Besides the obvious environmental benefit of saving water, stormwater management greatly helps the larger environment well beyond the subject site.
Rainwater tanks must first fill up with roof runoff before the overflow starts to divert the excess into the local stormwater system. The result is a major delay on the flow of water through the stormwater infrastructure. This means a reduced, but prolonged flow into natural waterways and a reduction in the risk of flooding. The graphs below show rainwater flow at different points along the catchment. The pink curve demonstrates the high quantity over a short time that stormwater runoff creates without any rainwater collection implemented. This is common in many states other than NSW where rainwater storage is not mandated in new developments. The blue curve shows the impact that residential stormwater harvesting has. The delay of flow caused by collection creates a minor difference in overall runoff quantity, but a major difference in drainage time, greatly reducing degradation of rivers, wetlands and oceans.