What causes cooling water temperature gradients in a forested stream reach?

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-columnatmosphere 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.

The influence of riparian vegetation shading on water temperature during low flow conditions in a medium sized river

“…On a clear day, the maximum values of the energy balance differed by 159 W/m 2 between areas without vegetation and fully vegetated areas, whereas on cloudy days differences decreased to 39 W/m 2 . Garner et al (2014) also recorded the dramatically reduced energy gains of the river at vegetated sites compared to open sites. These differing energy inputs to the river were reflected in the different WT max during the clear sky days.…”

“…Cooling occurs when energy gains are reduced by riparian vegetation, which minimizes water temperature increase, so that the influence of cooler water is prevalent (Garner et al, 2014). The cooling effect is greater at high water temperatures, typically occurring during heatwaves and at daily temperature peaks (Moore et al, 2005) whereas there is only a minor effect of vegetation shading on daily temperature minima (Rutherford et al, 2004).…”

“…The effect of shading might be different for larger rivers where water temperature fluctuations are normally lower (Poole and Berman, 2001) and solar radiation may play a different role, but these effects are not fully explored yet. Moreover, studies investigating riparian shade and its impact on stream water temperatures have been carried out in most temperate regions of the world, but up to date only a few studies have been conducted in Europe (Ghermandi et al, 2009;Broadmeadow et al, 2011;Garner et al, 2014;. So far, studies, focusing on effects of riparian vegetation on water temperature of medium sized rivers are missing, especially for Central European climates where climate change impacts are predicted to be high (Kovats et al, 2014;Laaha et al, 2016).…”

Can riparian vegetation shade mitigate the expected rise in stream temperatures due to climate change during heat waves in a human-impacted pre-alpine river?

“…3f), while during the V100 scenario the corresponding figure was only 91 W m −2 . The positive energy balance can explain the gradual warming of the stream temperature along the river (Garner et al, 2014) which can be seen in Fig. 3g.…”

“…Net shortwave radiation is the dominant energy input causing diurnal and seasonal water temperature variability. Longwave radiation flux (Benyaha et al, 2012) as well as the turbulent fluxes of evaporation and convection, which are controlled by air temperature, vapour pressure, wind speed and net radiation, play an important role (Caissie et al, 2007;Garner et al, 2014;Hannah et al, 2008;Johnson, 2004).…”