An evapotranspiration model, based on the FAO Penman-Monteith method, taking into account the surface temperature, has been developed and implemented in the ground temperature model of the CitySim software. A case study was conducted in a district of Geneva, Switzerland, which consists of 704 buildings and 714 ground surfaces, to understand the influence of evapotranspiration on the ground surface temperature and water requirement for the irrigation of these areas at urban scale. The simulations were conducted to investigate the cooling potential of green areas in present and future climate scenarios. To handle the large quantity of inputs needed by CitySim and the large amount of results produced, a PostgreSQL database and QuantumGIS software have been used for this case study.