ABSTRACT 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 artificial 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), fixed/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 identified by principal component analysis (PCA). The waters of the Pereira de Miranda reservoir were classified as hyper-eutrophic throughout the study period and at each of the collection points. The PCA identified 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 intensified 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, intensified by climate factors.

ABSTRACT Water reservoirs were built in the semi-arid region to mitigate the effects of drought and water scarcity. However, these reservoirs are subject to variables that impact the volume and quality of the stored water, with the contribution of such nutrients as Total Nitrogen (TN) and Total Phosphorus (TP) being one of the main factors in intensifying the eutrophication process. The aim of this study was to quantify the principal inputs and outputs of nutrients (N and P) and calculate the nutrient balance, in order to verify the influence of seasonality on the import and export of these nutrients in surface reservoirs of the semi-arid region of Brazil. Seven campaigns were carried out between 2015 and 2016, with water samples collected at five points in the Pereira de Miranda reservoir and four in the General Sampaio reservoir. The inflow of nutrients was greater when the inflow rate of the water was higher. The balance showed the accumulation of 22.8 tons of TN and 2.8 tons of TP in the Pereira de Miranda reservoir, and 13 tons of TN and 1.5 tons of TP in the General Sampaio. These reservoirs are therefore retaining nutrients, acting as a storage site for nutrients transported by surface runoff in the basin of the Curu River. This increase in storage occurred mainly during the rainy season, when nutrient retention by the reservoirs impaired water quality, resulting in eutrophication, and making it difficult to meet the uses for which the reservoirs were intended.