What causes cooling water temperature gradients in forested stream reaches?

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.

Modelling temperature change downstream of forest harvest using Newton's law of cooling

“…A previous work has shown that q radiation may be so reduced within heavily shaded reaches that it no longer dominates the net heat flux (e.g. Garner et al ., ). Considering this result, we hypothesized that for sections of streams that are sufficiently shaded and lacking tributaries, the q radiation and q mixing contributions to ∑ q may be so reduced relative to q convection and q conduction that NLC becomes applicable.…”

“…Stream temperature change due to forest harvest has been widely studied at both local scales (e.g. Gray and Edington, ; Brown and Krygier, ; Baillie et al ., ; Gomi et al ., ; Gravell and Link, ; Groom et al ., , ; Janisch et al ., ; Rex et al ., ; DaSilva et al ., ; Kibler et al ., ) and downstream scales (Brown et al ., ; Caldwell et al ., ; Zwieniecki and Newton, ; Story et al ., ; Rutherford et al ., ; Cole and Newton, ; Garner et al ., ; Johnson and Wilby, ). This abundance of studies reflects the concern over stream temperature impacts to aquatic ecosystems and has led to the evolution of stream protection rules for managed forests (e.g.…”

“…Efforts to quantify these site‐specific parameters necessary for estimating the heat flux components governing downstream temperature response have used combined reach‐scale micrometeorological, hydrological and stream temperature measurements (e.g. Roth et al ., ; Garner et al ., ). While these studies provide deep insight into processes occurring at a reach scale, the measurements they require are difficult and costly to repeat across the landscape.…”

“…For example, some studies indicate that groundwater is an important component of observed cooling (e.g. Brown et al ., ; Story et al ., ), while others observed cooling in the absence of significant groundwater or tributary input (Cole and Newton, ; Garner et al ., ). Achieving predictability in the physical processes governing reach‐scale downstream temperature response to forest harvest and other events that reduce riparian canopy cover is an important step in understanding the thermal behaviour of streams at the watershed scale.…”

T‐NET, a dynamic model for simulating daily stream temperature at the regional scale based on a network topology

“…River temperature controls are multivariate and nested at regional, sub‐basin, reach or site‐specific scale (Hannah and Garner, ). At regional scale, climate (solar radiation and air temperature) drives the thermal regime of rivers, but at smaller scales, the riparian shading (Moore et al, ) and groundwater inputs could strongly influence the local water temperature (Garner et al, ). However, most of the previous studies have been restricted to the sub‐basin scale or limited to studding the influence of site‐specific factors at a local scale, and there is a lack of research on spatial and temporal variability and controls on river temperature at a regional scale (Hannah and Garner, ).…”