Investigating Abrupt Changes and Periodically Changes Pattern of Reference Evapotranspiration in Iran with Wavelet Analysis and Moving t Test

Analysis of the temporal variations of reference evapotranspiration (ETo) declares climate changes and their effects on water demands. In this paper, an analysis of the monthly ETo (calculated by the FAO Penman– Monteith method) was done at 19 stations, located in different climatic regions, during 1961– 2003 in Iran. The annual ETo time series were derived, and the moving t test and Morlet wavelet were conducted to analyze the comprehensive time series for characterizing abrupt change and period in the ETo data sets. The annually and monthly spatial distibuations of ETo were produced by the inverse distance weighting (IDW) method. Based on the moving t test over annual ETo time series there were 4 abrupt changes in 1981,1982,1983, and 1984 years which were coiencidnce with ETo trend. Also moving t test detected abrupt changes in 1976- 1986 period in arid and semi-arid climates. KhoramAbad station in semi-arid climate with 21 abrupt changes had the largest changes and Isfahan, Tehran, and Birjand stations with 3 changes had the smallest changes. The presented methodology in this paper can be extended to hydrological time series, water resources and drought studies to infer their abrupt changes and monitoring periodical changes pattern.