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

Abstract. This paper uses hindcasts (1981–2010) to investigate the sources of skill in
seasonal hydrological forecasts for Europe. The hindcasts were produced with
WUSHP (Wageningen University Seamless Hydrological Prediction system). Skill
was identified in a companion paper. In WUSHP, hydrological processes are
simulated by running the Variable Infiltration Capacity (VIC) hydrological
model forced with an ensemble of bias-corrected output from the seasonal
forecast system 4 (S4) of the European Centre for Medium-Range Weather Forecasts (ECMWF). We first analysed the meteorological forcing. The
precipitation forecasts contain considerable skill for the first lead month
but hardly any significant skill at longer lead times. Seasonal forecasts of
temperature have more skill. Skill in summer temperature is related to
climate change and is more or less independent of lead time. Skill in February
and March is unrelated to climate change. Different sources of skill in
hydro-meteorological variables were isolated with a suite of specific
hydrological hindcasts akin to ensemble streamflow prediction (ESP). These
hindcasts show that in Europe, initial conditions of soil moisture (SM) form the
dominant source of skill in run-off. From April to July, initial conditions of
snow contribute significantly to the skill. Some remarkable skill features
are due to indirect effects, i.e. skill due to forcing or initial conditions
of snow and soil moisture at an earlier stage is stored in the hydrological
state (snow and/or soil moisture) of a later stage, which then contributes to
persistence of skill. Skill in evapotranspiration (ET) originates mostly in the
meteorological forcing. For run-off we also compared the full hindcasts (with
S4 forcing) with two types of ESP (or ESP-like) hindcasts (with identical forcing
for all years). Beyond the second lead month, the full hindcasts are less
skilful than the ESP (or ESP-like) hindcasts, because inter-annual variations in the
S4 forcing consist mainly of noise which enhances degradation of the skill.
