Groundwater dominates terrestrial hydrological processes in the Amazon at the basin and subbasin scales

The hydrology of the Amazon River basin (ARB) has been extensively studied; however, critical scientific gaps remain regarding key processes that govern hydrologic dynamics and the resilience of the rainforest. This inhibits the understanding of hydrological considerations needed for sustainable forest management under climate change and growing human stressors. Here, using high resolution (similar to 2km), long-term simulations from a process-based hydrological model (LEAF-Hydro-Flood), we investigate the dominant hydrological processes across the ARB, their key roles in shaping basin functions, and the decadal evolution therein. Results indicate that shallow groundwater (<5m deep) strongly modulates the seasonality of the surface fluxes across the ARB and at least 34% of the Amazonian Forest is supported by groundwater during the dry season. A two-month lag between the seasonal peak of evapotranspiration (ET) and river discharge is a key mechanism that potentially prevents the rainforest from tipping into savanna. The dry season in the ARB is getting drier and the wet season is getting wetter, pointing to an accelerating hydrologic cycle. The ARB is dominantly energy limited, however, our results imply that in the absence of groundwater support, and with less than similar to 125 mm/month of precipitation, the basin could have become water-limited over some regions. The long-term basin-averaged ET-dominated by transpiration-changed with a split pattern of +/- 9% in the past three decades. Similarly, water table depth (+/- 19%) and runoff (+/- 29%) changed with heterogenous patterns across the ARB. Further, the contribution of canopy interception loss and ground evaporation changed heterogeneously across the ARB in response to deforestation. River discharge did not change substantially due to the crucial buffering role of groundwater, but terrestrial water storage (TWS) decreased (increased) in the 2000s (2010s) compared to that in the 1990s. Although groundwater is the dominant contributor to total TWS, the dynamics of TWS over the major river channels are controlled by flood water, given relatively shallow groundwater. This study provides crucial insights on the dominant hydrological processes in the ARB to inform forest management practices.