Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 14 8 2010 10.5194/hess-14-1623-2010 http://www.hydrol-earth-syst-sci.net/14/1623/2010/ http://www.hydrol-earth-syst-sci.net/14/1623/2010/hess-14-1623-2010.html http://www.hydrol-earth-syst-sci.net/14/1623/2010/hess-14-1623-2010.pdf 1623 1637 2010-08-24 A past discharges assimilation system for ensemble streamflow forecasts over France – Part 1: Description and validation of the assimilation system G. Thirel guillaume.thirel@jrc.ec.europa.eu E. Martin J.-F. Mahfouf S. Massart S. Ricci F. Habets CNRM-GAME/GMME, Météo-France, URA 1357, Toulouse, France IES, Joint Research Centre, European Commission, Ispra, Italy URA CNRS/CERFACS No. 1875, Toulouse, France UMR Sisyphe, UPMC, ENSMP, CNRS, Paris, France Two Ensemble Streamflow Prediction Systems (ESPSs) have been set up at Météo-France. They are based on the French SIM distributed hydrometeorological model. A deterministic analysis run of SIM is used to initialize the two ESPSs. In order to obtain a better initial state, a past discharges assimilation system has been implemented into this analysis SIM run, using the Best Linear Unbiased Estimator (BLUE). Its role is to improve the model soil moisture by using streamflow observations in order to better simulate streamflow. The skills of the assimilation system were assessed for a 569-day period on six different configurations, including two different physics schemes of the model (the use of an exponential profile of hydraulic conductivity or not) and, for each one, three different ways of considering the model soil moisture in the BLUE state variables. Respect of the linearity hypothesis of the BLUE was verified by assessing of the impact of iterations of the BLUE. 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