Can catchment scale implementation of green infrastructure protect the flow regime of an urban stream facing urbanisation and climate change ? A modelling study in Lyon, France.

Urbanisation and climate change jeopardize the health of peri-urban streams, by yielding decreased baseflow and increased peakflows. Green infrastructure can help protecting and even restoring urban streams by storing, infiltrating and losing urban runoff to evapotranspiration. However, whether green infrastructure implementation at the catchment-scale (and how much) can counter future urbanisation and climate change remains a question of interest for urban managers. We modelled the hydrology of a 20 km² peri-urban catchment in the western suburbs of Lyon, France with the physically-based, spatially distributed hydrological model J2000P, at the hourly time step. We created 12 future urbanisation scenarios with stepwise increases of impervious cover as well as 36 climate change scenarios based on one climate projection (CNRM-CM5-ARPEGE- ALADIN63-RCP 8.5) and the observed temperature and precipitation records from the city of Orange, which is located 200 km south of Lyon in France. We applied a delta method to transform current hourly rainfall and evapotranspiration timeseries into potential future climate timeseries. We coupled these scenarios to stormwater management strategies, through the integration of a site-scale model of green infrastructure into J2000P. Five stormwater management strategies with increasing implementation of green infrastructure were tested: from ‘no green infrastructure’ to ‘all impervious areas drained into green infrastructure’. 640 scenarios coupling urbanisation, climate and stormwater management scenarios were simulated. For each simulation a range of hydrological indicators were calculated. We found that catchment-scale implementation of green infrastructure could mitigate the hydrological impacts of urbanisation. Sewer overflow were particularly sensitive to green infrastructure and urbanisation. Green infrastructure was however unable to mitigate the impact of climate change on the stream flow regime, because green infrastructure only impacted the urban parts of the catchment that accounted for less than 15% of the whole catchment. Non-urban areas (forests, pastures), which contributed very strongly to the flow regime, were impacted by climate change but not significantly by urban stormwater management strategies. These results can inform urban planners and water managers of the great potential of green infrastructure (reduction of sewer overflows, compensation for urbanisation) but also its limitations (little impacts on catchment scale induced flow peaks and droughts).