2. DATABASE OF INTERMITTENT RIVERS
The database of discharge time series of ephemeral and intermittent
streams have been collected in the framework of the SMIRES COST action
(Datry et al., 2017) in the different European countries in addition to
individual contributions and stations from the GRDC
(https://www.bafg.de/GRDC)
database including countries outside of Europe such as Morocco, Tunisia
and Israel. The selected rivers are characterized by natural or
moderately influenced regime, and catchment area smaller than 2000km².
The absence of dams or reservoirs upstream of the station gauge has been
verified from a GIS analysis using the GRanD database
(http://globaldamwatch.org/grand/).
The metadata originating from different countries should be analyzed
with care and can be misleading since the definition of “natural”, and
the distinction between “little influenced” and “heavily” influenced
rivers may vary strongly between different countries. Also, since this
study is focusing on zero-flow days, it is possible that zero values are
put in place of missing data; this is the reason why the data had to be
checked carefully in the absence of metadata for many rivers.
Instead of using zeros, a threshold of 10-4m3 s-1 (0.1 L s-1)
is considered to identify zero-discharge days to account for
measurements errors of very small discharge values. However other
thresholds, such as 5 L s-1 recommended by Gustard et
al. (1992) or Delso et al. (2017) have also been tested, yielding
similar results. In addition, the individual time series have been
checked to verify if the smallest reported non-zero flows were below 10
L s-1l/s. If there were no non-zero flows below 10L/s
in a series, the zero flows of that series were interpreted as wrongly
reported missing values. The analyses performed in the present work are
focusing on the annual and seasonal timescales, a hydrological year
starting April 1 through March 31, and considering summer from April to
September and winter from October to March, since this is common
practice in low-flow analysis. The definition of the hydrological years
was governed by a preliminary analysis on the seasonality of zero-flow
events in Europe (mainly in summer and autumn but also in winter). This
reduces the chance of observing a flow event in two consecutive
hydrological years. The database includes 452 stations with at least two
years with five consecutive zero-flow days. This criterion has been
chosen to avoid including in the database some missing data in place of
actual river intermittence, since it is unlikely that river flow will
cease only one day in one year. Indeed, if for an annual time series
only a single day with zero-flow is recorded, it could be missing data
not properly reported in the metadata. For all stations, all years with
more than 5% missing data have been removed. Across most stations
(452), there is a common period for analysis between 1970 and 2010 when
data are available. Two annual and seasonal metrics of duration, - the
duration of the longest no-flow event (maximum length of zero-flow days)
and the total duration of no-flow days (sum of zero-flow days) - and the
mean date of no-flow days were studied too.