A hydrological model to assess current and future freshwater availability: application to climate change impacts on hydrology in the Amazon River Basin through mid-century

• First application of an enhanced version of Hydro-BID over the entire Amazon basin. • Hydro-BID: useful tool to assess present and future water supply in the Amazon basin. • Hydrological simulations driven by the latest generation of climate forcing data. • Climate change is projected to lower mean annual river flows and runoff in the basin. • Severity of impacts worsens under high-emissions scenarios and low-water months Climate change is projected to alter the water balance of the Amazon River Basin (ARB), which will affect the water resources availability in the region. Investigating the resulting climate impacts using process-based hydrological models is challenging due to their complexity. To overcome this challenge, an enhanced version of the Hydro-BID hydrological modeling system was developed and applied over the entire ARB. Hydro-BID is a conceptual semi-distributed model with minimum data requirements and tractable computational costs. Here, Hydro-BID is used to investigate alterations of freshwater availability in the ARB through mid-century. To do so, Hydro-BID was forced by state-of-the-art global climate models under a low-emissions (SSP126) and two high-emissions (SSP370 and SSP585) scenarios. Historical simulations demonstrate Hydro-BID’s reasonable predictive skills in annual and monthly scales spanning a range of spatial scales. Future projections show diminished freshwater availability across the basin with large spatial variability and intensification of impacts under high emissions. ARB predominantly faces negative runoff anomalies (10–20%) with drier conditions in the south and northeast under high-emissions scenarios. For major tributaries, impacts on annual runoff and discharge lie within −20%–−5% range under SSP370 and SSP585. Under high emissions, the Abacaxis, Javari and Trombetas subbasins may face the largest impacts (between −20% and −14%) although the Jurua, Madeira, Negro, Purus, Tapajos and Xingu subbasins show non-trivial negative anomalies (11–14%). Projected flow duration curves highlight a shifting pattern toward a drier hydrological state with reductions in streamflow tending to be more accentuated during low-water months. These impacts could create considerable societal and environmental challenges. This study thus underscores the value of Hydro-BID in supporting important decision-making of key water policies and investments in the ARB.