Evaluation and calibration of operational hydrological ensemble forecasts in Sweden

Daily operational hydrological 9-day ensemble forecasts during 18 months in 45 catchments were evaluated in probabilistic terms. The forecasts were generated by using ECMWF meteorological ensemble forecasts as input to the HBV model, set up and calibrated for each catchment. Two kinds of reference discharges were used in the evaluation, "perfect forecasts" and actual discharge observations. A percentile-based evaluation indicated that the ensemble spread is underestimated, with a degree that decreases with increasing lead time. The share of this error related to hydrological model uncertainty was found to be similar in magnitude to the share related to underdispersivity in the ECMWF meteorological forecasts. A threshold-based evaluation indicated that the probability of exceeding a high discharge threshold is generally overestimated in the ensemble forecasts, with a degree that increases with probability level. In this case the contribution to the error from the meteorological forecasts is larger than the contribution from the hydrological model. A simple calibration method to adjust the ensemble spread by bias correction of ensemble percentiles was formulated and tested in five catchments. The method substantially improved the ensemble spread in all tested catchments, and the adjustment parameters were found to be reasonably well estimated as simple functions of the mean catchment discharge. (c) 2007 Elsevier B.V. All rights reserved.