HYDROLOGY OF FLOODS IN

Shafiqul University of Cincinnati Cincinnati, Ohio, USA

DRAFT

SaciWATERs Workshop on FLOODS IN SOUTH ASIA’ BRAC Centre Inn, , 28-30 November 2002 1. Hydrology of Floods in South Asia: An Overview We will provide a review of hydrology of floods in south Asia with a focus on the -Brhmaputtra-Meghna basin. Then, bring in recent advances in remote sensing and computational capabilities to reexamine our traditional understanding of causes and impact of flood in south Asia from a hydrologic perspective. We will also discuss hydrologic vulnerability of the region to potential climate change. Finally, we will provide a connection between El-Nino and Floods in the region and how this information can be used to develop a long-range forecasting methodology for integrated large scale water resources planning and management.

For this initial draft, our focus is to provide an outline of this chapter and raise some questions to be addressed within each subtopic. These questions will form the basis of our discussion during the workshop. With comments and feedback from the workshop, these questions will be refined, new questions will be defined and expanded in the final write-up of this chapter.

2. History and Statistics of Floods in South Asia Flood and water scarcity, may seem paradoxical and incongruous, are real challenges for the prosperity and sustainability of South Asia. Perhaps nowhere in the world is water so abundant and yet so scarce than in this region of the world. From food to fish to fiber, livelihood of primarily rural population of this region critically depends on water. For example, in a typical year, over half of Bangladesh is flooded during the Monsoon season; yet, during the dry season water is important not only for food production but also to maintain navigation in the interior rivers, minimize salinity intrusion in coastal areas, preserve wetlands from continued shrinkage due to extensive use of groundwater resources for irrigation, and sustain a complex matrix of terrestrial and aquatic production systems. Here, we begin with a few statistics of floods:

• While the number of geophysical disasters has remained fairly steady, the number of hydro-meteorological disasters has increased over the last decade. During the past decade over 90 per cent of those killed by natural disasters lost their lives in hydro- meteorological events such as droughts, windstorms and floods. Floods accounted for over two-thirds of the annual average of 211 million people affected by natural disasters, worldwide from 1991-2000 (WDR, 2001). • During the 1990s, 140,000 Bangladeshis were killed by cyclones. But the Cyclone Preparedness Program evacuated and sheltered 2.5 million more people before the cyclones struck – almost certainly saving their lives (WDR, 2002). • Catchment area of the Ganges-Brahmaputra-Meghna System is 1.55 million sq. km; 90% of this area is outside of Bangladesh (in , Tibet/China, and Bhutan). • “Normal” yearly monsoonal flooding has little adverse impact on rice cultivation; these lesser floods also add to the soil fertility. These floods begin to have negative effects when their duration increases beyond “normal.” • The 1987 major floods covered 40% of the land area in Bangladesh, 40 million people were affected and 1800 people died. The 1988 major floods covered 60% of the land area, affecting 45 million people and killing 2379. • In 1998, “the flood of the century” covered around 68% of the total area of Bangladesh, affected 31 million people but killed 918.

Hydrology of Floods in South Asia 1 Draft Report Islam University of Cincinnati

Questions: Are floods getting worse in South Asia? What are the metrics to quantify floods?

3. Hydrology of Floods in South Asia 3.1 Ganges, Brahmaputra and Basins The Brahmaputra originates on the Chinese side of the Himalayas and after traversing about 1800 km through Tibet and India, enters Bangladesh through the northern border. Within Bangladesh it is called the Jamuna and flows for an additional 275 km, up to its junction with the Ganges. The Ganges flows for about 2000 km through India, enters the Western side of Bangladesh and flows to the South-East for another 250 km to confluence with the Brahmaputra. The Meghna originates in one of the rainiest regions of the world, the Shillong Plateau in . The headwaters of the Meghna comprise a number of streams that meander through Assam for about 400 km and then enter Bangladesh from the North-East in the form of two major tributaries – Surma and the Kushyara – which reunite to form the main channel and flows to the Meghna (Chowdhury and Sato, 1996).

The characteristics of peak discharges of Ganges, Brahmaputra and Meghna rivers are unique. From Table 4 is seen that though basin area of Ganges is twice that of Brahmaputra but mean annual peak discharge of former is considerably lower than latter. The coefficient of variation (CV) of peak discharge of Ganges is much higher than of the other two major rivers. While CV of peak discharge of Brahmaputra and Meghna indicates similar precipitation pattern in their basin areas.

