Introduction: Groundwater is a key source for plants when the depth to the water table (WT) or the capillary fringe is within the reach of roots (i.e. in shallow water table sites), but this source has been neglected in ecological studies. The depth of the water table and its temporal fluctuation, both seasonally and inter-annually, may be key to understanding the responses of vegetation to climate variations. However, forests on shallow WT are under-sampled in the Amazon basin, what may bias the conclusions about forest responses to climate change. To guide a research program that explicitly considers this water source and its relationships with climate affecting forests, it is necessary to understand the spatial and temporal patterns of variation of WT.

Objective: Describe WT patterns as a function of climatic and terrain properties across the Amazon. We assume that most of the time, groundwater at high depths is not accessible to roots, and thus focus on shallow WT (6 months). Younger and flatter formations experience much higher seasonal variation in WT (up to 8 m) with the amplitude less coupled to local climate seasonality.

Conclusions/Implications: Amazonia has a mosaic of WT depth variation in space and time that needs to be taken into account in the efforts to understand climate change. Several combinations of WT depth/climate/geomorphology are not yet included in coupled vegetation/hydrological monitoring programs and should be prioritized.