To what extent does hydrological connectivity control dynamics of faecal indicator organisms in streams? Initial hypothesis testing using a tracer-aided model

The role of hydrological connectivity in driving the dynamics of faecal indicator organisms (FIOs) in streams is poorly characterised. Here, we demonstrate how a tracer-aided hydrological model can be used within a coupled modelling approach to explore the role of connectivity in governing stream faecal coliform (FC) dynamics. To do so, we tested a hypothesis that in northern upland catchments, the dynamics of hydrological connectivity between major landscape units (hillslopes and riparian zone) and the stream exert a dominant control on stream FC loads by facilitating generation of runoff-driven FC fluxes. This hypothesis was conceptualised within a simple FC model that was coupled to a tracer-aided hydrological model developed for a small (3.2 km2) data-rich catchment in NE Scotland. The model was dual-calibrated to daily discharge and stable isotope data for the period August 2008 to September 2009; stream FC loads were also simulated but not used as a calibration target. Behavioural models successfully captured the general dynamics of the discharge and isotope data (average Kling-Gupta efficiencies of 0.72 and 0.53, respectively), providing confidence in the realism of simulated hydrological processes. The models simulated a seasonally-varying role of connectivity in driving stream FC loads. In summer, connectivity of the catchment hillslope was crucial in providing a source of FC to the riparian zone for transfer to the stream; this countered the decline in fresh FC input to the riparian zone in summer which reflected the seasonal movement of red deer (the principal source of FC) onto higher ground. In winter when this seasonal movement caused FC to be predominantly stored in the riparian zone, simulated hillslope connectivity primarily provided water to the riparian zone that permitted increased runoff generation and associated mobilisation of FC. Comparison of observed and simulated stream FC loads revealed model performance to be variable (R2 range: 0-0.34). The better performance of the model in summer was consistent with hydrological connectivity being a dominant control on stream FC loads at this time. However, failure of the model to capture low FC loads in winter indicated that additional processes not considered in the model may also govern stream FC dynamics during this period. Incorporating the impact of freeze-thaw cycles on FC mortality, or a dilution effect of hillslope connectivity in winter, could be potential next steps in refining the hypothesis conceptualised in the FC model presented here. The novel coupled modelling approach used in this study successfully allowed a hypothesised role of connectivity in driving stream FC dynamics to be tested, contextualised by the accuracy of discharge and isotope-tracer simulations as indicators of hydrological process realism. Therefore, coupling FIO and tracer-aided hydrological models has clear promise for furthering understanding of FIO dynamics, which is a vital precursor to the successful management of microbial water quality. Based on the experiences in this study, a “roadmap” for the future development and application of coupled approaches is also presented.