Hydrological response to an abrupt shift in surface air temperature over France in 1987/88

During the last few decades, Europe has seen a faster increase of observed temperature than that simulated by models. The air temperature over Western Europe showed an abrupt shift at the end of the 1980s, still insufficiently documented. The aim of this study is to assess the characteristics of this shift and its potential impacts on the hydrological cycle over France. Such an assessment is essential for a better understanding of past and future climatic changes and their impact on water resources. A subset of 119 temperature, 122 rainfall, and 30 hydrometric stations was studied, over the entire French metropolitan territory. Several change-point detection tests were applied to temperature, rainfall and runoff time series. A shift in annual mean air temperature was detected in 1987/88, for more than 75% of the stations, and for both minimum and maximum temperatures. An abrupt increase of about 1 degrees C in minimum and maximum temperature provides evidence of this shift, which shows strong seasonality, with significant increases for DJF, MAM and JJA. Its detection is not affected by the length of the time series or any potential artefacts associated to the conditions of measurement. Cluster analysis of the rainfall stations was used to take account of regional variability in rainfall evolution. Two climate areas were obtained from this analysis: Mediterranean and temperate. No shift was detected in rainfall for either area. However, at annual and quarterly scales, several changes in runoff were observed between the periods 1969-87 and 1988-09. The significant changes occurred from January to July, in agreement with maximum increases in temperature. Evapotranspiration could well play a key role in these changes in the hydrological cycle, as a response to temperature increases in the watersheds studied.