Table 3.7.2 outlines potential environmental watering and expected water use under a range of planning scenarios.
The upper Barwon environmental entitlement can only support a small proportion of the environmental flow recommendations for the upper Barwon River. However, high carryover volumes from 2021-22 and the forecast of high allocations mean that the volume of water available to support environmental flows in the upper Barwon system in 2022-23 will be greater than in previous years under all climate scenarios.
Planned watering actions for the upper Barwon River are derived from recommendations in the upper Barwon River Environmental Flows Study. The Corangamite CMA is aware that many of the flow magnitudes recommended in the environmental flows study cannot be delivered without inundating private land, so the planned watering actions presented in Table 3.7.1 are deliberately less than the known channel capacity constraints. Also, water levels will be monitored during any planned delivery of water for the environment in 2022-23, especially if significant rain is forecast, so that release rates can be promptly adjusted to avoid affecting streamside landholders. The flow rates presented in Table 3.7.1 are expected to provide a lower environmental benefit than the full environmental flow recommendations. The Corangamite CMA will continue to work
with relevant agencies and landholders to investigate options that will allow some of the recommended environmental flows to be delivered closer to their target magnitude in future without affecting private land.
Under all climate scenarios, water for the environment will be used to maintain a continuous flow in the east and west branches during summer and autumn. Delivery of water for the environment in the east branch is prioritised over the west branch when supply is limited because the east branch has greater environmental values and relatively small flows in the east branch have the potential to deliver significant environmental outcomes. In the east branch, the priority will be to deliver summer/autumn low flows under all climate scenarios and summer/autumn freshes under dry, average and wet scenarios. The summer/autumn freshes will help to improve water quality and provide opportunities for fish and platypus to disperse throughout the system, breed and take advantage of increased food and habitat under wet and average climatic conditions.
The increased volume of water for the environment available under average and wet climate scenarios will be shared between the east and west branches and will be used to supplement natural events. The summer and autumn low flows in the west branch can be of greater magnitudes, as presented in Table 3.7.1, under average and wet climate scenarios to achieve better outcomes for fish, platypus and vegetation. Any remaining water for the environment under an average or wet climate scenario will be used to supplement winter and spring low flows in the east and west branches and flows further downstream. Winter and spring freshes in reaches 3 and 4 are essential for the system, but due to channel choke points from willow and glyceria and channel capacity and delivery constraints, these are not planned to be delivered by water for the environment and are not included in this seasonal watering plan.
The tier 1a and 1b watering actions presented should help to maintain current environmental values and conditions in the upper Barwon River. However, a larger environmental entitlement and complementary works that address non-flow-related impacts in the catchment (such as constrictions) will be needed to significantly improve environmental conditions.
The carryover reserve for the upper Barwon River is 500 ML for 2023-24.