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Quels apports hydrologiques pour les modèles hydrauliques? Vers ...

Quels apports hydrologiques pour les modèles hydrauliques? Vers ...

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LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTS X - 25without it revealed a nearly systematic gain in the Nash-Sutcliffe criteria in validationmode. Of course, exceptions exist where the introduction of upstream flow measurements460fail to improve downstream simulations but these cases were very limited.2. The ratio between upstream and downstream flows appeared as the key driver forthis performance gain. Simple relationships between different formulations of this ratiowere proposed to give a preliminary estimate of the expected performance gain.3. A surprising result was obtained while conducting a sensitivity analysis on the quality465of upstream flow data: the corruption of these data with a power transform that increasestel-00392240, version 1 - 5 Jun 2009470475480the high upstream flow regime lead to a significant performance gain. It was found thatthis artifical increase induced a reduction of the rainfall-runoff component to the profitof the routing component and thus an increase in the upstream flow measurement weightin the downstream simulation. The value of upstream flow measurements then appearedhigher than the rainfall data to simulate streamflows at a catchment outlet.4. A final experiment was conducted to investigate situations where these upstreamflow measurements are no longer available in validation mode. Strinkingly, all performancegains obtained previously were annihilated due to compensations between the differentcomponents of the model. This conclusion revealed how difficult it is to build spatiallyexplicit hydrological model in the abscence of measurements of internal streamflow.To continue this work, we are particularly interested in testing different strategies toweight upstream flow measurements in the semidistributed model, and to extract as muchinformation as possible from this additional input. Also, tests could be enlarged to finertime-steps where flow routing becomes more complex and could introduce additional errorsthat were not significant in our experiments.D R A F T July 24, 2008, 3:43pm D R A F T

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