Development of an integrated catchment-stream water quality model

Water quality models are extensively used for the estimation of water quality parameters of an aquatic environment, which is essential for the development of efficient pollution mitigation strategies. However, due to the lack of specific local information and poor understanding of the limitations of various estimation techniques and underlying physical parameters, modelling approaches are often subjected to producing gross errors. Moreover, the usual practice of water quality modelling is performed through separate models in isolation, which leads to inconsistencies and significantly biased results in the prediction of water quality parameters. This research project presents the development of an integrated catchment-stream water quality model to be able to continuously simulate different water quality parameters, i.e. SS (suspended sediments), TN (total nitrogen) and TP (total phosphorus). The integrated model is comprised of two individual models, i.e. the catchment water quality model and the stream water quality model. The catchment water quality model estimates the amount of pollutants accumulated on catchment surfaces during the antecedent dry days, and their transportation with surface runoff into nearby waterways and receiving water bodies throughout storm events. The stream water quality model estimates the amount of transported pollutants through a particular stream reach. Considering the time-area method, a rainfall-runoff model was developed. Water quality parameters were incorporated with the rainfall-runoff model, which represents the catchment water quality model. A stream water quality model was developed with the same water quality parameters as used in the catchment water quality model. Finally, the catchment water quality model and the stream water quality model were integrated for the continuous simulation of previously mentioned water quality parameters. For calibration and validation of the model, different published data and reliable source data collected from Gold Coast City Council (GCCC) were used. The calibration results demonstrated the suitability of the developed model as a tool to help with water quality management issues. Sensitivity analysis of the model parameters was performed to assess the most sensitive parameter and to enhance understanding of the modeller’s knowledge about the adopted modelling approach. The major advantage of the developed model is the continuous simulation of water quality parameters associated with surface runoff during any rainfall event using an integrated modelling approach. The preparation process of the input data for the model is simple. The capability of the model to simulate surface runoff and pollutant loads from a wide range of rainfall intensities make the integrated model useful in assessing the impact of stormwater pollution flowing into waterways and receiving water bodies and to design effective stormwater treatment measures.