C1.5 Ecosystem assessment
This tool involves the management of water, primarily in river systems and groundwater reserves, in order to minimise the impact of water use on the ecological functions of aquatic ecosystems. The principle is that natural riverine biota will survive and reproduce if natural flow regimes are maintained or simulated.
In the particular case of allocating water for rivers, an approach is needed which will assess the water requirements of the complete ecosystem, including such components as the source area, river channel, riparian zone, floodplain, groundwater, wetlands and estuary, as well as any particularly important features such as rare and endangered species. This inclusive concept is the ôriverine ecosystemö. Current thinking suggests that the management of river flows should be undertaken in ways to Ĺmimicĺ natural flows (in both temperate and dryland river regimes). This means that water extractions through allocation mechanisms should be undertaken to replicate the natural flows of rivers, not purely for economic production purposes. This requires a thorough understanding of river flow regimes and the need for long-term, accurate data on river flows.
The science of the assessment of environmental flows is emerging. The majority of contemporary ecosystem theories that have been developed to explain how rivers function originate from research on temperate, perennial streams (as does river management and water policy). Several methods have been developed to mimic natural flow regimes and these need further evaluation for determining environmental water requirements for riverine systems. They include the Instream Flow Incremental Methodology developed by the US Fish and Wildlife service (Bovee, 1982). The physical habitat component of IFIM (PHABSIM in the USA and RHYHABSIM in New Zealand (Jowett, 1982)) is seldom useful in an ecosystem or at community levels.
Examples of ecosystem assessments include:
Murray Valley wetlands (see Murray-Darling Basin Commission, Australia; Arthington et al., 1992); the Sabie River Catchment (see Foundation for Research Development, South Africa); the Peel-Harvey Estuary, Western Australia (see Environmental Protection Authority, Western Australia); Thompson River (see Victorian Department of Natural Resources, Victoria, Australia); New Zealand (see Jowett, 1982); USA (see Bovee, 1982).
In short, methodologies for both temperate and dryland environments need to be developed, due to significantly different prevailing rainfall/run-off regimes. Any derivative assessment is best made using expert panel determinations and an effective user community participation procedure.