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

摘要

Previous studies have suggested that shading by riparian vegetation mayreduce maximum water temperatures and provide refugia for temperature-sensitive aquatic organisms. Longitudinal cooling gradients have beenobserved during the daytime for stream reaches shaded by coniferous treesdownstream of clear cuts or deciduous woodland downstream of open moorland.However, little is known about the energy exchange processes that drive suchgradients, especially in semi-natural woodland contexts without confoundingcool groundwater inflows. To address this gap, this study quantified andmodelled variability in stream temperature and heat fluxes along an uplandreach 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 mreach using a spatially distributed network of 10 water temperaturedata loggers, 3 automatic weather stations and 211 hemisphericalphotographs that were used to estimate incoming solar radiation. These dataparameterised a high-resolution energy flux model incorporatingflow routing, which predicted spatio-temporal variability in streamtemperature. Variability in stream temperature was controlled largely byenergy fluxes at the water-column–atmosphere interface. Net energy gainsoccurred along the reach, predominantly during daylight hours, and heatexchange across the bed–water-column interface accounted for <1%of the net energy budget. For periods when daytime net radiation gains werehigh (under clear skies), differences between water temperature observationsincreased in the streamwise direction; a maximum instantaneous difference of2.5 °C was observed between the upstream reach boundary and 1050 mdownstream. Furthermore, daily maximum water temperature at 1050 mdownstream was ≤1 °C cooler than at the upstream reachboundary and lagged by >1 h. Temperature gradients were notgenerated by cooling of stream water but rather by a combination of reducedrates of heating in the woodland reach and advection of cooler (overnightand early morning) water from the upstream moorland catchment. Longitudinalthermal gradients were indistinct at night and on days when net radiationgains were low (under overcast skies), thus when changes in net energygains or losses did not vary significantly in space and time, and heatadvected into the reach was reasonably consistent. The findings of the studyand the modelling approach employed are useful tools for assessing optimalplanting strategies for mitigating against ecologically damaging streamtemperature maxima.