4.3 Links with SPEI anomalies
In this section, the findings of the relationships between the
occurrences of zero-flow days and SPEI anomalies are presented. In the
first step, a trend analysis has been performed on SPEI over the
different basins for different aggregation periods with 6, 12, 18 and 24
months. The results show a clear pattern with positive trends at
stations located north of 45°N and negative trends in the south, such
findings are fully consistent with Spinoni et al. (2017). To check if
the SPEI anomalies could be explanatory covariates for the inter-annual
variability of zero-flow day occurrences, the SPEI with different
aggregation time has been correlated with the number of zero-flow days
and the maximum dry spell lengths. Since it has been shown previously
that these variables and SPEI may exhibit trends, the time series have
been de-trended in the cases where significant trends were found.
Results show a strong association of zero-flow days with SPEI anomalies
in particular at the annual and summer timescales (Figure 5), with a
lower number of significant correlations during winter. The annual or
seasonal sums of zero-flow days are more strongly associated with SPEI
anomalies than the maximum length of dry spells, a metric that is
probably strongly affected by the hydrogeology and the memory of the
basins. Figure 6 shows the map of correlation coefficients obtained with
the SPEI18. The correlations are negative for all SPEI timescales,
indicating that negative SPEI anomalies, i.e. pronounced precipitation
deficits, are linked with a larger number of zero-flow days. For about
one-third of stations (36%) there is no significant correlation these
stations are mostly located in Spain, southern France, North Africa,
Cyprus, but also Belgium. A strong spatial variability of the pattern is
once again evident basins indicating local influences. The strength of
the correlations is higher for basins with a larger annual average
number of zero-flow days for almost all indicators. This demonstrates
that strongly intermittent basins are more influenced by climatic
variations to determine the annual number of zero-flow days. This is
probably due to the fact that these catchments are often close to the
wet/dry threshold and consequently immediately impacted by a variation
of precipitation availability. There is also statistically significant,
but moderate (rho=0.2), the correlation between the strength of the
correlations with the SPEI12 and basin size, indicating that the
influence of SPEI12 might be stronger for smaller basins since larger
basins can be more influenced by human activities. Yet, this
relationship is not significant for the SPEI18 and SPEI24. Smaller
basins have a reduced buffer capability in the context of climate
variability so this behaviour is expected.