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X - 2 LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTSAbstract.5This paper deals with the introduction of streamflows measuredon an upstream interior point as an additional input in a semidistributedhydrological model. Most studies in the literature indicate that this procedurecan improve the simulations of streamflow at the catchment outlet. Thispaper aims at quantifying this potential performance gain in three situations.First, upstream flow measurements are introduced as a straightforward model10input in paralell with rainfall and potential evapotranspiration during thetel-00392240, version 1 - 5 Jun 2009152025calibration and validation phase. Results are analyzed to identify the conditionsfor these data to influence downstream simulations significantly. Second,upstream flow measurements are corrupted to assess the sensitivity ofthe procedure to upstream biaises. Last, our semidistributed model is calibratedwith upstream flow measurements but run in validation mode withflows calculated from an upstream sub-model to check if the performance gainmentioned previously still holds in these degraded conditions.The results of the different experiments show that (i) the ratio betweenthe high-flow magnitude on the upstream and downstream stations constitutesthe key driver to explain the impact of upstream flows on downstreamsimulations, (ii) substential corruption of upstream data does not destabilizethe semidistributed model, but rather can even be beneficial when thiscorruption induces an overestimation of upstream flow measurements. Thisunexpected result can be explained by the increased weight taken by theseoverestimated flow values in the modeling scheme, which artificially reducesthe need for rainfall-runoff modeling on the remaining part of the catchment.D R A F T July 24, 2008, 3:43pm D R A F T