Numerical modelling of spatio-temporal thermal heterogeneity in a complex river system

Accurate quantification and effective modelling of water temperature regimes is fundamental to underpin projections of future Arctic river temperature under scenarios of climate and hydrological change. We present results from a deterministic two-dimensional hydrodynamic model coupled with a heat transfer model that includes horizontal advection and vertical water surface energy fluxes. Firstly, we model longitudinal, lateral and temporal thermal heterogeneity of a braided reach of an Arctic river; Kårsajökk, Sweden. Model performance was assessed against water temperature data collected at 11 monitoring sites for two independent 1-week time periods. Overall, model performance was strongest (r values >0.9, RMSEs ∼ 0.6 °C and ME < 0.4 °C) for main channel sites with relatively deep fast-flows where water temperature was comparatively low and stable. However, model performance was poorer for sites characterised by shallow and/or temporarily-stagnant streams at the lateral margins of the braidplain, where a lag of 60–90 min persisted between the modelled and measured water temperatures. Secondly, we present novel automated statistical analyses and quantify channel thermal connectivity and complexity. Our results lead us to suggest that with further development our modelling approach offers new opportunities for scenario-based predictions of response to environmental change and to assess anthropogenic impacts on water temperature.