Methodology
The methodology for GIS based screening tool of potential stormwater harvesting sites is described in the following four main steps, which can be applied to greenfield areas as well as existing urban areas.
2.1. Step 1 – evaluation of suitability criteria
Three tasks are involved in this step: a) Criteria identification for stormwater harvesting suitability, (b) Data acquisition and processing to create spatial maps for identified criteria, and c) Estimation of suitability indices.
In task (a), annual runoff and non-potable demand are considered as the suitability criteria, as they are the principal drivers for any stormwater harvesting scheme. It should be noted that social, economic and environmental considerations also play an important role in selecting overall suitable stormwater harvesting sites. However, suitability at the screening stage of planning process needs to consider first if there is a reasonable match between supply and demand before proceeding to more detailed assessment.
The runoff criterion considered runoff generated from impervious and pervious areas within the study region. The water demand is calculated from potential residential and non-residential water uses, such as irrigation of parks.
The stormwater harvesting catchments can also be considered as the ‘accumulating catchments’ with their runoff and demand. The accumulated catchment concept is explained using Fig. 1. For example in Fig. 1, catchments a and b are upstream catchments which drain at outlet-1 and outlet-2 respectively. The catchment c which drains at outlet-3 is an accumulated catchment, consisting of catchments a and b with an additional drainage area of c.