Catchment Characteristics as Indicators of Stormwater Quality in the Sydney and Illawarra Regions

The relationship between catchment characteristics (percentage urban landuse, catchment area and type of drainage system) and event mean concentrations (EMCs) of faecal coliforms, suspended solids, total and filterable phosphorus, total uncombined ammonia, oxidised and total nitrogen in stormwater was investigated using a multiple regression model. The model was developed from water quality and flow data collected from 23 catchments in the Sydney and Illawarra regions between January 1993 and July 1994. The catchment characteristics were found to be highly significant in explaining variation in EMCs, and in combination explained between 11 and 47% of the total variation accounted for by the model. Despite the strong relationship between catchment characteristics and EMCs, significant variation between sites still remained. To fully describe variation between sites and events, a large amount of detailed catchment and event information would be required. For large heterogenous catchments, such as those in this study, this information may be impossible to obtain. The overall model explained between 58% and 73% of variation in EMC data, indicating that the model is capable of predicting EMCs for unsampled events. Due to the large proportion of unexplained variation between events, the estimation of data is limited to events where data has been gathered for at least some sites.