The Standardized Precipitation Evapotranspiration Index (SPEI) is widely used for climatological and hydrological studies, in which the estimation of potential evapotranspiration (PET) is of great importance. As many different models exist in estimating PET, the question that arises is in which way the selection of the PET model affects the calculated SPEI and the drought assessment. This study, on the basis of evaluating drought conditions over the Hexi Inland River Basin in China with long-term climate data of 18 stations by using SPEI, compared three types and eight kinds different PET models with respect to their sensitivity to the calculation of SPEI, and to drought events and drought characteristics. The results showed that the study area experienced a drying trend over the past 56 years, and the extreme drought events occurred more frequently after 2000 as a whole. All the investigated PET models were sensitive to the estimation of SPEI and to the drought assessment. When considering the alternatives of the Thornthwaite model in the calculation of SPEI for drought identification, the Blaney–Criddle equation among the temperature-based models and the Makkink equation among the radiation-based models are recommended due to the comparable results in determining the drought trends, drought events, and drought characteristics.
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