On the existence of multiple states of low flows in catchments in
southeast Australia
Abstract
Hydrological variables of a catchment and their corresponding extreme
characteristics have a possibility of switching regimes, particularly
when a catchment undergoes protracted dry periods. This can result in a
catchment experiencing a flow anomaly that is even more extreme than
what was historically considered an extreme low flow event for the
catchment. Catchments in southeast Australia have been shown to exhibit
multiple states of mean annual flows. Given this and studies that
suggest that extreme events may be changing with time, it is important
to understand whether extremes in flows also have the potential to exist
in multiple states. To investigate this, we studied intensity, duration,
and frequency (IDF) of low flows for 161 unregulated catchments in
southeast Australia. A Hidden Markov Model-based approach was used to
examine shifts in the low flow characteristics. We found very strong
evidence of low flow intensity exhibiting two distinct states for at
least 34 (21%) catchments in the region, providing convincing reasons
to believe that extremes in low flows can and have undergone regime
changes. The second state of these catchments is often associated with
higher values of low flow intensities. Simulation of the duration and
frequency of these events, however, needs improvement with the current
approach and may be better studied by accounting for climate indicators
that may more suitably explain them. Impacts from a changing climate may
enhance the triggering of low flows into alternate states, which calls
for water managers to plan for changing regimes of extremes.