Basin-scale evaluation of 2021–22 Commonwealth environmental water: Vegetation.

Strategic management of Commonwealth water for the environment by the Commonwealth Environmental Water Holder (CEWH) is key to achieving the Commonwealth’s (Murray–Darling) Basin Plan 2012 environmental objectives. The Commonwealth Environmental Water Holder’s Science Program invests in Flow Monitoring, Evaluation and Research (Flow-MER) to demonstrate Basin-scale outcomes of Commonwealth water for the environment, support adaptive management and fulfil the CEWH’s legislative requirements under the Basin Plan. This evaluation describes groundcover vegetation outcomes from the use of Commonwealth water for the environment for 2021–22 in Selected Areas in the Murray–Darling Basin (the Basin), as well as the 8-year cumulative outcomes since monitoring began in 2014–15. It addresses the following 2 questions related to broad expected outcomes for Biodiversity defined in the Basin Plan: • What did Commonwealth environmental water contribute to plant species diversity? • What did Commonwealth environmental water contribute to vegetation community diversity? Commonwealth watering actions are often in partnership with states and other environmental water holders – in these cases, outcomes cannot be apportioned and evaluation is for the combined management of environmental water, not Commonwealth water for the environment alone. The evaluation is based on vegetation data collected under the Long Term Intervention Monitoring Project (2014–15 to 2018–19) and Flow-MER (2019–present) from floodplain–wetlands and river channels. Descriptions of the vegetation responses to environmental water are framed in terms of species and community responses and are described in terms of a range of structural and functional attributes. For the purposes of the evaluation: • species diversity encompasses the presence and abundance of individual plant species; here, we use species richness (number of species) • community diversity includes the composition and structure of vegetation assemblages occurring in different habitat types (riverine, wetland and floodplain). Structural and functional attributes include water plant functional groups (submerged, amphibious, damploving, woody flood-dependent and terrestrial), species growth forms (e.g. forbs, grasses, ferns), native and exotic species, rare and threatened species, and species that are known to be used by Aboriginal people. These attributes are commonly used to describe vegetation community composition, providing information about habitat diversity as well as plants with specific social and cultural values. The data are evaluated in the context of watering history (both natural and managed) and spatial patterns of observed vegetation responses at monitored sites are used to infer responses in vegetation across the Basin. Most of the evaluation focuses on floodplain and wetland vegetation responses to environmental water as we lack in-channel hydrology indices relevant to all riverine sites to support Basin evaluation. Vegetation monitoring data from floodplain and wetland habitats across the Basin are used to describe the vegetation assemblages associated with groups of sample points that have experienced similar inundation regimes over the past 8 years (known as hydrological groups). The resulting associations between the hydrological group and the vegetation are then used to predict the vegetation assemblage that would occur in the absence of environmental water, where the modelled counterfactual hydrological regime is used to define the absence of environmental water. Differences in the vegetation assemblages that are predicted to occur with and without environmental water can then, in part, be attributed to environmental water.
The approach taken in this evaluation is largely qualitative due to the sampling design and the coarse temporal resolution of inundation data. Improvements to future sampling design should consider the sampling point locations related to ANAE types as well as other complementary data (e.g. elevation data). The outcomes from the evaluation are used to infer the contribution of Commonwealth water for the environment to Basin Plan objectives and identify adaptive management responses to issues raised in the evaluation.