Comparing trends in European streamflow records to hydrological change in a large-scale model intercomparison experiment

In Europe, an overall appraisal of runoff changes at a continental scale has long been hindered by the paucity of readily-available runoff data. Recently, a coherent picture of hydrological trends in the most recent decades (1960-2000) has emerged from regional analyses of streamflow observations. This study created maps of relative change in annual and monthly runoff, and high and low flows across Europe based on an ensemble of eight large-scale hydrological models from the EU project WATCH. These modelled changes were validated against trends from 293 discharge records showing that the ensemble mean provides the best representation of changes. Estimates of change are particularly reliable for annual runoff, winter runoff, and high flows. The model ensemble maps reveal valuable details of a pronounced gradient between positive (wetter) trends in the Northwest and negative (drier) trends in the Mediterranean and in the Southeast, which are much stronger in winter. They provide a considerable improvement over previously published maps of observed trends covering only parts of Europe. An expansion to the whole 20th century has to rely on fewer observations and fewer model simulations. However, a product "WATCH 20th Century ensemble" is available and used for comparison. Based on the results of these analyses the potential of the ensemble the to fill gaps to assess decadal changes in runoff in Europe where long-term observations are unavailable is assessed. Relative changes in runoff are also compared to climate-model driven runs of the hydrological models for the 20th and 21st century.