Patterns and drivers of fecal coliform exports in a typhoon-affected watershed: insights from 10-year observations and SWAT model

Abstract

Fecal contamination from sewage and agricultural runoff has been a prevailing problem in coastal watersheds owing to accelerating human stressors and climate change in the Anthropocene. However, the patterns and drivers of fecal indicator bacteria (FIB) exports in typhoon-affected watersheds remain poorly understood. Here, the spatiotemporal variation of FIB and its drivers were systematically investigated in a subtropical coastal watershed, namely the Jiulong River Watershed (JRW), by integrating a decade of monthly in-situ observations and applying them to the Soil and Water Assessment Tool (SWAT) model. The SWAT model was calibrated and validated for streamflow and fecal coliform (FC) simulations with satisfactory performance (Nash-Sutcliffe Efficiency of 0.69 and 0.47, R2 of 0.72 and 0.60 for streamflow and FC in validation period, respectively). Observations of FC in both the North River (220–5580 cfu·100 mL−1) and the West River (3649–46,293 cfu·100 mL−1) of the JRW showed significant declining trends from 2010 to 2020. Source attribution analysis of FC export indicated that livestock manure emissions were the largest contributor (>60%) with urban domestic sewage being the least important contributor (<5%). Furthermore, our observation and modeling results show that FC exports were markedly enhanced during high rainfall events and typhoons, while FC exports decreased during the winter when the climate is dry and cold. The FC loads ranged from 2.6 × 107 to 2.8 × 1010cfu∙L−1∙s−1, with the average loads in agriculture, nature, and urban watershed accounting for 6.3 × 109, 8.3 × 108, and 7.4 × 109cfu∙L−1∙s−1, respectively. Our data support that the FC loads are closely related to anthropogenic indicators and are significantly higher in urban and agricultural watersheds than in natural or pristine watersheds. Future climate change with more frequent rainfall extremes will likely intensify FC exports. Such findings have important implications regarding mitigating FIB pollution for subtropical coastal watersheds under the context of climate change and increasing human impacts.