4.1 Spatiotemporal patterns and seasonality
Figure 1 shows that for a large number of stations, there are only a
small number of years with more than 10% zero-flow days. Only a few
stations have zero-flow 90% of the years with available data. As shown
on the map (Figure 1), the annual percentage of years with zero-flow
days can vary strongly even for neighboring stations. This highlights
the influence of local characteristics (geology, land cover, water
use…) on zero flow occurrences. Snelder et al. (2013) previously
observed over France that the high spatial heterogeneity in small-scale
processes associated with intermittence partly explains the low spatial
synchronization of zero-flows. There is a weak latitudinal gradient in
the occurrence of zero-flow days, with the higher mean annual number of
zero-flow days in the South (rho= -0.36 with latitude, significant at
the 5% level), but with a very strong spatial variability even for
neighboring catchments. This implies that most intermittent streams are
not necessarily associated with the aridest climate conditions in
Southern Europe.
Clustering has been applied using the variables θ , the mean
direction of zero-flow and the variability around this date, r ,
computed for the winter and summer seasons. As shown in Figure 2, three
different seasonality patterns were identified. The largest one, Cluster
2, is composed of 376 stations having a mean date of occurrence for
zero-flow days between May and November, at the end of summer. The
location of the stations composing this cluster are scattered all across
Europe, in different climatic zones ranging from Continental to
Mediterranean climate types. The second-largest cluster, Cluster 3,
contains 47 stations with a mean occurrence of no-flow events between
January and March. It includes stations with a nival regime, such as the
Pyrenees of Scandinavia, that experience cessation of flow due to
freezing. The Cluster 1 (29 stations) mean corresponds to late fall
(November to January). As shown in this analysis, for most stations
(Clusters 1 and 3), the zero-flow conditions are more frequently
observed in summer months or during winter or early spring due to snow
and ice cover. Yet, as shown in the map in Figure 2, there are no clear
spatial patterns that could be identified from this analysis though
Cluster 3 stations are located predominantly in mountainous or northern
areas.