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LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTS X - 9solution to overcome this limitation is to calibrate an upstream rainfall-runoff model andfeed the downstream model with upstream simulations when no measurement is available140(we assume here that model application is not limited by precipitation data availability).145This procedure corresponds to the sequential calibration of a two-subcatchment semidistributedmodel and defines different sets of parameters within the modelled catchment.Unfortunately, using a model on the upstream subcatchment introduces errors and theperformance gain we mentioned previously may not hold up under these degraded conditions.Indeed, several authors [Boyle et al., 2001, Ajami et al., 2004] have shown thattel-00392240, version 1 - 5 Jun 2009150155even where flow conditions are spatially variable, using spatially varying parameters forsubmodels did not provide significant simulation improvement over simple homogeneousparametrization schemes. Other authors presented opposite views [Feyen et al. , 2008].In a previous study, Andréassian et al. [2004] showed that rainfall input distribution (andto a lesser extent parameter distribution) was beneficial for downstream flow simulations,but this result was obtained on watersheds with artificially exaggerated heterogeneity anddoes not hold on actual catchments. The debate will not move forward unless a comparisonis made on a large and varied catchment set. Thus, the third objective of our paperis to generalize the conclusions of the above-cited authors with the help of a large set ofcatchments.3. A two-subcatchment model to investigate the value of upstream flow dataIn this section, we present three versions of the semidistributed daily rainfall-runoffmodel applied on 192 nested catchments (see section 4) showing a wide variety of config- section 4urations.D R A F T July 24, 2008, 3:43pm D R A F T