Interlinking may dry rivers post-monsoon and render it "dysfunctional": Study

Escalating global warming, population growth, pollution, and changes in land use are pushing India to a rapidly growing problem of water stress. As per the Central Water Commission, Government of India, the current per capita availability of water in India is around 1400 cubic metres. This is slated to reduce to about 1200 cubic metres by 2050. A large portion of the country is already classified as water-stressed. Irrigation in agriculture consumes a significant fraction of India's water resources. With the rapid intensification of agriculture, the water demand will only further increase in the coming decades.

In such a scenario, India will need most of its runoff to meet its urban and agricultural needs within the next 20 years. To cater to the water demand of the growing population and to offset the observed increase in droughts and floods in India, massive river interlinking projects are proposed. These projects intend to transfer water from surplus to deficit river basins. The goal is clear: to minimise the amount of water flowing from river basins into the oceans through a vast network of reservoirs and canals. And, harness it on land to meet the increasing water requirements of the country.

However, a recent study published in Nature Communications raises concerns over the proposed projects lacking an in-depth understanding of the hydrometeorological consequences of river interlinking. The study contends that river basins have atmospheric connections. Hence, feedback from linking a basin to other river basins can bring significant disturbances in the rainfall patterns within the river basins. The study uses climate models and simulations to show that land-atmosphere feedback generates causal pathways between river basins in India. It also stipulates that increased irrigation from the transferred water can reduce mean rainfall in September by up to 12% in already water-stressed regions of India.

Researchers believe that "reduced September precipitation can dry rivers post-monsoon, augmenting water stress across the country and rendering interlinking dysfunctional." The findings highlight the need for model-guided impact assessment studies of large-scale hydrological projects in the country and across the globe. 

Context

Large international rivers, such as the Ganga, the Brahmaputra, and the Indus, are central to the development of agriculture-dominated India. However, like river basins around the globe, Indian river basins are also under severe stress due to global climate change, massive population growth, increased uncontrolled human water use, and pollution. Indian summer monsoon (ISM) from June to September is the primary source of water in Indian river basins. This accounts for almost 80% of the country's annual rainfall and governs the country’s  Gross Domestic Product (GDP). Over the last few decades, ISM has experienced a decline in mean precipitation and an increase in the intensity, occurrences, and spatial variability of extreme rainfall. Such changing meteorological patterns have increased hydrologic extremes, floods, and droughts in India. These hydrologic changes have augmented the water stress across the country, elevating the risk of disasters or extreme weather events.

As an adaptation measure to combat the increasing hydrologic extremes, India came up with the National Perspective Plan (NPP) for inter-basin transfer. The plan has identified 16 links under the peninsular rivers component and 14 links under the Himalayan component with a proposed budget of USD 168 billion. The proposal involves a network of canals with an approximate length of 15,000 km and 3000 reservoirs with a capacity to transfer 174 billion cubic meters of water each year from surplus to deficit basins and generate 34 million kilowatts of hydropower along with benefits like flood control, drought mitigation, and navigation.

The experiences of river interlinking in China are known to achieve the stabilisation of groundwater. However, previous research has also shown that such an ambitious plan may significantly impact the ecology of the aquatic ecosystem and fish diversity.

However, this study is one of its kind in exploring the possibility of feedback from the inter-basin water transfer to the water cycle and their impact on donor or adjacent basins through land-atmosphere feedback.

Hydrological interdependence between river basins

The research attempted to develop a causal network between the atmosphere and land variables (soil moisture, latent heat flux, sensible heat flux, precipitation, relative humidity, wind speed, incoming shortwave radiation, and temperature) across the Ganga, Godavari, Mahanadi, Krishna Narmada-Tapi, and Cauvery river basins in India. 

