Operational flood-forecasting for Bangladesh

Short-term (1 to 10 day) forecasts were developed as part of the Climate Forecasting Applications for Bangladesh (CFAB) project aimed at providing operational real-time forecasts of river discharge into Bangladesh. For the last two monsoon seasons CFAB has disseminated operational flood forecasts to the country of Bangladesh to provide advanced warning of severe flood-stage discharges in the upper catchments of the Ganges and Brahmaputra basins over a wide-range of time scales. Here we concentrate on the model that provides the short-term forecasts. In order to generate fully-automated probabilistic river discharge forecasts, the scheme utilizes the European Centre for Medium-Range Weather Forecasts ensemble weather forecasts, near-real-time satellite and rain gauge precipitation estimates, and near-real-time discharge estimates from the Bangladesh Flood Forecasting and Warning Centre, in conjunction with statistical dressing and the use of a suite of hydrological models.; A brief history of the CFAB project is presented describing its evolution from forecasting sea-level influenced flooding to rainfall driven flooding for Bangladesh. Three of the primary technical issues that needed to be dealt with in the short-term discharge forecast model development are discussed: (1) A method to correct precipitation forecasts to remove model biases and other discrepancies with "observations" (while retaining spatial and temporal covariances) using a quantile-to-quantile mapping technique; (2) The issue of appropriate hydrologic model selection for this particular forecasting application was dealt with through the implementation of a flexible multi-model discharge forecasting approach which tries to capture the "best of both worlds" of the lumped and distributed hydrologic modeling techniques; (3) A technique to account for all aspects of discharge forecasting error (while simultaneously making a model correction) so that more statistically correct probabilistic discharge forecasts can be made.; Finally, concluding remarks on the CFAB project and the possible extension of its techniques to other sectors in the monsoon regions will be discussed along with the following outstanding related research problems covered in the appendices: a calculation to estimate backwater effects on upstream flooding; observed statistical differences between the "control" and ensemble members of the European Centre for Medium-Range Weather Forecasts (ECMWF) numerical weather prediction model; the theoretical relationship between ensemble "skill" and "spread"; and the dependence of precipitation forecast skill on spatial scale, and the ability of watersheds to extend forecast skill.