Hydrology and Earth System Sciences Discussions www.hydrol-earth-syst-sci-discuss.net 1812-2108 1812-2116 6 4 2009 10.5194/hessd-6-4891-2009 http://www.hydrol-earth-syst-sci-discuss.net/6/4891/2009/ http://www.hydrol-earth-syst-sci-discuss.net/6/4891/2009/hessd-6-4891-2009.html http://www.hydrol-earth-syst-sci-discuss.net/6/4891/2009/hessd-6-4891-2009.pdf 4891 4917 2009-07-07 An evaluation of the canadian global meteorological ensemble prediction system for short-term hydrological forecasting J. A. Velázquez juan-alberto.velazquez.1@ulaval.ca T. Petit A. Lavoie M.-A. Boucher R. Turcotte V. Fortin F. Anctil Chaire de recherche EDS en prévisions et actions hydrologiques, Université Laval, Québec, Canada Centre d'expertise hydrique du Québec, Québec, Canada Recherche en prévision numérique environnementale, Environnement Canada, Montréal, Canada Hydrological forecasting consists in the assessment of future streamflow. Current deterministic forecasts do not give any information concerning the uncertainty, which might be limiting in a decision-making process. Ensemble forecasts are expected to fill this gap. <br><br> In July 2007, the Meteorological Service of Canada has improved its ensemble prediction system, which has been operational since 1998. It uses the GEM model to generate a 20-member ensemble on a 100 km grid, at mid-latitudes. This improved system is used for the first time for hydrological ensemble predictions. Five watersheds in Quebec (Canada) are studied: Chaudière, Châteauguay, Du Nord, Kénogami and Du Lièvre. An interesting 17-day rainfall event has been selected in October 2007. Forecasts are produced in a 3 h time step for a 3-day forecast horizon. The deterministic forecast is also available and it is compared with the ensemble ones. In order to correct the bias of the ensemble, an updating procedure has been applied to the output data. 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