1 Introduction
Evapotranspiration (ET) acts as a linkage between the energy and
hydrological balance which is the key contributing factor of water vapor
to the atmospheric demand (Salam and Islam 2020). Reference
Evapotranspiration (ETref) is a pivotal element for hydrological
practice, water resources designing, watershed management, and
agricultural sector (Islam et al. 2018; Li et al. 2018; Chu et al.
2019). It is not only the most critical factor hydrological research but
also a critical factor in climatic research (Roudier et al. 2014).
Understanding the variations in ETref is essential for water resource
practices, hydrological processes and implementing adaptation of climate
change measures (Islam et al. 2019).
ETref relies on some meteorological parameters like sunshine hour
duration (SD), air temperature, relative humidity (RH), rainfall (R),
and wind speed (WS). Moreover, the variabilities in ETref rely on those
parameters and their exchanges. For example, the average temperature has
elevated by 0.8°C since the 1980s, whereas the SD has declined by up to
20% during the several decades worldwide (Dodgshun 2017). The
inconsistent variations in terms of radiative and aerodynamic parts of
ETref have triggered a contradiction in the ETref trend in the world.
Despite an increase in mean temperature, ETref has been observed to be
declined in most regions of the globe in recent decades (Chu et al.
2019). Decreasing SD and WS have been found as the main driver of the
”evapotranspiration paradox” in most other parts of the earth. The
variations in climatic factors are not happening consistently in the
globe. As for instance, change in RH is not significant globally, it is
observed to enhance up to 2.0% per decade over the main part of the
USA, China, and India in the last four decades. Diverse variations in
climatic variables have triggered a significant increase in ETref in
different countries e.g., Argentina (D’Andrea et al. 2019), South Korea
(Ghafouri-Azar et al. 2018; Hwang et al. 2019), eastern Pakistan (Ahmad
et al. 2019), and central India (Kundu et al. 2017), whereas a reduction
in ETref has been observed in various countries e.g., Brazil (Cabral
Júnior et al. 2019), Iran (Nouri and Bannayan 2019) and major regions of
China (Li et al. 2018; Yang et al. 2020). There is no variation in ETref
noticed in several parts like Peninsula Malaysia (Pour et al. 2020) and
the Mountains of Qilian in China (Lin et al. 2018). Both ascending and
descending trends in ETref were also reported in several areas including
Tibetan Highland (Zhang et al. 2019), and Huahe River basin (Chu et al.
2017).
The implications of ETref variation is much elevated in the subtropical
sub-humid region, especially Bangladesh. Bangladesh hosts the different
environments and an elevated content of species. These species have a
slight climatic position and thus, highly susceptible to climatic
variabilities. A small variation in ETref might have a huge effect on
biodiversity and the ecosystem. However, only a few cited works have
examined the changes of ET over Bangladesh. For instance, Rahman et al.
(2019) reported a decreasing trend in ETref in most of the regions of
the country. Published cited works
associated with variations in ETref and the influential meteorological
factors of such variations are yet scarce in Bangladesh. Considering
this fact, and our research intends to close the gap.
Hydro-geologically, Bangladesh has an abundance of freshwater but
excessive withdrawal reducing the level which becoming an alarming issue
for the past few decades (Acharjee et al. 2017; Islam et al. 2019). Many
studies have been reporting that the occurrence of drought is being
frequent nowadays with a threat to agriculture as well as food security
for Bangladesh reported by National Drought Mitigation Center (Uddin et
al. 2020). For an agriculturally based country like Bangladesh, climate
change is becoming a great threat to meet food security with an emerging
huge population. Though climate change has both positive and negative
impacts on agriculture, the negative effect is more prominent (Islam et
al. 2017). Though a vivid number of studies have been performed to
insights into the variations in rainfall and temperature in Bangladesh,
appraisal of trends in ETref and the influencing forces behind those
trends has not still investigated. Thus, it is essential to figure out
ETref variations extensively and how the meteorological factors
affecting these alternations as ETref playing a major function in the
water cycle as well as hydrological processes. ETref is highly sensitive
to variations in meteorological parameters that are pivotal for its
computations. Now the open question arises in this context: Are all
sites of Bangladesh changed in ETref trend or part of the country both
annually and seasonally? Which meteorological variables caused ERref
variations? What are the meteorological variables that influencing the
ETref most? Is there any periodicity of ETref in the seasonal and annual
scales? The answers to these questions remain unsolved. To design a
water resource plan for Bangladesh, a comprehensive study of ETref
trends and the affecting meteorological variables across Bangladesh is
needed. Keeping these in mind, the key objectives of our work are (1) to
quantify spatiotemporal trends of ETref and other climatic variables in
Bangladesh from 1980-2017; (2) to analyze periodicity of ETref at the
annual and seasonal scales under changing climate condition; (3) to
detect the climatic variables that are most influencing ETref in
Bangladesh.