Artificial reservoirs and their waters are vulnerable to physical, chemical and biological processes capable of promoting imbalances in these ecosystems, such as eutrophication, due to the high hydraulic retention time. This vulnerability is stressed under semiarid condition. The present research focused on water quality of the Pereira de Miranda reservoir (also known as açude Pentecoste), Ceará, located in the Northeast Region of Brazil. The study used the traditional Water Quality Index (WQI), Trophic State Index (TSI) and an adapted Water Quality Index for Reservoirs in Semiarid (WQIRSA). Prolonged drought favored deterioration of water quality. This was pointed out via association between available volume (as percentage of the reservoir capacity) or mean depth with water quality parameters. The results showed divergent trends regarding the applied indices. The use of WQIRSA may offer a more realistic interpretation. However, it requires improvement.
