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Availability of water resources, especially irrigation water in the Southwest (SW) region of Bangladesh, is in threat due to climate change impacts, reduction of upstream flow, and excessive groundwater consumption. It is essential to assess the evapotranspiration ( $$\mathrm{ET}$$ ), especially reference evapotranspiration ( $${\mathrm{ET}}_{0}$$ ) to ensure efficient irrigation demand for this region. However, direct measurement of $${\mathrm{ET}}_{0}$$ is a difficult task as empirical equation-based assessment requires a complex set of input data. As remote sensing (RS) technology has evolved as a viable alternative to the ground-based estimation of ET0, this study investigated whether it is possible to attain a reliable assessment of $${\mathrm{ET}}_{0}$$ for the SW region of Bangladesh using satellite data. Land surface temperature (LST) data retrieved from the moderate-resolution imaging spectroradiometer (MODIS) satellite have been evaluated as an alternative of air temperature in three empirical $${\mathrm{ET}}_{0}$$ assessment methods, namely the Hargreaves–Samani (HS), Thornthwaite (TH), and Blaney–Criddle (BC) methods. $${\mathrm{ET}}_{0}$$ assessed from these methods have been calibrated and validated against the FAO-56 Penman–Monteith (PM) method. Several statistical indices, viz. root mean square error (RMSE), mean absolute error (MAE), mean relative error (MRE), and coefficient of determination ( $${R}^{2}$$ ) have been used to evaluate the performance of $${\mathrm{ET}}_{0}$$ assessment from the above-mentioned methods. MODIS-LST data have been found strongly correlated with air temperature ( $${R}^{2}$$ = 0.95). A good correlation in $${\mathrm{ET}}_{0}$$ assessment has been found between the PM method and HS ( $${R}^{2}$$ = 0.85), TH ( $${R}^{2}$$ = 0.81), and BC ( $${R}^{2}$$ = 0.79) methods. With the calibrated result, the calibrated HS method provides the most accurate results compared to the other methods, with an average RMSE of 0.44 mm/day, MAE of 0.39 mm/day, and MRE of 12.34%. Considering the consistency of the findings with the previous studies, this method has been recommended for assessing $${\mathrm{ET}}_{0}$$ under conditions of limited data availability.

The recently reached Paris Agreement at the 21st Conference of the Parties (COP21) of the United Nations Framework Convention on Climate Change (UNFCCC) in 2015 includes a goal of pursuing efforts to limit the global warming at 1.5 °C. Following this, the Intergovernmental Panel on Climate Change (IPCC) has accepted an invitation by the UNFCCC to create a special report in 2018 which will include the impacts of 1.5 °C global warming on various Earth systems. It is therefore a priority now for the scientific community to quantify these impacts at regional scales. As a contribution to this effort, this study assesses the impacts of 1.5 and 2 °C global warming on the extreme flows and water availability of the Brahmaputra River, which is both an essential source of freshwater for its lowermost-riparian Bangladesh and also an unavoidable source of disastrous floods. Future flows are simulated with the Soil and Water Assessment Tool (SWAT) and bias-corrected weather data of an ensemble of 11 climate projections from the Coordinated Regional Climate Downscaling Experiment (CORDEX). Results indicate that floods will be more frequent and flood magnitudes greater at 2 °C specific warming level (SWL) than at 1.5 °C SWL. On the contrary, low flows are expected to be less frequent and low flow values to be higher at 2 °C SWL than at 1.5 °C SWL. Water availability will likely be greater at 2 °C SWL than at 1.5 °C SWL from January to August. For the remaining months, water availability will likely be greater at 1.5 °C SWL rather than at 2 °C SWL.

In the middle of 2007, a severe flood affected the People’s Republic of Bangladesh. This is a natural disaster that takes people’s lives, destroys livestock, infrastructures and communication systems and, damages crops and fish ponds. Despite many adverse impacts, the flood situation is an accepted phenomenon to the citizens of Bangladesh, due to the immense increase of soil fertility due to the flood, plus, the recharge of aquifer, ecosystem and fish. The flood of 2007 was the 5th major flood of the last 20 years when more than thirty-five percent of the area of the country was inundated with flood water. As in the past, the flood of 2007 had its own significance. The geography of the country contains a floodplain delta of three major river basins: the Ganges, the Brahmaputra and the Meghna (GBM). The mean monthly rainfall plot from the TRMM satellite data has shown that for both the Meghna and Brahmaputra basins, the rainfall was higher during July 2007 than any other months of the last 2 years. This excess rainfall had accumulated in the Brahmaputra and Meghna rivers and carried downstream to Bangladesh. This was the main cause of the flooding in 2007. The first crossing above the danger level of the river waters was observed at Durgapur station of the Someswari and at Sunamganj station of the Surma on the nineteenth of July, 2007 inside Bangladesh. In terms of magnitude of the peak and duration of the flood, the Brahmaputra was higher in 2007 than during 2004. However, the Ganges river water level never crossed the danger level during flood of 2007. The Meghna was lower during the flood peak for the duration of the flood in 2007. The year–to-year variability in both the magnitude and duration of the flood suggests changes in rainfall and landuse pattern of the catchment.