Quantifying Spatiotemporal Variation of Reference Evapotranspiration and Its Contributing Climatic Factors in Bangladesh during 1981-2018

Reference evapotranspiration is a vital research item for hydrologic pattern systems in drought-flood risks of a country like Bangladesh. The present research examines spatiotemporal fluctuations in reference evapotranspiration and influencing factors of these fluctuations by applying the Mann-Kendall test with the Theil-Sen's estimator, Inverse Distance Weighted interpolation method, differential equation and detrending method depending on diurnal meteorological data sets attained from 25 weather observatories between 1981 and 2018. By current research, the FAO 56 Penman-Monteith (PM) model is executed for determining seasonal and annual reference evapotranspiration (ETPMF), and differential equation and detrending method is implemented for assessing correlation, sensitivities, and benefaction of four climatological variables to ETPMF. The MK trend analysis has been performed jointly with the Theil-Sen's estimator to discover movement of ETPMF and meteorological variables. ETPMF displayed a conspicuous descendant tendency (p < 0.01) at post-monsoon (-0.87 mm/year), winter (-1.13 mm/year), and annual time scale (-3.31 mm/year) in whole Bangladesh, but at the same period, a non-significant descending tendency of ETPMF was observed in pre-monsoon (-1.47 mm/year) and monsoon (-0.07 mm/year) season in the study area. These changes of ETPMF could be explained by a notable rising tendency in average temperature (TA) and actual vapor pressure (VP) and noteworthy declining tendencies in wind speed (WS) and sunshine duration (SD). ETPMF had been maximum sensitive to TA, a bit less, to VP, SD, and WS in the annual and seasonal periods. TA, WS, and SD produced a positive effect, whereas the VP negatively affected seasonal and annual ETPMF. Despite the negative effect on ETPMF, VP was the second most sensitive to the annual and all seasonal ETPMF after TA. Crosswise the entire study area, WS had the greatest influence to ETPMF, after that TA and SD. At the same time, the positive contribution of VP to ETPMF could not counterbalance the negative contributions of WS, TA, and SD. While the consequence of VP on variations in ETPMF is minor, it has to be considered particularly in the cold period. The counter association among inclining TA and declining ETPMF, which is known as the "evaporation paradox," happenned in Bangladesh. These research outcomes will be beneficial to develop forthcoming water scheduling and regulation of feasible water resources to lessen climate change effects on agronomic water prerequisites and regulate human induced actions.