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LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTS X - 19amplitude between the UP and TOTAL gaging stations. Figure 5 shows the REff effi- Figure 5ciencies plotted against three of these descriptors: (a) the surface ratio between UP andTOTAL catchments, (b) the ratio between the mean annual discharge on UP and TOTAL335stations, and (c) the ratio between the 99 th percentile of the discharge time-series on UPand TOTAL stations. This last indicator gives the ratio between high flows on the twostations. The choice of the 99 th percentile appeared as the best compromise between lowerpercentiles that do not capture the appropriate flood regimes and higher percentiles thatare influenced by sampling problems on extreme values.tel-00392240, version 1 - 5 Jun 2009340345350Figure 5 clearly shows that REff(UP-Meas/Benchmark) increases when upstream flow Figure 5measurements constitute a singificant part of the downstream flows. Large measuredupstream contributions reduce the importance of errors introduced by the UP-Meas modelon the intermediate area INT. As a result, the downstream simulation improves. Itis remarkable that the REff criterion does not converge toward 1 (perfect simulation)when the different descriptors tend toward 1 (corresponding to a nested station locatedimmediately upstream of the downstream station): when upstream contribution increasessubstantially, any inconsistency in the upstream flow data will have a critical impact onthe downstream simulation and the performance gain becomes more uncertain. This wasnoted in several cases (outliers showing null or negative REff values with a 99 th percentiledischarge ratio close to 1, see Figure 5 (c)). Upstream flow data quality appears to be animportant factor in our analysis; it will be studied in greater detail in section 5.2.Linear regressions between the descriptors and REff values were performed as indicatedin Figure 5: the coefficient of correlation increases from 0.62 with the surface ratio up to Figure 50.67 with the 99 th percentile discharge ratio. Note that a combination of several descriptorsD R A F T July 24, 2008, 3:43pm D R A F T