Inter‐annual variability in the effects of riparian woodland on micro‐climate, energy exchanges and water temperature of an upland Scottish stream

Abstract

Abstract:The influence of riparian woodland on stream temperature, micro-climate and energy exchange was investigated over seven calendar years. Continuous data were collected from two reaches of the Girnock Burn (a tributary of the Aberdeenshire Dee, Scotland) with contrasting land use characteristics: (1) semi-natural riparian forest and (2) open moorland. In the moorland reach, wind speed and energy fluxes (especially net radiation, latent heat and sensible heat) varied considerably between years because of variable riparian micro-climate coupled strongly to prevailing meteorological conditions. In the forested reach, riparian vegetation sheltered the stream from meteorological conditions that produced a moderated micro-climate and thus energy exchange conditions, which were relatively stable between years. Net energy gains (losses) in spring and summer (autumn and winter) were typically greater in the moorland than the forest. However, when particularly high latent heat loss or low net radiation gain occurred in the moorland, net energy gain (loss) was less than that in the forest during the spring and summer (autumn and winter) months. Spring and summer water temperature was typically cooler in the forest and characterised by less inter-annual variability due to reduced, more inter-annually stable energy gain in the forested reach. The effect of riparian vegetation on autumn and winter water temperature dynamics was less clear because of the confounding effects of reach-scale inflows of thermally stable groundwater in the moorland reach, which strongly influenced the local heat budget. These findings provide new insights as to the hydrometeorological conditions under which semi-natural riparian forest may be effective in mitigating river thermal variability, notably peaks, under present and future climates.

Human Impact on Spatial Water Temperature Variability in Lowland Rivers: a Case Study from Central Poland

“…This confi rmed the conclusions of [27] that river water temperatures are not very sensitive to sewage infl ows in warmer periods, mainly due to the fact that infl ow temperatures are similar to those of natural fl ow. A clear increase of mean, maximum, and minimum water temperature metrics, although generally smaller in their magnitudes than those from sewage infl ow, were observed also due to the lack of riparian trees and shrubs resulting from channel regulation (between U6 and U7), which was responsible for short-wave radiation heating of the water surface [18,28]. In turn, there was a noted cooling effect in terms of mean and maximum temperature metrics between sites S3 and S5-S6 due to the appearance of riparian shade in the nature reserve section.…”

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 (e.g. Brown et al, 1971;Story et al, 2003;Moore et al, 2005;Leach and Moore, 2011;Garner et al, 2014a). Cooling gradients have also been observed in forested reaches downstream of open land use in which groundwater inputs are considered to be minimal (e.g.…”

“…Heat from fluid friction is negligible in this reach (see Garner et al, 2014a) and was therefore omitted. Herein, energy fluxes are considered to be positive (negative) when directed toward (away from) the water column.…”

“…Turbulent energy exchanges were calculated from micrometeorological measurements using commonly used methods (e.g. Webb and Zhang, 1997;Hannah et al, 2004Hannah et al, , 2008Moore, 2010, 2014;MacDonald et al, 2014c;Garner et al, 2014a); these equations are semiempirical (Ouellet et al, 2014) and thus do contain parameters, but they are not sitespecific. A sensitivity analysis was not performed on these parameters because, in the absence of direct measurements of latent and sensible heat (e.g.…”

“…Numerous studies have demonstrated that the presence of riparian woodland can decrease diurnal variability and mean and maximum stream temperatures (e.g. Malcolm et al, 2008;Brown et al, 2010;Roth et al, 2010;Imholt et al, 2012;Garner et al, 2014a) or, conversely, that forest removal results in temperature increases (e.g. Macdonald et al, 2003;Rutherford et al, 2004;Danehy et al, 2005;Moore et al, 2005; Published by Copernicus Publications on behalf of the European Geosciences Union.…”