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 decompositi on of organic matter. This work describes an oxygen model of Amazonian Reservoirs, focusing on the role of decomposition of organic matter o riginating from drown vegetation in the oxygen balance. The model enco mpasses the following state variables: three fractions of organic matt er accounting for differences in biodegradability, dissolved oxygen an d ammonium. The temperature is also considered to account for the stra tification. The model has been tested on a case study, Tucurui Reservo ir, which has a surface area of 2430 km(2) and a volume of 45 km(3). T he residence time is 45 days in average. The filling started in Septem ber 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 d ifferent hydrological conditions, i.e., at high and low water conditio ns. The modelled results of ammonium showed, however, a too small conc entration in hypolimnion during anoxic conditions probably due to an u nderestimation of the anaerobic release rate for ammonium from the sed iment to the water column.