Ground cover plants to aid rainfall interception and improve soil moisture storage capacity.
The effects of rainfall intensification caused by climate change and rapid urban development (with subsequent loss of soil permeability) are likely to increase the incidence of urban flooding in future. The use of green infrastructure has been cited as helping mitigate this risk.
The potential of trees to capture rainfall and slow runoff has been well documented. Less interest has been shown in the roles of low-growing plants (i.e. shrubs, groundcovers) because they are perceived to intercept and store less moisture due to their overall lower surface areas (Tromble, 1983). Nevertheless, within an urban context, there may not be space for trees in all locations, and more information is required on the ability of ground cover plants to intercept rainfall, slow surface runoff and re-charge the soil’s capacity to store water after previous rainfall events (i.e. to ‘re-dry’ the soil through evapotranspiration). The advantage of low-growing groundcover plants is that they can be readily utilized across a range of different landscape typologies and scales, e.g. within driveways, along pavements, and on green roofs.
In this research, outdoor experiments were conducted to determine the water-holding capacity of low-growing shrub and herbaceous species, and to determine whether certain leaf traits affect raindrop capture and retention. A small range of genotypes with contrasting leaf types and traits (viz. Bergenia cordifolia, Vinca minor, Dianthus ‘Haytor White’) were chosen to determine the variability of leaves to retain water.
Scenarios were created to determine how much moisture was retained on the canopy of these plants when exposed to a 4 mm rainfall simulation, and when the soil was already at field capacity. Retention was determined by measuring runoff generated from the system.
Preliminary results suggest that a system with 1m2 of low-growing vegetation can retain between 37% and 49% of the applied rainfall, with the proportion varying depending on the traits associated with different genotypes. Fine-leaved species with linear-shaped leaves retain a proportionally greater volume of rainwater than broad-leaved species (largely agreeing with work on trees). This indicates that plant leaf types play an important role in influencing water retention in the canopy, and that the role of low-growing plants should not be neglected.
Careful selection of plants in implementing small-scale green infrastructure could potentially contribute to a relatively large runoff reduction.
In addition to investigating the volume of water retained in plant canopies, the ability to recharge this storage capacity between subsequent rainfalls is also under scrutiny. Plants not only help to detain, retain and store moisture, but a part of an important process is releasing moisture back into the atmosphere. Potential rates and values of evapo-transpiration of these plants will be investigated in future experiments.
It is hoped that this study will contribute to current scientific knowledge of the roles of leaf morphology and plant physiology in influencing stormwater management, and the potential application of small-scale green infrastructure interventions to help mitigate flooding risk. This research is lead by Siti Nur Hannah Ismail, PhD Candidate at the Department of Landscape, University of Sheffield, and supervised by Dr. Ross Cameron (Department of Landscape) and Dr. Virginia Stovin (Civil and Structural Engineering Department).
Tromble, J. M. (1983). Interception of rainfall by Tarbush. Journal of Range Management, 36(4), p. 525-526.