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.