Abstract
Abstract. Medium-term hydrologic forecast uncertainty is strongly dependent on the forecast quality of meteorological variables. Of these variables, the influence of precipitation has been studied most widely, while temperature, radiative forcing and their derived product potential evapotranspiration (PET) have received little attention from the perspective of hydrological forecasting. This study aims to fill this gap by assessing the usability of potential evaporation forecasts for 10-day-ahead streamflow forecasting in the Rhine basin, Europe. In addition, the forecasts of the meteorological variables are compared with observations. Streamflow reforecasts were performed with the daily wflow_hbv model used in previous studies of the Rhine using the ECMWF 20-year meteorological reforecast dataset. Meteorological forecasts were compared with observed rainfall, temperature, global radiation and potential evaporation for 148 subbasins. Secondly, the effect of using PET climatology versus using observation-based estimates of PET was assessed for hydrological state and for streamflow forecast skill. We find that (1) there is considerable skill in the ECMWF reforecasts to predict PET for all seasons, and (2) using dynamical PET forcing based on observed temperature and satellite global radiation estimates results in lower evaporation and wetter initial states, but (3) the effect on forecasted 10-day streamflow is limited. Implications of this finding are that it is reasonable to use meteorological forecasts to forecast potential evaporation and use this is in medium-range streamflow forecasts. However, it can be concluded that an approach using PET climatology is also sufficient, most probably not only for the application shown here, but also for most models similar to the HBV concept and for moderate climate zones. As a by-product, this research resulted in gridded datasets for temperature, radiation and potential evaporation based on the Makkink equation for the Rhine basin. The datasets have a spatial resolution of 1.2×1.2 km and an hourly time step for the period from July 1996 through 2015. This dataset complements an earlier precipitation dataset for the same area, period and resolution.
Highlights
Hydrologic forecasting has the aim of predicting the future state of important hydrologic fluxes, most notably streamflow
We find that (1) there is considerable skill in the European Center for Medium-Range Weather Forecasts (ECMWF) reforecasts to predict potential evapotranspiration (PET) for all seasons, and (2) using dynamical PET forcing based on observed temperature and satellite global radiation estimates results in lower evaporation and wetter initial states, but (3) the effect on forecasted 10-day streamflow is limited
The response of the continuous ranked probability skill score (CRPSS) curve is flat for high P (X ≥ x) for temperature during summer, as all summer temperatures fall in the highest 60 % of temperatures of the whole year
Summary
Hydrologic forecasting has the aim of predicting the future state of important hydrologic fluxes, most notably streamflow. Throughout the process of forecasting, from model setup via initial state estimation to forecast run, meteorological forcing is a key component. Precipitation is known to be the main driver of hydrological processes, and most of the forecast uncertainty is attributed to inaccurate precipitation forcing (Cuo et al, 2011; Pappenberger et al, 2005). Evaporation is a result of the interaction between meteorological forcing and physical and physiological processes at the land surface. Meteorological forcing provides the potential energy (potential evaporation or PET) for evaporative processes to take place. There are many formulas to estimate the potential energy available for evaporation, which can be divided in three types of formulas based on their
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