• Amazon;
  • dry season;
  • ecosystem resilience;
  • evapotranspiration;
  • groundwater;
  • soil moisture

[1] We investigate the potential influence of groundwater on seasonal evapotranspiration (ET) in the Amazon using a coupled groundwater-surface water model (LEAF-Hydro-Flood) forced with ERA-Interim reanalysis, at 2 km grid and 4 min steps over 11 yrs (2000–2010), and validated with available soil moisture and ET observations. We find that first, the simulated water table is <2 m deep over a significant portion of the Amazon (20–40%). Second, shallow groundwater can reduce wet season soil drainage, leading to larger soil water stores before the dry season arrives. Third, capillary rises from the water table can reach the root zone and maintain high dry season ET near the valleys. Fourth, groundwater's delayed response to rainfall can buffer surface stress in the dry season, when groundwater is the shallowest. Fifth, this temporal delay can be seen as spatial patterns; continued drainage and convergence maintain moist valleys forming a structured mosaic of wet-dry patches in the dry season. Results from two parallel runs, with and without groundwater, suggest that overall groundwater made a large difference in modeled soil moisture where the water table is shallow, but it only made a difference in modeled ET where the seasonality is strong; over southeastern Amazonia, July–August ET differs by ∼1 mm/day. We note that our results are based on model simulations, which only suggest the potential importance of the groundwater system to the Amazon water cycle. The ultimate knowledge must come from carefully designed field observations linking vegetation, soil and groundwater with water balance studies and tracer tests, across a range of physical-biological settings.