Sources of spring predictability of the hydrological system over France werestudied on a seasonal time scale over the 1960–2005 period. Two randomsampling experiments were set up in order to test the relative importance ofthe land surface initial state and the atmospheric forcing. The experimentswere based on the SAFRAN-ISBA-MODCOU hydrometeorological suite whichcomputed soil moisture and river flow forecasts over a 8-km grid and morethan 880 river-gauging stations. Results showed that the predictability ofhydrological variables primarily depended on the seasonal atmosphericforcing (mostly temperature and total precipitation) over most plains,whereas it mainly depended on snow cover over high mountains. However, theSeine catchment area was an exception as the skill mainly came from theinitial state of its large and complex aquifers. Seasonal meteorologicalhindcasts with the Météo-France ARPEGE climate model were then usedto force the ISBA-MODCOU hydrological model and obtain seasonal hydrologicalforecasts from 1960 to 2005 for the entire March-April-May period. Scoresfrom this seasonal hydrological forecasting suite could thus be comparedwith the random atmospheric experiment. Soil moisture and river flow skillscores clearly showed the added value in seasonal meteorological forecastsin the north of France, contrary to the Mediterranean area where valuesworsened.
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