Session C1 - The role of plants in SCMs 

Bioretention systems are increasingly being used to manage stormwater in municipalities. If their hydrological performance is relatively well characterized, the contribution of plants in the performance of these systems remains to be studied, especially in northern climates. For example, a longer retention period in the system could stimulate plant growth and thus improve the hydrological performance of the system. In a greenhouse experiment, four perennial species (Deschampsia cespitosa, Hemerocallis fulva, Agastache foeniculum and Iris versicolor) were planted in mesocosms equipped with a drainage pipe controlled by a valve. Four growth cycles, each comprising a saturation period (0, 24, 48 or 96 h) followed by a 14-day drying period, were carried out, followed by three simulated rainfall cycles (25 mm of water in 32 h).  Plant growth (height and width) was measured at each cycle while the drained water was measured after each retention period and during each rainfall period. Preliminary results showed that a longer retention period improved the hydrological performance of the system by reducing drained water volumes. Plant species, however, had different abilities to withstand water saturation and drought conditions between events.

Stormwater tree pits are nature-based solutions for stormwater management, considered suitable for highly urbanized areas, and increasingly used. The contribution of the tree to the water balance (evapotranspiration) remains unclear due to measurement challenges. Our case study consists in the monitoring of five existing stormwater trees from the Vauban Street in Lyon (France). These five trees were retrofitted by improving the infiltration capacity with a trench, to increase collection of runoff from neighbouring streets and sidewalks. The study site also includes two control trees (receiving no runoff) located in the same street. The monitoring setup consists of a weather station, water level sensors at the inlet, soil water content and matric potential sensors in the tree pit, and sap flow sensors. The objective of the study is to characterize the overall water balance of the stormwater and control trees. This extended abstract presents preliminary results and feedbacks after 10 months' use of the system.

Implementing blue-green infrastructure (BGI) is a key strategy for urban adaptation to climate change. Here, we employ an integrated modelling framework in which each model component predicts specific performance indicators. Using this approach, we assess how BGI and surface decoupling measures can mitigate urban flood risk, improve the water balance, and reduce heat stress in the city of Innsbruck, Austria. Our results show that BGI measures help to reduce flood volumes and flooded area, while improving the water balance, particularly through increased groundwater recharge, additionally favouring drought resilience. However, even with citywide decoupling of 30% of impervious surfaces using BGI designed in accordance with Austrian guidelines, the increase in citywide evapotranspiration is modest, resulting in no measurable reduction in the Universal Thermal Climate Index (UTCI), a human heat stress indicator, at city nor inner-city district scale. Although local greening can lower UTCI by up to 2.5 °C and additional tree shading by as much as 13 °C depending on meteorological conditions, these cooling effects remain highly localised. Following current Austrian design guidelines to manage the stormwater from 30% decoupled area using BGI converts only 3.2-5.1% into green space, which is insufficient to have a measurable effect on citywide heat stress reduction. Our findings emphasise on the strong influence of shading for local heat mitigation and on combining water-managing BGI with shading vegetation as an effective strategy for enhancing urban climate resilience.