Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 12 6 2008 10.5194/hess-12-1323-2008 http://www.hydrol-earth-syst-sci.net/12/1323/2008/ http://www.hydrol-earth-syst-sci.net/12/1323/2008/hess-12-1323-2008.html http://www.hydrol-earth-syst-sci.net/12/1323/2008/hess-12-1323-2008.pdf 1323 1337 2008-12-10 From near-surface to root-zone soil moisture using an exponential filter: an assessment of the method based on in-situ observations and model simulations C. Albergel C. Rüdiger T. Pellarin J.-C. Calvet calvet@meteo.fr N. Fritz F. Froissard D. Suquia A. Petitpa B. Piguet E. Martin CNRM/GAME (Météo-France, CNRS), Toulouse, France LTHE (UMR 5564), Grenoble, France now with: Department of Civil and Environmental Engineering, The University of Melbourne, Melbourne, Australia A long term data acquisition effort of profile soil moisture is under way in southwestern France at 13 automated weather stations. This ground network was developed in order to validate remote sensing and model soil moisture estimates. In this paper, both those in situ observations and a synthetic data set covering continental France are used to test a simple method to retrieve root zone soil moisture from a time series of surface soil moisture information. A recursive exponential filter equation using a time constant, <i>T</i>, is used to compute a soil water index. The Nash and Sutcliff coefficient is used as a criterion to optimise the <i>T</i> parameter for each ground station and for each model pixel of the synthetic data set. In general, the soil water indices derived from the surface soil moisture observations and simulations agree well with the reference root-zone soil moisture. Overall, the results show the potential of the exponential filter equation and of its recursive formulation to derive a soil water index from surface soil moisture estimates. 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