A model for decomposition of the drown vegetation in an Amazonian reservoir

The terrestrial vegetation is in most cases not removed before filling of an Amazonian reservoir, causing oxygen problems due to decomposition of organic matter. This work describes an oxygen model of Amazonian Reservoirs, focusing on the role of decomposition of organic matter originating from drown vegetation in the oxygen balance. The model encompasses the following state variables: three fractions of organic matter accounting for differences in biodegradability, dissolved oxygen and ammonium. The temperature is also considered to account for the stratification. The model has been tested on a case study, Tucurui Reservoir, which has a surface area of 2430 km² and a volume of 45 km³. The residence time is 45 days in average. The filling started in September 1984 and it is estimated that the drown vegetation was 40 billion tons of carbon. The model results have been compared with measurements of oxygen taken close to the dam. The modelled oxygen and temperature profiles showed a reasonable agreement with the measurements at two different hydrological conditions, i.e., at high and low water conditions. The modelled results of ammonium showed, however, a too small concentration in hypolimnion during anoxic conditions probably due to an underestimation of the anaerobic release rate for ammonium from the sediment to the water column.