Hydroclimatic Trends Suggest Declining Water Resources in the Cascade Reservoirs of the Tocantins River

ABSTRACT Research on hydroclimatic variations explains the relationships between water masses and global climate factors. Climate change causes changes in river flow regimes and impacts ecosystems, the economy, and society. In this study, we characterised the hydroclimatology of the seven reservoirs of the Tocantins River, along 1500 km of river and during more than 12 years of sampling, where we analysed climatic variables such as precipitation, global solar radiation, and air temperature, in addition to hydrological variables such as discharge of the reservoirs. We identified that the discharge of the reservoirs recovered more slowly after the dry period and that these discharges decreased at a rate of 575 m 3 /s between 1995 and 2023, followed by a negative and significant downward trend. As with discharge, precipitation showed a downward trend. The water deficit caused by prolonged droughts between 2015 and 2017 resulted in lower flows and higher air temperatures. In addition to climatic factors, the socioeconomics of the reservoir areas demand high water withdrawals, associated with population growth and agricultural production. We conclude that the reservoirs have hydroclimatic variations. These gradients are mainly due to differences in precipitation and flows but are highly dependent on temperature conditions, solar radiation, and water withdrawal. These factors are important and should be discussed to mitigate the ecological and socioeconomic impacts on the Tocantins River basin.