CatStream: an integrated catchment-stream water quality model

Abstract

Water quality modelling is the primary tool used for catchment and stream water quality investigations. The general architecture of a typical water quality model is the integration of the pollutant processes with the hydrologic and hydraulic approaches. 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. Most of the available water quality models are too simple and/or stochastic in nature. Many of those models perform water quality estimations in isolation, i.e. separate water quality models for catchment and stream analyses. Isolated models may lead to inconsistencies and biased results in the prediction of water quality parameters. On the other hand, there are some integrated water quality models, which are very complex requiring huge physical and chemical data as well as determining many model parameters. This paper presents the development of a simple, integrated and deterministic catchment-stream water quality model to be able to continuously simulate different water quality parameters. The integrated model is comprised of two individual models: the catchment water quality model and the stream water quality model. The catchment water quality model consists of two sub-models: rainfall-runoff model and pollutant processes model. The rainfall-runoff model was developed by considering the time-area method of runoff routing. The model estimates amount of surface runoff generated from a specified catchment for which rainfall data is provided. Water quality parameters were incorporated with the developed rainfall-runoff model, which represents the catchment water quality model. This model estimates the amount of pollutant accumulated on catchment surfaces during the antecedent dry days, and their transportation with surface runoff into waterways and receiving water bodies throughout storm events. Similar to the catchment water quality model, the stream water quality model comprised of two sub-models: the stream flow model and stream pollutant processes model. The stream flow model was developed by considering the Muskingum-Cunge method of stream routing. The stream flow model estimates the rate of water flow into the downstream sections of a particular stream reach. The processes of the same water quality parameters as used in the catchment water quality model were incorporated with the stream flow model which represents the stream water quality model. Final output of the stream water quality model is the concentration of transported pollutants into different downstream sections of a particular stream reach. 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 by the Gold Coast City Council (GCCC) were used. Calibration of the catchment water quality model and stream water quality model was performed separately. The calibration results demonstrated the suitability of the developed model as a tool to help with water quality management issues. The major advantage of the developed model is the easy and continuous simulations of water quality parameters associated with surface runoff during any rainfall event. 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.