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X - 14LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTSfor simulations at the daily time-step has a low influence on model performance comparedto the wave celerity C. Therefore, D was fixed to a single value of 3000 m 2 . This value230was identified by trial and error and provided optimal performance on our sample of 192catchments. It is consistent with values obtained in the above-mentioned literature.As a result, the celerity C is the only parameter of the routing model that is calibratedin vectors ρ and ρ ′ mentioned in equations 1 and 2.3.4. Calibration and validation procedureequatioThe calibration algorithm is identical for the three tested models and aims at adjustingtel-00392240, version 1 - 5 Jun 2009235240245parameters to minimize the mean squared error (MSE) between calculated and observedflow. First, a regular sampling of the parameter space is applied by selecting severalinitial values for each parameter. Then all possible combinations of parameters are generatedand the MSE is calculated for all the parameters sets. The set having the lowestMSE is finally retained as a starting point for a local Levenberg-Marquardt minimizationalgorithm [Marquardt, 1963].Three initial values were identified for each of the four GR4J model parameters fromthe distributions obtained after calibration on a large set of French catchments [Le Moineet al., 2007]. For the celerity (C) of the diffusive wave model, three initial values of 0.5,1 and 1.5 m/s were selected in agreement with the literature [Moussa, 2008].A standard split-sample test is applied [Klemes, 1986] with two equal and contiguoussubperiods. Calibration is alternatively performed on each subperiods. The model’sperformance is assessed in calibration and validation mode with the Nash-Sutcliffe efficiency[Nash and Sutcliffe, 1970] (called here classical efficiency, CEff) comparing theD R A F T July 24, 2008, 3:43pm D R A F T