Climate change in the Amazon includes the intensification of the hydrological cycle in association with the increase of extreme events. This study aims to identify the impacts on groundwater (GW) within a pristine micro-scale catchment in the Central Amazon related to the observed alterations on hydrological cycle. Precipitation and GW levels from seven piezometers distributed along a hydrological transect covering three zones (lowland, slope and plateau) were collected for the period 2001-2021. External (evapotranspiration, GW storage and climate patterns) and computed (annual recharge) data were used to strengthen the analyses. We identified a generalized growth trend in all compartments (rainfall, evaporation and water table). There was a heterogeneity in water table fluctuations and annual recharge, depending on distance from creek channel and water depth. Surface processes influenced shallows piezometers associated with rapid overflow and low recharge. Conversely, deep piezometers involved slower processes related to water movement, and were more representative of regional GW. Those presented larger seasonal and inter-annual fluctuations, annual recharge and positive trend than shallow piezometers. Besides, all piezometers showed large inter-annual variations in recharge. The El Niño-Southern Oscillation (ENSO) influenced GW level, recharge and storage: positive phase contributed to lower all variables, with recovery in during neutral and negative phase. The larger positive trends in the deeper piezometers are a sign of the resilience of the Alter do Chão aquifer, subject to anthropogenic pressure. As water table play a key-role in shaping the structure and productivity of Amazon forests, further hydro-ecological studies should be conducted to gather information about the fate of GW-dependent Amazonian ecosystems.

The increasing pressure on water resources demands integrated multidisciplinary approaches to deal with environmental, scientific, and social issues related to water availability in watersheds. In large watersheds, many factors control hydrological processes, requiring the application of a methodology capable of describing water dynamics in complex situations. This study uses stable isotope (2H and 18O) modeling to characterize groundwater contribution in an area of heterogeneous hydrogeological framework and advanced anthropization, located in the Piracicaba-Capivari-Jundiaí (PCJ) and Tietê-Jacaré (TJ) water management units – central-eastern portion of the State of São Paulo, Brazil. Groundwater dynamics is controlled by the direct interaction of the rainfall regime with hydrogeological domains. The modeling performed in this study indicates that a portion ranging from 60 to 80% of the total volume of streamflow comes from groundwater discharge (for studied year). However, in the crystalline domain, the aquifer capacity of storing surplus water is smaller, increasing the importance of rainfall for the generation of direct runoff and maintenance of the streamflow. In most of the PCJ unit, there is a greater dependence on surface water and vulnerability to prolonged droughts. On the other hand, in the sedimentary areas, the contribution of groundwater discharge is around 80%, showing that the aquifer storage capacity guarantees homogenization of the discharge throughout the year, and ensures water security in drought periods. This study attests to the successful application of stable isotope modeling to large basins, as the method is able to describe natural hydrological processes occurring in watersheds with a heterogeneous hydrogeological framework and the effects of the anthropogenic action. It is concluded that the use of stable isotopes can aid water resources management in expanding the capacity of monitoring surface and groundwater resources to identify negative effects on water availability.