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.