Elsevier

Journal of Environmental Management

Volume 200, 15 September 2017, Pages 263-274
Journal of Environmental Management

Research article
Modeling washoff of total suspended solids in the tropics

https://doi.org/10.1016/j.jenvman.2017.05.091Get rights and content

Highlights

  • Rainfall and washoff data for more than 120 events were collected and analyzed.

  • Monte Carlo analysis was carried out to determine washoff model parameters.

  • We studied the correlation of washoff model parameters with rainfall characteristics.

  • Buildup of TSS did not correlate with the antecedent dry period.

  • Rainfall depth and catchment area affect model parameters most.

Abstract

Washoff behavior in the tropics is expected to behave differently from temperate areas due to differences in rainfall characteristics. In this study, rainfall, runoff and total suspended solids (TSS) were monitored from 9 catchments distinguished by different types of land use, in Singapore. The catchments ranged in size from 5.7ha to 85.2ha. Over 120 rain events were studied and more than 1000 storm samples were collected and analyzed. Monte Carlo analysis was applied to obtain the best fit values of the washoff model parameters consisting the washoff coefficient c3, washoff exponent c4 and initial mass on surface Bini. The exponent c4 was found to be approximately unity for all the events monitored, in agreement with other studies. The values of c3 and Bini were found to vary between events. Among all the rainfall and runoff characteristics studied, rainfall depth of the current event (d) was found to be the single parameter that significantly influenced the values of c3 and Bini. Contrary to expectations, Bini did not correlate well with antecedent dry period or with rainfall depth of the prior storm event. The results show that the common modeling practice where Bini is assumed to vary with antecedent dry period and previous rainfall depth should be reassessed when applied to catchments in the tropics. ANCOVA analysis showed that land use was not significant, but rather the variation of c3 and Bini with d was found to correlate well with the catchment area.

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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    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.

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