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 āIā 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.