5. Conclusions
In summary, we classified the natural hydrological regimes of NPRS
located in the western Mediterranean region using daily flow data sets
from 69 unaltered gauging stations. Using hydrological series of at
least 11 years of duration, a total of 315 hydrological indices were
calculated using different thresholds to define no-flow conditions (i.e.
0, 1, 2, and 5 l/s). We did not find notable statistical differences
between the different thresholds and after redundancy analysis. A neural
network model was calibrated with the 5 hydrological metrics that better
describe the attributes of flow magnitude, duration, frequency, and
timing of the zero-flow periods, and rate of change between flowing and
non-flowing conditions. The 5 hydrological metrics describe annual
attributes as number of days or percentage of months per year without
flow mean annual flows, coefficient of variation of Julian date of the
annual start of zero flow and annual rise rate. The analysis concluded
that three different categories of NPRS can be defined, namely:
ephemeral, intermittent, and temporal rivers. Ephemeral rivers are dry
for most of the year and would have a low flow during the wet phase with
water in their riverbed. Intermittent rivers have a dry phase not as
long as ephemeral rivers, and, as a rule, more water runs through their
bed. Temporal rivers are more briefly interrupted by a period of
drought. Additionally, our classification revealed ranges that are not
closed and different from the national legislation classifications of
Spain and Italy. Ephemeral rivers could be dry for more than 181 days or
7 months, intermittent rivers between 100 and 187 days or between 4 and
7 months, and temporal rivers less than 46 days and 2 months. Therefore,
our results could serve for the conservation and management of these
rivers, establishing a necessary typology in future studies of
ecological status of WFD, with well-defined groups based on statistical
analysis. The lack of studies addressing the hydrological diversity of
these freshwater ecosystems makes our study a basis for future efforts
aimed at improving management strategies of this type of river courses.
Additionally, future analyses guided to a better characterization of
biological communities associated with different types of NPRS will
allow a better understanding of the ecological implications of
temporality on the biota and will advance in better ecological status
classification and management of these ecosystems. Finally, due to the
relative low number of NPRS analysed, the resulting classification
should be considered as a first approximation and should be tested and
calibrated with other gauging stations not included in the study.