Decadal flood trends in Bangladesh from extensive hydrographic data

Bangladesh is a country that comprises much of the world’s largest delta, formed from the Ganges, Brahmaputra, and Meghna (GBM) rivers and their tributaries. Flooding is a fact of life in Bangladesh where up to two-thirds of the country is flooded annually from combined monsoonal rains and Himalayan snowmelt. For this reason, understanding flood dynamics on both local and regional scales is critical. However, flood hazard studies to date typically rely on single flooding events to create flood maps and to evaluate flood hazards using satellite imagery. Here we use geographic information systems to analyze weekly water level data from 304 river gauges and over 1200 groundwater gauges from the Bangladesh Water Development Board to determine the spatial and temporal changes in flood depth and extent. These data cover an eight year period from 2002 to 2010 and provide a temporal resolution that match or are better than that of available satellite imagery. Country-wide ground and surface water levels and corresponding annual flooding events were determined along with groundwater level, flooding, and precipitation trends in Bangladesh at multiple scales. We find that while precipitation within the GBM basin has steadily increased through the time series, the average country-wide inundation depth and absolute water level has been decreasing. These respective trends could be attributed to improved flood management strategies in Bangladesh and surrounding countries that are within the GBM basin, as well as fluctuating weather patterns, declining volume of Himalayan snowmelt runoff, dam construction upriver from the GBD both within and outside the Bangladesh border, and increased groundwater abstraction of shallow groundwater aquifers for sustaining life in the eighth most populous country in the world.