Assessment of the effect of PET calculation method on the Standardized Precipitation Evapotranspiration Index (SPEI)

Abstract

Standardized Precipitation Evapotranspiration Index (SPEI) is one of the more applied indices to evaluate drought characteristics. SPEI is based on precipitation (P) and potential evapotranspiration (PET) parameters. In PET calculation, many different equations have been introduced. In this research, monthly and seasonal SPEI values based on five more applied equations of PET calculation methods and Food and Agriculture Organization Penman-Monteith equation (FPM) (that is the standard equation of PET calculation and used as reference method) were compared. In this paper, meteorological data series of 24 meteorological stations (with suitable geographic distribution and different climate condition) in Iran from 1967 to 2017 were used to calculate PET values and SPEI in monthly and seasonal time scales. Results showed that according to normalized root mean square error index (NRMSE), similarities of estimated SPEI using Blaney-Criddle (BC) method with FPM method in monthly (at the 79.1% of evaluated stations) and seasonal (at the 75% of evaluated stations) time scale were more than other methods. According to the results of clustering analysis, the average of similarity of calculated SPEI between FPM equation and Jensen-Haise (JH), Modified Jensen-Haise (MJH), Thornthwaite (TW), Hargreaves-Samani (HS), BC equations were 85.46, 83.08, 82.17, 81.38, and 83.46% in monthly time scale and 84.08, 82.04, 80.38, 81.93, and 82.79% in seasonal time scale. Therefore, in areas with the least available meteorological data, it is suggested to replace the FPM equation with JH or BC equations.