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.