It was reported in August 2019 that a team of researchers from the Indian Institute of Technology, Bombay (IIT-B) has found the reasons behind the phenomena causing shift in monsoon rainfall to north-western India and extreme rainfall over central India. According to the researchers, any change in irrigation policy can result in shift of monsoon rainfall and intensify the event of extreme rainfall in India.
For conducting the study, the researchers developed a module of land-surface model in collaboration with the Pacific Northwest National Laboratory and Oak Ridge National Laboratory.
They took into consideration the actual soil and irrigation pattern followed in India and it was for the first time that a model especially designed for Indian conditions was used. Taking into account Indian conditions formed a prerequisite of the study due to the fact that irrigation pattern followed in India is different from what is followed in the west.
Generally, models used for such studies assume that irrigation starts only when soil moisture is very low (referred to as permanent wilting point) and stops when it reaches slightly below saturated soil moisture state (field capacity). But the situation is different in India as there is uncontrolled irrigation in India and paddy fields which account for about 50 per cent crop area in India remain in submerged conditions. So, there is a wide difference between the Indian conditions and that of the West as far as effect of moisture from the land to the atmosphere is concerned. So, in order to get appropriate results, any land-surface model used for India should take into consideration the Indian irrigation and agriculture system. Otherwise, incorrect outcomes will be derived.
According to researchers, during the month of September, when crops are matured, maximum evapotranspiration takes place due to high irrigation of agriculture lands, which results in highest contribution of moisture from the land to the atmosphere. This excessive irrigation over northern India causes the summer monsoon rainfall in September to shift towards northwestern India, even as rainfall over central India intensifies. The study further revealed that land-surface processes, including irrigation, affect heat fluxes—temperature-related and evapotranspiration. The combined effect of modified heat fluxes and changes in atmospheric moisture content and distribution result in a shift in rainfall towards northwestern India and increased extreme rainfall over central India during the month of September.
Significance of the Study
The study is the first to show that a change in irrigation management leads to a change in the moisture feedback to the atmosphere. The researchers arrived at the conclusion by studying three conditions—no irrigation, irrigation based on the level of soil moisture, and finally uncontrolled irrigation as is prevalent in India.
Some previous studies have already shown the effect of irrigation on the Indian summer monsoon. But the latest study by IIT-B gives a more detailed analysis of the phenomena pertaining to the effect of change in irrigation on change in moisture feedback to the atmosphere. The analysis underscored the importance of understanding the soil-to-atmosphere feedback.
Studies do incorporate the impact of rainfall on soil moisture content, but contribution of soil moisture to the atmosphere and rainfall conditions is neglected, as it cannot be easily visualised. In order to understand the phenomena in totality, it is essential that each of the component—atmosphere and hydrology—is studied together.
However, the study did not investigate the effect of irrigation and agriculture on monsoon in southern India. It was concluded that more detailed irrigation data on the southern regions would be needed for a proper analysis of such land-surface feedback in that region.