Modelling pavement evaporative cooling to mitigate heat wave

Abstract

As the intensity and frequency of urban heat wave increases in Europe, evaporative cooling from watering pavements has been considered as a promising strategy to regulate urban temperature. The aim of this research is to include evaporative cooling modelling and enhance the surface temperature model SURF-TEMP, which is in development at the Swiss Federal Institute of Aquatic Science and Technology (Eawag), to predict the optimal use of water resources towards mitigating urban heat. In order to test various evaporation models found in literature, we first developed a simple Python model to evaluate ten evaporation models. Differences among the simulated evaporative heat fluxes and surface temperature reduction can be around two folds under the same simulation condition for different evaporative cooling models. Besides the evaporation models, three equations to obtain the convective heat transfer coefficient were tested. We tested their uncertainties and determined the equation that provides the most neutral estimation on convective heat transfer coefficient. We proceeded to conduct various simulations to understand the sensitivity of these evaporation models with respect to four factors (wind speed, relative humidity of air, initial surface temperature, and initial water height). These factors influence the evaporative cooling efficiency and therefore the optimal watering rate. Two evaporative models that represent the higher and lower limit of the simulated evaporative heat flux are incorporated into SURF-TEMP. Results show that the model that estimates the highest evaporative heat flux is in good agreement with the lab measurement published by (Parison et al.,2020) when watering rate is equal or above 1 mm per hour. When watering rate falls below 0.75 mm per hour, the models studied underestimate the evaporative heat flux and surface temperature reduction. Discrepancies during low watering rate are caused by the absence of modelling of water conduction and infiltration with the pavement, non-linear relation of evaporation rate with vapor pressure difference, and the inherent error between simulation and experimental results even under dry condition.