4.3. Ecological status and typologies of non-perennial rivers and
streams.
The Water Framework Directive requires biomonitoring quality elements to
assess the ecological status of rivers. For this purpose, it is first
necessary to define the limits of the water bodies to be included in the
river basin management plans. However, NPRS water bodies are frequently
excluded because many of them have small drainage areas or hydrological
datasets are not available . In addition, NPRS are often omitted from
ecological status assessments because of inadequate knowledge about
their biological and hydrological characteristics (Mazor et al., 2014;
Stubbington et al., 2017). The methodologies in place focused on aquatic
organisms present during the wet phase, whereas dry-phase communities
remain poorly characterized and the metrics used to establish ecological
status in perennial rivers can only be applied in non-perennial rivers
when flow is present . Consequently, the European Commission recognized
the need to include ”temporary rivers” in national intercalibrations of
the status of their water bodies . However, the inclusion of NPRS in a
single type of water body (temporary rivers) may be imprecise for the
evaluation and transferability of ecological status in Mediterranean
rivers described by WFD, due to the high spatio-temporal variability of
their flows . Therefore, well-defined hydrological typologies that bring
together quality assessment, typologies and hydrology with zero-flow of
NPRS are needed .
Here, we have categorized Mediterranean rivers into three groups based
on the study of the variability of metrics related to their main
attributes of duration, frequency, magnitude, timing, and rate of
change. As expected, the comparison with the other three typologies
(Fig.5 and 6) used to classify NPRS reveals that our groups differ
slightly from the types reported by Spanish and Italian legislation and
the TREHS software . Moreover, our classification is strongly associated
with the temporal change gradient from ephemeral to temporary-types. It
is important to note that the denomination of the typology varies
between classifications, although many of the typologies coincide with
each other (Fig.5). For example, 100% of the stations grouped in
cluster III are intermittent in MATTM classification or temporary in IPH
classification, 100% of the episodic or intermittent rivers of MATTM
are ephemeral or temporary in IPH, respectively, and 100% of the
episodic rivers in MATTM or ephemeral in IPH are episodic in hydrotypes
TRivers classification. The use of a single variable without statistical
basis (number of days per year of zero flow) to classify NPRS makes the
Spanish regulation biased and does not adequately reflect the
hydrological diversity of these ecosystems.
Furthermore, described the difficulties of only using the hydrological
regime to classify non-perennial rivers: i) the main hydrological
features relevant to biological communities in temporary rivers are not
quantitative (i.e. discharges), but qualitative (i.e. the presence of
flowing water, stagnant pools or the lack of surface water); ii) flow
records do not report the permanence of isolated pools in periods of
zero-flow; iii) many NPRS are ungauged, and their natural flow regime
must be evaluated by alternative methods (e.g. remote sensing or
rainfall-runoff models); and iv) the complexity of sampling these
ecosystems due to the difficulty for recognizing the wet and dry phases.
In this context, they developed a novel approach to evaluate the
ecological status of NPRS. It defines four aquatic phases for NPRS, such
as perennial, intermittent-pool, intermittent-dry, and ephemeral. In
this way, used aquatic phases to propose NPRS classification that allows
evaluating their ecological status. Our study has also highlighted some
of these limitations, establishing a non-visual typology based on the
numerical data of the flow series to improve the ecological assessment
using quality elements connected with the hydrological patterns and
pressure gradients. It would also provide a starting point to improve
the management strategies guided to the achievement of the good
ecological status, based on measures related to improve hydrological
alteration and ecological connectivity .