Development of an Hourly Fecal Bacteria Simulation Framework in SWAT+

Fecal indicator bacteria, such as Escherichia coli (e.coli), pose significant public health risks in surface waters and are strongly influenced by hydrological processes, sediment dynamics, and land-use practices. Previous studies using the Soil and Water Assessment Tool (SWAT) have demonstrated the importance of modeling bacterial fate and transport at the watershed scale, including the role of sediment-associated processes. However, existing SWAT applications are largely limited to daily time steps, and the current version of SWAT+ lacks an explicit fecal bacteria simulation module, constraining its applicability for event-based and sub-daily microbial water quality assessments.This study presents the development and implementation of a novel fecal bacteria simulation framework within the SWAT+ modeling system, enabling hourly-scale representation of bacterial transport and fate. The study extends SWAT+ hydrological routing routines to support fecal bacteria transport. Hourly output routines are implemented to capture rapid fluctuations in bacterial concentrations during rainfall–runoff events, which are often underestimated in daily simulations.The enhanced SWAT+ model is applied to the Geumho River watershed in Korea, where mixed land use and episodic storm events strongly influence hydrological and water quality dynamics. Model calibration and validation are conducted separately for hydrological and microbial components using observed streamflow and e.coli data. The hourly simulation framework is used to examine event-scale variability in in-stream bacterial concentrations. This study represents the first attempt to enable fecal bacteria simulation in SWAT+ at an hourly time scale and provides a methodological foundation for the future development of microbial water quality modules in SWAT+ to support watershed-scale water quality management and public health risk assessment.