Assessing recent impacts of climate change on design water requirement of Boro rice season in Bangladesh

Water requirement is sensitive to the impacts of climate change, especially in Bangladesh because of limited freshwater availability in the dry season, despite the fact that the country's agriculture sector requires large quantities of water for the crop production. Hence, gaining a better understanding of changes in water requirements in Bangladesh during dry periods is important in the management of agricultural water resources. This study assesses the recent impacts of climate change on the design water requirement (DWR) of the Boro rice–growing season in Bangladesh using a frequency analysis over a 5-year period. The reference evapotranspiration (ET_ref), crop evapotranspiration (ET_p), effective rainfall (ERF), and gross irrigation water requirements (GIWR) of Boro rice were estimated based on daily weather data for the period of 1984–2013 using the CROPWAT8.0 model. The results showed the significant decreasing trends of ET_ref in most of these Boro rice growth stages in all districts. The GIWR of Boro rice and its trends demonstrated significant spatial heterogeneity in the last three decades due to significant changes in the ERF and ET_p. The DWR of Boro rice–growing season also supported the results of the GIWR, and the Weibull probability distribution function (PDF) is found to be an optimal PDF among eight PDFs for the estimation of DWR. Overall, the results indicate that a recent climate change does not only contribute to high water demands for the crop but also result in decrease water requirements due to variations in wind speed, sunshine hours, and relative humidity.