In recent years large-scale conversions of land use have been taking place worldwide to sustain the ever-growing population. As a result, parameters such as vegetation type, air temperature, relative humidity, soil moisture, and movement of the wind have been markedly affected. Many studies have examined the impact of urbanisation on temperature or relative humidity, but few have explored whether or not potential evapotranspiration (PET) is influenced by changes in population density and urbanisation. In this context, in the present study we attempted to find answers to three key questions. Is there a relationship between PET and urbanisation? If so, which parameter, able to change the pattern of PET, is the most sensitive to urbanisation in a given region? Lastly, can the impact of urbanisation on PET be measured without the application of any of the subjective models currently available and used worldwide to solve similar problems? Through analysing the recent literature, we found that a distinct relationship does indeed exist between PET and urbanisation. The most important parameter affected by urbanisation that also influences PET was investigated by the application of multi-criteria decision-making, and this was found to be air temperature. Finally, a neurogenetic model was applied to reveal the vulnerability of evapo-transpiration to urbanisation in four cities with differing levels of population density. The results confirmed that the vulnerability is greater in cities with denser populations and greater levels of urbanisation.