Modeling washoff in temperate and tropical urban catchments

Abstract Modeling the washoff of pollutants from urban catchments is a difficult task. Although the exponential washoff model has been in use for decades, there is a general lack of data on model parameters and much less is known about how the model parameters are influenced by storm and climate characteristics. This study was done to understand how model parameters vary between different catchments having vastly different rainfall characteristics. Such a study will elucidate model response to rainfall, which in turn provides better knowledge on model function. High quality rainfall, runoff, and water quality data for a total of 117 storm events were sourced from the literature, which included 48 storm events from temperate (Bellevue, Washington, US; Dublin, Ireland) and 69 storm events from tropical (Skudai, Malaysia; Singapore; Pampano Beach, Florida, US) catchments, respectively. All the data were sourced from catchments with residential land use. This dataset was supplemented by additional measurements done in two residential catchments in Geelong (temperate) where a total of 16 storm events were monitored. Monte Carlo analysis was used to obtain the best-fit values of the washoff model parameters consisting the washoff coefficient, C3, washoff exponent, C4, and the initial mass on surface, Bin . It was found that the spread of these parameter values was greater for the dissolved pollutants ortho-phosphate (OP) and ammonium- nitrogen (NH4 - N) than for total suspended solids (TSS). There are strong suggestions that Bin represents the mass available on the surface at the start of a washoff event, as evidenced by observations that Bin is significantly higher for the tropical dataset, a reflection of the greater storm energy associated with tropical rainfall. The small variation in C4 across all (temperate and tropical) sites is related to its dependence on local flow conditions, linked with the friction velocity and critical shear stress conditions. The parameters C3 and Bin on the other hand, are correlated with rainfall, with C3 having larger uncertainty. Of all the rainfall characteristics investigated, the rainfall depth for the current event, d was found to be the single parameter that correlated well with Bin and C3 for TSS. The strength of this dependence diminished for pollutants in mixed and dissolved forms. In temperate regions, Bin correlates strongly with both the average and maximum intensities whereas in tropical catchments, maximum intensity and depth were found to be significant. This correlation for the temperate catchments can be understood by the dependence of rainfall energy on intensity. Rainfall energy cease to increases once intensity exceeds 3 in/hr (76.2 mm/hr), which is uncommon in temperate catchments, but is common in the tropics.