Changes in meteorological patterns to drier conditions alter the hydrodynamic processes of reservoirs and can affect their thermal structure. The aim of this study was to assess the impacts of a severe drought period on the hydrodynamics of a tropical reservoir. One-dimensional hydrodynamic modelling was applied from 2009 to 2016 to simulate the thermal dynamics of the highly drought-impacted Serra Azul reservoir, a drinking water supply reservoir located in southeast Brazil. After calibration (r = 0.91, root mean square error [RMSE] = 1.33 °C) and validation of the model (r = 0.87, RMSE = 2.06 °C), physical indicators were calculated to evaluate the stratification and mixing processes in the water column. Results showed that the reservoir hydrodynamics were affected by the drought period through an increase in mixing frequency and duration. Negative implications for water quality are expected, such as higher concentration of dissolved substances and nutrients and increased turbidity, salinity, and electrical conductivity, as observed in most lakes and reservoirs subjected to drought. Different water withdrawal scenarios were simulated, and a critical water withdrawal volume that could prevent drought impacts on the Serra Azul reservoir thermal regime was identified. This study indicates the need for managers to understand potential impacts of extreme meteorological events in reservoir dynamics, particularly when the reservoirs are part of drinking water systems.

Droughts can impact ecosystem services provided by reservoirs. Quantifying the intensity of droughts and evaluating their potential effects on the thermal stability of reservoirs are subjects that demand greater attention, due to both the importance of temperature on aquatic metabolism and the climate change scenarios that predict an increase in the frequency of extreme weather events. This study aimed to investigate drought periods in ten Brazilian reservoirs and to discuss their effects on each reservoir’s thermal stability. The Standardized Precipitation Index at a twelve month timescale (SPI-12) was applied to identify the hydrological drought periods. One-dimensional vertical hydrodynamic modeling was used to simulate the water balance and the thermal dynamics in the reservoirs. Schmidt Stability Index (St) was calculated to assess the thermal stability of the reservoirs. The drought periods identified by the SPI-12 are related to decreasing water levels of the reservoirs, but the dam operating strategies and the upstream influence of cascading reservoirs are important drivers of fluctuations. A significant difference in St between wet and dry conditions was found only during summer for all reservoirs. Thus, this study identified alterations in thermal regime during drought periods according to the seasons and the reservoirs characteristics.