Effects of patchy shade on stream water temperature: how quickly do small streams heat and cool?

Abstract

Summer field observations in five 2nd order streams (width 1–2 m, depth 5–15 cm, velocity 5–10 cm s–1) in Western Australia and south-east Queensland showed that daily maximum temperatures changed by ±4°C over distances of 600–960 m (travel time 2–3 h) immediately downstream from 40–70% step changes in riparian shade. There was a strong linear relationship between the rate of change of daily maximum temperature and the change of shade such that downstream from a 100% change of shade the heating/cooling rates are ±4°C h–1 and ±10°C km–1 (upper bound ±6°C h–1 and ±15°C km–1) respectively. These high rates only apply over short distances and travel times because downstream water temperatures adjust to the new level of shade and reach a dynamic equilibrium. Shade was too patchy in the study streams to measure how long water takes to reach equilibrium, however, using an existing computer model, we estimate that this occurs after ~1200 m (travel time 4 h). Further modelling work is desirable to predict equilibrium temperatures under given meteorological, flow and shade conditions. Nevertheless, landowners and regulators can use this information to determine whether the presence/absence of certain lengths of bankside shade are likely to cause desirable/undesirable temperature decreases/increases.

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

“…In the present study, the influence of the mean VSIs of different lengths on WT was tested for a stream with 1-1.2 m 3 /s discharge and points to 4-7 km (peak correlation coefficient for WT max of À0.82 at 6 km VSI length) as being an appropriate length. The results are not directly comparable to those of most existing studies which deal with smaller streams with a discharge of up to 0.03 m 3 /s (Story et al, 2003;Johnson, 2004;Rutherford et al, 2004;Moore et al, 2005;Gomi et al, 2006). Rutherford et al (1997) mentioned 1st order streams need 250 m to 500 m; 2nd order 500 m to 1.5 km and 3rd order 1.5 km to 5 km unshaded sections in order to heat the water by 5°C.…”

“…Some studies were undertaken to quantify the influence of riparian vegetation on river water temperature, however, they have mainly explored small rivers, with a discharge of up to only 30 l/s (Story et al, 2003;Johnson, 2004;Rutherford et al, 2004;Moore et al, 2005;Gomi et al, 2006). The most widely discussed vegetation parameters of these studies are buffer width, vegetation height and its density.…”

“…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).…”

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

“…The Lagrangian modelling approach (after Bartholow, 2000;Boyd and Kasper, 2003;Rutherford et al, 2004;Westhoff et al, 2007Westhoff et al, , 2010Leach and Moore, 2011;MacDonald et al, 2014a, b) divided the reach into a series of segments (s) bounded by nodes (indexed by j ). For each time step, t (i.e.…”

“…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. Gomi et al, 2006).…”