Introduction
The flow regime of a river, defined as the magnitude, timing, duration
and frequency of flow events, has long been established as one of the
key driving influences on riverine ecosystems
(Naiman et al., 2008,
Poff et al., 1997). Previous efforts to
define the flow regime has provided a multitude of flow metrics that can
be used to quantify nearly any aspect of the flow regime
(Kennard et al., 2010,
VanLaarhoven and van der Wielen, 2009).
The altering of the flow regime through water resource development (e.g.
dams) has been linked to declines in riverine ecosystem condition, most
commonly associated with reductions in flows
(Poff et al., 2007,
Poff et al., 2010,
Poff and Zimmerman, 2010). It has been
further identified that some aspects of the flow regime have a
disproportionate impact on riverine ecosystems, in particular for
intermittent rivers (Datry et al., 2014a,
Datry et al., 2014b). It follows that the
assessment of impacts of changes to the flow regime of intermittent
rivers can be summarised using a suite of key, ecologically relevant
flow metrics, and that these changes in flow metrics can be meaningfully
translated into a level of potential ecological impact.
Seasonal rivers, as a class of intermittent rivers, are defined as
rivers that have a predictable cease to flow period. The vast majority
of these follow the typical Mediterranean flow regime of cease to flow
periods over the hot and dry summer months and a predictable flowing
period over the cooler, wetter winter months
(Kennard et al., 2010). These seasonal
rivers are the dominant type of river found in southern Australia and
are often heavily developed for their water resources to provide water
for agricultural activities (Malerba et
al., 2021). The level of development in some of these regions has led
to the need to regulate and manage the capture, extraction and use of
water through legislation (e.g. Landscape
South Australia Act, Government of South Australia, 2019).
The sustainable management of these water resources depends on archiving
an equitable balance between users, while keeping the total requirements
for water below a sustainable level. Generally these requirements are
broken down into social, economic, cultural and environmental water
requirements. For water planning in South Australia, there is a
significant emphasis placed on environmental water requirements as a
priority user for water resources. Generally, environmental water
requirements (EWRs) are defined as the water regime needed to sustain
the ecological values of aquatic ecosystems, including their process and
biological diversity, at a low level of risk
(VanLaarhoven and van der Wielen, 2009).
Previous assessments of environmental water needs have used various
different methods for describing the EWRs for seasonal rivers, ranging
from qualitative descriptions (e.g.
Barossa, Natural Resources AMLR, 2009) through to complex series of 50+
flow metrics (e.g. Western and Eastern Mt Lofty Ranges,
Natural Resources AMLR, 2013,
Natural Resources SAMDB, 2013).
The notion of establishing flow assessment methods is not novel, with
hundreds of methods developed and reported
(Tharme, 2003). The most recent attempt
at quantifying EWRs for seasonal rivers in South Australia provides a
pass/fail style threshold for 56 flow metrics. Subsequent assessment of
these flow metrics identified significant correlation between some of
the flow metrics, especially between flow metrics within the same flow
season (Maxwell et al., 2015). This
correlation was indicative of potential bias within the metrics,
especially when using percentage passing/failing as the overall
indicator, as metrics would fail in batches rather than independently.
There are also a considerable number of metrics using non-zero daily
flows which has been identified as an issue when comparing datasets. The
changing proportion of data between different scenarios included in a
non-zero flow assessment results in metrics that are not directly
comparable.
The goal was to define a suite flow metrics that were representative of
all aspects of the flow regime that were ecologically relevant and not
correlated with each other. This is in a similar vain to the functional
flows approach used by Yarnell et al.
(2020) with a focus both on function (e.g. intermittency, high flows)
and season (e.g. low flow season, transition seasons). The number of
metrics was determined by a combination of consideration of the aspects
of the flow regime that were deemed to be important along with
assessment of previous suites of metrics. A PCA of the metrics
(normalised) from VanLaarhoven and van der
Wielen (2009) suggested that the first six principal components
contained ~91% of the variance of the overall dataset.
While not directly relatable to individual metrics, six was chosen as
the number of metrics to develop.
Previous assessments of flow data from seasonal rivers in South
Australia has shown that there is significant correlation between flow
metrics per season (Maxwell et al., 2015)
so single metrics per season were identified to avoid this correlation.
Based on assessment of previous suites of flow metrics used (e.g.
Kennard et al., 2010,
Poff and Zimmerman, 2010,
VanLaarhoven and van der Wielen, 2009,
Yarnell et al., 2020), the six key areas
of interest were identified as (1) intermittency, (2) low flows, (3)
break of season, (4) spring flows, (5) medium flows and, (6) high flows.
The links between these flow metrics and riverine ecosystem function is
summarised in Table 1.
Table 1: summary of ecological functions identified for each of the flow
regime areas summarised from conceptual understanding of seasonal rivers
and literature review.