A comparison of different methods of impervious area estimation

A comparison of different methods of impervious area estimation

Estimation of impervious area is a pre-requisite for hydro-ecological research. Previous studies identified a linear relationship between catchment impervious fraction and stream ecosystem health indicators. Therefore, an accurate estimation of catchment imperviousness is of current interest to urban hydrology researchers. In this study, three methods were compared to estimate the imperviousness of eight catchments located in South East Queensland (SEQ), as part of the Stormwater Harvesting and Reuse Project. The techniques are: (a) manual digitisation of geo-referenced aerial photos; (b) automated image analysis using Erdas IMAGINE and ESRI ArcGIS software; and (c) rainfall runoff depth relationship. For five of the catchments, standard deviations of total impervious area (TIA) percentages are less than 3.6%. For the other three catchments, standard deviation fell between 5% to 8%. The automated image analysis method underestimated results for two catchments. Major constraints of the image processing technique were shading effects and different colours of surfaces, which can be reduced by appropriate selection of signature colours and multiple iterations of a supervised classification. The rainfall–runoff method, which is considered an accurate method from an hydrology perspective, depends on availability of event-based rainfall and runoff data, however a linear relationship was not observed for all catchments. The manual digitisation technique estimated TIA reasonably well, but requires careful selection of sample area for digitisation. Whilst the method requires less GIS skill than image analysis technique, it was time consuming.

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