Abstract in Englisch:

Water temperature is a key driver for riverine biota and strongly depends on shading by woody riparian vegetation in summer. While the general effects of shading on daily maximum water temperature Tmₐₓ are well understood, knowledge gaps on the role of the spatial configuration still exist. In this study, the effect of riparian buffer length, width, and canopy cover (percentage of buffer area covered by woody vegetation) on Tmₐₓ was investigated during summer baseflow using data measured in seven small lowland streams in western Germany (wetted width 0.8–3.7 m). The effect of buffer length on Tmₐₓ differed between downstream cooling and heating: Tmₐₓ approached cooler equilibrium conditions after a distance of 0.4 km (~45 min travel-time) downstream of a sharp increase in canopy cover. In contrast, Tmₐₓ continued to rise downstream of a sharp decrease in canopy cover along the whole 1.6 km stream length investigated. The effect of woody vegetation on Tmₐₓ depended on buffer width, with changes in canopy cover in a 10 m wide buffer being a better predictor for changes in Tmₐₓ compared to a 30 m buffer. The effect of woody vegetation on Tmₐₓ was linearly related to canopy cover but also depended on daily temperature range Trₐngₑ, which itself was governed by cloudiness, upstream canopy cover, and season. The derived empirical relationship indicated that Tmₐₓ was reduced by −4.6°C and increased by +2.7°C downstream of a change from unshaded to fully shaded conditions and vice versa. This maximum effect was predicted for a 10 m wide buffer at sunny days in early summer, in streams with large diel fluctuations (large Trₐngₑ). Therefore, even narrow woody riparian buffers may substantially reduce the increase in Tmₐₓ due to climate change, especially in small shallow headwater streams with low baseflow discharge and large daily temperature fluctuations.