1. INTRODUCTION
In streams and rivers, flow intermittence is characterized by the cessation of flow, followed or not by complete drying of the channels (Datry et al. 2016). The spatio-temporal patterns of flow intermittence can be extremely variable depending on climatic, geologic or topographic contexts (Costigan et al. 2017). While many studies have been focused on river low-flows characterization and, in particular, the possible long-term trends due to climate change (e.g., Marx et al., 2018), far less work has been dedicated to intermittent rivers and ephemeral streams. Recent studies indicate trends towards less severe climatic droughts over North-Eastern Europe, especially in winter and spring, and the opposite in Southern Europe where more severe droughts, are encountered (Gudmundsson and Seneviratne, 2015, Spinoni et al., 2017, Hertig and Tramblay, 2017). Globally, negative trends in streamflow in Europe have been reported by Stahl et al. (2010) and Blöschl et al. (2019), in Spain by Gallart and Llorens (2004), Coch and Mediero (2016), in Italy by De Girolamo et al. (2017), Germany by Bormann and Pinter (2017) and in Cyprus by Myronidis et al. (2018).
To our knowledge, no studies have explored the trends in flow intermittence across Europe. Snelder et al. (2013) analyzed French patterns in flow intermittence, using as indicators the mean annual frequency of zero flow periods and the mean duration of zero flow periods. Unsurprisingly, the highest values of the two characteristics coincided with the years of severe droughts. Besides climate influences, intermittence characteristics might be strongly influenced by processes operating at small scales, such as groundwater-surface water interactions and transmission losses (Beaufort et al., 2019, Costigan et al. 2017). Similarly, in different regions of the USA, Eng et al. (2015) classified 265 intermittent streams using as descriptors the number of zero-flow events, the median discharge and the 10thpercentile of daily flows, and they showed strong dependency of these metrics with temporal variations of precipitation and evapotranspiration. More generally, the probability of flow intermittence in rivers worldwide is likely to increase with the projected rise of temperature in future climate scenarios (Döll & Schmied 2012, Osuch et al., 2016, 2018, Snelder et al. 2013, Eng et al. 2015).
Previous classifications of European rivers based on their flow regime have usually not integrated flow intermittence, or in a relatively small sample of basins (Gallart et al., 2010, Oueslati et al., 2015). This is probably due to the difficulties in conceptually defining the intermittent, ephemeral and perennial states of streams (Gustard et al., 1992, Oueslati et al., 2015, Delso et al., 2017). For low flows and droughts, regional classifications at the European scale (Stahl and Demuth, 1999, Hannaford et al., 2010, Kirkby et al., 2011) or national scale (in Spain, Coch and Mediero, 2016) have been produced using most often the flow exceeded 90% of the time as a threshold for low flows or drought periods. Only a few classifications of intermittent rivers based on zero flow indicators have been proposed, in an attempt to relate their spatiotemporal variability with catchment characteristics or climatic variability (Kennard et al., 2010, Snelder et al., 2013, Eng et al., 2015, Perez-Saez et al., 2017, Tzoraki et al., 2016, De Girolamo et al., 2014, Dörflinger, 2016, Pournasiri Poshtiri et al., 2019). Identifying homogeneous regions and the drivers of flow intermittence, in terms of seasonality, catchment or climatic properties, could help to estimate intermittence characteristics and trends at the regional level (Pournasiri Poshtiri et al., 2019). Indeed, these intermittent and ephemeral streams are underrepresented in monitoring networks and often ungauged in Europe (Skoulikidis et al., 2017, Costigan et al., 2017).
Besides catchment characteristics, large scale climate variability may also exert an influence on intermittence patterns. Giuntoli et al. (2013) evaluated the relationships between low flows and large-scale climate variability in France, using climate indices such as the North Atlantic Oscillation (NAO), the Atlantic Multi-decadal Oscillation (AMO) and a weather typing approach. Their results indicated an increase of drought severity in Southern France, and the usefulness of lagged climate indices as predictors of summer low flows. Indeed, approaches based on weather typing or composite analysis with climatic data could help to evaluate the synoptic ingredients associated with dry periods and their long-term evolution and trends (Stahl and Demuth, 1999, Beck et al., 2015, Ionita et al., 2017). For the summer 2015 drought episode that hit large parts of Europe, Ionita et al. (2017) observed that this event was associated with positive anomalies in 500 hPa geopotential height and Mediterranean Sea surface temperatures. Since these climatic drivers are likely to have different influences in different regions of Europe, there is a need to perform such analysis at the regional scale.
The objectives of this study are: (i) to analyze the seasonal characteristics of flow intermittence in Europe, (ii) to test temporal trends in the number of zero-flow days at annual and seasonal scale and (iii) to analyze the possible relationships between the occurrence of zero-flows and climate indices. This study relies on an unprecedented database of intermittent rivers across Europe, which is presented in the next section, the methodology is presented in section three and the results in section four.