3. Results
3.1 Annual and seasonal trends ofreference evapotranspiration
Fig. 2 shows the spatial distribution patterns of ETref on the annual
scale, where the highest ETref was 1661 mm in Jessore station and the
lowest was 1137 mm in Mymensingh station during the observation period.
The higher spatial ETref variation was distributed in the southern
portion while the north-western portion of Bangladesh showed lowermost
ETref (Fig. 2a). In a seasonal scale, the ETref variations in the
western portion were comparatively higher in the pre-monsoon than that
of other seasons. Spatially, the dispersal pattern of pre-monsoon ETref
was analogous to the annual ETref pattern (Fig. 2b). The monsoon ETref
showed a heterogeneous spatial pattern during the observation period
(Fig. 2c). The highest ETref in post-monsoon can be found at the
Coxs-Bazar stations, which are located in the southern part of
Bangladesh and post-monsoon ETref also showed the analogous spatial
pattern of annual ETref (Fig. 2d). The northern and northwestern parts
of Bangladesh showed a narrower ETref spatial distribution and opposite
to eastern and southern regions in monsoon season. The winter ETref
exhibited a noteworthy spatial distribution in the southern part while
the northern part still shows the lowest value with a complex spatial
pattern (Fig. 2e). The ETref was lowest in the northern portion, however
the south and western parts indicated the maximum ETref on a yearly and
seasonal basis.
Table 1 showed the Modified
Mann–Kendall (MK) trend test results for each site of seasonal ETref in
Bangladesh. The ETref in the winter season showed a notable spatial
distribution in the southern part while the northern part showed the
lowest rate (Table 1). During the pre-monsoon, ETref declined 0.05 mm
per year and 89% of the sites showed a decreasing trend which was
mostly distributed in the northwestern part. Each region exhibited the
minimal rate of mean ETref trend in the pre-monsoon season except for
the western part. Western part showed a higher ETref in the pre-monsoon
season than the other seasons. Overall, the ETref had a decreasing trend
all over the country. The rise of mean temperature, sunshine duration,
and relative humidity in the western part of Bangladesh resulted the
rise of ETref in this region.
The results of linear trends showed a declining trend in ETref at a
significance level of 1.19 mm per decade (Fig. 3). The yearly ETref was
decreasing throughout the study period, but only 72% of the sites
showed a noteworthy level (p < 0.05) (Table 1). Excluding for
the post-monsoon, the periodical ETref rose in all seasons. The ETr
displayed a significantly declining tendency and the rate of magnitude
were -0.47 mm, -0.47 mm, and -0.50 per decade respectively in
post-monsoon, pre-monsoon and monsoon seasons except for winter. Across
the seasons, the southeast portion showed a noteworthy rising trend
(p< 0.05) whereas the central portion showed a declining
trend. The highest ETref in post-monsoon can be found at the Coxs-bazar
station, which are located in the southern part (Table S2). Spatially,
the ETref distribution were narrower in the north-eastern and western
parts than other regions, where the south and western sections exhibited
the maximum ETref annually and seasonally.