Vegetation Growth Response to Modified Flooding Regimes and Groundwater Depth on a Saline Floodplain

Dieback of native forests due to increasing salinity on river floodplains within the Murray- Darling Basin is widespread. Management proposals to reduce dieback include river flow management using upstream storages to increase flooding, and groundwater pumping with disposal to evaporation basins. This paper evaluates the long term growth response of Eucalyptus largiflorens (Black Box), on a saline floodplain of the lower River Murray, to modified flooding regimes and groundwater depth. The evaluation was conducted using a soil-vegetation-atmosphere continuum model (WaVES) which was calibrated at 5 soil-vegetation conditions during the 1993-94 floods. Management scenarios were evaluated using historic weather and river flow data for the 1970-94 period. The analysis indicates that in the long term, the vegetation health is determined by major flooding events as occurred in 1974-76. These events cause long periods (hundreds of days) of inundation which leach accumulated salt from the rootzone. The potential changes to the flooding regime using water releases from Lake Victoria are likely to produce some improvement in vegetation health as are groundwater pumping schemes lower the watertable by 1 m. These changes are likely to assist vegetation survive longer periods between major flood events but are unlikely to prevent dieback on their own. If the long term average relative flood duration (ie days flooded/days not flooded) were increased to approximately 0.05-0.1 then dieback could be prevented without the need for groundwater pumping.