Application of Build-up and Wash-off Models for an East-Australian Catchment Iqbal Hossain, Monzur Alam Imteaz and Mohammed Iqbal Hossain Abstract—Estimation of stormwater pollutants is a pre-requisite for the protection and improvement of the aquatic environment and for appropriate management options. The usual practice for the stormwater quality prediction is performed through water quality modeling. However, the accuracy of the prediction by the models depends on the proper estimation of model parameters. This paper presents the estimation of model parameters for a catchment water quality model developed for the continuous simulation of stormwater pollutants from a catchment to the catchment outlet. The model is capable of simulating the accumulation and transportation of the stormwater pollutants; suspended solids (SS), total nitrogen (TN) and total phosphorus (TP) from a particular catchment. Rainfall and water quality data were collected for the Hotham Creek Catchment (HTCC), Gold Coast, Australia. Runoff calculations from the developed model were compared with the calculated discharges from the widely used hydrological models, WBNM and DRAINS. Based on the measured water quality data, model water quality parameters were calibrated for the above-mentioned catchment. The calibrated parameters are expected to be helpful for the best management practices (BMPs) of the region. Sensitivity analyses of the estimated parameters were performed to assess the impacts of the model parameters on overall model estimations of runoff water quality. Keywords—Calibration, Model Parameters, Suspended Solids, To- tal Nitrogen, Total Phosphorus. I. I NTRODUCTION I T is widely recognized that watershed management is essential for the protection and improvement of the downstream environment from pollutions. However, efficient management of waterways and receiving water bodies heavily relies on the accurate estimation of the pollutants transferred from the catchment for the design of effective impact mitigation devices and management strategies. Inaccurate measurement of non-point pollutant loads can lead to the design of undersize and ineffective or oversized measures with excessive capital cost and maintenance requirements. The general strategies and programs for watershed management always depend on the modelling results of watershed responses, which involve both flow and pollutants processes [1]. During the last decade abundant hydrologic and water quality models have been developed for the prediction of the transported stormwater pollutants from a particular watershed area. However, the major problem in the application of any stormwater quality model is the selection of appropriate model parameters [2]. Hence, estimation of appropriate sets I. Hossain and M. Imteaz are with the Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia). M. Hossain is with Gold Coast City Council, Qld, 9729, Australia. of parameter values of stormwater quality models is essential to simulate the catchment responses accurately. Tsihrintzis and Hamid (1998) noted that parameter estimation is the most critical step in the practical application of any water quality model [3]. Different researchers have used different procedures for the estimation of model parameters. For example, Deletic and Maksimovic (1998) [4] and Kim et al. (2006) [5] used indirect methods for the estimation of these parameters. The parameters can be determined from the samples collected from each land-use [6]. However, the values of the parameters determined in this manner might only reflect the pollutant loads at a few sample points rather than all over the entire catchment. Other alternative approach for the estimation of these parameters is using the runoff quality data collected at the watershed outlet [7] by the calibration procedure, which reflects the combined effects of the whole catchment. However, Leinster and Walden (1999) discouraged the general application of water quality model parameters from overseas countries [8]. Chen and Adams (2006) and Baffaut and Delleur (1990) found that the model parameters vary not only catchment to catchment, but also differ among different rainfall events [9], [10]. Therefore, Pucket (1995) noted a watershed management plan needs to be developed on an individual watershed basis [11]. Hence, calibration of water quality model with local data is essential for the analysis, improvement and/or update of the existing BMPs for a particular catchment. Calibration procedure attempts not only identifying the best set of parameters, but also help to assess and reduce the uncertainty in parameter values [12]. This paper demonstrates the calibration of a catchment water quality model to obtain a set of parameters for which the predicted pollutant concentrations are close to the measured concentrations. Two major pollutants were analysed for the calibration of the model: TN and TP. These pollutants were chosen for the calibration of the model because of the data availability. Nonetheless, these parameters are generally used as indicators of water quality in receiving water bodies [13]. For the evaluation of the hydrologic outputs of the model, the peak discharges were compared with the calculated discharges from the WBNM and DRAINS models, which are widely used in Australia. Water quality data (TN and TP) was collected from several locations within the HTCC on the Gold Coast, Australia. The calibration of the model was demonstrated using the collected water quality data from the Gold Coast, Australia. The calibrated parameters are expected to be useful for the future use of the model to develop BMPs for pollution World Academy of Science, Engineering and Technology 51 2011 941