Modeling Washoff of Total Suspended Solids in Tropical Catchments

An attempt was made to assess the use of the empirical washoff model in predicting the concentrations of total suspended solids contained in storm runoff in the 12 sub-catchments of agricultural, residential and forest land use in the Kranji Reservoir catchment, Singapore through (1) an application of the washoff independent from the buildup model, (2) considering initial mass on surface B_ini as a calibration parameter, (3) considering washoff coefficient c₃ as a variable rather than a constant, and (4) investigation of the sensitivity of washoff parameters c₃, B_ini and the washoff coefficient c₄ with rainfall and runoff characteristics, land use and catchment size. The optimized model parameters c₃, c₄, and B_ini were obtained using the Monte-Carlo simulation. The results showed that the washoff model can be applied to catchments of all three mentioned types of land use with reasonable fit. B_ini for the agricultural catchments are significantly higher than that from the residential and forest catchments, which is in agreement with observations on the event mean concentrations of TSS. It was also found that the values of c₃ and B_ini for the same sampling station varied significantly with the rainfall depth (d). B_ini was found to be correlated mainly with d, but also to a lesser extent with average intensity, peak runoff volume, and runoff volume. Significantly, B_ini did not correlate well with ADP or with rainfall depth of the prior storm event. The results from this study show that the common modeling practice where the initial mass available for washoff B_ini is simulated using antecedence dry period and previous rainfall depth should be reassessed, at least when applied to tropical catchments. The study also shows that washoff parameters depend on catchment size. The slopes of regression between c₃ and B_ini with d increase with catchment area, implying that washoff behavior in smaller catchments is less sensitive to changes in rainfall depth than larger catchments.