Woody buffer effects on water temperature: The role of spatial configuration and daily temperature fluctuations

Abstract

AbstractWater 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.