Eventos extremos na Amazônia e seus efeitos na dinâmica hidrológica do rio e lagos de várzea amazônicos

Abstract

Amazonian várzea floodplains can be defined as humid areas bathed by rivers of white or muddy water. The main characteristic of these Amazonian várzea floodplains is the large amount of suspended nutrients brought to them by rivers during the seasonal high water period. Because they are fertile and productive areas, the Amazonian várzeas are of great importance for the population residing in these areas. The present study aims to analyze how the increase in the frequency of climatic extreme events is affecting the hydrological dynamics of the Amazonian floodplain lakes. We consider extreme events related to droughts and severe floods that have happened over the past few decades. Time series of 28 years of quotas from different hydrological stations were used: Lake Mamirauá, Lake Curuaí, Tefé and Óbidos. Data were obtained from the National Water Agency (ANA) and from Fluviometric Monitoring carried out by the Mamirauá Institute since 1990. In order to determine the effects of extreme events, statistical metrics were analyzed in a comparative way between floodplain lakes and the main courses of rivers, from the determination of the permanence quotas curves (referenced to sea level) for each hydrological year, with the segments of higher and lower quotas as well as the slope of the permanence curve. For the analysis of trends, the Linear Regression method and the coefficient of determination R² were used, as well as the non-parametric Mann-Kendall and Run tests with a significance level of 5%. It was obtained as results that: a) in the period of 28 years of analysis there were 8 extreme events, five related to droughts and three to severe floods, which corroborates the literature; b) the Mann-Kendall and Run tests did not show significant trends of extreme drought for any lake or monitoring station; c) a flood trend was observed for the Óbidos station in the lower Amazon with the Mann-Kendall test; d) the lower Amazon River showed a significant downward trend, as well as a trend towards maximum flood variation, by calculating seasonality; e) the Middle Solimões region did not show a trend for the number of flood days or for seasonality; f) the only significant trend observed was for precipitation at the Tefé station. Based on the results presented, it was concluded that the hydrological extremes of severe flood presented a significant tendency of increase in the region of the lower Amazon. However, due to the complexity of the river and lake system of the Amazonian várzea floodplains, as well as data series failures in some years of fluviometric monitoring and data series that did not reach 30 consecutive years, the results should be considered with caution.