ETL = average landscape evapotranspiration (m), A = irrigated land area (m2), D = days for the month.
It is assumed that the total monthly non-potable water demand is first fulfilled by the RWH system.
The generated runoff from the RWH system catchment is diverted to the storage tank.
The available storage capacity is compared with the accumulated runoff. If the accumulated runoff is greater than the available storage volume, excess water will be deducted from the accumulated runoff and will be released as excess water.
Total water demand is deducted from the accumulated/harvested rainwater.
If the total water demand is greater than the harvested rainwater in the tank, the model assumes that the remaining water demand is met by the public water supply or another source. A monthly water balance model, Equation (4), is set up using Microsoft Excel software to compute the average volume of rainwater used each month and the average volume left behind in the tank at the end of the month