4.2. Typology for Mediterranean non-perennial rivers and streams.
In this study, we considered only hydrological indices to categorize
Mediterranean NPRS based on statistical criteria. The patterns described
by the SOM classification suggest the existence of three groups
(clusters I, II, and III) that reflect the interaction between ranges of
the five attributes values (Fig.3 and 4, Table2). As expected, cluster I
encompasses rivers almost dry during the year with many days and months
without flow (usually with an onset of the dry phase in the first months
of the year), with low daily flows and high annual rise rate. The
cluster II lies between dry rivers (cluster I) and rivers with flows
occasionally interrupted by drought (cluster III) with fewer days and
months with zero-flow and with combinations of high flows. Our results
indicate that the number of days and the percentage of months per year
without flow are the most valuable attributes for Mediterranean NPRS
classification, since they form well-defined groups. Similarly, CV of
Julian date of annual start of zero flow has three compact and
differentiated groups, due to that in rivers with no flow during most of
the year, the dry periods tend to occur earlier . Conversely, the
metrics of mean daily annual flows and annual rise rate (Fig.4) showed
less variability, suggesting a lower discrimination capacity, a
relationship that has also been shown in other studies .
Similarly to other studies , we included the number of zero-flow days to
categorize NPRS. suggests a clear relationship between the duration and
frequency of cessation periods for different ecotypes of the Guadiana
basin, and other authors classified the NPRS according to the number of
days of the dry riverbed as intermittent, ephemeral or episodic. The
work by defines intermittent rivers cease seasonally or occasionally the
flow (usually for weeks to months), ephemeral streams flow only in
response to precipitation or snowmelt events (days to weeks) and
episodic streams maintain surface water only during very short periods
(hours to days), primarily after heavy rainfall events. In this line,
our results are similar to the national legislation of Italy and , where
days or months per year without flow are employed to distinguish between
permanent, temporal, intermittent, and ephemeral water bodies. For
example, in Italy , NPRS are classified according to the number of dry
months as intermittent (less than 8 months per year), ephemeral (more
than 8 months per year) and episodic (only water circulation after
intense precipitation events). In Spain , temporary rivers are defined
as those flowing for more than 300 days per year and possible dry
periods; intermittent rivers are those flowing between 100 and 300 days
per year; and ephemeral rivers are those flowing during less than 100
days per year. Similarly to national legislation, our results indicate
three groups (Fig.3 and 4) which we would define as ephemeral rivers
(cluster I), intermittent (cluster II), and temporal (cluster III), but
we acknowledge notable differences in the range of days and months
without flow ranges (Table 2). Apparently, the ranges of our
classification are not closed but cluster I (ephemeral) would have more
than 181 days or 7 months (59.6%) with zero-flows, while cluster II
(intermittent) between 100 and 187 days or between 4 (35.8%) and 7
(56.3%) months without flow, and cluster III (temporal) with less than
46 days and 2 (20.0%) months with zero-flows. As far as we know, this
is the first classification in which undisturbed gauging stations to
obtain ranges based on statistical analysis of real data were used,
providing new insights that improve the understanding of the
hydrological dynamics in these water bodies. Their integration into
national legislation, which currently classifies NPRS based on expert
judgement criteria (e.g., Spain and Italy), would undoubtedly lead to
better management strategies aimed at achieving the objectives set by
the WFD.
Nevertheless, the applicability of these results might be affected by
the limited number of NRPS water bodies controlled by gauging stations .
For example, 39% of France’s rivers were NPRS applying a regional
predictive model, but only 20% of the total gauging stations in France
had zero flow episodes . In addition, some caution is needed when
interpreting the results of this study. Our classification is based on
data from 69 gauge stations, which represent a small proportion of the
total NPRS located in the Mediterranean region. The use of a more
extensive dataset would undoubtedly contribute to a wider and more
robust classification. For example, we noticed a notable lack of
episodic rivers, defined by as rivers with flowing water only during a
few hours or days, depending on the rainfall event and its intensity.
Since our study has used flow data on a daily scale, it was not possible
to detect short-time-scale hydrological changes and none of the gauge
sites used in this study was classified as episodic. Given the
importance of NPRS in terms of the number of these freshwater ecosystems
and considering the limited gauge sites used to measure them, more
hydrological data would provide a more robust hydrological
classification. For this reason, alternative data acquisition of periods
of flow cessation would be necessary. The use of sensor loggers , remote
sensing techniques with unmanned aerial vehicles , empirical
observation, statistical approaches, process-based hydrological
modelling , and multi-temporal satellite data , may solve this problem
though notably increasing the costs .