The impact of urban green space irrigation on water-heat-carbon nexus in the built environment

Urban green spaces, e.g., lawns, trees, and green roofs, have been widely used as nature-based solutions to mitigate urban heat stress, reduce cooling energy consumptions of buildings, as well as reduce urban carbon emissions. Urban green spaces can modify the surface energy partitioning and the water-heat-carbon nexus through biophysical activities such as evapotranspiration, photosynthesis, and respiration. Urban irrigation is essential to maintain the biophysical and environmental functions of urban green spaces, while its impact on the heat, moisture, and carbon exchanges between land and atmosphere lacks comprehensive quantitative analysis. In this study, we first conducted comprehensive field experiments in the city of Wuhan via a newly established urban eddy covariance (EC) tower as well as a newly established green roof measurement systems in Wuhan University and collected anthropogenic activity data in the EC footprint area including urban irrigation scheme, etc. Secondly, we built a state-of-the-art single-layer Urban Canopy Model (UCM) featuring an enhanced ecohydrological module for green space, a building energy module to account for anthropogenic heat and moisture emissions, etc. Then we evaluated the UCM against field measurements of stratified soil temperature and moisture, air temperature and humidity, as well as uprising sensible and latent heat fluxes. After model evaluation, we conducted sensitivity test to study the impact of different urban green space parameters, particularly the greening fractions, soil moisture, soil temperature, irrigation scheme, etc. on cooling, humidifying and carbon-saving potential. In addition, we also quantified the trade-offs between water consumption and building energy-saving consumption. Overall, this study provides practical reference for city planners and managers to build a green and low-carbon city.

 

Scheme of Urban Canopy Model