Hydrology of Floods in South Asia 2 Draft Report Islam University of Cincinnati Hydrographs of the Ganges (lighter solid line) and Brahmaputra (thicker solid line) rivers for the typical water year 1967-68 (m3/sec) are shown here. Discharge of Brahmaputra river starts rising in March due to melting of snow in the Himalayas while Ganges discharge rises in July with the onset of monsoon. The flood peaks of the Brahmaputra occur in July and August, while the peaks in the Ganges occur in August and September.

TABLE 1 Percentage departures of monsoon rainfall and frequency Recently, the frequency of floods at the terminal G/D sites of Brahmaputra and Ganges river of floods during each systems in India (Dhar and Nandargi, 2001 monsoon season at their terminal gauge/discharge (G/D) sites in India (at Farakka for Ganga and Dhubri for Brahmaputra) have been studied (Dhar and Nandargi, 2001). Average aerial monsoon rainfall for each monsoon season was calculated for 14-year period and their percentage departure along with the number of flood observed each year are given in Table1. In this study, by flood it is meant the event when flood waters of these rivers overtop their respective danger levels at Dhubri and Farakka G/D sites. It is seen from Table1 that frequency is more or less of same order even though rainfall magnitudes received by each rainfall distribution are quite different. It was apparent that the size of basins does not play any major role as far as the frequency of floods in concerned at the terminal sites. Dhar and Nandargi (2000) studied that in the Brahmaputra basin in India, variability of annual rainfall over this region is low, in the order of 10-15%. Before the onset of monsoon, there is considerable

Hydrology of Floods in South Asia 3 Draft Report Islam University of Cincinnati thunderstorm activity over the region in the month of May due to the incursion of moisture in the region from the neighboring Bay of . The heavy rainfall is mostly caused when the eastern end of the monsoon trough shifts northwards of the Assam Valley or during the periods when ‘Break’ monsoon situations set in over the country with a northward shift of the monsoon trough to the foot of the Himalayas. In Figure 2, adapted from Dhar and Nandargi (2000), monthly and yearly flood frequencies are shown in the Brahmaputra River and its tributaries. It is seen that the maximum number of floods have occurred in July although the rainfall is at a maximum in the months of June.

To verify the claims of flood getting worse in GBM basin, a study on the peak discharge time series and flooded areas was carried out (Mirza et al., 2001). Four statistical tests were applied on the peak discharge time series and flooded area and the results did not indicate any conclusive changes in peak discharge or flooded area time

TABLE 2 Results of statistical tests applied to peak discharge data of Ganges, Brahmaputra and Meghna rivers and the flooded areas in India and Bangladesh (Mirza et al., 2001).

series within Bangladesh. However, at upstream stations in India two rivers showed an increase in peak discharge which was not registered at downstream stations in Bangladesh which was due to the regulation of discharge by the Farakka barrage. Factors other that increasing flood event characteristics (peak discharge and flooded area) which were identified were: (i) improvement in flood damage assessment techniques; and (ii) increases in human settlement in flood-prone areas.

To develop and analyze alternative regional water planning and management strategies, a structured planning framework is needed. Such a framework for the formulation, analysis and evaluation of alternative management strategies would allow accounting for both the short term and for the cumulative and long-term impacts on the performance of the water resource systems (WRS) in terms of its contribution to the regional development objectives such as food security and poverty alleviation.

In developing such a conceptual framework, it is important to pay particular attention todevelop process based transfer functions that can account for relevant impacts of changes in availability and utilization of water resources on ecosystems and on social and economic conditions of different users of the WRS. For example, hydrologic models that are in use for

Hydrology of Floods in South Asia 4 Draft Report Islam University of Cincinnati planning purposes in Bangladesh do not simulate some important characteristics of flood plain processes. Chowdhury (2002) identified some of these characteristics as inundation of floodplain by overflowing river during monsoon and their effects on groundwater recharge, rise of groundwater level close to the surface and its impact on the lateral movement of groundwater, and base flow contribution from subsurface water during dry season.

Issues of scales, both in time and space, are also important to develop these process based transfer functions. For example, water balance studies in GBM delta are typically based on a decad -- defined by dividing the month into first ten days, second ten days, and remaining days of the month – which does not match with the fortnightly neap-spring tidal cycle. These tidal cycles play an important role in residual flows, sediment transport, and saltwater intrusion for the tidal river networks. This mismatch of temporal scales and their influence on modeling river hydraulics, sediment transport, and wetland dynamics needs to be explored. In space, differences in the scale and dynamics of rivers and flood plain processes as well as the mismatch between the measurement scales and modeling scales needs to be understood.

