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.