Pan evaporation and reference evapotranspiration trend detection in western Iran with consideration of data persistence

It is important to identify the spatiotemporal trends of evaporation and evapotranspiration under the changing climate for use in regional water resources planning. This work aimed to investigate the trends of the Hargreaves reference evapotranspiration (ET_o), pan evaporation (E_pan) and pan coefficient (K_pan) series at 12 stations in the west of Iran by using the sequential Mann–Kendall, Kendall and Spearman tests after eliminating the influence of the significant lag-1 serial correlation from the time series by the pre-whitening method for the period 1982–2003. The approximate year of the beginning of the significant trends was detected by using the Mann–Kendall rank statistic. The spatial distribution of the trend magnitudes was obtained from the Inverse-Distance-Weighted (IDW) interpolation method. No significant trends were found in the ET_o time series, while an upward trend of 16 mm/year was observed in the E_pan series which began in 1998. Moreover, a downward trend was obtained in the K_pan series which started in 1994.