Questions: Should we look at hydrology of other basins from South Asia? How do we characterize the similarities and differences among different basins and different modeling strategies?

4. Remote Sensing and GIS for Flood Assessment in South Asia Remote sensing provides an unprecedented spatial and temporal coverage of critical land surface and atmospheric data that are logistically and economically impossible to obtain through ground based observation networks. Several studies have investigated remote sensing and geospatial processing methods for flood monitoring. Satellite-based synthetic aperture radar (SAR) imagery from RADARSAT has proven especially effective for monitoring the extent of flooding, due to its ability to image under typical monsoon cloud cover conditions. Most of the current monitoring and prediction systems are focused on the main river system. Coupling information from these systems with quantified flood plain dynamics derived from satellite radar remote sensing could be a key for flood management and planning in the region. The potential for resource management applications ranges from agriculture and fisheries management to disaster management. Martin et al., 1998 proposed such a method to develop a detailed flood depth maps on a daily basis by merging a time series of SAR imagery with a static digital elevation model (DEM) and water level data from gauges.

Hydrology of Floods in South Asia 5 Draft Report Islam University of Cincinnati Islam and Sado (2000) studied flooded area and flood hazard assessment in Bangladesh using remote sensing and geographical information system. Satellite images from the National Oceanographic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) were used to analyze Bangladesh's historical Flood event of 1988.

Recently, Xie et al. (2001) developed a new system and put into operation at the Climate Prediction Center of NOAA to produce real-time analyses of daily precipitation at 0.100 latitide/longitude grid over the South Asia (700E – 1100E; 500N – 350N). This automated system defines analysis of daily precipitation by merging satellite as well as four observation based individual data sets. Such a data set has the potential to fundamentally alter and significantly improve flood planning and management strategies in South Asia.

Questions: What is the status of remote sensing and geo-spatial analysis for flood monitoring and forecasting in the region? How do we improve flood forecasting methodologies using remotely sensed data and geo-spatial analysis techniques?

5. Hydrologic Vulnerability of the Region

The United Nation’s Intergovernmental Panel on Climate Change (IPCC) third assessment report in 2001 has highlighted the severe consequences that are likely to be faced by the developing countries of the world due to the continued emissions of greenhouse gases. This region, and Bangladesh in particular, is likely to be one of the most seriously affected, in terms of total population at risk due to climate change, and most vulnerable country in the world (Huq, 2001). Huq (2001) described several likely impacts of climate change on Bangladesh including (a) sea Level Rise, (b) more intense cyclones, (c) greater flood intensity, and (d) health impacts.

Mirza (2002) has studied the effect of global warming on changes in probability of floods. For various warming scenarios, like CSIR09, HadCM02, GFDL and LLNL , changes in the probability of exceedence of a current flood for the Ganges, Brahmaputra and Meghna rivers was calculated. Results suggest an increase in the probability of flood for the warming scenarios considered. (except for Ganges for HadCM02).

Hydrology of Floods in South Asia 6 Draft Report Islam University of Cincinnati Recent decades have exhibited an increase in extreme rainfall events over northwest India during the summer monsoon (Singh and Sontakke, 2001). Approximately 70% of the total annual rainfall over the Indian subcontinent is confined to the southwest monsoon season (June- September). The number of rainy days during the monsoon along east coastal stations has declined in the past decade. In , seven of 10 stations have shown a tendency toward increasing rainfall during monsoon season (Chaudhari, 1994).

Questions: What are the main hydrologic vulnerabilities and consequences of a possible climate change scenario? What other studies are available related to hydrologic vulnerabilities of South Asia?

6. A New Long Range Water Resources Planning Tool

Several recent studies have shown that El-Nino Southern Oscillation (ENSO) index has a significant influence on various climatic and hydrologic signals across the globe. A recent study attempted to identify the nature and strength of possible teleconnection between the Ganges river flow and ENSO and developed a model which can capture, at least in part, the natural variability of flow and provide a large forecasting lead time (Whitaker et al., 2001). The motivation came from the fact that in the past hydrologic forecasts of the basin through rainfall- runoff modeling could provide a lead time of the order of basin response time, which is on the order of several days or so. Such a short forecasting lead time is not adequate to hedge against extreme events (flood or drought) in large river basins. It appears that a significant relationship exists between the natural variability of the Ganges annual flow and ENSO index. Through further investigation it has been shown that the rate of change of ENSO index is also statistically related to the Ganges flow. This model uses current flow data, predicted ENSO data and its gradient to forecast flow in the Ganges with a forecasting lead time up to one-year (Whitaker et al., 2001). The model also provides a quantitative measure of forecasting uncertainty. A key advantage of this model is that it does not require rainfall and stream flow information from upstream areas and countries. It is encouraging to note that all four of the validation forecasts during the El Nino and La Nina (1982, 1988, 1991, and 1993) events are within the ninety-five percent confidence intervals. These results demonstrate the strength of the proposed model and suggest further exploration of this long-range forecasting methodology for other major rivers in South Asia. Particularly, utility of such a modeling framework for dry season flow needs further investigation.

