Climate change impacts on groundwater: a growing challenge for water resources sustainability in Brazil

Despite Brazil's substantial freshwater reserves, regional disparities and increasing anthropogenic pressures have led to recurrent water scarcity, particularly in the northeast and central-west regions. Climate change is expected to intensify these challenges by further stressing surface and groundwater resources. This study estimates the impacts of climate change on future water availability in Brazil using a GIS-based distributed water balance model, particularly the aquifer recharge. Changes in precipitation, temperature, surface runoff, and groundwater recharge (GWR) were evaluated under two emission scenarios (SSP245 and SSP585), based on bias-corrected CMIP6 projections for three future periods: 2025-2050 (F1), 2050-2075 (F2), and 2075-2100 (F3). Climate inputs were used to compute water balance parameters on a monthly basis, which were then aggregated to annual means and compared against a historical baseline (1980-2013). The results indicate an increase in average annual temperature under both emission scenarios. Under SSP245, the projected average temperature rise is 1.02 °C, 1.56 °C, and 1.94 °C for the periods F1, F2, and F3, respectively. Under SSP585, the temperature increase is more pronounced, with projections of 1.38 °C, 2.43 °C, and 3.66 °C for the same periods. Precipitation changes across the country are highly variable; however, the maps generally show a decrease in the northern and eastern regions. Changes in climate parameters are expected to impact annual runoff, with increases projected mainly in the southern and eastern regions. At the same time, decreases are anticipated in the north, west, and southeast regions, reaching up to - 261 mm/year. As surface water availability declines due to climate change, reliance on groundwater resources is expected to increase. However, climate change is projected to reduce GWR by up to - 666 mm/year directly. The Bauru-Caiuá Aquifer System is projected to experience the most severe reduction in GWR, with a decrease of up to - 27.94%. Other aquifers, such as Bambuí Cárstico, Furnas, Guarani, Parecis, Ponta Grossa, and Serra Geral, are also expected to face significant reductions in recharge. Therefore, an integrated approach to water resources management will be critical in these regions to effectively balance future water demand and supply.