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LERAT ET AL.: VALUE OF UPSTREAM FLOW MEASUREMENTS X - 5improved downstream flow simulations should be balanced with the limitations mentionedpreviously. This paper addresses how upstream flow measurements influence downstreamsimulations in three different situations showing a gradually decreasing availability of thesedata:551. With upstream flow data of acceptable quality, we investigate the conditions fortel-00392240, version 1 - 5 Jun 2009606570upstream discharges necessary to significantly improve downstream simulations,2. With low quality upstream flow data, we assess the sensitivity of downstream simulationsto these upstream errors,3. With limited upstream flow data, we check whether the performance gain mentionedpreviously still holds when measured values are replaced by values calculated by an upstreamsubmodel. “Good measurements are more valuable than good models” was themotto of many old practionners, and at a time where there is a strong tendancy to replacemeasurements by simulations [Silberstein, 2006], we revisit this motto in the context ofsemidistributed hydrological modeling.1.2. Experimental scheme based on nested catchmentsInvestigations were conducted with different versions of a semidistributed model appliedon pairs of nested gaging stations, allowing an explicit use of upstream flow measurements.We used a total of 192 pairs, covering a wide range of hydrological and climatic conditionsand daily time series over a 10-year period (1995–2005). Three models with different levelsof integration of upstream flow measurements were compared on our dataset:1. A model (of the downstream station) was calibrated and run using only simulatedupstream flows.This model will be considered as the benchmark model to which theD R A F T July 24, 2008, 3:43pm D R A F T