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. This paper uses hindcasts (1981–2010) to investigate the sources of skill inseasonal hydrological forecasts for Europe. The hindcasts were produced withWUSHP (Wageningen University Seamless Hydrological Prediction system). Skillwas identified in a companion paper. In WUSHP, hydrological processes aresimulated by running the Variable Infiltration Capacity (VIC) hydrologicalmodel forced with an ensemble of bias-corrected output from the seasonalforecast system 4 (S4) of the European Centre for Medium-Range Weather Forecasts (ECMWF). We first analysed the meteorological forcing. Theprecipitation forecasts contain considerable skill for the first lead monthbut hardly any significant skill at longer lead times. Seasonal forecasts oftemperature have more skill. Skill in summer temperature is related toclimate change and is more or less independent of lead time. Skill in Februaryand March is unrelated to climate change. Different sources of skill inhydro-meteorological variables were isolated with a suite of specifichydrological hindcasts akin to ensemble streamflow prediction (ESP). Thesehindcasts show that in Europe, initial conditions of soil moisture (SM) form thedominant source of skill in run-off. From April to July, initial conditions ofsnow contribute significantly to the skill. Some remarkable skill featuresare due to indirect effects, i.e. skill due to forcing or initial conditionsof snow and soil moisture at an earlier stage is stored in the hydrologicalstate (snow and/or soil moisture) of a later stage, which then contributes topersistence of skill. Skill in evapotranspiration (ET) originates mostly in themeteorological forcing. For run-off we also compared the full hindcasts (withS4 forcing) with two types of ESP (or ESP-like) hindcasts (with identical forcingfor all years). Beyond the second lead month, the full hindcasts are lessskilful than the ESP (or ESP-like) hindcasts, because inter-annual variations in theS4 forcing consist mainly of noise which enhances degradation of the skill.

Seasonal streamflow forecasts for Europe – Part 2: Sources of skill