Trends in flow intermittence for European rivers

Intermittent rivers are prevalent in many countries across Europe, but little is known about thetemporal evolution of intermittence and its relationship with climate variability. Trend analysis ofthe annual and seasonal number of zero-flow days, the maximum duration of dry spells and themean date of the zero-flow events, is performed on a database of 452 rivers with varying degreesof intermittence between 1970 and 2010. The relationships between flow intermittence andclimate are investigated using the standardized precipitation evapotranspiration index (SPEI) andclimate indices describing large-scale atmospheric circulation. The results indicate a strongspatial variability of the seasonal patterns of intermittence and the annual and seasonal number ofzero-flow days, highlighting the controls exerted by local catchment properties. Most of thedetected trends indicate an increasing number of zero-flow days, which also tend to occur earlierin the year, particularly in southern Europe. The SPEI is found to be strongly related to the annualand seasonal zero-flow day occurrence in more than half of the stations for differentaccumulation times between 12 and 24 months. Conversely, there is a weaker dependence ofriver intermittence with large-scale circulation indices. Overall, these results suggest increasedwater stress in intermittent rivers that may affect their biota and biochemistry and also reduceavailable water resources.