Modelling bacterial water quality in streams draining pastoral land

A model has been developed to predict concentrations of the faecal bacteria indicator E. coli in streams draining grazed hill-country in New Zealand. The long-term aim of the modelling is to assess effects of land management upon faecal contamination and, in the short term, to provide a framework for field-based research. A daily record of grazing livestock is used to estimate E. coli inputs to a catchment, and transport of bacteria to the stream network is simulated within surface and subsurface flows. Deposition of E. coli directly to streams is incorporated where cattle have access to them, and areas of permanent saturation ('seepage zones') are also represented. Bacteria are routed down the stream network and in-stream processes of deposition and entrainment are simulated. Die-off, both on land and in water, is simulated as a function of temperature and solar radiation. The model broadly reproduces observed E. coli concentrations in a hill-country catchment grazed by sheep and beef cattle, although uncertainty exists with a number of the processes represented. The model is sensitive to the distance over which surface runoff delivers bacteria to a stream and the amount of excretion direct to streams and onto seepage zones. Scenario analysis suggests that riparian buffer strips may improve bacterial water quality both by eliminating livestock defaecation in and near streams, and by trapping of bacteria by the riparian vegetation.