Modeling Stream Shade: Riparian Buffer Height and Density as Important as Buffer Width¹

Abstract

DeWalle, David R., 2010. Modeling Stream Shade: Riparian Buffer Height and Density as Important as Buffer Width. Journal of the American Water Resources Association (JAWRA) 46(2):323‐333. DOI: 10.1111/j.1752‐1688.2010.00423.x Abstract: A theoretical model was developed to explore impacts of varying buffer zone characteristics on shading of small streams using a path‐length form of Beer’s law to represent the transmission of direct beam solar radiation through vegetation. Impacts of varying buffer zone height, width, and radiation extinction coefficients (surrogate for buffer density) on shading were determined for E‐W and N‐S stream azimuths in infinitely long stream sections at 40°N on the summer solstice. Increases in buffer width produced little additional shading beyond buffer widths of 6‐7 m for E‐W streams due to shifts in solar beam pathway from the sides to the tops of the buffers. Buffers on the north bank of E‐W streams produced 30% of daily shade, while the south‐bank buffer produced 70% of total daily shade. For N‐S streams an optimum buffer width was less‐clearly defined, but a buffer width of about 18‐20 m produced about 85‐90% of total predicted shade. The model results supported past field studies showing buffer widths of 9‐11 m were sufficient for stream temperature control. Regardless of stream azimuth, increases in buffer height and extinction coefficient (buffer density) were found to substantially increase shading up to the maximum tree height and stand density likely encountered in the field. Model results suggest that at least 80% shade on small streams up to 6‐m wide can be achieved in mid‐latitudes with relatively narrow 12‐m wide buffers, regardless of stream azimuth, as long as buffers are tall (≈30 m) and dense (leaf area index ≈6). Although wide buffers may be preferred to provide other benefits, results suggest that increasing buffer widths beyond about 12 m will have a limited effect on stream shade at mid‐latitudes and that greater emphasis should be placed on the creation of dense, tall buffers to maximize stream shading.

Spatial and Temporal Variation in Soil Nitrous Oxide Emissions from a Rehabilitated and Undisturbed Riparian Forest

“…Tree density is often overlooked when evaluating nutrient and greenhouse gas dynamics in riparian forests. However, tree density plays a crucial role in understanding riparian forest function including nutrient input via litter fall, nutrient uptake, and shading of the forest floor (DeWalle, 2010). For example, Spies and Franklin (1991) found that young (40 yr) Douglas fir [ Pseudotsuga menziesii (Mirb.)…”

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

“…These are the parameters most commonly studied concerning global radiation reduction (Zwieniecki and Newton, 1999;DeWalle, 2010;Sridhar et al, 2004) but there are no clear results concerning the ranked importance of these. The VSI results in a significant correlation which had the same or even better correlation coefficients with WT as the individual vegetation parameters.…”

“…For each 50 m cross-section, VSI was calculated that integrates the parameters vegetation height (h R ), vegetation width (w), and vegetation density (d), as these parameters were found to influence water temperature (WT) most (DeWalle, 2010;Holzapfel et al, 2013). For the VSI of a northwards or southwards flowing river, all vegetation parameters were calculated as mean values of the orographic left and right river bank.…”

“…Buffer widths for east-west oriented rivers necessitate a minimum buffer width of 6-7 m, whereas for north-south orientated rivers the buffer width needs to be 18-20 m, and for meandering rivers no more than 12 m. According to Sridhar et al (2004), vegetation width exceeding 30 m distance has hardly any further influence on stream water temperature. 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.…”

“…They recorded total energy gains dominated by a net short-wave radiation of 97.6%. Solar radiation reaching the stream is influenced by different topographic factors, including the prevailing riparian vegetation buffer, which can decrease incoming energy by up to 95% (Rutherford et al, 1997;Moore et al, 2005;DeWalle, 2010). During summer heatwave periods in particular, solar energy plays a key role in influencing stream energy heat budget, and consequently stream water temperature (Johnson, 2004;Leach and Moore, 2010;Groom et al, 2011).…”