Simulating fecal coliform dynamics at the watershed scale using a modified SWAT model and surrogate model

Fecal coliform bacteria are widely used as an indicator of fecal contamination and associated human-health risk in water. This study simulated fecal coliform dynamics in the Bonghwang River using the Soil and Water Assessment Tool (SWAT). The SWAT bacteria subroutine, which considers in-stream bacteria die-off only, was modified to include solar radiation-associated die-off and concurrent growth and die-off within streambed sediments. To address the computational burden of SWAT, a surrogate model was developed using outputs from the modified SWAT model. The surrogate model enabled rapid watershed-scale prediction of fecal contamination by simplifying computations while preserving the predictive accuracy of SWAT. Sensitivity analysis demonstrated that solar radiation is one of the most significant fate factors of fecal coliform. The modified SWAT model improved watershed-scale estimates of bacterial concentrations, while the surrogate model enabled efficient prediction and analysis across the watershed. Overall, this approach provides predictive and reliable information on fecal contamination and can support effective watershed management.