Modelling the influence of River Murray management on the Barmah river red gum forests

Abstract

Three modelled sequences of estimated average monthly flows of the River Murray were used to study the likely effects of river management on the frequency and duration of flooding of the river red gum forests at Barmah in northern Victoria. One model gave monthly flows in the River Murray at Yarrawonga from 1890 to the present with no regulation or diversions. The second model gave flows with Hume Dam regulation and diversion. The third model gave flow with Dartmouth and Hume Dam regulation and diversion. This corresponds to “current” regulation. The Hume Dam (only) sequence was regarded as the one that corresponded best to the state of the river for which forest flooding data were available. A relation between the average monthly flow (expressed in gigalitres per day) and the percentage of forest flooded was used to transform the historical sequences of monthly flows into the corresponding maximum monthly percentages of forest flooded. This, in turn, was used as input in a grid cell model of the forest flooding based on historical flood maps. From the estimates of the maximum monthly extent of flooding between 1890 and 1984, statistical parameters describing the change in flooding regime for the “natural” and the “current” river were computed. The results showed an average annual reduction of 1–2 months duration in flooding and a substantial increase in the time between flooding for all sites. Many sites may suffer a 50% reduction in flooding frequency. This suggests that the vigor of the red gum within the Barmah Forest will be diminished, and some red gum areas may revert to dryland box forest.