Stormwater harvesting

The stormwater harvesting node serves to implement a representation of catchment-scale stormwater capture and reuse within the modelled project area. The node functions by first activating stormwater collection at the catchment level, which prompts the model to direct all non-roof impervious runoff generated by the project area to a conceptual storage body. Following this, the parcels contained within the GIS boundary layer used with the node are assigned a number between 0 and 100 – if this number is less than the value set for the stormwater harvesting percentage parameter, then the parcel is assumed as eligible for stormwater harvesting uptake.

A parcel’s actual utilisation of stormwater reuse is dependent on both its eligibility to harness other alternative water sources, and its current uptake of such initiatives. To begin with, rainwater harvesting tanks for non-potable reuse are given precedence, and will always be implemented over stormwater harvesting if available to a parcel. A lot that was assigned an alternative water source in a previous simulation also cannot employ stormwater reuse. Finally, if the potable stormwater harvesting node was used prior to the (non-potable or base) stormwater harvesting node, then stormwater capture for the base stormwater harvesting system will not be enabled and no parcel will be able to be marked eligible for non-potable stormwater reuse (this is to prevent the redundancy that would occur by having multiple stormwater reuse schemes collect the same runoff).

If a parcel has been specified as available to harness stormwater harvesting, and it has not been earmarked for rainwater harvesting or potable stormwater reuse, it is designated as participating in the stormwater harvesting scheme, and its corresponding non-potable and outdoor water demands are aggregated with others at the catchment scale and used as the demand stream for the conceptual storage body.

The storage body operates as a simple bucket or conceptual model, with the daily non-roof impervious runoff and aggregated water demands as the inflow and drawdown, respectively. Any surplus in the former is taken as stored volume for that time step, minus the spillage that accounts for the storage body volume (which is set to 20ML for stormwater harvesting systems); and the daily demand satisfied by the inflow/stored volume is summed to the annual level – this serves to quantify the amount of water from the water supply network that can be substituted with harvested stormwater (owning to the amount of households that participate in the stormwater harvesting scheme) on an annual basis.

Lastly, the catchment runoff is adjusted to take into consideration the water volume internalised by the stormwater harvesting system. Using the total inflow (the annual non-roof impervious runoff) and the spillage from the storage body, the combined amount of water servicing the demand stream and stored at the end of the simulation period is established, and this figure is subtracted from the overall runoff.

Parameter

Name

Type

Description

Default

Values

SH uptake (%)

DOUBLE

Percentage of buildings with stormwater

50

0 - 100