Intra-tidal variation of stratification in a semi-arid estuary under the impact of flow regulation

The Jaguaribe River is the largest semi-arid Brazilian river and represents an important water source for the state of Ceará. The Castanhão Weir, built to regulate the flow of the Jaguaribe, began operating successfully in the 1990s. As a result, the hydrodynamic regime of the estuary has undergone alteration and is no longer subject to the former annual climatological variability of the dry and rainy seasons. This paper introduces an assessment of the changes in the hydrological conditions resulting from the construction of the weir and investigates the structure of the density stratification in the estuary. A field experiment was conducted at the end of the dry season in November 2009, during which salinity and temperature were monitored at different levels for three tidal cycles. The temporal variations in salinity and temperature during the tidal cycles were 16 psu and 1.5°C, respectively. The salinity exhibited a vertical variation of four units, while the temperature variation was much lower. Several stratification indicators were calculated from the observed density: the stratification coefficient, the Brunt-Väisälä frequency and the potential energy anomaly. The results showed that the parameters behave similarly, differences being observed only in their relative magnitudes. The vertical structure of the estuary exhibited marked stratification during the ebb tide but vertical homogeneity during both flood and high tide. With the regularized flow existing under present conditions, the estuary exhibits hyposaline and a partially mixed structure for the period studied, despite the typical expectation of a hypersaline and well-mixed vertical structure. O rio Jaguaribe é o maior rio do semi-árido brasileiro, constituindo uma importante fonte de água para o estado do Ceará. Na década de 1990 entrou em operação o Açude do Castanhão com o objetivo de regularizar a vazão do rio Jaguaribe, o qual obteve grande sucesso. Como consequência foi alterado o regime hidrodinâmico do estuário, o qual atualmente não está mais sujeito a variabilidade climatológica anual de estações seca e chuvosa. O presente trabalho apresenta uma avaliação da mudança do regime hidrológico antes e após a construção do açude, e investiga a estrutura de estratificação da densidade nos estuários. Foi realizado um experimento de campo em novembro de 2009, no fim da estação seca, quando a salinidade e temperatura da água foram monitorados em diferentes níveis durante três ciclos de maré. A