5. DISCUSSION AND CONCLUSIONS
This study provides the first European-scale assessment of the trends in
intermittence over the last decades. The most striking results are (i)
the strong spatial variability of the detected trends, and (ii) the
seasonality and relationships with climate drivers. Overall, there is a
considerable spatial variability of flow intermittence, with possibly
strong dependencies of the different indicators with basin
characteristics. Most of detected trends are towards an increasing
number of zero-flow days, tending to occur earlier in the season, in
particular around the Mediterranean basin. For most basins, there is a
strong association of zero-flow days with SPEI negative anomalies, but
neighboring basins may exhibit different relationships showing again the
strong spatial variability. This implies that regional predictions or
generalizations for flow intermittence patterns should be interpreted
with caution. Any mapping or extrapolation from such regional results
may be prone to considerable errors if not considering basin
characteristics that are likely to play a strong role in determining
flow intermittence properties. In this study, the individual catchment
characteristics (ie. topography, geology, land use, soil types) have not
been analyzed, which would require a major work at such a
continental-wide scale. In particular, it would be particularly
interesting to distinguish basins with strong surface-groundwater
interactions that could explain some of the patterns described in the
present study. However, as noted by Snelder et al. (2013), flow
intermittence is also controlled by processes operating at scales
smaller than catchments, thus capturing these processes would require a
much more detailed investigation than classical regional approaches to
take into account the local physiographic characteristics (Tramblay et
al., 2010).
A major uncertainty of the present work and, to a greater extent,
applicable to all ecohydrological research on intermittent rivers is the
definition of the zero-flow days and the lack of regional
representativeness of the monitoring networks. With regard to the first
aspect, there is a wide variety of measurements procedures in different
European countries, leading to different accuracy of the measured
discharge values, in particular the minimal values. For example, in the
UK or in France the data is provided in m3 s-1 with an
accuracy of three decimals, but for many other countries, the minimum
reported discharge is sometimes much greater than 1 L s-1 due to
different measuring methods . In addition, many rating curves at open
channel stations have uncertainty at low flows caused by instability of
the riverbed. In the present work, we considered a strict criterion to
identify zero-flow days, but a more adequate selection could be made
possible if good quality metadata information were available for most
rivers, which is not currently the case for several countries. With
regard to the second aspect of regional representativeness, several
studies have highlighted the lack of measurements for intermittent
rivers (Skoulikidis et al., 2017, Costigan et al., 2017). The number of
monitored head water streams which are likely to be intermittent is
indeed much smaller than the number of perennial and large streams in
national and international databases, although their contribution to the
water resources is probably high. This questions the rationale behind
national measurement strategies for intermittent streams, in particular,
the most ephemeral ones, since they present a possibly reduced interest
in water resources management by comparison to perennial streams.
Depending on the national monitoring strategies of the river network, it
is possible that the selection of intermittent rivers to be monitored
may be biased towards a specific type of rivers within a given
geographic location of geological properties.
Another important aspect to take into consideration is the regulation
status of the monitored rivers, which may evolve over time and not be
available in the stations’ metadata. In the present work, the selected
basins are those described as natural or weakly altered based on their
metadata information. Yet, how to quantitatively define “weakly
altered” between different national networks and monitoring protocols?
This definition may differ from one country to another without a common
objective criterion to define the percentage of natural discharge being
diverted or used for water supply. Besides river alterations, often
assumed to be an expert judgement, the quantitative evaluation of the
water uptake would require extensive work to monitor and collect water
consumption data over time. Even for a river considered to be unaltered,
diffuse groundwater pumping may occur and therefore have impacts on the
groundwater-surface interactions, which could in turn strongly impact
intermittence occurrence. Two examples of the influence of river status
as described in the metadata on the trends results may be found in the
UK. The Coal Burn River is classified as broadly natural, but is an
experimental catchment set up to assess the impact of afforestation and
the trend analysis indicates an increase in zero-flow days, showing that
the influence of land-use change might indeed be significant on this
aspect of the flow regime. The limestone Slea River that conversely
experienced a decrease in zero-flow days was also classified as natural,
but further scrutiny revealed a discharge augmentation scheme installed
in 1995. Taking into account all these local specificities in addition
to the available metadata would require tremendous effort and the
information may not always be as readily available. The main findings of
the present study are an incentive to implement process-based studies on
the intermittency characteristics for different climatic and
physiographic environments.