Abstract
A simulation experiment was used to understand the importance of riparian vegetation density, channel orientation and flow velocity for stream energy budgets and river temperature dynamics. Water temperature and meteorological observations were obtained in addition to hemispherical photographs along a ∼1km reach of the Girnock Burn, a tributary of the Aberdeenshire Dee, Scotland. Data from nine hemispherical images (representing different uniform canopy density scenarios) were used to parameterise a deterministic net radiation model and simulate radiative fluxes. For each vegetation scenario, the effects of eight channel orientations were investigated by changing the position of north at 45° intervals in each hemispheric image. Simulated radiative fluxes and observed turbulent fluxes drove a high-resolution water temperature model of the reach. Simulations were performed under low and high water velocity scenarios. Both velocity scenarios yielded decreases in mean (≥1.6°C) and maximum (≥3.0°C) temperature as canopy density increased. Slow-flowing water resided longer within the reach, which enhanced heat accumulation and dissipation, and drove higher maximum and lower minimum temperatures. Intermediate levels of shade produced highly variable energy flux and water temperature dynamics depending on the channel orientation and thus the time of day when the channel was shaded. We demonstrate that in many reaches relatively sparse but strategically located vegetation could produce substantial reductions in maximum temperature and suggest that these criteria are used to inform future river management.
Highlights
It is anticipated that a changing climate will alter river temperature regimes
The orientation of the channel had a limited impact on total daily net solar radiation gains under: (1) the densest canopies (i.e. 70- 90 % density; Figure 4a), when limited portions of the stream remained unshaded (Figure 2c- 2f) and (2) under the sparsest canopies (i.e. ≤ 20 %; Figure 4a), when vegetation did not overhang the stream, cast minimal shade regardless of channel orientation (Figure 2a and 2b) and diurnal patterns were similar regardless of channel orientation
We compare two channel orientations under a 30% canopy density in order to demonstrate the drivers of this variability (Figure 6)
Summary
It is anticipated that a changing climate will alter river temperature regimes. Beechie et al, 2013; van Vliet et al, 2013; MacDonald et al, 2014a; Hannah and Garner, 2015]. Such changes, increased maxima, may diminish the spatial and temporal extent of suitable cool-water habitat for temperature sensitive organisms with potential impacts on the composition and productivity of aquatic ecosystems [Wilby et al, 2010; Leach et al, 2012]. Observational datasets, frequently in combination with deterministic modelling approaches, have demonstrated that the summer temperature of headwater streams is generally dominated by: (1) advected heat from upstream (2) heat exchange at the air-water column interface [e.g. Westhoff et al, 2011; Leach and Moore, 2014; MacDonald et al, 2014a; Garner et al, 2014], predominantly solar radiation gains [Hannah et al, 2008; Leach and Moore, 2010; MacDonald et al, 2014a], and at some locations (3)
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