Twenty-six years of uneven changes in low flows due to different uses and operation of a large dam in a semiarid river

Little is known about the hydrological impact of the dams in Brazil, despite the rising energy demand and untapped potential. The Paraguaçu River-s discharge has been greatly impacted since 1986 by a large dam built in the lower course of the river. In the 26 years of the history of the dam, the minimum discharges have followed different operational guidelines: 1) there was no established discharge minimum between 1987 and 1996; 2) the discharge minimum was 11 m?s-1 between 1997 and 2004; and 3) hydropower generation began in 2005, when local wet season discharges were either 80 m?s-1 or 160 m?s-1, and dry-season pulses of 10 m?s-1, which were adopted as minimum stream flow. The mean annual dam discharge between 1987 and 2012 was 17.4% less than the incoming river flow. While rare events of daily null flow exist in the historic records, hundreds of occurrences of daily null flow have been observed in the outgoing discharge since 1987. Other important hydrologic alterations (IHAs) have included: the one-day minimum, the 3-day minimum and the number of low-discharge pulses. Ecochange analysis indicates an overall discharge loss (ecodeficit) in the system. The period of dam operation has coincided with a noticeable drop in the natural river discharge, when the mean discharge was 32% lower than in the pre-dam period (1947-1986)