterization obtained in Évora. The atmospheric correction is done using the 6S (Second Simulation of the Satellite Signal in the Solar Spectrum) radiative transfer code, obtaining hence the water surface spectral reflectance, from level 1 MERIS full resolution data. These results are used to estimate the chlorophyll-a concentration and cyanobacteria density over the Alqueva surface area, using parameterizations already developed by the authors. The study will address the year of 2008, aiming at analysing the spatial and seasonal variations of water quality parameters over Alqueva reservoir.

Acknowledgements The work was funded by the Portuguese FCT through project PTDC/CTE-ATM/65307/2006. Image data has been provided by ESA in the frame of ENVISAT project AOPT-2357. We thank Prof. Ana Maria Silva for her effort in establishing and maintaining Évora AERONET site. We also thank the Water Laboratory of the University of Évora, EDIA and INAG for providing the water quality data used in this work.