Stormwater is urban surface water runoff from rain events.
In areas such as the sandy soils of the Swan coastal plain, rainfall and resultant stormwater naturally recharges the superficial aquifer. The Department of Water encourages infiltration of small rainfall events at or near source as a primary means of urban stormwater management to maintain the site water balance as close as possible to its pre-development hydrology.
Harvesting large quantities of stormwater for use as a water source (e.g. to irrigate public open space or for non-drinking uses in new residential developments) may be limited to:
- high-density development or commercial areas
- areas with limited infiltration potential (due to high groundwater levels or less permeable soils) or
- areas with existing large-scale stormwater discharge systems (such as traditional drains or large sumps).
Stormwater drainage systems are often former natural waterways and the harvesting of stormwater can impact on ecosystems and water bodies that are dependent on that water. Proponents need to consider these ecological water requirements of the catchment in determining the volume of water that can be collected for large-scale harvesting systems and the impacts on the local water balance.
- The use of stormwater can be incorporated in new urban developments using water sensitive urban design principles.
- The ecological water requirements of any ecosystems previously receiving the stormwater runoff or recharge must be accounted for.
- The quantity and quality of stormwater is highly variable and of a seasonal nature.
- Treatment requirements need to be considered to ensure the water quality is suitable for the intended uses.
- The quality of stormwater also depends on the land uses in the catchment area and treatment for non-drinking water use can be expensive and energy intensive.
- Stormwater will require storage if it is being used for irrigation, and collection areas will have to account for seasonal variation.
- Permission will be required from the owner (for example the Water Corporation or a local government authority) if the stormwater is taken from existing drains.
- For essential in-house non-drinking services, a back-up water source may be required to ensure continued supply.
- Governance issues surrounding ownership, operation and management of a community scale stormwater system can be complex.
Stormwater storage and use for irrigation
Stormwater collected from urban runoff or existing stormwater drains can be stored in tanks or underground storage cells for irrigation use. Since stormwater is of a seasonal nature, adequate storage is required to store sufficient volumes for irrigation supply during the drier months, which can be an expensive way of managing stormwater. Therefore, managed aquifer recharge can provide an economic means of storing stormwater for later abstraction and use. However, managed aquifer recharge may not be feasible on all sites, due to hydrogeological, environmental or cost limitations. There are also limitations to managed aquifer recharge within proclaimed public drinking water source areas. For more information see the department's managed aquifer recharge information.