Flooding and carbon dynamics on the lower Amazon floodplain

The Amazon basin stores and cycles large quantities of water and carbon, and floodplains are an important part of these processes. In this dissertation, I studied the interactions of hydrological and biogeochemical processes over a large floodplain unit in the eastern Amazon, the Curuai floodplain. I combined digital topography derived from the Shuttle Radar Topography Mission with extensive echo-sounding data to generate a hydraulically correct digital elevation model. Floodplain inundation was simulated over a period of 15 years (1995-2010) using LISFLOOD-FP, which combines one-dimensional river routing with two-dimensional overland flow, and a local hydrological model. An accurate filling and drainage of the floodplain was achieved with quantification of changes in water elevation, flooding extent, and river-floodplain flows. Levee height and channel topography were the main controls of river-floodplain exchanges during high and low-water, respectively. The inflow from the Amazon River corresponded to 82% of annual inputs from all sources, with dominant sources of inflow varying seasonally among direct rain and local runoff (November), Amazon River (December through August) and groundwater seepage (September and October). A water balance analysis and concentrations and transformations of carbon from other studies on the central Amazon floodplain were used to compute the fluxes through the major pathways of carbon loss from the Curuai floodplain. Carbon dioxide outgassing represented 86% of the total carbon loss and its seasonal and spatial variability was further investigated. The seasonal changes in lake mixing patterns exerted an important control over both carbon dioxide production and gas transfer velocities. The range of spatial variation in carbon dioxide concentration was least at low water and largest at high water. Gas transfer coefficients that take into account both wind and heat fluxes at the lake's surface were approximately four times higher than values previously used in regional estimates of gas evasion from lakes on the Amazon floodplain.