Abstract

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.

Highlights

  • Society may benefit from seasonal hydrological forecasts (Viel et al, 2016; Soares and Dessai, 2016; Crochemore et al, 2016), i.e. hydrological forecasts for future time periods from more than 2 weeks up to about a year (Doblas-Reyes et al, 2013)

  • A remarkable result of our work is the reduction of the skill in run-off beyond lead month 1, when annually varying system 4 (S4) forcing is used (FullSHs) instead of meteorological forcing that is identical for all years (InitSHs; see Fig. 4)

  • We first analysed the meteorological forcing, which consists of bias-corrected output from a climate model (S4), and found considerable skill in the precipitation forecasts of the first lead month but negligible skill for later lead times

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Summary

Introduction

Society may benefit from seasonal hydrological forecasts (Viel et al, 2016; Soares and Dessai, 2016; Crochemore et al, 2016), i.e. hydrological forecasts for future time periods from more than 2 weeks up to about a year (Doblas-Reyes et al, 2013) Such predictions can be exploited to optimise, for example, hydropower energy generation (Hamlet et al, 2002), navigability of rivers in low flow conditions (Li et al, 2008) and irrigation management (Ghile and Schulze, 2008; Mushtaq et al, 2012) to decrease crop yield losses.

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