Behind the Paper

Irrigation reduces the streamflow in the Bay of Bengal

Extensive use of groundwater for agricultural purposes in the Ganges basin reduces the streamflow in the Bay of Bengal despite the increase in precipitation in the basin.

Fadji Maina Apr 12, 2024

More than a billion people depend on the Ganges and the Brahmaputra rivers for survival and, for centuries, civilization has thrived along their tributaries. Originating in the glaciated areas of the Himalayas, these two transboundary rivers discharge into the Bay of Bengal, one of the largest river deltas in the world. By virtue of its strategic geographical location in Southeast Asia, the Bay of Bengal influences global geopolitics and the economy. Additionally, the bay also hosts one of the biggest marine ecosystems and the Sundarbans, the most expansive mangrove forests on Earth, and have vast deposits of hydrocarbon resources and mineral wealth.

Climate change and human management are having a substantial impact on the water cycle in the region. In fact, the warming rate in the Ganges-Brahmaputra basin is double the global average (0.32°C per decade compared with the global average of 0.16°C per decade), which makes it one of Earth’s most vulnerable basins on the planet. The unprecedented changing climate triggers glacier and snow melt, rain-on-snow, extreme monsoons, and sea level rise, which significantly impact groundwater and streamflow. Furthermore, the Ganges-Brahmaputra basin exhibits the most substantial groundwater consumption rate globally, as India annually extracts an estimated 230 billion m³ (equivalent to 5% of the total volume of Lake Michigan) of groundwater for irrigation purposes. Though irrigation has been practiced in the region since ancient times, population growth has dramatically increased the use of groundwater for irrigation in recent decades, with dramatic consequences on streamflow. Changes in streamflow have the potential to exacerbate water scarcity and food insecurity, as well as the already devasting floods that impact the economy and the lives of the inhabitants of the Ganges-Brahmaputra. Moreover, the decrease in streamflow in the Bay of Bengal could potentially hasten the impacts of sea level rise and seawater intrusion, with consequences on water quality, the environment, and human migrations.

In this study, we disentangle the impacts of climate change and human management (i.e., groundwater depletion caused by irrigation) on the streamflow of the Bay of Bengal by using a holistic and cumulative assessment of the impacts of the different factors driving the water availability over the bay. A land surface model reanalysis has been constructed over the past two decades by assimilating satellite-based observations of irrigation, terrestrial water storage (i.e., the total water storage consisting of groundwater, soil moisture, and snow), leaf area index (i.e., the stage of vegetation), and snow water equivalent to represent the hydrological changes that are influenced by both climatic and human management. The streamflow in the Bay of Bengal is profoundly impacted by groundwater depletion caused by irrigation, as demonstrated in our findings. Despite the increase in precipitation in the Ganges basin, the streamflow in the Bay of Bengal is reduced by as much as 1200 m³/s annually due to groundwater depletion caused by irrigation upstream in the Ganges basin. While a precipitation increase within the Ganges basin, reaching 315 mm/year in some locations, reduces the effects of groundwater depletion on the streamflow upstream, the reduction in precipitation at a rate of ~22 mm/year and the declining snowmelt in the Brahmaputra basin exacerbate the impacts of groundwater depletion on its streamflow. Consequently, while groundwater depletion is mostly pronounced in the Ganges basin, the Brahmaputra basin experiences the most substantial decrease in streamflow, which further amplifies its susceptibility to groundwater depletion and sea level rise. Such vulnerabilities intensify drought conditions, make the drought recovery longer, and worsen water scarcity.

With the onset of climate change, projections have become crucial as they provide guidelines for forthcoming strategies. However, the majority of future projections of the changes in hydrologic dynamics and water resources in the Ganges-Brahmaputra basin solely rely on climatic conditions. As such, these studies predict an increase in streamflow due to an increase in precipitation. However, groundwater depletion caused by irrigation could change the direction of the water availability trends, which could have profound consequences for water management as well as strategies for adapting to and mitigating climate change. This study provides a compelling example of how anthropogenic influences on the water cycle should be comprehensively accounted for during the estimation and prediction of water availability.