1. Introduction
Depending on the occurrence of flow or no flow periods of the natural flow regime, rivers could be classified into perennial or non-perennial. Although many terminologies have been used over time subject to research fields (e.g., arid, discontinuous, dry, ephemeral, episodic, intermittent, interrupted, irregular, non-permanent, seasonal, or temporary rivers), the term “non-perennial rivers and streams” (hereafter NPRS) refers to watercourses that cease to flow at the same point in time or space. Their defining characteristic is a temporary lack of surface flow, which leads to isolated pools or dry channels .
According to some estimates, these water bodies comprise at least 41% of the length of the global fluvial network . This percentage would increase in some regions of Africa, America, Australia, and Europe , depending on regional factors and local characteristics . Such is the case of the Mediterranean region, which includes temperate, semiarid or arid climates, where NPRS could represent 69% of the total length of the river network . This is because they are concentrated in the upper reaches of the basins with small drainage areas and steep slopes leading to a rapid delivery of water to the river channel, causing a lack of buffering from variations in precipitation . Considering that the future effects of global change (such as climatic and land use changes, impoundments, and increasing human water demands) are expected to be particularly severe in these freshwater ecosystems, the length and number of NPRS in the Mediterranean region is expected to increase over the next decades . As a consequence, non-perennial rivers have progressively received wider interest by water resource managers and the scientific community .
Recent studies in NPRS have advanced in characterizing geomorphological processes , biological communities , water quality , biogeochemical fluxes , status assessment and river management . All these studies highlight the importance of the different flow regime patterns (including the specific duration of the wet and dry phases) for biota and environmental processes in non-perennial freshwater systems. This is more evident in Mediterranean climate regions, as they are shaped by predictable seasonal events of flooding and drying over an annual cycle, with a strong inter- and intra-annual flow variation . The variability in the flow regime takes on a wide range of possibilities in terms of duration, frequency, and timing of the zero-flow periods, and rates of change between flowing and non-flowing conditions . The structure and functioning of each NPRS is historically adapted to the specific non-perennial flow patterns, which act as a major driver of their ecomorphological dynamics . The particular features of the components of non-perennial flow regimes also affect the human uses linked to the NPRS, enhancing a variety of socio-ecological non-perennial river systems, under distinctive biophysical contexts . Thus, ecological assessment methods of NPRS to evaluate the current status and historical trajectory of non-perennial freshwater systems require an advanced understanding of the flow-ecology responses . An improved understanding of the interaction among human uses, pressures, impacts, and responses of NPRS, could provide new insights into the most beneficial actions that should be selected to optimize management approaches to them . Indeed, developing knowledge about the dynamics that characterize this type of freshwater ecosystems could help change and improve the perception and attitudes of society towards NPRS . Both the ecological and the societal approaches to study different aspects of NPRS, require further interpretation of the hydrological gradients which characterize such systems. In this line, improving mechanisms to classify natural flow temporality would lead to better-informed management decisions about the NPRS.
The European water policy has been developed by the Water Framework Directive . The WFD states that, in order to evaluate the ecological status, the reference conditions for each type of water body have to be formerly defined. Subsequently, different biological quality elements (e.g. biological indices based on benthic macroinvertebrates assemblages, aquatic flora, and fish fauna) are compared with the reference conditions to establish the ecological status of the water body . However, there are limitations in the implementation of WFD in NPRS, as described by : (1) the establishment of a water body in the case of NPRS (a river stretch to be included in the river basin management plans) could be omitted due to their small catchment area or their limited annual discharge, and (2) the classification of river typologies considering the hydrological variability of NPRS is obviated due to the scarcity of hydrological data sets characterizing natural flow conditions. The aforementioned challenges have limited the capacity of EU member states to define both legally and technically non-perennial river typologies.
In the case of Italy or Spain, representative examples of Mediterranean climate conditions, despite the relative wide proportion of non-perennial rivers in their drainage network, there are no official studies aimed at classifying temporary flow patterns . In these countries, water authorities have used time thresholds (e.g., average number of days or months with no-flow), approved by legislation , to distinguish between temporary, intermittent, ephemeral and episodic river waterbodies. These thresholds were proposed based on initial assessments of flowing periods, on expert judgement or on external references instead of systematic analyses of continuous non-perennial flow gradients that could drive to statistically-based typologies (e.g., . Thus, the classification approach currently used by water managers faces several uncertainties. First, the best potential threshold to define the no-flow condition. Second, the suitability of defining specific non-perennial flow classes. Finally, the connection of types of non-perennial flows with ecological assessment methods for the optimization of restoration measures to mitigate the negative impacts derived from human activities.
In order to bridge the existing gaps in ecological assessment tools for NPRS, the ECOSTAT working group of the European Commission is currently working on a common standpoint to characterize, evaluate, manage, and monitor NPRS . With the purpose of developing a common method to analyse the ecological status of NPRS in the EU, research projects such as MIRAGE (https://cordis.europa.eu/project/id/211732), GLOBAQUIA (http://www.globaqua-project.eu/), LIFE+ TRivers (http://www.lifetrivers.eu/), and SMIRES (https://www.smires.eu/) have recently provided new insights into the functioning of this type of freshwater ecosystems. MIRAGE project started, for the first time, the study of temporal responses of rivers to hydrological, biogeochemical, and sediment transport events for the implementation of the WFD and specific management under the characteristics of flooding and drought of temporary streams. GLOBAQUA is an EU-funded project aiming to identify the prevalence and interaction between stressors under water scarcity, in order to improve water management practices and policies in rivers, including NPRS. The LIFE+ TRivers project provided the European river basin authorities a software (TREHS) to evaluate temporary river regimes as a first step for their ecological status assessment defined by the WFD in the NPRS. Finally, the SMIRES project was a COST Action that established a multidisciplinary network of scientists and experts from 32 countries, to consolidate and expand knowledge on NPRS and translate it into sustainable and science-based management of NPRS resources and biodiversity.
Stemming from these efforts, this study aims at providing a statistically-based classification of non-perennial flow patterns in rivers and streams of the Western European Mediterranean region. This region offers an interesting test bed for this analysis for at least two reasons. First, due to the relative abundance of dryland areas and NPRS. Second, due to the lack of homogeneous criteria for their hydrological classification. Specifically, this study assesses natural flow patterns of unaltered non-perennial rivers focusing on hydrological metrics related to the frequency, duration, timing, and rate of change in periods of flow cessation. We hypothesized that non-perennial Mediterranean rivers and streams can be classified according to a few hydrological attributes since many of them are redundant in reflecting inter- and intra-annual flow variability . We also hypothesized that there is a similar relationship among different thresholds to define zero-flow conditions (0, 1, 2, and 5 l/s). Linked to previous efforts in the UE to characterize ecological conditions of NPRS, we expect that the hydrological classification obtained in this study will provide useful insights to water authorities to improve the assessment of the ecological status in this set of water bodies.