Ecological impacts of water availability for Tasmania: A report to the Australian Government from the CSIRO Tasmania Sustainable Yields Project

This report describes how changes in the flow regime potentially impact on riverine condition and freshwater ecosystem values in Tasmania. To do this, the Flow Stress Ranking (FSR) for the five reporting regions of the CSIRO Tasmania Sustainable Yields Project was considered under four climate and development scenarios. The four scenarios are:

• Scenario A – historical climate (1 January 1924 to 31 December 2007) and current development
• Scenario B – recent climate (data from 1 January 1997 to 31 December 2007 were concatenated to make an 84-year sequence) and current development
• Scenario C – future climate (84-year sequence scaled for ~2030 conditions) and current development
• Scenario D – future climate (84-year sequence scaled for ~2030 conditions) and future development.

The Conservation of Freshwater Ecosystems Values (CFEV) database was interrogated to identify freshwater ecosystem values in Tasmania that are currently at risk. These values include, but are not limited to, the Striped Marsh Frog, Scrambling Ground Fern, Hydrobiid Snail, Forth River peppermint, Dwarf Galaxias, Purple Loosestrife, South Esk Pine, Golden Galaxias and Native Wintercress. These values were assessed taking into account the regional FSR results to determine whether these values would be further stressed due to varying water availability under the four climate and development scenarios. For an FSR of <0.2, the value would be considered severely modified, an FSR from 0.2 to <0.4 would be considered substantially modified, an FSR from 0.4 to <0.6 would be considered moderately modified, an FSR from 0.6 to <0.8 would be considered slightly modified and an FSR of 0.8 or greater would be considered largely unmodified. If the freshwater ecosystem values as derived from CFEV were located in an area where the FSR was in the moderately modified or substantially modified categories, or the inflows to the location were in these categories, the values were considered to be threatened.

Considering FSR assessments, overall, conditions under all scenarios potentially affect aquatic ecosystems, but the severity of the potential impacts and the types of ecosystem that are affected in different regions vary greatly. The greatest ecological effect is under the recent climate (Scenario B), with potential impacts on rivers and riverine wetlands in all five regions, on Ramsar wetlands in three regions, and on estuaries in three regions. Under the historical climate, future climate, and future development (scenarios A, C and D), less of an impact is expected, but as under the recent climate, rivers and riverine wetlands are more affected than are Ramsar wetlands and estuaries.

Under all scenarios, the FSR component indices associated with the potential ecological impacts were predominantly those that describe alterations to low and high flow components of the flow regime. Low and high flows are likely to be affected in impacted areas in most regions. Therefore, generally, instream components of ecosystems (such as macrophytes, macroinvertebrates, fish and platypus) and those that occur in riverine riparian and floodplain areas (such as riparian vegetation) are affected.

FSR results under the recent climate are the most striking and demonstrate the impact that the recent drought has had on Tasmanian freshwater resources. Under the recent climate, all regions show a decrease in the FSR index over some portion of their area. The overall trend across all regions (relative to historical conditions) is a reduction in the percentage of subcatchments within the largely unmodified FSR category by 14 percent, an increase in the number of subcatchments within the slightly modified FSR category by 64 percent, an increase in the number of subcatchments within the moderately modified FSR category by 21 percent, and a 1 percent incidence of subcatchments within the substantially modified FSR category. This is reflected particularly in the Midlands area of the South Esk and Derwent-South East regions, the north-east portion of the Pipers-Ringarooma region, and the central northern area of the Mersey-Forth region. The potential increased risk is exacerbated by irrigation pressures and dam construction in each region.

Under the historical climate, the FSR results indicate that, generally, most regions are in good hydrological condition. However, there are some catchments within the regions that show a slight decline in condition. These include the Duck and Black-Detention in the Arthur-Inglis-Cam region; the Rubicon and upper Tamar Estuary in the Mersey-Forth region; the Great Forester in the Pipers-Ringarooma region; the Macquarie, Brumbys, Meander and South Esk in the South Esk region; and the Clyde and Jordan in the Derwent-South East region. This reduction in FSR is linked directly with current irrigation and water supply allocations.

The FSR results under the future climate display similar outcomes as under the historical climate, particularly those under the wet extreme future climate (Scenario Cwet). Under the median future climate (Scenario Cmid) there is a slight reduction in the percentage of subcatchments within the largely unmodified FSR category and an increase in the percentage of subcatchments within the slightly modified FSR category in all regions relative to both the historical and wet extreme future climate. While there is an overall reduction, the FSR results are still in the range of largely unmodified or slightly modified condition, which has minimal impact on the ecology in the region.

Results under the dry extreme future climate (Scenario Cdry) show a greater departure from results under the historical climate with a reduction in the percentage of subcatchments within the the largely unmodified FSR category and an increase in the percentage of subcatchments within the slightly modified FSR category for all regions, with 37 percent of subcatchments in the latter category across all regions. This is highlighted in the Arthur-Inglis-Cam and South Esk regions where over 40 percent of the subcatchments are in the slightly modified category under the dry extreme future climate.

FSR results under future development (Scenario D) indicate that future commercial forestry plantations influence the FSR values most under the dry extreme future climate. Under future development, both the low flow indices and high flow indices are affected, which subsequently impacts the volume of water received. The major change in FSR values is from the largely unmodified FSR category into the slightly modified FSR category. There are instances where there is an increase in the moderately modified or substantially modified FSR categories highlighting potential regional areas of concern. The region which incurs the most changes in FSR under future development is the Mersey-Forth region. This is related to the greater projected increases in commercial plantation forests in that region.

Future irrigation development may have some ecological impact in the Tomahawk and Musselroe catchments in the Pipers-Ringarooma region where the FSR moves into the moderately modified category. In the Brumbys catchment in the South Esk region, the FSR may be reduced into the substantially modified category. These changes impact on low flow, high flows and annual flow, and affect instream ecosystem components (such as macrophytes, macroinvertebrates, fish and platypus) and those that occur in riverine riparian and floodplain areas (such as riparian vegetation).

In considering potential impacts to groundwater-dependent ecosystems, the results suggest that under the future climate, groundwater-dependent ecosystems would not experience conditions worse than those encountered historically. This indicates that the values identified in these regions are more likely to be under pressure from future development and competition for the groundwater resource if it is not managed appropriately.