Which environmental factors control extreme thermal events in rivers? A multi-scale approach (Wallonia, Belgium)

Managers need to know how to mitigate rising stream water temperature (WT) due
to climate change. This requires identifying the environmental drivers that influence
thermal regime and determining the spatial area where interventions are most
effective. We hypothesized that (i) extreme thermal events can be influenced by a set
of environmental factors that reduce thermal sensitivity and (ii) the role played by
those factors varies spatially. To test these hypotheses, we (i) determined which of the
environmental variables reported to be the most influential affected WT and (ii)
identified the spatial scales over which those environmental variables influenced WT.
To this end, the influence of multi-scale environmental variables, namely land cover,
topography (channel slope, elevation), hydromorphology (channel sinuosity, water
level, watershed area, baseflow index) and shade conditions, was analyzed on the
three model variables (day thermal sensitivity, night thermal sensitivity, and
non-convective thermal flux) in the model developed by Georges et al. (2021) of the
temporal thermal dynamics of daily maximum WT during extreme events. Values
were calculated on six spatial scales (the entire upstream catchment and the
associated 1 km and 2 km circular buffer, and 50 m wide corridors on each side of the
stream with the associated 1 km and 2 km circular buffer). The period considered was
17 extreme days during the summer identified by Georges et al. (2021) based on
WT data measured every 10 min for 7 years (2012–2018) at 92 measurement sites.
Sites were located evenly throughout the Wallonia (southern Belgium) hydrological
network. Results showed that shade, baseflow index (a proxy of the influence of
groundwater), water level and watershed area were the most significant variables
influencing thermal sensitivity. Since managers with finite financial and human
resources can act on only a few environmental variables, we advocate restoring and
preserving the vegetation cover that limits solar radiation on the watercourse as a
cost-effective solution to reduce thermal sensitivity. Moreover, management at
small spatial scale (50 m riparian buffer) should be strategically promoted (for
finance and staffing) as our results show that a larger management scale is not more
effective in reducing thermal sensitivity to extreme events.