Changing flow of rivers can alter monsoon patterns

Bengaluru city, for over a century now, has been depending on drawing water from rivers flowing in different watersheds. With the commissioning of Chamarajendra Water Works in 1894 and the laying of the first pipeline in 1896, the city started quenching its thirst for River Arkavathy.

Years later, in 1958, another committee proposed augmenting the water supply from River Arkavathy by drawing water from River Cauvery. Eventually, the Cauvery Water Supply Scheme was commissioned in 1974 that managed to draw water from Cauvery through the Torekadanahalli pumping station. Today, Bengaluru is dependent upon drawn from Cauvery and Kapila rivers planned through several stages and phases.

Inter-basin water transfer is not restricted to drawing drinking water to Bengaluru. We have examples from many other projects like the Hemavathy Canal Project or the Yettinahole Project.

A new study by a team of researchers has found that interlinking rivers across different river basins can alter the land-atmosphere interactions, and it can also alter the Indian summer monsoon. Prof Subimal Ghosh from the Indian Institute of Technology, Bombay, conceived and undertook this study along with his colleagues from the University of New South Wales, Sydney, Australia, the Indian Institute of Tropical Meteorology, Pune, and the University of Hyderabad.

Interlinking rivers involves connecting rivers in different watersheds through a network of canals, dams and other infrastructure. The basic premise has been to redistribute ‘surplus’ water in one basin to another that is experiencing a ‘deficit’. The notion of the deficit is derived from the higher water demand due to anthropogenic activities.

The interlinking of rivers could affect the natural flow of water besides impacting the ecosystem downstream and can have unintended consequences. However, the proponents of the interlinking of rivers assume that the river basins are hydrologically independent. And that there is no feedback from the interlinking of rivers across basins and thus it will not affect the rainfall patterns.

To date, no scientific studies had explored the impact of feedback on inter-basin water transfer and the water cycle. Addressing this gap, the researchers hypothesise that the water transfer may impact the source or adjacent basins through land-atmosphere interactions. The researchers tested this hypothesis by developing a causal network between the atmosphere and land variables across the river basins in India using the information theory-based transfer entropy and a causal network learning algorithm.

The variables considered in this study are: soil moisture, latent heat flux, sensible heat flux, precipitation, relative humidity, wind speed, incoming shortwave radiation, and temperature across major river basins of India.

The researchers carried out the causal analysis and represented the association between variables across different basins as networks. This analysis demonstrated that the causal relationships between land variables across basins through land-atmosphere, atmosphere-atmosphere, and atmosphere-land interactions are not hydrologically independent.

In other words, it implies that any perturbation in a river basin due to the interlinking of rivers can travel to neighbouring basins through the atmosphere.

The researchers have further used a modified regional climate model, Weather Research and Forecast coupled with Community Land Model 4 (WRF-CLM4) to ascertain the land-atmosphere feedback.

The model (WRF-CLM4) simulations reveal that increased irrigation from the transferred water reduces mean rainfall in September by up to 12% in many parts of India most of which are already water-stressed. This includes western arid regions (Rajasthan and Gujarat), central (Madhya Pradesh), central-eastern (Odisha and Chhattisgarh) and northern (Punjab, Haryana, and Uttarakhand) parts of India.

The researchers noted that a significant population in these regions are dependent on agriculture. And any reduction in monsoon rainfall would have an impact on the socio-economic fabric of these regions leading to increased climate vulnerability and risk.

Cause for water stress?

The researchers note that more drying occurs in La Niña years as compared to El Niño years. They argue that the reduction in September precipitation can lead to the drying of rivers post monsoon causing water stress across the country.

The findings of this study are important on multiple accounts. Since the interlinking of rivers has implications for the Indian summer monsoon, this can accelerate climate-induced vulnerability and risk. The study calls for a rethinking of the interlinking of river projects among policymakers and decision-makers. This also calls for model-guided impact assessment studies of large-scale hydrological projects across the globe considering land-atmosphere interactions.

While this study is conducted at a pan-India scale, locally, ‘surplus’ water from the Hemavathy watershed to Shimsha watershed is known to have altered cropping patterns. Farmers are transitioning from monsoon crops to more water-intensive commercial plantations (areca nut) in the semi-arid landscapes of Shimsha River watershed. And, there are variations in rainfall patterns, but it is too early to attribute it to inter-basin water transfer. More studies at a regional scale could shed more light on this.

Clearly, researches show the negative implications of projects such as the interlinking of rivers on the larger ecosystem within and beyond the river basin. It thus calls for a deeper introspection on the development pathways that would make us climate resilient.

(The writer is with Research Matters)