Flooding dynamics on the lower Amazon floodplain: 2. Seasonal and interannual hydrological variability

We analyzed seasonal and interannual variability in hydrological fluxes and inundation dynamics of a large floodplain unit (2440 km²) along the lower Amazon River over a period of 15 years (1995-2010). Floodplain inundation was simulated using LISFLOOD-FP, which combines one-dimensional river routing with two-dimensional overland flow, and a local hydrological model. Dominant sources of inflow varied seasonally among direct rain and local runoff (November), Amazon River (December to August) and seepage (September and October). Shifts in timing of dominance among the water balance components occurred conform variations in annual peak stage. The period of dominance of river inflow over total floodplain influxes began about 1 month earlier and ended 1 month later in the 2009 high flood year compared to the 1998 low flood year. On average, river to floodplain discharge represented 0.75% of the Amazon River discharge at Óbidos and 82% of the annual hydrological influxes to the floodplain. We observed an up to ninefold variation in river-floodplain annual discharge. Relatively small increments in main stem peak discharge cause disproportionately large changes in the flow routed through the floodplain. Despite the higher frequency of years with lower minimum stages, the intensification of the hydrological cycle of the Amazon Basin is causing substantially greater amounts of riverine water to flow across floodplain environments. Key Points Floodplain hydrological fluxes were analyzed with an inundation model Proportion of water supplied from river and local sources varied seasonally Up to ninefold interannual variation in river-floodplain exchange was observed