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flow management rules under the Murrumbidgee Water Sharing Plans, as commenced in
July 2004 (New South Wales Department of Infrastructure, Planning and Natural
Resources 2004).
The maximum one-day flow in August over the period 1900-99 at Balranald on the
Murrumbidgee River is indicated in Figure A for both flow regimes. The simulated data
support the observation that extreme high flow events, such as those when flows are more
than 30 gigalitres a day, are not greatly affected by infrastructure and flow regulation to
meet irrigation demands, but that moderately high flows, between 10 and 30 gigalitres a
day, are.
Maximum flows in August exceed 10 gigalitres a day in 54 of the 100 years under
modeled natural conditions — an average return period or interarrival time of 1.78 years.
Under current conditions, August flows are predicted to exceed 10 gigalitres a day in only
23 of the 100 years. This is equivalent to an average return period of 4.27 years but, more
significantly, the maximum period between these events is 15 years compared with only
four under natural conditions.
The duration as well as the frequency of these events is also important. An
experiment was carried out on the Murrumbidgee River near Wagga Wagga to investigate
the response of billabongs to natural inundation, with a view to exploring the likely
ecological outcomes of connectivity between flood plain wetlands and the river (Hillman
et al. 2003). An environmental contingency allocation was used to augment high tributary
flows, and volumes exceeding 30 gigalitres a day were briefly achieved allowing
connection with four billabongs. Although connection was soon lost, the research team
tracked several ecological parameters in the river and billabongs for several weeks
afterwards. The key results in this work that are of relevance to the current paper include
the following:
During periods when billabongs are disconnected from the river they contain
significantly higher levels of particulate organic carbon, support much higher levels of
bacterial activity and have zooplankton densities several orders of magnitude greater
than the river.
Connection with the river (and inundation) produces the following sequence of events:
Day 1: Concentrations of dissolved organic carbon increase rapidly.
Days 2-5: Levels of a range of microbial functions and therefore microbe densities,
reach maximum levels.
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