Abstract
Abstract. Previous studies have suggested that shading by riparian vegetation may reduce maximum water temperatures and provide refugia for temperature-sensitive aquatic organisms. Longitudinal cooling gradients have been observed during the daytime for stream reaches shaded by coniferous trees downstream of clear cuts or deciduous woodland downstream of open moorland. However, little is known about the energy exchange processes that drive such gradients, especially in semi-natural woodland contexts without confounding cool groundwater inflows. To address this gap, this study quantified and modelled variability in stream temperature and heat fluxes along an upland reach of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) where riparian land use transitions from open moorland to semi-natural, predominantly deciduous woodland. Observations were made along a 1050 m reach using a spatially distributed network of 10 water temperature data loggers, 3 automatic weather stations and 211 hemispherical photographs that were used to estimate incoming solar radiation. These data parameterised a high-resolution energy flux model incorporating flow routing, which predicted spatio-temporal variability in stream temperature. Variability in stream temperature was controlled largely by energy fluxes at the water-column–atmosphere interface. Net energy gains occurred along the reach, predominantly during daylight hours, and heat exchange across the bed–water-column interface accounted for <1% of the net energy budget. For periods when daytime net radiation gains were high (under clear skies), differences between water temperature observations increased in the streamwise direction; a maximum instantaneous difference of 2.5 °C was observed between the upstream reach boundary and 1050 m downstream. Furthermore, daily maximum water temperature at 1050 m downstream was ≤1 °C cooler than at the upstream reach boundary and lagged by >1 h. Temperature gradients were not generated by cooling of stream water but rather by a combination of reduced rates of heating in the woodland reach and advection of cooler (overnight and early morning) water from the upstream moorland catchment. Longitudinal thermal gradients were indistinct at night and on days when net radiation gains were low (under overcast skies), thus when changes in net energy gains or losses did not vary significantly in space and time, and heat advected into the reach was reasonably consistent. The findings of the study and the modelling approach employed are useful tools for assessing optimal planting strategies for mitigating against ecologically damaging stream temperature maxima.
Highlights
River temperature dynamics are of increasing interest to the scientific community, environment managers and regulators (Hannah et al, 2008), given climate change predictions (e.g. van Vliet et al, 2011; Beechie et al, 2013) and associated consequences for water temperature and, thereby, aquatic ecosystems (Poole and Berman, 2001; Caissie, 2006; Webb et al, 2008; Wilby et al, 2010; Leach et al, 2012)
Results are presented in four sections: (1) prevailing hydrological and weather conditions, (2) observed spatio-temporal water temperature patterns, (3) riparian canopy density and net energy flux patterns, and (4) modelled spatio-temporal water temperature patterns
On 2 July net energy gains were markedly lower due to overcast skies and associated low solar-radiation receipt (Fig. 2). This data window allowed the investigation of the influence of contrasting energy gain conditions on the spatio-temporal variability of water temperature and energy flux
Summary
River temperature dynamics are of increasing interest to the scientific community, environment managers and regulators (Hannah et al, 2008), given climate change predictions (e.g. van Vliet et al, 2011; Beechie et al, 2013) and associated consequences for water temperature and, thereby, aquatic ecosystems (Poole and Berman, 2001; Caissie, 2006; Webb et al, 2008; Wilby et al, 2010; Leach et al, 2012). G. Garner et al.: What causes cooling water temperature gradients in a forested stream reach?. The presence of net energy gains led both Brown et al (1971) and Story et al (2003) to attribute the generation of cooling gradients to groundwater inputs that were thought to be underestimated by unrepresentative energy exchange measurements made at a single point within the reach Cooling gradients have been observed in forested reaches downstream of open land use in which groundwater inputs are considered to be minimal An explicit conceptualisation of the processes driving observed patterns of cooling in forested reaches without groundwater inputs is lacking; this is essential if stream managers are to plan future riparian planting strategies that maximise benefits at minimal cost
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