The study demonstrates that the causal relationships between land variables across basins- land-atmosphere, atmosphere-atmosphere, and atmosphere and interactions- are not hydrologically independent. A perturbation in a river basin due to the proposed interlinking can travel to the neighbouring basins by atmospheric pathways. This result is at odds with the conventional assumption of the absence of atmospheric links between river basins while planning hydrological projects. The study also shows that through land-atmosphere feedback, river interlinking projects in India will affect the Indian summer monsoon, reducing September rainfall in the country's dry regions and further aggravating water stress.

How does land-atmosphere feedback affect other river basins?

Causal connections between land variables of two different basins, A and B, exist through a series of indirect links: land variable (river basin A) → atmospheric variable (river basin A) →atmospheric variable (river basin B) → land variable (river basin B).

For example, the link originating from the latent heat of Ganga goes to the temperature, precipitation, and humidity of Ganga, showing that the evapotranspiration of Ganga basin contributes to the moisture content in the air and consequently, temperature and precipitation received within the Basin. This precipitation is called recycled precipitation. The intra-basin land-to-atmosphere connection happens in the form of soil moisture contributing to the moisture content of the air through evapotranspiration (high evapotranspiration during high soil moisture) while also causing surface cooling.

The supplied moisture by evapotranspiration can either lead to recycled precipitation in the same basin or get transported to faraway regions by the action of wind. This evaporative cooling changes the thermal contrast between ocean and land or in between different land regions. Thereby, changing wind patterns and, subsequently, the moisture transport and rainfall. 

Based on the above causal analysis, the researchers show that a perturbation in the land variables of a receiver basin due to the proposed interlinking can further affect its neighbouring basins. For example, feedback between Ganga and Mahanadi can lead to changes in spatial patterns of the Indian Summer Monsoon.

Changes in spatial patterns of Indian Summer Monsoon 

The research establishes that the hydrological processes across river basins are not independent and that the assumptions made for the interlinking are not valid. The perturbed hydrological processes of the receiving river basins send feedback to the Indian monsoon, potentially changing the spatial patterns, specifically in September. Such changing patterns of monsoon, in turn, affect the hydrology of the neighbouring basins. This is a critical result that most large-scale hydrological projects across the globe, including river-linking projects in India, do not consider while planning. 

The researchers performed simulations to show that river interlinking could lead to a systematic reduction of the mean September precipitation of up to 12% in the western arid region (states of Rajasthan and Gujarat), central (state of Madhya Pradesh), central-eastern (states of Odisha and Chhattisgarh) and northern (states of Punjab, Haryana, and Uttarakhand) parts of India. This was attributed to the land-atmosphere feedback from interlinking. 

The researchers also separately analysed El Niño and La Niña years to understand the interannual variations of the land-atmosphere feedback from the excess irrigation proposed through interlinking. It was found that soil moisture drying due to excess irrigation is expected to be more prominent in La Niña years compared to the El Niño years. The whole central Indian belt from the desert regions of Rajasthan to the eastern coast shows a decline in rainfall and temperature increase. Hence, soil moisture declines in the La Niña years. The dry western region shows a decline in rainfall and soil moisture even for the El Niño years with an increase in temperature.

The drying of the arid region due to interlinking could be alarming. Hence, needs to be addressed in the planning for interlinking.

Why should we be worried?

As per the study, this reduction in September precipitation will dry up the rivers in the subsequent months, amplifying water stress manifolds in various parts of the country- an unexpected and unintended result of interlinking. The research warns that a reduction in precipitation of a donor basin can reduce its ability to send water to other deficit basins post-monsoon. Furthermore, a decrease in monsoon rainfall can cause reduced crop yields in regions dependent on agriculture, leading to increased climate vulnerability and risk.

The study only reiterates the concerns that several environmentalists and conservationists have raised over the decades- to not view river basins as isolated and independent entities existing in a vacuum. While the study acknowledges that such unexpected feedbacks were unforeseen in the planning stage, it underscores the need for a deeper evaluation of the land-atmosphere feedback processes in arriving at any policy decisions related to the interlinking. Hence, the researchers argue that the water balance of interlinked river basins needs to be carefully re-evaluated after including impacts of land-atmosphere feedback.

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