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 .