4. Discussion
This study aims to explore the
spatiotemporal variation of ETref and other climatic variables that
influence ETref. We focused to identify the seasonal and annual trend
patterns of ETref of the study area. It was observed that the MT had an
increasing trend in most locations in the country. Many studies also
reported that global temperature was increasing substantially over the
last few decades in an alarming rate which resulting not only altering
the global water budget but also the more frequent drought and flood
(IPCC 2007; Feng et al. 2014; Islam et al., 2019). Recently, a study by
Islam et al. (2017) also found frequently and intense droughts phenomena
occurring in Bangladesh that also support our result of an increasing
trend of MT which might result from the severe change of climatic
parameters throughout the country.
Wind speed and sunshine hours were reducing considerably in most of the
regions and it can be alarming under climate change scenario. Syeda and
Nasser (2012) reported that a significant decreasing trend of the
sunshine hour had been observed over Bangladesh that might be the result
of the increasing air pollution in our atmosphere and also the soared
anthropogenic activities for the past few decades. Lian et. Al. (2015)
studied that characteristics of changing climate ought to change the
water cycle which can be resulted in frequent occurring of drought and
flood also indicate the same changes occurring like our study. Most
areas in Bangladesh experiencing a considerable declining of SD coupling
with the rising temperature. Many scientists i.e. Jhajharia et al.
(2012) and Chu et al. (2017), considered that the rising of aerosols and
clouds resulting from air pollution was the main reason behind the
lowering SD.
A significantly decreasing trend of ETref also found in the study done
by Rahman et al. (2019) which also supports our findings. A slightly
increasing trend of ETref was found northeast to the southwest part of
Bangladesh that contradicted the studies done by Yang et al. (2013). But
Mojid et al. (2015) found an increasing trend of ETref in Bogra and
Rangpur stations similar to our studies. Some published works like Fan
and Thomas (2013) and Tao et al. (2015), it was evident that SD played a
pivotal role in quantifying ETref variations in several locations of
Bangladesh. According to Tao et al. (2015), it is evident that SD and WS
are the most prominent parameters than MT and RH which act behind the
disparity of ETref.
Nonetheless, various scholars in their works (Chu et al. 2017; Li et al.
2018) confirmed that SD and WS act as a key influencer about ETref which
is analogous to our study. On the other hand, The PCC analysis revealed
that SD, MT, and RH were the main drivers of ETref and WS had little
effect. The declining amount of RH in the air also influencing the
reduced ETref as like as the other climatic variables. Though an overall
decreasing trend of ETref was found in the study area coupled with
increasing temperature which should be taken under consideration by the
policymaker to manage water for future agricultural water practice and
drought risk reduction phases.
The decrease in SD and WS was found in the study area which should
resultant a decline ETref but a significant increase in MT offset the
impact and followed the “evapotranspiration paradox” in Bangladesh
which is supported the study conducted by Herath et al. (2017), Feng et
al. (2017). Studies by Roderick and Farquhar (2002) establish the fact
that a decline in SD resulted in the lessening ETref which were similar
to our study.
The studies done by Rahman et al. (2019) and Mojid et al.
(2015) showed that MT, SD, and WS had a significant influence on the
ETref occurring that also support our study. The WS and SD were
declining notably in some parts of Bangladesh. In the meantime, in
Bangladesh strange fluctuations were noticed in MT and SD that can be
triggered by global warming. Changes in hydrological processes resulted
from changing climate ought to enhance the frequency of drought, flood
and reallocating the water resources in Bangladesh (Islam et al.,
2019).
5. Conclusion
We found decreased in ETref with time across Bangladesh which is similar
to the “evapotranspiration paradox” in different portions of the
globe. Though a significant rise in MT, but SD, R, and WS was found as
the key influential factors of decreasing annual and seasonal ETref in
Bangladesh. This study indicates that the declining SD and WS would
gradually decrease ETref in Bangladesh. Spatially, the southern part of
Bangladesh reflected the higher ETref while the northwestern region
exhibited the lower ETref on an annual and seasonal scale. In a seasonal
scale, the ETref in the western part tends to be elevated in the
pre-monsoon compared to other seasons. Moreover, it is interesting to
note that decline in ETref across Bangladesh in the last four decades
has been prominent during the all seasons except for monsoon. The
declining ETref might have implications in atmospheric evaporation
balance, irrigation, and reservoir management required in different
regions of Bangladesh. A decrease in ETref and variations in water
shortage can influence the hydrologic cycle in the study region which
has created an environmental crisis. The outcomes of the research can be
useful for designing adaptation measures of climate change and also
mitigating drought effects on agricultural water resources and
ecosystems. The coupling of satellite-derived ETref and climatic
datasets can be utilized for better evaluating spatial variations in
ETref and climatic variables in the future study. Also, research should
be carried out to appraise the variations in soil moisture water balance
because of a decline in ETref for evaluating the effects of decreasing
ETref on irrigation practices, hydrological system, and drought
adaptation in Bangladesh, especially in the northwest region.