Increased water flow and sedimentation in the Ganga basin due to climate change will lead to more floods in India and Bangladesh

A photo taken from a drone of the Ganga River around Munger in Bihar that highlights the serious problem of sedimentation the increases the risk of flooding (Photo courtesy Rajiv Sinha)

A photo taken from a drone of the Ganga around Munger in Bihar that highlights the serious problem of sedimentation, which increases the risk of flooding (Photo courtesy Rajiv Sinha)

The Ganga basin, one of the world’s largest that is shared by Nepal, Bangladesh and India, will witness increased flooding and water flows due to climate change, new research shows.

The monthly flows during the monsoon are projected to increase under a future climate change scenario, the results show. The increased flows in the monsoon suggest there will be increased flooding in the future, which could have significant consequences for India and Bangladesh.

On the other hand, the dry season flow will be only slightly increased.

The study is part of a larger Deltas, Vulnerability and Climate Change: Migration and Adaptation (DECCMA) project that included a similar analysis on the Volta delta in Ghana, and looked at the impacts of climate change and other environmental drivers across contrasting deltas in Africa and Asia. The findings on the Ganga basin have been reported in a journal called Science of the Total Environment.

“Our work suggests that the water flows are likely to be enhanced in the monsoon period as well as during non-monsoon periods for different climate change scenarios for 2050s and 2090s,” Rajiv Sinha, one of the authors and professor at the department of earth sciences, Indian Institute of Technology, Kanpur, told indiaclimatedialogue.net. “This has huge implications in terms of water resource management, particularly in terms of flood risks. Many of these basins are already flood-prone and this will increase the flood risks quite significantly in future.”

“Apart from water fluxes, our work also shows an increase in sediment load for the Western Ganga system during the monsoon and a decrease in the sediment load for the Upper Ganga stations. These projected changes of sediment load provide a first-hand estimate of the potential future sediment regime of the Ganga River,” he added.

The study also looked at nutrient changes, mainly nitrogen and phosphorous, in these rivers. It found that though increased flows will somewhat dilute nitrogen and phosphorus, “the changing socioeconomic conditions will offset the dilution effect and there would be a net increase in these fluxes as well,” said Sinha.

Greatest deltaic plain

Bangladesh and the eastern part of West Bengal in India are part of “the greatest deltaic plain in the world at the confluence of the Ganga, Brahmaputra and Meghna (GBM) rivers and their tributaries,” the report observes. The GBM river system and the Hooghly River serve a population of over 780 million, and provide fresh water for people, agriculture and industry. GBM and the associated Hooghly river system is a large transboundary river basin spreading across India, Nepal, China, Bhutan, and Bangladesh, with a total catchment area of 1,612,000 sq. km.

A team of scientists from India, the UK and the US used a combination of models to study the relative impacts of climate change and socioeconomic factors on river flow and water quality. They downscaled data from three different widely used climate models to represent local-scale climate processes.

They also used an integrated catchment model to simulate water flow pathways in the surface and groundwater systems, and track daily fluxes of pollutants and other chemicals in both the terrestrial and aquatic portions of catchments. They looked at two conditions — high and low water flow in the rivers.

The scientists correlated the water flow and water quality with socioeconomic changes such as population changes, water for public use, irrigation water demands, hydropower, industry water demands and effluents discharge. Other socioeconomic factors include land use change, atmospheric deposition driven by industrial development and water transfer plans.

Increased flooding

The scientists report a “modest increase” in high flow conditions in the 2050s in all rivers and “a very large change” in the 2090s, “suggesting that flooding will significantly increase.”

The Brahmaputra, Meghna and Mahanadi rivers will experience “significant increases” in low flows by the 2050s, and all rivers will show increases by the 2090s, the research shows.

Changes to low flows are also likely to occur given projected increases in variability. Drought duration may become more frequent, while extreme low flows may actually increase in extent, the report says.

“However, if the effects of major water transfer schemes are considered, such as a 30% diversion of the Brahmaputra flows, then there is a significant reduction in flows in Bangladesh,” the report says. “This could be quite serious in Bangladesh and especially in the low flow period when the water is used in Bangladesh for irrigation and is a key freshwater driver for fisheries in the rivers and estuary systems.”

The results suggest that socioeconomic factors will have a fairly limited impact on water quality.

The scientists conclude that development of models for such large and complex river systems “provide an important planning tool” for exploring future scenarios, engaging stakeholders in dialogue on water resources management, and identifying gaps in knowledge and data.

Devising strategies

Experts studying climate and water concur on the study’s value in planning and management. “The study should be emphatically included in devising strategies for providing water for public supply, hydropower, irrigation water for agriculture, and policy making across GBM river systems,” Manish Goyal, assistant professor at the Indian Institute of Technology, Guwahati, who is working on water modeling and river basin management under climate change scenarios, told indiaclimatedialogue.net.

A.K.M. Saiful Islam, professor of hydrology and climate change, Bangladesh University of Engineering and Technology, says the study “showed not only the changes of the future flow but also changes in the water quality by studying changes of the nitrate and phosphorus in the major river systems.”

“Both flows and water quality for a range of climate and socio-economic scenarios presented in this study can assist in a more holistic management approach,” Islam said.

The latest study does have some limitations too, which the authors, and Goyal and Islam note. These include uncertainties within the model, input data on flow and water quality, and the socio-economic factors.

A few other studies in Bangladesh suggest that due to increase of the monsoon flow in future, floods in the Brahmaputra are likely to become more frequent, and their magnitude will also become more severe, said Islam.

“This will enhance the chances of flash floods in the northeast areas of Bangladesh. Severe flash flood in the northeast areas can bring havoc by damaging Boro crops during the pre-monsoon season. Moreover, heavy rainfall in the pre-monsoon season is often associated with thunderstorms and lightning, which cause damage to crops and human fatalities,” he told indiaclimatedialogue.net. “Adaptation to reduce flood risks during the pre-monsoon and monsoon seasons will be necessary for climate vulnerable countries like Bangladesh.”

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