No abstract

This paper focuses on time-space characterization of drought conditions in the São Francisco River catchment, on the basis of wavelet analysis of Standardized Precipitation Index (SPI) time series. In order to improve SPI estimation, the procedures for regional analysis with L-moments were employed for defining statistically homogeneous regions. The continuous wavelet transform was then utilized for extracting time-frequency information from the resulting SPI time series in a multiresolution framework and for investigating possible teleconnections of these signals with those obtained from samples of the large-scale climate indexes ENSO and PDO. The use of regional frequency analysis with L-moments resulted in improvements in the estimation of SPI time series. It was observed that by aggregating regional information more reliable estimates of low frequency rainfall amounts were obtained. The wavelet analysis of climate indexes suggests that the more extreme dry periods in the study area are observed when the cold phase of both ENSO and the PDO coincides. While not constituting a strict cause effect relationship, it was clear that the more extreme droughts are consistently observed in this situation. However, further investigation is necessary for identifying particularities in rainfall patterns that are not associated to large-scale climate anomalies.

Because it is assumed that the impacts in the natural streamflow regime promoted by Small Hydropower Plants (SHP) are negligible, environmental licensing for such facilities is usually easier. Nonetheless, recent studies have shown that the operation of SHPs may disturb the natural flow conditions, mainly when the plants are placed in a cascade arrangement. In this context, the main objective of this study is investigating the alterations in flows periodic behavior in a system of six hydropower plants, being five of them SHPs. Daily discharge time series were extracted from eight streamflow gauging stations located in the Jauru River catchment, Brazil, whose period-of-record spans from May/2016 to Aug/2017. By using the wavelet transform, dominant cycles along the time series were identified and their coherence in nearby stations was compared. Among the results, one may observe that, from upstream to downstream, the high frequency cycles became more important whereas the low frequency ones have weakened. Additional analyses indicate that such alterations are not directly related to meteorological factors or to the gradual increasing in the catchment's drainage area in the downstream direction, which suggests that the operation of SHPs may affect the streamflow natural cycles. RESUMOPor assumir que os impactos nos regimes naturais de vazão de Pequenas Centrais Hidrelétricas (PCHs) são desprezíveis, os licenciamentos ambientais para esses empreendimentos são relativamente fáceis de serem obtidos. Contudo, estudos recentes têm mostrado que a operação de PCHs pode afetar as condições naturais de escoamento especialmente quando os aproveitamentos hidrelétricos são alocados em cascata. Nesse contexto, o principal objetivo deste trabalho é investigar a alteração do comportamento das periodicidades de vazões em um sistema com seis aproveitamentos hidrelétricos, sendo cinco PCHs. Foram utilizadas séries de vazões diárias extraídas de estações localizadas na bacia do rio Jauru, na região hidrográfica do Alto Paraguai, Brasil, entre maio de 2016 e agosto de 2017. Por meio da transformada wavelet, foi possível identificar ciclos dominantes ao longo do tempo e comparar a coerência de séries de vazões entre estações adjacentes. Entre os resultados, observou-se que, no sentido de montante para jusante, os ciclos de alta frequência se tornam mais evidentes enquanto aqueles de baixa frequência ficam enfraquecidos. Análises adicionais indicaram que tais alterações não estão diretamente relacionadas a fatores meteorológicos ou ao aumento gradual da área de drenagem da bacia na direção a jusante, o que sugere que a operação das PCHs pode afetar o ciclo natural de vazões.Palavras-chave: Periodicidade de vazão; Transformada wavelet; Impacto de pequenas barragens; Usina a fio d'água; Análise de séries temporais.RBRH, Porto Alegre, v. 24, e33, 2019Effects of small hydropower plants in cascade arrangement on the discharge cyclic patterns 2/15