Catchments to the west of Melbourne generally experience drier climatic conditions than those to the east. In the Central Region, it is not uncommon for individual systems to experience different climatic conditions at the same time. Environmental water entitlements in the Yarra system are more reliable than the other systems, which makes water availability more certain, irrespective of catchment conditions. Water delivery infrastructure limits the volume
of water for the environment that can be delivered to the region's systems, and deliveries of water for the environment are usually planned to supplement unregulated flows and make up shortfalls associated with low flows and small-tomedium-sized freshes. Catchment conditions and seasonal hydrological patterns, rather than water availability, influence which flow components are prioritised in most years.
Annual water for the environment allocations in the Werribee, Tarago and Moorabool systems are linked to catchment inflows, so dry conditions will result in less water being available. Carryover of unused allocations from wet years helps meet environmental demands in these systems in dry years. As well, the Werribee and Maribyrnong (for which there is no environmental entitlement) systems rely heavily on trade from Melbourne Water shares to achieve priority watering actions.
Winter and spring are usually the wettest seasons in the Central Region and contribute most of the annual inflow to system storages. Average rainfall and higher-thanaverage temperatures are predicted for the region through winter 2018. Climate outlooks indicate the 2018–19 winter storage filling season in the region is likely to be later than average, owing to below-average soil moisture.
If 2018–19 is very dry, deliveries of water for the environment throughout the region will aim to maintain critical refuges for water-dependent plants and animals, to allow recolonisation when conditions are more favourable. If conditions are closer to average or wet, environmental releases will aim to improve the health of the environment by increasing the quality and quantity of aquatic habitat for animals and trigger native fish to migrate and possibly spawn.