Guidelines for Riparian Vegetative Shade Restoration Based Upon a Theoretical Shaded‐Stream Model¹

Abstract

Guidelines for riparian vegetative shade restoration were developed using a theoretical model of total daily radiation received by a shaded stream. The model assumed stream shading by nontransmitting, vertical or overhanging, solid vegetation planes in infinitely long reaches. Radiation components considered in the model were direct beam shortwave on the stream centerline, diffuse atmospheric shortwave, shortwave reflected by vegetation, atmospheric longwave, and longwave emitted by vegetation. Potential or extraterrestrial shortwave irradiation theory was used to compute beam shortwave radiation received at the stream centerline, and view factor theory was used to compute diffuse radiation exchange among stream, vegetation, and atmospheric planes. Model shade effects under clear skies were dominated by reductions in receipt of direct beam shortwave radiation. Model shade effects with cloudy skies were dominated by the “view factor effect” or the decreases in diffuse shortwave and longwave radiation from the atmosphere balanced against increases in longwave radiation from vegetation. Model shade effects on shortwave radiation reflected by vegetation were found to be negligible. The model was used to determine the vegetation height (H) to stream width (W) ratios needed to achieve 50, 75, and 90 % shade restoration for mid‐latitude conditions on clear and cloudy days. Ratios of vegetation height to stream width, for dense nontransmitting vegetation, generally ranged from 1.4 to 2.3 for 75% shade restoration at a mid‐latitude site (40°N). The model was used to show H/W needed for E‐W vs. N‐S stream azimuths, varying stream latitudes between 30° and 50°N, channels with overhanging vegetation, channels undergoing width changes, as well as the limits to shade restoration on very wide channels.

Which offers more scope to suppress river phytoplankton blooms: Reducing nutrient pollution or riparian shading?

“…Indeed the findings of Ghermandi et al (2009) and others (e.g. Davies-Colley and Quinn, 1998;Boothroyd et al, 2004;Caissie et al, 2007) are consistent with the Dewalle (2008) model but all these study sites are dissimilar to the Ouse, either in terms of latitude or river width. In the Ouse, the reduction in intensity of solar radiation reaching the water column due to tree shading was assumed to be 39%, as estimated using the method of DeWalle (2008).…”

“…Davies-Colley and Quinn, 1998;Boothroyd et al, 2004;Caissie et al, 2007) are consistent with the Dewalle (2008) model but all these study sites are dissimilar to the Ouse, either in terms of latitude or river width. In the Ouse, the reduction in intensity of solar radiation reaching the water column due to tree shading was assumed to be 39%, as estimated using the method of DeWalle (2008). The method calculates shading as a function of site latitude, river flow direction and the ratio of the height of the riparian vegetation to the width of the riverbank.…”

“…Whilst some experimental studies, as reviewed by Ghermandi et al (2009), appear to suggest that effects of tree shading on light availability are unlikely to be substantial in rivers as wide as those in the Ouse network, a recent model study (Dewalle, 2008) goes into much greater systematic and quantitative detail, indicating such a view to be unduly pessimistic. Indeed the findings of Ghermandi et al (2009) and others (e.g.…”

Modelling the time course of shade, temperature, and wood recovery in streams with riparian forest restoration

“…our simulations indicated that shading of streams depends strongly on stream size, because the ratio of canopy height to stream width is the single most important factor controlling stream light exposure (Davies-Colley & Rutherford 2005;DeWalle 2008). as a consequence, small streams in which thermal regime is most severely disrupted by light exposure following riparian clearance, recover most rapidlybecause trees growing on their banks rapidly achieve occlusion of a large proportion of the canopy gap.…”

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

“…However, the height and density of the riparian vegetation have an equal or even higher impact on water temperature (Sridhar et al, 2004;DeWalle, 2010). DaviesColley et al (2009) andDeWalle (2008) showed that the ratio of canopy height to stream width has particular high influence on stream light exposure and is therefore notably relevant.…”