Qualitative vulnerability of the waters of a surface reservoir subjected to drought in a tropical semi-arid region

Resumo

Irregular rainfall and the occurrence of droughts in tropical semi-arid regions have a negative impact on the quality and
availability of water stored in reservoirs. The aim of this study was to evaluate the quality of water stored in an artifi cial reservoir in the
tropical semi-arid region of Brazil during years of drought. The study area was the Pereira de Miranda reservoir, located in the district
of Pentecoste, Ceará, Brazil. Seven water-sampling campaigns were carried out between April 2015 and September 2016. The variables
to be analysed were chlorophyll-a (Cl-a), hydrogen potential (pH), electrical conductivity (EC), total nitrogen (Ntotal), total phosphorus
(Ptotal), transparency (SD), fi xed/volatile/total suspended solids (FSS/VSS/TSS), and temperature (T0
); the trophic state index (TSI) was
then calculated. The variables which better explained the variance in water quality were identifi ed by principal component analysis
(PCA). The waters of the Pereira de Miranda reservoir were classifi ed as hyper-eutrophic throughout the study period and at each of the
collection points. The PCA identifi ed that the variability in water quality was determined by suspended sediment and nutrients, showing
that the reduction in variables made it possible to obtain high explainability of the conditions of the reservoir during monitoring,
saving time and resources on the analyses. In addition, the occurrence of severe drought resulted in low accumulated volumes, which
intensifi ed and made vulnerable the quality of the waters. As such, the Pereira de Miranda reservoir proved to be in a process of
degradation, mainly due to anthropogenic action, intensifi ed by climate factors.