Evaluating severity-area-frequency (SAF) of seasonal droughts in Bangladesh under climate change scenarios

Drought is considered to be one of the most devastating natural hazards, causing widespread environmental and social damage in many parts of the world. Using standardized precipitation index, this work has assessed changes in the severity-area-frequency (SAF) relationship curve of seasonal droughts in Bangladesh. Changes were estimated for mild, moderate, severe and extreme droughts for the four climatic seasons; winter, pre-monsoon, monsoon, post-monsoon, and for the two major growing seasons; rabi (November to April) and kharif (May to October). Nineteen general circulation models (GCMs) of Couple Model Intercomparison Project 5 were used. The model output statistics approach was used to downscale GCM simulated rainfall for eighteen climate stations in Bangladesh. Changes in the SAF curve were computed for three periods (2010-2039, 2040-2069 and 2070-2099). The uncertainty band of the SAF relationship curve was then computed using the Bayesian bootstrap method at the 95% confidence level. The results reveal that moderate and severe drought categories have the highest return period and are likely to affect the region more than other types of droughts. The kharif season drought was found to be most pronounced and affected significant portions of the country during all return periods and severity categories. Projections also show that monsoon and kharif droughts would increase across Bangladesh in regards of severity and return period. Higher return period droughts were also projected to increase in aerial extent in the middle of this century (2040-2069).