Cooling streams with riparian trees: Thermal regime depends on total solar radiation penetrating the canopy

Abstract

Riparian planting is often recommended for stream restoration, notably to mitigate solar heating of stream waters. However, plant leaves shade photosynthetically active radiation (PAR, 400–700 nm) more efficiently than the near-infrared (NIR, 700–3000 nm) wavelengths that comprise about half of incident solar radiation and so contribute equally to water heating. Total solar radiation (NIR + PAR) exposure at the reach scale is needed to avoid bias when predicting the thermal response of streams. In this article, we alert stream ecologists and riparian restorers to the water heating contribution of NIR and provide a means to account for both NIR and PAR in total solar radiation penetrating riparian canopies. We used spectral pyranometers to simultaneously measure total solar radiation and its NIR component under different tree canopies over a wide range of shade conditions as indexed by PAR exposure. Measurements were made during full overcast (diffuse lighting) conditions so as to ‘sample’ transmission of radiation through the complete canopy. The NIR proportion of sub-canopy solar radiation increased steadily with increasing shade and was appreciably greater under willow than pine canopies, while NZ native broadleaf rainforest had intermediate (rather variable) NIR content. Our trend lines for different tree canopies permit total sub-canopy solar radiation to be estimated from PAR exposure for unbiased modelling of stream thermal regimes.