A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Hydrological dynamics of the varzea of Lago Grande de Curuai : water and sedimentbalance, influence of river stage and local rainfall, long term dynamicsPascal Kosuth (IRD)Pascal Kosuth, IRD, CP 70911 Lago Sul, CEP 711619-970 Brasilia DF Brazilkosuth.ird@apis.com.brFloodplains play a key role in Amazon River hydrology, sediment dynamics, carbon cycle,aquatic biodiversity and ecology. Nevertheless some basic questions related to floodplainsstill have not been answered : What fraction of Amazon River waters actually flows throughfloodplains ? What is the mean residence time of water in floodplains ? What percentage ofsediments entering a floodplain really stays trapped in it ? What is the signature offloodplains on river waters chemistry ? To contribute to the understanding and quantificationof water and sediment fluxes through floodplains a specific flooded system has beenmonitored since March 1999.The Varzea of Lago Grande de Curuai, south of Obidos, has a 3660 km² watershed area, ofwhich 800 km² to 1600 km² are flooded depending on the hydrological cycle phase. Thevarzea consists in interconnected lakes, linked to the Amazon River through eight majorchannels, 3 of them permanent. Annual amplitude of river level fluctuation at Obidos is 7meters, mean annual rainfall is 2400 mm and mean annual evapo-transpiration is 1400 mm.This varzea was selected for its size and morphological diversity allowing to monitor varioustypes of lakes with and without river inflow.Monitoring includes six daily measured gage stations, one simple meteorological station(rainfall and evaporation), eleven stations with ten to ten days surface water sampling forgeochemistry and sediment measurement. Additionally fourteen measurement campaignshave been realized since March 1999, at various hydrological stages, with dischargemeasurement on thirteen channels and systematic water sampling at 35 points.Measurement results enlighten the hydrological, sediment and geochemical dynamics of thevarzea. Water level inside the varzea changes regularly with river level with only slightgradients (tens of centimeters). Annual water inflow from the river and outflow to the river,estimated over November 1998 – October 1999 period, are respectively 10.6 10^9 m3 and13.8 10^9 m3. Over the same period estimated suspended sediment inflow and outflow arerespectively 1 350 000 t and 500 000 t, meaning an estimated net trapping of 850 000 t/year.A model of the varzea hydrological and sediment dynamics has been developed. Annualinflow from the river mainly depends on annual rainfall and celerity of river level raise.Percentage of flooded area in the watershed plays a crucial role in the balance betweenrainfall inflow and river water inflow and so controls the suspended sediment trapping.