Hydrology of Floods in South Asia 7 Draft Report Islam University of Cincinnati

Questions: Can we use ENSO related information to develop a long-range water resources planning tool for major rivers in South Asia? What are the potentials for such a toll for dry season flow forecasting?

7. Concluding Remarks It appears that many of the recent studies have focused on flood and hydrologic problems in Bangladesh. Based on our discussion in the workshop, we hope to expand different subtopics to cover other regions and river systems in South Asia. Our goal is to examine the similarities and differences among different river basin of the region to develop a comprehensive overview of hydrology of floods in South Asia.

Hydrology of Floods in South Asia 8 Draft Report Islam University of Cincinnati References Chaudhari, Q.Z., 1994: Pakistan's summer monsoon rainfall associated with global and regional circulation features and its seasonal prediction. In: Proceedings of the International Conference on Monsoon Variability and Prediction, Trieste, , May 9-13, 1994. Chowdhury MR, Sato Y. 1996. Flood monitoring in Bangladesh-Experience from normal and catastrophic floods. Hydrology(J. Jpn. Soc. Hydrol. Sci.) 26(4): 241-252. Dhar ON, Nandargi S. 2000. A study of floods in the Brahmaputra basin in India, International Journal of Climatology 20: 771-781. Dhar ON, Nandargi S. 2001. A comparative flood frequency study of Ganga and Brahmaputra river systems of north India – a brief appraisal. Water Policy 3: 101-107. Huq, S. (2001): “Climate Change: A Growing Impediment to Development in Bangladesh” Prepared for the The Bangladeshi-American Foundation, Inc (BAFI), Washington DC, 20 August 2001. IPCC, Third Assessment Report, Intergovernmental Panel on Climate Change, 2001. Islam MM, Sado K. 2000. Flood Hazard assessment in Bangladesh using NOAA AVHRR data with geographical information system. Hydrological Processes 14: 605-620. Lal, M, Harasawa H. 2001. Future climate change scenarios for Asia as inferred from selected coupled atmosphere-ocean global climate models. Journal of the Meteorological Society of Japan 79 (1), 219–227. Martin, T.C., K. Hasan, D.Werle and M.D. Rahman. 1998. Satellite-based synthetic aperture radar (SAR) for Bangladesh land and water resource applications. Proceedings of the Final Symposium: RADARSAT Advanced Data Research Opportunity (ADRO). Montreal. Mirza MMQ. 2002. Global warming and changes in the probability of occurrence of floods in Bangladesh and implications. Global Environmental Change 12: 127-138. Mirza MMQ, Warrick RA, Ericksen NJ, Kenny GJ. 1998. Trends and persistence in precipitation in the Ganges, Brahmaputra and Meghna basins in South Asia. Hydrological Sciences Journal 43 (6): 845–858. Mirza MMQ, Warrick RA, Ericksen NJ, Kenny GJ. 2001. Are floods getting worse in the Ganges, Brahmaputra and Meghna basins? Environmental Hazards 3: 37-48. Singh, N. and N.A. Sontakke, 2001: Natural and anthropogenic environmental changes of the Indo-Gangetic Plains, India. Climatic Change, (communicated). Whitaker DW, Wasimi SA, Islam S. 2000. The El Niño-Southern Oscillation and Lon-range Forecasting of flows in the Ganges. International Journal of Climatology 21: 77-87. WDR (2001, 2002): World Disaster Report, An annual publication of the International Federation of Red Cross and Red Crescent Societies. Xie, P., Yarosh, Y., Love, T., Janowiak, and Arkin, P. (2001): “A real-time daily precipitation analysis over South Asia”, Presented at the 16th Conference in Hydrology, AMS, 2001.

Hydrology of Floods in South Asia 9 Draft Report Islam University of Cincinnati