Impact of plant choice on rainfall runoff delay and reduction by hedge species

Abstract

Soil sealing and a decrease in vegetation cover in urban areas increase the likelihood and frequency of localised flooding. Populating the remaining green areas with vegetation, which can efficiently capture excess rainfall, is therefore important. We argue that urban hedges can be a useful tool in mitigating rainfall, so the understanding of optimal plant choice, and underlying traits which enable most rain attenuation, is needed. We tested the hypothesis that higher plant evapo-transpiration rates and larger canopy size can be linked with reduced rainfall runoff in urban hedge species. We first characterised seven hedge species grown in individual containers. These were both deciduous and evergreen species, with a range of inherent canopy sizes and water requirements. We assessed their plant water use, leaf stomatal conductance, canopy rainfall retention, and runoff delay and reduction capacity. The species showing highest and lowest potential for runoff reduction were then investigated for their outdoor performance, when planted in a hedge-like form. Our findings suggest that—after 3 days between rainfall events—species such as Cotoneaster and Crataegus with larger and wide canopies, and with high evapo-transpiration/water use rates, delayed the start of runoff (by as much to 10–15 min compared to bare substrate) as well reduced the volume of rainfall runoff. For example, < 5% of the applied rainfall had runoff with Cotoneaster and Crataegus, compared with > 40% in bare substrate. Substrate moisture content at the time of rainfall (which is linked to plants’ ET rate) was the key explanatory variable.