A study carried out by a team of researchers from IIT Madras and IIT Bombay has found a decrease in spatial variability of mean monsoon rainfall over major river basins in India.

While there is a statistically significant decrease in the monsoon rainfall over major water surplus river basins, there is no statistically significant increase in monsoon rainfall over major water deficit river basins. The results were published on July 28 in the journal PLOS ONE.

“Major surplus basins such as Mahanadi, Godavari, Brahmani and West Flow River-I are witnessing significant decrease in rainfall. Other surplus basins also exhibited decrease in rainfall but they are not statistically significant,” says Prof. Subimal Ghosh from the Department of Civil Engineering, IIT Bombay.

More than 10%

The decrease in water yield in recent periods in major surplus basins has been more than 10 per cent in the case of Mahanadi and West Flow River-I. In the case of other surplus basins, the decrease has been within 10 per cent.

Though water yields in major deficit river basins such as Indus, Ganga and East Flow River-II have increased during the period 1976-2000 compared with the previous 25 years (1951-1975), the trend is not statistically significant, says Prof. Sachin S. Gunthe, the corresponding author of the paper from the Department of Civil Engineering, IIT Madras.

Ganga, which is a major water deficit basin, has seen significant increase in rainfall, while Yamuna, Krishna and Cauvery river basins exhibit a decrease.

There has been an increase in extreme rainfall events in India but this does not play a significant role in water availability as the surplus water gets wasted.

“Stable distribution of rainfall during the entire monsoon period is what is most desirable,” says Prof. Gunthe.

Challenges to interlinking

As a previous study had pointed out, the latest study has found that the amount of summer monsoon rainfall over Indian river basins does not support the paradigm “wet gets wetter and dry gets drier”.

The changes in rainfall pattern over major river basins in India raises concerns regarding the suitability and viability of interlinking major river water basins.

“The water demand in a surplus basin first needs to be assessed and then met under decreasing water availability scenarios before transferring water to deficit basins. Hence, planning for inter-basin water transfer [in order to supply water from surplus to deficit river basins] necessitates an immediate reassessment with a systematic approach,” they write.

Published in The Hindu on July 29, 2016