Abstract for the VIIth IAHS Scientific Assembly, Foz de Iguaçu, 3-9 April 2005 Symposium 7-2: Model evaluation and comparison: uncertainty analysis and diagnostics.

50 year discharge simulation of the Amazon basin rivers by ORCHIDEE Monthly discharges are simulated during the 1949-2000 period using ORCHIDEE, the land-surface scheme of the Pierre Simon Laplace Institute (IPSL). Simulated discharge values are compared to the observed Hybam project (Hydrogeodynamics of the Amazon basin) discharge data set, to assess the ability of the model in reproducing:

The model tends to overestimate discharge values in the southern tributaries and underestimate them in the western rivers. As a result, the annual simulated discharge in Óbidos is close to the reality.

The Nash-Sutcliffe coefficient shows that good simulations are obtained in 0bidos, in the downstream Mamoré and along the Madeira river, and to a lesser extent in the downstream Negro and Solimões basins. The results are not so good along the Xingu and the Branco rivers and more generally in the upper basins rivers.

Interannual variability is quite well reproduced along the Madeira River and in Obidos. The long term variability and in particular the 1970 break in Obidos discharge is also observed in Orchidée data. The simulation results in being a good tool to determine the geographical origin of this important change in Obidos discharge.

The annual cycle is generally well reproduced in the whole basin. The sooner high flow discharge in Óbidos may be related to the above normal and on time water flux coming from the southern tropical basins and to the anticipated high flow along the Solimões. The late low flow in Óbidos may be associated with the delayed low flow in the southern tributaries.