Research articleModeling washoff of total suspended solids in the tropics
Introduction
Various models have been developed to assess storm runoff quality and examine its potential impacts on receiving water bodies. Among the models, the empirical washoff model has been widely used to simulate the washoff process (Barbe et al., 1996, Berrentta et al., 2007, Haiping and Yamada, 1996, Park et al., 2008). This model is based on experimental studies by Sartor et al. (1974), which showed a tendency toward an exponential, or first-order decay in total washed-off load. The use of washoff models requires proper calibration of model parameters; however, much uncertainty in predicted results remains even after calibration (Avellaneda et al., 2009, Bertrand-Krajewski, 2007, Gaume et al., 1998, Kanso et al., 2005, Kanso et al., 2006, Lindblom et al., 2007, Sriananthakumar and Codner, 1993, Sutherland and Jelen, 2003). One of the major problems is the non-uniqueness of the model parameters which can reduce user confidence in applying a calibrated model for new events. This has highlighted serious limitations of the model's ability to reproduce all aspects of the pollutograph equally well with a single parameter set (Beven, 2009, Gupta et al., 2005). For example, reduction in parameter uncertainty was achieved when calibration of the model was performed using storms with similar characteristics, such as high flow versus low flow events (Avellaneda et al., 2009, Sriananthakumar and Codner, 1993). Gaume et al. (1998) concluded that uncertainty is high especially if rainfall events have different peak intensities or shorter previous dry weather periods than the events used during the calibration phase. This suggests that calibration parameter sets are sensitive to rainfall characteristics such as peak intensity and antecedent dry period in addition to land use. Recently, Gamerith et al. (2013) also showed that parameter sensitivities depend on the chosen rainfall event.
Despite the aforementioned observations, our understanding of how rainfall characteristics can influence model parameters is still limited. Arguably, a proper understanding of these effects is more critical in the tropics, where the frequent and high intensity rainfall often leads to a flashier response and higher runoff volumes. This study investigated the sensitivity of washoff parameters to rainfall and runoff characteristics under tropical rainfall conditions. This study was focused on the washoff of total suspended solids (TSS) since TSS is found in large quantities in stormwater runoff and acts as a vehicle for the transport of other pollutants. The objectives of this study were: (i) investigate the variation of washoff model parameters as a function of rainfall and runoff characteristics for catchments with different types of land use, (ii) identify the significant factors that affect washoff behavior and hence washoff model parameters, and (iii) elucidate the results obtained in this study in the context of runoff behavior in the tropics.
Section snippets
Site description
Singapore is a small island nation (area ≈700 km2), located at the southern end of the Malaysian peninsula in Southeast Asia. The climate of Singapore is classified under the Koppen system as a tropical rainforest (Af) with no true dry season. Annual mean rainfall is approximately 2300 mm and annual mean temperature is 27 °C. The annual maximum 60 min rainfall intensity, 1980–2010, ranged between 70 mm/h and 130 mm/h. The mean number of days per month with rain ranges between 11 and 19 and the
Observed data
Box plots of the rainfall and runoff characteristics for all events are presented in Fig. 2. The boxes indicate the median, 25% and 75% quantiles, and the length of whiskers represent 1.5 times the inter quartile range. Data points that lie beyond the whiskers are considered outliers, and these are denoted by crosses. Fig. 2 shows rainfall characteristics that are unique to the tropics. For example, in comparison with the data collected for the buildup/washoff model for the summer months in
Variation of washoff model parameters with rainfall and runoff variables
The plots in Fig. 3 show that the washoff parameters can vary over wide ranges, in some cases by 3 orders of magnitude. Even for a given catchment, c3 ranges from 0.003 to 2.8 (KC07) and the difference between maximum and minimum Bini value may be up to 500 kg/ha (KC06). The high variation in c3 has also been reported in the literature. Studies by Alley (1981) and Berrentta et al. (2007) have shown that c3 varied for different events at the same site. Sutherland and Jelen (2003), in reviewing
Conclusions
The following can be concluded from this study:
- i.
The median value of c3 for agricultural catchments is significantly higher than that for residential catchments, which suggests a faster washoff rate and a more significant first flush effect in the agricultural catchments. Among agricultural catchments, catchments dominated by soil-based vegetable farming tend to show higher c3 than areas with limited soil-based farming activities. This is probably attributed to potential rapid erosion of the
Acknowledgments
The authors acknowledge the support and research funding provided by the Public Utilities Board - Singapore (PUB), Singapore and the National Research Foundation – Environment and Water Industry (NRF – EWI), Singapore.
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2022, Science of the Total EnvironmentCitation Excerpt :Furthermore, considerable sampling work is required. Results are influenced by many factors, for example rainfall characteristics, surface roughness, climate difference and urban-rural gradient (Hong et al., 2016; Le et al., 2017; Liu et al., 2018; Morgan et al., 2020; Zafra et al., 2017). Recently, some simplified methods have been proposed that focus on the direct link between RDS and TSS in surface runoff, only requiring characterizing RDS, such as amount, particle size, associated pollutants and mobility (Al Ali et al., 2016; Leutnant et al., 2018; Zafra et al., 2017; Zhao and Li, 2013; Zhao et al., 2014).
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2019, Journal of Environmental ManagementCitation Excerpt :ii) The ranges of c4 values were not significantly different between any pollutants. Nor did the values show any change associated with land uses which agrees with earlier studies for TSS in Le et al. (2017). ( iii) For ranges of Bini, all pollutants showed significantly different ranges between residential and agricultural land uses, with the ranges for mixed land use generally falling between these extremes.
- 1
Current address: Surbana Jurong Pte Ltd, Coastal Engineering, Infrastructure and Land Survey Department, #01-01 Connection One, 168 Jalan Bukit Merah, 150168, Singapore.
- 2
Current address: DairyNZ, Canterbury Agriculture & Science Centre, Gerald Street, Lincoln, 7608, New Zealand.