A note on some uncertainties associated with Thornthwaite's aridity index introduced by using different potential evapotranspiration methods.

Potential evapotranspiration (PET) is a key parameter for climate classification and aridity assessment. The widely used UNEP (1992) classification system is based on the estimation of the aridity index AI, which requires annual average values of precipitation P and PET. For the calculation of PET the Thornthwaite's (1948) formula is, in principle, suggested. Recent studies use more advanced and accurate methods for PET estimation but apply to AI the same thresholds proposed by UNEP for aridity classification. This work deals with the uncertainties introduced by the use of different PET methods in the estimation of the aridity index AI. Specifically, the Hargreaves-Samani (HS) method and four of its modifications, three modifications of Thornthwaite's formula and the equation of Hamon, are evaluated against the widely used Thornthwaite's original method, by assessing their impact on the AI. Climatic data as monthly average values of at least 30 years of measurements from 122 stations in the Greek peninsula are used. Results show that AI is highly affected by the PET method adopted, resulting thus to changes in climatic classification of a region. Further, results imply the need for an adjustment of the threshold values that determine aridity classes according to the method each time adopted. Therefore, new threshold values for the aridity classes are developed and presented in this work. The proposed threshold values cover a range of sites that belong to semi-arid SA, sub-humid SH and humid H aridity classes of the Greek peninsula. • UNEP Aridity Index AI is highly affected by the PET method adopted for its estimation. • 10 PET methods effect on AI is assessed for 122 sites over the Greek peninsula. • Uncertainty in aridity classification under Mediterranean climatic conditions. • Most sites were categorized to more arid classes compared to Thornthwaite's approach. • AI adjustment coefficients and threshold values are proposed for each PET method. [ABSTRACT FROM AUTHOR]