3.3 Periodicity of Annual and seasonal reference evapotranspiration
To examine the periodicity of ETref in Bangladesh, the yearly ETref series of the study period from 1980-2017 was displayed by the Morlet wavelet transform (MWT). Fig. 6 (a) shows the MWT of ETref which exhibits significant changes in the frequency bands of 1-3, and 3-5-year periods during 1988-1994 and 2006-2009. Fig. 6(b) shows the significant changes of ETref in pre-monsoon, prominent in frequency domains of 0-5 and 5-8 and 8-12 during 1994-1998, 1990-2004, and 1994-2002 periods of observation respectively, characterized by an ā€œIā€ and an elongated oval-shaped dark red color. Fig. 6(c) shows MWT of ETref in monsoon season, exhibiting small but significant changes in the frequency domains of 2-3.5, 2-3, and 2-4 years during 1987-1993, 2001-2003, and 2006-2009 periods respectively, denoted by the small and scattered islands of red color contours inside of the borders. Fig. 6(d) shows the little changes of ETref in post-monsoon, most prominent in the frequency band of 0-1.5 years during the 1988-1993 period of observation, represented by an edge dark red color half contour. Fig. 6(e) shows a continuous wavelet transform of ETref in the winter season, showing two small but significant changes in the frequency bands of 1-3 and 3-5 years during 1998-2001, and 2005-2009 periods respectively, characterized by the small and central and right sides island of red color contours along its margin.