The Lachlan valley has an area of 90,000 square kilometres, extending from the Great Dividing Range to the Great Cumbung Swamp on the Riverine plains. Nearly 1300 kilometres of the 1400-kilometre-long river is regulated by water storages, of which Wyangala Dam is the largest at 1220 gigalitres.
The Lachlan catchment recovered rapidly from drought conditions in 2020–21 with La Niña conditions bringing rainfall in August–September 2020. Two translucent flow events (environmental water under water sharing plan rules) occurred in August–September and October–November 2020, totalling 173,000 megalitres. Consequently, water for the environment was managed primarily to supplement translucent flows and extend the duration, depth and connectivity of floodplain inundation in the lower Lachlan.
Water managers prepared for the 2020–21 year with a forecast of nil to low water allocation. This corresponded with an expected very dry to dry resource availability scenario, as identified in the Lachlan catchment: Annual environmental watering priorities 2020–21. There was zero allocation for general security in 2019–20. The aim under these conditions was to maintain strategic drought refuges to avoid irretrievable loss of species and habitat.
Minimal inflows occurred in 2019 – close to the record minimum. However, substantial inflows in August increased Wyangala Dam capacity from 16% in July 2020 to 70% in April 2021 and water allocations were increased accordingly. The first general security allocation in 3 years occurred in September, and again in November 2020 and April 2021. This meant the environmental water allowance (20,000 megalitres) also became available in April 2021.
Supporting key refuge sites
Maintaining key drought refuges, such as Booberoi Creek and Great Cumbung Swamp, was the focus in the Lachlan early in 2020–21. Although drought intervention was not required, water for the environment was delivered from May to July to increase unseasonably low base flows. This helped maintain permanent refuge habitat for threatened species such as southern bell frog in Cumbung, and macrophytes and native fish in Booberoi Creek.
Unprecedented collaboration between the Department of Planning, Industry and Environment, WaterNSW, local landholders, local fish experts, Department of Primary Industries – Fisheries and the local Aboriginal community to remove constraints and increase channel capacity of the Booberoi Creek delivery system was successful, with subsequent translucent and licensed environmental water delivery exceeding expected flow efficiencies and ecological outcomes.