Establishment of the stage-discharge relationship of the River Amazon at Obidos: "normaI difference in level" method using "variable geometry"

This paper describes the way in which the daily discharges of the River Amazon were calculated at Obidos. The stage-discharge relationship is practically univocal in low and mean water. In high flow periods, the scatter of the stream gaugings compared to the univocal rating curve effects the correction of the discharges according to the levels at Santarem then at Parintins. The "normal difference in level" method is not well adapted for Obidos: it was replaced by a "normal difference in level with variable geometry" method, a function of the water levels at Obidos. While using only the Santarem station, the application of this new method reduced the scattering to 3.0% on the entire rating curve.