Expressive riverine fluxes over Amazon floodplain units revealed by high resolution 2D modelling

Abstract

Water fluxes in the Amazon River floodplain affect hydrodynamic and ecological processes from local to global scales. Nevertheless, these fluxes remain poorly understood due to difficult access and limited data. In this study, we characterize the hydrodynamics of eight floodplain units of the central Amazon River (40'000 km 2 ) using the 2D hydraulic model HEC-RAS.High resolution modeling improved the representation of river and floodplain discharge, water surface elevation (77 cm accuracy) and flood extent (˜80% -high water period, ˜52% -low water period). We have learned 13 lessons about river and floodplain hydrodynamics from the modeling. The most remarkable lessons are that the floodplain is organized in units of about 80 km with upstream inflow and downstream outflow. These gross flows are much larger than the net flows with values of up to 20% of the Amazon River discharge and a residence time around 6 days during floods (several months during low water period). Water extent does not a have strong interannual variability during floods as the volume stored in the floodplain, possibly due to topographic constrains. Significant flood extent and volume hysteresis, as well as active flow and storage zones on the floodplain, highlight the complexity of floodplain hydrodynamics. Extreme floods strongly impact the onset and duration of the flood of up to 2 months and, consequently, on the period of high connectivity with the river. These findings are important for understanding carbon and sediment fluxes, and the effects of climate change on water fluxes and riparian communities.