The uncertainity in stormwater quality modelling for temperate and tropical catchments

More research has been done on the uncertainty of stormwater quantity as compared to quality modelling. Although the washoff model has been in use for decades, there is a general lack of studies on the uncertainty associated with washoff modelling and how this uncertainty varies between catchments in different climate zones due to differences in rainfall conditions. This study investigated model structure uncertainty, in addition to gaining knowledge on model parameter uncertainty in water quality modelling of urban catchments located in two climate zones. Such a study not only adds to the body of knowledge but highlights the importance of distinguishing the difference in washoff behaviour, and their results, between catchments in different climate zones. Following an exhaustive search and assessment of data quality, the rainfall, discharge and water quality data sourced from 11 residential catchments, which include 72 events from temperate catchments and 59 from tropical catchments were analysed. Comparison of uncertainty between buildup-washoff model (BW) and washoff model (WO) results for particulate (Total Suspended Solids (TSS)), mixed (Total Phosphorus (TP), Total Nitrogen (TN)) and dissolved (ortho-Phosphate (OP), Ammonium (NH4-N)) phase water quality parameters were carried by using the Generalized Likelihood Uncertainty Estimation (GLUE) method. Higher Average Relative Interval Length (ARIL) values obtained for tropical catchments showed that uncertainty in model prediction is higher than temperate catchments. This result is confirmed for both the BW and WO modelling approaches. The threshold criteria for obtaining behavioural set chosen as likelihood measure of Tr = 0.7 (based on the Nash-Sutcliffe coefficient) and η ≥ 70 % (proportion of observed data falling within the 90 % uncertainty band of the simulated data distribution over the total number of observations) are mostly satisfied for the particulate phase and least by the dissolved phase water quality parameters, providing further evidence that the structure of the models are not universally applicable, and that model results should be understood in the light of this difference. Comparisons between the BW and WO approaches showed similar levels of uncertainty, irrespective of climate. Although this is consistent with other studies reported in the literature, the antecedent dry period (ADP), a key parameter in the buildup process, was not significantly (p =.07) different between the temperate and tropical datasets. This is likely an artifact of sampling, especially for temperate catchments, where field data is collected during the rainy season, when ADP is limited and similar with catchment in the tropics.