Assessing the Influence of PET Calculation Method on the Characteristics of UNEP Aridity Index Under Different Climatic Conditions throughout Iran.

Accurate assessment of changes in climate conditions and their impacts on different sectors including agriculture, animal husbandry, wildlife, environment, and so forth can affect the appropriate natural disaster management. The aridity index defined by the United Nations Environmental Programme (UNEP) is one of the most widely used indices in the evaluation of climate conditions based on the ratio of precipitation (P) and potential evapotranspiration (PET) parameters. In this study, the impact of changes on PET calculation methods (6 PET calculation methods) in values of UNEP aridity index were compared by analyzing the data series of 28 synoptic stations with various climate conditions on monthly and seasonal time scales in Iran from 1967 to 2017. Therefore, it is determined to apply the FAO Penman–Monteith (FPM) equation as the reference method to measure PET. According to the results, based on the GEE method (Backward Generalized Estimating Equations) within monthly and seasonal time scales, calculated UNEP aridity index based on Hargreaves–Samani (HS), Jensen–Haise (JH), and Blaney–Criddle (BC) equations, respectively had the most similarities with calculated UNEP Aridity Index based on FPM method. The clustering analysis indicated that values of calculated UNEP aridity index within the monthly and seasonal time scales using JH, HS and BC equations are strikingly similar to the values of calculated UNEP aridity index using the FPM method. The average similarity rates of the calculated UNEP aridity index using JH and HS equations and by FPM method were 98.35% and 98.34% within a 1-month time scale and 98.11% and 98.09% within a 3-month time scale, respectively. [ABSTRACT FROM AUTHOR]