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.