Flood Forecasting for Bangladesh

River Flooding Damaging Floods: large peak or extended duration Affect agriculture: early floods in May, late floods in September Recent severe flooding: 1974, 1987, 1988, 1997, 1998, 2000, 2004, and 2007 1998: 60% of country inundated for 3 months, 1000 killed, 40 million homeless, 10-20% total food production 2004: Brahmaputra floods killed 500 people, displaced 30 million, 40% of capitol city Dhaka under water 2007: Brahmaputra floods displaced over 20 million (World Food Program)

Improving Bangladesh flood warning lead time Problems: Limited warning of upstream river discharges Precipitation forecasting in tropics difficult The goal was to improve flood warning techniques (Georgia Tech PI institution, program called “Climate Forecast Applications for Bangladesh – CFAB) Why flooding problem exacerbated: India almost completely surrounds Bangladesh; historically no data sharing between countries => no advanced warning of severe flood stages until discharges reach the India-Bangladesh border (at which point extensively developed Bangladesh hydraulic model routes water downstream, giving Dhaka ~ 24- to 48-hrs in-advance lead-time of severe flood stages). CFAB goal: to extend out this lead-time to additional days/weeks/months in advance. What allows us to make skillful river discharge forecasts at long lead-times (especially since forecasting precipitation in the tropics notoriously difficult)? Large catchments (Ganges ~ 1,000,000km^2, Brahmaputra ~500,000km^2 -- red line separates the catchments; Combined Meghna/Brahmaputra/Ganges peak discharges can reach up to 150,000 m^3/s => ~10X larger than historic Danube peak flows [2006] & 5X larger than historic Mississippi peak flows [1993])) => roughly-speaking catchments spatially- and temporally integrate precipitation => utilize ECMWF GCM output (1 to 10 day 51 member EPS weather variable forecasts and 1- to 6-month in-advance 41-member ensemble seasonal forecasts) - one of the (or arguably “the”) premier weather forecasting center in the world partnership with Bangladesh’s Flood Forecasting Warning Centre -- send us near-real time border discharge measurements; capitalize on this data source by incorporating into our forecasting and error-correction data-assimilation schemes). Discharge forecast schemes went “operational” in 2003; focus on 2004 results in this talk (2005 and 2006 Ganges and Brahmaputra river flows remained relatively low throughout the monsoon seasons). Conceptual idea of catchments being able to spatially/temporally integrate … first, temporal integration idea: say, it takes 10 days for the water that falls on the top end of the catchment to reach the flow outlet point. So the discharge flowing out today would be the result of rainfall that fell 10days ago at the top, 9days ago a little lower down, 8 days a little further down still, etc., including todays rainfall that's falling right near the channel outlet. This means if I wanted to generate, say, a 5day discharge forecast, a lot of the discharge 5 days from now would be due to "observed" precipitation I am observing today, yesterday, etc., up to 5 days ago;and only part (“5 days worth”, if you will) of the discharge would be due to "forecasted" rainfall. In this way,loosely-speaking, large catchments "temporally integrate" rainfall (inputs). Similarly, they "spatially-integrate" in the sense that all the rainfall events that fall within the catchment will eventually (excepting evapotranspiration, dam holdings, etc.) reach the outlet: so in terms of forecast skill, if ECMWF forecasts that it will rain, say, 100km away from where it actually will rain, as long as it is still within the catchment (and still roughly equidistant to the channel outlet) this displacement error won't greatly affect the final discharge forecast; in this way (again loosely speaking) large catchments "spatially integrate" rainfall and this spatially intregration enhances the relevant skill of the precip forecasts themselves. Assets: Good data inputs => ECMWF weather forecasts, satellite rainfall estimates 2. Large catchments => weather forecasting skill “integrates” over large spatial and temporal scales 3. Partnership with Bangladesh’s Flood Forecasting Warning Centre (FFWC) => daily border river readings used in data assimilation scheme 4

Merged FFWC-CFAB Hydraulic Model Schematic Primary forecast boundary conditions shown in gold: Ganges at Hardinge Bridge Brahmaputra at Bahadurabad Benefit: FFWC daily river discharge observations used in forecast data assimilation scheme (Auto-Regressive Integrated Moving Average model [ARIMA] approach) In 2006, CFAB 1-10day forecasts of the Ganges (Hardinge Bridge) and Brahmaputra (Bahadurabad) were integrated into the FFWC hydraulic model. Shown in gold are the first entry-point gaged locations for each of these rivers that are being forecast. Although by far most of the water flowing into Bangladesh flows through these two gaged locations, other tributary boundary conditions are also accounted for in the hydraulic routing model, as shown by the blue boxes (through combined FFWC-CFAB efforts, not discussed further here). 5

Daily Operational Flood Forecasting Sequence Schematic of the 1-10day discharge forecasting sequence for the Brahmaputra and Ganges rivers (which was fully-automated in 2004). Next we talk about the orange box to account for all sources of uncertainty in the discharge forecast. 6

2004 Brahmaputra Ensemble Forecasts and Danger Level Probabilities 7-10 day Ensemble Forecasts 7-10 day Danger Levels 7 day 8 day 3 day 7 day 8 day 4 day 3 day 4 day 5 day 9 day 10 day 5 day 2004 (shown here) discharges for severe Brahmaputra flooding was well forecasted (left), as were the cumulative probabilities above danger level (right). 9 day 10 day 7

Five Pilot Sites chosen in 2006 consultation workshops based on biophysical, social criteria: Rajpur Union -- 16 sq km -- 16,000 pop. Uria Union -- 23 sq km -- 14,000 pop. Kaijuri Union -- 45 sq km -- 53,000 pop. Bhekra Union -- 11 sq km -- 9,000 pop. Because of the skill achieved in the 2004 forecasting event, there are now 5 operational 5 pilot areas where 1- to 10-day forecasts are actively being disseminated this year along the Brahmaputra (government entity called a “union”-- pink areas are the larger districts, yellow areas are the next subdivision down, called a “thana”, and the smallest orange with red outline patches are the “unions”). Bangladesh is a very poor country with limited communication. But through the work of the ADPC (Asian Disaster Preparedness Centre; our partner in this project), the flood warning information channels have been established inside these unions so that the information can reach the local farmer, fisherman, craftsperson, etc. As well, workshops have been conducted in these areas to help the people understand how best to use the probabilistic information we are providing them. Very exciting to finally have these forecasts have the potential to significantly affect the lives of the local Bangladeshi. Gazirtek Union -- 32 sq km -- 23,000 pop. 8

2007 Brahmaputra Ensemble Forecasts and Danger Level Probabilities 7-10 day Ensemble Forecasts 7-10 day Danger Levels 7 day 8 day 7 day 8 day 9 day 10 day Severe flooding in 2007 was well forecasted. 9 day 10 day 9

Community level decision responses for 2007 flood forecasts (Low lands) “… on 25th July we started communicating the information to as many people as possible … especially those people living in river islands (“chars”)...” “On the 28th and 29th, meetings were organized in villages near Rangpur … they perceived that the river water level would fall, but our forecasts showed a rising trend…[with] significant chance of overflow and breaches [of weak] embankments ... We engaged … an evacuation plan urgently” “We communicated the forecast to another pilot union … on July 26th … to mobilize resources for evacuation ... All the six villages in the union were later flooded to a height of 4-6 feet on July 29th… about 35% of the people in the union were evacuated in advance.” “The communities in Rajpur Union … were able to use the forecast for … mobilizing food, safe drinking water for a week to 10 days, protecting their … rice seedlings, fishing nets, and … fish pods.” Responses from Bangladeshi’s receiving advanced flood warnings from the system