Thornthwaite's approach for assessing aridity changes during the last seven decades in the urban environment of Heraklion-Crete in Greece

Changes in aridity, especially to more arid classes, can have a strong impact on vegetation development in urban environments affecting also the inhabitants’ health and quality of life. In this work, water related parameters (i.e. Potential evapotranspiration PET, water deficit D and surplus S) and aridity index AI were examined according to Thornthwaite's approach, in the city of Heraklion – Crete in Greece during the period 1955-2017. The decadal analysis of seven decades (1950s to 2010s) indicates that S remains negligible, however PET and D have increased values varying from 938mm in 1980s to 1007mm in 2010s for PET and from 396mm in 1960s to 568mm in 2010s for D, whereas precipitation has decreased presenting maximum in 1960s (579mm) and minimum in 2010s (422mm). Therefore, the recent decade (2010s) is more arid compared to the past, displaying the lowest AI value (0.42). During the previous decades, AI was much higher with values between 0.60 in 1960s and 0.44 in 1990s, indicating that the region was at the edge of the threshold (0.50) for changing aridity zone from sub-humid (SH) to semi-arid (SA), according to UNEP’s classification.