4. Discussion
Hydrological classification is a fundamental tool for the ecological
status assessment of WFD by water managers. The main objective of this
study was to develop a classification with relevant hydrological indices
in order to categorize non-perennial rivers and streams (NPRS) of the
western Mediterranean region. Using unaltered streamflow data, our
analysis suggest that the natural flow regimes of NPRS can be
characterized through the selection of a few indices from a wide set of
attributes of duration, frequency, magnitude, seasonality, and rate of
change of periods of flow cessation . The results highlighted the strong
correlation between variables for the different thresholds used to
define zero-flow condition set at 0, 1, 2, and 5 l/s. This means that
some variables of a few attributes could envelope the inertia of NPRS
flow diversity and be used to classify them. Our analysis of
hierarchical clusters (based on the correlation distance matrix of the
same attribute) and the repetition of indices at each principal
component (for the different thresholds) allowed us to select 5 relevant
metrics of different hydrological attributes, such as number of days per
year without flow (duration), annual percentage of months without flow
(frequency), mean daily annual flows (magnitude), coefficient of
variation of Julian date of the annual start of zero flow (timing) and
annual rise rate (rate of change). Our SOM classification, developed
from a set of 69 gauging stations and the selected 5 hydrological
indices, showed three groups associated with the temporality of the flow
cessation. However, beyond this hydrological characterization of
temporality patterns, there is incomplete development of river typology
in the context of WFD to evaluate the relationship between hydrology of
NPRS and ecology of biological communities, and there is a need to
improve tools to evaluate their ecological status, and integrate the
establishment of measures into national legislation. Below, we briefly
summarize the methodological considerations of the classification and
its relevance to the NPRS management. We also compare it with other
existing classifications to analyse further research needs.